AFRL is gearing up to launch a follow-on programme to IHPTET, which has driven US engine R&D for the last 15 years
It is no easy feat to follow a programme as successful as the Integrated High Performance Turbine Engine Technologies (IHPTET) initiative, but AFRL is drawing up tough goals for its successor, the planned Versatile Affordable Advanced Turbine Engine (VAATE) programme.
As IHPTET winds down over the next five years, VAATE will become the focus for AFRL's Propulsion directorate (PR) and US engine manufacturers. Richard Hill, chief of PR's turbine engine division, says improving engine thrust-to-weight ratio while reducing fuel consumption, as well as cutting production and maintenance costs, will be goals for VAATE just as they were for IHPTET. But other key criteria have emerged, such as durability and sustaining the engine over a long service life.
For the past 15 years, IHPTET has been a highly effective focus for advancing turbine engine technology in the USA. A companion programme, the Integrated High-Payoff Rocket Propulsion Technology (IHPRPT) initiative, has as one of its goals improving reusable launch vehicle payload by 95% by the end of phase three around 2003.
PR's five divisions also drive research and development of advanced propulsion systems such as pulse detonation engines (PDEs) and scramjets, as well as power cells, fuels and lubricants. PDEs can use liquid or solid fuel and can operate from subsonic speeds to Mach 4, says PR deputy director Fred Oliver. They promise to be affordable, costing around $25/lb-thrust compared to $100/lb for turbines, making them attractive as expendable powerplants for cruise missiles. Combined propulsion systems, either rocket/scramjet or turbojet/scramjet, are also being considered, Oliver says.
PR has been involved in "more electric aircraft" studies, says Oliver. AFRL believes it will be possible to replace an engine's mechanical systems with simpler electrical equivalents and a single mechanical system - the integrated power unit, which combines an auxiliary power unit and generator, providing both shaft and electrical power. Improvements in magnetic materials and generator design have led to the internal starter-generator, which acts as a motor to spin the engine shaft up at start but becomes a generator when the engine is running.
Another development is JP8+100 jet fuel, which has additives which improve its capacity as a heat sink. This new fuel has a 38íC (100íF) thermal stability, which PR intends to increase to over 100íC and, eventually, to 480íC. JP8+100 also cleans the engine.
For rocket motors, PR is developing high energy density matter (HEDM) with improved specific impulse. If HEDM provides a 10% specific impulse improvement, Oliver says, it would double the payload of a single-stage-to-orbit launch vehicle. PR's Space Propulsion division at Edwards AFB, California, has also experimented with launching light - 30-50kg - satellites using ground-based laser power. The vehicle sits on the laser, which heats the air underneath, causing it to rise.
The major focus for PR over coming years will be VAATE. While IHPTET has placed a heavy emphasis on improving turbine engine performance, affordability will be the prime driver for the new programme. For the first time, development costs will also be considered. But affordability must be viewed as the ratio of capability to cost, Hill says, adding: "Cheap doesn't win battles - capability does."
GOTCHA
VAATE follows a methodology known as "GOTCHA" - goals, objectives, technical challenges and approaches - to determine which emerging technologies hold the most promise. Split into three elements of durability, intelligent engine and versatile core, VAATE poses issues for current and new engines, says Hill. Durability affects both readiness - a weapon system cannot be unavailable because of engine problems - and maintenance costs.
AFRL is striving to understand more completely failure modes, how engines are operated and how to best use data from health and usage monitoring systems. A major consideration is high-cycle fatigue, still the dominant failure mode in turbine engines, says Hill.
PR is reviewing the little-understood physics of high rotational speed and vibration coupled with hot environments, as engines have become less forgiving. The introduction of thinner aerofoil leading edges and tighter tip clearances, for instance, have improved engine efficiency but the blades are more susceptible to damage and wear, reducing engine performance and increasing support costs.
Linked with durability is VAATE's intelligent engine concept, which will use the control system to monitor performance and degradation and make self-adjustments to provide optimum performance without damaging the engine. Emerging technologies such as the internal starter-generator, magnetic bearings and fluidic nozzles - a fixed device with the throat area determined by the fluid flow - will be considered, says Hill. The intelligent engine should be able to tailor its performance using active inlet, compressor and nozzle controls; intelligent flow control to provide cleaner, distortion-free airflow; and automatic vibration control for rotors and blades. These techniques could be controlled by microelectromechanical systems (MEMS) and wireless communications, suggests Tim Lewis, performance analysis group leader, PR turbine engine division.
The versatile core element of VAATE acknowledges that military engine programmes could become too small to be financially viable, says Hill. A versatile core would allow military propulsion systems to be aligned with the higher production rates of commercial engines. Two cores would be developed: a smaller unit for expendable engines, turbofans up to 15,000lb-thrust (67kN) and turboshafts: and a larger versatile core applicable to large military turbofan/turbojets and high bypass ratio engines.
Hill points out two pervasive areas: one is unmanned air vehicles and the second is HITs, or high-impact technologies. UAVs are generally accepted to have a bright future, but questions remain, such as: should foreign object damage, birdstrike and icing requirements be similar to man-rated powerplants, and should they have 100h or 4,000h lives?
The second area, HITs, are revolutionary R&D programmes that come to fruition during VAATE. HITs include pulse detonation engines, acoustic compressors and inner turbine burning. Acoustic compressors use sound/vibration to compress the air and, Lewis says, "we could see benefits for extremely high pressure ratios" of around 60.
Acoustic fatigue
Aircraft improvements may also affect the engine's life and support costs. The Lockheed Martin/Boeing F-22 will supercruise, or sustain supersonic speeds without afterburner. The extra forces resulting from the sustained higher speed may become an issue, AFRL believes. Higher thrust engines have higher jet loads that can cause acoustic fatigue, while running at higher temperatures means that more basic research into creep may be needed.
A VAATE demonstration programme will have two phases with defined goals (see tables) ending in 2010 and 2017. Hill says there has been a concerted effort to consider all participants from the outset so that the VAATE plan is right "from day one". Six US engine manufacturers are involved: Allison Advanced Development (part of Rolls-Royce), General Electric, Honeywell, Pratt & Whitney, Teledyne Continental and Williams International. All principal US airframe manufacturers are also involved. The continuing drive towards low-observable platforms and integrated systems means the propulsion system is more closely linked to the airframe, and that all players must be involved, says Hill.
R&D programmes today, Hill adds, will affect USAF well into the middle of this century. "We're looking at what to do today to give the 'next force' and the 'force after next' their capabilities at an affordable price," he says. PR is also talking with the US Department of Energy and NASA to establish common technologies for power generation and marine turbines as well as aircraft propulsion.
"There's lots of room for collaboration", he says, adding that the GOTCHA process is being applied to each organisation to determine where the fertile technologies lie, so it is possible to "leverage each organisation's funding to everyone's benefit". Nine areas have been identified: structures; materials; compressors; combustors; turbines; mechanical components - seals, bearings and drives, for instance - design tools, including modelling and simulation; the validation of testing methods; and instrumentation controls and health monitoring.
Some work will be joint, other areas will be jointly funded while data will be shared for other technologies. A fifth and final meeting is scheduled for early December to finalise the collaboration. VAATE's goals are "significant and challenging", Hill says, and the 2010 and 2017 end points are based on the pace of funding. If the funding is reduced, AFRL will slip the end dates - but not reduce the goals.
Source: Flight International
