Author Topic: Alternative propellants for Nuclear Thermal Rockets  (Read 12311 times)

Offline Propylox

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Re: Alternative propellants for Nuclear Thermal Rockets
« Reply #20 on: 10/20/2018 10:40 pm »
edit: I don't know what happened to my post. It looked good, then vanished so I'll restate.

  If two propellants operate at the same Temperature, yes the lighter will have higher velocity and thus isp and Thrust. Temperature = (molecular mass * velocity^2)/2 ; so if the mass is (double) that of Hydrogen the velocity is 1/sqrt(double), or around 70%.
  However, Pressure and Temperature are interchangeable in ideal gasses. Pressure * vol = Temperature ; so if the core Pressure is (double) for a mass it will have the same velocity, isp and Thrust as Hydrogen while keeping the same operating Temperature.
  I'd be remiss if I didn't point out Helium has a critical pressure 1/6 that of Hydrogen, easily becoming a supercritical fluid. In this state, and at core pressure (double) that of Hydrogen, the Helium will conduct heat better and can therefor use a smaller core to achieve the same performance. The critical pressure decline from Halogen to Noble gasses is also true, but around 1/2 ; so Neon would need to operate at around 30atm and Krypton 55atm to be in a supercritical state and compare to the performance of Hydrogen.
« Last Edit: 10/21/2018 03:33 am by Propylox »

Offline speedevil

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Re: Alternative propellants for Nuclear Thermal Rockets
« Reply #21 on: 10/21/2018 12:37 am »
Here is an interesting web-site that argues that for dv < 3km/s water is the ideal propellant.

http://neofuel.com/staiff1999/index.html

I wonder though what the maximum isp of monopropellant HTP is in a vacuum and whether it would be better to keep the reactor on the moon and use it to supply a fleet of ships with HTP. Or perhaps use a two stage system with the first stage using nuclear produced HTP, and a water NTR second stage.

Quote
The cost of rocket fuel and other mass in space can drop by orders of magnitude because Earth can launch "pumps." A pump delivers orders of magnitude more mass than its own mass.  Like a pump, each ton of hardware sent to space to operate a steam rocket architecture can return between 10,000 and 100,000 times its mass in rocket fuel at Low Earth Orbit. This is completely equivalent to orders of magnitude drop in costs.  A chemical architecture would only return 100's of times its mass.

This does rely on the cost deltas being similar.
If the reactor costs 'only' $500M for 20 tons of very high power reactor, and you can launch 2500 tons of chemical stuff for the same price, suddenly you're back in the same ballpark.

I would really love a political climate in which launching uncritical reactors made from low enriched uranium was not an intractably expensive thing to do, and did not have political risks to missions, I don't see any realistic prospect of this any time in the near future.
« Last Edit: 10/21/2018 12:43 am by speedevil »

Offline ArbitraryConstant

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Re: Alternative propellants for Nuclear Thermal Rockets
« Reply #22 on: 10/21/2018 09:57 pm »
I would really love a political climate in which launching uncritical reactors made from low enriched uranium was not an intractably expensive thing to do, and did not have political risks to missions, I don't see any realistic prospect of this any time in the near future.
FWIW I think any Earth launched reactor is going to be highly enriched. The design considerations are similar to naval reactors except even more so. And I don't think it's politics except inasmuch as they have to keep cutting things to fund shuttle derived launchers.

Online edzieba

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Re: Alternative propellants for Nuclear Thermal Rockets
« Reply #23 on: 10/22/2018 01:39 pm »
All the current work on the notional SLS Nuclear Upper stage engine is on adapting existing NTR designs to operate on low-enriched fuels.

Offline speedevil

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Re: Alternative propellants for Nuclear Thermal Rockets
« Reply #24 on: 10/22/2018 04:23 pm »
I would really love a political climate in which launching uncritical reactors made from low enriched uranium was not an intractably expensive thing to do, and did not have political risks to missions, I don't see any realistic prospect of this any time in the near future.
FWIW I think any Earth launched reactor is going to be highly enriched. The design considerations are similar to naval reactors except even more so. And I don't think it's politics except inasmuch as they have to keep cutting things to fund shuttle derived launchers.
High enriched is significantly worse than low enriched, as it adds proliferation as a concern, and makes sourcing fuel much harder.
The political aspect arises as you absolutely can't do such a mission privately and have an assurance you will be allowed to launch, or even to proceed to fueling.

This also means that if it's a government mission, with a high power nuclear reactor, it's not just going to be ~$1B or so for 20 tons, but much much more, if the usual set of prime contractors gets their hands on it.

 

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