GUY NORRIS / CHINA LAKE
Deep in the Californian desert, engineers are working on a project that could revolutionise the next generation of supersonic and subsonic craft
Several seconds after the siren's warning wail dies away, an unusual, but powerful, sound shatters the silence of the desert. Resembling the noise from a giant buzz-saw, the staccato burst lasts less than a second but reverberates around the tiny block house at China Lake Propulsion Laboratory's T-Range test site in California, where Pratt & Whitney engineers are controlling the firing.
The source of the disturbance, almost hidden by a confusion of piping and wiring, is widely considered to be the most advanced pulse detonation engine (PDE) in the world. Hidden away in the vast area of California's US Naval Air Weapons Center, a few kilometres from where Second World War scientists worked in the "Secret City" on the critical explosives technology for the Manhattan Project, the work on ITR-2 (integrated test rig) could be paving the way for the next step-change in propulsion.
The PDE offers the chance of a revolutionary leap for a huge range of potential developments, ranging from supersonic missiles and unmanned combat air vehicles (UCAV) to single-stage-to-orbit space vehicles and even conventional subsonic transports powered by hybrid PDE/turbofans. However, proponents of PDEs have suffered from a chronic lack of data to substantiate any of the predicted results from what should be a highly efficient and simple system. Flight International was invited to an ITR-2 test to see a PDE in action, witness the dramatic progress made so far and assess the effort going into proving the concept.
ITR-2 is the culmination of a step-by-step build-up by a wide group of companies, organisations and US government agencies dating back more than a decade. They include P&W, the P&W Seattle Aerosciences Center (formerly PDE pioneer Adroit Systems - ASI), Boeing Phantom Works, US Naval Air Systems Command, the Office of Naval Research (ONR) and United Technologies Research Center (UTRC). Each of them began work on individual PDE concepts in the early 1990s, and they teamed in 1999 on the PDE risk-reduction programme for ONR. In 2000, P&W signalled its long-term intent to follow PDE research when it acquired ASI.
From the start, ASI and later P&W treated the PDE concept almost as a living organism in which components were coupled strongly, and the slightest perturbation would have serious knock-on effects for the shock/detonation-wave interaction. It therefore began a slow build-up in which each component was developed to a level of maturity before being integrated into the core flow.
Component development
"Each step is critical," says P&W Seattle Aerosciences Center division manager Tom Bussing. "It required us to demonstrate the appropriate level of performance and operability at each stage. So far we've worked out the performance of the rotary valve, initiator, combustion tubes and now the nozzle. We're working from left to right through the flow path."
The roots of today's ITR-2 work go back to 1992 when ASI began studies, under a US Air Force small business initiative research (SBIR) grant, that looked at the core of the PDE concept, the deflagration to detonation transition (DDT) process, in vertically mounted shock tubes. In 1994 it followed this up with work on a single combustor device in Redmond, Washington, under a NASA Langley Research Center SBIR with the goal of demonstrating the use of hydrogen as a fuel for a multiple-cycle mode PDE operating at 2Hz.
Success in both efforts prompted development of the first rotary-valved, single-combustor PDE under another NASA SBIR. The rotary valve was a key breakthrough in metering steady inlet air into the tube and separating the inlet from the unsteady combustion process. The test demonstrated multiple-cycle operations at 6Hz for 10s duration.
Increasing interest from the US Air Force Research Laboratory (AFRL) then sparked a further SBIR covering development of the first rotary-valved, multiple-combustor PDE in 1996 which went on to demonstrate an overall cycle rate of 22Hz. The success of this concept then led to a further AFRL contract to demonstrate the PDE in a flight-scale, near-flight frequency with targets of 30s running time and a cycle time of 40Hz per combustor. The targets were met over 1997-9 during tests at the Naval Postgraduate School with four 12.7mm (0.5in) inlet diameter water-cooled tubes burning ethylene/air fuel.
The stage was now set for the formation of the Boeing, P&W and UTC team to tackle the first multi-year ONR PDE risk-reduction programme, and the development of a five-tube ITR-1 at China Lake. This tested a higher-performance air valve, seals, fuel injection, the initiator, a thermal management system and the nozzle. The six-month programme included tests with ethylene, ethane and propane fuels, as well as a liquid fuel (JP-10) that would prove more supportable for military use.
ONR interest was prompted by the potential of PDE as a power source for a future family of anti-ship and ground-attack missiles variously known under several programme titles, including High Speed Strike Missile, HyStrike, HyFly and Fast Hawk. Although most have fallen by the wayside, a potential US Navy missile still represents the nearest-term application opportunity for a PDE, as indeed do other similar efforts under way in France and Japan, among other countries.
Missile capabilities
In the case of the US Navy, initial operational capability (IOC) remains the 2010 target originally set out in the HyStrike timetable. The new missile would be capable of Mach 2.5 to M4, altitudes of up to 40,000ft (12,200m) and a range of 1,300-1,500km (700-800nm), depending on the chosen cruise speed and altitude profile. The PDE-powered missile would be able to accelerate and decelerate like a gas-turbine powered vehicle, and would be a high-speed replacement for weapons such as Harpoon and SLAM-ER.
An IOC of 2010 would require flight tests of a PDE around 2006 and forms the mid-term focus for P&W which came close to flying a scaled PDE on NASA Dryden Flight Research Center's F-15B propulsion flight-test fixture with NASA Glenn under the cancelled HyStrike hypersonic programme. For the moment, the team is concentrating on completing the current ITR-2 test phase, and preparing for a probable follow-on navy programme starting in 2004. Tests of ITR-2 are focused on engine operation and performance demonstration. They include a direct connection to an air supply that simulated M2.5 inlet pressures and temperatures.
"We are now testing at M2.5 with heated flow with the existing individual nozzle system," says P&W Seattle Aerosciences Center propulsion programmes manager Gary Lidstone. After a break, tests will restart in September with a focus on the back-pressure effects of a compound nozzle. "The primary advantage of the nozzle will be control of the back pressure of all the tubes with one system," says Lidstone. As detonations are pressure-sensitive, the effectiveness of the back-pressure system is vital. As pressure decreases, so does fluid density and the energy required to initiate a detonation increases. Also, detonation is related to pressure ratio, so the higher the pre-detonation pressure, the higher the post-detonation pressure, and therefore the thrust.
Predicted performance
The key is to back-pressure the detonation chamber as close to inlet pressure as possible, avoiding "blowdown" to ambient pressures. Overall test results to date are on track, says Bussing. "Performance is very close to what our analysis predicted." Key parameters include measurements of thrust, fuel flow, pressures inside the tubes, and temperatures in the tubes and exhaust nozzles, he says.
Initial tests of operability involved perfecting the timing of fuel injection, ignition sparks and scheduling of the bow tie-like rotary valve, which spins at 1,800RPM to match the 60Hz operating frequency of each of the five tubes. The fuel mixture of ethylene, oxygen and compressed air is ignited by a Formula 1 racing car spark plug, with each tube firing every 16.6ms. Divided by five, this results in an overall engine firing rate of 3.5ms. Initial thrust levels with the unheated "cold" flow were close to 600lb (2.7kN) in each 0.7s-long firing test, although higher thrust levels are thought to be within reach with the revised nozzles. Earlier tests with a compound nozzle show that they work, says Bussing. "The question is, can we get the performance we're looking for?"
While tests continue, ITR-2 is being visited by various interested parties looking for new propulsion sources. "We have had several integrators out to the test site and we are working with them now," says Lidstone. Applications range from UCAVs and other unmanned systems to hybrid PDE/turbofan studies, he adds.
P&W is confident of meeting such a wide variety of requirements through the extended experience gained via its partnership with Boeing and UTRC, as well as early pulse-detonation rocket engine development work for the AFRL and NASA Marshall Space Flight Center.
Boeing's PDE work dates back to 1992 and first focused on a technical feasibility study using a boilerplate test rig installed at the Boeing High Pressure Test Cell. Later work included simulated altitude testing with NASA Glenn at the agency's Engine Component Research Facility and, in 2001, the adaptation of the entire rig into Glenn's Aero-Acoustic Propulsion Laboratory to test acoustic energy levels under varying operating frequencies, chamber pressures and fuel-to-air ratios.
Boeing and P&W have also combined efforts to develop a large-scale test rig at West Palm Beach, Florida. Here, tests have been conducted on a notional 0.2 x 0.8m ogive-shaped inlet for a supersonic UCAV. The rig is "the largest PDE ever successfully operated", says P&W, and has provided data showing that an "aggressively shaped" high-speed UCAV with a PDE would have up to 12% less supersonic drag and more than 14% lower transonic drag than a conventionally powered vehicle.
Apart from potential missile and UCAV applications, P&W's $20 million-plus 10-year PDE experience is also aimed at several advanced hybrid turbofan developments. P&W is managing a small NASA hybrid study as part of the RASER programme and the USAF is considering the concept under the Versatile Affordable Advanced Technology Engine programme. P&W itself is "cautiously optimistic" that PDE technology could play an important role in many of its future products. These range from hybrid commercial and military engines in which the PDE replaces the conventional combustor and core, to augmented military turbofans with a PDE-based afterburner.
Fuel-saving advantage
The hybrid is estimated to have a fuel consumption advantage of up to 10% over traditional engines, says P&W, which points out that much work remains to mature the concept to a reasonably high technology readiness level (TRL). "The state-of-the-art of the PDE is relatively low, mostly 3-4 TRL, with some subsystem levels higher at around six," says Bussing. TRL 6 is normally considered the right level for the prototype stage.
P&W's work reflects the increasing tempo of other PDE activities around the world, including plans by arch rival General Electric, which is preparing to run a PDE on its experimental Global Research Center Laboratories in Niskayuna, New York. Configuration studies are due to be completed this year, and first runs could be made in 2005-6.
In France, EADS is studying the concept for low-cost tactical missiles and even for space launchers, while Japanese and Russian groups are evaluating possible missile and high-speed target applications. P&W, meanwhile, hopes its pioneering work will give it a head start to be the first major manufacturer to truly get "fired up" about PDEs
Source: Flight International
