That effect was implemented through just a thruster and an exhaust stream, e.g.:
void clbkSetClassCaps(){
// Other vessel definition code \\
FXFuel = CreatePropellantResource(1);
fx_explosion[0] = CreateThruster(_V(0, 0, 0), _V(1, 0, 0), 1, FXFuel, 15);
AddExhaustStream(fx_explosion[0], _V(0...