Space Shuttle RTLS in 3-engine-out: what does it use for propulsion?

Question

For a three-engine-out abort, there's a couple of great diagrams provided in the answer at https://space.stackexchange.com/a/39733/11532 showing different abort contingencies. One contingency plan is the "RTLS" (Return To Launch Site), where the spacecraft would reverse direction and fly back to the spaceport. However, this raises the question: with the main engines out, what provides the thrust for the flyback maneuver (the "power" for the PPA or "Powered Pitch Around")? For that matter, the diagram mentions "MECO" (Main Engine Cut-Out); what "main engines" are going to undergo MECO if the SSMEs are all already out? Is that the SRBs (Solid Rocket Boosters), or the OMS (Orbital Maneuvering System), or...?

The SRBs seem like the most likely answer - they've got far more raw power than the OMS, and they are steerable - but they're attached to the external tank, which I expect you'd want to jettison for weight and aerodynamic control reasons even if the lack of a balancing thrust from the SSMEs didn't cause center-of-gravity issues. They also don't have controllable throttle or shutdown (aside from the fight termination system, which basically just blows their casing apart). Additionally, you'd need to jettison the external tank (and thus the SRBs) at some point before landing - can't deploy the landing gear with the tank in the way - but that could possibly be the "MECO" point as I expect that the relative lack of aerodynamics and lifting surface mean that they wouldn't make it back to land even if the orbiter could.

On the other hand, the OMS seems like a much more conventional choice. It's underpowered compared to the SSMEs or SRBs, but with the external tank and SRBs jettisoned, and the orbiter flying on aerodynamic control, it wouldn't surprise me that much if the OMS has enough power to get the orbiter home. They're fueled from an internal tank (they're mainly used in orbit, when the external tank has already been jettisoned), and actually carry quite a bit of fuel. They are throttlable, steerable, and restartable. I'm pretty sure their nozzle expansion isn't too great for sea-level use. Finally, the orbiter is clearly designed for powered maneuvers under OMS alone (can't use the main engines in orbit once the external tank separates), unlike the SRBs.

Answer 1

Forget about the SRBs, they play no role in an abort. You don't even select the RTLS abort until after SRB sep.

The earliest RTLS selection is made at 2m30s allowing time for SRB sep induced transients to be damped out, and for second stage guidance to converge. Therefore, an abort could be initiated before SRB sep, but the vehicle would not begin the RTLS flight profile until after 2m30s anyway. The result is that the RTLS abort is always delayed until at least 2m30s.

Section 6.3 from Intact Abort Training Manual

The OMS will only be used for dumping propellant for CG control in these contingency cases.

If you are in a three-engine-out case, you are not going to be doing an RTLS.

Those black zone graphs show the parts of the trajectory that are going to be unsurvivable if you are already doing an RTLS and lose your last engine. Similarly, the color zone graph shows which type of contingency abort to execute based on when you lose that last engine during an RTLS.

If you refer to the top line of boxes in the ascent checklist cue card, you will see the types of entry that result from three-engine-out cases:

• CONT ENTRY - Contingency entry. Results in bailout, if you are lucky.
• ECAL ENTRY - You attempt to make it to an East Coast landing site by turning back towards the coast
• Only the topmost box results in a TAL - Transatlantic Landing - this is shown in the 3-engine-out uphill graph, and happens when you suddenly lose all three engines right before Main Engine Cutoff. This is the only "intact abort" on the card for 3-engines-out.

The bottom part of the card (white background) is for 2-engine-out contingency cases.