Ramming Speed

Aerospace engineers are crossing a cannon with a jet engine and hitting five times the speed of sound without leaving the ground.

By Gregory T. Pope|Tuesday, March 01, 1994
RELATED TAGS: WEAPONS & SECURITY
Range 18 at the Aberdeen Proving Ground in Maryland speaks in the low-tech, steel-and-gunpowder grunt of a U.S. Army weapons test range. Tank guns aim out over placid Chesapeake Bay. Ammunition storage bunkers lurk half buried behind barbed-wire fences. Scarred concrete slabs and armor plates line the earth. In the distance, experimental artillery booms like thunder.

A few steps to the right of one of the menacing oversize weapons, however, stands a piece of equipment that appears to have an attitude problem. It's a clunky contraption with a shiny new paint job, and next to the dull tank guns it looks like a Thanksgiving Day float that's strayed into a Veteran's Day parade. Its chief component is a 40-foot-long metal pipe that's stretched along a huge girder propped up by hydraulic jacks. The pipe is made of used tank-gun barrels joined end to end. They've been pierced at intervals by slim metal plumbing lines that lead away to an armored shed filled with bottles of compressed gas. This ungainly device is called a ram accelerator. And despite its unsophisticated looks, it packs a wallop that would put any normal tank cannon to shame.

Like its slam-bang neighbors, the ram accelerator's purpose in life is to boost projectiles to blinding speeds. But the ram accelerator isn't exactly a weapon, and its closest relatives aren't howitzers and cannons. It is more an aeronautics laboratory, a wind tunnel, and an engine test-bed rolled into one, and its kissing cousins are howling aircraft engines called ramjets and their even faster brethren, scramjets. These are the engines that purportedly will power the aircraft of the future at hypersonic speeds--speeds, that is, greater than Mach 5, or five times the speed of sound. No aircraft has ever flown that fast with an air-breathing engine.

Achieving these heady velocities has long been a goal of aircraft designers. But the quest has been fraught with technical failures and ungodly expense. Ramjets and scramjets are envisioned for experimental aircraft with billion-dollar price tags; the recently canceled X-30, a proposed plane expected to reach 25 times the speed of sound--about 18,000 miles per hour--is an example. Ram accelerators, though, offer engineers a firsthand glimpse of the complex physical phenomena that take place in a ramjet engine, without anyone's having to shell out a fortune.

This is possible because ram accelerators and these hypervelocity engines essentially play by the same rules. In each case, gases are compressed, ignited, and expanded rapidly to supply thrust. The difference is that a ramjet or scramjet supplies thrust to an entire aircraft, whereas a ram accelerator supplies thrust to a small projectile.

Like any jet engine, ramjets and scramjets work by mixing fuel with compressed air. But unlike a conventional jet, they lack mechanical air compressors. Those compressors don't work well when the jet gets up to speeds around Mach 2, and that's the point at which ramjets are designed to begin operating: as the screaming flow of inrushing air enters the engine, it's compressed by a cone-shaped diffuser that protrudes from the engine's front. Then it is forced--rammed--through narrow inlets to a combustion chamber located just behind the diffuser. That's where the air is mixed with fuel, usually hydrogen, and ignited by a sparking device. The combusting mix expands and shoots out the back of the engine, supplying the thrust.

Ramjets can, in theory, take planes up to about Mach 5. Up to that speed the incoming air, as it's compressed, is also significantly slowed down, to subsonic speeds. As the plane hits higher speeds, however, the air rushes into the engine so fast that it's hard to keep the fuel mixture burning. To get past a ramjet's limitations, a plane needs to be outfitted with scramjets (supersonic combustion ramjets), which are designed to further squeeze the incoming air and, again in theory, allow the mix to burn even when the air is moving at supersonic speeds. Trouble is, scramjets themselves have yet to be perfected: keeping the gas ignited in such a rapid airflow has so far proved an insurmountable challenge.

This is where devices like the one at the Aberdeen Proving Ground enter the picture. Inside the dark tube of a ram accelerator, the problems of supersonic ignition seem to disappear.

The world of ram acceleration revolves around a lanky, dignified 50-year-old aerospace engineer named Adam Bruckner. Bruckner has been in the forefront of the field ever since he and his colleagues built their own ram accelerator in a bunkerlike basement lab at the University of Washington a decade ago.

Bruckner credits now-retired University of Washington professor Abe Hertzberg with the germ of the idea. In 1983, during a train ride across China, Hertzberg whiled away the hours by trying to envision a method for smashing heavy projectiles together so hard that nuclear fusion would take place. Instead of relying on a one-shot explosion to jackrabbit a projectile, he mused, why not chase it down a launch tube with a traveling fireball of expanding gases that pushes constantly on the projectile's tail?

"Abe is a tremendous idea guy," says Bruckner with clear affection. "It's our role as his colleagues to work out the details." And the details were devilish, to say the least.

Hertzberg had portrayed his concept as a ramjet engine in a tube, with the tube serving as the engine's cowling, and the projectile as the diffuser. Since a ramjet must be traveling at a supersonic clip before it can start to generate thrust, the projectile in the tube had to be moving at a similar speed. Then, once it was in the tube, the gases behind the projectile had to be ignited.

The plan Bruckner, Hertzberg, and colleague David Bogdanoff eventually struck on called for kicking the projectile to an initial velocity of 1,500 miles per hour with a blast of compressed helium. Inside the tube, the helium would push against an "obturator," a flat-faced cylindrical plug that resembled a hockey puck. The obturator in turn would push against the projectile. As both started to accelerate, they'd encounter a volatile gaseous cocktail of premixed fuel and air. The gases would sweep neatly past the streamlined projectile but smack into the blunt obturator. The sudden compression would create enough heat to ignite the explosive brew. That would fire the projectile--a six-inch-long, Buck Rogers-ish silver missile with fins.

As the projectile hurtled down the length of the accelerator tube, the tube's gaseous mix, forced through the ring-shaped gap between the projectile and the tube's inner wall, would get compressed. Emerging from the gap, the gases would be ignited by the continuing combustion taking place behind the projectile's tail. The projectile would therefore surf through the tube on a wave of combustion pressure, enjoying nonstop acceleration the whole way.

At first the whole idea drew guffaws. "We got the biggest laughs from people outside the university," Bruckner recalls. "A pipe filled with a combustible mixture? The slightest thing will set it off like a bomb! But they didn't understand the process, and we were very confident it would work."

Right off, it didn't. Bruckner, Hertzberg, and Bogdanoff stumbled through several false starts before the concept lived up to their claims. Getting the gases to ignite behind the projectile proved especially tricky. (The trio first tried an onboard ignition system before they hit on the idea of the obturator.) It also took months to come up with a fuel mix that would burn at a cool enough temperature so that the whole tube wouldn't, indeed, explode like a giant pipe bomb on ignition. They finally settled on a mix of methane, nitrogen, and oxygen. By 1986 they had a working 16-foot, inch-and-a-half-diameter ram accelerator and a small network of interested sponsors--the Air Force, NASA, and one aerospace corporation.

Since then the group has tried to juggle two imperatives. With staunch academic discipline they've plunged headlong into the fundamental physics of ram acceleration. What's going on as the projectile charges through the tube? Just how apt a metaphor is ram acceleration for the operation of a high-speed aircraft engine? At the same time, however, they've tried to please their funders, who reacted like excited children in the backseat of a sports car, urging the researchers to push the accelerator to higher and higher velocities. In satisfying their sponsors, the group found that the ram accelerator defied their own predictions.

In particular, Bruckner learned he'd underestimated the ram accelerator's ultimate speed limit. He'd believed the projectile wouldn't accelerate beyond a gas mixture's detonation speed--the velocity at which, after ignition, a combustion wave propagates through the gas (analogous to the speed at which an explosion travels through an ignited stick of dynamite). If the projectile did overshoot the detonation speed, he reasoned, the pressure imparted by the combustion wave would lag behind the projectile's tail. Without this pressure, the thrust would simply fall off.

The theory proved to be simplistic. As it turns out, the projectile has no problem outstripping the detonation speed. "We were trying an experiment with one very long tube," Bruckner recalls, "because we wanted to see how the thrust falls off. But the damn thing didn't fall off--it got a second wind and took the projectile through the detonation speed, higher and higher."

To understand what was happening, says Bruckner, imagine the projectile as it begins sailing down the tube. Recall that as the compressed gases squeeze through the ring-shaped gap around the projectile and meet the zone of combustion behind it, they ignite and expand, adding to the force pushing against the projectile's tail. The release of energy from all this combusting gas causes the formation of a shock wave--a continuous wave of high pressure--that sits about halfway up the projectile's tail, perpendicular to the axis of the ram accelerator's tube.

But now the projectile is closing in on the mixture's detonation speed. What appears to happen, Bruckner and his companions believe, is that the shock wave begins shifting from its perpendicular stance to an oblique one, for reasons they still don't clearly understand. The gases whipping around the speeding projectile and through the gap are now compressed so hard by this "oblique detonation wave," as they call it, that the heat produced by this added compression ignites the gases before they even reach the combustion zone behind the tail.

No longer is combustion taking place behind the projectile. Instead a thin, supersonic flow of burning gases now squeezes the projectile's conical tail, like two fingers squirting a watermelon seed. In this mode the Washington ram accelerator has reached speeds over 5,200 miles per hour, much faster than the mixture's average detonation speed of about 3,800 mph. Bruckner says the theoretical ceiling may loom higher than 18,000 mph.

What really astounded Bruckner, though, was that the ram accelerator made a smooth transition from one mode of acceleration to another--from pushing the projectile by the combustion in back of it to driving it by the oblique detonation wave. That discovery is more than just an academic curiosity. If Bruckner is right about what's actually happening in the tube, then he and his colleagues may have discovered a way to overcome the biggest problems in hypersonic flight.

The appearance of the oblique detonation wave may allow for the solution of the problem that's hindered scramjet development--the difficulty of supersonic combustion. Burning fuel in a supersonic flow of air, says one scramjet expert, "is like trying to keep a match lit in a speeding convertible." And yet, Bruckner believes, supersonic combustion does occur when the oblique detonation wave takes over in the ram accelerator.

Since the University of Washington device divulged its secrets, the ram accelerator club has swollen. Two accelerators began firing last year at the French-German Research Institute in Saint-Louis, France, a laboratory financed by both nations' defense ministries. "I was salivating at their equipment," Bruckner recalls wryly of a trip there last year. Now also on the ram bandwagon is U.S. Army mechanical engineer David Kruczynski, who inspected the University of Washington facility in 1990. "I came back totally impressed," says Kruczynski. He proceeded to lobby his employer, the Army Research Laboratory, for funding. Early this year his machine at the Aberdeen Proving Ground, with its 120 mm barrel--4.7 inches in diameter, the size of a tank gun, and three times the diameter of the Washington device--achieved success on its very first shot. With ram accelerators, diameter's the thing: the bigger the bore, the bigger the object that can be fired out of it. The Army's machine is currently the largest in the world, and it has proved that the ram accelerator can work at larger scales.

Obviously a concrete military aim lies behind the Army's and France and Germany's ram accelerators. Both are funded with an eye toward developing a system to crash projectiles into armor so that the damage wrought by futuristic weapons can be studied. But in an era of dwindling defense budgets, Kruczynski is quick to play up the versatility of this technology. "I especially like the idea of launching small payloads into orbit with a larger ram accelerator," he says.

Not surprisingly, larger ram accelerators have also captured the fancy of NASA, but plans for a functional device are on hold. Not long ago, investigators at NASA's Langley Research Center were interested in building a ram accelerator so powerful that it would leap right into the oblique detonation stage. They wanted it for a huge testing structure called the Advanced Hypervelocity Aerophysics Facility, or AHAF. The AHAF accelerator was to be a giant tube about 2 feet wide and 1,000 feet long, designed to hurl a 2- to 3-foot-long sensor-studded model aircraft at speeds up to 27,000 mph. The model projectile would have zipped out of the ram accelerator and through a chamber containing the thin air of the stratosphere.

"We don't have a facility that simulates what really happens when you fly 8, 10, or 12 times the speed of sound in real air," explains Langley aerospace engineer Robert Witcofski, who helped develop the idea. "Ram acceleration had the most promise for AHAF. You can control the acceleration, there are no limits to the size you could build, and the biggest thing? It's cheap." Not cheap enough, however--NASA has back- burnered the facility.

Meanwhile, a new crop of ram accelerators are on their way at two Japanese universities and at Eglin Air Force Base, seeded respectively by interest in hypersonic engines and high-speed missile defense. The burst of activity gratifies Bruckner. "My only concern," he adds, "is that the serious money in this has shifted overseas. We're operating on a shoestring budget now."

While Bruckner and his colleagues win high marks from NASA and the Pentagon for their pioneering efforts, they're having trouble winning dollars in the austere 1990s. "Still," he says, "we were the first group to build one and prove the principle and go on from there. And no one can take that away from us."
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