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Respond to details of off nominal GRTLS vectors
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Challenger Truth
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At this point the Challenger is high and pointed away from the KSC landing strip. I believe the Gliding RTLS FSW would be able to handle the situation from MECO onward. This The Gliding RTLS TAEM (Terminal Area Energy Management) program would bemake the turns necessary to line up with the HAC (heading alignment cone).

*TAEM guidance is divided into four sections or phases. The four phases are:

• Acquisition phase
• Heading alignment phase
• Prefinal phase

The key to understanding how TAEM works is the concept of "range-to-go." In order for TAEM guidance to work, it must know the accurate distance the shuttle must fly before it can land. It is not enough to know the straight line distance from the shuttle to the runway. This is obvious when one considers that the shuttle must approach the runway at the proper speed and direction. Therefore, the turn necessary to get the shuttle lined up for landing must be taken into account.

To model these turns, the shuttle computers project what is called a “nominal” RTLS once MECOheading alignment cone or HAC. This HAC is setan imaginary cone in space which is located 7 n. mi. from the end of the runway. The projection of this cone at any altitude is a circle which describes a turn the shuttle must make to get lined up with the runway. By approaching the HAC on a tangent and then turning on the HAC, the shuttle will complete the turn lined up with the runway center line. For each runway, there are two HACs, one on each side of the runway. The shuttle is normally targeted for the farther HAC, which is called the overhead HAC since the shuttle must make a long overhead turn to get lined up on the runway. The nearer HAC is the straight-in HAC, and the shuttle makes a shorter turn to get lined up. It can also be seen that the overhead HAC requires more energy. Therefore, the selection of these HACs is partly a function of energy*.

https://www.aerospacearchives.tk/rtls-abort/grtls-guidance.html

At this point the Challenger is high and pointed away from the KSC landing strip. I believe the Gliding RTLS FSW would be able to handle the situation from MECO onward. This would be a “nominal” RTLS once MECO is set.

At this point the Challenger is high and pointed away from the KSC landing strip. I believe the Gliding RTLS FSW would be able to handle the situation from MECO onward. The Gliding RTLS TAEM (Terminal Area Energy Management) program would make the turns necessary to line up with the HAC (heading alignment cone).

*TAEM guidance is divided into four sections or phases. The four phases are:

• Acquisition phase
• Heading alignment phase
• Prefinal phase

The key to understanding how TAEM works is the concept of "range-to-go." In order for TAEM guidance to work, it must know the accurate distance the shuttle must fly before it can land. It is not enough to know the straight line distance from the shuttle to the runway. This is obvious when one considers that the shuttle must approach the runway at the proper speed and direction. Therefore, the turn necessary to get the shuttle lined up for landing must be taken into account.

To model these turns, the shuttle computers project what is called a heading alignment cone or HAC. This HAC is an imaginary cone in space which is located 7 n. mi. from the end of the runway. The projection of this cone at any altitude is a circle which describes a turn the shuttle must make to get lined up with the runway. By approaching the HAC on a tangent and then turning on the HAC, the shuttle will complete the turn lined up with the runway center line. For each runway, there are two HACs, one on each side of the runway. The shuttle is normally targeted for the farther HAC, which is called the overhead HAC since the shuttle must make a long overhead turn to get lined up on the runway. The nearer HAC is the straight-in HAC, and the shuttle makes a shorter turn to get lined up. It can also be seen that the overhead HAC requires more energy. Therefore, the selection of these HACs is partly a function of energy*.

https://www.aerospacearchives.tk/rtls-abort/grtls-guidance.html

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Challenger Truth
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The key to any successful RTLS is energy management. The reason for the dangerous PPA maneuver in a nominal RTLS is to bleed off the excessive velocity prior to ET sep and gliding. This must be done within the proper boundary conditions to succeed. In simple terms they must use the SSME to slow down. The problem facing the Challenger pilots was similar but if they took no action the STS stack without the SRB’s would naturally lose velocity. The situation at 74 seconds requires the remaning 51L stack to bleed off excess velocity and get in the correct configuration for post MECO coast and ET sep while not getting too far away from KSC.

So the answer to the first question is that the choice of MECO after SRB sep at 72 seconds is not critical, so long as it is not delayed too long. The vehicle in post SRB sep configuration is doing what you want it to do, lose velocity. Without the SRB thrust the stack is losing velocity at the rate of 10 feet per second even with all three engines running. My very rough calculation with a starting velocity of 2900 feet per second indicate that the shuttle will still be gaining 3800 ft/sec in altitude and moving 1962 feet downrange every second.

So after successful SRB sep give Dick and Mike 15 seconds to regain situational awareness. That puts them at 90,000 feet and 15 miles downrange. Velocity has dropped to 2618 feet per second.

It is at this point seat of the pants pilot skills come into play. Dick Scobee was a Shuttle Carrier Aircraft flight instructor and was familiar with the out of nominal release conditions for the Shuttle and the SCA. He would have been the best astronaut in the program to make this off nominal ET sep decision.

The best move is to increase the loss of velocity quickly so that you can execute the ET sep and start the glide back toward KSC. The most obvious way to bleed off velocity is to do a MECO. With no thrust from the SSME and a 1.6 million pound vehicle, the stack will rapidly lose energy. First, step would be to roll the shuttle into a “shuttle up position” MECO would occur at 95 seconds, at 2240 feet per second, altitude 111,000 feet, 18 miles downrange. The RTLS FSW would take over and control the ET sep process At 1300 feet per second the ET sep would occur. My calculation indicates that this would occur at 18 seconds after MECO or MET 113 seconds. Altitude would be 138,000 feet and 26 miles downrange.

At this point the Challenger is high and pointed away from the KSC landing strip. I believe the Gliding RTLS FSW would be able to handle the situation from MECO onward. This would be a “nominal” RTLS once MECO is set.