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Nuclear thermal rocket

Discussion in 'Fred's House of Pancakes' started by bwilson4web, Jan 26, 2023.

  1. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Recent articles about NASA working with DARPA on a nuclear rocket had me wondering what is going on. The the nuclear heat source generated electricity for ion acceleration, it would make sense but a thermal rocket had me wondering because there are limits to high temperature materials per Wiki:

    As thermal rockets, nuclear thermal rockets work almost exactly like chemical rockets: a heat source releases thermal energy into a gaseous propellant inside the body of the engine, and a nozzle at one end acts as a very simple heat engine: it allows the propellant to expand away from the vehicle, carrying momentum with it and converting thermal energy to coherent kinetic energy. The specific impulse (Isp) of the engine is set by the speed of the exhaust stream. That, in turn, varies as the square root of the kinetic energy loaded on each unit mass of propellant. The kinetic energy per molecule of propellant is determined by the temperature of the heat source (whether it be a nuclear reactor or a chemical reaction). At any particular temperature, lightweight propellant molecules carry just as much kinetic energy as heavier propellant molecules and therefore have more kinetic energy per unit mass. This makes low-molecular-mass propellants more effective than high-molecular-mass propellants.

    Because chemical rockets and nuclear rockets are made from refractory solid materials, they are both limited to operate below ~3,000 °C (5,430 °F), by the strength characteristics of high-temperature metals. Chemical rockets use the most readily available propellant, which is waste products from the chemical reactions producing their heat energy. Most liquid-fueled chemical rockets use either hydrogen or hydrocarbon combustion, and the propellant is therefore mainly water (molecular mass 18) and/or carbon dioxide (molecular mass 44). Nuclear thermal rockets using gaseous hydrogen propellant (molecular mass 2) therefore have a theoretical maximum Isp that is 3x-4.5x greater than those of chemical rockets.    ​

    Ok, this sort of makes sense but powering an ion engine makes more to me.

    Bob Wilson
     
    #1 bwilson4web, Jan 26, 2023
  2. CR94

    CR94 Senior Member

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    Meaning, I think, the theoretical exit velocity of the propellant would have to be 3 to 4.5 times as high (so its kinetic energy would be 3² to 4.5² times). Is that even possible, given the limits of the engine refractory materials?
     
    #2 CR94, Jan 26, 2023
  3. fuzzy1

    fuzzy1 Senior Member

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    I think it means they will be using the very same engine refractory materials already used with liquid H2-O2-fueled engines (H2O exhaust, molecular mass 18). By reducing the exhaust molecule weight by 9X (to a weight of 2 atomic units), they increase the exhaust speed by 3X.

    The other figure, 4.5², also comes from re-using the same engine material, but with C(x)H(z) fuels and oxygen, producing an exhaust mixture of CO2 (weight 44) and H20 (weight 18). Same engine chamber, somewhat slower fuel than H2-O2.

    Reducing the exhaust molecular weight to just 1 unit, i.e. monotomic hydrogen, would be even more efficient, a 40% speed boost. But splitting H2 into H1 may require a higher temperature than any thermal engine chamber can handle.
    Agreed, for high specific impulse. The ion propulsion gets around the material temperature limits of thermal rockets by using electric-magnetic confinement and acceleration, so the exhaust is not in direct thermal contact with engine chamber walls.

    But it seems that ion engines can't (yet) be built to similar total thrusts as thermal rockets, so it takes far longer to get up to speed. Can't work for ground launch stages.
     
    #3 fuzzy1, Jan 26, 2023
  4. bwilson4web

    bwilson4web BMW i3 and Model 3

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    I suspect a nuclear rocket either thermal or ion needs to be in low earth orbit before going critical. But there are some interesting molten salt, thorium cycles that appear very interesting.

    Bob Wilson
     
    #4 bwilson4web, Jan 26, 2023
  5. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    https://en.wikipedia.org/wiki/Lead-bismuth_eutectic ??

    I remember this was a 'thing' in the 80's when a third world nation with a large military produced a submarine that could out run and out dive torpedoes (some said...)
    Liquid metal cooled reactors......
    hmmmm.....

    Awful dang COLD in space!
    Arrange heat transfer and you're in business.
    Also, if you do not have squishy, fragile cargo your environmental and acceleration limitations are not as limited.
    (NOT that acceleration is a problem for ion drive! ;) )
     
    #5 ETC(SS), Jan 26, 2023
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