Supersonic Turbine Stepping To Hypersonics
Aviation Week & Space Technology Sep 18, 2015 , p. 46
Guy Norris, Graham Warwick Los Angeles and Washington
High Mach
Graham Warwick/AW&ST
Developers hope to ground test a turbine engine at Mach 3.2 in the coming months, paving the way for long-range supersonic cruise missiles as well as potentially laying the foundation for a viable combined-cycle hypersonic propulsion system.
Testing of high-speed engines is being conducted separately by Rolls-Royce Liberty Works and Williams International under the U.S. Air Force Research Laboratory’s (AFRL) supersonic turbine engine for long-range (Stelr) program. A follow-on effort to the joint AFRL and Darpa (Defense Advanced Research Projects Agency) high-speed turbine engine demonstration (Histed) program, Stelr is targeted at the development of Mach 3-plus weapons and vehicles. These include long-range standoff missiles, air-launched cruise missiles, unmanned air vehicles and advanced cruise missiles capable of sustaining flight at maximum Mach number for 1 hr.
NASA’s combined cycle test later this year will focus on turbine engine compatibility and begin to characterize the inlet at lower Mach numbers. Tests in 2016 will focus on mode transition schedules and operation at lower Mach numbers. Credit: NASA
Rolls-Royce’s Stelr engine has already operated for “more than two hours at Mach 2-2.5, and will run up to Mach 3.2 in the next few months,” says John Kusnierek, director of business development and strategy for the company’s Liberty Works unit. Although Rolls has applied lessons learned on its Histed engine, the YJ102R, to the Stelr project, Kusnierek emphasizes the newest development “is not the same engine.”
Speaking to Aviation Week at the Air Force Association convention in Washington, Kusnierek explained that although the Stelr engine is designed for a lower Mach number than the YJ102R, it has longer endurance. The engine has been developed to “run at Mach 2-3.2 continuously.” The design mission is to operate for 1 hr. at speeds up to Mach 3.2, or sufficient to provide a range of more than 2,000 mi. The same system would also, therefore, have the ability to fly 1,000 mi. in 30 min., which is a “capability of interest,” he adds.
Displayed in mockup form at AFA, the engine is similar in size to the YJ102R, which was earmarked for the canceled Lockheed Martin revolutionary approach to time-critical long-range strike (Rattlrs) missile flight demonstrator. Just like YJ102R, the Stelr is nonafterburning, providing longer range at supersonic speed. “At Mach 3.2 the inlet air temperature is 800F, so there is a lot of materials technology in the engine,” says Kusnierek.
Although designed for expendable weapons, the engine’s baseline durability could make it useful for wider, reusable roles. “The need to have enough life to qualify the engine means it can be reusable, so it could probably do 50 missions in an ISR [intelligence, surveillance and reconnaissance] role,” he adds.
Rolls-Royce aims to test the Stelr, seen at AFA in mock-up form, to Mach 3.2 in coming months. Credit: Graham Warwick/AW&ST
Beyond supersonic missiles, Stelr also offers new hope for developers of hypersonic engines. Stelr technology could provide the missing piece of the puzzle for engine-makers looking to close the gap between turbines and high-speed dual-mode supersonic combustion ramjets/scramjets for hypersonic flight. “It fills the niche between subsonic and hypersonic propulsion. There is no companion vehicle program. It is a chicken and egg situation, no one can design a missile until there is an engine, so first we have to demonstrate the engine,” Kusnierek says.
Stelr is one of three active Air Force and NASA high-speed propulsion efforts underway to support development of reusable turbine-based combined-
cycle (TBCC) engines. In these propulsion systems a turbine engine would provide the power from takeoff to Mach 4, with a ramjet/scramjet taking over for higher-speed flight. Speaking at the AIAA International Space Planes and Hypersonics Systems and Technologies Conference in July, Darpa deputy director Steven Walker says Stelr “would enable state-of-the-art ramjet takeover.” The program has “identified four turbine engine options and completed vehicle synthesis for each,” he adds.
Companion efforts include AFRL’s medium-scale critical components (MSCC) program, which is exploring first generation, larger-scale scramjet engine characteristics beyond those of the pioneering X-51A hypersonic demonstrator, which last flew in 2013. Designed to evaluate engines with 10 times the airflow rate, performance, operability and thermal management capabilities of the X-51, the MSCC “will test takeover, acceleration and cruise conditions,” says Walker. The Air Force’s Aerodynamic and Propulsion Test Unit at the Arnold Engineering Development Center in Tennessee, has been modified to conduct the first direct connect tests of these larger scramjet engines, and calibration testing began in July. Combustor testing will begin at the site in February 2016.
NASA, supported by funding from AFRL and Darpa, has meanwhile been testing a large-model TBCC under the combined-cycle large-scale inlet mode transition (CCE-LIMX) program. Conducted in the 10 X 10-ft. wind tunnel at NASA Glenn Research Center in Ohio, the test unit consists of a high-Mach turbine simulator or engine paired with a scramjet simulator. A modified Williams WJ38-15 turbojet, similar in size to the company’s XTE88 Histed engine, was made available for the tests, though it was limited to Mach 3. Flow to the engines, depending on the operating speed and mode, is controlled via a set of low- and high-speed ramps and flowpaths.
The initial phases of the program focused on inlet performance and stability at Mach 4, which took up 95% of the early testing. Mode transition schedules were developed during tests in 2011-12, and a Mach 3 bleed configuration was created to help solve a high steady state distortion that was discovered at Mach 3. The goal of the latest phase was to focus on smooth and stable mode transition at Mach 3 and test a closed-loop inlet control system in the process. Walker says the program completed system identification of inlet dynamics for development of controls algorithms and “successfully demonstrated a fully autonomous mode transition with no unstarts.” This latest phase of testing was completed in May.
Stelr is also one of the propulsion options included in a NASA-funded Lockheed Martin study in support of the proposed SR-72 hypersonic, ISR strike aircraft. The study has been looking into the viability of a TBCC propulsion system with several combinations of “near-term turbine engine solutions” and a very-low-Mach ignition dual mode ramjet. Unlike the Mach 4 takeover range of most ramjets conceived to date, this study, together with another similar contract recently awarded by NASA to Aerojet Rocketdyne, is evaluating take-over velocity to be reduced to Mach 2.5 and below.
www.airforce-technology.com
I thought that the first flight of SR-72 will be 2018. ;D
