On Sat, 22 Feb 2003, Randall Clague wrote:
> >80%+ for heat-exchanger rockets, like Jordin Kare's current concepts.
>
> I didn't know he'd gone past pulse detonation. Is there a URL?
Not that I'm aware of, although he talked about it at last year's Space
Access. The transition happened about a decade back, when it became clear
that SDIO (as it then was) wasn't going to build a big laser with the
right pulse characteristics for the detonation thruster. The nice thing
about heat exchangers is that they don't care much about pulse
characteristics.
> >Not nearly as good for the more elegant scheme which blows thin layers of
> >gas off a chunk of solid propellant: 10-30% and lots of fairly difficult
> >development needed to make it work well.
>
> That I also didn't know. Or is the difficult part the vehicle
> attitude control vs. beam propagation issue?
No, it's the thruster dynamics itself that are the problem. Experimenting
with it turns out to be quite difficult -- Jordin did some of that with
about the last of the funding he had for actual hardware work, and said it
added a significant number of gray hairs -- and the performance numbers
that came out were disappointing, your choice of good Isp plus lousy
efficiency, or lousy Isp plus marginal efficiency. It's not likely to get
up into the desirable part of the graph without a whole lot of slow,
difficult optimization work.
Heat-exchanger rockets don't have great Isp, but with LH2 they can have
reasonable Isp -- enough better than chemical that SSTO does not look too
difficult -- and they come standard :-) with high efficiency and near-total
insensitivity to laser details. And LLNL had previously done some work on
very-high-power-density heat exchangers for high-power semiconductors,
which fit right in.
Henry Spencer
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