by Ann Parker

From the doodlings of Leonardo da Vinci and the penned fantasies of Jules Verne to the tangible accomplishments of the Wright brothers and other aviation pioneers, mechanized flight has captured the imagination of humanity through the centuries. Even today, with atmospheric and space flight a reality, there are still aviation realms to dream about and conquer.

Hypersonic flight at speeds 5 to 12 times the speed of sound (Mach 5 to Mach 12) is one such area of interest to the commercial and defense communities. At Lawrence Livermore National Laboratory, aerospace engineer Preston Carter has invented a concept for a next-generation hypersonic aircraft, dubbed HyperSoar, that would fly efficiently, economically and cleanly.

Flying at Mach 10 (3 kilometers per second), HyperSoar could reach any point on the globe within two hours. For comparison, the fastest military plane, the SR-71, flies between Mach 3 and Mach 4, while the commercial Concorde reaches only Mach 2. HyperSoar would also have twice the fuel efficiency of commercial airliners, be three to five times more efficient in putting satellites in space than today's launch systems, and use liquid hydrogen fuel, which produces simple water vapor when burned. 

HyperSoar - a concept-development project funded through Livermore's Physics Directorate and the Laboratory Directed Research and Development Program - could transport people or cargo, strike enemy targets, or help put satellites into space.

Lawrence Livermore National Laboratory HyperSoar over Hawaii, on its 72-minute flight from Los Angeles to Tokyo

"The fact that HyperSoar has many potential uses is key," says Carter. "Developing an entirely new aircraft is expensive. However, if there is a large market for such an aircraft, the cost per plane goes down. It's like the difference between a 747 and the Stealth bomber. There are hundreds of Boeing 747s being used by commercial airline companies, airfreight companies, and so on. But the only market for the Stealth is the military, which only needs a few. That's why you'll never see a Stealth being built for much less than they cost today."

Skipping on the atmosphere
A 25-meter-long HyperSoar aircraft (about as long as the wingspan of a large business jet) could make a conventional takeoff from a standard runway. Using special combined-cycle engines that are air-breathing but based on rocket technology, it would ascend to 40 kilometers. Here - at the outer limit of Earth's atmosphere - its engines would be turned off. HyperSoar would then coast up to a high point of 60 kilometers, before beginning to fall back down to about 35 kilometers, well inside the upper levels of the atmosphere.

As the aircraft descends into denser air, it would be pushed up by the increased aerodynamic lift. The engines then fire briefly, propelling the plane back into space. Outside the atmosphere, the engines shut off and the process repeats. In this way, HyperSoar would skip off the top layer of the atmosphere every two or so minutes, like a flat rock skittering in slow motion across the surface of a pond.

Including the ascent and descent portions of a flight, a trip from Chicago to Tokyo (10,123 kilometers) would involve about 18 skips and 72 minutes. To travel from Los Angeles to New York (3,978 kilometers) would involve about 5 skips and take only 35 minutes. (Both flights require a total of about 2,450 kilometers and 27 minutes for take off and landing.)

By popping regularly out of the atmosphere and using the engines intermittently, HyperSoar would use less fuel and also solve a critical problem that plagues other hypersonic aircraft designs - overheating.

Beating the heat
Any object speeding through the atmosphere - airplane, spacecraft, meteorite - will compress and heat the air in front of it. This heat is inevitably absorbed by the surface of the object.

"Heat buildup just kills most designs for hypersonic aircraft," explains Carter. "The hotter the craft gets, the more material engineers add to the airframe to strengthen and shield it. Also, most other hypersonic concepts have trajectories that are strictly atmospheric, and the only way to get rid of the heat is to dump it into the fuel and then burn the fuel in the engines. The problem is, the faster you fly, the more fuel you must carry as a heat sink. Eventually, you end up carrying a significant amount of fuel just as a heat sink, and the engines end up running fuel-rich, that is, burning up more fuel than they really need. That's wasteful in and of itself. Also, more material and more fuel translate to more weight. After a while, the aircraft can no longer carry a decent cargo."

Because HyperSoar spends nearly two-thirds of its time out of the atmosphere, it can radiate the heat into space. Carter and colleagues at the University of Maryland have analyzed HyperSoar, compared it to other concepts, and found that - thanks to its trajectory and shape - HyperSoar has less heat load on its airframe and consumes less fuel.

From express mail to satellites
"The way HyperSoar blends flight and space access is revolutionary, opening up a world of potential applications," says Carter. Possibilities include using HyperSoar as a freighter, military aircraft, low-cost launcher, and, eventually, a passenger aircraft. According to Carter, HyperSoar would be capable of carrying more weight over longer distances than other planes of similar size and mass.

For reaching space, HyperSoar could be employed as the first stage of a two-stage-to-orbit launch system. At the high point of its skip, HyperSoar would eject an upper-stage vehicle and its payload into low-Earth orbit. For a given takeoff weight, this approach would deliver approximately twice the payload to orbit as any of today's expendable rockets.

A larger HyperSoar vehicle, the size of a Boeing 777 for example, could handle a 13.7 metric ton payload in addition to the weight of a typical second-stage launcher. At take-off, this version of the HyperSoar would weigh 255 metric tons, about half as much as the largest Ariane 4 rocket - but it would carry about 40 percent more payload. Because most satellites are smaller than this, in reality the HyperSoar vehicles will not need to be built as large vehicles but rather as smaller, less expensive ones.

Flying the paper plane
Even though HyperSoar is still in the "paper airplane" stage, it has garnered interest from organizations as diverse as Federal Express and the U.S. Air Force Strategic Air Command. HyperSoar has appeared in Jane's Defence Weekly, Aviation Week and Space Technology, Scholastic's Weekly Reader, and daily papers from the Los Angeles Times to the Washington Times to local newspapers such as the Valley Times.

Passenger flight would be one of the last applications to become reality, but it is the one that the media and the public are most interested in. "The public gets very excited about space and air travel," said Carter. "To the general public, HyperSoar looks doable. The technology is nearly there, the concept is proven on paper. The thing now is to make it economically feasible to the defense and commercial communities so HyperSoar can get the funding it needs to take the next step in development."

Carter estimates that about $500 million would be needed to develop the technologies needed, and to build and test a 16-meter-long flyable unmanned prototype. Lawrence Livermore is positioned to help bring HyperSoar into reality because of its expertise in thermal protection materials, large-scale computational fluid dynamics, ultrahigh pressure testing design, and modeling the environmental effects of high-speed supersonic aircraft.

The question of funding aside, the day when passengers can hop a HyperSoar to London is still a way off.

"When most people hear about HyperSoar," Carter adds, "they immediately think big - building big airplanes to carry lots of passengers or cargo. But that's not economically feasible. I propose building small airplanes to justify the market and then building up from there, according to the need. That's how all the different flight technologies - airplanes, jets, helicopters - got started. It's the way that fledgling technologies like HyperSoar take wing."