Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Rocketing or flying into space?
Date: Tue, 13 Feb 2001 21:16:36 GMT

In article <3A8928A9.C06C5A16@knutsen.dk>,
Peter Knutsen  <peter@knutsen.dk> wrote:
>...states that the cheapest way to get into orbit is to
>fly there. Instead of rocketing.

There is a popular belief, largely without foundation, that rockets with
vertical takeoff are immensely inefficient.  In fact, the reason why they
need so much fuel is mostly just that reaching orbit is *hard*.

>But is it valid for Earth? Would it be cheaper to build a plane-
>like vehicle and fly through the lower atmosphere, instead of
>blasting straight up in a rocket?

There are two semi-separate issues:  propulsion and wings.

The liquid oxygen for a rocket engine is heavy.  It is tempting to think
that if you're flying through an oxygen-containing atmosphere, then
carrying all your oxygen with you is wasteful -- that you should use the
air instead, with jet engines instead of rockets.  This is *much* harder
than it looks, especially at very high speeds; almost certainly it is
actually harder to reach orbit that way than with rocket engines.  The one
exception is if the jets are involved only very early, when the air is
thick and speeds are low, and they stay behind, e.g. by air launch off a
large aircraft.

The other question is whether it's better to use wings and start out
flying roughly horizontally, rather than going up vertically.  That is
more debatable.  You still end up climbing quite steeply, and doing much
of the actual accelerating outside the atmosphere, if you are sensible.
But using wings early in ascent does reduce engine thrust requirements,
at the cost of having to haul relatively heavy wings all the way up.
(Note, the sort of wings you need for descent, assuming you're planning
a horizontal landing, will not be nearly strong enough for use on the
way up, when the vehicle is much heavier.)  Whether this is better than
a vertical pure-rocket ascent is subject to argument; the vertical
ascent does waste some fuel, but it doesn't waste very *much*, because
rockets accelerate so quickly, and slightly larger fuel tanks are lighter
than wings.

>If yes, why are we still doing things the rockety way?

Partly because that's how we started, and there has been little opportunity
to explore alternatives.  Partly because it's just plain better.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)