I while ago (middle of May 2010) I came up with following idea:
The solid propellant heats up the alcohol or water during combustion reducing thermal loading of the nozzle and combustion chamber. Because the casing is double - walled and liquids are so difficult to compress, the combustion chamber pressure can remain very high. As pressure rises, a nozzle made from ductile material might start to deform in a manner so as to optimize the expansion depending on the altitude (aka altitude compensation). Once burnout of the solid propellant is completed, the rupture disc bursts or is burned away by the solid propellant, and super - heated steam flows through the nozzle and expands, which results in some additional thrust. Using water or alcohol preheated prior to launch can warm up the solid propellant and improve its combustion performance as well.
I furthermore proposed a design, using copper fittings:
Rick Maschek from the SS2S team and his rocketry friends built a first prototype end of the year 2010, using hard soldered copper fittings and tubes:
A case bonded sugar/KNO3 propellant with cone shaped core was used for the static test firing. As they were afraid the motor would CATO, the nozzle was removed prior ignition:
Rick gave the motor a new name: SAM, an abbreviation for Steam Assisted Motor.