From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Space Balloons?
Date: Thu, 28 Dec 1995 22:52:53 GMT
Organization: SP Systems, Toronto
Lines: 41

In article <4bt4pr$bvr@lace.colorado.edu> fcrary@rintintin.Colorado.EDU (Frank Crary) writes:
>>The major problem for reaching orbit is velocity, not altitude.  There is
>>some advantage to a high-altitude launch, but it is mostly in velocity-
>>related things like lower air pressure...
>
>Not entirely. A rocket launched from the ground has to waste
>a large amount of fuel fighting gravity...
>That problem goes away if you drop-launch a rocket.
>The rocket can accelerate horizontally, without 
>compensating for gravity, assuming it has enough
>thrust to get up to orbital velocities before hitting
>the ground.

Unfortunately, it's not that easy.  The dominant requirement is not "don't
hit the ground" but "keep clear of the atmosphere", to avoid air drag and
aerodynamic heating, both of which become very serious problems at major
fractions of orbital velocity.

>...stay as high as possible, to reduce air drag, but
>a moderately aerodynamic rocket shouldn't have trouble
>doing so: A trivial lift to drag ratio would let
>it accelerate horizontally and keep it from falling.

At the price of a lot of drag and a lot of heat.  Staying high to minimize
air drag also minimizes available lift; you need a *non*trivial L/D ratio
to get useful amounts of lift without incurring excessive drag -- that's
what the ratio means!  And if memory serves (my references aren't handy),
drag losses are already more important than gravity losses even for a
traditional rocket trajectory.  As for heat, observe that lift and drag
rise as the square of velocity, while heating rises (roughly) as the cube
of velocity -- because lift and drag are caused by momentum but heating
is caused by kinetic energy -- and so it gets harder and harder to get
useful aerodynamic forces without burning up. 

Note that even Pegasus, which has wings and a good L/D ratio, commences
powered flight by pulling up into a steep climb to get clear of the
atmosphere.  And even so it needs ablative thermal protection on parts of
the first stage, notably near the wing-body junction. 
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu