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Artist's view enhances the peculiar trail of the pulse engine.

The ungainly configuration of the 'Borealis' is always certain to attract curious visitors at air meets.

The four exhaust pipes extend way behind the airframe.

Customer: USAF's Air Force Research Laboratory (AFRL) / Air Vehicles Directorate
Main contractor: Innovative Scientific Solutions, Inc. (ISSI)
Subcontractor: Scaled Composites

Type:  pulse-detonation engine test-bed

Program:  Borealis (pulse detonation powered aircraft)

Powerplant: 1 x modified 4-cylinder General Motors automobile engine

First flight: (as a Long-EZ) 1991, (as Borealis) April 2004

"Today's commercial aircraft engine will be replaced by Nazi-era technology in the next decade. We'll be flying from L.A. to China in six hours. The jet engine that powers an aircraft is a pretty distant cousin to the engine that powers a car. Someday these two internal combustion engines may become far more alike." Dramatic though these assertions may sound, it is true that engineers are tinkering with jets that burn fuel in a sequence of miniature explosions much like the explosions that move a car engine's pistons. Today's jet engines suck in air, compress it, combine it with fuel and then ignite the fuel. The air pushes out the back, providing thrust and also spinning the turbine blades that power the compressor. The pulse detonation engine dispenses with the compressor and the turbine, thus potentially saving on weight and maintenance. Existing jets can power a commercial aircraft at up to 500mph of airspeed at an altitude of 30,000 feet; the pulse engine should be able to go at four times the speed and as high as 40,000 feet.

Pulse-detonation engines, by contrast, are essentially tubes in which fuel and air are admitted into one end and then detonated, creating a high-pressure wave that travels at supersonic speeds down the length of the tube, causing burning rates of thousands of meters per second and a pressure rise of 30-100 atmospheres. As exhaust gases are expelled at the far end of the tube, the process repeats. In comparison, pulsejets used by Germany during World War II did not use a detonation process and peak pressure rises were only about 2-3 atmospheres. PDEs offer the potential to propel missiles and manned aircraft at speeds up to about Mach 4. Commercial gas turbines also could benefit from pulse detonation technology; if substituted for the core of a conventional gas turbine engine, the resulting hybrid powerplant could show a fuel efficiency improvement of 10% or more.

Pulse propulsion technology has been around since the 1930s, when Nazi engineers developed it to power the infamous V-1 missiles that blitzed England in World War II. U.S. military engineers built knockoffs from captured parts, but the design received little attention outside the Air Force, where it was used to develop high-speed cruise missiles. In the 1980s Boeing engineer Thomas Bussing decided to give pulse detonation another spin. "I was really fascinated with the V-1 technology,"he says. "I wanted to make a viable engine out of the idea." Bussing, who has a doctorate in aerospace engineering from MIT, started work on the concept in his Seattle garage. In 1992 he left Boeing to launch an aerospace research arm of the privately held military contractor Adroit Systems. He raised $24 million in private capital and government grants and built his first working pulse detonation engine in 1995.

Pratt & Whitney bought Bussing's business in 2001, invested another $16 million and began working out possible engine configurations. GE quickly followed P&W's lead. Both now have several working prototypes. Pulse engines use the same materials as conventional jet engines and can be fitted onto the current aircraft fleets. But if they are to reach speeds beyond the sound barrier (761mph), Boeing and Airbus will have to redesign passenger planes completely. In the meantime the new engines will likely be used on cruise missiles and as afterburners to provide extra thrust for conventional supersonic fighter jets. Burt Rutan planned to fly his Long-EZ plane fitted with a pulse detonation engine built by the Air Force using elements of Bussing's design. Says GE's Correa: "People need to go to Bangalore or Shanghai. There's this unmet need building up, much like when people first needed to cross the oceans and continents on a routine basis. If you could deliver five times the speed of sound at a reasonable price and without a disastrous effect on the environment, people would like to do this.

The Air Force has been examining PDEs, in an in-house program, since the late 1990s, the Navy even longer, in the form of the Borealis program. This uses a standard Long-EZ kitplane which was manufactured by a Timothy R. Binder in 1991 and purchased by Scaled Composites for the PDE program. The flight tests were a joint effort between the Air Force Research Laboratory's (AFRL) Combustion Science Branch, Innovative Scientific Solutions Inc. (ISSI), the AFRL Air Vehicles Directorate and Scaled Composites. The engine used in the tests was a modified four-cylinder General Motors automobile engine that ran on standard aviation gas. Four PD tubes replaced the cylinders and two sets of cylinder heads were installed so that there were eight valves per tube. "It's basically an off-the-shelf engine," said Fred Schauer, a research engineer within the Laboratory's Propulsion Directorate. The flight test engine was similar to pulse detonation powerplants the Combustion Sciences Branch had been researching for several years.

Three engines were fabricated for the tests. The first was run under static conditions on a teststand, while the second was installed in a LongEZ for structural and acoustic tests. The third engine was the flight test article. Installed, it is capable of propelling the LongEZ at about the same cruise speeds as the 108 hp. Lycoming engine it replaces, Schauer said. "When the engine detonates, it provides 1,200 lb. at peak thrust. Of course, the average thrust is lower and also depends on ambient temperatures and pressures," he explained. Regardless, researchers classify the flight test PDE as "basically functionally equivalent" to the powerplant it replaces. That's why, during the flight trials, the aircraft stayed in the middle of its flight envelope, reaching a speed of about 150 kt.

As part of a joint Propulsion Directorate/Air Vehicles Directorate program to evaluate the feasibility of using pulsed detonation engine (PDE) propulsion with manned aircraft, a complete engine was shipped for assembly to Scaled Composites in Mojave, California, for vehicle integration, following successful testing of a prototype flight-worthy pulsed detonation engine. It is believed that the test demonstrated the first self-contained PDE in operation. The prototype engine consisted of a PDE assembly and pod which contained everything required to make a self-contained propulsion system. This included an auxiliary power unit, oil system, fuel pumps and fuel injection system, alternator, battery, throttles, and control computers, as well as superchargers to enable static starts and nonself aspirated operation. The complete engine wass constructed from off-the-shelf components and was designed to meet FAA durability requirements for experimental propulsion systems.

The shipped engine assembly consisted of a PDE and pod integrated together to create the second of three flight engines. This engine was integrated with an experimental Long-EZ airplane using a mount designed by Scaled Composites. Following fabrication of the engine mount, a duplicate mount was shipped back to Wright-Patterson AFB for installation on a ground test Long-EZ using flight engine number three. Engine number one, which had additional test instrumentation, took part in outdoor acoustic testing and was used as a spare. Although the joint program between was studying integration issues with a manned subsonic airframe, pulsed detonation technologies were expected to have performance benefits in the Mach 0-4+ regime and for hybrid/combined-cycle applications. The program addressed structural, acoustic, and durability concerns while maturing this potentially revolutionary propulsion technology.

Engine noise is a significant issue with PDEs and the flight tests provided significant data in this area. "You have a Mach 5 shock wave coming out of each tube 20 times a second," Schauer said. Noise around the aircraft is about 150 dB., equivalent to the noise generated by a conventional gas turbine using its afterburner. Noise in the cockpit is about 130 dB., so hearing protection is required by the pilot. Overall, USAF research has been directed at examining detonation, instrumentation and measurement techniques as well as developing the methods used to study PDEs. "All this has been shared with industry," Schauer said. USAF work also has been focused on exploring the potential operating envelope of PDEs. "We've been successful in stationary detonations; we've also been doing some collaborative tunnel testing with NASA to see if we can do detonations under supersonic conditions," he explained. Nozzle work also is a priority item for researchers. "You can't efficiently use conventional nozzle designs because they're for steady-state systems," Schauer said. PDEs are currently anything but that. In a PDE, "you go from an under-expansion to an over-expansion many times a second," Schauer noted.

The experimental pulse-detonation-powered aircraft began test flights in April 2004 at Mojave, California, after the FAA had issued it an airworthiness certificate. Powered by a PDE developed by the Air Force Research Laboratory's Combustion Science Branch, the aircraft, a Rutan LongEZ, was flown by pilots from Burt Rutan's Scaled Composites. The aircraft made a series of short flights, each time flying a few circles then landing, to show that the PDE concept was viable. The purpose of the tests were three-fold: to propel an aircraft using a PDE, to investigate the acoustical and vibrational effects of a PDE on an aircraft and pilot, and to help demonstrate the potential of PDEs. The tests were not part of an engine technology demonstration; the flight test powerplant was merely a research tool.

It is rather amusing to notice that the program is called 'Borealis' by the Air Force, as in "aurora borealis". Of course, "Aurora" is a highly-secret and still unacknowledged program covering various so-called "Black" projects, prominently centered around pulse-detonation technology and the alleged XR-7 "Gaspipe" vehicle. An amusing way, perhaps, to signify that Borealis is kind of Aurora's alter ego in the "white" world.

Population: 1 [03-001] [also registered with Scaled as N90EZ]

Specs: unknown

Crew/passengers: 1 (originally 2)

Early flight tests were conducted by Dick Rutan himself.