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From: Doug Jones <>
Subject: Re: Whoa. Am I Dreaming?
Date: Wed, 23 Aug 2000 10:52:58 -0700

Aaron Smith wrote:
> Actually, I ran it through again and I have "optimized" the design for my
> purposes
> 2 atm chamber pressure
> 200 expansion ratio
> 4.5 o/f
> 470 ISP ( ! )
> SCORES gave a t/w of 60 (IIRC, RL-10 = ~45).  Because of the low pressure,
> this thing could be made pressure fed.  I think most tanks nowadays are
> already built for something like 35 PSI, with safety margin.  So the stage
> would be a teensy bit heavier (for making the tanks stronger) but that might
> be made up for by the high ISP.

Yep, you've rediscovered the fact that engines for use in vacuum don't
need high chamber pressure unless the base area of the vehicle is too
small for the large nozzle.  You would almost certainly need an
extensible nozzle, much of it radiation cooled.  The deployment
mechanism for your nozzle may hurt your T/W, though.

The vapor pressures of the propellants would suffice to provide the feed
pressure, but regen cooling is a bit problematic for boiling
propellants- except hydrogen which is so damn effective that it doesn't
matter.  It also doesn't hurt that the heat flux is much lower due to
the low Pc, but the total integrated heat load will be high due to the
large area/volume ratio (not many W/m2, but lots of m2).

The biggest problem with a low pressure engine like this is that it's a
bear to test at sea level- you need a very good vacuum for it to exhaust
into, better than a steam aspirator can provide, so it would have to be
tested with a truncated nozzle.

It's a shame nobody ever tested the idea back in the days when wacky
things were being tried out routinely.

Doug Jones
Rocket Plumber, XCOR Aerospace

From: (Henry Spencer)
Subject: Re: Whoa. Am I Dreaming?
Date: Wed, 23 Aug 2000 14:17:36 GMT

In article <>, Aaron Smith  <> wrote:
>    I just ran something through SCORES that blew me away.  LOX/LH2 at 1
>atmophere (yes, 14.7 psi) chamber pressure in a vacuum with 1000
>expansion ratio, 5 o/f ratio and I got an ISP of 470.  Can somebody tell
>me, did those mushrooms I ate somehow make me hallucinate?

It's not ridiculous.  The chamber pressure is *irrelevant* for first-cut
analysis; it's only the expansion ratio that counts.  And yes, LOX/LH2
with a very high expansion ratio does have a pretty high Isp.

On more detailed analysis, some other considerations need attention, like
wall friction in the nozzle, and whether water will start condensing in
the exhaust (yes, it's possible and it can be a limiting factor).  There
will also be some reduction in performance due to dissociation -- that's
one place where a low chamber pressure hurts significantly.  And that low
mixture ratio means high LH2 tank mass; *vehicle* performance is often
better with smaller tanks, even though it means some reduction in *engine*

>But if these
>numbers are true, why hasn't anyone built an engine like this before?
>(maybe a freakin low t/w ratio, but in space that shouldn't matter all
>that much).

Even in space, T/W does matter.  Since long-term LH2 storage is hard, most
in-space LOX/LH2 applications are for boost out of LEO, and more generally
for applications involving operation in substantial gravity fields... and
in that context, the performance penalty for low-thrust operation can be
large.  (The Van Allen belts also add nasty complications for anyone
trying to get out of LEO with low-thrust propulsion.)

Bottom line:  the reason why nobody has built such an engine is that we
already have space-proven LOX/LH2 engines with nearly that Isp and much
better T/W.  If Isp is the dominant concern, there have been designs, and
sometimes prototype tests, of pump-fed LOX/LH2 space engines with even
higher Isp; they have not been pursued because nobody wanted them badly
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

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