RailGun

A high-powered electromagnetic launch system. Basically a linear, single turn electric motor. Two parallel rails carry a HighCurrent? pulse (1 MA+ for a few ms), with a small armature between them. The resulting LorentzForce? is enough to propel the armature / slug at upwards of 4 km/s.

       Current ->
 + --------------------------
 Power!              | --->>>> Force!!!  = i L x B
 - --------------------------
       <- Current

Typically powered by large CapacitorBank?s, HomopolarGenerator?s, or CompensatedPulsedAlternator?s. Launcher performance is primarily limited by the power supply.

• To be precise, the terminal velocity is proportional to the current going through the system because of the resistive force dicatated by Lenz's Law. The issue of power supply is really not the limiting factor in how fast the slug can be propelled, but rather how much power you can put into the system before destroying it since at terminal velocity all power into the system is dissipated as heat. At 1 MA and a resistance of .01 Ohms, the system would be generating 10 GW of power. If 1 GW of that went into accelerating the slug, you'd still have 9 GW of power left, more than 7 times the power needed to power the DeLorean? from BackToTheFuture?. Sustained for 1 ms, that amount of energy would be about equivalent to 2 kilograms of TNT.

The advantage is that such a system would be relatively simple to make and the exit velocity doesn't depend on the length, making it ideal if you're short on space. The primary disadvantage then is that you can only fire it once before it melts down. A MassDriver? system would probably be more powerful since you have a virtually no resistive forces (this is especially true in a vacuum). Plus exit velocity is proportional to the length of the MassDriver? so you can potentially increase the power of the weapon indefinitely as long as you keep making it longer. MassDrivers? tend to be a lot more complicated and expensive to make, though.

• In the theoretical limit, yes, but good luck building a power supply to store more than a few kilojoules of rapidly deliverable energy. Taking your example (1 MA * 10 mOhm = 10 kV power supply rising to 1 MA in 1 ms), we'd have to have an inductance budget of (much) less than 10 μH total for the launcher, power supply, any connectors / switches, etc. For anything with a realistic budget, I suspect you're more likely to run into charring/scoring/pitting of the rails, rather than vaporization. (Welding the projectile to the rail can also be a problem.)
  • Lots and lots of inductors in parallel? I think we can both agree, though, that a RailGun isn't very practical because it'd badly damage itself everytime you fire. I think we should focus our efforts more one building a Trebuchet or Ballista. But that's just me; I'm ME.
  • The military has built a railgun it wants to use to kill people who don't look like us, but currently it needs 100 kilowatts of power.

AlexUtter really really wants to build one, but hasn't found the time or materials and would welcome any assistance. Has thought of forming a RailGunClub.

ProfessorEckert suggested in the first section of Physics24A? (in Spring 2008) that the Physics24A? students build a rail gun.