Nuclear Thermal Rocket Engines

[enigma]

One of the pleasant side-effects of the unpleasant fact that the US pulled out of the nuclear non-proliferation treaties is that NASA is now free to restart research into nuclear thermal rockets.

For those not familiar with the concept, a lightweight gas (helium or hydrogen) gets passed through a "combustion chamber" which doesn't combust anything. Instead of oxodizer and fuel getting burnt to raise it to high temperatures and pressures, it merely sucks the heat out of a nuclear reactor and gets accelerated through a standard converging/diverging nozzle.

The main upside is that nuclear rocket motors provide two to three times as much thrust as conventional rockets, enabling much faster transit times. Another plus is the capability to siphon off heat which can be converted to electricity, providing the potential for large missions to the outer planets where solar cells don't cut it.

What does everyone think about the possibility of us building one of these? It's a nuclear reactor, and the guys at NASA would be the ones flying it... still, the potentials are huge if all goes well.

[Jonathan]

I don't like the idea of nuclear anything riding piggy-back on a giant explosion machine, we already have enough radioactive waste to deal with, let alone if a rocket explodes and throws a bunch of it all over the place. On the other hand, without any actual combustion, it's less likely there'll be an explosion.

[drag]

Greetings !

I'm in favour of nuclear propulsion, abviously as long
as it's used in space(not crazy stuff like the Orion
project). A rocket with an Isp of 900 can go a long way
beyond our best Isp 450 today and can get people to Mars.

However, it won't be of much use for lower mass spacecraft,
although I did see some designs of small reactors with a
few MWs and no moving parts to power primary electric
propulsion on medium sized spacecraft, like interplanetary
probes.

If I'm not mistaken it would even be better to use the
reactor on large vessels (like manned spacecraft) to power
an MPD since it has a considrably higher efficiency and
not very massive by comparison when a large vessel is used.
It could also power a VASIMR engine if its found preferable
to the MPD for a mission.

Anyway, with what I know today it would take me some
time to connect the figures. So, Enigma can you make
an estimate of what would be the best of these three
options for primary propulsion of a manned Mars mission
with a nuclear reactor already serving as a power plant
(I'm mainly unsure about MPD vs VASIMR comparisson at
their present levels of development) ?
(In terms of costs - mass to orbit, and lenght of the trip.)

Also, how adaptable would a nuclear thruster like
this be in terms of propellant matter ? Could we use
matter on Mars or the Moon and just throw it "in
the pot" and let it fly out without too much refining/
conditioning/collecting specialties involved ?

Thanks. [:)]

Live long and prosper.

[enigma]

Originally posted by drag
If I'm not mistaken it would even be better to use the
reactor on large vessels (like manned spacecraft) to power
an MPD since it has a considrably higher efficiency and
not very massive by comparison when a large vessel is used.
It could also power a VASIMR engine if its found preferable
to the MPD for a mission.


To be honest, I don't know too terribly much about the VASIMR or MPDs, other than they are both electric engines.


Anyway, with what I know today it would take me some
time to connect the figures. So, Enigma can you make
an estimate of what would be the best of these three
options for primary propulsion of a manned Mars mission
with a nuclear reactor already serving as a power plant


If you've already got the nuclear reactor, then the nuclear propulsion would probably be the way to go. Electric engines are HEAVY, and although they are more efficient, it takes a long time to ramp up the velocity, because their thrust is so low. If you've spent the money launching one power plant, if it can be easily tied to the propulsion system, why spend the money to launch a seperate system?

Another nice feature would be the radiation shielding required. Radiation from galactic cosmic rays (GCR) and solar particle events (SPE) are huge problems for interplanetary missions, particularly if you're sending "squishies" out there. We're protected planetside and in orbit from SPE's by the Earth's magnetic field. GCRs are blocked by the atmosphere, and half the time for orbitting craft by the Earth itself. When you're in deep space, you get the full brunt from both. If you use a nuclear reactor, then you'll need radiation shielding for the engine's constant radiation, so it's an added bonus that it covers against random events.

If the probability is that you may get hit by X dose, you design to cover based on the risk of the event. If you know you'll be exposed to X+Y radiation, you need the shielding regardless, so there isn't any wiggleroom with the higher-ups.


Also, how adaptable would a nuclear thruster like
this be in terms of propellant matter ? Could we use
matter on Mars or the Moon and just throw it "in
the pot" and let it fly out without too much refining/
conditioning/collecting specialties involved ?


You know, I'm honestly not sure. That would have to be analysed by the engineering team.

I do know that the best fuels are those with the smallest atomic masses, so hydrogen is A#1. Theoretically, I don't see anything that would keep us from using heavier gases harvested in-situ, but I don't think you'd want to 'wing it' with something like that. The containment vessels and engine would need to be designed to operate on both.

[NEOclassic]

Originally posted by enigma
For those not familiar with the concept, a lightweight gas (helium or hydrogen) gets passed through a "combustion chamber" which doesn't combust anything.

Hi Enigma,
Back in the 60s when Los Alamos tested, in Nevada, the KIWI rocket engine the apparant idealized propellant was ammonia gas. No matter what propellant was used, the inability to recycle the gas meant that any reasonable amount would likely be soon exhausted for any mission beyond the MIR. Cheers, Jim

[FZ+]

If I remember the Orion programme properly, it was estimated that each launch would kill about 6 people due to increasing the level of radiation in the atmosphere. So, is this safe? And can you persuade the public that this is safe?

[enigma]

Orion is a completely different animal.

Orion was a launch vehicle which spat bomblets out the back which were then detonated, and the craft rode the explosion wake up.

The nuclear thermal rocket is more like a nuclear reactor on a sub. It heats a fuel and spits the material out the back, not the radioactive material.

Most plans propose using the rocket as a third stage for use once it is out of the Earth's atmosphere, not as a primary launch phase.

[selfAdjoint]

It seems to me a reasonable criterion that a space nuclear engine would not be turned on till the basic speed was above circular velocity. This would ensure that the active engine could not accidentally fall back to Earth. And the "fueling" could be done a little at a time so that a rocket accident at any stage would have low pollution potential.

For a proposed Mars mission these don't seem like overly restrictive constraints.

[drag]

Greetings !
Originally posted by enigma
If you've already got the nuclear reactor, then the nuclear propulsion would probably be the way to go. Electric engines are HEAVY, and although they are more efficient, it takes a long time to ramp up the velocity, because their thrust is so low. If you've spent the money launching one power plant, if it can be easily tied to the propulsion system, why spend the money to launch a seperate system?

Well actually you'll be spending a lot more money on
launching the fuel. A Mars manned (quick) mission would
require probably about 13 miles/sec total velocity change
which with a nuclear thruster Isp of 900 would require
about 7 times more fuel mass than the total remaining
ship mass. Not to mention that you'll also need to
carry the fuel for the return trip (at least for the
first manned missions) of a serious part of the
spacecraft (reactor, thruster, crew, return samples
the returning modules themsleves and more).
Also, you'll need stuff like the nozzle for example,
either way.

A VASIMR or MPD thruster can operate at greater efficiency
and at the advised, according to a number of studies I
saw, Isp ranges of 4000 to 5,500 they will require
a lot less fuel - about equal to the remaining spaceship
mass for going both ways. Further more, I heard that
gradual thrusting can shorten the trip, but I'm not certain
about this part. Also, the thrusters are not that large,
since the thrust is gradual you don't need a big thruster.

In addition, a VASIMR engine in high thrust/low Isp mode
could, theoreticly, also be used to land and launch
small modules on Mars (though I doubt that it would be done
since it envolves risking and relying upon the primary engine/s
for all the manned mission's stages).
Originally posted by selfAdjoint
It seems to me a reasonable criterion that a space nuclear engine would not be turned on till the basic speed was above circular velocity. This would ensure that the active engine could not accidentally fall back to Earth. And the "fueling" could be done a little at a time so that a rocket accident at any stage would have low pollution potential.
Well, that kin'na beats the purpose, doesn't it ?
What's so dangerous about an orbital accident, even
if it's the worse case scenario - total meltdown
and explosion (which is BTW pretty hard to "achieve"
in space). Most of the debris will stay there, as
for some of it that will reach the atmosphere - I
don't know about this, wouldn't it moslty melt during
reentry ?

Live long and prosper.

[selfAdjoint]

At all the usual orbital altitudes there is still some atmospheric drag; orbits do decay. I was proposing to avoid that.

As for boosting out of orbit on chemical or other and then pulling the rods when the speed is high enough, I don't think there's too bad a penalty for that.

[selfAdjoint]

BTW I googled on VASIMR, and I note in the description phrases like "the gas will be ionized" and "the gas will be heated". How? By nuclear power?

[drag]

Greetings !

So what happens to that stuff when it experiences
reentry - will it just normally disperse in the atmosphere ?
Originally posted by selfAdjoint
BTW I googled on VASIMR, and I note in the description phrases like "the gas will be ionized" and "the gas will be heated". How? By nuclear power?
Doesn't matter. The gas is heated by RF coils
and plasma containment is electromagnetic. One of
the cool things is that the nozzle is variable and so
you can produce high thrust with low Isp like that
of even chemical propulsion or go all the way up
to 200 miles per sec (though I can't imagine what
a mission would be worth wasting so much energy on -
unless you've accidently almost ran out of fuel or somethin').

http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/develop.pdf
(Sorry for the PDF file, there used to be those nice htmls
with pictures and all, but now the access to those sites is restricted - I guess they're afraid of space terrorists...[:D])

Live long and prosper.

[FZ+]

Originally posted by enigma
The nuclear thermal rocket is more like a nuclear reactor on a sub. It heats a fuel and spits the material out the back, not the radioactive material.

Most plans propose using the rocket as a third stage for use once it is out of the Earth's atmosphere, not as a primary launch phase.
Ah... this sounds much more interesting. But I still fear public opposition to the idea of strapping lots of nuclear material to the top of a lot of explosives...

Is the first stage of the launch vehicle itself reliable for nuclear material carrying? An accident could be nasty....

[LURCH]

Does anyone here remember the public outcry against the launching of Cassini? And it only had a very small amount of fuel and no reactor; just relying on radiative decay to generate ellectricity. This is one of the reasons to go back to the moon, IMO. Send inert materials there for enrichment and launch them as refueling tankers from the surface. If a launch fails (as one innevitably must) it will be in an environment where there is no atmosphere to contaminate.

[enigma]

Originally posted by FZ+
Is the first stage of the launch vehicle itself reliable for nuclear material carrying? An accident could be nasty....

They use the same types of launch vehicles for ICBM's...

Yes, there will be public outcry. The public is stupid.

There is enough radiation in outer space to kill your ordinary human in several days (if not sooner) unless radiation shielding is built into the craft. If Apollo 8 had been launched three days earlier, the entire crew would have been killed from the radiation from a solar flare.

People greatly overestimate the damage we frail humans can cause to the universe.

[BCRion]

Nuclear Reactors for space propulsion would use Uranium-235 as their fuel. U-235 and U-238 are quite low on the radioactive scale and you will find traces (on parts per billion) in natural seawater -- enough to power the entire planet with breeder reactors for well over a billion years.

In nuclear propulsion, one does not activate the reactor until the craft is safely out of orbit, so in the worst case scenario, all that would happen is there would be some relatively safe Uranium crashing back down to earth which even if it did escape containment (which tests show that it would not), there would be negligible environmental impact.

You would probably not want to have the reactor re-enter since fission products are a lot worse than the Uranium fuel. It may be best to eject the spent rods back into outer space. As far as the whole pollution argument, space is really a sea of radiation, some spent fuel rods pale in comparison to the radiation already present.

[kerimek]

Originally posted by BCRion
Nuclear Reactors for space propulsion would use Uranium-235 as their fuel. U-235 and U-238 are quite low on the radioactive scale and you will find traces (on parts per billion) in natural seawater -- enough to power the entire planet with breeder reactors for well over a billion years.

In nuclear propulsion, one does not activate the reactor until the craft is safely out of orbit, so in the worst case scenario, all that would happen is there would be some relatively safe Uranium crashing back down to earth which even if it did escape containment (which tests show that it would not), there would be negligible environmental impact.

You would probably not want to have the reactor re-enter since fission products are a lot worse than the Uranium fuel. It may be best to eject the spent rods back into outer space. As far as the whole pollution argument, space is really a sea of radiation, some spent fuel rods pale in comparison to the radiation already present.
I think that the best option for spaceship is reactor with thorium232 - uranium 233 breeding cycle. It requires only a bit of uranium for start and rest of fuel is stable thorium (thorium is as common as iron). u233 is fissionable by rapid neutrons, which allows minimal size and weight of the reactor. U233 is similar fuel as plutonium239 (used in nuclear submarines), but without its radioactivity and toxicity. Main problem of the nuclear spaceship is the fact that it must be build on orbite, because of no nuclear engine can overwhelm earth gravity.

[drag]

Originally posted by kerimek
Main problem of the nuclear spaceship is the fact that it must be build on orbite, because of no nuclear engine can overwhelm earth gravity.
What do you mean ?

[kerimek]

Originally posted by drag
What do you mean ?
Large rocket engine has thrust typically in meganewtons and power output in tens of gigawats. If spaceship has 1000tons, then you require about 10MN thrust to get up it from earth surface. This is impossible to reach with any nuclear reactor (without its destruction). Adventage of nuclear fuel is in its total energy (and final speed), not in acceleration. Chemical rocket can accelerate on example 100s on 10G. Then final speed will be 10km/s. Nuclear ship can accelerate millions of second, but only on 0.1G Final speed will be thousends of km/s.

[drag]

Well, actually we weren't discusssing the use of
nuclear rockets in the atmosphere. They are too polluting
and too dangerous for that. Also, although spaceships
and launch vehicles today have much lower mass than
the one you've mentioned they'll still require up to
a few GW for the really massive ones - which today
means an average building size and mass reactor which
indeed makes it unfeasable. Although, you could use small
nuclear explosions like the Orion project suggested -
if you don't care about the planet you're leaving... [:D]

(BTW, acceleration due to rocket thrust is never linear.
As you burn your fuel the spaceship becomes less massive
and the acceleration increases. That's why the G forces
on astronouts are very low at launch and then grow
to barely managable levels before the rockets burn-out.)

Live long and prosper.

[kerimek]

Originally posted by drag
Well, actually we weren't discusssing the use of
nuclear rockets in the atmosphere. They are too polluting
and too dangerous for that. Also, although spaceships
and launch vehicles today have much lower mass than
the one you've mentioned they'll still require up to
a few GW for the really massive ones - which today
means an average building size and mass reactor which
indeed makes it unfeasable. Although, you could use small
nuclear explosions like the Orion project suggested -
if you don't care about the planet you're leaving... [:D]

(BTW, acceleration due to rocket thrust is never linear.
As you burn your fuel the spaceship becomes less massive
and the acceleration increases. That's why the G forces
on astronouts are very low at launch and then grow
to barely managable levels before the rockets burn-out.)

Live long and prosper.

I consider nuclear ship only for interplanetar manned flights. And the interplanetar ships probably take a few thousands tons. Regardless, they will be substantially smaller than todays submarines or carriers. So i think that some 100-1000MW is sufficient. We must only wait for general restart of nuclear industry... Maybe China try it at first, communists have their Greens under control...

[ATCG]

Forget Nuclear, anti-matter is the coming future for rocket propulsion. In fact, I have the schematics for a hybrid anti-matter rocket. Unfortunatly anit-matter is the most expensive substance in the world and it is predicted that the rocket would let off immense gamma radiation thousands of miles long. Another drawback is the weight. The current design for this type of rocket weighs about 400 tons. On the plus side, the rocket could fly to mars on one-billionth of a gram of anti-matter in a few weeks instead of months. This is attainable because antimatter/matter annihilation produces the highest known physical reaction in the world. The rocket technology will really revolutionize space travel and will possibly make travel to other regions of deep space possible.
[8)]
-ATCG

[BCRion]

Unless we have some great technological revolution in antimatter production, it is out of the question for any serious mission. There are not even milligram of that stuff on the planet. Until antimatter becomes economical (which has a very bleak forcast) nuclear propulsion is the best we can do.

[TonySlim]

just siphon some anti-matter from the multiverse. how far away could it really be?

[Beast]

Anti matter is the ideal fuel. It is a fuel like hydrogen(H, D, or T) is a fusion fuel, uranium is a fission fuel, and chemical rockets use a chemical fuel.

You can either mine or generate the fuel. Either the way the magic thing you need is energy. So you can burn oil or use hydrogen fusion to generate the anti-matter.

Its really like charging a battery. And an anti-matter engine would be dead simple. It the anti-matter containment that is a tiny challenge.

[russ_watters]

Originally posted by Beast
Its really like charging a battery. And an anti-matter engine would be dead simple. It the anti-matter containment that is a tiny challenge. Manufacturing it is also a "tiny" challenge. And no, you can't mine it.

[Beast]

Sorry I didn't mean you can mine antimatter. I mean you mine iron or coal, and use the energy to create hydrogen or antimatter. More resposible would be to use solar power.

Surely an antimatter engine is easy. You have a parabolic mirror, and burn a positron and electron at the focal point. Then high energy gamma rays are relected out the back pushing the ship forwards. No matter is left so no shock wave, just make sure the mirror will properly reflect high energy gamma rays.

The positrons can be cold and contained in a magnetic bottle.

[drag]

Greetings !

Well, first of all, like was mentioned before
generating any significant amount of anti-matter
(significant meaning miligrams) is practicly impossible
today, unless you've got a few US GDPs to spare. [;)]
Second, storing the anti-matter in considrable
amount aspecialy on a spaceship would be extremely difficult,
unless you actually produced anti-matter atoms -
which would even further increase the costs.
Third, using it for propulsion is also rather difficult,
there's no "mirror" you can use for such high energy
radiation, and there's no reason for you to do this
anyway since it will provide poor impulse. The more
reasonable idea is to use it to power some high Isp
plasma propulsion. Even today we have ideas and patents
of such systems but it is unreasonable to use them
unless you have a major power source. Extracting the
energy of matter-antimatter reactions again, however,
presents a considrable technological difficulty because
of the highly energetic radiation one has to capture and
effectivly convert to usable energy. As far as I know,
we have no such technology for now, but we may
build it in the future if and when we'll be able to
produce large quantities of anti-matter and handle it.

Live long and prosper.

[Mirabilia]

Hm...anti-matter....natural substance.......can be made on earth.......NO! Can't possibly be true. Could it?

[Mandrake]

I did my graduate thesis on a cavity core reactor design. At that time there were a number of nuclear rocket engine concepts. One that you will see in old documents is the nuclear light bulb engine. The idea of these designs is to produce a substantial thrust and to do it with fuel that is expelled from the engine. Hexifloride gasses of U or Pu were considered. The nuclear light bulb was supposed to generate about 1.2 million pounds of thrust per engine. Gasseous fuels were discussed as early as 1955.

The design I analyzed was not intended for rocket application; it was a breeder reactor, fueled with Pu hexifloride. The core was essentially a cavity (pressure tank) in which the reaction rate was controlled by gas density. It was designed to produce 3000 MWt. If anyone happens to be interested in such a strange design, here are a few of my findings:
fissile mass = between 4000 and 5000 Kg (depending on other factors)
pressure = somewhat over 600 psia
breeding ratio = about 1.4

[pervect]

Sorry I didn't mean you can mine antimatter. I mean you mine iron or coal, and use the energy to create hydrogen or antimatter. More resposible would be to use solar power.

Surely an antimatter engine is easy. You have a parabolic mirror, and burn a positron and electron at the focal point. Then high energy gamma rays are relected out the back pushing the ship forwards. No matter is left so no shock wave, just make sure the mirror will properly reflect high energy gamma rays.

The positrons can be cold and contained in a magnetic bottle.

This isn't anything like the anti-matter rocket proposals I've seen.

I don't think we know how to make a gamma ray mirror for normal incidence gamma rays. Multilayer dielectric mirrors are the highest frequency mirrors I'm aware of, and they only go up to soft x-rays. The prospects for going up higher are worse.

Current designs use the nucleus of the anit-matter (with the positrons, too, but they only contribute 1/2000 of the energy). The main decay mode is into pions. About 2/3 of the pions are charged, which turns out to be very convenient.

Take a look at

http://www.islandone.org/APC/Antimatter/02.html

for solid core, gas core, plasma core, and the beam core anti-matter rocket designs.

Alas for these designs, we still don't have a cheap enough way of manufacturing anti-matter.

Probably the most efficient use known of anti-matter would be the anti-proton induced fission fragment designs, such as in

http://niac.usra.edu/files/library/fellows_mtg/oct02_mtg/pdf/740Howe.pdf

This is all currently highly theoretical, though.

[Orion1]

Data results from some nuclear rocket programs:

NERVA-Derived Reactors:
Mass: 2,555 kg
Thrust: 71.6 kN to 981 kN
Hydrogen Flow: 8.5 kg*sec^-1
Thermal Power: 354 MWt - 4500 MWt
Fuel: UC-ZrC-C in a graphite matrix
Exhaust: 5500 F

KIWI Series:
85 MWt-900 MWt
Exhaust: 2683 K
Fuel: cylindrical Uranium Oxide elements in graphite modules

Phoebus Series:
1090 MWt - 5,000 MWt
Exhaust: 2370 K
Fuel: Niobium Carbide coated

PEWEE:
503 MWt
core power density: 2340 MWt*m^-3 average and 5200 MWt*m^-3 peak
Exhaust: 2550 K
Fuel: Zirconium Carbide coated

Nuclear Furnace Series:
44 MWt
core power density: 4500 to 5000 MWt*m^-3
Exhaust: 2500 K
Fuel: Uranium Carbide

NRX Series:
1100 MWt - 1200 MWt
Thrust: 334 kN

XE':
1100 MWt

SNRE:
Thrust: 73 kN


"One overriding lesson from the NERVA program is that fuel and core development should not be tied simply to a series of engine tests which require expensive nuclear operation. Definitive techniques for fuel evaluation in loops or in non-nuclear heated devices should be developed early and used throughout the program..."

Reference:
http://en.wikipedia.org/upload/2/22/NASA-NERVA-diagram.jpg
http://www.f104g.demon.co.uk/space/images/nerva.jpg
http://grin.hq.nasa.gov/IMAGES/SMALL/GPN-2000-000697.jpg
http://www.space.com/images/h_ntr_diagram_072000_02,0.gif
http://en.wikipedia.org/upload/thumb/7/7f/180px-NASA-KIWI-A-prime.jpg
http://en.wikipedia.org/wiki/Nuclear_thermal_rocket
http://www.fas.org/nuke/space/c04rover.htm
http://www.lascruces.com/~mrpbar/rocket.html

[marcus]

I did my graduate thesis on a cavity core reactor design. At that time there were a number of nuclear rocket engine concepts. One that you will see in old documents is the nuclear light bulb engine....

that is interesting
I remember seeing a "nuclear light bulb" design discussed in a nasa document from the 1970s IIRC
the walls of the cavity were (intended to be) protected by a flow of gas
along the walls IIRC
the fuel mass was achieved in a gas phase---just as you say here.

If memory serves, also at that time "pebble bed" designs were considered somewhat like a HTGR (high temperature gas-cooled reactor) for propulsion.

Mandrake please tell me what you think: suppose someday for a Jupiter mission----landing on an icy Jovian moon----NASA or some such agency wants a rocket which the crew can resupply with propellant derived from locally available water ice.

Could you,without a lot of work, roughly compare some practical alternatives----some kind of nuclear rocket using a reactor of some type to heat water or to heat hydrogen (derived from the ice)----or a chemical rocket.

I am interested in knowing if it is possible to have a low maintenance
robust propulsion technology whose propellant can be resupplied from materials found on site, perhaps in low or moderate gravity.

[Mandrake]

that is interesting
I remember seeing a "nuclear light bulb" design discussed in a nasa document from the 1970s IIRC
the walls of the cavity were (intended to be) protected by a flow of gas
along the walls IIRC
the fuel mass was achieved in a gas phase---just as you say here.
Yes. The big advantage of a fissile gas fueled reactor is its simplicity. If the gas can be expelled, there are other advantages, but there is the obvious concern for spewing fission products around.

If memory serves, also at that time "pebble bed" designs were considered somewhat like a HTGR (high temperature gas-cooled reactor) for propulsion.
I retired from the nuclear business in 1996 and have only occasionally kept in touch with the subsequent work. To the best of my knowledge, the pebbel bed design (a very old concept) remains as one of the most attractive concepts for future fission reactors. For space applications, I am not sure how it would work. Weightlessness would introduce factors that are not present in other designs.

Mandrake please tell me what you think: suppose someday for a Jupiter mission----landing on an icy Jovian moon----NASA or some such agency wants a rocket which the crew can resupply with propellant derived from locally available water ice.
I can only guess, but anything that could be heated and expelled could be used to provide thrust. Water would work, but I don't see any reason why methane or liquids could not be used. The idea is to simply heat the propellant to a very high temperature and let it go!

Could you,without a lot of work, roughly compare some practical alternatives----some kind of nuclear rocket using a reactor of some type to heat water or to heat hydrogen (derived from the ice)----or a chemical rocket.
The alternatives that come to mind fall into two categories:
1 - a reactor used as a heater. This is basically what we have with existing power reactors. The only difference is that the coolant would be ejected. This category could involve a gas core (per my prior comments) or a solid core (as in existing reactors). The general problem is that reactors, even if they used a hexifloride fuel, are heavy and rather complicated. There is also the obvious concern that various safety systems are required to prevent the reactor from going prompt critical.

Low powered reactors, operating in a gravitational field, can be used for such things as heating with relatively simple passive controls. This is one attraction of the pebble bed. Such reactors would be useful for electric power generation, ice melting, etc.

2 - an open system. The big attraction of an open system (fuel expelled) is that it could provide very high thrust. This might be possible in some special situations.

I am willing to write more later, but have to shut down the computer now.

SCRAM!

[enigma]

I can only guess, but anything that could be heated and expelled could be used to provide thrust. Water would work, but I don't see any reason why methane or liquids could not be used. The idea is to simply heat the propellant to a very high temperature and let it go!


You'd want to have the coolant be as light as possible. Higher thrust is attainable through lighter reactant masses. You could use water - which provides the thrust for the highest performance (non-polluting... H-Fl works better) chemical rockets. Even better would be to remove the oxygen from the equation entirely and simply use hydrogen.

On the other side of the coin, hydrogen is near impossible to store large enough quantities for propulsion in gasseous form, and cryogenics are extremely unattractive for long duration missions. For that reason, water may be a better option. It would depend on what the specifics of the mission are (Do note that I haven't done any trade studies on the various options either).

[drag]

Current designs use the nucleus of the anit-matter (with the positrons, too, but they only contribute 1/2000 of the energy). The main decay mode is into pions. About 2/3 of the pions are charged, which turns out to be very convenient.

Take a look at

http://www.islandone.org/APC/Antimatter/02.html

for solid core, gas core, plasma core, and the beam core anti-matter rocket designs.
Anybody got an idea about the efficiences of such systems ?

[Orion1]

The efficiencies for some anti-matter thrusters are already listed in the first reference.

The efficiencies for these design types is high, however, if you incorporate the 'industrial efficiencies', that is, the total amount of energy required to produce the anti-matter catalyst fuel as well as the thruster efficiency, you probably produce an 'industrial efficiency' that is in the micro-percentage.

These designs are interesting for theoretical study, however, it would seem more efficient, given any amount of anti-matter, to be used as a nuclear fusion catalyst for power generation, rather than exhausted as a form of thruster propulsion. The designs themselves make no reference about where the power is produced for the necessary powerful magnetic fields required to store the anti-matter, (another seperate nuclear reactor probably).

Also, such theoretical designs are not conducive for private sector development and advancement, which is a major key if the ultimate goal is to advance science and explore space. It is not an appealing thought to have major world governments with absolute monopolisational control of space travel.

From such a prospect, we as scientists would only result in the expansion of the world governmental conflicts into the very areas of space that private sectors would want to explore the most.

Design simplicity, reliability and industrial efficiency are the real keys here, as well as the promotion of private sector development. This is why thermo-fission rockets are the best design strategy.


Reference:
http://www.islandone.org/APC/Antimatter/apc55.gif
http://www.islandone.org/APC/Antimatter/apc54.gif
http://www.islandone.org/APC/Antimatter/apc56.gif
http://www.islandone.org/APC/Antimatter/apc53.gif

[drag]

Greetings !

Oh... didn't notice the efficiencies listed, thanks.

Provided that anti-matter is stored in the form of anti-hydrogen
and with advanced superconducting containment technology, I believe
that future containment problems could be solved and draw relatively little
power. Production efficiency is doubtedly an issue, if anti-matter
is used it is likely to be produced on Earth and trasnported to
the spacecraft that will exploit its energy.

Don't wan'na go into politics.

Live long and prosper.

[Morbius]

One of the pleasant side-effects of the unpleasant fact that the US pulled out of the nuclear non-proliferation treaties is that NASA is now free to restart research into nuclear thermal rockets.


enigma,

This is puzzling - since the USA has not pulled out of any nuclear
non-proliferation treaties. The USA is still a signatory of the NPT -
the nuclear non-proliferation treaty.

The U.S. Senate did not ratify the CTBT [ Comprehensive Test Ban Treaty ]
that President Clinton signed - so the CTBT will not go into effect; since
under the terms of the CTBT as negotiated by President Clinton, the USA
is one of the 40+ countries that has to agree to the treaty for it to go
into force.

Although the CTBT never went into effect, the USA has abided by, and
continues to abide by the terms of the CTBT since 1992 when the USA
stopped nuclear testing.

The only arms control treaty that the USA has pulled out of is the
ABM [ Anti Ballistic Missile ] Treaty. However, the ABM treaty never
constrained nuclear weapons development.

No treaty constrains the development of nuclear thermal rockets.

There is a constraint against exploding nuclear weapons in space
like in the Orion concept. That constraint remains in force.

Dr. Gregory Greenman
Physicist LLNL

[Morbius]

Sorry I didn't mean you can mine antimatter. I mean you mine iron or coal, and use the energy to create hydrogen or antimatter. More resposible would be to use solar power.

Surely an antimatter engine is easy. You have a parabolic mirror, and burn a positron and electron at the focal point. Then high energy gamma rays are relected out the back pushing the ship forwards. No matter is left so no shock wave, just make sure the mirror will properly reflect high energy gamma rays.

Beast,

What material did you have in mind that reflects high energy gamma
rays?

The wavelengths of gamma rays are too short to reflect off any
material I know of in a "mirror-like" manner.

Gamma rays can be scattered, i.e. deflected by matter - by Compton
scattering for example. But that process does not obey the angle of
reflection equals the angle of incidence like in a mirror - and that's
the whole idea behind a parabolic reflector. Unless you have the
reflection angle equal to the angle of incidence - there's nothing
special about a parabola.

Additionally, if you are only exhausting photons - the amount of
energy you have to expend for the amount of momentum in the photons
[ which produces the thrust ] is extremely large.

You don't get much momentum from a photon for the energy you
expend.

Dr. Gregory Greenman
Physicist LLNL

[Colby]

Remember, if you use oxygen or carbon dioxide as a fuel, oxidation may occur with the uranium fuel rods. The uranium will corrode and flake off, leaving you with less and less nuclear energy. Eventually, it would just shut down completely. There are I believe reactors out there that would be capatible with oxygen or carbon dioxide, but they would not produce as much energy, and oxygen and carbon dioxide have a lot more mass then hydrogen, so there wouldn't be much thrust produced.

I am only telling you what I read last night on nuclear thermal rocket engines. :smile: I was looking to see if a nuclear rocket could use hydrogen and oxygen propellant because that would be vastly easier for a transfer vehicle between the Earth and the Moon. You could load up on LH2 at Earth and then refuel with LOX at the Moon, but it seems that just won't work.

What I'm interested in is thrust-augmented nuclear thermal rocket engines. They inject liquid oxygen in the nozzle after the hydrogen has passed the reactor, and combustion takes place. It gives a big blow to its Isp, but it produces more thrust.

[enigma]

enigma,

This is puzzling - since the USA has not pulled out of any nuclear
non-proliferation treaties. The USA is still a signatory of the NPT -
the nuclear non-proliferation treaty.

Sorry I missed this before. I was referring to the CTBT and anti-ballistic missile treaties, not the non-proliferation treaty.

The point I was trying to make was that research into nuclear engines is ramping up again.


Although the CTBT never went into effect, the USA has abided by, and
continues to abide by the terms of the CTBT since 1992 when the USA
stopped nuclear testing.

As far as we know :wink:


No treaty constrains the development of nuclear thermal rockets.


Yes, but that doesn't mean that it hasn't been extremely un-PC to consider nuclear engines for many years. That's starting to change.

Thanks for catching my errors. :smile:

[enigma]

What I'm interested in is thrust-augmented nuclear thermal rocket engines. They inject liquid oxygen in the nozzle after the hydrogen has passed the reactor, and combustion takes place. It gives a big blow to its Isp, but it produces more thrust.

Why would you need more thrust when you're in space? Fuel efficiency is the be-all-and-end-all in space.

[Colby]

Well you can get to the Moon in 24hr with the technology. ;) It's just interesting, especially someday for space tourism. I know I wouldn't want to be stuck in a module with a bunch of tourists for more than day.

[Morbius]

As far as we know :wink:



enigma,

You can be sure Las Vegans would know - they got jolted by the
underground nuclear tests that the USA conducted at the Nevada
Test Site.

Currently, the USA conducts "sub-critical" tests - that is tests in which
the experiment does not result in a self-sustaining nuclear chain reaction.

Because there's no chain reaction - there's no great release of energy.

The only energy release is due to the chemical explosives. Since there
is no great release of energy - these sub-critical experiments are not
detectable.

However, when President Clinton negotiated the CTBT - he negotiated
it so that sub-critical experiments were PERMITTED. Additionally, the
release of nuclear energy in inertial confinement fusion [ laser fusion ]
was also permitted in the CTBT as negotiated by President Clinton.

I bring this up - because one often hears that the USA is violating the
CTBT because of its laser fusion activities. First, the CTBT is NOT in
effect - so you can't violate it. Second, even if it were - President Clinton
negotiated the CTBT to PERMIT laser fusion.

Dr. Gregory Greenman
Physicist

[NEOclassic]

Currently, the USA conducts "sub-critical" tests - that is tests in which
the experiment does not result in a self-sustaining nuclear chain reaction.

Because there's no chain reaction - there's no great release of energy.

The only energy release is due to the chemical explosives. Since there
is no great release of energy - these sub-critical experiments are not
detectable.

Hi, You might be interested to know that the plutonium that is used to mock-up weapons grade Pu-239 is the very safe nonfissionable isotopes, Pu-242 and Pu 244, that are 90+ percent of the plutonium content of spent reactor fuel rods. The Pu-239 that is "breedered" from the U-238 dilutent of the reactor-grade U-235 is fissioned to the extent of over 70 percent. The cross section (n, fission) for Pu-239 is 742 barns and that for (n, gamma) that creates Pu-240, is 286 barns. The Pu-240's (n, fission) is negligibl and (n, gamma) is 250 barns; that results in Pu-241. 70% of the Pu-241 fissions, (n, fission) - 1010 barns and (n, gamma) is 390 barns. Most of the plutonium that was created has been burned; what remains becomes Pu 242, Pu-243 and Pu-244. Do you think North Korea can make a bomb that works?
In the late 1970s, my hydrodynamics group at Los Alamos was using Pu-242 in 4-pi spherical bombs without fear that, even being compressed, would go nuclear. Of course, these experiments were conducted in confinement vessels so that the Pu-242 could be recycled. It was also important to not scatter it around the firing site for health reasons. Thanks for your audience,
Jim

[Morbius]

Hi, You might be interested to know that the plutonium that is used to mock-up weapons grade Pu-239 is the very safe nonfissionable isotopes, Pu-242 and Pu 244, that are 90+ percent of the plutonium content of spent reactor fuel rods. The Pu-239 that is "breedered" from the U-238 dilutent of the reactor-grade U-235 is fissioned to the extent of over 70 percent. The cross section (n, fission) for Pu-239 is 742 barns and that for (n, gamma) that creates Pu-240, is 286 barns. The Pu-240's (n, fission) is negligibl and (n, gamma) is 250 barns; that results in Pu-241. 70% of the Pu-241 fissions, (n, fission) - 1010 barns and (n, gamma) is 390 barns. Most of the plutonium that was created has been burned; what remains becomes Pu 242, Pu-243 and Pu-244. Do you think North Korea can make a bomb that works?
In the late 1970s, my hydrodynamics group at Los Alamos was using Pu-242 in 4-pi spherical bombs without fear that, even being compressed, would go nuclear. Of course, these experiments were conducted in confinement vessels so that the Pu-242 could be recycled. It was also important to not scatter it around the firing site for health reasons. Thanks for your audience,
Jim


Jim,

I'm afraid that you are in error in your post above. The subcritical tests
DO use Pu-239 Please see:

http://www.llnl.gov/str/Conrad.html

which states:

"In the Livermore experiments, chemical high explosives are detonated
next to samples of weapons-grade plutonium (plutonium-239) to obtain
new insights about plutonium and its alloys in the ensuing microseconds."

Subcritical tests are not tests of weapons - so you don't have a weapons
configuration. There's no chance of the experiment going critical in any
case.

You are also in error in your accounts of what the isotopic mix of spent
reactor fuel is. It is more complex than the simplistic analysis you give
above using just the neutron cross-sections.

The isotopic mix doesn't just depend on the cross-sections, but on the
positioning of the material in the reactor, the flux of neutrons that
the fuel sees and the length of time the fuel spends in the reactor...

In order to determine the isotopic mix of the spent fuel, a priori; one
needs to use a complex reactor fuel cycle analysis computer code.
Argonne National Laboratory has written one such computer code
called "REBUS":

http://www.rae.anl.gov/codes/rebus/

One will get a higher percentage of the higher mass isotopes if one
leaves the fuel in the reactor longer for higher "burnup".

Dr. Gregory Greenman
Physicist

[NEOclassic]

"In the Livermore experiments, chemical high explosives are detonated
next to samples of weapons-grade plutonium (plutonium-239) to obtain
new insights about plutonium and its alloys in the ensuing microseconds."

Subcritical tests are not tests of weapons - so you don't have a weapons
configuration. There's no chance of the experiment going critical in any
case.

I have no argument with LLNL's pursuit; however, LANL's quest was the safe study of the Equation of state of Pu in real weapon configuration. Until Pu-242 became available, the fuel of our bombs were mocked with uranium.

You are also in error in your accounts of what the isotopic mix of spent reactor fuel is. It is more complex than the simplistic analysis you give
above using just the neutron cross-sections.

The isotopic mix doesn't just depend on the cross-sections, but on the
positioning of the material in the reactor, the flux of neutrons that
the fuel sees and the length of time the fuel spends in the reactor...

The point I make is that the presence of U-238 in the fuel mix of reactor grade fuel has more to do with the creation of Pu-239 than with neutron flux and geometric considerations. Much of it burns and my use of cross-sections is merely to calculate the possible path of the natural conversion to the Pu- 242. I don't believe I need REBUS to do that . I do agree that the age as well as the amount of U-238 in the mix have a lot to do with the buildup of other isotopes. Cheers, Jim

[airbuzz]

Well, nobody here talked enough about Nuclear Electric Propulsion (NEP).
Nuclear Termal Rockets (NTR) are a very good possibility to low down the costs to go from ground to space, but are at all not good for interplanetary missions.
I explain:
Chemical propulsion can have a maximum specific impulse around 450 sec. To explain easily the specific impulse it is how many seconds the propulsor can work with 1 Kg of propellant to give a thrust of 1 N. The specific impulse is the gas exhaust speed divided for g, so it is about 1/10 of the exhaust speed.
In chemical propulsion this il limited by the energy of the reaction.

NTR can arrive to 1000-1500 sec, but not more. So, they can half the initial weight of the rocket and be very good do take off form ground.
However, for long missions, the fuel consumption is still too big.

Electric propulsion, like MPD, don´t use termodynamics to accelerate the propellant, but electro-static/magnetic forces. So the exhaust speed has no limit. With Vasimr it is possible to arrive to specific impulse on the order of 15000 sec, ten times NTRs. The problem is that to do this they need very high electric power.
The great idea of NEP is to make a nuclear power plant in the spacecraft, t give MPD the necessary electric energy.
The problem is that with electric propulsion is impossible to take off, because the acceleration will ever be less than g (to have more power you need more weight...)
So: use NTR to go to GTO (the work that now is done by shuttle, ariane and so on) and take there the materials to construct an orbitant spacecraft in which you can construct a nuclear power plant to use to transfer to other planets.
Recently NASA gived something like 400.000.000 of dollars to Lockheed to start a NEP project. I think this means that it is possible to do this, and if we wait some 10 years maybe we will see it.
Byez

[Morbius]

I have no argument with LLNL's pursuit; however, LANL's quest was the safe study of the Equation of state of Pu in real weapon configuration. Until Pu-242 became available, the fuel of our bombs were mocked with uranium.

Jim,

The Equation of State [ EOS ] of Plutonium has absolutely NOTHING
to do with the configuration of the experiment - it is an intrinsic
property of the material. Since the objective is to study the EOS
of the material used in the weapons - Pu239 - then why not use the
actual material you want to study in your experiment?

The only reason why one would use a proxy material is if it would lead
to the chances of a nuclear criticality. If the experiment is not a
weapons configuration - then there's no chance of a criticality - and
one can use the actual material that one wants to study.

By using a weapons configuration - LANL may have complicated their
experiment. By using a configuration in which a criticality was a
possibility meant they couldn't use the material they actually wanted
to study and forced them to use Pu242 as a proxy.


The point I make is that the presence of U-238 in the fuel mix of reactor grade fuel has more to do with the creation of Pu-239 than with neutron flux and geometric considerations.

ABSOLUTELY, POSITIVELY, CATEGORICALLY WRONG!!!

The neutron flux has every bit as much to do with the creation of Pu239
as the presence of U-238!

Pu239 is created by neutron absorption on U-238 - and the production
rate is equal to the product of the cross-section and the flux. So they
are EQUALLY important. It is erroneous to say "the presence of U-238
in the fuel mix of reactor grade fuel has more to do with the creation
of Pu-239 than with neutron flux..." as you contend above.

[ Before I joined Lawrence Livermore National Laboratory, I spent the
first few years of my career doing nuclear reactor design and analysis
for Argonne National Laboratory - doing precisely the type of analysis
that we are discussing here. Your experience in the field of nuclear
reactor fuel cycle analysis is ??? ]


Much of it burns and my use of cross-sections is merely to calculate the possible path of the natural conversion to the Pu- 242. I don't believe I need REBUS to do that . I do agree that the age as well as the amount of U-238 in the mix have a lot to do with the buildup of other isotopes. Cheers, Jim

Yes, much of the Pu239 does burn. In fact, in the average three years
that a typical fuel assembly spends in a typical Light-Water Power
Reactor - about 40% of the energy that is derived from that fuel
assembly is due to the fissioning of Pu239 that was created in situ.

So the neutron flux both creates the Plutonium, and then burns it.
The distribution of the neutron flux is dependent on the distribution of
the fissile materials - which is dependent on the distribution of the
neutron flux.

So you have a coupled, inter-dependent problem and you think you can
do an accurate calculation without a computer code? BALONEY!!

You can get an extremely rough estimate with your "back of the envelope"
calculations. But to do anything approaching reality - paper and
pencil methods won't cut it.

In your previous post. you stated your doubts about the North Koreans
being able to use the Plutonium from their reactors to build a bomb.

Where do you think we got the Plutonium for our bombs? Reactors!!!

If you operate the reactor with frequent refueling so that the time the
U-238 laden fertile material is exposed to the neutron flux is limited -
then you can build up the concentration of Pu239 and not burn too much
it.

You have to do a time-dependent and space-dependent analysis of the
Plutonium accreation and depletion by the neutron flux.

This is why your "back of the envelope" - using neutron cross-sections
only gives you the WRONG answer! You didn't take the temporal
and spatial natures of the problem into account.

I would proffer that the North Koreans do a better job of reactor
analysis than you just did - and know how to extract weapons usable
Plutonium from a nuclear reactor fuel cycle - exactly the way we do!!

Dr. Gregory Greenman
Physicist

[CharlesP]

Nuclear rockets were given up many years ago and they haven't been taken seriously since. The main reasons were possible release of radioactive material and building material shortcomings. Just for a start hot gas playing on metal will cut through it causing catastrophic failure. Intense radioactivity destroys all structures. After Casini only miniscule amounts of radioactive material are contemplated for space work. The challenges involved in assembling any type of nuclear rocket in high orbit are beyond today's technology. The costs are prohibitive.
The solar system can and will be explored by small chemical powered robots. Anything beyond that is speculation.
Several laboratories are attempting to make a few atoms of anti-hydrogen to measure its properties.

[hitssquad]

After Casini only miniscule amounts of radioactive material are contemplated for space work.... The solar system can and will be explored by small chemical powered robots.NASA’s Nuclear Focus Aimed At 2009 Mars Lander (http://www.space.com/businesstechnology/technology/nuclear_focus_040218-1.html)

By Brian Berger
Space News Staff Writer
posted: 07:00 am ET
18 February 2004

WASHINGTON, D.C. - NASA’s nuclear future promises more maneuverable, longer-lasting spacecraft and rovers with more onboard power than scientists know what to do with.

Nuclear propulsion and power systems also could greatly reduce travel times to distant planets and supply energy to future planetary settlements, said Al Newhouse, director of NASA’s Project Prometheus nuclear power and propulsion program.

In the near term, Newhouse said, NASA’s nuclear ambitions are focused on building a better battery for an unmanned lander launching to Mars in 2009 and a nuclear-electric propulsion system for a planned 2015 robotic tour of Jupiter’s icy moons. NASA plans to spend more than $480 million in 2005 to continue work begun last year on a new generation of radioisotope power generators as well as nuclear-electric propulsion systems capable of producing thrust over long periods of time.

Boeing Co. and Lockheed Martin Corp. are working on competing nuclear battery designs for NASA’s 2009 Mars Science Laboratory and other missions in the early planning stages.

[Morbius]

Nuclear rockets were given up many years ago and they haven't been taken seriously since. The main reasons were possible release of radioactive material and building material shortcomings. Just for a start hot gas playing on metal will cut through it causing catastrophic failure.


No - it depends on the temperature of the gas.

Look at metal tubes in a commercial power plant boiler. There's hot
gas surrounding those tubes all the time - yet the hot gas doesn't cut
through the tubes as you've stated above.

When the nuclear rocket is designed - the design will take the heating
of the materials into account and provide proper cooling.

After all, the reactor is hotter than the gas that's cooling it - so why
doesn't that fail?


Intense radioactivity destroys all structures


Again, you are mistaken. The internal structures of nuclear reactors
are exposed to intense radioactivity and they are not destroyed.

There is an effect in which the radiation can cause embrittlement of
steel. However, that process takes decades, and the damage can be
repaired by annealing - that is one heats the metal to the point where
the atoms can return to their proper positions in the metal's crystal
structure after having been dislodged into interstitial locations.



. After Casini only miniscule amounts of radioactive material are contemplated for space work.


After all the scare stories, the Cassini mission came off without a hitch,
and did not endanger anyone, contrary to the predictions of the
anti-nuclear crowd.

Actually, NASA comtemplates expanded uses of RTGs like those aboard
Cassini.

Dr. Gregory Greenman
Physicist

[Astronuc]

Nuclear rockets were given up many years ago and they haven't been taken seriously since.

NASA and others still take them seriously, so much so that funding has been provided to various labs and manufacturers like BWXT and Northrop-Grumman to design and build a system for JIMO (Jupiter Icy Moons) mission.

Just for a start hot gas playing on metal will cut through it causing catastrophic failure.

Like Morbius stated, it depends on temperature, and a good designer can select the appropriate materials and flow rates to mitigate erosion of metals by hot hydrogen. Besides, NTR's are usually designed for relatively short periods of thrust (on the order of hrs) because of the relatively low Isp.

Intense radioactivity destroys all structures.

Actually one should use the term radiation in that statement, but it is still incorrect. Again as Morbius stated, radiation (particularly neutron radiation) causes the displacements (dislocations) in material structures which embrittle, but may also strengthen the material (much the same as cold-working does). The effect is well known and can be accomodated in the design.

Operating at high enough temperatures will also anneal some of the dislocations, so the designer can choose an operating temperature where the thermal conditions amerliorate the effects of radiation.

As for steel - it would not be used in a nuclear rocket. Alloys of niobium, tantalum, molybdenum, tungsten and rhenium are under consideration for high temperature rockets - both nuclear thermal and nuclear electric. The nuclear electric designs use compact liquid metal (Li) cooled, fast reactors. The reference alloy is a Nb-1Zr alloy.

[CharlesP]

I don't know of any materials that would survive at the high enough temperatures that you will need to get adequate specific impulse out of a nuclear engine. Power plants run cool in comparison. There are so many tough problems, it is a long list. All I see is research and speculation. We were promised a SCRAM jet to replace the Shuttle but I don't see one in production.

I was on "NASA's side" over Cassini when certain lay folk worried that the craft would be a threat on the return pass. That was far fetched.

[Astronuc]

Nuclear thermal rockets are temperature limited and therefore the Isp is limited - but that is a design issue. If one needs a greater Isp, then one must use an electric propulsion system.

Designing (including optimization) of the power plant and propulsion system is a challenge - but not impossible.

Actually, the mission requirements will dictate which type of propulsion systems is appropriate.

As for SCRAMJETs - NASA is testing the X-43 - http://www.nasa.gov/missions/research/x43-main.html

NASA is getting there - but perhaps not as fast as they would like. :biggrin:

[CharlesP]

Thanks for the link.

[u235]

BaH!

Again my poor friend's, the scientists working on nuclear rocket propulsion are decieving us all. The idea of Fissionable reactions boast ISP'S at a minimum (when engineering plans are acuratley created) of over 1million. These ISp's for 4500 - and 5000 are merley for gas core and liquid core rocket which have barley minimum success - because they over heat too quickley, they will all eventually become expendable, and Lh2 is extremely diffucult propellant to work with, even in space. The theory of using Lh2 + LOX in the future of space flight dynamics if extremely idiotic and will not go ahead. The E.T (External Tank) has to constantly refuel itself until launch, most LH2 turns of useless Hydrogen vapour and such systems of keeping the fuel cyrogenic are so complex that in the end the con's really outway the Pro's.
Chemical Fuels should not be mixed into Fission, Fusion or Antimatter propulsion ever, thus as seen with the current NERVA design's, the mobility and power of Nuclear Physics as we know it will be lost.

[u235]

I don't get it. What's the point of using Nuclear Fusion (or to that matter fission) Reactors for Nuclear Propulsion. Surley there has to be a better way. Increasing temprature of fuel's such as Hydrogen is good, and leads to faster velocity energy to transfer and channel into thrust, but thermal issues are huge and so is weight. If nuclear physics has tought me one thing, and it has'nt, it is that > why can't we use the direct energy from fissioning fuels such as Uranium of Plutonium and use the atom's energy to expand and channel the thrust? Surely this would create much more power and the energy from the (MeV) would provide power sources?

Use a nuetron generator for fissioning of the Uranium then the energy will expand and produce the required thrust. Later Fusion could be introduced - D-T and from the Temprature and Pressure these nucleis would create He4 and even more energy for thrust. Off course too much could not work because such a thruster would blow up>?

[Lord Flasheart]

Chemical Fuels should not be mixed into Fission, Fusion or Antimatter propulsion ever, thus as seen with the current NERVA design's, the mobility and power of Nuclear Physics as we know it will be lost.

Who says that chemicals are used as fuels? Show me a source where it says that nuclear fission and/or fusion use liquid hydrogen as fuel.

Antimatter propulsion is excluded in this case, due to the fact that some varients of the design require ice or hydrogen.

Furthermore, there is a NERVA with LOX-augmentation, but it can be considered as two different propulsive schemes, as the LOX augmentation does not take place inside the reactor at all.

[u235]

Der........What do you think the hydrogen escaping from the Nuclear thermal rocket and minute percentage relatavistic velocities is. Hydrogen is used as the fuel in this concept due to the overheating of it causing critical tempratures which induce it to travel at above supersonic velocity (even some engines depict subsonic) directing out of the thruster for thrust propulsion. The uranium inside the reactor would not be the fuel, even if it can be classified as the fissioable fuel, the escaping Hydrogen is the classified fuel. The reason that their is such poor energy outputs for todays designed NTR (Nuclear Thermal Rockets) is because they use nuclear reactors. Thats why it was canned back is 1972 or whenever - due to the output of thrust (per pound) from between 50,000 to 250,000 pounds. Even boeing 747,s outputted more pounds of thrust, some common rocket engines outputted 400,000 and obviously the SSME (Space Shuttle Main Engine) outputs 512,000 - only in LEO (Low Earth Orbit).

Some scientists from Glenn Reasearch Center in Cleveland - NASA - have depicted the new NTR that they are working on - right now - to only output 15,000 pounds of thrust. ''What the hell's the point". The enviroment risk would never allow it. Plus constant continous thrust is required, so to reach the escapes of earth's gravity is about at a point of 900 million km.
People today spaceflight requires large velocity boosts - has anyone heard of momentum. Yes earth can pull you back ever so slightly in terms of gravity - decreasing your initial acceleration - however, a large enough boost would only require to start the engine once inside earth gravity boundary to get into solar orbit. Than mean velocity can be kept by a small amount of large velocity increases (about 1 -2 ) between earth and mars - then mars's gravity will bring you in. On the way home the trip is even easier, with escaping mars grav being extremely easy, (3rd of earth's) and once coating out of solar orbit is complete earth will pull you back in. The use for 15,000 pound is extremely weak, and requires continuity. That's why NTR engines harnessing Nuclear Reactor do not work.

+ their is no such a thing a fusion based nuclear design's today - except possibly for my depicted version of a hybrid (nuclear fission / fusion engine.)
see. Nerva - nuclear engineering www.physicsforums.com

[Lord Flasheart]

Der........ What do you think the hydrogen escaping from the Nuclear thermal rocket and minute percentage relatavistic velocities is.

Wrong.

What I know is that the H2 is propellant. There is a difference between fuel and something one propels. The modern chemical rockets and theoretical antimatter drives are the only propulsion systems that utilize chemicals as both fuel and reaction-mass.

Hydrogen is used as the fuel in this concept due to the overheating of it causing critical tempratures which induce it to travel at above supersonic velocity (even some engines depict subsonic) directing out of the thruster for thrust propulsion.

Once again, H2 is not the fuel in a fission/fusion drive.

Why would Hydrogen be used if it has a harmful effect to rocket-plumbing? Check your notes.

The uranium inside the reactor would not be the fuel, even if it can be classified as the fissioable fuel, the escaping Hydrogen is the classified fuel.

The Uranium inside a NERVA-esque propulsion system is the true fuel, providing the energy to the reaction-mass. As stated before, AMAT/MAT and conventional chemical engines provide both energy and the propellant when operated.

Oh, and lastly...

Der........ What do you think the hydrogen escaping from ... and minute percentage relatavistic velocities is.

Hydrogen escapes from minute % relativistic velocities? I'd have never seen the day.

Cheerio!

[u235]

The uranium is known as the fissionable fuel, which microfissions inside the reactor realeasing heat and other pressure properties that force the hydrogen fuel to exert out of the thruster nozzle.

The hyrogen is the classified accelerent which is indeed the propellant and classified fuel.

"Once again, H2 is not the fuel in a fission/fusion drive."

- this statement is wrong, fusion is not a component in today's NTR. (Nuclear Thermal Rockets)

The hydrogen is harmful because with the properties of uranium inside the fuel the temprature and chemicals can corrode the inside of the chamber. Thus some smaller solid fragments of unfissioned uranium also exist and cause plumbing problems. This is why most NTR's are expendable.

My note -relativistic- from the text only suggest's mi-nute, as written, percentage of velocity compared to anything in the equations of c - speed of light.

and no the uranium in not a core fuel component, the generator itself provides heat and pressure. that's why we are only going to get NTR's From NASA which have about 15,000 pounds or so of outputed energy. This is due to cost cutting and reducing mass.

[hitssquad]

The uranium is known as the fissionable fuel....

and no the uranium in not a core fuel componentIn nuclear fission reactors that burn uranium as fuel, uranium is the fuel but it is also not the fuel?

[u235]

In NTR's (Nuclear Thermal Rockets) harnessing nuclear reactors

the uranium is the fissionable fuel,
the hydrogen is the propellant that is heated to supersonic velocities,

in a classic example, the SSME (Space Shuttle Main Engine) uses both fuels, one categorized as the accelerent - the hydrogen, the other categorized as the oxidizer - LOX.

In this NTR example, the uranium is the fissionable fuel, which heats and pressurizes the accelerent which can also be classified and connected in spaceflight terminology to propellant. - the h2 is the substance being propelled at either supersonic - or - subsonic velocities (designs differ).
The uranium cannot be classified as the fuel, because it is not in direct use.
The fissioning energy which is a expanding sphere of ionization energy, which is derived from nuetronic bombardment, is not being directly used for means of propulsion.

Watters and Engima closed the thread Nerva, which depicted one of my designs that specifically used this direct fission energy.
Yet, the classification for primary fuel is the accelerent which is the analogous term for propellant.

[Astronuc]

U235, by convention, i.e. generally accepted and customary usage, the term 'fuel' is defined as:

A substance that consumed to produce energy, especially:

- A material such as wood, coal, gas, or oil burned to produce heat or power.
- Fissionable material used in a nuclear reactor.


In LH2/LO2, the combustion reaction of 2H2 + O2 \rightarrow 2H2O provides the heat (thermal energy). You are correct that H is the fuel (accelerant) and O is the oxidizer, however the propellant is primarily the reaction product H2O with some excess H2.

Incidentally, the pressure is provided by cryogenic turbopumps. The pressure drops as the reactants pass from the combustion chamber and out the nozzle.

In the case of a nuclear thermal rocket motor, hydrogen is the propellant (or the working fluid).

The uranium, which is fissioned, from which the thermal energy is supplied, is the 'fuel'.

It would be well worth reading James Dewar's book, "To the End of Solar System: The Story of the Nuclear Rocket". It covers some of the technical aspects and political/policy aspects of the program.

Basically, 'direct use of fission for propulsion' is seemingly impractical, but I first have to complete some calculations in order to support that statement.

[u235]

So basically you are re-establishing what i stated in my previous post...
That Hydrogen is the primary fuel - the accelerant which is the propellant...

However, the terms for hydrogen and liquid oxygen for use, hypothetically, inside the space shuttle, are scripted as LH2\LOX for their cyrogenic -200.C liquid properties.

Yes the pressure is provided by cryogenic turbopumps, however, the main pressure supply is either helium (for line pressure) and/or nitrogen for (purge pressure). Mechanical pumps do not exist in space flight.

- my referal to pressure properties within uranium are stated because the fissionable fuel inside the core is used to heat as well as pressurize the hydrogen to supersonic (even sometimes, subsonic) velocities. This is the main reason for the nuclear reactor inside these common NTR's (Nuclear Thermal Rockets), - specifically to heat Lh2 to tempratures common checmical reactions can't so that exhaust velocity is faster, thus over all velocity of the thrusters energy output is greater. - Remember when Uranium is fissioned - it not only produces extreme heat - but also pressure.

(note: the pressure from the uranium is not used for pushing fuel through the lines, just some properties exist for accelerating hydrogen into the combustion chamber for higher pressure (PSI).

Quote:
"Basically, 'direct use of fission for propulsion' is seemingly impractical, but I first have to complete some calculations in order to support that statement."

-what?, - yeh you should do some calculations...
Listen, its impractical to use fission for heating fuel. Thats what is impractical. Using energy from direct fission is much more clearer for propulsion than using generators. The output is thousands of times greater...

I will be posting a new thread, named NERVA2..... look out for it, it will more clearly articulate my point on NTR's and 'direct fission(+)fusion propulsion'.

(P.s some nuclear reactors differ -NTR's May be affected for analysis....
- some reactors can use nuetrons to increase hydrogen atoms velocity and some use uranium for micro-fissioning purposes.)

[selfAdjoint]

The original NERVA had its limitations, I believe it couldn't be on for very long before it developed cracks. Are the newer designs free of this? What is the longest any nuclear thermal engine has remained active continuously?

[Astronuc]

So basically you are re-establishing what i stated in my previous post...
That Hydrogen is the primary fuel - the accelerant which is the propellant...

Not exactly - hydrogen is the fuel in a chemical rocket, used in conjunction with oxygen (oxidizer) to provide energy. The propellant is the reaction product water with some excess of hydrogen.

In a Nuclear Thermal Rocket, hydrogen is simply the propellant and uranium (primarily U-235) is the fuel by virtue of the fact that the fission of U provides the thermal energy.

Mechanical pumps do not exist in space flight.
Incorrect - the turbo pumps in the space shuttle are powered by hot exhaust gases from powerhead above the combustion chamber of each shuttle engine. Pay attention to the following:
================================================== ==
"Two-Duct Powerhead (source - http://www1.msfc.nasa.gov/NEWSROOM/background/facts/ssme.html )

Considered the backbone of the engine, the powerhead consists of the main injector and two preburners, or small combustion chambers. Liquid oxygen and hydrogen are partially burned in the preburners, generating hot gases. The liquids continue to move through ducts into the main combustion chamber, while the gases created in these chambers drive the high-pressure turbopumps, which give the Shuttle thrust.

The two-duct hot gas manifold is a new powerhead design that first flew on the Shuttle in July 1995. It significantly improves fluid flows in the system by decreasing pressure and turbulence, thus reducing maintenance and enhancing the overall performance of the engine.

The previous powerhead featured five tube-like ducts -- three on one side of the engine where hot gases flow from the fuel turbine, and two on the side where hot gases flow from the oxidizer turbine. The two-duct hot gas manifold replaced the three small fuel ducts with two enlarged ducts -- smoothing the fuel flow, reducing pressure and turbulence, and lowering temperatures in the engine during operation. This design reduces stress on the main injector and requires fewer welds, eliminating potential weak spots in the powerhead.

================================================== ===

In a NERVA type rocket, the turbo pumps are powered by has gases which are bleed off the nozzle.

- my referal to pressure properties within uranium are stated because the fissionable fuel inside the core is used to heat as well as pressurize the hydrogen . . .
Incorrect - turbo pumps pressurize the hydrogen. The thermal energy heats the hydrogen causing a considerable decrease in density (conversely increase in specific volume) (due to change in temperature), and in accordance with the continuity equation (mass flow into core = mass flow out), the resulting high exit velocity provides the propulsive thrust.

This is the main reason for the nuclear reactor inside these common NTR's (Nuclear Thermal Rockets), - specifically to heat Lh2 to tempratures common checmical reactions can't so that exhaust velocity is faster, thus over all velocity of the thrusters energy output is greater. - Remember when Uranium is fissioned - it not only produces extreme heat - but also pressure.
Incorrect - Combustion temperature in SSME - ~6,000 °F (3,315 °C). NERVA/Rover fuel approached 3000°C, so the hydrogen coolant/propellant temperature was somewhat less.

the fact that the propellant is hydrogen with a molecular mass of 2 amu is much lighter than H2O (molecular mass = 18 amu) provides much greater specific impulse. For a given thermal energy, the hydrogen molecule achieves a higher velocity by virtue of

v = \sqrt{\frac{2E}{m}}
where E is the molecular kinetic energy.

-what?, - yeh you should do some calculations...
The calculations to which I am referring have to do with fission density as related to neutron flux or current density, which would then show that 'direct fission for propulsion' is impractical. However, it is not a high priority item at the moment.


(P.s some nuclear reactors differ -NTR's May be affected for analysis....
- some reactors can use nuetrons to increase hydrogen atoms velocity and some use uranium for micro-fissioning purposes.)
I am sorry to say that this last statement makes little sense.

All nuclear reactors based on fission produce neutrons. In high temperature hydrogen gas the free mean path for neutron scattering is so large that the neutrons would contribute little to the velocity of the hydrogen atoms.

It is very hard to take one seriously when one states the following:
[qoute]If nuclear physics has tought me one thing, and it has'nt, . . . . [/quote]

[Astronuc]

Are the newer designs free of this? What is the longest any nuclear thermal engine has remained active continuously?

A summary of the US program in the 60's and early 70's can be found at -
http://www.fas.org/nuke/space/c04rover.htm

Of the tests, the longest running seem to be (from the FAS site)

NRX-A6

This 1100 MWt engine was operated in December 1967 for 60 minutes at full power, exceeding the NERVA design goal.

XE'

This 1100 MWt engine was a prototype engine, the first to operated in a downward firing position. It accumulated a total of 28 start cycles in March 1968 for a total of 115 minutes of operations. Test stand coolant water storage capacity limited each full power test to about 10 minutes.

--------------------------------------------------------

There may be better references available, but at the moment I can't put my hands on one.

[u235]

Quote:
"Not exactly - hydrogen is the fuel in a chemical rocket, used in conjunction with oxygen (oxidizer) to provide energy. The propellant is the reaction product water with some excess of hydrogen"
"In a Nuclear Thermal Rocket, hydrogen is simply the propellant and uranium (primarily U-235) is the fuel by virtue of the fact that the fission of U provides the thermal energy."

We are not discussing chemical rockets here. Hydrogen -is- the propellent which in turn is also the accelerent - giving it the classification as the primary fuel. Uranium inside a nuclear reactor only provides thermal and pressure properties to the Hydrogen flowing through the internal finned passages and thus causes it to accelerate, cause pressure inside the combustion chamber, and an actual higher velocity for H than possible with chemical reactions. You should check your information, heat loads for NERVA based NTR's are much higher than the SSME. The NTR also uses the cryogenic liquid Hydrogen to cool down the engine and reactor structure, however nuclear fuel being processed through a state of fission produces much high tempratures.

I refer to pressure properties of Uranium not for fuel pressurization, line feed or re-pressurization, but for additionally accelerating H velocity. Fission provides both thermal and heat pressure. Unless you've made a break through to stop fission from producing pressure, even at a micro-fissioning level then the breakup of U235 (92 nuetrons + 143 protons) = 2 lighter nuclei usually krypton - barium + [1 - 3] 'spare' nuetrons. This cause 188MeV of energy + about -- PSI - can't find the PSI for 1 atom? - Help? But this should be the pressure that i have stated at the 'added pressure properties' for light H acceleration.

(Something on the order of 200 MeV (million electron volts) is released by the decay of one U-235 atom (if you would like to convert that into something useful, consider that 1 eV is equal to 1.602 x 10-12 ergs, 1 x 107 ergs is equal to 1 joule, 1 joule equals 1 watt-second, and 1 BTU equals 1,055 joules).



Quote:
Mechanical pumps do not exist in space flight.
- I re-state my position.

Hydrogen is a bi-modal fuel that can sort of ''look after itself.''
In some parts of the SSME, the LH2 is boiled off to provide a gaseous pressure, which is returned to the ET to provide re-pressurization.
The term 'Mechanical Pump' specifies a turbine based suction unit that is driven mechanically by rotors, motors, fans and other solid structure entities.
I state that - space flight - does not use mechancal pumps because the pumps inside such systems as NTR's, inside the OMS, inside the RCS, inside the SSME use gas (either GN2 - He - or a Bi-modal appraoch) to pressurize / re-pressurize. The ''two duct powerhead'', and other designs, that you talk of, are all built around pressure systems that act out on traps, pipes, and other non-classified mechanical structures...Mechanical systems are not used in space because they incur a great weight burden on designs....

Almost 2 dozen different Nerva based NTR designs exist, most harnessing different approaches from gas core to solid core. My statement on nuclear reactors is clear, different designs are likely to variate for analysis. Yes fission reactors may work different to fusion reactors, however many variations for reactor based technology exist.

Quote:
"All nuclear reactors based on fission produce neutrons. In high temperature hydrogen gas the free mean path for neutron scattering is so large that the neutrons would contribute little to the velocity of the hydrogen atoms."

And no, why would someone build a hydrogen accelerating reactor and compell it to undergo the exact simalar processes as a fission based reactor. Obviously this would'nt work. But if reactors are changed, then direct process
for nuclear fuel - acceleration production must also be changed....

BTW, how do you quote: properly ?
sad,

[hitssquad]

how do you quote: properly?Click the Quote icon on the lower right corner of the post you want to respond to. The BBCode tags it places in the message can also be manually entered.

[u235]

how does em+W+S meet G in one unified equation?

[luiscar]

Concerning what you say that electrical engines are heavy, I strongly disagree. In fact, the problem with the MHD and the ion propulsion is that it needs a high level of power; we are speaking from 15MW to 5GW (depending on the efficiency you want and the time to accelerate you have). That means that you need heavy nuclear reactors. The electrical part, ionisation/acceleration of propellant has an insignificant mass (less than 1%).
On the other hand, with the Nuclear thermal rocket, there is the problem of the maximum exhaust speed. Since the speed in the NTR is caused by temperature, there is the limit that you mention. Higher than that (9500m/s) would have no possible container and the exhaust pipe would burn.
In the ion and MHD, you can achieve exhaust speeds up to 150000m/s (which is under development) which is 15 times more (although, achieving near c would be possible, it would not be recommended due to the power needed and therefore, the weight of the power reactor needed, which would be problem)
On the other hand,in an NTR since the exhaust gases pass through a radioactive material, atoms of this radioactive material would also go out with the exhaust gases. You would have a radioactive gas!!.
In the ion and MPHD approach, the radioactive core remains confined and the only thing you send out is the gas (maybe Xenon) de-ionised, as in the final part of the exhaust pipe, there is the de-ionization step.

[luiscar]

Mandrake, About youir reactor, for how long could it maintain 3000MWt?
Which was the total estimated reactor mass?
Was it possible to make it work to let's say 3MWt when not propelling the spacecraft?

[Coolblueflame]

is it possible the last gentleman was saying the hydrogen was super heated to a nuclear reaction and directed for thrust ...thereby transforming a plain propellant into a propellant/fuel ?

[Astronuc]

NTR seems to be favored by the Griffin administration over NEP (Nuclear Electric Propulsion).

NTR and Lunar Surface Reactor Programs will likely get priority.

[~()]

Any re-development of previous programs which promote nuclear reactor based rocket engines will likely fail. It fundamentally won't work because the conversion of using nuclear fission to create heat and transfer this to a working fluid is quiet an in-effective design for maximum force and thrust output. Future development will probably need to opt into direct fission fragement propulsion (i.e using the fissioned products directly as expellant), or possibly fusion. Smaller systems need to be built which can undertake efficiently fissioning and fusing particles but thats likely a long way away. Just the amount of energy needed to operate something is enourmous (size of a powerplant).

[Astronuc]

Future development will probably need to opt into direct fission fragement propulsion (i.e using the fissioned products directly as expellant), or possibly fusion. Not necessarily so with respect to fission, and fusion has yet to be perfected as a useful energy source.

[superczar]

NTRs, NEP, MPD, Orion, Gaseous Fission Reactors, etc., etc. all worked out disappointingly to anyone interested in a workhorse propulsion system designed to make interplanetary manned exploration more feasible. I, for one wouldn't want to be an astronaut on a flight to Mars driven solely by NTRs. I have nothing against a fission plant (such as a pebble-bed reactor) which can deliver 10GW in under 20 tons. I just don't think you're taking full advantage of your brain if all you want to do with that energy is double the ISP over a chemical rocket. Some of the lastest thinking today better exploits the technological possibilities, such as the VASIMR concept (although I think the actual VASIMR concept will fail).

VASIMR has been in development for decades and still dosen't seem to have produced a prototype which I think will be the workhorse of the future. I think ion propulsion (or plasma propulsion) is a great way to achieve high ISPs. My problem is that no one has been immediately successful in demonstrating a concept that achieves an ion propulsion system which can be scaled-up to the kind of thrusts we'll need to perform missions such as near-Earh asteroid deflection, manned interplanetary and near-interstellar probe missions. I think the main problem with VASIMR is that it tries to get too fancy with heating the ions instead on just using an electric field to accelerate them to the necessary energy. A helicon wave machine is an excellent way to make ions. In fact rough calculations show that if this method were used an engine of sufficient power for a 30 - 45 day trip to Mars could be constructed in approximately 5 tons. Also, current reasearchers in helicon wave technology seem certain ion densities could be increased above current levels with increase in magnetic field strength, thus reducing engine mass until it becomes a non-factor.

Apparently, after taking the existence of the pebble-bed reactor of 10 GW (and assuming problems with clogging due to frits can be worked out) and assuming the helicon wave engine can be constructed, all's we need is a converter (and a closed-cycle MHD system would appear to provide a low-mass answer). This appears to be the most efficient possible propulsion system likely to be constructed in the near future whose promise is far beyond what is currently available. Boasts have been made for superior systems employing technologies which I don't see as feasible yet, such as fusion, etc. With only a modest research & engineering effort a small start-up corporation might be able to demonstrate the feasibilty of the propulsion system in a short time. Any advance in ion-propulsion technology is likely to affect the current market for low-payload interplanetary probes like DS-1. So, there's money to be made in this field and ample opportunity of entrepreneurship. Question is, would this lead to a massive effort to privately fund a manned interplanetary mission which could be made profitable by sample return from say Mars or Europa, government funding of space exploration & asteroid deflection, a Mars colonization effort or an interstellar probe.

Although there are a number of other technological hurdles to work out with any particular mission configuration. It is almost a certainty that many missions will not be feasible without such a propulsion system. The types of missions which include the chance for a large and profitable economic expansion, the chance to protect civilization from devastating meteor impacts, the chance to gather data on our solar system and other solar systems, and a furtherance of man's push into space to provide for an expansion of population without overcrowding Earth.

[Astronuc]

Actually, NTR's have been and are the only nuclear propulsion technology successfully demonstrated!

NEP certainly have very high Isp, but low thrust, and they still need to be demonstrated over the lifetime of a trajectory (i.e. 1-many years). There are still issues with cathode and anode erosion, and fault tolerance.

VASIMR has yet to produce any significant thrust.

There is still a lot of work to be done in order to develop a reliable and safe nuclear propulsion system.

Forget Gas Core and Orion!

[~()]

Although nuclear energy is a viable and much more greater source of energy than chemical , the designs for NTR's need to be drastically refined in order to extract, transfer and control the energy into direct thrust. The general-design method of using a fission reactor which transfers heat to a working fluid (commoly hydrogen) just does not realistically produce any significant differences in thrust performance. The NERVA program failed because of low thrust performance, commonly only reaching values between 50,000 - 250,000 pounds of thrust. We must develop new ways in order to directly extract nuclear energy into thrust if we are to harness a larger potential of the energy source.

[Morbius]

Although nuclear energy is a viable and much more greater source of energy than chemical , the designs for NTR's need to be drastically refined in order to extract, transfer and control the energy into direct thrust. The general-design method of using a fission reactor which transfers heat to a working fluid (commoly hydrogen) just does not realistically produce any significant differences in thrust performance. The NERVA program failed because of low thrust performance, commonly only reaching values between 50,000 - 250,000 pounds of thrust. We must develop new ways in order to directly extract nuclear energy into thrust if we are to harness a larger potential of the energy source.

Short of creating a "Maxwell's demon" - how do you do that?

The nuclear energy released in a fission comes out principally as kinetic energy of the
byproducts - i.e. "heat".

Dr. Gregory Greenman
Physicist

[~()]

I've been working on, for a while, my own design for a NTR. I have tryed to stay away from the general approach of a fission reactor creating and transferring heat to a working fluid and rather tryed a new approach of directly creating larger initial and then sustaining fusion reactions. I have been looking into the generation of an initial fusion reaction confined in a combustion chamber, then a addition of a fuseable aneutronic fuel added to this reaction to create a sustaining reaction which mimics the way in which a chemical reaction in a standarnd rocket engine works. However there is many complex facets to this problem, such as creating accelerating structures which can initiate a initial reaction, confining the reaction inside a sufficient EM structure, establishing whether a smaller yet sufficient power source exists (much smaller than a power station) and whether it is generally theoretically possible to create and sustain a fusion thrust in a simalar way to a conventianal chemical reaction (burn).

Essentially the SSME (Space Shuttle Main Engine) is the most sophisticated working hybrid chemical engine to date and runs of an efficiency of about 80% in converting the H2 / LOX propelland and oxidizer into energy for direct thrust. It operates within the ranges of about 400,000 to 500,000 pounds of thrust which tells us that even in the next decade with little refinement, the standard chemical rocket will have reached its maximum performance ability. Unfortunately, these engines which rely on a chemical energy source are insufficient for space propulsion outside of LEO. This is why it is most probable that sustaining nuclear reactions which use direct fragments as exhaust thrust will naturally replace the chemical ones if they are theoretically possible. Although we know these reactions are possible inside suns and stars which initiate these reactions under their own gravitational fields, maybe we will be able to replicate this with man made engineering systems.