I typed a post about this yesterday morning but before I could finish one of our cats sat on the mouse and deleted it (at least I think that's what happened) and then I had to leave for work so I couldn't finish. Whether there is a nozzle or not doesn't answer the question we've been ruminating (don't get to use THAT word very often) about. In a pure action/reaction event whether there is an explosion or a nozzle controlled impulse, we're not clear on why the "launch" surface has a significant impact on the altitude attained. I knew he answer had to be in the physics and I think that you are both right. If it were a nozzle controlled impulse, the force of the jet from the nozzle pushes against the air/atmosphere and so the surface wouldn't matter, but why does the surface matter? An explosion action/reaction *should* be like the astronaut/baseball example. The explosion just pushes the lid away transferring the force of the transferred energy to the *rocket.* Since Micromeister's experiments clearly show that the surface matters, it is obviously not quite so simple.
What I think is happening:
It *is* like (or at least similar to) the astronaut/baseball example but with an important difference. In this case, the astronaut is standing next to a brick wall. If he is sufficiently close to the wall, then when he "throws" the ball (the explosion) the ball goes nowhere (and is not a part of the mathematics/physics problem at all) but the astronaut "pushes off" against the wall. All of the energy of the "push" is transferred to the astronaut/rocket since the wall obviously isn't going anywhere.
If the astronaut is a step (maybe a half step) away from the wall and throws the ball, the energy is transferred differently. Instead of pushing off against the wall and *all* of the energy being transferred to the astronaut, some of the energy is transferred to the astronaut/rocket and some to the baseball/lid. Given a fixed impulse, conservation of energy demands that the more things that are made to move from the force of the explosion/push, the less energy each of them can have.
If this is what is happening, then in order to maximize the altitude of the "rocket," one would want to design your base so that the film can lid does not move at all (or as little as possible).
Does that sound right?