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From: Henry Spencer <>
Subject: Re: Air Breathing Spaceplanes?
Date: Fri, 9 Aug 1996 14:11:20 GMT

In article <> Peter Bellini <> writes:
>>    ...try taking off the airbreathing part (and it's intakes!) and
>>    just replacing it with rocket propellant.  You buy back a lot of the
>>    delta-V you wanted to get from airbreathing....
>Actually, you don't.  Because of airbreathers' (ramjet,scramjet) high Isp...

Isp is not everything; in some ways it's not even the most important
thing.  (This is heresy, yes, but it's also true.)  The point of high Isp
is low mass ratio, but low mass ratio does not automatically correlate
with ease of design and construction.  It does *if other things are equal*.
But they aren't.  For a given vehicle configuration (engine type, fuel
choice), you certainly want maximum Isp, but Isp is not a good metric for 
comparing different configurations.

For example, if you're trying to build an SSTO, do you want to use liquid
hydrogen or a hydrocarbon fuel?  The LH2 has much higher Isp.  But the
goal is not high Isp, but ease of design and construction... and dense
fuels win big there, because high mass ratios are easier to achieve with
them.  The US flew an SSTO-class rocket stage using dense fuels in 1961
(the Titan II first stage) but has never managed to build one with LH2
(the S-II achieved the necessary mass ratio, at great cost and pain, but
its engines are not up to the job).
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |

From: Henry Spencer <>
Subject: Re: design approaches (Launch efficiency)
Date: Mon, 5 May 1997 14:07:52 GMT

In article <>,
Edgar Zapata  <> wrote:
>One point about mass fraction - stepping out of the rocket limits with
>airbreathing can get us to numbers of about and below below 0.70 (in

However, one has to be really careful about that, because a mass fraction
of 0.70 can be harder to achieve with airbreathing propulsion than one of
0.95 with rockets.  Airbreathing engines have to use oxidizer which is
four or five orders of magnitude less dense than LOX; they are inherently
big and heavy because they have to handle so much volume.  (An airbreathing
engine with a thrust:weight ratio of 15:1 would be a remarkable advance; a
LOX/kerosene rocket engine with a T:W of 150:1 would be only a modest
improvement on current ones.)

That 0.70 mass fraction is also going to be mostly hydrogen... which is
bad news, because hydrogen is terribly bulky and hydrogen tanks are big
and heavy.  And the airbreather will need more hydrogen than the rocket,
because of increased losses.  Trading off cheap, compact LOX which burns
off on the way up for expensive, heavy airbreathing engines and huge
hydrogen tanks that have to be carried all the way is a questionable

>The recent X vehicle (future X, was X-37) that HR 1275 has
>slated apropriations for (with mention of "more advanced than X-33") is
>just such a system as an RBCC (rocket based combined cycle)...

While the airbreathing mafia would undoubtedly like to make X-37 (or
whatever its name is this week) into a billion-dollar project for their
particular hobby shop, it remains to be seen whether that will happen.
Notably, other parts of NASA are saying that if one is going to build
another X-megaproject, it should have orbital capability, no excuses
allowed.  Airbreathing technology is not up to that, not for a reasonable
amount of money, as witness the demise of the X-30.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

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