Wednesday, March 10, 2004

Is the Shuttle Unsafe?

Clearly it isn't perfect and in fact is badly compromised, although no one talks about this any more, by military requirements set at the beginning of the program. But is it a killer as some would have it?

Assumptions that the space shuttle is dangerously unsafe would be easier to accept if the loss of Columbia had been a killer freak. Unfortunately, like the loss of Challenger, the events that led to the loss of Columbia had telegraphed themselves for years - in fact, since the seventh flight of the shuttle system.

The External Tank is the large, orange object familiar to viewers of space shuttle launches. It fulfills two important functions: it contains liquid hydrogen and liquid oxygen to fuel the Space Shuttle Main Engines, and serves to tie the entire vehicle together. The two solid rocket boosters are fastened to opposite sides of the External Tank and the shuttle orbiter is attached to the tank's "back." The orbiter is fastened to the tank in three places: by two struts on each side of the tail end of the orbiter, and by a bipod structure under the orbiter's nose (fuel lines also attach to the orbiter, but these are not structural connections). The bipod is a V-shaped metal strut that fastens in two places to the ET, and has a single attachment to the orbiter. The attachments of the bipod to the fuel tank are actually designed to rotate slightly so as the tank is filled with supercold propellents the slight change in tank size does not affect the orbiter. Each of these attach points is insulated and aerodynamically faired by a foam ramp called the bipod ramp.

The entire ET is covered by an orange foam insulation to prevent the formation of ice on the tank that might fall off during launch and strike and damage the orbiter. This foam is so effective that when the tank is full it is only slightly cool to the touch. Much of the foam is applied by automatic sprayers, but a few areas with complex shapes need to be sprayed by hand and one such area is the two bipod ramps.

Although the foam insulation on the ET has overall been very successful, it has been prone over the history of the shuttle system to shedding. Most shedding events are small, and result in tiny pieces of foam coming loose, mostly to fly into the airstream and be lost. Occasionally these pieces hit the orbiter, sometimes causing damage to the tiles on the orbiter's underside. Changes have been made in the foam over the years that have lessened the loss of foam and in fact have been continuing all through the period before and after the loss of Columbia.

The most dangerous foam loss, however, is associated with the bipod ramps. According to the report of the ET Working Group to CAIB, "Of the 113 STS space shuttle flights, 79 flights had usable imagery of the +y axis," and of those 79 flights, five are known to have had foam loss from the left (- y) bipod ramp (the right, or +y bipod ramp, has never been known to shed foam. It appears to be protected by the liquid oxygen feed line that runs beside it), loss that appears to be correlated, according to the ET Working Group, to "sideslip angle, q-beta, and out-of-plane wind velocity." Again, the right bipod ramp appears to be shielded by the large fuel line routed alongside it.

Loss of foam from the bipod ramps is especially dangerous because it has a good chance of striking the vehicle, and because the pieces tend to be much larger and heavier than foam loss events elsewhere on the ET. Charts reproduced on the ET Working Group report's page 41 demonstrate this.

Before Columbia's loss, these are the five known instances of left bipod ramp shedding:

STS-7/ET6 Size of foam: 18" x 12" Estimated weight: 0.6 lb. This instance of foam loss was attributed to a repair on the left ramp and following this incident the ramp repair criterion was changed to limit the size of repairs to less than 3 square inches. STS-7 flew in 1982. It was the seventh shuttle flight.

STS-50/ET50 Size: 26" x 10". Weight: 0.98 lb. This loss "encompassed the majority of the bipod ramp" and in fact may be similar in size to the foam that doomed Columbia. This instance was attributed, according to the ET Working Group report, to "voids/debonds in the Isochem bond layer of the non-vented two-tone TPS area. ET50 was the last ET built with this intertank TPS configuration." Clearly, once again someone thought the problem might have been fixed.

STS-52/ET55 Size: 8" x 4" Estimated weight: 0.02 lb. This small loss appears not to have caused any alarm in spite of the fact that this happened shortly after the insulation was changed.

STS-65/ET62 Size: 3" x 1" Estimated weight: 0.001 lb. This smallest known bipod shedding event was referred to as a "small divot in the after face of the ramp."

STS-112/ET115 Size: 7" x 12" Estimated weight: 0.3 lb. STS-112 was the flight directly before STS-107 (flights are not uncommonly flown out of order). This particular loss of foam caused damage to one of the solid rocket boosters. Interestingly, photos of this damage or detailed descriptions of its nature are totally absent from the CAIB report. After this occurrence, even NASA sat up and took notice and "initiated plans to evaluate the materials, design, and processes used for the ramps." The final corrective action was to have been presented on February 6, 2003 - five days after the Columbia disintegrated during reentry.

And finally...

STS-107/ET93 Size: unknown. Weight: estimates are from 1.19 to 1.72 pounds. This instance of foam shedding struck the leading edge of Columbia's left wing causing damage that led to the loss of the orbiter on reentry.

This was not a killer freak. The shuttle is not particularly unsafe - the culture that operates it is, and has been, the problem, and I think the evidence proves that. For future ETs, the bipod ramp will be completely eliminated and replaced by heaters that will prevent ice from forming. The shuttle has flown safely, and can continue to fly safely, and damn it, it can fly to Hubble.


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