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Ground-based Laser + Liquid Nitrogen Rocket Engine?

  1. May 11, 2015 #1
    No idea why this popped into my head on the drive home tonight, but here we go: Would it be possible to build a rocket filled with either liquid Nitrogen, or some other super-cooled substance, then use a battery of high-energy, ground-based laser beams pointed at the super-cooled propellant to evaporate/sublimate the propellant with enough force to achieve lift off or sustained flight or even break orbit?

    If nothing else, would this be a novel way to nudge a satellite with a deteriorating orbit back into position as it passes over the laser base?

    Would such a propellant system contain potential energies even remotely close to more standard flammable rocket fuels?
  2. jcsd
  3. May 12, 2015 #2
    For nitrogen, the latent heat of evaporation is 200 kJ/kg, this is the energy you get when you evaporate nitrogen.
    For hydrazine, a rocket fuel, the heat of combustion is 20000 kJ/kg, this is the energy you get when you burn hydrazine.

    So the difference between evaporating stuff and burning stuff is quite large.

    And why evaporate it with a laser beam? As long as you're close to earth, the nitrogen will evaporate itself, and if you're in space, the nitrogen will instantly freeze again as soon as you eject it, which doesn't seem very convenient. Also, the energy needed to evaporate nitrogen is... 200 kJ/kg.
  4. Jan 21, 2016 #3
    I was thinking of something similar, it's been vaguely discussed as a form of non-rocket space launch though I don't know why it isn't used. There's a thing called lightcraft where the idea was to heat up a point to achieve thrust. https://en.wikipedia.org/wiki/Lightcraft

    My thinking for using a laser was so we can separate power generation from working mass. A laser/power system on the ground is weight that doesn't need to be on the craft. The reason to use a laser is because it can cause the nitrogen to evaporate much faster. I think the reason for using nitrogen instead of something combustable is for safety, if there was a critical failure with only nitrogen propellant onboard it's safer than hydrogen or oxygen.

    I like the idea...
  5. Jan 22, 2016 #4


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    but as you can see from the post above yours, it would be extremely inefficient
  6. Jan 22, 2016 #5
    He did have good information though it leaves me with some more questions. It seems like an incompatible comparison between the energy released through burning and energy required to vaporize a liquid. It seems like a good portion of the burn energy goes to not just vaporizing the next kg of fuel but to increasing the temperature of the exhaust gas.

    I think the equivalent comparisons for latent heat are 200 for nitrogen and 1250 for hydrazine in kJ/kg. The equivalent comparisons for input energy from fuel burning are 20,000 for hydrazine and 0 for nitrogen because it isn't burning. The input energy for both vaporizing and increasing exhaust temperature of the nitrogen would be from ground based lasers tracking the rocket.

    In the end the idea was less about fuel efficiency and more about construction cost and safety. If a nitrogen propellant rocket was overall cheaper per launch and safer to operate that'd be the ideal.
  7. Jan 22, 2016 #6


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    abd that id the whole point of why burning fuel is better ..... its gives a huge thrust ratio over just plain vaporising nitrogen

    and do you also understand/realise that the power of the laser decreases in power by the inverse square law as the distance from the laser emitter increases ?

    you are flogging a dead horse :rolleyes::wink:
  8. Jan 22, 2016 #7
    And if the laser also heated the nitrogen gas in addition to just vaporizing it?

    Agreed, the laser power decreases by an inverse square though the reason for using a laser is it can keep the beam divergence small which might keep the constant factor small enough to still be useful.
  9. Jan 23, 2016 #8


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    as I said, you are flogging a dead horse

    if you don't know what that saying means, google it :wink:
  10. Jan 25, 2016 #9
    For anyone else looking NASA did a beam energy propulsion study. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120002761.pdf

    For launching payloads, the study concluded that using BEP to propel vehicles into space is technically feasible if a commitment to develop new technologies and large investments can be made over long periods of time.
  11. Jan 26, 2016 #10
    Your idea is good but the is it has drawbacks
    1.1.the amount of expansion of gases will be very less compared to real rockets
    2 the amount of power will it consume will be Unimaginable
    3 the Specific impulse (Isp) will be very low and
    Isp= thrust/mass flow rate of propallant
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