Use this section when you are using a launch tube - a rod that
goes up the inside of the nozzle and both helps to steady it at
the beginning of launch and also provides a bit of extra thrust.

It does not have to be a snug fit in the nozzle to work but it
is most efficient if it does not let any water past it so the
model assumes that no significant amount of water will leak
past the launch rod whilst it is in the nozzle. In reality, you can
lower any friction by using some grease of some sort.