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From: ST401385@BROWNVM.BITNET
Newsgroups: net.space
Subject: Next generation shuttle: electrically assisted take-off?
Message-ID: <8602010854.AA02457@s1-b.arpa>
Date: Fri, 31-Jan-86 15:40:26 EST
Article-I.D.: s1-b.8602010854.AA02457
Posted: Fri Jan 31 15:40:26 1986
Date-Received: Sat, 1-Feb-86 20:31:45 EST
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     There have recently been several postings relating to
electromagnetic launch from the Earth's surface.
Why not use an accelerator boost for a next-generation shuttle?
     Rockets are not very energy efficient.  Most of the energy of the
fuel goes to the exhaust.  The other problem is that they have to carry
their reaction mass with them.  Propulsion is much more efficient
if you can push against something else, like the earth.
In fact, propulsion energy efficiency approaches unity as the mass of
what you're pushing gets large compared to your mass.
     Suppose we boost a shuttle from a linear accelerator attached
to the side of a mountain.
For an example, let's suppose this is Mount Kenya, a 5 Km
tall mountain on the equator (the most efficient place to launch
from, at least if you want equatorial orbits.)
A track slanting 45 degrees up the side of the mountain has a path
length of 7.4 km.  Use an electromagnetic accelerator ("mass driver")
to boost it at one g.  At the end of the track, the shuttle will be
moving at 380 m/sec, or mach 1.2 (speed of sound at 5 km is 320 m/sec).
Boost time is 39 seconds, and the acceleration on board (vector sum of
applied acceleration and gravity) is 1.85 g--not very stressful at all.
Certainly this is gentle compared to what most of the mass-driver
people design for, and with much smaller problems with how fast
power switching is required.
     Now, 380 m/sec is not a large fraction of what is needed for orbit,
and the 5 mile altitude is small compared with orbit.  However, keep in
mind that we are substituting cheap electrical energy for expensive
rocket propulsion at the most important part of flight, where we are
lifting not only the shuttle, but also a huge load of fuel.
I don't have good figures
for the shuttle weight and impulse, (how much of the fuel is needed to
get it to the first 380 m/sec?) but a rough calculation tells me that
this will save about 33%.   You could use only one SRB, or
reduce the size of the ET to about a quarter of what it now is.
In fact, a tank that small would probably be better incorporated into the
shuttle (or into the SRB's): no throw-away parts.
     How much energy would such a thing take?  Lets see, if the vehicle
masses say 2000 metric tons; 150 billion joules.  40 megawatt hours?
That's tiny.  Average launch power is 4 Gigawatts: high, but not
unreasonable.
     Is this idea totally nuts, or would it work?

                                    --Geoffrey A. Landis
                                      Brown U.