The Accident to SpaceShip Two

Alister Macintyre noted in the Risks Forum 28.83 that the

US National Transportation Safety Board (NTSB) released results of their investigation into the October 31, 2014 crash of SpaceShipTwo near Mojave, California.

The NTSB has released a preliminary summary, findings and safety recommendations for the purpose of holding the public hearing on July 28, 2015. All those may be modified as a result of matters arising at the hearing. This is standard procedure for the Board.

Their summary of why the accident happened is

[SpaceShip2 (SS2)] was equipped with a feather system that rotated a feather flap assembly with twin tailbooms upward from the vehicle’s normal configuration (0°) to 60° to stabilize SS2’s attitude and increase drag during reentry into earth’s atmosphere. The feather system included actuators to extend and retract the feather and locks to keep the feather in the retracted position when not in use.

After release from WK2 at an altitude of about 46,400 ft, SS2 entered the boost phase of flight. During this phase, SS2’s rocket motor propels the vehicle from a gliding flight attitude to an almost-vertical attitude, and the vehicle accelerates from subsonic speeds, through the transonic region (0.9 to 1.1 Mach), to supersonic speeds. ….. the copilot was to unlock the feather during the boost phase when SS2 reached a speed of 1.4 Mach. …. However, …. the copilot unlocked the feather just after SS2 passed through a speed of 0.8 Mach. Afterward, the aerodynamic and inertial loads imposed on the feather flap assembly were sufficient to overcome the feather actuators, which were not designed to hold the feather in the retracted position during the transonic region. As a result, the feather extended uncommanded, causing the catastrophic structural failure.

This, the Board notes, represents a single point of catastrophic failure which could be instigated, was in this case instigated, by a single human error.

A hazard analysis (HazAn) is required by the FAA for all aerospace operations it certifies. It classifies effects into catastrophic, hazardous, major, minor and “no”, and certification (administrative law) requires that the probability of events in certain classes is ensured to be sufficiently low, through avoidance or mitigation of identified hazards.

HazAn is a matter of anticipating deleterious events in advance. The eternal questions for HazAn are:

  • Question 1. Did you think of everything? (Completeness)
  • Question 2. Does your mitigation/avoidance really work as you intend?

These questions are very, very hard to answer confidently. Imperfect HazAns are almost inevitable in novel operations. In aviation, sufficient experience has accumulated over the decades to ensure that the HazAn process fits the standard kinds of kit and operations and the answers to the questions are to a close approximation yes-yes. In areas in which there is no experience, for example use of lithium-ion batteries for main and auxiliary electric-power storage in the Boeing 787, answers appeared to be no-no . In commercial manned spaceflight, there is comparatively a tiny amount of experience. Certification of a new commercial transport airplane takes thousands of hours. Problems are found and usually fixed. SS1 and SS2 have just a few hours in powered spaceflight so far.

As soon as the accident happened it was almost inevitable that the answer to either Question 1 or Question 2 was “no”. The NTSB summary doesn’t actually tell us whether it was known that unlocking the booms too early would overstress the kit, but given Scaled Composites’ deserved reputation, as well as the strong hint from the NTSB that human factors were not sufficiently analysed, I would guess that the answer is yes; and the answer to Question 2 is partially no: the mitigation works unless the pilot makes an error under the “high workload” (performing many critical tasks under physical and cognitive stress) of transonic flight.

I emphatically don’t buy Macintyre’s suggestion that anyone “cut corners” on test pilot training and HazAn.

These are brand-new operations with which there is very little experience and (contrary to marketing) are inevitably performed at higher risk than operations with thousands or millions of hours accumulated experience. Nobody, in particular no one at Scaled, messes around in such circumstances. Scaled has a well-deserved reputation over three decades for designing radically new aerial vehicles to enviably high standards of safety. But things do sometimes go wrong. Voyager scraped a wingtip on takeoff and nearly didn’t make it around the world (they had 48kg of fuel remaining when they landed again at Edwards after nine days of flight in December 1986, enough only for a couple hours more). Three people were killed during a test of a rocket system in 2007 which was based on a nitrous oxide oxidiser, apparently a novel technology. OSHA investigated. An example of some public commentary is available from Knights Arrow. Scaled has been owned by Northrop Grumman since 2007 (before the rocket-fuel accident). And now a test pilot has lost his life and the craft by performing an action too early.

It may be more apt to note that, like many such analyses of complex systems with proprietary features, the HazAn for WK2/SS2 space operations is substantial intellectual property, whose value will increase thanks to the NTSB’s suggestions on how to improve it.

The purpose of the NSTB’s investigation is to look minutely at all the processes that enabled the accident and to suggest improvements that would increase the chances of a yes-yes pair of answers to the HazAn questions as well as all other aspects of safety. They said the human factors HazAn could be improved. Since human error was presumed to be the single point of failure, that conclusion was all but inevitable. The NTSB also suggested continuity in FAA oversight – the FAA flight-readiness investigation was carried out by different people for each flight so there was reduced organisational learning. As also some other stuff about how to improve the efficacy of oversight, and organisational learning such as the mishap database. And the NTSB suggested proactive emergency readiness by ensuring a rescue craft is on active standby (it usually was, but this wasn’t the case for the accident flight).

One wonders what else in the HazAn isn’t quite right. There are plenty of places to look (witness the Knights Arrow report above on the fuel choice). It doesn’t mean the HazAn is bad. But it will be improved. And improved, all with the goal of getting to yes-yes.