From: Adrian Tymes (wingcat@pacbell.net)
Date: Sat Jan 20 2001 - 22:43:31 MST
Samantha Atkins wrote:
> Adrian Tymes wrote:
> > "Ross A. Finlayson" wrote:
> > >
> > > Hello,
> > >
> > > How about a rocket that you can carry around and set on the ground,
> > > where it will then launch itself into space. Here I am talking about a
> > > rocket that weighs less than fifty pounds. It would have a
> > > self-contained levelling stand so it could point in the good direction
> > > to achieve orbit. I am thinking it would have a propellant or ion
> > > exhaust. If it was fusion powered, it could escape the plasma into the
> > > launch vesicle, and power itself from the via ramscoop once it hit
> > > velocity. You would have to stand fifty or a hundred meters away.
>
> Exactly what kind of rocket can acheive the necessary thrust to reach
> orbit on less than fifty pounds of possible reaction mass/fuel
> (including the engine)?
A fifty pound rocket? ^_^;;;
Mass used to achieve orbit is a percent of takeoff mass as determined
by rocket's delta velocity and exhaust's velocity relative to rocket.
It is not an absolute quantity. Specifically (citing from notes) the
equation for single stage rockets is:
Mi/Mf = exp(dV/Ve)
where
Mi = intial mass
Mf = final mass
dV = velocity change (~9000 m/s for standard Low Earth Orbit)
Ve = exhaust velocity (~4500 m/s for standard explosive fuel)
Using the examples, dV/Ve=2, so Mi/Mf=exp(2)=7.39. So 1/7.39, or about
13.5%, of the initial liftoff mass is left in this case. Note that
this is independent of the actual liftoff mass. So, if the rocket were
50 pounds on liftoff, just under 7 pounds would reach orbit, and just
over 43 pounds (but less than 50) of fuel would be used in the process.
This would probably have to be solid fuels to make the non-consumed
part of the engine less than 7 pounds.
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