[AR] Re: DARPA responsive launch challenge

  • From: Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
  • To: arocket@xxxxxxxxxxxxx
  • Date: Sun, 22 Apr 2018 19:41:00 -0700

On 4/22/2018 4:51 PM, Henry Spencer wrote:

On Sun, 22 Apr 2018, Henry Vanderbilt wrote:
I'm not greedy - I'd settle for injection velocity around 20,000 m/s...
And that'd also be about 2000 seconds Isp even before you combust the propellant...

Reminds me of several of Heinlein's 1950s space-travel stories, in which the propulsion systems are solid-core nuclear-thermal rockets whose propellant is stabilized atomic hydrogen (just how it's stabilized is, of course, never explained).

If you could actually do that, the first thing that would happen is that when somehow-stabilized H hits your hot reactor core, the stabilization pretty much inevitably comes undone.

The second thing that would happen is that your very expensive reactor core melts and washes out the nozzle.

And the third thing would be that engine performance improves as a result, because all that graphite and uranium was just getting in the way. 2H->H2 all by itself is good for about 2000_s Isp (if the stabilization doesn't add a significant inert component) -- *much* hotter than you're going to get out of any solid-core reactor.

But may the gods have mercy on your soul if your fuel tank ever takes a micrometeorite hit, because no conceivable escape system will get you away from *that*.  Stabilized or not, that stuff would be the great-grandmother of all energetic monoprops, with all that implies...

Mind, given the stabilization is SFnal handwaving, you could assume it's *stable*. IE, not prone to detonating if looked at cross-eyed, and comes out of the reactor still monatomic.

At which point with monatomic hydrogen exhaust and materials temp limits similar to various later NTR proposals, you should get sqrt2 times the 800-900 seconds Isp of those NTRs with H2. 1100 to 1250ish seconds isp, not too shabby.

But if we're going for SFnal high-energy monoprops, I'd prefer metallic hydrogen to monatomic. The storage density on single-H must be truly abysmally low...

Henry






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