From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Single Stage to Orbit - How does it work?
Date: Fri, 28 Aug 1992 16:53:29 GMT
Organization: U of Toronto Zoology

In article <1992Aug28.090640.29092@mullet.gu.uwa.edu.au> phew@mullet.gu.uwa.edu.au (Phew) writes:
>All the mentions to Single Stage To Orbit now make me wonder what
>is different. My understanding was that a kilogram of fuel didn't
>have enough chemical energy to get itself into orbit...

It doesn't have to.  Most of the fuel comes nowhere near getting into
orbit; its energy is used up putting more (kinetic) energy into the
last little bit of fuel, so that *it* can reach orbit.

Staging is not magic; it's just a way of reducing the weight of tanks
and engines carried into orbit.  If your engines are good enough and
your tanks are light enough, you don't need to stage.  Getting into
orbit with one stage is not impossible, and the numbers say it is not
ridiculously impractical, although it is difficult.

Atlas, burning LOX and kerosene, can put 2000kg or so into orbit with
one and a half stages (it drops two of its three engines halfway up).
All the orbital Mercury missions were launched that way.  (Nowadays
Atlas generally doesn't fly without a Centaur upper stage, but that
wasn't part of the original configuration.)  The first 1.5-stage-to-
orbit mission, Project Score, was launched on an Atlas in 1958.

People have been discussing SSTO spacecraft for a long time.  Ed
Heinemann, Douglas Aircraft's weight-reduction genius, thought he
could put a single-stage expendable launcher into orbit (with very
little payload) circa 1950, although he never got the chance to try.

The possibility of making the Shuttle SSTO was studied very seriously.
This concept lost out on the fundamental problem of SSTO:  most of
the mass carried into orbit will be structure, engines, etc., so any
serious weight growth in those items could wipe out the payload
entirely... and there is always weight growth.  Circa 1970, for a
program that politically had to succeed, the risk was felt to be
unacceptable.

Two things are different about the current SSTO program:  we now have
better lightweight materials, and this program is willing to take some
risks.  The estimates say it can be done; it's high time we found out.

>I realize that there are many other possible solutions, such as better
>fuels and fuel combustion, nuclear engines, ramjets which convert
>to closed rockets etc. What method/s have been adopted for the SSTO...

Basically, none of the above.

As currently planned, SSTO will use very ordinary engine technology,
except that it will probably use either a "plug nozzle" (which puts the
entire base area of the rocket to work as the nozzle) or telescoping
nozzles that can be made longer in flight.  Both of these basically just
optimizes the nozzle design to match changing outside pressure; neither
has flown, although both look workable.  Last I heard, the feeling was
that telescoping nozzles looked less risky.

It will burn liquid hydrogen and liquid oxygen; no exotic propellants
or nuclear engines.

It will make no attempt to use airbreathing propulsion, which involves
a whole slew of nasty problems in its own right.  If what you want is
to get into space, rockets are better.

It *will* make use of some of the very lightweight materials developed
for the X-30.  They probably aren't necessary, but they help.
-- 
There is nothing wrong with making      | Henry Spencer @ U of Toronto Zoology
mistakes, but... make *new* ones. -D.Sim|  henry@zoo.toronto.edu  utzoo!henry
Newsgroups: sci.space