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'how do the shuttle thrusters work?'
2002\12\21@015306 by Russell McMahon

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>  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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2002\12\21@023120 by rad0

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> 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
>
OK, so it's a question of efficency,

don't we have free electricity, from solar panels....

and so isn't it a question of the weight of the solar panel and the electric
motor and the compressor,
versus all that hydrozine?

I yield to efficency,if the numbers add up....

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2002\12\21@085514 by Russell McMahon

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> > 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.

> OK, so it's a question of efficency,
> don't we have free electricity, from solar panels....
> and so isn't it a question of the weight of the solar panel and the
electric
> motor and the compressor,
> versus all that hydrozine?

Note that the figure of merit, Isp, is thrust per unit mass. You need to get
the mass from somewhere to add energy to it. If you can add enough energy to
mass such as water you could improve on burning Hydrogen and oxygen. But
it's not easy. Doing it in conventional ways - eg gas compression, is not an
easy or really realistic task. Non conventional means do actually work. An
ion drive uses an electric potential to accelerate charged ions to very high
velocities. The resultant motors typically have Isp's in the several
thousand range - far better than chemical combustion powered motors.
However, Murphy saw them coming and such motors are NOT suitable for typical
main propulsion systems because the amount of thrust available from
practical ion motors is very very small by main engine standards - small
even by typical thruster standards. For long duration  / deep-space missions
an ion motor is often s good solution as the very very low thrust operating
over long periods makes better use of the available reaction

Comsats in geosynchronous orbit and costing hundreds of millions of dollars
usually have a useful lifetime measured by the ability to "station keep" in
the position they are meant to be in. This in turn is governed mainly by how
much fuel they can carry. Typically more than half a comsat's mass is
station-keeping fuel.

See the "propellantless thrust" thread with a message sent 10 minutes after
the one I am replying to now for an idea which MAY have an effectively
infinite Isp. If this proves to be the case (and it probably won't) it will
revolutionise comsat station keeping.


       Russell McMahon

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2002\12\21@101318 by Bob Ammerman

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A lot more delta-V for the mass they have to carry.

Bob Ammerman
RAm Systems



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Sent: Saturday, December 21, 2002 1:28 AM
Subject: [ot]: how do the shuttle thrusters work?


> watching too much sci fi
>
> but any way,
> 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,
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2002\12\21@133331 by Wagner Lipnharski

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That's simple;

A gallon of compressed air, can generate some explosion if ruptured, right?  Let
say it can lift a brick to 5 meters high.

The same gallon of fuel, gasoline, black powder, hydrogen + oxygen mix, can
generate much powerful explosion with much more energy drawn.

Just a little part of this fuel is necessary to generate such explosion to lift
the same brick 5 meters high.

It means the gallon of fuel can generate many consecutives explosions, what will
push the brick many times further than the whole compressed air gallon could.

The whole gallon of fuel, used in small quantities each time and generating
consecutives explosions could lift the same brick as high as 500 meters or more.

So, why don't use much more compressed air?

1) Bigger the air cylinder, heavier it will be, requiring more and more
compressed air to lift it by itself. Goes to a point that it will be unable to
leave the ground.

2) To generate such compressed air on site, it will require a compressor and
compressor fuel.  Then you will be talking basically about turbo-fan, or jet
systems.  But they only work nicely (vertical lift) in small weight vehicles,
like few jet fighters and such.  Even the F15 can stand vertical lift for only
few moments.  The iF-22 can do it better, but the AFT will consume a tremendous
amount of fuel.  Not even talking that to compress air, "air" is necessary, our
planet's air envelope is very thin, what would limit the maneuvers of such
engine.

The main idea for turbo-fan (or jet system) is to push a vehicle without much
friction. It means, just push something that is already sliding over an air bed.
The stronger friction will be only the front air, this is why speedy jets use a
state-of-the-art aerodynamics - even a monster 747 when front view, is just
small and offer as less as possible volume of frontal air to be pushed away when
flying.

To push against the planet's gravity is another and completely different thing.
When you push a paper airplane forward it can fly longer (if lucky), and you
don't even need to apply much push energy.  Now try to make it fly up, lift
vertical, even with a strong arm push this little paper plane will go no more
than 3 yards high, then down.

Unfortunately, until we find a way to generate anti-gravity energy, we will be
blasting a tremendous amount of vapors everytime a space shuttle lifts.

From my home front door - 50 miles from Kennedy Space Center:

That is not my front neighbor house's smoke, that is the vapor trail of the
STS98 Feb/2002.
You can see 3 little dots at top left, the shuttle and the already detached two
auxiliary rockets.
http://www-pao.ksc.nasa.gov/kscpao/shuttle/summaries/sts98/index.htm


Wagner.

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