I just noticed that the TRF thread on the episode has been joined by Rick Maschek, who was involved with the making of the rocket motors. He has two Facebook albums that are viewable: Building the motors; and, on the playa. In the thread, he confirms what I thought I had observed - that 38mm motors were used to ignite the P motors.
There is more about the episode on the aftershow video. Asked about the possibility of a rocket car episode 4, a car with wings, both Adam and Jaime both said "no". Adam says that would be a car shaped rocket, not a rocket car. Jaime says you can make a brick fly, but what's the point? Guys, if you have to ask, you don't get it :)