4 ms·

Rail guns wouldn't be very useful for delicate things, but you could, in theory, use one to launch bulk supplies like fuel and water. The problem is the projec

by tsotha 16y ago

Rail guns wouldn't be very useful for delicate things, but you could, in theory, use one to launch bulk supplies like fuel and water. The problem is the projectile comes out of the gun so fast most of the weight is going to be taken up by shielding. If you have to put up with a sucky mass factor you may as well build a rocket instead. A ram accelerator is probably more practical in any case.

stcredzero 16y ago
The problem is the projectile comes out of the gun so fast most of the weight is going to be taken up by shielding. Just how long does the shielding have to withstand high temps? We already solve a much harder problem with reentry of orbital vehicles. The problem here is much simpler, as we don't actually want to shed most of the velocity. In fact, we want to lose as little of our kinetic energy as possible. If ablative shielding can handle reentry, it can handle launch. AFAIK, the mass ratio for reentry shielding is quite good.
- tsotha 16y ago
  The difference is where you start. Coming back from orbit you're going to start your run at orbital speed in no air and gradually increase air density. You'll dissipate most of your energy by the time you reach the thick air of the lower atmosphere. The rail gun cargo is going in the opposite direction - you start in the thick air at a speed greater than orbital velocity. It's been years since I could grind through the numbers, but it makes a big difference
  - stcredzero 16y ago
    You'll dissipate most of your energy by the time you reach the thick air of the lower atmosphere. This speaks to my point. In launch accelerator mode, you want to dissipate as little energy as possible. In reentry mode, you want to dump the majority of a big honking chunk of kinetic energy. I'm sure there are all sorts of optimizations to consider. How about a mostly vertical trajectory, with most of the horizontal velocity provided by lasers heating a heat-exchanger? By keeping the trajectory mostly vertical, one can avoid most of the lower atmosphere by stationing the opening to the atmosphere miles above the surface. This also limits the downrange danger-zone for falling malfunctioning cargo pods. Additionally, it makes the cargo pods easier to target with the lasers, since the powered flight would be entirely in the upper atmosphere. It would also make the rocket nozzles easier to design.

by tsotha 16y ago

stcredzero 16y ago

The problem is the projectile comes out of the gun so fast most of the weight is going to be taken up by shielding. Just how long does the shielding have to withstand high temps? We already solve a much harder problem with reentry of orbital vehicles. The problem here is much simpler, as we don't actually want to shed most of the velocity. In fact, we want to lose as little of our kinetic energy as possible. If ablative shielding can handle reentry, it can handle launch. AFAIK, the mass ratio for reentry shielding is quite good.

tsotha 16y ago
The difference is where you start. Coming back from orbit you're going to start your run at orbital speed in no air and gradually increase air density. You'll dissipate most of your energy by the time you reach the thick air of the lower atmosphere. The rail gun cargo is going in the opposite direction - you start in the thick air at a speed greater than orbital velocity. It's been years since I could grind through the numbers, but it makes a big difference
- stcredzero 16y ago
  You'll dissipate most of your energy by the time you reach the thick air of the lower atmosphere. This speaks to my point. In launch accelerator mode, you want to dissipate as little energy as possible. In reentry mode, you want to dump the majority of a big honking chunk of kinetic energy. I'm sure there are all sorts of optimizations to consider. How about a mostly vertical trajectory, with most of the horizontal velocity provided by lasers heating a heat-exchanger? By keeping the trajectory mostly vertical, one can avoid most of the lower atmosphere by stationing the opening to the atmosphere miles above the surface. This also limits the downrange danger-zone for falling malfunctioning cargo pods. Additionally, it makes the cargo pods easier to target with the lasers, since the powered flight would be entirely in the upper atmosphere. It would also make the rocket nozzles easier to design.