[Snail]

And I could have sworn that I flew a Sekhmet in bog standard retail FS2 in Bearbaiting once upon a time.  I do distinctly remember having two banks of Helios for that mission.

Sure you weren't playing one of the many fan remakes?

[LordPomposity]

Due to relativistic corrections, E=mv²/2 actually exceeds E=mc² at somewhere around 87% of the speed of light (the full kinetic energy equation for a fast-moving object is E=mv²/[2*SQRT(1-v²/c²)]). The chief disadvantage of particle beams is that they would lose cohesion much more rapidly than lasers and would therefore have a much shorter range. Still, by "short range," we're talking thousands of kilometers versus several light-seconds. Not really enough of a limitation to make trouble for FS2 combat.

No it doesn't.  Relativistic KE pulls away from Newtonian KE pretty quickly as you get close to c.  The actual formula for relativistic KE is T = mc^2/sqrt(1-(v/c)^2) - mc^2, by the way.  Tell Wolfram Alpha to graph x^2/2 and 1/sqrt(1-x^2) - 1 and you'll see what I mean.

Yep, you're right, I had the equation wrong. Oops.