An interesting scenario I ran a lot to test Powered Explicit Guidance convergence for a TAL abort after a droop procedure.
It is basically a double engine failure between 12000 and 14000 km/h where we can avoid a contigency abort with a tough entry G wise, and transition towards a more nominal Entry after a specific procedure to avoid to fall too deep into lower atmosphere layer (below 265kfeet).
Some informations from the Ascent/Abort handbook:
When a substantial loss of thrust occurs (second main engine failure, or one engine fail with
another stuck in the throttle bucket), the Orbiter can continue downrange if past the “droop”
boundary. This boundary is described as follows: Whenever two engines fail early in ascent, the
thrust to weight of the shuttle is instantly reduced to less than 1.0 and a dip (or droop) in the
trajectory begins. As fuel is burned by the remaining engine, the vehicle thrust to weight increases
until it is once again greater than 1.0 and a climb can resume. If the vehicle descends no lower
than 265,000 ft in this process, it does not violate ET heating constraints. Staying above 265,000
ft allows the abort to continue in powered flight until fuel depletion, thus getting closer to TAL
sites. Prior to this droop boundary, if two engines fail, the shuttle does not have sufficient energy
to stay above 265,000 feet, so separation from the ET must be immediate, terminating powered
flight with fuel still remaining. This boundary is usually near 12,000 fps.
If two engines fail prior to the droop boundary as in line a (fig. 2.5.2-1), the vertical thrust
attitude cannot “turn around” the sink rate until below 265,000 ft where the ET begins
overheating. The first point in the nominal trajectory where (if two engines fail) a near vertical
thrust attitude can prevent descending below 265,000 ft is the point called the droop boundary.
The droop in the trajectory bottoms out when thrust to weight returns to greater than one due to
fuel burnoff. Achieving the droop boundary allows a safe downrange abort and is the basis of the
two engine out droop maneuver (contingency abort cue card).
Morning Launch
Second stage and closed loop guidance
Double engine failure past the droop boundary and before the single engine TAL boundary (14000 fts ish)
OMS propellant is dumped using the RCS nozzles for Center of Gravity considerations.
We can see all the AFT RCS jets working at once.
Max throttle is also selected (109%)
Once the vertical speed is positive and we are out of the danger zone, Droop logic is automatically exited and we can declare a TAL abort.
PEG guidance MECO parameters are changed and guidance reconverges quite fast.
Single engine operations view from the rear
Manual MECO when propellant is under 2% to avoid fuel starvation in the feedlines and some damages to the Main Engines
Velocity is quite low ( 22500 fts) instead of 24000 ftsish for a nominal TAL. It is like expected as we had the failure before the single engine TAL boundary.
There is a logic for the case where there is an underspeed at MECO and when a low energy condition arises, called Low Energy Logic.
Low energy logic is activated (Yellow item 3) and first pullout after a tough entry in the denser atmosphere.
Almost at the TAEM interface and still a bit low in energy
TAEM interface and Straight In downmode is selected (with the corresponding message OTT ST IN)
with item 6.
It will be a direct entry into the HAC instead of a 300 ° Alignement Cone. A very good help to get rid of a low energy condition.
View from Istres tower
Closing in
Into the HAC
Pre Flare
Short final
Touched
Welcome in France, Atlantis