Lest we forget: 30 years ago today, the Challenger fell

Here’s some suitable music for reading this.  It’s Dernier Rendez-Vous (Ron’s Piece), from the Jean-Michel Jarre album “Rendez-Vous”.  I’ll explain why it’s suitable as we go, and why it’s subtitled “Ron’s Piece”:

30 years ago, on January 28, 1986, seven men and women climbed aboard the Space Shuttle Challenger for the last time.  The mission had been delayed repeatedly; under the naming convention NASA has just introduced, STS-51L meant that this, the 25th mission of the Space Shuttle Program, would be the twelfth manifested mission from Kennedy Space Center in 1985.  (The “5” denoted the year, the “1” denoted KSC as opposed to Vandenberg, where at the time SLC-6 was undergoing conversion to support the Space Shuttle, and the letter indicated the sequence.)  Crews had been shuffled, and shuffled again.  The final crew were Commander Francis Scobee, Pilot Michael Smith, Mission Specialist 1 Elison Onizuka, Mission Specialist 2 Judy Resnick, Mission Specialist 3 Ron McNair, Payload Specialist 1 Greg Jarvis, and Payload Specialist 2, Christa McAuliffe, for the Teacher In Space Program.  (Her backup, Barbara Morgan, would not get to fly to space until over twenty years later, on a mission to the ISS.  But she would use some of McAuliffe’s lesson plans on that flight, finally completing the TISP mission.)  And even after STS-51L finally got manifested for January of 1986, it would still face several more delays and scrubs on the pad, slipping from January 22, then 23 & 24 due to delays with the previous mission, then delayed on a day-by-day basis after that due to bad weather either at KSC or at the trans-atlantic abort (TAL) sites.

It was a cold morning, unusually so, with temperatures well below the freezing point and citrus farmers fretting about their crops.  On the launchpad, huge amounts of ice now decorated the vehicle and pad structures thanks to the interaction of the moist Florida weather with the super cold propellants that had been piped into the Orbiter on January 27 and then detanked again.  This caused considerable alarm, and the launch was delayed a full hour while the Ice Team inspected and reinspected to determine whether any of the icicles could pose a threat if they shattered off of the pad and struck the vehicle at liftoff.

Icicles_on_the_Launch_Tower_-_GPN-2000-001348

Ultimately, they gave it the all-clear, despite serious reservations by Rockwell (the Orbiter’s manufacturer) and although people like to simplify the Challenger accident to a failure of upper management to listen, I think it’s quite likely that hyperfocus on this very visible threat blinded them to more subtle threats.  For although the ice looked like a very serious threat, it ultimately was not a factor in the accident.  The air temperature that day was very low for Florida, lower than on any other flight, and at the very limits of what the SRB manufacturer, Morton Thiokol, had documented as allowable.  Indeed, overnight it had dropped well below the allowable limit, but by launch time, it had crawled just above, and the “go” decisions were made.  Several engineers at Morton Thiokol tried to express concerns, because they were worried that the O-rings sealing the joints between the segments of the solid rocket boosters could become inflexible, allowing them to leak as the boosters flexed during ascent, but for whatever reason, their concerns never reached the ears of those who needed to hear them.

After a few more pad delays, the final “go” decision was made, and launch proceeded.

At T-6.6 seconds, the SSMEs ignited, precisely according to specs.  At T=0, with the SSMEs at 100% thrust, the SRBs ignited and the hold down posts were released, freeing the Shuttle to leave the pad.  It looked good, but by looking carefully at the video in slow motion afterwards, bad signs were present almost immediately.  I’ll do the rest of the timeline as a list:

T+0.678 – puffs of black smoke are seen near the starboard SRB’s aft strut, which attaches the rest of the Shuttle stack to it via the External Tank, as the SRB segments flex and expand in the heat and from the aerodynamic forces of ascent.  This is smoke from the O-ring being incinerated by the extremely hot gasses inside the SRB.

T+2.733 – the last puff of black smoke is seen; by this time, the O-ring in the vicinity of the leak has been vaporized, and the extremely hot gasses have begun to melt the SRB casing itself; by happenstance, however, some of the rubbery solid propellant has extruded into the gap and temporarily sealed it.

T+28 – Challenger’s main engines begin throttling back to reduce aerodynamic stresses while passing through max-Q (the region of maximum dynamic pressure, or more simply, the sound barrier).

T+35.379 – SSMEs reach 65% throttle.

T+37 – Challenger begins to experience unexpected wind shear events, exceeding any seen on previous flights.

T+40 – Challenger reaches Mach 1.

T+51.860 – SSMEs begin throttling back up towards the intended 104% throttle.

T+58.788 – A plume is visible at the aft attach strut of the starboard SRB; it is believed that the torsion of the wind shear events has flexed the SRB enough to defeat the temporary seal created by the solid propellant, and now the blow-by will remain through the rest of the short flight.

T+60.238 – Pressure inside the starboard SRB has dropped considerably due to the leak, and the plume is now a flame.

T+64 – The wind shear events end.

T+64.660 – The plume seen by the aft attach strut changes shape.  The blow-by of hot gasses from the SRB have now penetrated the liquid hydrogen tank, which is leaking into the plume.

T+66.764 – Liquid hydrogen tank pressure drops due to the leak.

T+68 – CAPCOM Richard Covey calls out “Go at throttle up,” a normal callout to indicate the Shuttle returning to full thrust; there is nothing for the crew to actually do, as this is carried out by the onboard computers.  Commander Scobee acknowledges the callout with “Roger, go at throttle up.”  This is the last communication heard over the radio from the crew.

T+72.284 – The leak from the starboard SRB has burned through the aft attach strut, and the SRB tears free in back.

T+72.525 – Flight data records a sudden lateral acceleration, likely due to the shifted SRB.  The cockpit voice recorder captured Pilot Michael Smith saying “Uh oh”, the last recording of any of the Challenger crew’s voices.  So at this moment at least those on the flight deck likely understood something was badly wrong.

T+73.124 – The external tank aft dome fails, dumping the entire remaining contents of the liquid hydrogen tank into the plume, creating thrust that propels the hydrogen tank upwards into the liquid oxygen tank.  Simultaneously, the starboard SRB pivots around its forward attach point and strikes the intertank region.  The LOX tank ruptures as well, and all the propellants immediately go up in a huge fireball.

T+73.162 – The vehicle begins to break up.  Altitude is 48,000 feet.  The offcenter thrust of the starboard SRB plus the wildly erratic thrust of the exploding ET  push Challenger offcourse and into the airflow, which imparts 20 Gs to the vehicle.  It’s only rated for 5, so it is ripped apart.  The SRBs are stronger, and tear away from the disintegrating vehicle and continue to climb.  As they are normally steered by the Orbiter’s main computer, they are now climbing entirely out of control.  This produces the famous cloud we all know entirely too well.

T+75.237 – Crew module, the strongest portion of the Orbiter, is observed later on video exiting the cloud of debris, still ascending on a ballistic trajectory.

T+89 – In mission control, the flight dynamics officer (FIDO) reports that radar is detecting multiple objects, the first clear indication to MCC that the Orbiter may not have survived.

T+97 – Crew module reaches apogee at 65,000 feet, and begins to fall towards the Atlantic Ocean.

T+110.250 – The Range Safety Officer triggers the self-destruct packages on the two errant SRBs, judging their erratic flight as a hazard to the ground.  They are very near burnout anyway, so the explosion it small.  A solid rocket is destroyed by small shaped charges that essentially split the casing down its length; the sudden increase in surface area accelerates burnup of propellant, while the loss of pressure out the nozzle terminates the thrust.  How much of a boom you get depends on how much propellant is left, and there was not much; if you watch the video, you will see only a brief flash.

T+149 – Ground control reports negative contact, greatly dimming hopes that any part of the Orbiter survived, although the flight controller tells his team to watch carefully for any sign that it has.  Sadly, it has not.

Over the next few days, recovery teams removed much of the structure of the Shuttle stack from the sea floor, and some of the crew remains.  Identifiable remains were returned to the astronauts’ families; unidentifiable remains are interred with Dick Scobee in his plot at Arlington National Cemetery.  Analysis of the wreckage ultimately rules out the ice theory and reveals the real cause: the cold weather had rendered the O-rings inflexible, and one of them had leaked.  The SRB propellant itself had filled the hole, but high winds aloft broke this temporary seal and doomed the flight.  To this day, bits of Challenger still wash up along the Atlantic coast; the debris is stored largely in an abandoned missile silo at Cape Canaveral Air Station.  The crew, meanwhile, were not killed in the initial explosion or the sudden breakup a moment later; their actual cause of death could not be determined, but it is most likely that they were killed only on impact with the ocean, which would’ve been a 200G impact and definitely not survivable.  At least four of them activated their emergency oxygen packs, and severals switches had been set to positions not normal for launch; the only explanation is that Commander Scobee had been attempting to restore electrical power after the crew module lost power as it was ripped away from the fuel cells in the midfuselage.  However, as the cabin would have been depressurizing and the emergency oxygen was unpressurized (much like the emergency oxygen on a commercial airline flight), it is quite possible they lost consciousness before reaching the ocean.

 

I said I’d explain why the song at the start of this article is called Ron’s Piece, and here it is.  Jean Michel-Jarre has always had a fascination with space, and in 1985 he arranged for McNair to bring a saxophone along on his mission.  McNair was an accomplished sax player, and was happy to prepare for what would become the first professional music recording from space.  Alas, it was not to be, and the sax solo above was recorded later, on the ground, by a non-astronaut musician.  (In the end, the first professional music from space would not be recorded until Chris Hadfield created his album “Space Sessions: Songs from a Tin Can”, recording vocals and acoustic guitar during the ISS Expedition 35 in 2013.)  Jarre would later dedicate the entire album “Rendez-Vous” (the title of which is a play on both romance and spacecraft operations — when two spacecraft fly in formation, it is called a rendezvous, and in French, the word for a date is “rendez-vous”) to the crew of STS-51L.

Several major changes were made following the Challenger accident.  The biggest was a change to the SRB field joint design.  A third O-ring was added, along with heaters and an interlocking mortise and tang structure to reduce flexing of the joint.  They also banned the flight suits and crash helmets worn for ascent and entry on early Shuttle missions, reverting to pressure suits that would keep the crews alive and conscious in the event of a high altitude accident, to preserve possibilities of escape.  And the flight rules were changed, to increase the minimum temperature, require a minimum temperature for a certain number of hours prior to the flight instead of just during the actual launch, and also placing new restrictions on wind shear.  The Shuttle returned to flight in 1988 with Discovery on STS-26; NASA had jettisoned their short-lived naming convention and returned to sequential numbers.  The system was safer than ever.

And yet, in 2003, there would be another fatal accident, and eerily, the same time of year, although this time the weather had nothing to do with it.  But that’s a story for another day.  In the meantime, we have to keep reaching high, and not let our tragedies turn us permanently from the goal of leaving Earth.  As Konstantin Tsiolkovsky said, long ago, “The Earth is the cradle of humanity, but mankind cannot stay in the cradle forever.”  So I will leave us with So Far Away (Welcome to the Mission), from the Alan Parsons album “On Air”, which alludes to the Challenger at the very end:

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