A Historical Systems Study of Liquid Rocket Engine Throttling Capabilities

Abstract

This is a comprehensive systems study to examine and evaluate throttling capabilities of liquid rocket engines. The focus of this study is on engine components, and how the interactions of these components are considered for throttling applications. First, an assessment of space mission requirements is performed to determine what applications require engine throttling. A background on liquid rocket engine throttling is provided, along with the basic equations that are used to predict performance. Three engines are discussed that have successfully demonstrated throttling. Next, the engine system is broken down into components to discuss special considerations that need to be made for engine throttling. This study focuses on liquid rocket engines that have demonstrated operational capability on American space launch vehicles, starting with the Apollo vehicle engines and ending with current technology demonstrations. Both deep throttling and shallow throttling engines are discussed. Boost and sustainer engines have demonstrated throttling from 17% to 100% thrust, while upper stage and lunar lander engines have demonstrated throttling in excess of 10% to 100% thrust. The key difficulty in throttling liquid rocket engines is maintaining an adequate pressure drop across the injector, which is necessary to provide propellant atomization and mixing. For the combustion chamber, cooling can be an issue at low thrust levels. For turbomachinery, the primary considerations are to avoid cavitation, stall, surge, and to consider bearing leakage flows, rotordynamics, and structural dynamics. For valves, it is necessary to design valves and actuators that can achieve accurate flow control at all thrust levels. It is also important to assess the amount of nozzle flow separation that can be tolerated at low thrust levels for ground testing. Nomenclature

Expander and Coolant-Bleed Cycles of Methane-Fueled Rocket Engines

“…3, 2018 ber pressure. 17,21) Under the condition of maximum throttle, when a pressure drop in the oxygen injector remains at 15%, it will drop further as throttling is reduced. This large pressure drop in injection affects the necessary pump power and operating range of the engines.…”

“…Several studies have been conducted on the thrust control ability of liquid rocket engines, most of which targeted hydrogen-fueled engines and bipropellant engines. [17][18][19][20][21] For example, it was found that the throttled operation of a rocket engine can degrade excessive acceleration during launch while reducing the thrust required when landing a reusable vehicle. Boccaletto conducted studies on the throttling operation of methane-fueled engines ranging from 40% to 125% when using the SC cycle and the GG cycle.…”

Closed-Loop Thrust and Pressure Profile Throttling of a Nitrous-Oxide HTPB Hybrid Rocket Motor

“…Proper fuel and oxidizer atomization is critical for stable combustion in liquid rockets. 55 Maintaining a sufficiently high pressure drop across the injector for satisfactory atomization sets a practical lower limit to the depth of throttling that can be achieved. Specialized injectors are required to maintain combustion stability during deep throttling.…”

Conceptual Study of Throttled Operation of Rocket Engine Turbopump

“…The shift of the coefficients may cause pump operation to be unstable, where the slope between the flow and head coefficients is positive. 3,11,13) As a result of the shift of the coefficients, pump efficiency will become lower. To keep operation of the pump around its design specific speed, a circulation system is integrated to the pump for the throttled operation.…”

“…Throttling has been investigated and several engines were tested in deep and shallow throttled conditions. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Throttling is also necessary for the rocket-based-combined-cycle engine (RBCC) to change its operation to the ramjet mode. 15) In the rocket engine, thrust is proportional to the combustion chamber pressure.…”