>  how do the shuttle's thruster jets work?
>  .. and my real question is why don't they just use compressed air?
>
> with compressed air, they could just draw down some of the crew
consumables
> and use some sort of mechanical compressor, to compress the hell out of
it,
> and then use this pressure to blast themselves into the wanted position?
> In zero G --
> what does their thruster fuel give them over compressed air??

Energy density is a primary consideration.
A thruster is a rocket motor which achieves its effect by ejecting mass at a
velocity. The higher the mass x velocity product the more effect it has.
Compressed air is an entirely viable thruster mass BUT the energy taken to
compress it has to come from somewhere. In a (semi) closed system such as
the Shuttle the energy source and the mass have to be taken along for the
ride. The more dense the energy source is, both in energy per mass and
energy per volume terms, the better.

Typical thrusters for space use use a "monopropellant, such as
mono-methyl-hydrazine (MMH). This has considerably better energy density
than is liable to be achieved by using a machine to compress air. An
important of merit for a rocket propellant is "Isp" or Specific Impulse.
This is the number of units of thrust which can be obtained in a second for
a unit mass of propellant (using consistent units). In simplistic terms Isp
has the units of 'seconds" as eg a pound of propellant which produced 100
pound-seconds of thrust per pound would have an Isp of 100 lbf-second/lbm =
100 seconds. The mixing of mass and force units leads to extended debate
which is not worth entering into here.

The Shuttles main engines burn Hydrogen and Oxygen and have an Isp of about
450. A MMH thruster may have an Isp of only about 100. A "cold gas"
compressed gas thruster will typically have an Isp of around 10 or 20. (Lots
of considerations beyond immediate scope of this conversation).
Monopropellants are generally preferred because they are infinitely
restartable and if desired can more easily be throttled over a wide range.

For seriously heavy duty thrusting an MMH system is undesirably inefficient
(due to its low Isp) and the Shuttle has a "Reaction Control System" which
is a small motor set which burns the same fuel as the main engines. Due to
its less complex design (and especially lower pressure operation) the RCS
Isp is not as high as for the main engines but still far superior to MMH
thrusters.

An engine could well be used which runs on the oxygen/hydrogen fuel and
compresses air (or its own water exhaust) for thruster purposes but burning
it directly in a rocket motor is far more efficient.



        Russell McMahon

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