In a previous question, I explored whether or not, with a call up from an observant Flight Controller, the SRB’s could be separated from the STS-51L stack before the destruction of the ET tank at 73 seconds.
Without commenting on the likelihood of success, I believe it was possible. Was a method available to save the Challenger Crew?
Removing the SRB’s from the stack, does not however save the Challenger crew. At 74 seconds, with the Challenger still attached to the ET, a successful RTLS must be executed.
The boundary conditions for the RTLS are unique:
Altitude 47,000 feet, velocity 2900 feet per second ( 1,977 mph), Downrange 9 nm Orbital inclination 28.5
ET tank at 87.7% O2 and 87.9% H2 Challenger disaster: how full was the external tank at the time of destruction?
ET tank with a leak of unknown flow rate and unknown size hole
SRB’s flying uncontrolled for another 50 seconds nearby
I see Dick Scobee and Mike Smith facing two almost insurmountable issues:
With the shuttle mass changing in an unpredictable fashion due to LH leak the FSW for RTLS will be nearly useless.
From the training manual for RTLS
If the RTLS abort is declared early in the flight, some fuel wasting is necessary, so the fuel dissipation task takes control of guidance. The fuel dissipation task works by assuming an immediate pitch around. From this assumption, software predicts the trajectory of PPA and flyback phases to compute the shuttle mass at MECO. This burnout mass is then compared to the desired 2 percent ET propellant remaining. If the predicted mass is less than the desired mass, then the pitcharound is initiated immediately. If the predicted mass is still greater than the desired mass, then further fuel wasting is necessary. The difference in mass between computed and desired is then used to compute how much more fuel must be wasted.
I believe the predicted MECO would be well after the actual engine fuel starvation. Further, with large quantity of fuel in the ET, the fuel dissipation maneuver would take the damaged stack far away from the KSC return target.
Question #1: What if anything could a shuttle pilot do in this situation to choose a MECO that is survivable?
The ET tank was breached by the SRB flame at 66 seconds and has a hole in the aft wall. It further was of unknown structural integrity.
The PPA maneuver requires exposing the aft end of the ET to the slipstream. I believe the actual ET tank failure on 51L occurred because the RH SRB rotated into the top of the ET and punctured a hole. This hole then created high pressure airflow into the ET tank and it literally “blew up” like a balloon and popped. The PPA maneuver might expose the aft ET leak to the same circumstance.
Question #2: Given these risks could a RTLS be executed by rolling the STS into a shuttle up position, burning enough fuel to gain sufficient altitude for a “safe” ET separation sequence. What would be the safe altitude to execute this task, how far downrange and what velocity would the Shuttle be at?
The leak in the LH intertank was sufficiently small that it was compensated for by the ET system until the moment of ET destruction at 72 seconds. As the timeline shows this event began at 72.204 seconds when the RH and LH SRB paths diverged. This was the point where the ET tank forward dome breach occurred and ET structural breakup began and LH tank destruction began. It held nominal pressure (above 32.8 psi) until that event and only the last valid data point shows a downward trend.
I did some thrust to weight ratios over time using the known data:
This data tells me that on 51L with no SRB attached after 72 seconds the thrust to weight ratio would result in the vehicle losing velocity. However, after MET 124, the thrust to weight ratio between 51L with a leak and a nominal flight would be similar. I provide 3 examples of the numbers used for a nominal calculation at the bottom of this spreadsheet.
Clearly with the loss of SRB thrust the stack would have been in a ballistic arc. I wonder if ET sep could have been accomplished as the Stack reached the top of that arc? Velocity would have been low and conditions would have been similar to the nominal RTLS post MECO coast, with the exception of the fuel load not being at 2%.