Hypersonic Ammo Could Increase Lethality Of Gunships And Helicopters

Space-based interceptors might have jumped to the top of the U.S. defense wish list, but a group of researchers argues it is time to bring aerial gunnery into the 21st century.

These researchers are calling for new operational requirements for aerial gunnery that would spark development and production of hypersonic ammunition that could significantly increase the capability of gunships, attack helicopters and even future fighters.

• A 20mm hypersonic round has the armor penetration of a conventional 30mm
• The flight-safe discarding sabot encloses a dart-like flechette penetrator

The appeal for support is based on academia’s successful prototyping of discarding-sabot ammunition for hypersonic aerial gunnery that dramatically reduces time of flight and increases range, accuracy and lethality.

The researchers bemoan a three-decade lack of investment in ammunition development since leadership of the aerial gunnery mission was transferred to the U.S. Army from the U.S. Air Force. They say the time is ready, as a first step, to develop new hypersonic ammunition to upgrade the lethality of Air Force Special Operations Command Lockheed Martin AC-130s and Army Boeing AH-64s.

Standard 20mm and 30mm aerial rounds used by the U.S. and NATO trace their lineage back to the early 1970s. Ballistic coefficients have not improved, while those for Army and U.S. Navy mid-caliber ammunition are now “an order of magnitude better,” Ronald Barrett-Gonzalez, professor of aerospace engineering at the University of Kansas (KU), told the American Institute of Aeronautics and Astronautics’ SciTech Forum in Orlando, Florida, in January.

Barrett-Gonzalez blamed the lack of innovation on Defense Department procurement practices, which treat aerial ammunition as a commodity. “We have a split, almost dual-source, no-compete contracting mechanism,” he said. “Northrop Grumman gets roughly half of it. General Dynamics gets the other half. They don’t have to evolve anything. So we have antique ammunition and I think it’s time to change.”

The kinetic energy and supersonic drag of current ammunition determine its range, time of flight, accuracy and penetration. With the aerodynamic center ahead of the center of gravity, gusts destabilize the round, creating windage effects, such as precession and nutation, that increase circular error probable (CEP), degrading accuracy.

The hypersonic ammunition invented by Barrett-Gonzalez and Lauren Schumacher, senior systems engineer at RTX, uses an aeromechanically stable flechette. The dart-like penetrator is long and thin for lower supersonic drag. The center of gravity is ahead of the aerodynamic center, so a gust creates a stabilizing moment, nulling flightpath deviations and reducing CEP. The result is improved accuracy even at longer ranges.

The sabot is a jacket that fits around the narrow flechette so it can be fired from a larger-bore barrel for higher acceleration and kinetic energy. The sabot is discarded after the projectile exits the muzzle.

Discarding-sabot ammunition decouples barrel bore and projectile caliber. For the same muzzle energy and recoil force, large gains in muzzle velocity are possible. But impact with the rapidly decelerating sabot poses a hazard to the launching aircraft.

The Air Force abandoned development of discarding-sabot aerial ammunition in 1998, when leadership was transferred to the Army. “This has led to a 27-year gap in aerial gunnery ammunition,” Barrett-Gonzalez said. “The only place where research was kept alive was in academia.

“Then, in 2016, we made a breakthrough,” said Barrett-Gonzalez, director of KU’s Adaptive Aerostructures and Aircraft Design Laboratories. “We discovered how to make discarding-sabot aerial gunnery ammunition. The Air Force had tried for 50 years and failed.”

Conventional sabots are unstable so they will separate from the projectile, and they tumble and can strike the aircraft. Barrett-Gonzalez’s team has developed ballistic aeromechanically stable sabot (BASS) and maneuvering aeromechanically stable sabot (MASS) ammunition that can be unguided or guided.

On firing, while the flechette flies downrange at hypersonic speed, the sabot separates, flips around and turns away. The launching aircraft makes a maximum-G turn that is safely outside the widest and tightest turns the sabot can make (see diagram). “So the sabot is an aircraft unto itself,” he said.

Combining the flight-safe sabot with a traveling charge—solid propulsion that accelerates the projectile along the barrel—would produce “hard launch” hypersonic ammunition with much shorter time of flight than today’s rounds over longer ranges with increased lethality at range.

The researchers have prototyped 10mm, 20mm and 30mm rounds with tungsten flechette penetrators and conducted ground firing tests. “These rounds go 1-2 mi. per sec.,” Barrett-Gonzalez said. “When you are firing them from the AC-130, your windage effects essentially go away. Your CEPs are a lot tighter.”

A 20mm hypersonic round eclipses the penetration capability of the Fairchild Republic A-10’s 30mm round. “T-72 [tank] top armor is sliced through with a 20mm round,” he said. “A 20mm round being more lethal than a 30mm round is unheard of. It means the gun doesn’t have to be as big, and recoil forces are less.

“We can put the AC-130 well outside [man-portable air defense system] threat zones,” Barrett-Gonzalez added. “Conventional rounds run out of kinetic energy quickly. [With BASS rounds], you can make the entire ground you’re fighting over safe with one aircraft.”

Hypersonic rounds are compatible with muzzle flash suppressors. “At night, AC-130s light themselves up. They basically become flying targets. Because these rounds are suppressor-compatible, you swallow that fireball and the AC-130 stays dark,” he said.

For short-range counter-missile use, “we fire these at a Mach greater than 5,” he added. “The missile homes on the flechette, and the flechette homes on the missile. They are drawn toward each other. Be it an infrared or a radio-frequency seeker, you can tune them to act as an attractant.”

At very short range, the flechette flies line-of-sight out to 10 km (6.2 mi.). “Long range is different,” Barrett-Gonzalez said. “You fire a trio of rounds, at least, and they form a synthetic aperture that guides them on to the target beyond 100 nm. We are working on all of these.”

BASS rounds can also be used to counter drones. “You have to reduce the caliber so that you undermatch the aircraft you are going after in terms of cost, weight and size,” he said. “So the 10mm round is intended for counterdrone, and it can also go after soft targets.”

In an advanced fighter application, higher-caliber hard-launch hypersonic ammunition could provide ranges in excess of 100 km, Barrett-Gonzalez said. But he does not expect to see application to air combat before a putative seventh-generation fighter because of institutional barriers. “It’s not for sixth-gen,” he said. “There are too many firewalls.”