When the X-15 rocket plane pilots got to the edge of space some 60 years ago, they carried two assumptions along with them: (1) that they would keep going, and (2) that their destiny lay above.
The idea of going into space had special appeal for the Air Force, which saw the “aerospace” domain as an extension of air operations closer to Earth. Airmen expected to fly in space and fight there.
It was understood, of course, that aerodynamics would not apply in deep space but hard vacuum was 1,000 miles out. That left plenty of margin to operate in low earth orbit, 60 to 300 miles above the ground.
It is technically feasible for manned spaceflight to become routine in a very few years.Gen. Thomas White, Air Force Chief of Staff in 1958
“It is technically feasible for manned spaceflight to become routine in a very few years,” said Gen. Thomas D. White, Air Force Chief of Staff in 1958. “In the not-too-distant future, efficient ballistic missiles and true piloted spacecraft will enter our forces as operational weapons.”
Testifying to Congress in 1959, White said, “We foresee that we are not only going to have manned bombers and missiles, but that eventually we will have manned space vehicles as combat weapons in the future.”
Early on, “the commonly held perception was that the airplane would itself evolve into a winged spaceship,” said Richard Hallion, former historian of the Air Force.
The fighting vehicle of preference would be the “spaceplane” or “aerospaceplane,” and USAF pursued the concept for the next half-century. Some variants had pilots; some did not. Their wake is littered with failures and cancellations.
As it turned out, the military has numerous vital interests in space: communications, navigation, surveillance, early warning, weather. So far, however, it has not been established that there is a combat mission in space, or a military mission that requires a human crew.
Except for the space shuttle, no spaceplane yet has broken away from the X (experimental) designation.
To the Moon Instead
The National Aeronautics and Space Act in 1958 directed that U.S. activities in space be “devoted to peaceful purposes” and created NASA, which—rather than the armed forces—would lead the leap into space.
Air Force’s “Man in Space Soonest” program was canceled, replaced by NASA’s Project Mercury. Also cut was the “Aerospaceplane”—all one word, no X designation—a loosely structured study conducted by the Air Force between 1957 and 1964 with 17 companies submitting proposals. Nothing came of it, and it ended when the funding was eliminated in 1964.
The Air Force managed to hang onto a few of its manned space programs, such as the X-20 Dyna-Soar spaceplane and the Manned Orbiting Laboratory, but these were overshadowed by and canceled in deference to NASA’s Mercury, Gemini, and Apollo projects.
The X-15 continued flying until 1968. It still holds the world’s record for highest speed ever reached by a manned, powered aircraft—4,520 mph, or about Mach 6.72. Its highest altitude was 67.0 miles. Astronaut wings were awarded for X-15 flights exceeding 50 miles.
After the moon landings, the manned space program lost focus. For NASA, the expedition into space was an end in itself and there was no real objectives other than science and exploration.
Emphasis eventually settled on the space shuttle, extremely useful but hardly the space combat vehicle that White had predicted.
The streamlined, elegant design of the X-15 was replaced with the boxy fuselage of the X-20 and the blunt configuration of various “lifting bodies,” wingless spacecraft that could glide back from orbit and land on a runway. Aerodynamic lift, needed for flight in the atmosphere, was provided by the shape of the vehicle body itself.
One such was the manned X-24. It was carried aloft under the wing of a modified B-52, dropped off, and climbed from there on its own. It made 28 powered flights between 1969 and 1970, reaching Mach 1.6 and an altitude of 71,400 feet.
The Air Force had high hopes for the X-20 Dyna-Soar, which was to be launched atop a Titan missile but returned from orbit to an unpowered landing by a gliding style called “dynamic soaring.” The X-20 carried a single pilot in a forward cabin and was intended to perform multiple missions, including research, reconnaissance, and weapons delivery. It was envisioned as flying across the Soviet Union at an altitude of 50 miles, providing intelligence imagery faster and with better resolution than was possible from satellites.
In 1962, Secretary of Defense Robert S. McNamara stripped the X-20 program of weapons capability. Dyna-Soar never flew. It was canceled outright in 1964 before the first spacecraft could be built.
About the same time, the Air Force introduced the high-flying SR-71 Blackbird, with a design every bit at sleek as the X-15. However, it was a reconnaissance airplane, not a spaceplane. Its maximum speed was Mach 3.2 and its best altitude was 16.4 miles.
In 1969, the Air Force and NASA agreed to develop a reusable vehicle that would meet both civilian and military space requirements. This was the space shuttle. It was of enormous value, but it was basically a truck. It did not inspire visions of streaking across space on missions of glory. Nor was speed the primary consideration.
Between its first flight in 1981 and the last in 2011, the shuttle flew 135 missions in space, transporting people and cargo into orbit, launching and recovering satellites, and enabling access to the International Space Station.
The crown jewel of spaceplane concepts is the “aerospaceplane,” which takes off from a runway on Earth, reaches orbit in a single stage, flies through the transatmosphere at hypersonic speed—Mach 5 or faster—reenters the atmosphere, performs a mission, reenters space, and returns to land at a runway on Earth.
The most ambitious such program was the X-30 National Aerospace Plane, proposed by the Air Force in 1986. An Air Force Magazine cover from that year depicts the X-30 boiling up from a runway and headed straight for space. “As planned, the aerospaceplane will be capable of flight in two modes—single-stage into low-Earth orbit and hypersonic (Mach 12 to Mach 25) cruising in the transatmosphere at altitudes between 100,000 feet and 350,000 feet,” the magazine said.
President Ronald Reagan was a strong supporter. In his 1986 State of the Union address, he predicted “a new Orient Express that could, by the end of the next decade, take off from Dulles Airport and accelerate up to 25 times the speed of sound, attaining low-Earth orbit or flying to Tokyo within two hours.”
The expectation was that the X-20 might perform roles that varied from carrying satellites into orbit to reconnaissance, global interdiction, and interception of attacking forces in space and in the air.
“It offers strategic force survivability,” said Gen. Lawrence A. Skantze, commander of Air Force Systems Command. “A fleet could sit alert like B-52s. We’re talking about the speed of response of an ICBM and the flexibility and reliability of a bomber, packaged together into 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.”
By the 1990s, though, the X-30 was behind schedule and 500 percent over budget. No compelling military requirement for it had been established, and budget cuts forced the termination of the program in 1994.
“Military Man” in Space
The phrase “military man in space” was rendered obsolete in the United States in 1983 when Sally Ride flew on a Shuttle crew. The Russians had sent women in space even earlier. Nevertheless, the term continued in use for some time.
A “Military Man in Space” program was introduced in 1985 at the behest of Air Force Undersecretary (soon to be Secretary) Edward Aldridge. He acknowledged that efforts thus far had failed to identify a mission for military man in space, but he was determined to try again.
Studies and analyses over the next few years did not find such a requirement, and the program faded away in 1991. Uncrewed platforms did the job better and more reliably.
“Just look at the nature of things we do in space—communications, surveillance, warning systems, navigation. We don’t use man for most of those things down on Earth, so why would we put man in space to do them?” asked Gen Donald Kutyna, commander of U.S. Space Command in 1990.
Kutyna noted that a great many satellites are at 22,300 miles distant, in geosynchronous orbits, and that it would cost far more to build the orbital transfer vehicles needed to “take Mr. Goodwrench out to those orbits’ than it would to ‘build reliability into the vehicles [satellites] in the beginning. Every study we have ever done tells us to put the reliability into the vehicles, because we probably wouldn’t be able to fix them once we got out there anyway.”
In the 1990s, NASA and the Air Force sponsored research on a reusable launch vehicle, the X-33 VentureStar. It was to take off vertically and land horizontally, but no firm decision was made about whether there would be a pilot. Beset by delays and technical problems, the X-33 was canceled in 2001.
Hypersonics and Scramjets
Research into hypersonics—defined by scientific convention as movement at Mach 5 or higher—began in the 1950s. Speed was of inherent military interest and value, not only in aircraft and spacecraft but also in missiles and weapons.
“A hypersonic missile enables the seizure of opportunity against distant and perhaps fleeing targets,” said historian Hallion. “In one minute, at Mach 6.5, a hypersonic missile flies approximately 73 miles, roughly eight times further than a conventional subsonic cruise missile. Thus in 10 minutes it can engage and destroy a target in excess of 700 miles from its launch point.”
Hypersonics evolved toward new possibilities with the emergence of the scramjet, or supersonic-combustion ramjet, in the early 2000s.
Unlike a standard ramjet—in which the air intake reduces the flow before mixing it with fuel to ignite—a scramjet gulps in air at full supersonic strength. The difficulty of combustion, Hallion says, has been compared to “lighting a match in a hurricane.”
Whereas ramjets operate from Mach 3 to Mach 5, scramjets enable flight at Mach 12 to Mach 15. A boost from an initial power source is necessary to accelerate the vehicle to supersonic speed, where the scramjet kicks in.
In 2004, NASA’s X-43A technology demonstrator made aviation history with two successful flights of a scramjet aircraft at hypersonic speeds. The X-43A was launched off the coast of California by a B-52 and boosted further by a Pegasus rocket before the scramjet took over. It reached Mach 6.6 on the first flight, and Mach 9.6—a speed record that still stands—on the second.
The scramjet X-51A Waverider, a collaborative effort of the Defense Advanced Research Projects Agency and the Air Force in 2010, was likewise launched by a B-52 above the Pacific and traveled more than 230 nautical miles in just over six minutes, reaching Mach 5.1 before crashing into the ocean.
True believers have not given up entirely on the notion of an aerospaceplane. In 2003, DARPA and the Air Force began taking proposals for the HTV-3X Blackswift “hypersonic cruise vehicle.” It was to be a fighter-sized aircraft—no pilot aboard—that would take off from a runway and fly at speeds up to Mach 15. It did not receive the necessary funding and was canceled in 2008.
The Department of Defense has given hypersonics a high priority among critical modernization technologies. Extensive prototype testing of hypersonic missiles is underway in crash programs by all of the services to catch and surpass Russia and China, who have a head start. The Russians claim that their Avangard hypersonic missile can zoom at Mach 27.
The U.S. prototypes are mostly boost-glide systems rather than scramjets. A rocket boosts the weapon to hypersonic velocity at high altitude, after which it uses the speed of its gliding descent to hit and destroy targets. The advantage is not only in time alone but also the range that such a vehicle can achieve.
More reliance on scramjets is coming. “I entered this job thinking scramjet would probably be a step behind boost-glide, and I’m delighted to say that I was wrong. Scramjet is more mature and ready to go than I originally thought,” USAF acquisition chief Will Roper said in April.
Repeatedly over the past several years, the Air Force has expressed its commitment to hypersonics, particularly in weapons. In February 2020, USAF announced that it would proceed with development of the Air-launched Rapid Response Weapon (ARRW). It will be carried by a bomber or fighter to altitude, where the glide body will detach and be accelerated to hypersonic speed by a booster.
Spaceplane development continues, although at a lesser priority, and the lineup of contenders has narrowed. In January, DARPA scrapped its XS-1 Phantom Express, an experimental spaceplane program in progress since 2013 as an effort to develop a reusable launcher to put medium-sized payloads into low-Earth orbit.
The XS-1 was to be an aircraft-like plane, taking off vertically with an expendable upper stage mounted on top of the fuselage and carrying the payload. After releasing the upper stage, the suborbital vehicle was to glide back to a runway landing. It was intended to fly a suborbital trajectory of about Mach 10.
The X-37 in Orbit
Current attention to spaceplanes is focused on the Air Force’s X-37B “Orbital Test Vehicle.” It began in 1998 as a NASA technology demonstration, transferred to DARPA in 2004, and was taken over in 2006 by the Air Force, which developed its own variant, the unpiloted X-37B.
The X-37B is launched vertically into low-Earth orbit by an Atlas V rocket, returns to the atmosphere on command from the ground, and lands horizontally on a runway.
USAF has two X-37B research vehicles, which between them have flown five missions in space since 2010. The spacecraft is shrouded in secrecy. The most astounding known feature is the duration of its flights.
At first launch in 2010, the Air Force forecast an on-orbit time of 270 days. Four of the five missions so far have done better than that. In October 2019, the X-37B landed at the Kennedy Space Center shuttle facility in Florida after a record-breaking 780 days on orbit. The flight has been nominated for the Collier Trophy, which recognizes great aerospace achievements.
The X-37B has been called “Space Shuttle Jr.” Indeed, it does resemble the shuttle, although much smaller, and with one rocket bell instead of the shuttle’s three. Two X-37Bs would fit inside the shuttle’s huge payload bay. Their own cargo bays are about the size of a pickup truck bed.
USAF says little about the X-37B’s purpose except that it is “testing out technologies for future spacecraft.” Officials have denied that it is a weapon designed to attack or capture enemy satellites.
Last year, former Secretary of the Air Force Heather Wilson said the X-37B “can do an orbit that looks like an egg, and when it’s close to the Earth, it’s close enough to the atmosphere to turn where it is, … which means our adversaries don’t know—and that happens on the far side of the Earth from our adversaries—where it’s going to come up next.”
Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics added that, “The dip into the atmosphere causes a change in the timing of when it comes up next overhead. So [trackers’] predictions are off and [they] have to search for it all over again.”
Since the creation of the U.S. Space Force, there has been speculation that the X-37B and the overall domain of spaceplanes will be transferred to the new service, but there has been no official indication of that thus far.