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.

Author: M. Bindhammer, Team Selene