Thrust calculation is incorrect when core or bypass are choked

[ngomezve]

In tfsize(), the core mass flow rate is calculated using
$$\dot{m}_c = \frac{F_e-F_i}{(1-f_o+f_f)u_6 - u_0 + BPR(u_8-u_0)+f_0 u_9}$$,
where the far-wake speeds $u_6$ and $u_8$ are calculated by isentropically expanding the nozzle flow to ambient pressure. If the nozzles are only convergent, this equation is incorrect if either stream is choked.

The thrust equation for a turbofan engine with a convergent nozzle is, using the TASOPT station notation,
$$F_e = F_i+ \dot{m}_c\left[(1-f_o+f_f) u_5- u_0+BPR(u_7 - u_0) + f_0 u_9\right] + A_5(p_5-p_0)+A_7(p_7-p_0)$$,
where stations 5 and 7 are the core and fan nozzle throats.

If the nozzle is not choked and the flow expands to atmospheric pressure, the TASOPT calculation is correct as the pressure terms will drop to 0. Otherwise, they have to be included. The calculation in TASOPT would be equivalent to a variable-area convergent-divergent nozzle whose pressure is always matched.

https://github.com/MIT-LAE/TASOPT.jl/blob/36196792dfac68bc83ac1f821eaa8a3142db0e1e/src/engine/tfsize.jl#L629C13-L630C76