Human factors in commercial suborbital flight: Failure modes and survival strategiesby John Jurist
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| The various failure modes and their probabilities of occurrence must be defined before survival issues can be addressed rationally. |
If an accident occurs with damage to uninvolved people or property, or injury to commercial passengers, AST would most likely revisit this issue intensively. Given Rep. James Oberstar’s attempts during the creation of the recently enacted Commercial Space Launch Amendments Act of 2004 (CSLAA) to require commercial spaceflight to be essentially as safe as commercial aircraft operations, an accident would almost certainly result in rather hostile Congressional scrutiny. Interestingly, CSLAA actually prohibits AST from issuing regulations to protect the safety of passengers, other than for medical screening or training, for the next eight years unless there is an accident that either results in or has the potential to result in, serious or fatal injuries.
The various failure modes and their probabilities of occurrence must be defined before survival issues can be addressed rationally. There is not much rationality in spending, for example, several million dollars to protect against a one-in-a-million failure possibility while ignoring a one-in-a-thousand risk that can be abated for a few thousand dollars. This kind of failure analysis will be done by a prudent suborbital vehicle developer in any event. In addition, design efforts will be expended to assure graceful rather than catastrophic failures whenever feasible.
For example, one critical decision is whether a commercial RLV is launched vertically (VTO) or takes off horizontally (HTO) like an aircraft. If a motor fails or is shut down during the first few seconds of flight in a VTO RLV, the vehicle will be lost. A HTO RLV with motor shutdown during the first few seconds of flight may be able to initiate a runway abort, a go-around procedure, or survive the event in some other fashion.
If a VTO RLV uses a cluster of motors, engine-out capability may exist in the absence of a catastrophic failure. The probability of a motor failure, along with propellant leaks, etc., for a multiengine cluster is greater than for a single motor of similar reliability. If a single motor is 99.9% reliable for a given flight profile, the probability of a motor failure in a single mission is 0.1%. For a five-motor cluster, the odds of a failure involving at least one motor is (1-0.999)5 or about 0.5%. In order to increase odds of vehicle survival in a clustered VTO vehicle, designing in various motor shutdown scenarios to avoid catastrophic failures may be desirable.
For a VTO RLV, what are some human factors considerations? First, ejection seats are essentially useless during a very low altitude abort because the vehicle will be lost and the crew and passengers must be transported clear of the almost fully-fueled vehicle’s potential fireball. This implies that, if abort capability is desired during this part of the flight envelope, some type of rocket-powered escape capsule must be used. That is why the Apollo system, for example, had an escape tower attached to the command module. The tower was ejected after the vehicle was outside its useful operating envelope. The only manned experience with an escape capsule during a launch abort occurred on September 26, 1983 with an abort of Soyuz T-10-1 as a consequence of a prelaunch booster fire. The 20 G escape of the capsule saved the crew.
| If passengers are free to unstrap from their seats at the completion of the propulsion burn and float around, how does one ensure that they are all back in their seats before the start of atmospheric re-entry? |
During the intermediate portion of the flight profile, say up to Mach 2.5 or so, there is some limited usefulness for ejection seats in non-catastrophic vehicle failures (explosions). Ejection seats have been designed for so-called zero-zero ejections (zero altitude, zero speed). These seats could be potential lifesavers for a HTO RLV emergency during the early phases of a flight, through climb out and through the intermediate portions of the near-vertical portions of the propulsion burn. In general, ejection seats are of limited value above about Mach 0.9 at sea level to perhaps Mach 3.7 at 20,000 meters because of high dynamic pressures and/or stagnation temperatures. Above 20,000 meters, high stagnation temperatures are problematic for survival without a capsule above speeds of about Mach 2.5 to 3.7.
It is unlikely that a catastrophic failure would occur after the propulsion burn and before the recovery phase of the flight. Failure of the cabin environmental system is probably not going to occur catastrophically unless associated with a propulsion system failure.
After the propulsion burn, even if prematurely terminated, a suborbital RLV is committed to a ballistic trajectory for up to several minutes depending on how close the burn was to completion at termination. Staying with the vehicle is most likely a favored survival strategy during this phase of the flight even if cabin pressure is lost since the cabin provides some protection against the high stagnation temperatures encountered during the return to denser atmosphere. This was mentioned in the previous column (See “Human factors in commercial suborbital flight: Impact acceleration: an extreme skydiving experience”, July 18, 2005). One scenario leading to cabin pressure loss near the end of the propulsion burn would involve a motor explosion with fragments penetrating the cabin. Provision of blast shielding around the motor(s) can abate this risk as can use of stored make-up gas to compensate for cabin leaks.
If the cabin design involves use of a pure or nearly-pure oxygen atmosphere at relatively low pressure, O2 prebreathing should take place before flight to reduce the risk of aeroembolism as discussed in a previous column (See “Human factors in commercial suborbital flight: The limits of supplemental oxygen”, April 25, 2005). If cabin depressurization at altitude is a significant risk, passengers should be equipped with either partial or full pressure suits or the environmental system must provide sufficient make-up gas to maintain pressures at no higher than perhaps 10,000 meters. If cabin depressurization occurs at altitude and goes above this value, passengers in shirt sleeves will die shortly.
| Operators must be abundantly aware that although spaceflight is inherently risky, they will have battalions of lawyers (and members of Congress) minutely examining every decision with the benefit of 20/20 hindsight if an accident results in injury or death to passengers or people on the ground. |
During the coasting portion of the flight, the passengers could potentially be left free to float around in reduced gravity. Alternatively, they could remain strapped in their seats. A typical HTO suborbital concept might entail several minutes of microgravity. If passengers are free to unstrap from their seats at the completion of the propulsion burn and float around, how does one ensure that they are all back in their seats before the start of atmospheric re-entry? Does a commercial suborbital operator need a cabin attendant to deal with this issue? Will it work? This also affects marketing considerations in any business plan since experiencing a few minutes of microgravity while strapped into a seat may not be as attractive to a prospective paying passenger as floating free.
For suborbital commercial operations, the risk of a post-abort water landing must be considered and balanced against training any passengers for such an eventuality. Fire, smoke, and emergency egress after emergency landings should also be considered by the suborbital operator. Appropriate training and familiarization procedures should be incorporated into the pre-flight training period.
As discussed above, AST’s position is that it is up to the operators to make the appropriate decisions related to these factors in terms of passenger training and familiarization. Operators must be abundantly aware that although spaceflight is inherently risky, they will have battalions of lawyers (and members of Congress) minutely examining every decision with the benefit of 20/20 hindsight if an accident results in injury or death to passengers or people on the ground.
