I mounted 2 Estes Rockets (Bullpups, the thick white and black military model) using a bent coat hangar as the launch rail (one under each wing). I put a 9 volt battery on board with a micro switch and a flap servo that would turn to close the switch, thus making the circuit to heat each ignitor for both rockets. The first run they both left the plane at the same time - since I was actually using the motors with the explosive last stage to blow the chute, I just pulled the 7 foot wingspan, powered Lanier glider straight up, then fired them so they wouldn't hit the ground and start a fire. Both rockets flew, popped the chute and floated safely to the ground - success! This was before the days of YouTube, so it is all on VHS tape, but we did it 3 times and it was successful each time. I have a friend who is an actual rocket scientist at NASA here in Huntsville, AL, and he assisted me with the setup - mostly held the wires while I soldered.
NOW I know that the AMA frowns on this, so I haven't done it since. We were at a huge coal strip pit so we were in the safest place I could find. I know you're asking for rocket assist but I just thought I'd put in 2 cents here. Besides, they say confession is good for the soul...
I have a Hangar9 P51 kit that flies really well, and oh how I want to mount 3 of the 3 inch long Estes micro rockets under the wings, then surprise some of my flying club buddies as I position the nose behind their planes... oh, well. But I promise each year when I get my AMA license that I won't involve myself in such behavior so I don't do it anymore. But it was FUN!!!!!!!!!!!!!!
Best,
RP