dietz@SLB-DOLL.CSNET (Paul Dietz) (12/20/85)
A problem with shooting matter into orbit with ground-based guns is that the orbit will either escape from earth completely or will intersect the ground. There is no way to get into a stable elliptical orbit without some additional acceleration in space. I suggested some months ago that larger payloads (hundreds of kg) be sent to the moon by electric gun; this could provide a cheap way of sending rare volatiles into space. Harvesting would be simplified if the payloads were made to crash in a small area on the moon's surface. The large payloads are needed to make on-board maneuvering rockets practical; these rockets are needed to correct for velocity errors introduced by the passage throught the atmosphere. To get matter into low earth orbit one would shoot it into a very elongated elliptical orbit (with perigee beneath the earth's surface); at apogee a rocket would fire to bring the perigee above the atmosphere. The farther out the apogee is the less of a burn is needed, since more angular momentum will be supplied. After the orbit is stable aerobraking could be used to lower the apogee. This again requires projectiles of substantial mass to be practical.
dietz@SLB-DOLL.CSNET (Paul Dietz) (01/09/86)
> The major losses would be atmospheric drag, and all the friction of the > tubes or whatever you are using for "rail guns". Actually, current rail guns are not very efficient (30%?). Much of the energy ends up in a residual magnetic field after the projectile leaves the launcher. Using distributed energy input to the gun would help (this is equivalent to a chemical gun in which the propellant is distributed along the barrel). The biggest technical obstacle to rail guns and other electric launchers is, suprisingly, not the launcher itself but rather the power source. Launching a 100 kilogram payload to orbital velocity at 1,000 gees requires a peak power of something like 10 gigawatts (for a brief time). The average power will be much less (depending on the launch rate). Rail guns could be useful here: an "inverse railgun" can generate a pulse of power by using a chemical explosion (natural gas and air, say) to push a metal armature into a magnetic field. Several thousand such generators would be used in a launcher.
ems@amdahl.UUCP (ems) (01/14/86)
> Actually, current rail guns are not very efficient (30%?). Much of the > energy ends up in a residual magnetic field after the projectile leaves > the launcher. ... > The biggest technical obstacle to rail guns and other electric launchers > is, suprisingly, not the launcher itself but rather the power source. > Launching a 100 kilogram payload to orbital velocity at 1,000 gees > requires a peak power of something like 10 gigawatts (for a brief > time). The average power will be much less (depending on the launch > rate). Why not use capacitors or something similar to store the energy? Charge them up over night, then POW, discharge for a launch. The peak size of the *generator* is reduced by orders of magnetude ... -- E. Michael Smith ...!{hplabs,ihnp4,amd,nsc}!amdahl!ems This is the obligatory disclaimer of everything.
dietz@SLB-DOLL.CSNET (Paul Dietz) (01/15/86)
>Why not use capacitors or something similar to store the energy? >Charge them up over night, then POW, discharge for a launch. >The peak size of the *generator* is reduced by orders of magnetude ... Actually, you'd charge them up over a period of minutes using the power grid. A 100 kg payload launched to 10 km/sec needs 15 billion joules of energy (at 33% efficiency). Electrolytic capacitors these days have a storage density of around 100 joules/cubic centimeter, so that's a capacitor roughly 16 feet on a side! Building something that big out of hundreds of thousands of smaller capacitors doesn't sound impractical. Handling the power is not just a matter of storing the energy, but also switching it. Designing switches and cables capable of handling tens of gigawatts of power in short pulses is nontrivial. It might be an interesting project to build, at home, a small railgun or coil gun powered by some of those large capacitors found in computer power supplies. A 100 milligram mass could be accelerated to 10 km/sec with just 15 kilojoules of energy (at 33% efficiency). Don't aim it at anything valuable. (I take no responsibility for the results if someone actually tries this...)
Carter@RED.RUTGERS.EDU (_Bob) (01/16/86)
From: Paul Dietz <dietz%slb-doll.csnet at csnet-relay.arpa>
It might be an interesting project to build, at home, a small railgun
or coil gun powered by some of those large capacitors found in computer
power supplies. A 100 milligram mass could be accelerated to 10 km/sec
with just 15 kilojoules of energy (at 33% efficiency). Don't aim it
at anything valuable. (I take no responsibility for the results if
someone actually tries this...)
Is there an appropriate description of such a small railgun anywhere
in the literature? Just what physical principles do you have in
mind? Are you speaking of what amounts to a linear motor only, or do
you also mean a device that would collapse the rails (and their
associated fields) with a linear chemical explosion as well?
If the latter, it would be advisable to consult the law governing
explosive devices. Things that go bang are regulated everywhere
and completely illegal in many places. Even a purely magnetic
railgun would probably be a "firearm" under the laws of my state.
_Bspace@ucbvax.UUCP (01/19/86)
Try the Space Studies Institute in Princeton. They'll send you papers on the mass driver. The first one was built with photographic flash capacitors, I believe. Bob Hettinga University of Chicago Computation Center