Did he not think that if a steam powered rocket was viable then the Nazis, Russians, USA etc would all have tried it.
The Germans did use steam powered rockets in a manner of speaking, but where the late Mr Hughes was effectively using a pressurised kettle, they instead used high test peroxide (HTP, which the Germans called T-Stoff).
The first method was a so-called "cold engine" where the T-Stoff was decomposed with an aqueous catalyst to produce a stream of superheated steam and oxygen to provide thrust. This type of rocket engine was used to power
take-off assist packs and
boosted glide bombs.
The second, "hot engine", method was to use T-Stoff as an oxidiser for C-Stoff fuel (a blend of methanol, hydrazine and water). T-Stoff and C-Stoff were hypergolic - that is, they reacted violently on contact without the need for an ignition source:
The violent combustion process resulted in the formation of water, carbon dioxide and nitrogen, and a huge amount of heat sending out a superheated stream of steam, nitrogen and air that was drawn in through the hole in the mantle of the engine, thus providing a forward thrust of approximately 17 kN (3,820 lbf).
https://en.wikipedia.org/wiki/Walter_HWK_109-509#Design_and_developmentThis type of rocket motor was used in the Messerschmitt Me 162 Komet and Bachem Ba 349 Natter interceptors.
The British used the "cold engine" type of HTP-fuelled rocket to power a scaled-down model of the cancelled Miles M.52 to test a number of concepts for supersonic flight. In doing so, it was proved that officialdom's fears about the safety of the M.52 - one of the excuses given for the project's cancellation - were unfounded.
Not propulsion in a direct sense, but a number of rocket engines have used decomposing HTP to generate steam to drive the fuel and oxidiser turbo-pumps*, and the X-15 hypersonic rocketplane used HTP "cold engines" for its reaction-control system, which controlled where the plane pointed at altitudes where the air was too thin for aerodynamic control surfaces to be of use.
*
So in a sense they are "steam powered" on the basis that if the pumps "run out of steam", the rocket engine stops working from fuel starvation.