You have to play with vorticity and velocity dampening to dive the explosion the zero gravity effect. But in this case, some explosions moving outward is a good thing because we want the explosions to have the appearance of bursting through the Lucifer hull. I'm creating some explosions that move outwards and some that stay put.
Like I posted earlier though, this way may not look 100% realistic but not much of the scene does anyway. The biggest advantage here is that the scene geometry can truly interact with the explosions.
Can you not set the mass of the projectiles that FumeFX generates to prevent them from falling back? (Note, I'm just guessing that FumeFX uses some kind of Newtonian physics model or something, I don't really know)
Yes, except it isn't fume that generates them. For the projectiles, Fume works in conjunction with any particle system. This works well because you have much greater control over where the projectiles go.