Combustion devices described herein comprise a compliant combustion chamber wall or segment. The compliant segment deforms during combustion in the combustion chamber. Some devices may include a compliant wall configured to stretch responsive to pressure generated by combustion of a fuel in the combustion chamber. A coupling portion translates deformation of the compliant segment or wall into mechanical output. One or more ports are configured to inlet an oxygen source and fuel into the combustion chamber and to outlet exhaust gases from the combustion chamber.