The Air Force’s Secretive Robot Space Plane Is a Mashup of Old and New
X-37B builds on Space Shuttle, X-20, old Russian engines
High overhead, one of the U.S. Air Force's two known drone space planes is nearing a year in orbit, doing … well, who knows. Run by the Air Force's Rapid Capabilities Office, which surely has the boldest motto in government—Opus Dei Cum Pecunia Alienum Efficemus, Latin for “doing God’s work with other people’s money”—the X-37B provokes paroxysms of speculation.
But the concept, components and capabilities of the tiny space plane date back to the glorious dawn of the Space Age. And history may offer clues to the X-37B’s missions.
The X-37B is like a self-driving space truck with no crew cab. In fact, at 5.5 tons and 29 feet in length, it’s about the same size as an actual delivery truck. Without the need to protect and house a human crew, the X-37B compresses the Space Shuttle's characteristics into a spacecraft one-fourth the size.
Just what a difference this makes can be seen in videos of the small space plane landing at Vandenberg Air Force Base. Its approach to the runway is steeper and faster, and its stopping distance shorter than nearly any manned aircraft.
The infrared cameras recording the landing captured the advanced state of the X-37B's thermal protection. Only a few minutes after a meteoric re-entry over the Pacific, the nose and belly of the drone glowed fire-orange in infrared while the top and sides were already a cool blue.
The orange visibly fades in the footage as the spacecraft coasts to a stop in the early California dawn, and the visible light video shows no scorching or discoloration on its fuselage.
The X-37B’s lightness and strength are achieved through special materials a generation or more removed from earlier substances. “Toughened unipiece fibrous re-inforced oxidation-resistant composite,” or TUFROC, cover the wings’ leading edges. Pieces of “toughened unipiece fibrous insulation,” or TUFI, have taken the place of the Space Shuttle's fragile, problematic ceramic tiles, while also reducing weight.
The X-37B’s protective layer is much more rugged than the Space Shuttle’s. It’s less likely to suffer the kind of damage that led to Columbia’s demise, and much harder to damage deliberately. Strong, lightweight composites also make up much of the space plane’s structure and components.
Like the F-22 and F-35 stealth fighters, the X-37B sports two canted “ruddervator” control surfaces, a layout that fits more compactly within payload shrouds atop rockets. A speed brake like that on a fighter jet pops up between the control surfaces upon touchdown. No more sexy drag ‘chute.
Hydraulic control systems requiring gobs of on-board power have been replaced with all-electric controls. Juice comes from lithium-ion batteries recharged in orbit by a large gallium-arsenide solar array. Gallium arsenide was once the hot technology back in the 1980s; solar arrays made of the stuff are more efficient than silicon-based ones, and are also more resistant to radiation damage.
The drone carries a large supply of the tried-and-true storable hypergolic propellants hydrazine and nitrogen tetroxide. “Hypergolic” is a $10 word for stuff that ignites spontaneously upon contact, no spark needed. These chemicals have been used for decades because they’re potent, reliable and have long shelf lives. They don’t boil away like liquid oxygen or hydrogen. They’re also as toxic as Congress, which is why you see ground crews approach the stopped vehicle in moonsuits.
When large propellant loads are combined with high electrical capacity and divorced from the need to keep humans alive, an Earth-orbiting spacecraft can achieve amazing endurance. The second X-37B test flight logged 435 days in space, during which the vehicle made two considerable changes to its orbit while maintaining precise altitude control.
But even these technical advances—new composites, fly-by-wire controls and robust electrical power—pale in comparison to gains delivered by the tremendous growth of computing power. The Space Shuttle’s computers, even after their final upgrade, had only a fraction of the power of an iPhone.
The systems aboard the X-37B are capable of controlling the entire de-orbit burn, re-entry and landing autonomously. Such automated descents and re-entries have existed since the dawn of the Space Age, but they were used mostly for ballistic capsules, not winged things.
No school like old school
The components of the space plane’s booster herald from the heroic age of rocketry of the 1950s and ‘60s. Though the Atlas V booster core shares only its name with America’s first ICBM, its Centaur second stage is the current model of the world’s first rocket stage powered by liquid hydrogen.
The Altas-Centaur combination has a storied history that includes lofting early communication satellites and the lunar Surveyor probes. A whopping big launch-pad explosion of an Atlas-Centaur in 1965 gave NASA an idea of how bad a Saturn V explosion could be.
The first-stage engines have the strangest pedigree. The Russian RD-180 engines, imported and prepared by Aerojet Rocketdyne, are direct descendants of the high-performance closed-cycle liquid-fuel engines originally developed for the Soviet Union’s moon shot.
The N-1 rocket’s first stage ran 30 NK-33 engines compared to the Saturn V first stage’s five F-1s; the Soviets couldn’t build engines as big as American ones, but their engines were more advanced, higher-performance designs thought impossible to build in the West.
When the Politburo cancelled the Soviet manned lunar program in 1974, the Americans had already won the game and left the field. Everything built for the huge project was ordered scrapped, including dozens of rocket engines.
The engines’ maker mothballed them in secret, and 20 years later offered them to astounded American engineers. Tests, negotiations and deals resulted in a steady supply of these advanced rocket engines from Russia to the U.S., despite occasional political grumblings.
The X-37B really descends not from the Space Shuttle, itself a compromise between NASA and the Air Force, but from the Air Force’s never-realized DynaSoar program, which ran from 1957 to 1963.
The X-20 DynaSoar was to be America’s first spaceship, a winged glider launched atop a ballistic missile and flown by an airman-astronaut. Itself directly descended from the World War II Saenger-Bredt “Silverbird” concept, the X-20 nearly made it into production before running afoul of Pentagon politics.
Planned missions for the DynaSoar included intercontinental manned bomber, orbital reconnaissance platform and space taxi. The space-bomber mission was problematic given America’s professed peaceful goals in space, but a highly maneuverable, unpredictable spy plane was appealing. As the program evolved, the X-20 would have become a mini-Shuttle, able to transport astronauts and cargo to space and back.
Mark Wade of Encyclopaedia Astronautica notes that “the Air Force was especially interested in exploring the possibilities of ‘synergistic’ orbital maneuver. This would involve the X-20 entering the upper atmosphere, and using its aerodynamic maneuverability to change the orbital plane.”
A powered variant of the Silverbird’s “skip-glide” technique, a synergistic maneuver could allow a space plane to radically change its orbital trajectory, greatly complicating enemy plans to avoid overflights or interceptions.
The drone’s mission control and basing were recently shifted to the Tonopah Test Range, where Sandia Labs tests the non-nuclear components of nuclear weapons — the bomb and missile casings, avionics and parachutes.
The X-37B could be a prompt precision strike platform. With its great orbital endurance the drone could fly quietly like a submerged ballistic missile submarine, ready to deliver its payload with little warning. But its 500-pound payload would need to be mighty small and powerful, and the target mighty important, to justify the use of such an expensive delivery system.
With no more cargo capacity than a compact pickup, it doesn’t seem like the X-37B could be much of a space truck, but the same miniaturizing factors at play in the space plane are driving satellite design. Nanosats and swarms are the the new thing in spaceflight, where constellations of small probes in formation flight can do what a bus-sized satellite once could.
One intriguing potential mission may be related to the Defense Advanced Research Project Agency’s Phoenix program, which seeks robotic techniques to capture, disassemble and repair satellites in orbit. Such operations are analogous to earlier manned repair missions such as those to the Hubble Space Telescope, but will be much more nimble. Of course, one nation’s roadside repair job is another’s carjacking, and that path flies into classified space.
The DynaSoar program was on track for initial flight tests when Defense Secetary McNamara killed the project in December 1963. Fifty years later, America is taking to the idea all over again.