FROM the doodlings of daVinci 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 aviatory 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 could fly efficiently, economically, and cleanly. Flying at Mach 10 (3 kilometers per second), HyperSoar could reach any point on the globe within two hours. (The fastest military plane, the SR-71, flies between Mach 3 and Mach 4, while the commercial Concorde only reaches 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. "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 air-breathing, rocket-based, combined-cycle engines, it would ascend to 40 kilometers-at the outer limit of Earth's atmosphere. Once there, its engines would be turned off, and (as shown in the figure below) it would coast up to a high point of 60 kilometers before beginning to fall back down to about 35 kilometers-well inside the atmosphere's upper level. As it descends into denser air, the aircraft would be pushed up by the increased aerodynamic lift. The engines would 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. Inclusive of the time taken and distances covered by 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, and to travel from Los Angeles to New York (3,978 kilometers) would involve about 5 skips and take 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 solve a critical problem that plagues other hypersonic aircraft designs-heat.

Beating the Heat Any object-airplane, spacecraft, asteroid-speeding through the atmosphere 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," Carter said. "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 Espress 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 planes of similar size and mass. As a freighter, it could make four round-trips to Tokyo daily versus one or less for today's aircraft. This speed would be a boon to the $4-billion-per-year commercial intercontinental package delivery market. "The speed of today's aircraft has limited the growth of this market," says Carter. "The express delivery industry requires central intracontinental hubs that are about two hours' flying time apart. Current technology allows express mail, for instance, to move between these hubs in close to that time. Now, imagine the possibilities if you could fly between Memphis and Singapore in close to two hours." Carter estimates that a HyperSoar aircraft flying express mail between Los Angeles and Tokyo could generate ten times the daily revenue of a similar-size subsonic cargo plane. As a military aircraft, a HyperSoar bomber the size of an F-22 could take off from the U.S. and deliver its payload from an altitude and at a speed that would defy all current defensive measures. It could then return directly to the continental U.S. without refueling and without the need to land at forward bases on foreign soil. HyperSoar could also be employed as the first stage of a two-stage-to-orbit space launch system. This approach would allow approximately twice the payload-to-orbit as today's expendable launch systems for a given gross takeoff weight. At the high point of its skip, HyperSoar could eject an upper-stage vehicle and its payload into low-Earth orbit. A larger HyperSoar vehicle, the size of a Boeing 777 for example, could handle a 13,700-kilogram payload in addition to the weight of a typical second-stage launcher. At a 255,000-kilogram gross vehicle weight, the HyperSoar would weigh about half as much as the largest Ariane 4 expendable launch vehicle but could carry about 40 percent more payload. Of course, these lower weight-to-payload requirements mean that 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 STRATCOM (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 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 ways off. "When most people hear about HyperSoar," Carter added, "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." -Ann Parker

Key Words: HyperSoar, hypersonic aircraft.

For further information contact Preston Carter (925) 423-8293 (carter17@llnl.gov).