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A spaceplane has been part of the Air Force's long-range vision for more than 40 years. Advocates say a reusable spaceplane could cut launch costs from $10,000 per pound of cargo to $1,000 per pound and give the Air Force much greater flexibility in access to space, whether for maintaining satellites or performing other missions.

It would also provide the ultimate counter to any adversary's anti-access strategies; a spaceplane that can fly at Mach 25, reach orbit, and return to Earth would be virtually impossible to stop before reaching its objective.

But today, there is no single "spaceplane" on the drawing boards. Several experimental vehicles are seeking to demonstrate the technologies needed for a spaceplane. A December 2000 report from the Air Force's Scientific Advisory Board laid it out: "If the Air Force vision of 'controlling and exploiting the full aerospace continuum' is to become reality, the Air Force needs a comprehensive plan for hypersonics."

Yet the Air Force has been stymied in its efforts to get Washington behind a stated requirement for a spaceplane or to fund the extensive research that is still needed to make the concept a reality. The recent demise of the X-33 spaceplane project signaled that once again, the technology hurdle is high and the gap between dollars and rhetoric is deep.

Ideas for a spaceplane date back to German research on rocketry before World War II. In the Air Force, a reusable spaceplane has long been part of the vision for full control and exploitation of air and space.

Schriever's Vision

In 1962, Gen. Bernard A. Schriever described a set of requirements for space capabilities that included the ability to orbit, maneuver, rendezvous, de-orbit, re-enter, and land on a routine basis. Today, USAF is still at least a decade away from acquiring a reusable spaceplane that can do the jobs Schriever described.

Technology hurdles remain at the heart of the issue. Hypersonic flight-defined as flying faster than Mach 5-began to tantalize aerospace engineers in the 1950s. One early success was the North American X-15, tested at speeds up to Mach 6.7 in the 1960s. But for the most part, programs dealing in hypersonics and reusable spaceflight made only limited progress. One such was the Boeing X-20 Dyna-Soar, a boost-glide vehicle designed to become a manned, orbital plane. The Air Force funded it in 1957, but Secretary of Defense Robert S. McNamara canceled the X-20 in 1963, and Phase 1 of the hypersonic spaceplane era was over.

Dyna-Soar and other programs contributed to the manned space shuttle program. NASA's space shuttle first flew in April 1981 and has logged more than 100 successful missions, sometimes flying on a monthly basis. Still, the shuttle's need for expendable tanks to help it reach orbit and the continued high cost of each launch differed from the concept of a true spaceplane. Better access to space continued to be a driving issue.

In 1986, President Reagan reinvigorated the idea of an airplane-like transatmospheric spaceplane. In 1986, he called for "a new Orient Express" that could, by the end of the 1990s, "take off from Dulles Airport and accelerate up to 25 times the speed of sound." In Reagan's concept the transatmospheric plane could attain low Earth orbit or stay in the atmosphere, "flying to Tokyo within two hours."

Behind Reagan's sensational announcement was hope for a technological breakthrough in the field of hypersonics. Research from a Defense Advanced Research Projects Agency-funded secret program called Copper Canyon suggested that active thermal management could boost the power of a scramjet engine. Instead of succumbing to a heat barrier around Mach 8, the friction from the atmospheric drag would be used as part of a system to superheat hydrogen fuel then inject it into a scramjet engine. Using this technique, a spaceplane might overcome the thermal drag barrier by dissipating heat, while using the energy to boost engine performance.

As a result, the National Aerospace Plane was to be a revolutionary advance: a transatmospheric craft that would provide cheaper space launch and the ability to exploit space in military operations. Plans called for NASP to fly as a single stage to low Earth orbit and to cruise at hypersonic speeds of Mach 12 to Mach 25 in the transatmosphere-between the altitudes of 100,000 to 350,000 feet.

With the advent of NASP, the spaceplane concept branched into two roles.

First, a reusable spaceplane might replace the space shuttle as a launch platform carrying heavy payloads for customers like the Strategic Defense Initiative Organization, which contributed heavily to NASP research funding. Air Force Brig. Gen. Kenneth E. Staten, NASP program manager, said in 1986 that NASP might be able to deliver payloads to orbit for "between one percent and 25 percent of the expense of doing it with the shuttle."

Second, for the Air Force, NASP could also be a lightning-fast bomber. Gen. Lawrence A. Skantze in 1985, as commander of Air Force Systems Command, said NASP might have "the speed of response of an ICBM and the flexibility and reliability of a bomber, packaged together in a plane that can scramble, get into orbit, and change orbit so [that] the Soviets can't get a reading accurate enough to shoot at it." As a satellite truck and a strike platform, the spaceplane would be a revolutionary leap.

No "Golden Mission"

However, research on NASP stalled when it failed to meet performance goals. By the early 1990s, NASP was projected to be a decade late and 500 percent over budget. NASP was "fully capable of hypersonic flight," according to the Science Advisory Board, but could not reach orbital velocity. Advanced hypersonic technology remained out of reach. "On the basis of current knowledge, it is hard to defend previous DOD plans for NASP," concluded a Rand report in 1989. "No compelling 'golden mission' exists for NASP."

Cuts in the defense budget and the end of the Cold War sealed the fate of NASP and the program was canceled in 1994. "These are exciting ideas," said Martin Faga, assistant secretary of the Air Force for space at the time, "but they are not ready for commitment."

Even before the death of NASP, researchers were focusing on a more cautious approach that divided up the technology hurdles of hypersonic flight and reusable systems.

The next "spaceplane experimental" was an early success that raised hopes for both military and commercial applications for a spaceplane. McDonnell Douglas won a contract in 1991 to build what became the DC-X Delta Clipper. This single-stage-to-orbit vehicle grew out of an SDI requirement for a single-stage, reusable vehicle that could put Brilliant Pebbles, a component of a ballistic missile defense system, into orbit at a reasonable price. It was managed by the Air Force for SDIO, later the Ballistic Missile Defense Organization.

Although the program was handed off to NASA, the Delta Clipper stirred Air Force thinking on the possible uses of a spaceplane. The commercial potential and simplicity of the program seemed to foreshadow a new era when commercial launch demand would help fund spaceplane technologies.

The Delta Clipper was not a hypersonic scramjet spaceplane but a single-stage rocket with advanced lightweight materials and directional control. Its charter was to demonstrate the ability to take off and land vertically, using controlled, rocket-powered flight. In its full concept, the Clipper would be a reusable vehicle that could be launched and recovered at the same site by a small ground control team. Maintenance would be streamlined, leading to lower operating and support costs that would bring about a dramatic reduction in the price of launching payloads into orbit. The subscale demonstrator and an advanced version, the DC-XA, successfully completed a series of flights in the period 1993-96, demonstrating control and maneuverability at the White Sands Missile Range in New Mexico.

Then trouble struck. During landing on July 31, 1996, a landing strut failed to extend. The Clipper tipped over and its liquid oxygen tank exploded, causing a fire that destroyed the vehicle. "Like any good experimental vehicle, the DC-XA flew until it was destroyed," commented McDonnell Douglas. "We will always be impressed by the lessons this little rocket taught us."

NASA's X-Planes

During the 1990s, NASA took the lead in research on spaceplane technologies. No single program was attempting to pair single-stage launch to orbit with hypersonic transatmospheric flight. Instead, a series of X-planes sought to test various aspects of spaceplane operations, ranging from thermal material to advanced propulsion to autonomous landing under different weather conditions, but not a full-scale demonstration.

All programs shared the same philosophy: rapid development of prototypes, with no more than a few years passing between contract award and demonstration. Some, like Boeing's X-37 and X-40, were demonstrators for a vehicle that would be ferried into orbit, operated by its own rocket engine, then would return to land on a runway. In contrast, Lockheed Martin's X-33 was designed to take off vertically, fly a suborbital path, and then land horizontally at a US base. Orbital Sciences' X-34 was a rocketplane designed to be launched from a jetliner, reach Mach 8, then return and land on a runway. X-43A, also from Orbital Sciences, was built to ride into the air on a B-52 bomber, separate from the bomber, then from a boost rocket, and fly a Mach 10 trajectory before crash-landing in the Pacific.

The Air Force was a junior partner in deals with NASA and aerospace industrial firms to fund these X-planes. For example, the X-37 program was financed by roughly $75 million from Boeing, $72 million from NASA, and just $16 million from the Air Force.

Despite its limited financial participation, the Air Force closely watched the X-planes. X-33, in particular, looked like it could push the envelope on spaceplane design and give the Air Force a chance to evaluate suborbital space operations concepts. X-33's linear aerospike engines were a significant evolution from the bell-shaped engines of the space shuttle program. The linear aerospike was designed to increase power and, more importantly, perform with maximum efficiency at a greater range of altitudes.

Test of the linear aerospike engines proceeded smoothly through a series of test runs in 2000. The aerospike engine project manager, Donald Chenevert, praised the performance of the engines, noting that "few new, much less innovative, engines even get to full power in so few tests," but with X-33's engines, "we met or exceeded a number of significant objectives during the first phase of the program."

X-33's big test was to be a series of suborbital "hops" where the demonstrator would take off, fly to another point, and land. But the hops never took place. X-33 suffered a setback in a November 1999 test, when the composite material layers of a liquid hydrogen fuel tank peeled apart during a stress test. An agreement signed in the fall of 2000 kept work going on X-33 until March 2001. However, the delays caused by the fuel tank problems slowed work on X-33, so it never picked up momentum to become a priority for NASA, where many regarded the single-stage-to-orbit concept as too difficult. NASA canceled the $1.3 billion program in March 2001. "We are going to take off our silk scarves and retire them for a while," said Daniel Goldin, NASA administrator, in a Washington Post interview.

NASA's cancellation of X-33 set up the first major challenge for the Air Force's decade-long practice of letting NASA take the lead in spaceplane development. The commander of Air Force Space Command, Gen. Ralph E. Eberhart, wrote to Goldin and said that the Air Force wanted to review the situation and perhaps take over support of the X-33 program. Estimates for completing the prototype X-33 ran to about $400 million, while developing and testing a full-scale spaceplane force might cost between $3 billion and $7 billion by 2015.

However, Samuel L. Venneri, a top NASA technology official, told the Washington Post, "We're not interested in spending any additional money out of our technology program, if it is not associated with a strong commitment from the Air Force."

For the Air Force, X-33 raised important questions. To begin with, as the SAB concluded, USAF needed a reusable spaceplane because it was "unlikely that the Air Force will ever be able to achieve an aggressive aerospace force vision by relying on [expendable launch vehicles] for its access to space."

Indeed, spaceplane concepts had become a central part of the Air Force's vision of its future. Long-range plans written by Air Force Space Command and by the Directorate of Strategic Planning at USAF headquarters both called for a new generation of reusable space vehicles to provide space control, including assured launch capabilities, surveillance, protection of assets in space, and the prevention of hostile operations. If necessary, space control would extend to negation: using military force against an enemy's space capability.

Air Force plans envisaged acquisition of a Space Operating Vehicle and a Space Maneuvering Vehicle. The Space Operating Vehicle would be a single-stage-to-orbit vehicle that could launch to low Earth orbit or employ a second, pop-up stage to put payloads into medium Earth orbit or beyond. The SOV would launch vertically on demand, deliver payloads or conduct surveillance or any other type of combat support mission, and return to Earth and land horizontally. The Space Maneuvering Vehicle would be an on-orbit vehicle that might perform missions after being launched by a reusable launch vehicle or a Space Operating Vehicle. The Space Maneuvering Vehicle could act as a temporary satellite itself or maneuver to perform missions such as deploying or retrieving satellites. According to USAF officials, it would stay in orbit for four to six months, carrying anything from weapons to replacement satellites.

X-33 tested some of the technologies that might be used in a follow-on Space Operating Vehicle, and X-37 (X-40A) prototyped some of the concepts for a Space Maneuvering Vehicle.

Serious Ops ... On Demand

However, the planned X-33 demonstrations also rekindled Air Force interest in the spaceplane's potential. Senior leaders saw in it the capability "to do serious operations in space on demand-from space control to space operations," said retired Lt. Gen. Marvin R. Esmond, a former deputy chief of staff for air and space operations. A suborbital craft-flying at Mach 10 or 12 instead of the full Mach 25 needed to reach orbit-could evolve into the next long-range strike aircraft. As the SAB said, "The pressing utility for a hypersonic aircraft is rapid time-to-target, the survivability provided by increased speed, some loiter and search capability, and increased weapon penetration and kill capability."

If the X-33 suborbital hops worked, it could have demonstrated the concept of operations for a suborbital, hypersonic strike platform which would make the most of swiftness and increased survivability and perhaps replace long-range bombers. For example, a suborbital vehicle could launch rapidly, reach speeds high enough to travel to the upper edges of the atmosphere, then launch weapons, all while remaining over the sovereign territory of the United States or open oceans.

In that sense, the spaceplane would be the ultimate anti-access weapon, requiring no diplomatic overflight clearances and no serious threat of opposition. A spaceplane could travel so high and fast that it would be well beyond the tracking abilities of current surface-to-air missiles. Hypersonic velocity would increase the depth of weapons impact, enhancing the Air Force's capabilities for attacking hardened and deeply buried targets. If conflict arose, the spaceplane could "send a message right from Vandenberg [Air Force Base in California] in less than an hour," said Esmond. Compared with the B-2 bomber's average 17-hour one-way flight time to its targets during the Kosovo crisis, a suborbital strike craft would seem to be almost instantaneous. It would transform the aerospace force.

At the end of the summer, Air Force Space Command briefed Air Force Secretary James Roche on its $2 billion proposal to keep X-33 alive and to extend funding of Boeing's X-37 beyond 2002. "My feeling was, it's expensive, but you don't know until you try," Esmond said of X-33. "This had, to date, the best chance of success."

However, the plan fell victim to budget constraints. "Both programs have made significant contributions toward understanding achievable vehicle performance, cost, and integration issues and have provided valuable information on the dynamics of launching space vehicles," the Air Force said officially on Sept. 7, 2001. Neither X-33 nor X-37 provided "a level of military utility needed to continue development and funding by the Air Force."

The X-33 decision was a surprise, not the least because it came after the Air Force had declared a renewed focus on the development of military space power. A spaceplane with responsive capabilities to replenish satellites could be the most useful item in the inventory in the event of a "space Pearl Harbor" that takes out on-orbit systems.

Even in a less catastrophic scenario, a spaceplane seemed to offer the potential for real transformation of US forces over time. In the near term, X-33 "would have given us the vehicle behind which to have a serious discussion" on doctrinal and political aspects of joint space operations, explained Esmond. In the long term, spaceplanes serving as strike platforms could change the equation of US defense planning by making it possible to launch flexible, rapid strike missions from United States territory.

"I think it's truly the answer [for] full global reach, global power," said Esmond. A fully developed, suborbital, hypersonic spaceplane could ultimately "stand on alert and provide a deterrent force. Then you could shape the Air Force to be truly expeditionary and take care of smaller-scale contingencies," he added.

Critics contend that today's fighters and bombers can, for vastly less cost, do the job of a fleet of spaceplanes. True, the initial cost of fielding a spaceplane would be extremely high. However, revolutions do not come cheap. Total investment to date in stealth aircraft programs exceeds $100 billion.

More to the point, said one Air Force official, is this question: "How long can we penetrate [enemy air defenses] with stealth? It won't last for the next 50 years. Why penetrate with a bomber when the weapon could be delivered from a suborbital spacecraft?" A spaceplane could carry out immediate attack operations against targets harboring weapons of mass destruction, for example.

The Saga Continues

As illustrated by the demise of the X-33 program, the path toward a spaceplane remains difficult, and constant demands on USAF aerospace power will make it hard to find significant streams of investment needed to develop the technologies. But given its central role in the Air Force vision, the spaceplane concept is not finished yet. "With Rumsfeld, who understands space, and this Administration, which is excited about transformation and becoming more efficient, we are closer than ever," Esmond said.

The choice about when to push hard for a reusable, hypersonic spaceplane cannot be put off indefinitely.

"As with air operations, the Air Force must take steps to create a culture within the service dedicated to developing new space system concepts, doctrine, and operational capabilities," said the Rumsfeld Space Commission Report in January 2001. The Air Force's Scientific Advisory Board concluded in its December 2000 report that the demand for reusable space access would grow as USAF became "a true aerospace force."

Even so, the bulk of the technicians who have experience in hypersonic experimentation-not just theory-is aging rapidly. According to the Scientific Advisory Board, "The hypersonics workforce is at a crossroads," and "the majority of its members will retire in the next five to 10 years." Foreign competition may also emerge. Russia, China, France, India, Germany, and several other nations are working diligently on hypersonics. In fact, the Air Force Research Lab, headquartered at Wright-Patterson AFB, Ohio, and its Office of Scientific Research in Arlington, Va., have funded joint research with Russian agencies. If their research bears fruit, the United States could find itself behind the pack and on the wrong side of an asymmetric capability.

Most of all, the spaceplane remains a good fulfillment of long-term Air Force requirements. Expendable launch vehicles will not meet future demand for space access. Even a suborbital spaceplane could also serve the demands of expeditionary operations and homeland security more efficiently in several roles. According to the SAB, reusable launch vehicles like the X-33 concept "offer immense potential to meet all the requirements of the future US aerospace force." Combining hypersonics and a reliable, reusable platform is the path to dramatic improvement in the Air Force's core competencies.

With a suborbital hypersonic craft or one that can reach orbit, USAF would gain a rapid-response capability of phenomenal power, free of much of the infrastructure needed for expeditionary warfare. To be sure, an Air Force base would have to be heavily modified with a new ground support structure to accommodate spaceplanes, but the asymmetric capabilities of a spaceplane would trump all remnants of 20th century warfare.