Yesterday, I came across a fascinating and hair-raising audio transcript from 1988 of NASA conducting a full-blown exercise in which a Space Shuttle enroute to orbit suffers a complex serious of failures and attempts to reverse course and return to its launch site. There were three immediate contexts for this abort exercise:
- The Challenger disaster in 1986, in which crew and craft were lost when in mid-ascent searing hot gases from a booster made it past protective seals, torching the shuttle’s main fuel tank and causing the vehicle to disintegrate.
- The implementation post-Challenger of new hardware and flight software in the shuttle fleet to introduce a new abort mode called Contingency Abort. A contingency abort is all about giving the crew a greater probability of survival when it is a given that the shuttle vehicle has no hope of making a controlled landing due to any series of failures or emergencies. Prior to the Challenger disaster, no such crew-bailout capability existed — the Return to Launch Site (RTLS) abort scenario had been the last, worst (most complex) option in the event of major problems during ascent. Return to Launch Site is exactly what it is means: in mid-flight, the shuttle turns around and begins rocketing back toward Kennedy Space Center to attempt a controlled landing. Post-Challenger, mission controllers realized that there could be multiple RTLS scenarios where stuff happens and the shuttle is unable to produce the energy necessary to fly back home. The Contingency Abort was added for just those circumstances in which a RTLS went bad: if needed, the shuttle could assume a new flight profile allowing the crew to parachute away from the vehicle at a survivable altitude and speed. As it so happened, the shuttle program concluded without experiencing a real RTLS or Contingency Abort.
- The upcoming “return to flight” launch (STS-26) of Shuttle Discovery in 1988, getting the shuttle back into flight following the loss of Challenger.
Prior to the STS-26 launch, mission planners and controllers wanted to play out the contingency abort scenario in a full-blown exercise, with a real countdown, crew in their seats and mission controllers at their consoles — to see what might happen. The audio transcript, which is available online, begins with a normal liftoff and then unfolds as follows:
- 40 seconds into the ascent, one of the redundant electrical systems on board the shuttle fails; controllers and the shuttle commander agree in the moment that there is no need to alter the shuttle’s ascent program.
- At approximately 80 seconds into the flight, the starboard main engine experiences an uncommanded shut down. At this point in the ascent scenario, the loss of the engine leaves Discovery with insufficient energy to reach orbit; the crew and mission control have no choice but to prepare for a Return to Launch Site abort. But before the RTLS sequence starts, the shuttle must jettison its booster rockets which happens at 130 seconds into the flight.
- After the boosters are jettisoned, the shuttle makes a powered (rocket-powered) pitch-over (turn around) to change the direction of thrust from up (toward space) to down (toward Kennedy Space Center). In this posture, it’s two remaining main engines continue to fire, drawing fuel from the giant orange tank still attached to the belly of the shuttle. It will take a significant amount of time for the redirected main engines to stop the climb to orbit and begin accelerating the shuttle toward Florida.
- In the STS-26 contingency abort scenario, the initial RTLS pitch-over goes great but after a minute the shuttle’s port main engine cuts out. At this point, the Flight Director calls out “Two Engine Out Blue” and immediately asks the Flight Dynamics Officer “do we have any chance of getting the vehicle back (to a runway). The immediate answer is “there is no way to get the shuttle to a safe landing.”
- At this point, the newly-developed contingency abort scenario becomes the best, last option for the crew. Voice traffic in Mission Control because constant as does the frequency of communications between Mission Control and the shuttle crew. And the scenario worsens as the center (and last) main shuttle engine shuts down, leaving the unpowered shuttle attached to its large and very heavy external tank. If the shuttle is to maintain sufficient energy to get to a safe altitude and location for the crew to bailout, it must now jettison its main fuel tank sooner than planned. The audio communication traffic underscores this.
- Within a few moments, the shuttle has separated from the main tank and the crew and flight software are trying to manage the flight profile of the shuttle to get the craft to a point approximately 197 miles east and north of Kennedy Space Center — over the wide-open Atlantic Ocean. That is where the crew will bailout (parachute drop).
- The audio transcript suggests that shuttle crew resources are stretched to the limit by the accumulation of failures and rapidly changing flight plans. At one point, mission control must repeatedly and urgently instruct the shuttle commander to pay more attention to the shuttle’s flight profile — to drop (pitch down) the nose of the shuttle substantially or else lose the energy necessary to get to a safe bailout point.
- The shuttle crew successfully pitches down the craft’s nose. The shuttle then begins a serious of steep banking maneuvers to navigate to the planned RP — rescue point. The crew is feeling 3+ G’s of force, much higher than experienced during normal descents but not debilitating.
- At this point in the exercise, Mission Control is unable to make voice contact with the crew although flight telemetry (data) continues to be received uninterrupted. Ultimately, mission controllers begin broadcasting “in the blind” in the hopes of getting some response from the crew. (In the early years of the shuttle program, voice communications were susceptible to cutouts when the craft made steep turns that blocked the line of sight to its voice antenna.) Voice communication is not regained before the end of the audio transcript which is very disconcerting. I would like to think that the crew, working through this incredible challenge, had defaulted to flying the shuttle first and communicating last — prioritizing their attentional “bandwidth” to getting to the rescue point.
All in all, an interesting glimpse into both the way NASA prepares for contingencies and the extent of the space shuttle’s capabilities under worst-case circumstances.