I never fully bought the claim that the FSI ****** could really go supersonic. Note I am not saying I think the claim was 100% bogus, but I tend to think it is a lot more likely be untrue than true.
If a person could get hold of some truly accurate engine data for the F100 and D20, not just the FSI claimed thrust curves and N-sec, but REAL data, and also account for the real-world engine masses and so forth, it would be interesting to run the whole ****** system in Rocksim and see if it was likely it really could do it. Keep in mind that the F100 was only about 50 n-sec, but the actual N-Sec may have been less than that (The "E5" was supposed to be 22 N-sec but by sometime in the early 1980's the NAR certified the E5 as a "D", at something like 17, or 18, or just maybe 19 N-sec. That was symptomatic of some other motor claims as well, I do not think the real performance delivered by the F7 or F100 was over 45 N-sec). Also keep in mind those were BP motors, and their casings were cardboard, pretty darned heavy.
Since the D20 was a friction-fit inside of the F100, I think that most likely the "Sonic Pop" some claimed to hear, was literally the D20 POP-ing out of the F100 at staging. Or even just hearing the D20 itself when it staged. Several others speculated those two as well.
Edit - I am not totally sure if the booster was an F100, or an E60. If an E60, even worse.
The composite Enerjet engines of the early to mid 1970's, their time-thrust curve was very bad for rockets overall, with relatively low thrust at liftoff, and progressively stronger thrust as they burned. But for a rocket intended to go supersonic, that would be the kind of curve you'd want - more and more thrust as the drag built up more and more from the increasing velocity. I think someone claimed to have gone supersonic using an Enerjet F67, which might have been possible (and it was close to a full F motor) but I do not think they had a way to prove it.
It would ALSO be interesting to run a sim of an F67 inside of a minimal 30mm rocket to see how that would have compared against the FSI ****** system or even today's "F" class motors regarding Supersonic flight.
- George Gassaway