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Ramjets
How
Ramjets Work
BY NICHOLAS GERBIS
As
anyone who's ever belly flopped off a high dive can tell you, when you hit a
fluid without giving it time to get out of the way, it tends to hit back.
Divers
beat physics by taking a more streamlined plunge, and faster cars and aircraft
do it by sporting more aerodynamic shapes.
But
there comes a point, near the sound barrier, where streamlining is not enough
-- a speed at which the very air that keeps your plane aloft begins to hammer
you with seemingly insurmountable drag, teeth-rattling turbulence and brutal
shock waves.
Indeed,
many believed this sound barrier unbreakable until, on Oct. 14, 1947, Chuck
Yeager's rocket-powered Bell X-1 proved them wrong.
But
what if you could turn all that piled-up air to your advantage?
What
if, instead of churning through it with propellers or burning through it with
rockets, you could pack it into a specially shaped tube, pump it up with an
explosion and fire it out a nozzle at supersonic speeds, all with no major
moving parts?
You'd
have a very special type of jet engine, a "flying stovepipe" fit for
slicing through the sky at thousands of miles per hour. You'd have a ramjet.
But
the ramjet's apparent simplicity is deceptive; it takes cutting-edge
aeronautical engineering, modern materials and precision manufacturing to pull
one off -- which partly explains why an idea nearly as old as powered flight
was repeatedly taken up and cast aside for decades before achieving limited
success during the Cold War.
Unlike
its main speed competition, the rocket, which burns fuel using onboard
oxidizers like ammonium nitrate, potassium chlorate or ammonium chlorate,
ramjets breathe air.
Thus,
while rockets can operate in the near vacuum of space, ramjets must fly through
the atmosphere.
They
must do so at very high speeds, too -- around Mach 2.5-3.0, or three times the
speed of sound -- because ramjets work by harnessing ram pressure, the
natural air compression brought on by an aircraft's high speed.
In
other words, ramjets make allies of the very shock waves and compression forces
that once opposed high-speed flight; they literally go with the flow [sources: Encyclopaedia Britannica; NASA].
Ramjets
are more efficient over long distances than rockets but suffer a significant
disadvantage: They are useless at low velocities.
Consequently,
they rely on booster rockets or other vehicles to get them up to speed.
Standalone ramjet aircraft typically use hybrid engines [source: NASA].
If
that explanation flew past you at supersonic speed, it's probably because we
skipped over a lot of cool and interesting stuff. Let's look at how jet engines
have developed to produce this modern marvel.
Detonations
and Arrivals
Jets
run on controlled explosions. That sounds strange until you realize that most
car engines do, too: Pull in air, compress it, mix it with fuel, ignite it
and bang! You've pushed a piston.
But
whereas gasoline and diesel engines involve cyclical or intermittent
combustion, jets entail continuous combustion, in which
fuel and air mix and burn nonstop.
Either
way, burning more rubber means guzzling more gas, and that means sucking in
more oxygen to get the mixture right. Souped-up cars do this with
superchargers; in jet engines, it's more complicated [source: Encyclopaedia
Britannica].
The
first operational jet aircraft zoomed into combat near the end of World War II
using turbojet engines, a straightforward but ingenious design
based on the Brayton (or Joule) Cycle:
As the plane flies, air streams through an intake into a diffuser,
a chamber that slows airflow and inhibits shock waves.
It
then passes through a series of bladed disks: spinning rotors,
which force air backward, and stationary stators, which guide
airflow.
Together,
they act as a compressor that pumps up pressure within the jet's combustion
chambers. There, fuel mixes with pressurized air and ignites, blasting
temperatures into the 1800-2800 F (980-1540 C) range or higher [sources: Encyclopaedia
Britannica; Krueger; Spakovszky].
Pressure
rises with temperature, so this explosion creates a lot of force with nothing
to do but seek a quick exit. As the exhaust shoots through the rear nozzle it
generates thrust to move the aircraft.
En
route to this nozzle, the exhaust also shoots through a turbine connected to
the rotors by a torque shaft. As the turbine spins, it transfers energy to the
compressor blades in front, completing the cycle.
In
airplanes with turboprops or helicoopters with turboshaft engines,
the turbines also transfer power to a propeller or helicopter rotor via a
series of gears.
Turbojets
pack a lot of power but struggle at low speeds. Consequently, in the 1960s and
1970s, low-supersonic aircraft began trending toward the turbofans that
most private jets and commercial airliners still use.
A
turbofan is the turducken of engines -- essentially a turbojet wrapped in a
larger cowling with a big fan slapped on its front.
The
fan pulls in more air, which the engine then splits into two streams: Some air
moves through the nested turbojet, while the rest flows through the empty space
around it.
The
two streams reunite when redirected cooler air mixes with the turbojet's
exhaust and slows it down, creating a larger, slower thrust stream that is more
efficient at low speeds [sources: Encyclopaedia
Britannica; Krueger].
Meanwhile,
around the time that turbofans came into their own, research into ramjet
aircraft was finally hitting its stride. It had been a long road.
AFTERBURNERS
Some turbojets and turbofans are coupled
with afterburners, which eke out more energy by injecting fuel into
exhaust after it passes the turbine and reigniting it. This process, also known
as reheat, is inefficient but can boost turbofan thrust by as much
as 50 percent [sources: Encyclopaedia
Britannica; Pratt & Whitney].
Afterburners come in handy during takeoff
or in unfavorable, low-speed or low-pressure conditions. They are mostly found
in supersonic fighter aircraft, although the Concorde SST used them on takeoff
as well [sources: Encyclopaedia
Britannica; NASA; Pratt & Whitney].
Ramjets, Ahead of Their Time?
Whoever said that you have to
walk before you can run never met Frenchman René Lorin. He saw the
possibilities of ram-pressure propulsion as early as 1913, when pilots were
still flying glorified wooden kites.
Aware of the design's
uselessness at subsonic speeds, he instead designed a ramjet-assisted flying
bomb. The French military waved him off.
Hungarian engineer Albert
Fono, another ramjet pioneer, pursued a similar idea in 1915 and received a
comparable reception from the Austro-Hungarian Army [sources: Gyorgy; Heiser
and Pratt; Wolko].
Ramjets designs enjoyed a
short vogue between world wars.
Soviet engineers made
early strides in rocket-based ramjets, but interest burned out before 1940.
The German occupation
interrupted French engineer René Leduc's early work, but his persistence and
secrecy paid off on April 21, 1949, when his Lorin-inspired 010 model made its
first powered flight of a ramjet aircraft.
Carried aloft atop a
Languedoc 161 airliner, it flew for 12 minutes and reached 450 mph (724 kph) at
half power [sources: Siddiqi; Ward; Wolko; Yust
et al.].
And, for a while, that was
that. Despite Leduc's success, lack of funds ended official support for his
research in 1957 [sources: Siddiqi;
Ward; Wolko; Yust et al.].
The ramjet was beginning to
look like an invention with no application.
Meanwhile, World War II had
ushered in the first generation of operational turbojets: the British Gloster
Meteor, the German Messerschmitt Me 262 and the American Lockheed F-80 Shooting
Star [sources: Encyclopaedia
Britannica; Encyclopaedia
Britannica; Encyclopaedia
Britannica; National
Museum of the USAF; van
Pelt].
As the war ended and the Cold
War heated up, it became clear that turbojets and turbofans presented more
practical subsonic and low-supersonic solutions than ramjets.
Thereafter, most U.S. and
Soviet work in ramjets focused on building intercontinental missiles.
In 1950, American engineer
William H. Avery and the Johns Hopkins University Applied Physics Laboratory
produced Talos, the U.S. Navy's first ramjet missile.
Future generations would
refine and streamline the design, introducing hybrid ramrockets capable
of achieving high supersonic speeds (Mach 3-5) [sources: Hoffman; Kossiakoff; Ward].
Despite intriguing designs
like the Hiller XHOE-1 Hornet helicopter, the proposed Republic XF-103 bomber
interceptor and the short-lived Lockheed D-21B unmanned reconnaissance drone,
ramjet aircraft languished until the 1964 debut of the Lockheed SR-71
Blackbird.
The fastest manned aircraft
until its retirement in 1989, the Mach 3+ Blackbird also used a hybrid engine,
sometimes called a turboramjet [sources: National
Museum of the USAF; Smithsonian; Ward].
We'll dive into the SR-71 and
other ramjet hybrids and subtypes in the next section.
RUDI RAMJET?
By the end of World War II,
Germany had begun research into numerous jet craft, including a rocket-assisted
ramjet, the Fw 252 "Super Lorin," and the ramjet-powered Sänger-Bredt
antipodal bomber.
Most famously, they
successfully built the V-1 Buzz Bomb, a steam-catapult-launched, pulse-jet-driven
guided bomb.
A pulse jet is not a ramjet,
but they share qualities in common, including simplicity and a minimum of
moving parts [sources: Encyclopaedia
Britannica; Encyclopaedia
Britannica; Encyclopaedia
Britannica; National
Museum of the USAF; van
Pelt].
Ramjets:
Making Mock of Mach
If ramjets are so fiddly,
then why bother? Well, at the pressures and temperatures generated at Mach
2.5+, most jet engines become hugely impractical -- and utterly pointless.
Even if you could make one
work, doing so would combine the hazards of running a windmill in a hurricane
with the pointlessness of hauling a wave machine to Oahu's North Shore.
Ramjets take the basic
principles of other jets and crank them up to 11, all without major moving
parts.
Air enters a ramjet's
diffuser at supersonic speeds, assaulting it with shock waves that help build
ram pressure.
A diamond-shaped center body
in the intake further squeezes the air and slows it to subsonic speeds to more
efficiently mix with fuel and combust.
Combustion occurs in an open
chamber akin to a giant afterburner, where liquid fuel is injected or solid
fuel is ablated from the chamber's sides [sources: Ashgriz; Encyclopaedia
Britannica; SPG; Ward].
Ramjets' speed limitations
gradually inspired hybrid engines that could fly at lower speeds and accelerate
to supersonic velocities. The most famous example, the SR-71 Blackbird, used a
turbojet-ramjet hybrid called, appropriately, a turboramjet.
Such engines work like an
afterburning turbojet until well past Mach 1, after which ducts bypass the
turbojet and redirect the ram-compressed airflow into the afterburner, making
the engine behave like a ramjet [source: Ward].
Missile designs, meanwhile,
gradually did away with boosters by moving them inside the ramjet itself,
creating ramrockets, aka integral rocket ramjets.
During rocket acceleration,
plugs temporarily seal the ramjet's intake and fuel injectors. Once the rockets
are spent and the ramjet is up to speed, these pop off, and the empty rockets
act as combustion chambers [source: Ward].
Looking forward, crossing the
Mach 5 line into hypersonic speeds will likely entail scramjets
(supersonic combusting ramjets).
Unlike other ramjets,
scramjets do not need to slow air to subsonic speeds in their combustion
chambers.
To pull off ignition and
expansion in the 0.001 seconds before the pressurized air shoots out the
exhaust, scramjets typically use hydrogen fuel, which has a high specific
impulse (change in momentum per unit mass of propellant), ignites over
a wide range of fuel/air ratios and releases a huge burst of energy when burned
[sources: Bauer; Encyclopaedia
Britannica; NASA].
Scramjets remained
theoretical before the past few decades, and work remains mostly experimental.
In November 2004, NASA's eight-year,
$230-million Hyper-X Program produced a scramjet that reached Mach 9.6 on its
final flight.
Some analysts believe the
technology could reach Mach 15-24, but air travel at hypersonic speeds means
overcoming forces unlike those faced by even the fastest supersonic craft. In
short, we have a long way to go before we can commute from New York to Los
Angeles in 12 minutes [sources: Bauer; DARPA; Fletcher; NASA].
THE INTERSTELLAR RAMJET
One major obstacle to
rocket-powered space travel is the exponential relationship between
acceleration and fuel. The faster you go, the more fuel you need; the more fuel
you carry, the more mass you add, the more additional fuel you need to overcome
it [sources: Long; NASA].
With this in mind, physicists
have proposed other solutions, including everything from solar sails to
exploding ejected nuclear bombs.
In 1960, physicist Robert
Bussard proposed an interstellar ramjet that would collect charged particles in
space via an electromagnetic field, converge them, create a fusion reaction and
use the energy for propulsion [sources: Long; NASA].
Author's Note: How Ramjets Work
I'm often enchanted by
stories of great innovations that failed to find an application when they were
first invented. While writing this article, for example, I was repeatedly
reminded of the laser, which was once called a solution looking for a problem.
Oh, what a difference a few
decades make.
On the other hand, sometimes
weird inventions make millions. Other times we invent things for one purpose
that turn out to have unforeseen applications. Among its many contributions,
the American space program invented the ribbed swimsuit and changed diapers
forever.
Today, materials scientists
are discovering properties for which we have yet to find uses. With luck,
they'll fare better than Lorin.
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