GRAHAM WARWICK / HUNTSVILLE

Interest in aircraft and weapons able to strike rapidly over long ranges is driving engine technology

The US military's growing need to reach out and strike back quickly could boost NASA efforts to develop new high-Mach propulsion systems. Potential applications include long-range strike aircraft, hypersonic weapons, high-speed transports and reusable launch vehicles (RLVs).

Last year, US high-speed research was brought together under the National Hypersonics Plan. After 11 September, hypersonics became part of the US government's new National Aerospace Initiative, which embraces high-speed aircraft, space access and in-space technologies.

"Hypersonics has moved up a notch," says Steve Cook, acting director of NASA's Advanced Space Transportation Programme, which is funding research into high-Mach propulsion systems. "There is enthusiasm at senior levels for hypersonics and there is more significant investment than at any time in the past 20 years."

NASA is spending $130 million a year on hypersonic research and this will be combined with US Department of Defense funding under the jointly managed National Aerospace Initiative. While NASA's research is aimed at an air-breathing RLV which could enter service around 2025, it could spin off propulsion systems for a Mach 4+ strike missile by 2012 and a M2 to M4 strike aircraft by 2020. A near-term product could be an engine for a supersonic business jet.

DoD involvement could change the direction of NASA's research. The space agency is pursuing both rocket-based combined cycle (RBCC) and turbine-based combined cycle (TBCC) propulsion systems, with the intention of selecting one concept for flight demonstration by 2010. The DoD favours the TBCC approach because of its near-term applicability to high-speed missiles and aircraft, says a NASA official.

RBCC work comes under the Integrated System Test of an Air-breathing Rocket (ISTAR) programme, led by NASA's Marshall Space Flight Center, while Glenn Research Center is leading TBCC work under the Revolutionary Turbine Accelerator (RTA) programme. "We have a race between RBCC and TBCC. It is too early to make a downselect," says Uwe Hueter, manager of NASA's hypersonics investment area.

The agency plans to make its selection after ground-testing both the RBCC and TBCC concepts in 2007. The winning engine would then be flight-tested in the X-43B, a scaled-up version of NASA's X-43A hypersonic experimental vehicle. The X-43B is scheduled to fly by 2010, but is unfunded. "We'd like to bring it forward by a couple of years and we're working on the funding," says Hueter.

Military interest

DoD funding could be the answer. Cook says the RTA is a candidate for early demonstration under the National Aerospace Initiative, reflecting the military's interest in the high-Mach turbine engine, which he calls the first breakthrough in high-speed propulsion since the Lockheed SR-71. "They are very interested in a global reach aircraft," he says.

While the M3-plus Blackbird's Pratt & Whitney J58 turbojets had a thrust-to-weight (T/W) ratio of four, NASA's goal is a T/W of 10-15 for a turbine engine capable of M4-5, says RTA programme manager Paul Bartolotta. The challenge will be in developing lightweight components that can operate at high temperatures with the required efficiency and longevity.

Air entering the inlet will reach 620°C (1,175°¡F) at M4 and 950°C at M5, with temperatures in the engine approaching 2,200°C - beyond the capability of today's alloys and requiring the development of improved materials, cooling strategies and coatings. Bearings, seals and lubrication systems will have to operate at temperatures of 750-800°C, compared with 200-250°C in today's turbine engines.

Thermal management will be an issue as high compressor temperatures will prohibit the use of bleed air for cooling. The ability to use the fuel as a heat sink will also be stretched to its limits. Weight will be critical, as studies show a 1kg reduction in rotating component weight will save 8kg in propulsion system weight and 160kg in vehicle weight, says Bartolotta. Inlet and nozzle will need to be highly integrated with the airframe for efficiency.

NASA is evaluating two RTA cycles. A turbofan-ramjet capable of M0 to M4.4 is the most efficient, says Bartolotta, while a turbojet-ramjet capable of M0 to M5 is best suited to the X-43B, where it would be integrated with a dual-mode ramjet-scramjet that could take the vehicle from M5 to M8. A competition is under way to select contractors for the ground and flight demonstrations, with a decision expected by July.

Mid-scale demonstrator

The agency cannot afford to develop a full-scale, 1.5m (5ft)-diameter, 55,000lb-thrust (245kN) engine, so it plans a mid-scale, 1m-diameter, ground-based demonstrator and a small-scale, 0.4m-diameter, flight-test engine. General Electric and Pratt & Whitney have submitted bids to build the mid-scale RTA, as has Rolls-Royce's US arm, which is also up against Williams International to build the smaller engine.

Critical component rig tests would lead to demonstration of a complete engine in 2007. Two further builds of the ground demonstrator engine are planned with the goal of a thrust-to-weight ratio of 10 by 2010, when NASA hopes to begin design work on a large-scale RLV demonstrator. This would fly by 2018 when the agency hopes to have matured RTA technology to a T/W of 15 and hot life of 750h, compared with around 100h for the J58.

After the ground tests, if TBCC is selected over RBCC for flight demonstration, the engine could fly by 2009 in the X-43B. This would be powered by four small-scale RTAs integrated with a stretched version of the US Air Force's HyTech dual-mode scramjet.

The hydrocarbon-fuelled, regeneratively cooled HyTech engine is to be flight-tested first in 2006 in the expendable X-43C. This is a slightly stretched version of the X-43A, which is powered by a NASA-developed hydrogen-fuelled dual-mode scramjet. Both the A and the C are designed to be air-launched from NASA's Boeing B-52 and rocket-boosted to hypersonic speed, but the reusable X-43B would be released from the B-52 to accelerate from M0.8 to M7.

The mission profile will be similar if NASA elects to power the X-43B with the ISTAR hydrocarbon-fuelled RBCC, which is under development by a consortium of Aerojet, Boeing Rocketdyne and Pratt & Whitney. But the DoD and some NASA officials are rooting for the RTA.

Whether or not NASA selects TBCC over RBCC for flight demonstration, there may be useful spin-offs from the RTA programme, says Bartolotta. The small-scale high-Mach compressor could form the basis of a powerplant for high-speed missiles, unmanned vehicles and a small business jet, while the mid-scale demonstrator could lead to an engine for a long-range strike aircraft or high-speed transport.

Source: Flight International