Investigation into the Operation of Thermally Regulated Autogenously Pressurized (TRAP) Engine Cycles

Abstract

Since the development of liquid rocket engine technology in the early decades of the 20th century, many different methods (engine cycles) to feed the propellants into the combustion chamber have been developed. One of the simpler methods to transport the propellants from the tanks to the combustion chamber is using a pressure fed system [1]. However, one of the main disadvantages of this method is the inevitable pressure decay over the length of the burn due to the expansion of the gas used for pressurization. This problem can be circumvented by storing more of the pressurizing gas at veryhigh pressures such that the pressures in the tank can be maintained at the appropriate levels for the entire burn through a pressure regulator, but at the cost of mass increases and complexity for the pressurization system. Alternatively, it is theoretically possible to take advantage of the autogenous pressurization of the propellants themselves in combination with heat transfer from the nozzle to the tanks to maintain tank pressurization throughout a full duration burn. This engine cycle is called a Thermally Regulated Autogenously Pressurized (TRAP) Cycle. TRAP cycle engines draw on the established physics of pump fed expander cycle engines. However, instead of using an expanding fluid to power a turbine which drives pumps to feed the combustion chamber, TRAP cycle engines remove the pump and send the fluid back to the tanks and through a heat exchanger. This delivers the necessary heat flux into the tanks which will evaporate the autogenously pressurized propellant, thereby maintaining pressurization. TRAP cycle engines may be ideally suited to environments where the outside pressure is less than 10 kPa (moon, mars, or 2nd stage engines) where a low combustion pressure would require little extra mass to safely contain the pressure fed propellants. Considering the vast complexity decrease commensurate with the removal of the turbo-pump assembly from the propellant feed system, it is expected that the TRAP cycle engines may be cheaper and more reliable to develop while sacrificing little in performance compared to their pump fed expander cycle counterparts. Furthermore, the vast reduction in moving parts makes TRAP cycle engines preferable in an environment where re-usability is necessary, as is becoming more the case inmodern rocketry.