John Carmack wrote:
> If plug nozzles actually work to allow automatic altitude compensation, it
> seems like it should be possible to do an SSTO with blowdown pressurization
> and light tanks.
>
> My reasoning is that the bulk of the delta-V that an SSTO needs is
> generated outside the atmosphere, and with no back pressure, a very large
> expansion ratio nozzle can extract essentially the same Isp regardless of
> chamber pressure. If you had a huge plug nozzle over the entire base of a
> vehicle, you could start out with a 150 psi tank pressure and get a crappy
> 200 or so Isp at sea level (98% peroxide / kerosene), but it would rise to
> 300 when out of the atmosphere. The time spent at the lower Isp is not
> actually all that great -- under 20% of the time in the sensible
> atmosphere, with the full Isp at the end where you really need it.
>
> If you have an engine capable of deep throttling, say, with a moveable
> pintle injector, starting with some ullage and letting the pressure decay
> to 20% of its starting pressure probably isn't a bad thing at all, since
> you are going to throw away 95% of your mass by that point. Making a
> pressurization subsystem just go away is a big deal optimization.
>
> Extremely low engine pressures also make the cooling task a lot easier --
> the plug nozzle could almost certainly live with a pretty lightweight heat
> barrier / high temp metal fabrication for a six minute burn to orbit.
>
> Composite tanks with a 200 psi burst pressure can be fabricated with SSTO
> mas fractions fairly easily.
>
> John Carmack
I know Randall will be thinking "Oh no. Here he goes ..." but - Yes, a
pressure fed system should be capable of delivering SSTO performance
if you have a working aerospike engine and very light composite tanks.
PROTO was spec'ed for 500 psi talk pressure and 300 psi engine
pressure. We've had to push the tank pressure up on KISS because we're
getting higher pressure drop in the plumbing than a full-budget
optimized system should be capable of giving. That being said, if you
can get the tank makers to sell you a tank with the right pressures
and weights (we tried but their lawyers talked them out of it) you
should be able to build about as you've described.
We're working towards that goal still - with engine tests, KISS
flights, GizmoCopter flight software, POGO construction and Spike
design, but there's still a lot of work to do. Additionally, we need
to get more 98+ % HTP and get hard data on the cermet catalyst (we've
fired it with 99.2% HTP but didn't have proper data collection systems
in place at the time). And then there's the whole issue of making more
catalyst because the current "hand pressed" manufacturing method
definately won't get us to orbit (and likely won't get us to POGO).
Ah ... so much to do and so little time and money. * Sigh *
Michael
----------------------------------------------------------------------
Michael Wallis KF6SPF (408) 396-9037 [EMAIL PROTECTED]
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