r_rocketTrail
0 = No rocket trail 1 = Normal 2 = Grenade trail 3 = Alt normal trail 4 = Slight blood trail 5 = Big blood trail 6 = Tracer1 trail 7 = Tracer2 trail 8 = Plasma trail 9 = Lavaball trail 10 = Fuel Rod Gun trail 11 = Plasma Rocket Mk II trail
Note that if you use the rocket trails, you will have to change the explosion colours and things like that yourself. And you might want to try hiding the rockets with the new cl_hiderockets cvar for some trails are intended to be "beam" trails.