Nuclear Thermal Rocket Engines

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  • #1
enigma
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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.
 

Answers and Replies

  • #2
Jonathan
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Great, but...

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.
 
  • #3
drag
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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 length 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. :smile:

Live long and prosper.
 
  • #4
enigma
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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 separate 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.
 
  • #5
NEOclassic
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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 apparent 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
 
  • #6
FZ+
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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?
 
  • #7
enigma
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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.
 
  • #8
selfAdjoint
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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.
 
  • #9
drag
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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 separate 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.
 
  • #10
selfAdjoint
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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.
 
  • #11
selfAdjoint
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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?
 
  • #12
drag
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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... )

Live long and prosper.
 
  • #13
FZ+
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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...
 
  • #14
LURCH
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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.
 
  • #15
enigma
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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.
 
  • #16
BCRion
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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.
 
  • #17
kerimek
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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.
 
  • #18
drag
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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 ?
 
  • #19
kerimek
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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.
 
  • #20
drag
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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...

(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.
 
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  • #21
kerimek
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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...

(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...
 
  • #22
ATCG
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Anti-matter propulsion

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.

-ATCG
 
  • #23
BCRion
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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.
 
  • #24
TonySlim
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just siphon some anti-matter from the multiverse. how far away could it really be?
 
  • #25
Beast
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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.
 
  • #26
russ_watters
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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.
 
  • #27
Beast
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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.
 
  • #28
drag
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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. :wink:
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.
 
  • #29
Mirabilia
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Hm...anti-matter...natural substance...can be made on earth...NO! Can't possibly be true. Could it?
 
  • #30
Mandrake
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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
 
  • #31
pervect
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Beast said:
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 [Broken]

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 [Broken]

This is all currently highly theoretical, though.
 
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  • #32
Orion1
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Unclear Nuclear...


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 [Broken]
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 [Broken]
http://en.wikipedia.org/upload/thumb/7/7f/180px-NASA-KIWI-A-prime.jpg [Broken]
http://en.wikipedia.org/wiki/Nuclear_thermal_rocket
http://www.fas.org/nuke/space/c04rover.htm
http://www.lascruces.com/~mrpbar/rocket.html [Broken]
 
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  • #33
marcus
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Mandrake said:
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.
 
  • #34
Mandrake
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marcus said:
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!
 
  • #35
enigma
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Mandrake said:
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).
 

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