wagner

Have you considered making the hollow structural fuselages actually hollow (and marked as cargo bays for FAR)? As it is, you can clumsily force things in there, but the SPH often doesn't play well when you do that, and FAR accounts for all those parts in its drag calculations.

An MHD/MPD (same thing) thruster uses magnetic fields to accelerate an ionized fluid (plasma). VASIMR is a traditional thermal rocket, using an RF emitter as a heat source instead of a chemical reaction, and using shaped magnetic fields as a virtual nozzle instead of a physical element. In thermal rockets, ISP is dictated by combustion chamber temperature. The higher your temperature, the higher Mach number you can aerodynamically expand your exhaust up to. It's variable specific impulse because for a given electrical energy input, you can vary the temperature of the "combustion" chamber, more mass at lower temperature for high thrust, or less mass at higher temperature for high ISP.

Aerodynamic expansion means it's supposed to be a VASMIR-style magnetic nozzle, rather than a MHD plasma thruster?