Monday, February 18, 2013

Remembrance: Challenger

by Stephanie Osborn

You may remember that I started my career as a payload flight controller for first Shuttle, and then Station. And that I had a friend aboard Columbia during her final flight. For those who have read my all-too-painfully prescient book, Burnout: The mystery of Space Shuttle STS-281, you know I dedicated that book to my friend Kalpana Chawla, her crewmates, and all those who have died in pursuit of space. "Ad astra, per ardua." ("To the stars, through struggle/adversity.")

We've just passed through a period of time of which most people are unaware. You see, all of the major space disasters that America has experienced all occurred within a 2-3 week span on the calendar. And interestingly, they occurred in chronological order on the calendar.

Apollo 1 Fire - January 27

Space Shuttle Challenger disaster - January 28

Space Shuttle Columbia disaster - February 1

Challenger - What Happened?

On January 28, 1986, Space Shuttle Challenger launched on mission STS-51L. It carried the first teacher in space, Christa McAuliffe, as well as astronauts Mike Smith, Dick Scobee, Ron McNair, Ellison Onizuka, Greg Jarvis, and Judy Resnick. At MET (Mission Elapsed Time) 00/00:01:13 (T+73sec), Challenger exploded. There were no survivors.

Some personal background

On January 28, 1986, I was about a month and a half into my first full-time job post-graduate school. I was the resident astronomer on a defense project that was in Phase 1 R&D. (I can't tell you much more than that, 'cuz then I'd have to kill ya. *jk* But you get the picture.) Phase 2 was to have been prototype development and testing, including prototype flight and testing aboard the Shuttle. They needed a payload specialist for that flight. My hat was in the ring and I was a likely candidate.

The Challenger disaster grounded the fleet indefinitely. This was a factor in the cancellation of Phase 2 of my project. So my PS candidacy dream exploded with that shuttle.

I ended up transferring over to the space programs division of my company and working for many years as a payload flight controller, scheduling activities for Shuttle missions, and even training astronauts (more about that next week). I fit in an entire career between two of only three major space disasters that NASA has had in its existence.

I remember the day clearly. I was sitting in my cubicle working when my friend from grad school and coworker, Jim, leaned over the top of the partition (Jim is really tall).

"Steph? Steph!"

"Yeah, Jim? What's up?"

"The Shuttle just blew up."

"Ha. Ha. Very funny."

"I'm serious."

It wasn't until he said that, that I truly diverted my full attention from my work and looked him in the face. And I knew. He really WAS serious.

I flung my pen at the desk, shoved my chair back, and rounded the cubicle partitions at a full sprint. Our boss had a small TV in his office, and when I got there, most of the branch was crammed in his office watching. There was nothing else on - everyone was covering it. We all sat in silence and watched the reports come in, and the replays of the explosion. To this day I can't stand to watch footage of that explosion anymore.

Later I went down the hall on some errand, and across the building to the cafeteria for lunch. All the halls were empty except for offices that had TVs, where people spilled out into the hall trying to watch said TV. One or two doors were closed and I thought I heard the sounds of weeping from behind them - or else similar sounds from behind restroom doors. It was not something I'll ever forget. Sometimes I wish I could.

So what happened?

This is something I know a fair amount about, because I used the Challenger disaster information as part of my research for writing my first book, Burnout: The mystery of Space Shuttle STS-281.

The gist of it is that the entire vehicle is only certified to to 40F, and they launched in 18F temperatures. Management felt it was a high profile mission with McAuliffe aboard, and ignored all warnings from the scientists and engineers who knew better.

What many do not know is that equipment on the launch pad also failed, possibly due to the cold. In the end, those were ruled out as part of the cause of the accident, but they so easily COULD have been, that it is worth mentioning. Now, on to what actually happened.

It had been discovered in previous flights that SRB ignition caused the first segment of the SRBs to warp, the casings ballooning out from the stress, and opening the joint between it and the next segment.

Wait. Back up, I hear you say. Why is the SRB in segments in the first place? Because the solid propellant has to be poured, cast, and cured like concrete. And just like concrete, if you pour too big a slab, it will crack during the curing process. This crack will act as a fuse for the fuel burn to travel along, and cause not only uneven thrust (a significant danger in a craft whose thrust must remain balanced on each side), but also a burn-through of the casing. So it's cast in segments, and those segments stacked.

Ok. So the joint opened up. Regularly. It was found, however, that the primary O-ring tended to shift, compensating for the warp and sealing the opening. This took a certain amount of time, but it was generally short enough to prevent anything but hot gases from escaping - no actual flame ever got past. Now granted, the hot gas was around 5000F, but it still wasn't flame, and it was only for fractions of a second, maybe a half-second at most, and all within 3-4 seconds of liftoff, so that was considered more or less okay. This behavior actually ended up being retroactively added into the specs.

However, the flexibility of the O-rings, as we all know now, is temperature-dependent. The colder the temperature, the less flexible the rings, and the longer it takes for them to shift into position to block the blow-by, as the escaping gas was termed. Turns out that 18F pretty much "de-flexed" the O-rings into rigidity. The primary O-ring didn't flex into the "new" position - evidently at all - and the secondary O-ring was unseated by the warping of the casing. There was nothing to stop the blow-by until the aluminum oxide components from the solid propellant essentially clogged the opening. By that time the O-rings had been burned away for nearly a quarter of the circumference of the starboard SRB. But the metal oxides had sealed the hole. No problem.

Not quite.

From approximately T+37sec (launch plus 37 sec) to T+74sec, the Shuttle encountered a layer of heavy wind shear, stronger than ever encountered before. This wind shear broke open the aluminum oxide seal that was holding the starboard SRB closed. A plume of exhaust immediately formed and became well defined as blow-by resumed. This in turn ate away at the casing, enlarging the hole and allowing for more and more blow-by. The thrust of the SRB began to drop, as part of it was being redirected out the hole in the side.
A recovered piece of the starboard SRB, showing the hole and blow-by scorching.

Soon the plume struck the external tank (ET) and began eating into it. (Hot gas and plasma has a way of doing that.)

The plume develops on the side of the SRB, near the ET.

It didn't take long (~T+66sec, two seconds after the ET plume formed) before the liquid hydrogen tank began leaking fuel, adding to the whole mess by burning as it entered the plasma exhaust stream. This led to two additional problems. One, the pressure in the LH2 tank was dropping steadily, which would cause problems in operation of the Shuttle main engines (SSMEs or Space Shuttle Main Engines). Two, this would cause an additional thrust vector - and remember what I said earlier about needing to have balanced thrust? Different thrust angles means that there are now torques (twisting forces) being applied to the Shuttle "stack" that it's never experienced, and is not designed to experience.

At this point neither the crew nor the flight controllers have registered that the drops in pressure are due to something other than normal ascent; the Shuttle is passing through "Max Q," the segment of the trajectory that experiences the maximum stress from the atmosphere, and breaks Mach 1. Once it has done this, the aerodynamic forces drop (the SSMEs have throttled back to about 65% for this, to minimize stress), and at T+68, CapCom Dick Covey called, "Challenger, you are go at throttle-up."

Commander Dick Scobee replied, "Roger, go at throttle-up," indicating that they would increase the SSME operations back to 104%. It would be the last words heard on the Air-to-Ground loop from Challenger.

At ~T+72, the rear strut attaching the starboard SRB to the ET... broke, pulled loose, whatever...and the inevitable accelerated into its awful climax. The craft slammed to the right, and the onboard black box recorder caught Pilot Mike Smith remarking, "Uh-oh." This appears to have been the first indication the crew had that anything was amiss. It was far too late.

At fractions of a second past T+73, the rear of the LH2 tank ruptured and the spilling LH2 apparently ignited, causing the tank to act like a rocket. It slammed forward into the rear of the liquid oxygen (LOX) tank. At the same time the starboard SRB pivoted on its remaining strut, slamming into the ET. The ET failed (aka "busted open") and the LH2 and LOX mingled, igniting and producing a huge cloud of water vapor. Everything went to hell in a handbasket as the Shuttle veered off course, experiencing forces far beyond its specs (20G as opposed to 5G), and it essentially shattered.

Interestingly, this did not indicate demise of the crew, only the Orbiter and stack. (The ET had already broken up, and once the Range Safety Officer determined the SRBs were in free, uncontrolled flight, he initiated detonation to protect ocean vessels and land inhabitants.) There is some debate about whether or not the mid-deck crew survived the breakup, as that area caught a considerable amount of force during the devastating disintegration. It is beyond doubt, however, that the flight deck did in fact emerge intact. Three of the four emergency oxygen systems for the flight crew were found activated, with sufficient oxygen used to just equate to the free fall. For the commander and pilot to be wearing them, it was necessary for the mission specialists to don theirs, unstrap, put on the masks for the CDR and PLT, then sit back down and strap in - they were found masked and strapped in. Also instrument settings on the console had been changed, and could only have been changed by the pilot in an effort to reaquire cabin electricity.

The crew cabin during free-fall.

However, the emergency oxygen system was not pressurized, and at that altitude unconsciousness would have occurred quickly. This is probably merciful because the impact on the ocean surface generated forces in excess of 200G, which neither the crew nor the remains of the cabin could survive.


The Shuttle Fleet was grounded for a significant period of time, approximately three years. During this time, investigations into exactly what happened and why were extensive. So were redesigns, including a new joint design for the SRB segments, which included a mortise and tang design that was significantly reinforced with thick layers of steel. A new bailout procedure and equipment was developed; this was actually depicted in the movie, Space Cowboys (but it would not have saved the crew of Challenger). A new abort procedure was developed, known as Return To Launch Site, or RTLS (which would also not have saved the Challenger crew). The Office of Safety, Reliability, and Quality Assurance was created within NASA. The crew returned to wearing pressure suits for ascent and re-entry. Numerous other changes were made within the organization of NASA, to foster a different attitude in the management. Those of us who worked the actual missions took it very seriously.

Unfortunately, as we'll see next week, NASA management failed to remember the lesssons learned from this catastrophe.

-Stephanie Osborn