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I mostly agree that beamed propulsion has a lot of promise, but I'm not totally sold on our ability to actually make it happen. Like I was saying in the other thread aiming and targeting are really very hard and there's a lot of potential downside if you get it wrong.
Here's a wacky idea: What about an electron engine?
The idea is that exhaust velocity of an ideal gas is proportional to the inverse of the square root of the molar mass of the propellant. That's why hydrogen is best for this kind of rocket, because it dissociates to individual H molecules with a molar mass of 1.
Electrons, though, have a much lower molar mass. An electron has a molar mass of 0.00055 amu. That means that for similar operating temperatures and pressures an electron engine will have an exhaust velocity more than 42 times higher. So, for example, a NERVA-like system might get an Isp of 41,000 s.
The system might be somewhat ramjet-like: If you're flying through a plasma (for example in the ionosphere) you might suck in the electrons through a positively charged plate, heat them up, and push them out through an engine nozzle.
I'm sure there are 10 billion problems with this idea but it's still pretty interesting.
Here's a number, for reference. 1 kg/s of electrons is a current of 175 GA (that's "giga-amps"). Given the numbers above that will generate 410 kN.
-Josh
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The other option would involve using a gas gun, rail gun or coil gun to provide an initial velocity increment for your SSTO. If it can shave 1.5 km/s off of your dV, i.e. a muzzle velocity of 1.4km/s (high end rifle speeds) at a height of several km, then an SSTO with exhaust velocity 4500m/s will reach orbit with a mass ratio of 5.92.
The catch is that the capital cost of the gun would be quite high - it would be cheaper than a lower stage only above a specific launch rate.
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