Redundancy ends with the pilot
Peter Garrison – 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 programme progressed with speed and smoothness that were a credit to Rutan himself and to the talents of the team of engineers and pilots he had assembled.
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 a few potentially life-threatening problems arose, the programme ended without mishap and gained the $10 million Ansari X Prize, 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. Tier1 required speeds on the order of Mach 3, or around 2,000 mph. Injecting something into low earth orbit requires 17,000 mph and escape from the gravitational field of the earth requires 25,000 mph. Attaining those higher speeds is exponentially more difficult, as is decelerating from them while re-entering the atmosphere.
‘propelled by laughing gas and tyre rubber’
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 and a half, 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 up at $250,000 apiece – was right around the corner. That launch finally occurred in September, 2021, shortly followed by an announcement that, because of undisclosed problems, no further flights would occur for at least eight months.
‘mishaps could occur in ways and at times nobody anticipated’
There were several reasons for the slow pace of progress. One was that carrying passengers required a much lower tolerance for risk. Rutan himself, who for health reasons ceased to have a leading role a few years into the programme, 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 oxidiser 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 X Prize days, of the fact that SS1 was propelled by “laughing gas and tyre 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 in-flight burn attempted so far on one of SS2’s rubber-based engines was 18 seconds.
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. An October 31, 2014 flight, whose purpose was examine feathered behaviour at supersonic speed, was to be its first airborne test.
‘depend on the pilots to fly correctly’
The “feather” system consists simply of a hinge running across the wing at around 70 percent of chord. It allows the entire spaceplane to jack-knife, 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 stabilises 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 centre of lift of the wing shifts aft, the aerodynamic force on the tail surfaces is downward. Earlier in a flight, however, when the centre of gravity is behind the centre 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 mother ship, 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 Mach 0.8. This was an eventuality against which no redundancy had been provided. It was unimaginable.
‘the speed reached Mach1.4, about 25 seconds after release‘
But the unimaginable happened. The tail rotated and the spacecraft pitched up with extreme violence. The motor broke away from the oxidiser 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 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 in-flight fatality occurs. The October 31 accident was the first such fatality in commercial space flight, and the FA did exercise its right to demand a number of modifications to the design.
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, what is true in atmospheric flight also holds true in space: You have to depend on the pilots to fly correctly, even if every now and then one of them may make some catastrophic mistake.
