What Happened to SpaceShipTwo | Flying Magazine

What Happened to SpaceShipTwo

Redundancy ends with the pilot.

Technicalities SpaceShipTwo

Technicalities SpaceShipTwo

SpaceShipTwo

During 18 months in 2003 and 2004, SpaceShipOne, Scaled Composites' original air-launched spaceplane, made 14 free flights of which six were powered, the rest glides. Although SS1 was a novel design with an untried type of motor and was venturing into inhospitable territory last visited by the X-15 almost 50 years earlier, the privately funded program progressed with speed and smoothness that were a credit to Burt Rutan himself and to the talents of the team of engineers and pilots he had assembled.

Although a few potentially life-­threatening problems arose, the program ended without mishap and gained the $10 million Ansari XPrize, for which it was, in fact, the only realistic competitor. The reported program cost, met by Microsoft co-founder Paul Allen, was $25 million.

At Scaled Composites, the SS1 program went by the name Tier 1, the first of several "tiers" that represented, metaphorically, a staircase out of this world. Tier 1 meant suborbital flight just past the "edge of space," arbitrarily defined as 100 km or about 62 miles above the earth. Rutan had grander visions, however, and spoke of orbiting hotels and "affordable" tourist trips to the moon. Privately funded manned orbital flight was to be Tier 2, and everything beyond low Earth orbit was Tier 3.

The steps, however, were of different sizes. Tier 1 required speeds on the order of Mach 3, or almost 2,000 knots. Injecting something into low Earth orbit requires 17,000 mph (space travelers do not share our ridiculous infatuation with knots), and escape from the gravitational field of Earth requires 25,000 mph. Attaining those higher speeds is exponentially more difficult, as is decelerating from them while re-entering the atmosphere.

On the Tier 1 to Tier 3 scale, the step from the three-person SS1 to a larger copy carrying half a dozen paying passengers to the same height appears practically trivial. Tier 1b, commonly known as SpaceShipTwo, is a scaled-up and restyled version of SS1. The technology is proven. There are some significant changes, notably the switch from a high to a low wing and a much sleeker "look" — the watermelon-shaped SS1 was rather homely — but the essentials, including the so-called "hybrid" rocket engine and the novel "feather" system for dissipating energy during the descent, remain the same.

Nevertheless, SS2 development has dragged on for a decade, its slowness paradoxically underscored by Virgin Galactic founder Richard Branson's repeated public assurances that the first launch of would-be "space tourists" — 700 of whom had signed on at up to $250,000 apiece — was right around the corner.

There were several reasons for the slow pace of progress. One was that carrying passengers implied a much lower tolerance for risk. Rutan himself, who for health reasons ceased to have a leading role a few years into the program, warned that commercial space flight could not be as safe as airline travel; but Scaled engineers felt morally and professionally obliged to make it as safe as they could. A deadly oxidizer explosion in 2007, during an ostensibly innocuous ground test, drove home the lesson that mishaps could occur in ways and at times that nobody anticipated. The self-imposed discipline of engineering SS2 came to resemble a certification program, even though the FAA was not involved and there is not yet anything like a Part 23 for spacecraft.

The principal cause of delay, however, was the hybrid rocket engine, which turned out not to scale well. Much had been made, during the XPrize days, of the fact that SS1 was propelled by "laughing gas and tire rubber." The implication was that the engine was not delicate and temperamental, but very primitive and simple and just about bulletproof. Scaled had cast the motor's solid-propellant cores in-house with reasonable success, but they were still prone to uneven burning and unnerving thrust fluctuations. These problems proved to be more severe on SS2, which required 75,000 pounds of thrust to SS1's 18,000. It had to produce that thrust for about 80 seconds, but the longest inflight burn so far on one of SS2's rubber-based engines was a quarter of that.

Finally Scaled switched to a different fuel formulation, commonly described as a plastic resembling nylon, which was said to be more powerful and better behaved and to have performed well in ground tests. The Oct. 31 flight, whose purpose was to examine feathered behavior at supersonic speed, was to be its first airborne test.

The "feather" system consists simply of a hinge running across the wing at around 70 percent of chord. It allows the entire spaceplane to jackknife, raising the nose and the fore part of the wing so that they present a bluff surface to the air. The lift/drag ratio drops by a factor of more than 10, and the craft stabilizes in a very steep descent with the fuselage more or less horizontal and the tail surfaces and their supporting booms aligned with the direction of flight. Once the spaceplane enters sufficiently dense air, it unfolds back to a normal configuration and glides to a conventional landing. What is remarkable about the system is that it is equally stable at subsonic and supersonic speeds. It has even been used, on one occasion, to recover from an inadvertent flat spin.

At supersonic speed, because the center of lift of the wing shifts aft, the aerodynamic force on the tail surfaces is downward. Earlier in a flight, however, when the center of gravity is behind the center of lift because of the weight of the as yet unburned fuel, aerodynamic forces at high subsonic speed lift the tail with sufficient leverage to overpower the pneumatic actuators. A separate locking system is therefore required to preclude premature feathering.

In SS1, the pilot would never unlock the feather system until the engine burn had ended. In the highly unlikely event that the feather failed to unlock, SS1 would have had to make a dangerous, but probably successful, gliding re-entry. In the heavier SS2, an unfeathered re-entry would have involved higher speeds and looked too chancy for paying passengers.

A new flight protocol was consequently introduced. When the speed reached Mach 1.4, about 25 seconds after release from the mothership, the pilot would call for unlock and the copilot would perform the action. If the feather failed to unlock, the engine would be shut down immediately and the spaceplane would coast back without the need for a Mach 3 re-entry.

This procedure appeared safe. What no one could anticipate was that the copilot, for reasons that are not understood and probably never will be, would unlock the feather at subsonic speed. This was an eventuality against which no redundancy had been provided. It was unimaginable.

But the unimaginable happened. The tail rotated and the spacecraft pitched up with extreme violence. The motor broke away from the oxidizer tank; the airframe disintegrated. The pilots, first crushed down by enormous positive G, were then flung out in their seats as the top of the fuselage tore away. Some piece of structure struck the copilot, killing him; the pilot, one shoulder broken, fell for more than a minute through intense cold, without oxygen, but came to in time to unbuckle his seat belts. As his seat fell away his parachute, set to deploy automatically at 14,500 feet, opened. A chase plane came to look at him; he gave it a thumbs-up with his good arm.

While the FAA, as the regulator of atmospheric flight, issues launch licenses to operators of commercial space vehicles, it is barred by a piece of Ronald Reagan-era legislation from interfering in their technical development. The idea was to allow the technology to evolve naturally, like that of early airliners, before codifying a set of certification standards for passenger-­carrying spacecraft. A later law, the Commercial Space Launch Amendments Act of 2004, permits the FAA to step in if an inflight fatality occurs. The Oct. 31 accident was the first such fatality in commercial space flight. If the FAA exercises its right to impose its scrutiny on the feather system, which is the central design feature of SS2, the spaceplane's development, already slow, will likely be further retarded, and its ability to attract investment impaired.

It might seem simple enough to add a system to prevent feather unlock at subsonic speed during motor burn. But that system would have its own failure modes, and would require its own increasingly complex backups — and on and on. In the end, it is as in atmospheric flight: You have to trust the pilots to fly correctly, even though every now and then one of them may make some catastrophic mistake.

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