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I was surprised when reading this:
DARPA moving forward with development of nuclear powered spacecraft.
by Sandra Erwin — May 4, 2022
https://spacenews.com/darpa-moving-forw … pacecraft/
The article discusses that DARPA is funding nuclear powered propulsion to cislunar space. This is space in the vicinity of the Moon. The only reason why you would want it nuclear powered is you want to get there rapidly, in a matter of hours instead of days. What military purpose could there be for getting to the Moon in hours?
Robert Clark
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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Hi Bob:
My guess would be larger payload fractions to some "thing" they want to put in that region of space, for the same delta-vee otherwise.
Using the difference between escape (11 km/s) and orbital speeds (8 km/s) as a crude measure of the one-way delta-vee there (3 km/s), a two-way trip requires twice that (6 km/s). For the sake of argument, I assume the same payload is carried round trip.
For 375 s Isp, Vex ~ 3.678 km/s. For 800 s Isp, Vex ~ 7.845 km/s.
For 375 s, the required mass ratio = exp(6/3.678) = 5.111, for a single stage propellant fraction of 80% of initial ignition mass. If the inerts are 5% of initial ignition mass, that leaves 15% mass fraction for the payload. A 5% inert fraction is pretty typical for a modern chemical stage.
For 800 s Isp, the required mass ratio = exp(6/7.845) = 2.149, for a single stage propellant mass fraction of 53%. The reactor is quite heavy, so guess an inert fraction nearer 10%. That leaves a payload fraction near 37% of the initial ignition mass.
That's more than twice the payload, despite the heavier reactor engine, for the same mass delivered up from the surface to LEO. I would suspect that more than-double the deliverable payload is why they are looking at nuclear.
It was not really any different with the NERVA-powered alternate third stage proposed for the old Saturn-5 decades ago. It more than doubled payload capacity. That was to be baseline for the 1980's manned Mars mission NASA had planned, before Nixon's executive order restricted all human spaceflight to LEO in 1972. Which in turn is why NASA cancelled NERVA just as it became ready for flight test in 1974.
Yeah, you could use it to fly faster Earth-to-moon, but with only a 3 day trip one way, why bother? Slow down and carry a lot more. That makes much more sense.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Both NASA and DARPA know that chemical propulsion is too inefficient for the types of missions they want to undertake, so this should not be too surprising. Specific impulse will be about double that of LOX/LH2, and with modern technical ceramics combined with the use of LEU instead of HEU fuel, there's at least a chance that larger privately-funded vehicles, such as Starship, could be outfitted with a considerably more efficient propulsion system that makes transit to the moon and Mars a more practical proposition than LOX/LH2 or LOX/LCH4. I'm waiting to see the very first test flight in space before I believe that this project will succeed where NERVA failed.
BWXT notes that the thermal power that their nuclear-powered RL-10 equivalent provides actually heats the Hydrogen to a lower temperature than LOX/LH2 combustion, but that the lower molecular weight of the exhaust product creates the difference in Isp. Their reactor is 550MWt power output for 100kN of thrust, with Isp at 893s and propellant mass flow rate of 12.9kg/s. Reactor temperature can be varied to produce Isp between 830s and 1,000s. The RL-10 is around 300kg, whereas they're projecting that this engine is around 4,800kg.
RL-10 has a mass flow rate of 24.1kg/s at 110kN thrust level. Burn time on a Centaur upper stage is about 700s, so 16,873kg propellant in total. The nuclear-powered engine would burn through 9,030kg over the same burn duration, but weighs 4,800kg, so 13,830kg vs 17,173kg. That's significant, but not a game changer for smaller missions. However, if we made both stages equal in mass, then the nuclear engine can fire for another 259s, so a 37% improvement in total burn time or total impulse delivered for equivalent stage mass. If we adjust for the fact that RL-10 produces 10% more thrust, then that total impulse improvement is 23.3% at most. That's certainly better than nothing, as there are not many opportunities for 20%+ total impulse improvement, but it's not a game changer unless the nuclear engine's burn durations are rather lengthy, on the order of a half hour or more.
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