Unexpected behavior if thrust is re-calculated between engine sizing and off-design

[ngomezve]

The engine is sized just before mission!() gets called. In mission, the engine state is calculated using tfoper!() and the design thrust. Interestingly, the thrust gets re-calculated but never stored. This is an issue in off-design missions because the thrust at start of cruise does not get stored properly and its not the same as in the design point. If the thrust is stored, the aircraft converges to a slightly different point. I believe this may be the reason why it was not getting stored in the first place.

I'll do a quick fix by not running this portion of the code in the design cases, but this should be investigated further.

https://github.com/MIT-LAE/TASOPT.jl/blob/d2cd9ed3a973c80dad6d539eadcdfbb8b83c58e6/src/mission/mission.jl#L447-L461

[ngomezve]

I have traced this issue to the fact that para[iafracW, ipcruise1] gets recalculated between sizing and the off-design call in ipcruise1. The balance function is also run and this affects thrust. If all these are moved to after the tfoper call so the design and off-design thrusts are exactly the same, the aircraft converges to the same point as before.

Interestingly, all of these parameters converge to a consistent value as the sizing loop converges. For the default aircraft, the current version gives a PFEI of 0.9186795; if the off-design cruise point engine call is run storing the thrust, the PFEI is 0.9160889. It's unclear why this behavior occurs.