The U.S. Air Force (USAF) and its rocket engine contractor Aerojet Rocketdyne (AJRD) have achieved a major milestone toward a new U.S. state-of-the-art capability to develop powerful next generation hydrocarbon rocket engines. The achievement involves completion of the first in a series of hot-fire tests on a sub-scale oxygen-rich pre-burner, built by ARJD for the USAF’s Hydrocarbon Boost Technology Demonstrator (HBTD) program.
While America once led the world in kerosene and RP-fueled rocket engine technology, the U.S. has lost such hydrocarbon rocket infrastructure and lags behind Russia, specifically with the Energomash RP-1/liquid oxygen RD-180 that powers the proven and reliable United Launch Alliance (ULA) Atlas-V.
There are two major rocket engine cycles. One is called a “gas generator cycle,” where gases used to drive an engine’s turbopump are exhausted by the pump, giving a somewhat ragged looking rocket plume due to this flaming exhaust vented beside more distinct rocket nozzle plumes.
The other cycle is the “staged combustion cycle,” where a share of the propellant—be it kerosene or oxygen or both—is first burned in a pre-burner. The resulting hot gas is first used to power the engine’s turbines and pumps, then, instead of being dumped as with the gas generator cycle, that exhausted gas is injected into the main combustion chamber, along with the rest of the propellant to generate powerful thrust. The two stages that make up the staged cycle propulsion are from the pre-burner stage, then combustion chamber stage.
A key advantage of staged combustion is that it gives an abundance of power, which permits very high chamber pressures and the use of high expansion ratio nozzles. These nozzles give better efficiencies at low altitude critical to the flight in the moments after liftoff.
The disadvantages of staged cycle engines include harsh turbine conditions, exotic plumbing to carry the hot gases, and complicated feedback and control. The U.S. mastered all of these for the Space Shuttle RS-25 Main Engine design that used cryogenic oxygen and hydrogen propellants and a preburner for each.
According to Aerojet Rocketdyne an oxidizer preburner combusts hydrogen and oxygen at an extremely fuel-rich mixture ratio, and thus supplies hot gas at variable rates to drive the engines high-pressure oxidizer turbo pump. The operating level of the oxidizer preburner is controlled by regulating the oxidizer flowrate by means of the oxidizer preburner oxidizer valve. Welding the injector into the top of the engine’s Hot Gas Manifold forms the combustion area and places it immediately above the pump turbine.
“Throughout the sub-scale fabrication and facility checkouts, we’ve documented a number of lessons learned that have directly influenced a full-scale pre-burner design. We are looking forward to what more we will learn during the hot-fire test series,” said Joe Burnett, program manager of the Hydrocarbon Boost Technology Demonstrator program at Aerojet Rocketdyne.
AJRD states, “In coming months, multiple injector configurations will be tested to evaluate the performance and stability parameters that are critical for a high-performance, high-reliability liquid oxygen/kerosene rocket engine.”
According to the USAF, typical parameters for an oxygen preburner are:
- Mixture: A full 100 percent of the engine’s oxygen flow will be be mixed with 4 percent of the RP flow.
- Losses: There are no secondary gas flow losses en route to the thrust chamber.
- Performance: The resulting high pump and turbine speeds will equate to higher combustion chamber pressures producing higher thrust.
The sub-scale test series will be used to aid the design and development of the full-scale pre-burner and engine development. An oxygen-rich pre-burner is one of the enabling technologies of the Oxygen-Rich Staged Combustion (ORSC) cycle needed to provide high thrust-to-weight and performance regardless of hydrocarbon fuel type, both USAF and AJRD documentation says.
Under program direction of the Air Force Research Laboratory (AFRL), Aerojet Rocketdyne is designing, developing, and testing the HBTD engine. Its technologies are directed at achieving the goals of the Rocket Propulsion for the 21st Century (RP21) program, formally known as Integrated High Payoff Rocket Propulsion Technology, or IHPRPT.
Designed to generate 250,000 pounds of thrust, the engine technology uses liquid oxygen and liquid kerosene (RP-2) in the first U.S.-developed demonstration of the ORSC cycle. It has been designed as a re-usable engine system, capable of powering up to 100 flights, and features high-performance long-life technologies and modern materials, said the Air Force and its contractor.
Burn-resistant, high-strength alloys, manufactured using novel technologies, will be used throughout the engine. Manufacturing parameters of some of the alloys have been developed under a joint effort with the Air Force, known as the Metals Affordability Initiative or MAI, said AJRD.
These advanced technologies will be matured sufficiently throughout the program to support the next generation of expendable launch system development efforts. It also will help in the rapid turn-around usability for future re-usable launch systems.
The data from this test effort will be used by other Air Force development programs such as the Advanced Liquid Rocket Engine Stability Tools program (ALREST) to further advance the state-of-the-art capabilities in combustion stability modeling.
Previously, Aerojet Rocketdyne designed and supplied the oxygen-rich and fuel-rich pre-burners for the Air Force’s Integrated Powerhead Device (IPD) demonstration engine, the world’s first full-flow staged combustion rocket engine.
“The design lessons learned and test approach from the IPD pre-burners have been leveraged for the HBTD pre-burner architecture,” Aerojet Rocketdyne believes.