[ExI] specific thrust

spike spike66 at att.net
Sat Dec 11 09:19:57 UTC 2010

On Behalf Of Adrian Tymes
Subject: Re: [ExI] specific thrust

On Fri, Dec 10, 2010 at 11:10 PM, spike <spike66 at att.net> wrote:

>> Ja, ion engines are only good for enormous specific thrust.

>Why is this?

There is a lot of energy thru-put there, so the stuff to handle it is
inherently heavy.

Think of a particle accelerator like SLAC up the street here at Stanford.
That is an ion engine in a way: it accelerates mass to enormous velocities.
If that whole rig were up in space, it could be used to create a small
thrust while using verrry little mass, so its specific thrust would be off
the charts.  If we had that up there and arbitrarily much energy to
accelerate a tiny amount of matter, then the momentum still needs to be
conserved, to the entire SLAC would be gently pushed the other way, so then
we could go all over the place out there using so very little propellant.
So if you want to think of it as an engine, SLAC might hold the record for
the highest specific thrust anywhere.

Chemical propellant carries all its energy in the propellant itself, but
uses a hell of lot of it to go anywhere.  The equipment needed to do it is
very small, light and simple: a tank, a pressurization mechanism such as a
turbine, a combustion chamber, a nozzle and off ya go.  Solid propellant is
even simpler still: just a pressure vessel and a nozzle.  But those have
even lower specific thrust.  High peak thrust.  The highest peak thrust ever
I think is with solids, but I would need to look that upwards.  Are the
space shuttle solids higher peak thrust than a Saturn V first stage main

Back to particle accelerators, it might be interesting to try to estimate
the specific thrust to figure out how fast SLAC or CERN could accelerate if
you did have that device out in space.  A centimeter per square week?  

Imagine we created an MBrain which wanted to haul its star elsewhere.  This
is an exercise we did here about a decade ago, when Robert Bradbury's
imaginative writings kept us calculating into the night.  I vaguely recall
calculating it, and it came out on the order of a few meters per square
century or something like that, so that one could navigate to the escape
velocity of the Milky Way galaxy in about a couple billion years, as I
recall, or make it to the nearest star in 10 million or so.  In that case,
the sun with its MBrain would have a very high specific thrust, and even its
total thrust is high by the numbers we are accustomed to using, but it has a
lot of mass to get moving.

I might have that written in my green notebooks somewhere, or if I get in
the mood tomorrow I may repeat those calcs.


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