Some guys from HARA built one back in the 90's. I saw it fly in Manchester, IIRC. One of the guys from that project was involved in building the motor for Rutan's Spaceship One, Tim Pickens. The steam rocket was fairly conventional. It used a heating element to bring the pressure vessel's water above the boiling point so that it would turn to steam upon release. I can't recall the rating they calculated, but the rocket was pretty darn heavy, so I'd guess maybe an "L"???
http://timpickens.com/tim-pickens-p.../steam-rockets/

The steam rocket was an extreme extension of the water rockets that Dan Coon (also HARA at the time) had been building. His water rockets flew with Polaroid camera payloads, and eventually, gimballed nozzle flight stablization.

BTW, this was more or less the same group of guys that were determined to put a "dirt" rocket into space. The project was a rockoon with an asphalt hybrid motor. It was tested in a conventional flight at Manchester, but I think their final design used a more standard fuel for the hybrid motor. It was launched off a barge in the Gulf, IIRC. I was at their asphalt motor test stand runs in Alabama and saw the conventional flight at Manchester. Those were fun times.

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