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Space Rockets and Space Aircrafts

  1. Jan 7, 2007 #1
    I was wondering if nuclear power is being used in pushing and thrusting Space Rockets when launching out from the earth?

    Cheers. :smile:
    Amir Fahd.
     
  2. jcsd
  3. Jan 7, 2007 #2

    Astronuc

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    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
     
  4. Jan 11, 2007 #3
    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?
     
  5. Jan 11, 2007 #4
    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 spacecrafts had nuclear powered batteries. But the thrust required to launch would be pretty dangerous IMO.
     
  6. Jan 11, 2007 #5

    Astronuc

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    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.
     
  7. Jan 11, 2007 #6

    Morbius

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    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
     
    Last edited: Jan 11, 2007
  8. Jan 11, 2007 #7

    Morbius

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    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
     
  9. Jan 11, 2007 #8
    Could you elaborate on this? Heat transfer is in the negative thermal gradient direction, so why would the pellet heat up further?
     
  10. Jan 11, 2007 #9

    LURCH

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    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.
     
  11. Jan 11, 2007 #10

    Morbius

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    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
     
  12. Jan 11, 2007 #11

    Morbius

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    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
     
    Last edited: Jan 11, 2007
  13. Jan 11, 2007 #12
    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?
     
  14. Jan 12, 2007 #13

    Astronuc

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    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!


    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.
     
  15. Jan 12, 2007 #14

    Morbius

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    sanman,

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

    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.

    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
     
  16. Jan 12, 2007 #15
    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.


    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.


    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.
     
  17. Jan 12, 2007 #16

    Morbius

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    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.

    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.

    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?

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

    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
     
  18. Jan 13, 2007 #17
    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.


    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.


    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.



    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.
     
  19. Jan 13, 2007 #18

    Astronuc

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    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.

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


    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.
     
  20. Jan 13, 2007 #19

    Morbius

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    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!!!
    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!!

    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
     
    Last edited: Jan 13, 2007
  21. Mar 12, 2008 #20
    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...)
     
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