El triste fin del Columbia

La NASA ha publicado el Columbia Crew Survival Investigation Report, donde se detalla la causa de la muerte de la última tripulación del primer transbordador de la historia. No debemos confundir este informe con el Columbia Accident Investigation Board, que explicaba las causas del accidente, pero aún así contiene mucha información que daría para varios posts, aunque podemos resumir hoy lo más importante. A diferencia de lo que apuntaban algunos informes, el fin tuvo lugar de forma bastante rápida:

After loss of control at GMT4 13:59:37 and prior to orbiter breakup at GMT 14:00:18, the Columbia cabin pressure was nominal and the crew was capable of conscious actions. The depressurization was due to relatively small cabin breaches above and below the middeck floor and was not a result of a major loss of cabin structural integrity. The crew was not exposed to a cabin fire or thermal injury prior to depressurization, cessation of breathing, and loss of consciousness. The depressurization incapacitated the crew members so rapidly that they were not able to lower their helmet visors.

El traje ACES y los procedimientos relacionados con él no salen muy bien parados:

The majority of the SCSIIT findings related to the first lethal event were connected to the operational incompatibilities of the advanced crew escape suit (ACES) with the orbiter. The launch and entry suit was added in response to the Challenger accident, rather than as a part of the original vehicle design. The ACES was the successor to that suit. The suit protects the crew in many scenarios; however, there are several areas where integration difficulties diminish the capability of the suit to protect the crew. Integration issues include: the crew cannot keep their visors down throughout entry because doing so results in high oxygen concentrations in the cabin; gloves can inhibit the performance of nominal tasks; and the cabin stow/deorbit preparation timeframe is so busy that sometimes crew members do not have enough time to complete suit-related steps prior to atmospheric entry. As Columbia entered the atmosphere, one crew member was not yet wearing the ACES helmet and three crew members were not wearing gloves. Per nominal procedures, the crew wearing helmets had visors up. There was a period of about 40 seconds after the orbiter loss of control (LOC) but prior to depressurization when the crew was conscious and capable of action. Part of this short timeframe was undoubtedly employed in recognizing that a problem existed, as the indications of LOC developed gradually. The crew members could have closed their visors in this timeframe but did not. The SCSIIT attributed this to the training regimen, which separates vehicle systems training from emergency egress training and does not emphasize the transition between problem resolution and a survival situation. Once the cabin depressurization began, the rate of depressurization incapacitated the crew so quickly that even those crew members who had fully donned the ACES did not have time to lower their visors. Although circulatory systems functioned for a brief time, the crew could not have regained consciousness upon descent to lower altitudes due to the effects of the depressurization.[...] The change (from the crew's vantage point) from a nominal entry profile to the LOC and subsequent separation of the forebody from the orbiter all occurred in approximately 40 seconds.[...] The exact time and sequence that the crew and seats separated from the crew module is unknown. A comprehensive evaluation of ballistic analysis of debris, crew member remains, and crew worn equipment indicates that the middeck crew remains were separated from the crew module prior to the flight deck crew remains, supporting the conclusion that the flight deck stayed intact a few seconds longer than the middeck.[...] The dynamic pressure environment exposure caused the mechanical failure of the crew suits (common to high-speed accidents, but somewhat unexpected given the aerodynamic pressure of only 450 to 550 psf). The suit is designed to maintain structural integrity when exposed to a windblast that is up to 560 knots equivalent air speed (KEAS) (806 psf). This assumes that the helmet visor is down. The helmet visors being in the up position is the most likely explanation for the hastened disruption of the suits. Although suit disruption was primarily due to aerodynamic (mechanical) loads, the thermal environment and atomic oxygen in the atmosphere may have been a contributing factor.

Tampoco salen muy bien los asientos de la tripulación, los cuales no la protegieron de las fuerzas de aceleración cuando el compartimento se separó de la nave. No es que unos asientos mejor diseñados hubiesen salvado a los desafortunados astronautas, pero es un dato a tener en cuenta para el diseño de la Orión:

The orbiter lost control, probably when the hydraulic systems failed due to hot gas intrusion in the left wing. The resulting motion was not lethal but did require bracing by the crew. The forebody (crew module and forward fuselage) eventually separated and the crew module lost pressure at orbiter break-up. When it separated, the forebody began a multi-axis rotation at approximately 0.1 revolution/second. Loads due to deceleration significantly decreased at the moment of breakup due to the change in ballistic number, but began to climb as the forebody continued to decelerate. After the crew module depressurized and the crew lost consciousness, the seat inertial reel mechanisms failed to lock despite the off-nominal motion. The reels were not defective; they were simply not designed to lock under the conditions the forebody experienced. The upper harness straps failed at some point prior to the forebody breakup, causing the straps to recoil back into the inertial reel mechanism. Because the reel mechanisms did not lock, the unconscious or deceased crew members were exposed to cyclical rotational motion while their upper bodies were inadequately restrained. Helmets that did not conform to the head and the lack of upper body restraint resulted in injuries and lethal trauma.

Resumiendo, los cinco sucesos letales del accidente fueron (el último es obvio):

• The first event with lethal potential was depressurization of the crew module, which started at or shortly after orbiter breakup.
  
• The second event with lethal potential was unconscious or deceased crew members exposed to a dynamic rotating load environment with nonconformal helmets and a lack of upper body restraint.
  
• The third event with lethal potential was separation from the crew module and the seats with associated forces, material interactions, and thermal consequences. This event is the least understood due to limitations in current knowledge of mechanisms at this Mach number and altitude. Seat restraints played a role in the lethality of this event.[...]Although the seat restraints played a significant role in the lethal-level mechanical injuries, there is currently no full range of equipment to protect for this event. This event was not survivable by any means currently known to the investigative team. All circulatory functions had ceased by the end of this phase.
  
• The fourth event with lethal potential was exposure to near vacuum, aerodynamic accelerations, and cold temperatures.
  
• The final event with lethal potential was ground impact.

Ad astra per aspera. Semper Exploro.