How the "Challenger" tragedy could have been avoided

The Presidential Commission's inquiry concluded "the cause of the Challenger accident was the failure of the pressure seal in the aft field joint of the Solid Rocket Motor."

In fact, the aft field joint had two O‑ring seals, a primary seal and a secondary seal as a back-up, therefore both seals must have failed and the Pres. Comm's report should have read "the failure of both pressure seals". Had this been stated in the 'plural' rather than the 'singular' a lot of thinking would have been different.

The O‑rings performed exactly as O‑rings would be expected to perform under the circumstances, that is, when frozen rock solid and inflexible by the extreme cold prevailing at the launch site. How, then, can O‑rings, that performed normally, have been the CAUSE of the accident?


Theory 1:
In the very first moments of launch, videos show smoke coming from the aft field joint, therefore both O‑ring seals were leaking. The smoke was the only evidence of what was happening. The smoke was coming in puffs caused by the cyclic vibration of the launch process.

The first puff of smoke was seen so soon after ignition there hardly seemed time for the SRB's exhaust gases to do sufficient damage to create such a quantity of smoke. Here is the sequence:
  1. Ignition of SRB.
  2. cyclic vibration as launch begins.
  3. cyclic vibration opens the primary seal.
  4. some flaming exhaust gases pass through the gap.
  5. exhaust gases burn a little of the primary O‑ring and make a bit of smoke.
  6. smoke partially fills the annular gap between the primary and secondary O‑rings.
  7. cyclic vibration opens the secondary seal.
  8. a puff of smoke comes into view.
  9. both seals* close.
*Clearly, the secondary O‑ring seal was being vibrated open and shut, which suggests it was frozen, or partially frozen, and, therefore, inflexible and thus unable to close any gap so as to maintain a constant seal. Whether the primary O‑ring seal was also being vibrated open and shut, or was already open before launch, is impossible to say.

Conclusion: It is doubtful all this could have happened in so short a time.


Theory 2:
Assumption: Before the launch process began, the primary O‑ring was not sealing the aft field joint. This allowed the SRB's exhaust gases, prior to launch but immediately following ignition, to flow over the primary O‑ring, burning it and making smoke. Here is the sequence:
  1. Ignition of SRB.
  2. flaming exhaust gases bypass the primary seal.
  3. exhaust gases burn much of the primary O‑ring and make considerable smoke.
  4. smoke fills the annular gap between the primary and secondary O‑rings.
  5. cyclic vibration as launch begins.
  6. cyclic vibration opens the secondary seal.
  7. a puff of smoke comes into view.
  8. secondary seal closes.
Conclusion: Because it happened so quickly, the primary O‑ring was unlikely to have been sealing before launch!

The problem was not the O‑ring, itself, but rather that the O‑ring was frozen solid because of the extremely cold conditions at the launch site.

Because of the freezing temperature the O‑ring was slowed in its expansion to its ideal cross-section. The O‑ring thus had insufficient time to seal the gap between the SRB tang and clevis and it was through this gap that the burning gases escaped, leading to the destruction of the shuttle.

The disaster could have been prevented quite easily. If the O‑ring had been defrosted the flight would have been normal. Why didn't the technicians defrost it? Apparently, nobody thought of defrosting the O‑rings and, to this day, defrosting the O‑rings has never entered anyone's mind.

All 12 O‑rings (there are two in each field joint and three field joints on each SRB) could have been defrosted within an hour. The equipment capable of doing the job was already on the launch site and had recently been used on "Challenger" for pressure-testing of the field joints. During the 38 days "Challenger" stood on the pad preparatory to launch, its field joints were periodically tested for leaks. All 12 O‑rings were pressure-tested to be sure they had sealed. High-pressure air (nitrogen?) is injected into the annular space between the primary and secondary O‑rings. When the pressure is 100 psi (auto tyre pressure 30 psi approx) the air supply is sealed off. If the pressure gauge connected between the O‑rings then shows the pressure to be dropping it indicates a seal is leaking.
The test connections could have been used to defrost the frozen O‑rings. All that needed to be done was replace the compressed air with warm air!
The only extra thing needed was for a small vent to be drilled in the annular gap diametrically opposite the air input. This vent is necessary to enable exhaust air to escape after having flowed freely around the annular space, defrosting the O‑rings as it passed by. The O‑rings could have been heated to whatever the launch directors wanted. The vent would also act as a drain-hole for water that sometimes seeped into the joint and would only need to be plugged when pressure testing.
pressure test
defrost O-rings

Graeme Lindridge   © 2009

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