KFL@MIT-MC.ARPA ("Keith F. Lynch") (12/21/85)
Note that not all electromagnetic guns are railguns. One problem with railguns is they have to be in contact with the projectile at all times. This is not practical at velocities of several miles per second. It's not just for launching things that electromagnetic guns are useful. They are equally good at catching things. You just have to make sure things are lined up right. It takes relatively little energy to get into space. It's just that to stay in space, you have to have orbital velocity, 5 miles per second. If you go straight up from Earth at just 1.5 miles per second, which only takes 9 percent as much energy as going into orbit, you will reach an altitude of 194 miles before falling back to Earth. Lets say that there is a satellite with an electromagnetic gun orbitting 194 miles above the Earth. The projectile can be accelerated to orbital velocity by passing through this gun. Since it is in space, the gun could be very long, which means the accelerations could be low enough for people to make the trip. A 200 mile long gun would be necessary if people are to endure no more than 10 Gs. Alternatively, two 50 mile long guns in different orbits could also work. Or four 12.5 mile long guns. One problem with this is that the satellite with the gun would tend to fall out of orbit after transfering its momentum to the projectile. It might be able to do several before this happens if it is much more massive than the projectile, but that doesn't gain you anything since you had to use proportionately more energy to launch the more massive gun satellite in the first place. The solution is to run the gun in reverse as often as forward. Whenever a satellite wants to reenter, instead of using atmospheric braking, it would decelerate using the gun satellite, and drop straight down to Earth at a relatively gentle 1.5 miles per second. Much smaller heat shields would be needed. Possibly no heat shields at all. If not enough satellites want to reenter, pieces of asteroids and comets could be gradually maneuvered into low Earth orbit, perhaps with light sails. They could then be dropped into the ocean by the gun satellite. Note that the savings are actaully much greater than that 9% figure would suggest. The main reason rockets are so large is that they must lift so much of their own fuel so far. The ratio between the rocket exhaust velocity and the desired velocity is the natural log of the ratio between the payload mass and the total (payload plus fuel) mass. Since the exhaust velocity of a hydrogen-oxygen rocket is at most about 1.8 miles per second, the mass of the fuel must be at least 15 times the mass of the payload in order for the payload to reach 5 miles per second (orbital velocity). If only 1.5 miles per second are needed, you only need 1.3 times more fuel than payload. It is only improvements of THIS magnitude that can make space travel no more expensive than air travel is today. ...Keith
henry@utzoo.UUCP (Henry Spencer) (12/22/85)
> Note that not all electromagnetic guns are railguns. One problem > with railguns is they have to be in contact with the projectile at all > times. This is not practical at velocities of several miles per > second. As I recall, the problem is not that *physical* contac* is required, but that *electrical* contact is required. The two are not necessarily the same. There have been serious studies of using electric arcs as conductive paths for electrically-powered trains! Not as silly as it sounds -- arc plasma is a far better conductor than any metal. Maintaining the arc does involve losses, though. And an arc might behave strangely in the intense magnetic fields of a railgun. -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry