* Originally By: Richard Lackey
 * Originally To: All
 * Originally Re: Hypersonic Propulsion
 * Original Area: FidoNet UFO Echo
 * Forwarded by : Blue Wave v2.12 OS/2

The follow Aviation Leak article may be of some interest:

      AVIATION WEEK & SPACE TECHNOLOGY / May 15, 1995  (Pg 66-67)
                      STANLEY W. KANDEBO/NEW YORK

Researchers at Rensselaer Polytechnic Institute believe they 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, bluntbodied, 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 2ft.-dia. test section of Rensselaer's Mach 25-class, 60
h.-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
condition-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 (see photo).

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 1 950s 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 l-cm.
interspace between the 0.1 25-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 an 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 Silo'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 pulsejet 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 spi--- TBBS v2.1/NM

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