Space Rockets and Space Aircrafts

[amirfahd]

I was wondering if nuclear power is being used in pushing and thrusting Space Rockets when launching out from the earth?

Cheers.
Amir Fahd.

 

[Astronuc]

I was wondering if nuclear power is being used in pushing and thrusting Space Rockets when launching out from the earth?

Cheers.
Amir Fahd.

Hi, Amir. In short the answer to your question is - No. Nuclear rockets, and those would be nuclear thermal, are not used in Earth's atmosphere.

sanman provided a link to Project Pluto, a nuclear ramjet concept. That had links to other programs. The relevant programs for nuclear rockets are the NERVA/ROVER and KIWI programs -

http://en.wikipedia.org/wiki/Nuclear_thermal_rocket

http://www.astronautix.com/project/nerva.htm

http://www.fas.org/nuke/space/c04rover.htm

http://www.lascruces.com/~mrpbar/rocket.html

 

[sanman]

And I think it's really too bad that nobody is pursuing nuclear power for this purpose. Regarding the fear of radioactively contaminating the sky in the event of some launch/re-entry disaster, I think that the reactor could be designed solidly enough to prevent release of radioactive contaminants. Technologies like pebble bed could be used, which have high stability and lesser risk for heat buildup, while also providing good surface area for contact heating of an airstream/propellant stream. If subcritical fuels like thorium are ever harnessed, these could further lower risks.

Based on what we know today which we didn't know 50 years ago, I wonder what would be the optimal choice for a nuclear launch vehicle design? Anyone have any speculations or comments on that?

 

[wxrocks]

I think the problem with a nuclear launch is the large output of power needed over a short amount of time. Nuclear would be fine for getting around in space... Pardon if this is in error -- but I think the Voyager spacecraft s had nuclear powered batteries. But the thrust required to launch would be pretty dangerous IMO.

 

[Astronuc]

If subcritical fuels like thorium are ever harnessed, these could further lower risks.

No. A subcritical system would require a massive driver, so its specific energy would be way too low.

We don't know much more than we did 40 years ago, which is both sad and frustrating.

Nuclear rockets require compact critical cores which require a balance between the nuclear physics and heat transfer. In addition, high thrusts require high mass flowrates at rates that are not necessarily for pebble beds. One issue for the pebble bed is the force of the bed on the pressure vessel. An important safey issue would be core dispersal in the event of catastrophic failure.

I don't see that nuclear rockets will be proposed for lauch from Earth's surface, but rather would be proposed for GEO (maybe LEO) to Mars or further destination.

 

[Morbius]

Technologies like pebble bed could be used, which have high stability and lesser risk for heat buildup,

sanman,

A pebble bed would be the WORST design to use for a rocket.

A pebble bed has a high heat transfer area. That turns out to be a double-edged sword here.

If the heat source that you are concerned with is internal to the pellet; then that high heat
transfer area promotes heat transfer and that's why the pebble bed is at lesser risk for heat
buildup due to an INTERNAL source - there's lots of heat transfer area for heat to get OUT.

However, in case of a rocket failure, the heat source will be EXTERNAL - it will be the
friction of the fast moving reactor with the atmosphere. That high heat transfer area
now works to effectively conduct heat INTO the pellet - which is the LAST THING you
want to do if you want the pellet to survive instead of being dispersed.

So the same thing that makes a pebble bed desireable as a power reactor, in essence;
dooms its use in a rocket.

A pebble bed would be the WORST design for a rocket reactor.

Dr. Gregory Greenman
Physicist

 

[Morbius]

. Pardon if this is in error -- but I think the Voyager spacecraft s had nuclear powered batteries.

wxrocks,

You are correct. The Voyager spacecraft - or any spacecraft that we send to the other
reaches of the solar system uses "nuclear batteries" or RTGs. A simple battery wouldn't
last long enough without recharging. Solar cells are no good in the outer reaches of the
solar system because of the decreasing solar radiation with distance from the Sun.

So these craft have RTG - Radiolytic Thermal Generators. They use a radioisotope,
usually Pu-238; that generates heat via radioactive decay. The heat is converted to
electricity. There are two methods for that thermionic, and thermoelectric. The
thermoelectric means simply that you heat one end of a thermocouple.

Dr. Gregory Greenman
Physicist

 

[theCandyman]

However, in case of a rocket failure, the heat source will be EXTERNAL - it will be the
friction of the fast moving reactor with the atmosphere. That high heat transfer area
now works to effectively conduct heat INTO the pellet - which is the LAST THING you
want to do if you want the pellet to survive instead of being dispersed.

Could you elaborate on this? Heat transfer is in the negative thermal gradient direction, so why would the pellet heat up further?

 

[LURCH]

Unfortunately, even if we set aside the risk of atmospheric contamination caused by catastrophic structural failure, the use of a nuclear reaction for thrust to launch a rocket into space requires venting radioactive waste directly to the atmosphere (through the thrust nozzles). This would produce a radioactive cloud, no different from that which one would get from an aboveground nuclear test (or operating a reactor without any walls). For many people, this constitutes an unacceptable level of contamination.

 

[Morbius]

Could you elaborate on this? Heat transfer is in the negative thermal gradient direction, so why would the pellet heat up further?

Candyman,

Consider a scenario similar to the ill-fated Columbia. If the craft is moving fast enough
through the atmosphere - you get heating of the craft due to atmospheric friction. That's
why the Columbia burned up.

Suppose the ascending craft built up a lot of speed, and is high in the atmosphere. Now
it experiences control failure and tumbles and aerodynamic forces tear it apart.

Now all the energy the rocket has expended so far is now in the form of kinetic and
gravitational potential energy of the craft. That energy, both kinetic and potential will
be transformed into heat as the parts of the craft fall. There can easily be more heat
generated by atmospheric friction on the parts of the craft than the pellets generate.

Again consider Columbia. A lot the energy that the rockets expended launching Columbia
was dissipated as heat when Columbia re-entered. That heat turned the air around the
Columbia into a plasma - much, much hotter than a fuel pellet gets. When the damaged
heat shield couldn't protect Columbia from this heat - it burned up; and unfortunately the
7 astronauts perished.

If we had a similar accident with a nuclear propelled rocket ascending; in addition we
would have pieces of reactor raining down that would not be able to survive the high
heat due to frictional forces either.

Dr. Gregory Greenman
Physicist

 

[Morbius]

Unfortunately, even if we set aside the risk of atmospheric contamination caused by catastrophic structural failure, the use of a nuclear reaction for thrust to launch a rocket into space requires venting radioactive waste directly to the atmosphere (through the thrust nozzles). .

LURCH,

Not neccesarily. It depends on what is used as the propellant. If the propellant has
a low neutron capture cross-section - then it won't be made radioactive in going through
the reactor. This assumes direct heating of the propellant by the reactor.

Suppose the reactor is designed similar to a PWR; the primary coolant cools the reactor,
and then that heats the propellant which is expelled; and the primary coolant returns.
Just as the water in the secondary loop of a PWR - the one that turns to steam to drive
the turbine is not radioactive; the analogous propellant in a rocket won't be radioactive
either.

The reactor fission products won't be expelled, since they are locked up in the fuel
material just as in a commercial power reactor.

Dr. Gregory Greenman
Physicist

 

[sanman]

Regarding force of the pebble bed on the pressure vessel, that would only be the result of the force generated by the propellant flowing through the bed. Surely if the force of heated propellant can be handled by the walls of the combustion chamber of a conventional rocket, then it can be handled by the walls of the pebble bed pressure vessel. Both are forms of pressure vessel, aren't they?

In the event of a catastrophic failure, then the pebbles would be released and not stay concentrated together, so they would no longer have critical mass. The thermal throttling principle would also prevent a runaway reactor meltdown. But if the pebbles were released in orbit, then their higher surface area would make them more likely to vaporize on re-entry, rather than survive as concentrated hazardous chunks of fallout. The diffuse dispersal of the radioactive material into the natural environment from which it came (we didn't manufacture U-235, Mother Nature did) would not be likely to change any cancer rates, especially if it happened over the ocean.

So build a large floating single-stage nuclear-powered reusable launch vehicle the size of an oil supertanker, and then use it to transport large quantities of materiel from the Earth to the Moon.

The pebbles could be ballistic shaped, in order to better accommodate the high speed propellant flow, but you could also have the propellant flowing at lower density through the bed, and then later being concentrated into higher velocity at a nozzle throat.

What other nuclear reactor design could be more accommodating than this?

 

[Astronuc]

Regarding force of the pebble bed on the pressure vessel, that would only be the result of the force generated by the propellant flowing through the bed.

Not quite. The rocket is 'accelerating', and the mass of the pebble bed resists acceleration, and that resistance provides the force on the support structure (core support) which transmits that force to the pressure vessel. That is in addition to the coolant pressure!

In the event of a catastrophic failure, then the pebbles would be released and not stay concentrated together

And that would be unacceptable in the environment/atmosphere. Dispersal of a heavy metal (U) would be undesirable, as would dispersal of radioactive fission products in the atmosphere.

 

[Morbius]

Regarding force of the pebble bed on the pressure vessel, that would only be the result of the force generated by the propellant flowing through the bed.

sanman,

As Astronuc already pointed out - the above is incorrect. You missed considering the
"inertial force" of an accelerating object.

In the event of a catastrophic failure, then the pebbles would be released and not stay concentrated together, so they would no longer have critical mass. The thermal throttling principle would also prevent a runaway reactor meltdown. But if the pebbles were released in orbit, then their higher surface area would make them more likely to vaporize on re-entry, rather than survive as concentrated hazardous chunks of fallout.

You don't want the pellets to vaporize - that just disperses the radioactivity - which is
what you DON'T want to do. You want the reactor to survive intact without dispersing
its radioactivity. You WANT the reactor to remain in one chunk for recovery. If it stays
in one chunk, then you clean up by recovering the chunk. You can't clean up the
radioactivity if it is dispersed.

The diffuse dispersal of the radioactive material into the natural environment from which it came (we didn't manufacture U-235, Mother Nature did)

You are not considering the radioactive material we DID make; namely the fission
products. It's not just the U-235 from the ground. A fresh nuclear power plant fuel
element is only slightly radioactive, and you can handle it and stand next to it with
no problem.

However, after the fuel has been irradiated - it now contains in addition to U-235, the
fission products; the remnants of fissioned U-235 atoms. There are now materials
like Iodine-131, Iodine-135, Strontium-90, Cesium-135... The spent fuel element is
now INTENSELY radioactive. You can no longer stand next to it or handle it directly.

It has to be kept under at least 20-30 feet of water as shielding in spent fuel pools.
It is this INTENSELY radioactive material that would be dispersed - not just relatively
benign U-235. You DON"T want to disperse the fission products.

The pebbles could be ballistic shaped, in order to better accommodate the high speed propellant flow, but you could also have the propellant flowing at lower density through the bed, and then later being concentrated into higher velocity at a nozzle throat.

What other nuclear reactor design could be more accommodating than this?

Nuclear rockets have already been designed and tested. LLNL's "Project Pluto":

http://www.llnl.gov/str/Hacker.html
[scroll down about half-way]

There were also the NERVA and Kiwi reactor rockets:

http://en.wikipedia.org/wiki/NERVA
http://www.fas.org/nuke/space/c04rover.htm
http://www.daviddarling.info/encyclopedia/K/KIWI.html

NONE of these reactor designs are pebble beds.

Dr. Gregory Greenman
Physicist

 

[sanman]

sanman,

As Astronuc already pointed out - the above is incorrect. You missed considering the
"inertial force" of an accelerating object.

To be frank, a conventional rocket has to also contend with inertial forces on the chemical fuel load. There's nothing special there. In the case of nuclear fuel, it's going to have less mass than chemical fuel, hence less inertial force to contend with.

You don't want the pellets to vaporize - that just disperses the radioactivity - which is
what you DON'T want to do. You want the reactor to survive intact without dispersing
its radioactivity. You WANT the reactor to remain in one chunk for recovery. If it stays
in one chunk, then you clean up by recovering the chunk. You can't clean up the
radioactivity if it is dispersed.

You are not considering the radioactive material we DID make; namely the fission
products. It's not just the U-235 from the ground. A fresh nuclear power plant fuel
element is only slightly radioactive, and you can handle it and stand next to it with
no problem.

However, after the fuel has been irradiated - it now contains in addition to U-235, the
fission products; the remnants of fissioned U-235 atoms. There are now materials
like Iodine-131, Iodine-135, Strontium-90, Cesium-135... The spent fuel element is
now INTENSELY radioactive. You can no longer stand next to it or handle it directly.

It has to be kept under at least 20-30 feet of water as shielding in spent fuel pools.
It is this INTENSELY radioactive material that would be dispersed - not just relatively
benign U-235. You DON"T want to disperse the fission products.

Waitasec -- how long is this nuclear-powered ascent taking, anyway? The typical Space Shuttle ascent takes about 8 minutes from ground to orbit. I can't believe that 8 minutes worth of fission-reaction power is going to produce horrendous amounts of radioactive waste.

Since pebble bed chain reaction is based on proximity of the pellets to each other, then you don't move them together until you're ready to initiate your launch.

Nuclear rockets have already been designed and tested. LLNL's "Project Pluto":

http://www.llnl.gov/str/Hacker.html
[scroll down about half-way]

There were also the NERVA and Kiwi reactor rockets:

http://en.wikipedia.org/wiki/NERVA
http://www.fas.org/nuke/space/c04rover.htm
http://www.daviddarling.info/encyclopedia/K/KIWI.html

NONE of these reactor designs are pebble beds.

Dr. Gregory Greenman
Physicist

True, but none of them is younger than 50 years old, either.

Only nuclear power offers the wide energy margins necessary for versatile and convenient access to space.

 

[Morbius]

To be frank, a conventional rocket has to also contend with inertial forces on the chemical fuel load. There's nothing special there. In the case of nuclear fuel, it's going to have less mass than chemical fuel, hence less inertial force to contend with.

sanman,

Conventional rocket fuels DO deal with this. However, consider the liquid fueled
shuttle main engines. The fuel is a liquid - it's not going to have problem with the
inertial force. The liquid fuel doesn't have to maintain any structural integrity.

However, the pellets and the reactor do.

Waitasec -- how long is this nuclear-powered ascent taking, anyway? The typical Space Shuttle ascent takes about 8 minutes from ground to orbit. I can't believe that 8 minutes worth of fission-reaction power is going to produce horrendous amounts of radioactive waste.

Time is only part of the issue. What's the power of the reactor? The amount of fission
products is going to be dependent not on just time; but on the total energy the reactor
has to deliver.

THINK about it. I could use your argument above with a nuclear bomb:

How long is it going to take for this nuclear bomb to explode, anyway. The typical bomb
explodes in a fraction of a second. I can't believe a fraction of a second worth of
fission-reaction power is going to produce horrendous amounts of radioactive waste.


However a nuclear bomb DOES produce a lot of radioactivity - because it produces so
much ENERGY! The time is NOT the determining factor.

Since pebble bed chain reaction is based on proximity of the pellets to each other, then you don't move them together until you're ready to initiate your launch.

Right, and then you do move them together and the rocket takes off, and then sometime
during the ascent, there is some type of failure, it loses directional control and the
aerodynamic forces tear your rocket apart. What then? How do you prevent the
contamination due to the radioactivity produced?

True, but none of them is younger than 50 years old, either.

So - the laws of physics haven't changed in 50 years. What's your point?

Only nuclear power offers the wide energy margins necessary for versatile and convenient access to space.

We may have to forego launching from Earth with nuclear rockets, and transport
people and material to space with chemical rockets as we have done for the past
40 some years. Then use the nuclear rocket assembled in orbit to take it from there.

Dr. Gregory Greenman
Physicist

 

[sanman]

sanman,

Conventional rocket fuels DO deal with this. However, consider the liquid fueled
shuttle main engines. The fuel is a liquid - it's not going to have problem with the
inertial force. The liquid fuel doesn't have to maintain any structural integrity.

However, the pellets and the reactor do.

Look, the fact is that chemical fuel tanks don't deform horrendously in a chemical rocket. With nuclear fuel, you're talking about a much lower mass of fuel, and in pellet form if it's a pebble bed. The containment system for chemical fuel is going to weigh much more than the containment for nuclear fuel used to provide 8 minutes of power.

Time is only part of the issue. What's the power of the reactor? The amount of fission
products is going to be dependent not on just time; but on the total energy the reactor
has to deliver.

THINK about it. I could use your argument above with a nuclear bomb:


However a nuclear bomb DOES produce a lot of radioactivity - because it produces so
much ENERGY! The time is NOT the determining factor.

Look, I said 8 minutes worth of power -- that's clearly enough to derive the energy for a known launch mass. Suppose we talk about the mass of the Space Shuttle, for the sake of argument. I don't believe that lifting the mass of the Space Shuttle to orbit would generate the amount of radioactive waste as a Hiroshima bomb. For one thing, your radiation is inside an enclosed structure, and it's going through a moderator, etc.

Right, and then you do move them together and the rocket takes off, and then sometime
during the ascent, there is some type of failure, it loses directional control and the
aerodynamic forces tear your rocket apart. What then? How do you prevent the
contamination due to the radioactivity produced?

Again, how much radioactive waste is going to be produced from lifting a Space Shuttle sized mass to orbit? I don't think it's going to be a lot. Anyhow, your trajectory could be over the ocean.

We may have to forego launching from Earth with nuclear rockets, and transport
people and material to space with chemical rockets as we have done for the past
40 some years. Then use the nuclear rocket assembled in orbit to take it from there.

Dr. Gregory Greenman
Physicist

Oh, and how pray tell will we get the nuclear elements of that nuclear rocket to orbit? Or will we have to harvest the ore from space? What happens if the orbital nuclear rockets suffer some accident, and tumble towards Earth? Again, I think that we can handle the safety issues of a nuclear launch vehicle just fine, and it will end up being a more robust system for transport, since it will have the higher energy margin necessary to provide a safer trip.

 

[Astronuc]

Look, the fact is that chemical fuel tanks don't deform horrendously in a chemical rocket. With nuclear fuel, you're talking about a much lower mass of fuel, and in pellet form if it's a pebble bed. The containment system for chemical fuel is going to weigh much more than the containment for nuclear fuel used to provide 8 minutes of power.

Let's not forget that in addition to the core, one still needs the hydrogen propellant of approximately the same mass as one would have in a chemical system - not however to provide chemical energy, but simply to serve as the working fluid/propellant.

I don't believe that lifting the mass of the Space Shuttle to orbit would generate the amount of radioactive waste as a Hiroshima bomb. For one thing, your radiation is inside an enclosed structure, and it's going through a moderator, etc.

Please provide calculations comparing the fission product yield of a Hiroshima size weapon and a pebble bed core providing the same energy of the Space Shuttle. Keep it simple - just calculate the MCi or I-131, Cs-137 and Sr-90 for the given energy produced.

Again, how much radioactive waste is going to be produced from lifting a Space Shuttle sized mass to orbit? I don't think it's going to be a lot. Anyhow, your trajectory could be over the ocean.

Again please provide the calculations - other one is making unsubstantiated statements.

Oh, and how pray tell will we get the nuclear elements of that nuclear rocket to orbit? Or will we have to harvest the ore from space? What happens if the orbital nuclear rockets suffer some accident, and tumble towards Earth? Again, I think that we can handle the safety issues of a nuclear launch vehicle just fine, and it will end up being a more robust system for transport, since it will have the higher energy margin necessary to provide a safer trip.

Individual fuel assemblies would be pack in special containers - something we considered in the past. It is quite easy to test also with dummy fuel assemblies made of W-Mo alloy or WC-Mo cermet of roughly the same density as fuel. The containers are designed to have high drag and then impact limiting heads.

Current pebble bed designs core commerical systems are very different in terms of design and operating conditions from nuclear thermal rockets. NTRs have much high power density and therefore will generate a higher specific activity.

 

[Morbius]

Look, I said 8 minutes worth of power -- that's clearly enough to derive the energy for a known launch mass. Suppose we talk about the mass of the Space Shuttle, for the sake of argument. I don't believe that lifting the mass of the Space Shuttle to orbit would generate the amount of radioactive waste as a Hiroshima bomb. For one thing, your radiation is inside an enclosed structure, and it's going through a moderator, etc.

sanman,

When Challenger blew up; it produced an explosion equivalent in yield to a small
nuclear bomb. [That was due just to the shuttle's main engine fuel which provides a
minority of the energy required. The majority of the energy is due to the boosters.]
If you had a nuclear bomb of the equivalent enerrgy as the space shuttle,
it wouldn't be the same size as the Hiroshima bomb which was 15 KT; but
it would be fairly substantial. As I recall, the Space Shuttle represents an amount
of energy of roughly about 1 KT.

As Astronuc advised; do the calculation!

Again, how much radioactive waste is going to be produced from lifting a Space Shuttle sized mass to orbit? I don't think it's going to be a lot.

You're GUESSING! You haven't done the calculation. That's NOT the way to do
science and physics.

Consider just the energy of the orbiting orbiter. Under "Technical Data" at:

http://en.wikipedia.org/wiki/Space_Shuttle

The gross weight of the orbiter is 109,000 kg = 1.09e5 kg
The speed is 7,743 m/s

Therefore the energy of the orbiting shuttle is:

E = 1/2 * (1.09e5 kg)(7,743 m/s)^2 = 3.27e12 Joules

The conversion from Joules to Kilotons may be found at:

http://en.wikipedia.org/wiki/1_E12_J

1 Kiloton = 4.186e12 Joules

Therefore the energy in the space shuttle represents

E = 3.27e12 Joules / ( 4.186e12 J/KT ) = 0.78 KT

Which is about 1/19-th of the yield of the Hiroshima bomb.

That's a SUBSTANTIAL amount of radioactivity if that
amount of energy is produced by nuclear fission!

So much for uninformed guessing!

Oh, and how pray tell will we get the nuclear elements of that nuclear rocket to orbit? Or will we have to harvest the ore from space? What happens if the orbital nuclear rockets suffer some accident, and tumble towards Earth? .

We get the nuclear elements up to orbit the same way we build the space station; you
ferry them up in chemical rockets.

Just like my previous example with fresh nuclear reactor fuel, the unirradiated
nuclear components are only slightly radioactive - as you point out - no more
radioactive than when they were dug out of the ground. Therefore, a failure during
launch would only disperse very slightly radioactive material - similar to what is found
naturally.

Contrast this with what you are proposing of operating the nuclear rocket as a lift
vehicle. When the reactor is operated, it produces HIGHLY radioactive material,
similar to the spent fuel that's unloaded from nuclear power reactors. A launch
accident would disperse HIGHLY radioactive material in that case.

So there's a world of difference in the degree of contamination between the two schemes.
My scheme risks dispersing slightly radioactive material; your scheme risks dispersing
HIGHLY radioactive material. Why would one risk doing the latter, when one can do the
former?

Dr. Gregory Greenman
Physicist

 

[cmc21us]

I think there are several applications that could be beneficial to the space industry.

1) I've always wondered what the potential would be for a nuclear powered rail gun typ launcher on the side of a mountain. Not reasonable for the delivery of personnel; but a possibliity for material?

2) My second thought would be a nuclear core as a heat source connected to a sterling engine. Not useful for propulsion but high potential for power on longer operations (Moon, Mars or beyond...)

 

[sanman]

We definitely need to consider some kind of newer higher-energy density material. Some kind of new fuel that would pack more energy per unit mass than what's currently in use.

The question is -- what would that new fuel be?

 

[russ_watters]

Consider just the energy of the orbiting orbiter. Under "Technical Data" at:

http://en.wikipedia.org/wiki/Space_Shuttle

The gross weight of the orbiter is 109,000 kg = 1.09e5 kg
The speed is 7,743 m/s

Therefore the energy of the orbiting shuttle is:

E = 1/2 * (1.09e5 kg)(7,743 m/s)^2 = 3.27e12 Joules

The conversion from Joules to Kilotons may be found at:

http://en.wikipedia.org/wiki/1_E12_J

1 Kiloton = 4.186e12 Joules

Therefore the energy in the space shuttle represents

E = 3.27e12 Joules / ( 4.186e12 J/KT ) = 0.78 KT

Good start, and I know you know that's not all the energy required, but I think I have an easier and more accurate way:

The shuttle external tank holds 109,000 kg of hydrogen and 630,000 kg oxygen.
The combustion energy is 120,000 kJ/kg (H2).
Hydrogen heat of vaporization .449 kJ/kg.
Oxygen heat of vaporization 54.4 kJ/kg.

Total energy expended: 1.30E13 Joules

SRB's (combined): 1,000,000 kg, 16% aluminum.
Aluminum heat of combustion: 31 MJ/kg
Energy: 5.0 E12 MJ

Total: 4.3 kT

Now one of the main difficulties (already sort of discussed) of using nuclear propulsion is the energy output just doesn't fit what a rocket does. You can either have fuel that lasts for years or you can have it last for nanoseconds and it is difficult to do anything in between. Those solid rocket boosters together put out 700 MW for 2 minutes, putting them in the ballpark of a commercial nuclear reactor. But you wouldn't want to try to heft a commercial nuclear reactor into space.

I don't think there is a way to make a reactor smaller but use its fuel faster than a normal reactor. It's either just barely self-sustaining or a runaway chain reaction and there is little wiggle-room in between.

Now once you get it into space, a nuclear-powered ion engine would be an ideal way to slowly accelerate a large spacecraft for a decade-long interplanetary trip.

[edit - I see this thread is a year old. And I just spent a half hour researching that data...]

 

[Astronuc]

I think there are several applications that could be beneficial to the space industry.

1) I've always wondered what the potential would be for a nuclear powered rail gun typ launcher on the side of a mountain. Not reasonable for the delivery of personnel; but a possibliity for material?

2) My second thought would be a nuclear core as a heat source connected to a sterling engine. Not useful for propulsion but high potential for power on longer operations (Moon, Mars or beyond...)

A rail gun just needs a very large current source, and it doesn't have to be nuclear, but possibly a dedicated peaking plant or high energy storage system, since a rail gun represents an electrical transient compared to most loads.

Re - point 2), a Brayton cycle is perhaps more likely compared to Stirling cycle. NASA's Glenn Research Center has a program on the Stirling cycle.

 

[cmc21us]

I like the Nuclear option for the rail gun power soley do to size. Problem though would be waste of an asset if it were dedicated. Some other peaking station might be a better option, large frame gas turbines can do this pretty well.



On the space power supply, I like the sterling as it is a closed cycle. I think Brayton is an open circuit using some uncontained gas supply. (Is this correct?) Ideally, a sterling would have only one charge of working gas ever. In any case, a small make up source would be prudent. Added benefit, close into the sun it could actually be run on solar radiation via a concetrator (same machine multiple fuel/power source). Big fan.


When I get the time I will likely build a similar prototype running off concentrated solar and biogas.

 

[Astronuc]

On the space power supply, I like the sterling as it is a closed cycle. I think Brayton is an open circuit using some uncontained gas supply. (Is this correct?) Ideally, a sterling would have only one charge of working gas ever. In any case, a small make up source would be prudent. Added benefit, close into the sun it could actually be run on solar radiation via a concetrator (same machine multiple fuel/power source). Big fan.

The Brayton cycle may be closed (using recirculation) or open (using atmosphere). The Brayton cycle refers to a gas cycle in which a compressor increases the pressure on the working fluid, which is passed through the heat source and then passed through a turbine. A Brayton cycle based on a combustion (air-fuel) energy source, which exhauts to the atmosphere is obviously open. A system which used a nuclear or solar heat source, and which recirculates the gas working fluid is closed. One major aspect of the Brayton cycle is the gas working fluid, which requires a compressor to increase pressure and density of the working fluid which is then heated as passed into a turbine, where the fluid's energy is transformed into mechanical energy by the turbine.

 

[cmc21us]

Brayton vs Sterling

I did think about that after the post. With the extreme temperatures of space cooling the working gas after heating should not be too difficult. Do you think the Brayton would provide a higher power for the mass? I think the generator coupling might be mare easily accomplished (microturbines are using a high speed generator directly coupled to the mover without reduction gears)?

 

[Astronuc]

I did think about that after the post. With the extreme temperatures of space cooling the working gas after heating should not be too difficult. Do you think the Brayton would provide a higher power for the mass? I think the generator coupling might be mare easily accomplished (microturbines are using a high speed generator directly coupled to the mover without reduction gears)?

In space, the heat transfer has to be accomplished by radiation, and there is a mass penalty associated with the fact that for a given power level, the mass of the radiator increases at T(reject) decreases. There is a trade of between thermal to mechanical conversion efficiency (which is maximized by minimizing T_cold) and overall system mass, which is often dominated by the radiator.

NASA Glenn folks have looked at reciprocating (opposed) Stirling engines whereby the momentum of two cancel. Power conversion is acheived with linear induction generators. But the Stirling cycle would still need a radiator.

 

[cmc21us]

Good stuff

Hey Astronuc

I have not yet purchased the paper but is this something like what we were discussing?



A Brayton cycle solar dynamic heat receiver for space
Sedgwick, L.M. Nordwall, H.L. Kaufmann, K.J. Johnson, S.D.
Boeing Aerosp. & Electron., Seattle, WA;


This paper appears in: Energy Conversion Engineering Conference, 1989. IECEC-89., Proceedings of the 24th Intersociety
Publication Date: 6-11 Aug 1989
On page(s): 905-909 vol.2
Meeting Date: 08/06/1989 - 08/11/1989
Location: Washington, DC, USA
References Cited: 13
INSPEC Accession Number: 3676704
Digital Object Identifier: 10.1109/IECEC.1989.74576
Posted online: 2002-08-06 16:52:43.0





Abstract
The detailed design of a heat receiver developed to meet the requirements of the US Space Station Freedom, which will be assembled and operated in low Earth orbit beginning in the mid-1990s, is described. The heat receiver supplies thermal energy to a nominal 25 kW closed-Brayton-cycle power conversion unit. The receiver employs an integral thermal energy storage system utilizing the latent heat of a eutectic-salt phase-change mixture to store energy for eclipse operation. The salt is contained within a felt metal matrix which enhances heat transfer and controls the salt void distribution during solidification

 

[Astronuc]

Hey Astronuc

I have not yet purchased the paper but is this something like what we were discussing?

A Brayton cycle solar dynamic heat receiver for space
Sedgwick, L.M. Nordwall, H.L. Kaufmann, K.J. Johnson, S.D.
Boeing Aerosp. & Electron., Seattle, WA;

This paper appears in: Energy Conversion Engineering Conference, 1989. IECEC-89., Proceedings of the 24th Intersociety
Publication Date: 6-11 Aug 1989
On page(s): 905-909 vol.2
Meeting Date: 08/06/1989 - 08/11/1989
Location: Washington, DC, USA
References Cited: 13
INSPEC Accession Number: 3676704
Digital Object Identifier: 10.1109/IECEC.1989.74576
Posted online: 2002-08-06 16:52:43.0

Abstract
The detailed design of a heat receiver developed to meet the requirements of the US Space Station Freedom, which will be assembled and operated in low Earth orbit beginning in the mid-1990s, is described. The heat receiver supplies thermal energy to a nominal 25 kW closed-Brayton-cycle power conversion unit. The receiver employs an integral thermal energy storage system utilizing the latent heat of a eutectic-salt phase-change mixture to store energy for eclipse operation. The salt is contained within a felt metal matrix which enhances heat transfer and controls the salt void distribution during solidification

Yeah, that's along the lines of the closed Baryton system. In this case, the primary thermal source is solar energy so there is a solar collector which focuses the light onto the heater. That heater could just as easily be a nuclear reactor, so the primary system is not too different, and the balance of plant is much the same.