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Nuclear Thermal Rocket Engines

  1. Sep 17, 2003 #1

    enigma

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

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

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

    What does everyone think about the possibility of us building one of these? It's a nuclear reactor, and the guys at NASA would be the ones flying it... still, the potentials are huge if all goes well.
     
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  3. Sep 17, 2003 #2
    Great, but...

    I don't like the idea of nuclear anything riding piggy-back on a giant explosion machine, we already have enough radioactive waste to deal with, let alone if a rocket explodes and throws a bunch of it all over the place. On the other hand, without any actual combustion, it's less likely there'll be an explosion.
     
  4. Sep 17, 2003 #3

    drag

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    Greetings !

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

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

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

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

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

    Thanks. :smile:

    Live long and prosper.
     
  5. Sep 17, 2003 #4

    enigma

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

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

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

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

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

    I do know that the best fuels are those with the smallest atomic masses, so hydrogen is A#1. Theoretically, I don't see anything that would keep us from using heavier gases harvested in-situ, but I don't think you'd want to 'wing it' with something like that. The containment vessels and engine would need to be designed to operate on both.
     
  6. Sep 17, 2003 #5
    Hi Enigma,
    Back in the 60s when Los Alamos tested, in Nevada, the KIWI rocket engine the apparant idealized propellant was ammonia gas. No matter what propellant was used, the inability to recycle the gas meant that any reasonable amount would likely be soon exhausted for any mission beyond the MIR. Cheers, Jim
     
  7. Sep 17, 2003 #6

    FZ+

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    If I remember the Orion programme properly, it was estimated that each launch would kill about 6 people due to increasing the level of radiation in the atmosphere. So, is this safe? And can you persuade the public that this is safe?
     
  8. Sep 17, 2003 #7

    enigma

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    Orion is a completely different animal.

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

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

    Most plans propose using the rocket as a third stage for use once it is out of the Earth's atmosphere, not as a primary launch phase.
     
  9. Sep 18, 2003 #8

    selfAdjoint

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

    For a proposed Mars mission these don't seem like overly restrictive constraints.
     
  10. Sep 18, 2003 #9

    drag

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

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

    In addition, a VASIMR engine in high thrust/low Isp mode
    could, theoreticly, also be used to land and launch
    small modules on Mars (though I doubt that it would be done
    since it envolves risking and relying upon the primary engine/s
    for all the manned mission's stages).
    Well, that kin'na beats the purpose, doesn't it ?
    What's so dangerous about an orbital accident, even
    if it's the worse case scenario - total meltdown
    and explosion (which is BTW pretty hard to "achieve"
    in space). Most of the debris will stay there, as
    for some of it that will reach the atmosphere - I
    don't know about this, wouldn't it moslty melt during
    reentry ?

    Live long and prosper.
     
  11. Sep 18, 2003 #10

    selfAdjoint

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

    As for boosting out of orbit on chemical or other and then pulling the rods when the speed is high enough, I don't think there's too bad a penalty for that.
     
  12. Sep 18, 2003 #11

    selfAdjoint

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    BTW I googled on VASIMR, and I note in the description phrases like "the gas will be ionized" and "the gas will be heated". How? By nuclear power?
     
  13. Sep 18, 2003 #12

    drag

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    Greetings !

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

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

    Live long and prosper.
     
  14. Sep 18, 2003 #13

    FZ+

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    Ah... this sounds much more interesting. But I still fear public opposition to the idea of strapping lots of nuclear material to the top of a lot of explosives...

    Is the first stage of the launch vehicle itself reliable for nuclear material carrying? An accident could be nasty....
     
  15. Sep 18, 2003 #14

    LURCH

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    Does anyone here remember the public outcry against the launching of Cassini? And it only had a very small amount of fuel and no reactor; just relying on radiative decay to generate ellectricity. This is one of the reasons to go back to the moon, IMO. Send inert materials there for enrichment and launch them as refueling tankers from the surface. If a launch fails (as one innevitably must) it will be in an environment where there is no atmosphere to contaminate.
     
  16. Sep 21, 2003 #15

    enigma

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

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

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

    People greatly overestimate the damage we frail humans can cause to the universe.
     
  17. Sep 25, 2003 #16
    Nuclear Reactors for space propulsion would use Uranium-235 as their fuel. U-235 and U-238 are quite low on the radioactive scale and you will find traces (on parts per billion) in natural seawater -- enough to power the entire planet with breeder reactors for well over a billion years.

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

    You would probably not want to have the reactor re-enter since fission products are a lot worse than the Uranium fuel. It may be best to eject the spent rods back into outer space. As far as the whole pollution argument, space is really a sea of radiation, some spent fuel rods pale in comparison to the radiation already present.
     
  18. Sep 25, 2003 #17
    I think that the best option for spaceship is reactor with thorium232 - uranium 233 breeding cycle. It requires only a bit of uranium for start and rest of fuel is stable thorium (thorium is as common as iron). u233 is fissionable by rapid neutrons, which allows minimal size and weight of the reactor. U233 is similar fuel as plutonium239 (used in nuclear submarines), but without its radioactivity and toxicity. Main problem of the nuclear spaceship is the fact that it must be build on orbite, because of no nuclear engine can overwhelm earth gravity.
     
  19. Sep 25, 2003 #18

    drag

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    What do you mean ?
     
  20. Sep 25, 2003 #19
    Large rocket engine has thrust typically in meganewtons and power output in tens of gigawats. If spaceship has 1000tons, then you require about 10MN thrust to get up it from earth surface. This is impossible to reach with any nuclear reactor (without its destruction). Adventage of nuclear fuel is in its total energy (and final speed), not in acceleration. Chemical rocket can accelerate on example 100s on 10G. Then final speed will be 10km/s. Nuclear ship can accelerate millions of second, but only on 0.1G Final speed will be thousends of km/s.
     
  21. Sep 25, 2003 #20

    drag

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

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

    Live long and prosper.
     
    Last edited: Sep 25, 2003
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