Army Tests Scramjet to Power Kinetic Energy Tank Rounds
by Frank Colucci
The U.S. Army is testing a supersonic projectile that could drastically increase the killing power of future tanks.
The Armaments Research Development and Engineering Center, at Picatinny Arsenal, N.J., will test a supersonic combustion ramjet—scramjet—designed to improve the penetrating power of tank guns.
The scramjet, like common jet engines, burns fuel mixed with compressed atmospheric oxygen. However, unlike traditional jets, the scramjet has no compressor disks or other moving parts to compress the air. Hot air entering the scramjet inlets at four times the speed of sound, ignites the fuel and sustains combustion, so the scramjet itself contains pure fuel without wasting weight and volume of a separate oxidizer.
As it emerges ignited from a cannon barrel, a scramjet-powered tank round could produce thrust in flight to extend its range or sustain its penetrating power all the way to the target.
Compared to unpowered kinetic energy tank rounds that slow down and lose penetrating power to aerodynamic drag, a scramjet powered round could sustain its tank-penetrating power over longer ranges, or enable a smaller, lighter gun to achieve the same result.
Laboratory flight tests of a 101 mm demonstrator engine, scheduled from April to July of this year, may lead to a live-fire demonstration of a 120 mm round in a tank gun by 2005. Army researchers believe a scramjet-powered kinetic energy penetrating round will help give lighter fighting platforms an improved large caliber, direct fire capability.
With no moving parts, the scramjet engine burns fuel with the compressed, superheated air encountered at Mach 5—the muzzle velocity of existing tank guns.
Timing fuel combustion to the desired flight profile makes it possible to sustain the kinetic energy and penetrating power of tank rounds at extended ranges in direct-fire applications. Alternatively, the scramjet could extend the range of cannon rounds for indirect fires.
Unlike rockets, the scramjet wastes no fuel volume carrying oxidizer. “It’s a way to provide more thrust per pound of round,” explains Joe Snyder, aerospace engineer at the ARDEC Advanced Systems Concepts Office.
Accelerated to Mach 5 by the time it leaves the 120 mm gun of the Abrams tank, the standard M829A2 kinetic-energy round uses a finned “dart” to penetrate opposing tank armor and devastate the crew compartment without an explosive warhead. In Operation Desert Storm, one such penetrator reportedly pierced two Iraqi T-72 tanks parked side-by-side. Despite their acknowledged effectiveness, kinetic-energy rounds lose about 100 miles-per-second velocity over 2 kilometers, due to drag.
Since kinetic energy declines with the square of the velocity, armor penetration falls off at extended ranges. A scramjet propelled round could sustain the velocity and penetrating power of a tank gun round all the way to its target.
Oversight for the Picatinny ARDEC is transitioning from the Tank and Automotive Armaments Command to the new Research, Development and Engineering Command, but the center remains the research focal point for gun armament systems.
The ARDEC Advanced Systems Concepts Office broadened a scramjet program initiated by the Defense Advanced Research Projects Agency and the Office of Naval Research.
Tyrus Cobb, chief of the ASCO requirements analysis division explains, “That’s what we’re all about, looking at futuristic stuff that will enhance our armament applications.”
Scramjet technology has potential applications in anti-ship missiles and other platforms. Allied Aerospace received a contract under the DARPA/Navy Small Business Innovative Research program to demonstrate scramjets in the supersonic wind tunnel at the Arnold Engineering and Development Center at Tullahoma, Tenn.
“We were looking to find a means to test scramjets in flight for lower cost,” explains vice president of engineering Robert Bakos. He notes a gun-launched scramjet could be tested in the 1,000-foot tunnel at the AEDC for a fraction of the cost of a rocket-boosted vehicle.
Four test firings from a 101 mm light gas gun in July 2001 proved a scramjet engine could sustain Mach 7 in thin air at a simulated 100,000-foot altitude. Informal networking by engineers at the Picatinny ASCO and DARPA resulted in a Cooperative Research and Development Agreement with Allied Aerospace to produce a “gun-hardened” engine for Mach 5 launch at sea level.
The company first demonstrated scramjet technology in 1961 and has worked on the National Aerospace Plane, the Hyper-X, and other hypersonic research applications. Subsonic ramjets transition to supersonic scramjets around Mach 5—five times the speed of sound. Fuel metered into the hypersonic airstream ignites spontaneously. According to Bakos, “What you’re trying to do is control the supersonic flow through the engine, and add heat to it, and make it produce thrust. It’s a delicate balance of many opposing large forces.”
The demonstrator engine burned gaseous ethylene in eight combustion chambers. Engineers optimized the scramjet inlet ducts and fuel injectors for a Mach 7 flight with 10,000 G acceleration. The Army’s follow-on effort refines the design to withstand the 60,000 G load of a main tank gun and the sustained heat of hypersonic flight. “You’re flying very fast with a lot of heat transferred to the vehicle,” observes Bakos. “The vehicle has to sustain those loads without weakening.”
In DARPA tests, the 101 mm (4-inch) diameter titanium demonstrator engine flew 260 feet before it vaporized on impact with the steel plates at the end of the test tunnel. Though the missile shape was aerodynamically unstable, the engine produced thrust throughout its 30-meter flight. “There wasn’t enough time for the projectile to turn over,” says Snyder.
The ARDEC-sponsored demonstration will combine a refined motor with a penetrator, stabilizing fins and discarding sabot designed by Army engineers at Picatinny.
Building a scramjet round able to withstand acceleration forces six times those encountered in the original demonstration presents design and material challenges. Simply thickening the walls between the combustion chambers would reduce fuel volume and air flow. High strength materials are essential to produce a gun-hardened scramjet round. Target throw weight of the engine plus penetrator is 23 pounds. “We’re looking to lighten-up anywhere we can,” says Snyder.
The ARDEC demonstration in the Arnold tunnel calls for two unpowered aerodynamic shapes of a representative mass to validate the important stabilizing fins. Three scramjet-powered rounds will then test the integration of the engine, penetrator and fins. A four-piece composite sabot like that used on standard tank rounds seals the gun tube to capture the pressure from burning launch propellants and increase muzzle velocity, then breaks away in flight.
The powered rounds are expected to travel the full 1,000-foot length of the test tunnel at Mach 5 at sea level (1,700 miles per second). Successful demonstrations may lead to a tactical 120 mm round development program for the Future Combat System’s tank-like mounted combat system. Similar rounds could be incorporated in the Stryker brigade mobile gun system and Abrams tank, officials said. Advanced Systems Concepts Office director Eugene Del Coco, explains, “You can continue to improve the performance of existing weapons systems by adding new munitions and new technology.”
The test engine continues to use gaseous ethylene fuel for its short tunnel flight. A useable scramjet-powered kinetic-energy round would require solid propellant to reach 4 km or more, and to provide a safe, easily-handled round with a shelf life of 20 years or longer. The ARDEC Energetic Materials branch will evaluate alternative fuels for tactical trials, and Allied Aerospace is working with Alliant Tech Systems to identify suitable solid propellants.
The ideal line-of-sight, direct-fire kinetic energy round might discard its motor after fuel burn out to eliminate parasitic drag on the penetrator. While the current goal is a 120 mm round to fit future and existing guns, the technology could potentially boost the velocity and increase the lethality of smaller rounds to support development of smaller guns carried by lighter, more agile vehicles
http://www.nationaldefensemagazine.org/ ... fm?Id=1170
Edited by - rickusn on Jul 29 2003 4:52 PM
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