On Sat, 17 Apr 2004, Ian Woollard wrote:
> >There is *some* benefit to be had from high chamber pressure even in
> >vacuum.
>
> Well, thrust scales with chamber pressure; but I would expect that
> thrust isn't massively important for orbital taxis in a lot of cases.
It's not entirely insignificant, but usually not very important. In any
case, thrust is determined by the size of the engine and/or the number of
engines, which are secondary decisions. The benefits I was referring to
are size-independent.
> > Yes, you could get the same expansion ratio at lower chamber
> >pressure just by making the nozzle longer...
>
> Yes... if you need the same thrust due to the orbital mechanics.
The point of wanting the same expansion ratio is not getting the same
thrust, but getting the same Isp. Orbital taxis often don't care a lot
about thrust, but they care seriously about Isp.
> > but you would take a small
> >performance penalty from things like nozzle drag and minimum-gauge issues.
>
> You could avoid these by having multiple chambers.
Not really. Minimum-gauge penalties get *worse* with multiple smaller
engines. How the drag penalties scale I'd have to think about, but I'd
guess they get worse too, since the boundary layer is a larger part of the
flow cross-section in a smaller engine.
> On the plus side,
> your propellent feed is at a lower pressure; so the pumps and feeds
> weigh less.
Yes, the parts of the plumbing that are carrying liquids do get lighter.
(For gases it doesn't make much difference, given the same mass flow.)
And the pump power requirement is reduced in proportion.
> >Mind you, most designers would conclude that it's far more trouble than
> >it's worth. But NASA rocketry has a long history of obsession with
> >performance regardless of practical issues.
>
> Yes, although the Shuttle's OMS runs at 125 psi or so, so it doesn't
> /always /work out like that.
That wasn't the OMS system they wanted; it's just the one they decided
they could afford. The orbiter was originally going to be all-LOX/LH2.
Henry Spencer
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