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Had a space shuttle suffered a main-engine failure very early on during ascent, or had a time-critical emergency (for instance, a crewmember unexpectedly having a stroke) occurred before negative return,1 it would have performed an RTLS (Return To Launch Site) abort, whereby it would fly a normal first-stage2 trajectory, and, after the SRBs were exhausted and decoupled, continue straight downrange for a considerable distance before eventually pitching around, gradually nulling out its downrange velocity and acquiring considerable uprange velocity (losing a tremendous amount of altitude in the process), and finally jettisoning the ET and gliding to a landing back at KSC.3

This would seem to be a rather risky course of action, given that, if one SSME failed, especially early on, whatever took it out could easily knock out one or both of the remaining SSMEs,4 which would, for almost all of the standard RTLS trajectory, leave the orbiter unable to reach land; this would have been especially bad pre-Challenger-loss, when the shuttle crews had no bailout capability, forcing them into a likely-fatal ditching. The long second-stage downrange leg is especially odd, given that it would bring you further and further from safety at an increasingly fast rate.5 If anything, it would seem that one would want to avoid pitching over into the usual gravity turn, and instead fly as steep a flightpath angle as possible in order to maximise one’s reserves of altitude and minimise one’s velocity away from terra firma; failing that, turning to the north or south would keep one closer to a landing site than simply continuing straight onwards would, and would allow the final turn back to KSC to have been made via a bog-standard banked turn, rather than the supersonic vertical plummet of a standard RTLS (where the loss of even one additional engine could easily cause a loss of control). So why did the standard RTLS trajectory go so far downrange before turning around?


1: Negative return was the point in a shuttle ascent past which an RTLS abort would no longer be physically possible.

2: I’m going by the stage designations in NASA’s SS abort documentation, where “first stage” was before SRB burnout and jettison, “second stage” was from SRB jettison to SSME shutdown and ET jettison, and “third stage” was the OMS burn(s).

3: Shuttle launches from Vandenberg, had they not been cancelled, were planned to land at Edwards in the event of an RTLS, rather than back at Vandenberg (lots more room for error at Edwards, given that you can land basically anywhere on Rogers Dry Lake if you have to; this actually happened on at least one shuttle landing, which came down short of the paved runway and was on the lakebed for the first part of its landing roll).

4: This nearly happened on STS-51F), where a faulty sensor disabled one SSME and came within seconds of causing another to shut down before it [the sensor] could be deactivated. Had the second SSME been lost, Challenger would have come down in the middle of the Atlantic; as this was before bailout capability was added in the wake of STS-51L, the crew would have been forced to ditch in the ocean, and would likely not have survived. As it was, they were still able to reach (a much lower) orbit (known as an Abort to Orbit, or ATO for short).

5: Wikipedia claims that the extended downwind leg was to allow enough time to burn off all the propellant in the ET, but this explanation doesn't stand up under scrutiny; flying (nearly) straight up, or turning steadily to one side during the leg, would serve the same purpose, and not require killing off nearly as much downrange velocity or clawing back nearly as much distance from KSC.

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    $\begingroup$ You can't do anything during first stage because you are stuck on the SRBs till they burn out. Your footnote 5 is incorrect and Wikipedia is right, though simplified. I suggest you read section 6 here: nasa.gov/centers/johnson/pdf/… and section 6.3 here nasa.gov/centers/johnson/pdf/… then edit your question if not satisfied. Shuttle was highly alpha/beta constrained during ascent. $\endgroup$ – Organic Marble Feb 20 at 15:26
  • $\begingroup$ @OrganicMarble: Just because you can't jettison the SRBs before they burn out doesn't mean you can't start altering your trajectory before they burn out. $\endgroup$ – Sean Feb 20 at 22:16
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    $\begingroup$ It pretty much does because of the DOLILU alpha/beta constraints. See here: space.stackexchange.com/questions/21056/… and the linked documents. The shuttle flew through a pretty narrow alpha/beta "tunnel in the sky" for first stage. Also, the first stage trajectory was pre-programmed (basically a canned trajectory) prior to launch. $\endgroup$ – Organic Marble Feb 20 at 22:35
  • $\begingroup$ @OrganicMarble: Okay, canned trajectory I get, but, if that could have been "uncanned" somehow, I still don't see why the trajectory couldn't have been altered within the constraints of the tunnel. $\endgroup$ – Sean Feb 21 at 4:50
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    $\begingroup$ You are correct in that the first stage trajectory was altered slightly for engine out cases: A set of engine-out I-loads were incorporated into the day of launch trajectory design. There were tables for "top engine out" and "side engine out" cases. It could also be altered slightly for load relief. But a common problem with people outside the program asking why didn't shuttle do such and so instead is that they don't understand the incredible number of contraints placed on the system. I hoped that reading the references linked in my comments might give you a glimpse of some of these. $\endgroup$ – Organic Marble Feb 21 at 13:48
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Please refer to this diagram while reading this answer.

enter image description here

You mentioned modification of the first stage trajectory in response to initiation of an RTLS abort; this was in fact allowed for, within the stringent constraints of the first stage ascent. A pitch bias would be added to the pre-programmed pitch attitude to loft the trajectory above the nominal. You can see this lofting in the diagram above. If the abort was a 3-engine RTLS, the engines would be throttled down until the total thrust approximated that of two engines. Also, upon abort initiation, any I-Y (out of plane) steering would be cancelled.

The Space Shuttle was flown close to its design margins during a nominal ascent to maximize performance. Execution of a Return to Launch Site (RTLS) abort was an even more challenging maneuver. For successful completion of an RTLS, many criteria must be satisfied.

Here is a sampling of some of the criteria that had to be met.

  • The first stage ascent alpha/beta/qbar constraints and load factor rules must not be violated.
  • During the high-speed portion of second stage the vehicle's altitude should not droop below 265,000 feet. Below that altitude loss of control may occur or the ET may rupture due to overheating.

  • Powered pitcharound (the maneuver that ends the accelerating-away-from-the-launch-site phase and begins the process of deceleration) must be performed at a rate of approximately 10 deg/sec

  • The ET must impact no closer to land than 25 nautical miles

  • For a safe separation, the ET must contain a maximum of 2% propellant remaining at Main Engine Cutoff (MECO).

  • MECO must occur at a flightpath angle, altitude, and velocity that allows the Orbiter to glide to the runway without undershoot or overshoot.
  • Dynamic pressure at MECO must be between 2 and 10 lbs/ft^2

  • The angle of attack just prior to MECO must be approximately 30 degrees.

  • The angle of attack at ET Sep must be approximately -2 degrees.
  • The powered pitch down (transition between these two values of alpha) must be performed at a rate of approximately 8 degrees / second.
  • During the glide phase, Z axis acceleration must not exceed 2.5 gravities.

Any trajectory that met these constraints (and many more that I don't know about or have left off) could theoretically have resulted in a successful RTLS. I am confident that the decades that highly skilled engineers spent refining this abort mode resulted in the optimum design for this far-from-optimum situation.

Sources

Intact Abort Training Manual section 6

Space Shuttle Flight Rules A2-57, A2-62

Space Shuttle Day-of-Launch Trajectory Design

Shuttle Crew Operations Manual section 6.3

Upper stage structural loads on ascent?

Personal notes & experience

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