"The erosion was the most significant observed by the Space Shuttle program up to that point"

The previous blow through of the primary ring should have been enough warning, it's not as if the two o rings could possibly have had different failure modes.

What staggers me is that they thought having just two o-rings on each joint was sufficient. Knowing the forces and temperatures at work here - Yes, this IS rocket science - three should have been the bare minimum, as in terms of cost, what value do you attribute to a couple of SRB's when compared to the overall cost of an orbiter, it's crew and the payload?

Three? Not at all. They should have had four! No, they should have had SIXTY.

Two is fine. Nothing should get past the one O-ring. Choose the right O-ring and use it within spec and it'll generally be fine, with the second being a backup in case it isn't. The problem comes when you compromise both simultaneously- three O-rings wouldn't have helped if they were all too cold (as with this and Challenger).

They should not have been segmented at all. The original spec didn't allow segmented boosters, and the boosters were to be made in Florida near the Cape. But the Senator from Utah during the Shuttle design phase (either Bennett or Moss, I don't recall) wanted them built in Utah by Thiokol. To do that, they'd have to be segmented to fit on rail cars.

Years later, Utah Senator Orrin Hatch ensured that the Space Launch System (SLS) rocket would use the solid boosters from the Space Shuttle, again so they'd be made in Utah.

The Challenger disaster is due to politicians dictating the designs of space ships. Do stupid things, get stupid results.

three should have been the bare minimum

Adding additional seals would have either put them closer together, or increased the length of the joint. Both options would have changed the joint's characteristics and could have introduced new and different weaknesses or failure modes. There's no guarantee that the results would have been better.

The problem was the design of the joint meant that O-rings weren't a suitable way to seal it at all.

Due to combustion pressure and the effects of wind shear, the booster could flex, causing the tang/clevis joint to bend and creating a gap between the O-ring and the surfaces it was supposed to seal.

The cold reduced the ability of the O-ring to re-expand when the clamping force was removed as a result of that bending, eroding the safety margin to zero.

The book "The Challenger Launch Decision" mentions that the O-rings did re-seat after lift-off but turbulence from high altitude wind shear caused the to unseat again. If it wasn't for that wind shear, the launch would have been another near miss.

The book also mentioned that the team did a plot of air temperature at launch versus O-ring damage with the result that no obvious trend was noted. The fatal flaw of that plot was it only covered launches with ambient temperatures of less than 70ºF, where plotting all of the data showed no damage when ambient temperature was above 70ºF.

As for the non-segmented booster proposal, I would wonder about how they could pull off making the casting of the solid fuel grain.

The two o rings aren't meant to be a backup for each other. Two o rings spreads the joint load and helps make it more stable against be doing forces.

At the time, NASA management knew of the o ring erosion problem but since no o ring had ever eroded completely through, they considered it an engineering margin that was more than sufficient. The actual engineers saw it differently and viewed any o ring erosion as a failure and knew that there was a fundamental flaw that wouldn't have been solved with more o rings.

Interesting point.

Goes to show that armchair engineers are about as useful as armchair generals.

As I recall, the loss of the Challenger was some combination of engineering failures, hubris, and politics.

The engineering failures are well documented, as is the hubris (we got lucky before, we'll get lucky again).

As for the politics , recall that this was the flight of the " Teacher in Space 1 " and the launch was to roughly coincide with Ronald Reagan's 1986 State of the Union address 2 later that same day. Whether anyone would admit it or not, there was a lot of pressure to get that spacecraft into orbit.

I recall driving in to work that morning, while listening to the radio 3 coverage, thinking "they're trying awfully hard 4 to get this thing off the ground" and that something's going to go badly wrong. About the time I got to my office with my first cup of coffee, they certainly had.

Note that I'm not particularly blaming the Reagan Administration for the pressure. It's just human nature to want to impress (or curry favor with) the boss, make the boss happy, and allow theboss to point with pride at accomplishments of subordinates. It just shows what happens whey you let that impulse take over.

_______________

1 [1]Teacher in Space Project

2 [2]1986 State of the Union Address

3 Ask your grandparents.

4 [3]"My God, Thiokol," he said. "When do you want me to launch — next April?"

[1] https://en.wikipedia.org/wiki/Teacher_in_Space_Project

[2] https://en.wikipedia.org/wiki/1986_State_of_the_Union_Address

[3] https://www.science.org/content/blog-post/roger-boisjoly-and-management-hat

From the early days of the Shuttle program, the o-rings were being damaged. After every flight, they took the attitude "We didn't loose the shuttle, so it's OK". They became immune to the damage, not realising that damage to the o-rings signified fundamental problems.

Even after Challenger they decided not to fix the underlying problem, they just managed it.

If you're interested in a great read, check out Alan McDonald's book "Truth, Lies, and O-Rings." Alan McDonald was the Thiokol manager at Cape Canaveral who refused to sign off on the launch of STS-51L due to significant safety concerns. He played a crucial role as a whistleblower to the Rogers Commission, highlighting critical issues. His work on the Space Shuttle's return to flight, including his hands-on inspection inside a loaded Solid Rocket Motor (SRM) to examine the field joints, is particularly chilling.

Despite his efforts, similar issues persisted by the time of the Columbia disaster, characterized by poor management and the disregard of engineers' concerns.

The book also covers the redesign of the field joint, which now includes heaters and other enhancements to prevent the joint from separating under load. The problem wasn't just the cold O-ring; it was also the splaying of the field joint, which the O-rings were supposed to seal. This issue was exacerbated by a dangerous complacency from previous successful flights, leading to a disregard of the real risks involved.

BvB

Just because the penetration and hot gas exit was in another direction...

Another one for the list of "near misses".

Ellison Onizuka then had the misfortune to be on the Challenger mission that was lost. These stories demonstrate how what was portraited as a routine and safe launch vehicle was anything but.

Into the Black by Rowland White covers this and is a good read.

Both STS-51-C and the ill fated STS-51-L had Ellison Onizuka on board. Made me pause for a moment seeing his name connected to this flight.