Researchers at Rensselaer Polytechnic have demonstrated an "air spike" concept that could reduce the drag and heat transfer problems associated with hypersonic flight.
The concept was formulated by Leik Myrabo, an associate professor of mechanical engineering at Rensselaer and Yuri Raizer of the Moscow-based Russian Academy of Science's Institute for Problems in Mechanics. It was apparently demonstrated late last month in tests at Rensselaer's Mach 25 shock tunnel.
According to Myrabo, the concept would offer designers the capability to actively control the external aerodynamics and thermodynamics of an advanced transatmospheric vehicle by substituting directed energy for mass-typically in the form of a sharp nosed structure. With this capability, traditional hypersonic design rules would change, and ultralight, blunt-bodied, lens-shaped or saucer-shaped single-stage-to-orbit vehicles could emerge.
Traditional sharp-nosed hypersonic vehicles generate a conical bow shock wave that causes massive heating at the tip of the craft's nose. The air spike concept uses concentrated energy projected forward off a moving vehicle to drive air radially from the path of the craft and to transform the traditional conical bow shock into a weaker, parabolic-shaped oblique shock-one tilted strongly aft with respect to a hypersonic vehicle.
Using the air spike, a pocket of low density, low-pressure, hot air in the shape of a paraboloid of revolution is formed in front of the vehicle, reducing the drag and heat transfer effects normally encountered by a hypersonic craft. Estimates indicate that an air spike-equipped vehicle traveling at Mach 25 (orbital velocity) with respect to the exterior of the oblique shock wave would actually be subjected to Mach 3 conditions within the pocket formed behind the wave.
Another benefit of the directed energy air spike is that it can be used to help compress air for vehicle propulsion, particularly if the vehicle has a lens or saucer shape. According to Myrabo, the oblique shock generated by the air spike can be controlled to continuously pass the rim of the craft at a distance equivalent to one-tenth of the radius of the vehicle. As the craft moves forward, air inside the pocket gets compressed between the oblique shock and the rim of the vehicle. Although localized heating at the vehicle rim is severe, "there are ways to mitigate it by manipulating the geometry [of the structure], and we plan to examine them," he said.
According to Myrabo, an air spike formed by directed energy also has several advantages over a structural spike, the most important being the type of shock that is produced. "The air spike effect is best modeled as a cylindrical blast wave that expands into a parabola given the forward flight of a vehicle. Because it is a blast wave, a very low density air pocket forms behind it, and that in turn reduces heat transfer effects," he said.
In contrast, a structural spike generates a conical shock wave, and the air behind it is significantly denser than that found behind a blast wave. As a result, the drag and heat transfer effects associated with the air spike "are not replicated," Myrabo said.
In the recent laboratory tests a 6-in.-dia., blunt-bodied aluminum model similar in shape to an Apollo command module heat shield was tested in the 2-ft.-dia. test section of Rensselaer's Mach 25-class, 60 ft.-long shock tunnel. The energy to create the oblique shock waves associated with the air spike was provided by an electric arc plasma torch placed on a sting extending about 5.5 in. in front of the test model.
Calculations made prior to the experiments indicated that a conical shockwave impinging on the heat shield model would be generated by the plasma torch and its sting under Mach 10 free stream velocity conditions-if the torch were not operating .
With the tool operating at 34 kW., however, an oblique shock was formed under the same Mach 10 conditions. The calculations and oblique shock were confirmed in Schlieren images.
According to Myrabo, approximately 35 calibration and test runs were made in the shock tunnel. About 10 were dedicated to demonstrating the air spike concept "and they did," he said. Since the tests were conducted on a shoestring budget assembled by Myrabo, the models were not instrumented, and specific temperatures and pressures behind the oblique wave were not measured. "We were attempting only to determine the validity of the concept," Myrabo said.
The next step in his proposed test program is to conduct similar tests at speeds up to Mach 25. Follow-on tests with instrumented models would be next, depending on funding, Myrabo said.
Myrabo's air spike concept follows a wide range of work conducted in the late 1950s and in the 1960s. However, much of that work used chemical rocket exhausts, water and other "mass-intensive" on-board systems to manipulate shockwaves in front of a hypersonic vehicle. Myrabo and Russian researchers originally proposed air spikes generated by lasers; they now propose using microwaves.
Myrabo has been working on the "air spike" concept since 1993, and the conceptual work with Raizer was supported by the Space Studies Institute (SSI) near Princeton, N.J., to examine vehicles capable of driving earth-to-orbit transportation costs down by a factor of 100 to 1,000 in the next century. One important assumption made by the SSI study, however, was that an adequate space power infrastructure would exist. That includes orbiting satellites capable of transforming solar energy into microwaves that can be transmitted to Earth.
Using his air spike, Myrabo's SSI study proposes that a single-passenger, 10-meter-dia., double-hulled, single-stage-to-orbit craft fabricated from silicon carbide materials is possible in the next century. Helium, pressurized to two atm., would circulate in the 1-cm. interspace between the 0.125-mm.-thick double hulls to cool the lens or saucer-shaped vehicle.
Myrabo also has been associated with the former Strategic Defense Initiative Office. Work performed for the SDIO, USAF and NASA centered on pulse detonation wave engines powered by groundbased lasers. In tests at the Naval Research Laboratory about three years ago, the Pharos 3 laser was able to create enough overpressure above a flat plate to generate a pulse equivalent to about 180 Newtons per megawatt-about as efficient "as early jet engine," Myrabo said.
Similar subsequent tests in the U.S. reached pulse levels as high as 250 Newtons per megawatt, while the Russians claim to have reached levels as high as 500 Newtons per megawatt. Propulsion for Myrabo's lens or saucer shaped air spike vehicle at speeds up to Mach 1 is provided by a pulse detonation wave engine similar to that studied in the SDIO's laser propulsion program. However, the power used to accelerate and "explode" or expand the highly compressed air at the rim of Myrabo's craft would be provided by an off-board microwave system, not lasers. Pressures of 25 to 35 atm. should be achievable, he said.
For speeds above Mach 1, the vehicle would rely on a magnetohydrodynamic fan engine. The lens-shaped craft would have an interior rectifying antenna to absorb pulsed, focused, microwave power on the outside of the vehicle to ionize air forced to the rim of the craft by the air spike. The rectennas also would pulse electric power through the ionized air and, in conjunction with two superconducting magnets ringing the craft, accelerate the air aft past the vehicle. According to Myrabo, this drive system also tends to eliminate sonic booms by eliminating pressure discontinuities, so the vehicle is silent but very bright in hypersonic operation.
Citing calculations made by Brice Cassenti, a senior principal engineer at the United Technologies Research Center, Myrabo estimates that the gas between the vehicle's twin hulls, protected by the air spike, would rise in temperature only 25K during a flight to orbital velocity. Ultimately vehicles of this type could reach speeds as high as Mach 50.
Myrabo concedes the vehicle described in the SSI study is highly futuristic, but contends that the apparent confirmation of the air spike phenomenon could place it as little as a generation away. A step toward demonstrating the capabilities of a full-sized vehicle could be the construction and launch of a smaller satellite-sized vehicle using an airbreathing pulse-jet engine and a pulsed microwave chemical rocket. The microwave source would be a ground-based generator.
Preliminary estimates indicate this type of machine would weigh about 30 kg. (66 lb.) and have a payload capacity of 15 kg. (33 lb.). Average microwave power would have to be about 30 megawatts, while peak power would be about 3 gigawatts (3,000 megawatts). Myrabo believes this smaller satellite vehicle could be constructed as soon as five years after launch of a dedicated project, despite the heavy peak power demands.
NASA and Air Force officials are interested in the air spike concept. They recognize there may be no immediate payoff.
"NASA is interested in a variety of advanced space transportation candidates for development after RLV [recoverable launch vehicle], and this is one of them. However, some of the component technologies from the air spike vehicle may have more immediate significance," John Mankin, manager of advanced concept studies at NASA headquarters, said.
One area where the air spike could have more near-term effect is thermal protection systems, because the concept minimizes thermal effects on hypersonic bodies.
Others at NASA also view the "air spike" concept as one that is "interesting." "It's new, it's different and shows imagination, but it' s restricted in application, because there are limits to the amount of microwave power that can be transmitted through air. That [limit] tends to relegate this to fairly small payloads, on the order of 250 to 500 kg. (550 to 1,100 lb')," Dennis Bushnell, NASA-Langley's senior scientist said. Another drawback is that the vehicle would require a "technology stretch" in all areas, he added.
Still, Bushnell believes the concept is worthy of further study, particularly since an air spike has the potential to reduce drag and lessen sonic boom generation in a high-speed commercial transport (HSCT).
A potential stumbling block to this application may be the weight and size of the microwave generator needed to create the oblique shock wave. "No one has done the numbers yet [performed an energy balance], so we don't know what the answer would be [regarding system efficiency and drag reduction]," Bushnell said.
Also unknown is the effect of concentrated microwaves on the Earth's ozone layer , an important consideration in any HSCT application.
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