In Search of Spaceplanes

Dec. 1, 2003

For decades, the Air Force has wanted spaceplanes—craft that can take off from a runway, fly at hypersonic speeds through the upper atmosphere, reach low Earth orbit, and return in a conventional fashion. Yet, the practical application of the concept kept being pushed into the future because of high development costs, inadequate technology, and the lack of a truly pressing mission that would demand such an asset.

That is about to change. There now is an urgent mission for the capabilities of a spaceplane—or something like it.

USAF needs the ability to swiftly hit fleeting targets anywhere on Earth shortly after an order is given to do so. The target might be a terrorist camp, a ballistic missile launch site, a chemical weapons factory, or a leadership target. It also may be deeply buried. A spaceplane would offer the means to get to a target rapidly, soar high above defenses, and deliver munitions that would fall at high velocity and plunge far below the Earth’s surface.

Fueling the new urgency to produce an operational spaceplane is the proliferation of theater ballistic missiles and weapons of mass destruction, plus the belief that these and other “anti-access” weapons will only become more widespread in the future. In the hands of terrorists or states that support them, such weapons could prevent the US from getting close enough to an enemy to conduct operations—a prospect that the national leadership no longer will accept.

Moreover, the last few armed conflicts have illustrated how the US has become highly dependent on space systems for its entire range of military operations. The need to obtain cheap, reliable, rapid access to space has never been greater.

“We have an important interest—and airmen have always had an important interest—in speed,” said Gen. Gregory S. Martin, head of Air Force Materiel Command and former commander of US Air Forces in Europe. A spaceplane is “an activity that I think it is important for us to pursue.”

US Strategic Command, the “owner” of military space operations and the global strike mission, has established the requirement for a spaceplane. This fall, the Air Force and the Defense Advanced Research Projects Agency began accepting industry proposals for a project that in 2025 would produce a spaceplane—one that may look much like the defunct National Aerospace Plane conceptualized in the 1980s.

To Mach 15

The new craft, which is described as a Hypersonic Cruise Vehicle (HCV), would be capable both of launching satellites and deploying weapons. Plans call for it to fly at speeds up to Mach 15 and carry a mix of weapons comparable to the load carried by one of today’s fighter aircraft. It would probably be flown by a crew, but it could be flown remotely as well.

“The ability to reach out and touch somebody with great precision … at very long range is … a very attractive attribute,” said Brig. Gen. Stephen M. Goldfein, the Air Force’s director of operational capability requirements. “We are looking at what industry can tell us about the art of the possible … with great interest,” Goldfein said in an August interview.

Beyond offering the advantage of extreme speed from point to point, an orbital or suborbital spaceplane would also obviate the need to obtain overflight permission from other countries.

USAF wants a system before 2025, however. As an interim measure, the service is developing a hypersonic glide munitions delivery system, known as the Common Aerospace Vehicle. It could be fielded within eight years. The CAV is part of a program dubbed “Falcon” (Force Application and Launch from Continental United States), which also includes development of a Small Launch Vehicle to carry the CAV to an orbit altitude, from which it would coast to its “pierce point” location—that is, the point and velocity at which the vehicle enters the atmosphere. Additionally, the SLV would provide a quick-reaction launch capability for small satellites.

The Air Force and DARPA are collaborating on Falcon and expect substantial assistance from NASA, particularly on propulsion and vehicle control concepts. The service last summer asked US industry to turn in proposals and concepts this fall.

USAF wants to build the means to attack any target on the globe within 12 hours of an order to do so. That requirement stems from an April 2003 Air Staff study titled “Long-Range Global Precision Engagement.” In it, the Air Force—working with the Joint Staff and Office of the Secretary of Defense—put strike capabilities into three categories: prompt global strike, prompt theater strike, and persistent area strike.

USAF believes the products of Falcon will fulfill—to a great degree—the prompt global strike element. The ability to conduct prompt global strike would dissuade or deter enemies because they would know that the US could “hold at risk or strike high-value targets anytime and anywhere on the globe,” said the study. Such a technology would also eliminate the need for intratheater buildup before conducting a strike.

The study set the following parameters for the prompt global strike capability:

  • Availability on short notice.
  • Great standoff range with penetrating weapons.
  • Capacity for 200 strike effects within 12 hours.
  • Range of at least 3,400 miles

The study authors, despite seeing some partial workarounds, urged rapid development of hypersonic vehicles—be they new missiles or aircraft.

“In particular,” the study authors wrote, “the development of high supersonic/hypersonic weapons and delivery platforms significantly enables global strike from significant ranges and reduces the risks associated with forward basing.”

Additionally, they said, the reusability of these platforms increases their value in a variety of threat scenarios and makes them more cost effective.

The study concluded that, by 2015, even stealthy new systems such as the F/A-22 and F-35 fighters may lack the range to attack critical targets deep behind enemy lines in the early hours of a crisis. The ranges of the two fighters in the initial stages of a conflict would “not allow for penetration beyond approximately 400 nautical miles [460 miles] into an adversary’s battlespace without the use of external fuel tanks that may degrade some aircraft capabilities,” according to the study. Tankers that could refuel the two fighters would not be able to get close, if modern surface-to-air missiles were in the area.

The authors projected that some capability independent of forward bases will have to be in hand by 2015, if not sooner.

The Falcon program encompasses concepts that are near term—the Common Aerospace Vehicle and Small Launch Vehicle—and far term—the Hypersonic Cruise Vehicle.

Armed with 1,000 pounds of ordnance, the CAV hypersonic glide munitions dispenser could strike anywhere on an adversary’s territory. Descending from high altitude and at high speed, it would be extremely effective against hardened or deeply buried targets. Impact speeds “of approximately 4,000 feet per second” are expected, according to a USAF–DARPA summary.

CAV technology has been studied “since the mid-’90s,” USAF said. In that time, some workable concepts offered a downrange glide capability of nearly 3,500 miles. More advanced conceptual versions now promise ranges of up to 10,350 miles, but these enhanced systems would require significant technology development, “particularly in the areas of thermal protection and guidance, navigation, and control,” said the USAF–DARPA request for proposal.

Plans call for fitting the Common Aerospace Vehicle on the Small Launch Vehicle, which should be able to launch in less than 24 hours and, with the CAV as its payload, surge to 16 launches in 24 hours. It must lift a 200-to-2,200-pound payload for less than $7,500 per pound. Today’s benchmark is about $10,000 a pound.

The Air Force expects to have workable CAV and SLV systems by 2010.

The Falcon program is set up to use initial CAV and SLV work as well as NASA’s work on hypersonic technology in order to develop the Hypersonic Cruise Vehicle. This program is expected to culminate around 2025.

The Hypersonic Cruise Vehicle would be based on US territory. It would be reusable, having aircraft-like properties that enable it to take off from a conventional military runway. The system would:

  • Strike targets 10,350 miles away within two hours.
  • Carry a 12,000-pound payload.
  • Engage multiple, diverse, and widely dispersed targets.
  • Be retargetable and recallable.

The Hyper-X

NASA is about to resume testing of a vehicle that likely will be one of the Falcon program’s key technology pathfinders.

Under a program called Hyper-X, NASA will fly a miniature hypersonic vehicle up to Mach 10. The goal is to learn about the environment of very high speed and the performance of hypersonic engines. Such conditions can’t be duplicated in ground-based wind tunnels. The craft itself is called the X-43A. It has a hydrogen-based propulsion system.

David Reublish, deputy manager of NASA’s Next Generation Launch Technology Flight Vehicles and Systems Program Office, said NASA is “almost to the end” of its Hyper-X program, which was intended to be a seven-year technology project.

The initial program called for building three X-43A one-time-use vehicles, each about 12 feet long and mounted on a Pegasus booster. NASA uses the vehicles to explore ramjet and scramjet engine technologies at high Mach numbers. A booster problem caused the first vehicle test flight to fail. NASA scheduled the test flight of the second vehicle for this month. Plans called for it to fly at Mach 7 and then return to Earth. A third vehicle is slated to fly in the spring and reach Mach 10. The actual hypersonic segments of the flights will last only a few seconds, but they will provide mountains of valuable data, said Reublish. Completion of the third flight will mark the end of the first phase of Hyper-X.

Already under construction is the follow-on X-43C, developed in conjunction with the Air Force. Reublish said there would again be three vehicles. Each X-43C will be about 16 feet long and, like the A model, will ride to altitude on the front of a Pegasus rocket. The Pegasus will accelerate the X-43C to Mach 5. Then the vehicle will accelerate on its own power to Mach 7, where it will cruise. The first flight of the X-43C is expected in about 2007.

Unlike the X-43A, the X-43C will be fueled by JP-7 aviation gas. “The Air Force doesn’t want to build a new infrastructure to support hydrogen fuel,” Reublish said. Should the X-43C work, its derivatives could be refueled in flight—at fairly low speeds—by standard tankers.

After completing the X-43C, NASA will develop a 40-foot-long hypersonic vehicle called, for now, the Reusable Combined Cycle Flight Demonstrator. It will be carried aloft by a B-52H bomber. “The B-52 will get it to seven-tenths of Mach, then release it,” said Reublish. “The craft will accelerate to Mach 10, then land.”

Reublish explained that such air launches are used to cut costs. An air-launched craft can have lighter landing gear and smaller fuel tanks, he said. Development will cost about $1 billion. Giving the vehicle the ability to launch under its own power would increase the weight and boost the price tag to about $4 billion, he said, noting, “For every pound of weight you add, the cost grows geometrically.”

The demonstrator will feature engines of two kinds—the turbine for the thick air of the lower atmosphere and a scramjet for the thin air of the high atmosphere. It will be tricky, said Reublish, to accurately coordinate the closing of the turbine inlets with the opening of the scramjet inlets.

The project is not yet funded, but NASA and the Pentagon will undertake it jointly and fully expect to pursue it.

“NASA has a goal to develop a shuttle replacement by 2025,” Reublish explained. The pace of the program will make it possible to fabricate a test version of a full hypersonic craft around 2020 and an operational model in 2025. The vehicle would weigh about one million pounds, or one-fifth the weight of the space shuttle. It would have to be powered by hydrogen.

Because hypersonic vehicles can use the ambient oxygen in the atmosphere to burn fuel, they don’t need to carry along their own oxygen. This factor translates into tremendous weight savings.

Like an Airplane

NASA is also pursuing another spaceplane that would be airplane-like, taking off under its own power from a runway. Called Responsive Access Small Cargo Affordable Launch spaceplane (known as Rascal) it will be an 80,000-pound, winged reusable vehicle that could carry a 16,000-pound payload. The payload would consist of an internally carried two-stage rocket that could lift a 110-pound satellite into orbit.

In about a year, the US will begin construction on Rascal, which would fly to about 63,000 feet using four engines and then execute a sharp pull-up at supersonic speed. It then would release its payload—a satellite with an expendable booster. The booster would propel the satellite the rest of the way to its desired orbit. Meanwhile, Rascal would return to a runway landing.

Rascal’s four engines will probably be similar to the F100 power plants found on F-15 and F-16 fighters. At high temperatures, they would be cooled with water and liquid oxygen sprayed directly into the inlets. This technique improves thrust but keeps the engines at an acceptable temperature and provides oxygen for combustion. It is called mass injection precompressor cooling and was tried successfully in the 1950s. The technique would let the F100-type engines propel the craft to Mach 6.

Although both the Air Force and NASA see some urgency in developing hypersonic spaceplanes, there are valid cost and technology concerns, said Martin. The AFMC chief supports the concept of hypersonic vehicles, but he added, “We have to pursue it at the right pace.”