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From: dsmith@HPLABSC (David Smith)
Newsgroups: net.space
Subject: transatmospheric vehicle
Message-ID: <8512161706.AA05882@hplabsc>
Date: Mon, 16-Dec-85 12:06:13 EST
Article-I.D.: hplabsc.8512161706.AA05882
Posted: Mon Dec 16 12:06:13 1985
Date-Received: Wed, 18-Dec-85 02:24:25 EST
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Organization: The ARPA Internet
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> Question: what's there to be gained using airbreathing engines to get
> you as high and fast as possible first?

Rockets have to carry all of their reaction mass to the point of use;
airbreathing engines get most of their reaction mass at the point of use.
Therefore, the airbreathing vehicle can be a lot smaller.

An aerodynamic lift vehicle would probably have a lift-to-drag ratio
of 3 to 6 (wild guess, but an article I once saw in Astronautics and
Aeronautics estimated L/D at 3-4 for a spacecraft maneuvering at orbital
speed in the upper atmosphere).  The lift would reduce the burden carried
by the engine.

> On the thumbnail, if an aircraft can reach 100,000 ft that's only 18.9
> miles (out of 175?). I can't find a copy of the "Rubber book" and thus
> don't know what part of escape speed mach 8 is.

If I recall right, the Saturn V first stage burned out at an altitude of
25 miles (out of 230,000) and a speed of 3,500 mph (out of 24,500).
Yet it was a huge chunk of the vehicle.

			David Smith
			hplabs!dsmith
			dsmith%hp-labs@csnet-relay.arpa