posted on Jun, 20 2004 @ 07:46 PM
'AIR SPIKE' COULD
EASE HYPERSONIC FLIGHT PROBLEMS
'Aviation Week and Space Technology' May 15,
STANLEY W. KANDEBRO/NEW YORK
at Rensselaer Polytechnic have demonstrated an "air spike" concept
that could reduce the drag and heat transfer problems associated
with hypersonic flight.
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.
to Myrabo, the concept would offer designers the capability to actively
control the external aerodynamics and thermodynamics of an advanced
trans-atmospheric 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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 ground-based 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.
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
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.
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.
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.
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
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
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.
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).
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.
is the effect of concentrated microwaves on the Earth's ozone layer
, an important consideration in any HSCT application.
(reprinted with permission)