For some reason the few people that have used super heated water for rocket power, use 470 degrees F (500 PSI). As water is heated in a pressurized container beyond that point, things start to happen quite quickly. Here is a table of for temp/pressure of super heated water;
http://www.engineeringtoolbox.com/saturated-steam-properties-d_273.html Notice that at 705 degrees the pressure is 3,200 PSI. I have plans to construct a smaller test tank and see what these higher pressures and temps do for thrust. Of the small number of people that have used this process for rocket power, here is a link to a guy that is planning an "Evel Knievel" type, stunt jump some time in the future. There are three tests of his steam rocket from one-quarter power to two-thirds power.
http://www.madmikehughes.com/RocketPropulsion.html I'm thinking one quarter would be 125 PSI and two-thirds would be two-thirds of five hundred PSI or 330 PSI. The beauty of this rocket would be the fact that you can start off at a low pressure, anything above 212 F will start building pressure. As for the question of being able to shut it off, the air operated valve will work easily in both directions. I plan on doing several tests with the hydro safely tied down to the trailer before trying it on the water. The rocket tank will be tied in to several points, including the transom, of course, with right-left hand rods that can be pre-loaded to avoid shock to the boat. Notice how jumpy the guys are that are testing their rocket as they start to get above 1/4 pressure. At 1/4 pressure they simply walk up and trigger it by hand, but at higher pressures they cautiously try to open the valve with a long rod. In my hydro project, the tank will be directly behind my back with a thin aluminum fire wall to separate me from the rocket. As scary as it sounds, chromoly tubing with high strength rod ends have much more strength than should be needed to hold it in place. (a 3/4" 4130 rod end has a breaking strength of 26,000 lbs.)
https://www.screamandfly.com/archive/index.php/t-280403.html
