Could Nuclear Thermal Rockets Revolutionize Space Travel?

[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]

exhausted efficiency...


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 separate 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.

[/color]
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. 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

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.

 

[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

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]

Do you know where safe plutonium comes from?

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]

We are talking about 2 different things

"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]

NASA plans massive use of plutonium-238 to power space missions

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.

• http://www.space.com/businesstechnology/technology/nuclear_focus_040218-1.html [/size]
  
  By Brian Berger[/color]
  Space News Staff Writer
  posted: 07:00 am ET
  18 February 2004[/color][/size]
  
  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 accommodated 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.

 

[CharlesP]

Thanks for the link.

 

[u235]

BaH!

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 environment risk would never allow it. Plus constant continuous 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.