But we have a few problems here. First, if the compression is being done by a normal shockwave, I don't think there's any possible capture here.
You still have the air moving at much higher speed through the same area upstream of a normal shock than downstream, leading to a substantial increase in density downstream. At a free stream Mach number of 2.0 compression is about a factor of 2.7, giving virtually sea level density at 30k feet altitude.
But we have a few problems here. First, if the compression is being done by a normal shockwave, I don't think there's any possible capture here.
You still have the air moving at much higher speed through the same area upstream of a normal shock than downstream, leading to a substantial increase in density downstream. At a free stream Mach number of 2.0 compression is about a factor of 2.7, giving virtually sea level density at 30k feet altitude.
The issue is, what is compared to what?
This is basically a quantitative discussion. For instance, Mach 2.7 at 35k feet gives you roughly the same MFR as Mach .9 at sea level, assuming the inlet's pressure recovery is about the same at the different regimes. On the other hand, if you want to do Mach 1.2 at 35k feet with the same level of thrust as Mach .9 sea level, you'll need a larger inlet.
As an alternative, we could instead do a comparison between Mach .5 and Mach 2, which is a factor of 4 in terms of velocity. In this case, since air density drops by about 70% from sea level to altitude, you get a total factor of about 1.2x sea level at Mach .5.
So inlet area in fact becomes a tuning factor when it comes to engine performance, too small, bad performance at a variety of altitudes and speeds, too big, bad performance at low speeds.
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This originally came up in discussing as to whether the J-20's different inlet could allow the J-20 to supercruise on Al-31. The answer was ultimately no, even with the best case inlet area calculations (which could very well be wrong), except that it could hit a pseudo-supercruise (dive or AB into supersonic) at 35k ft of Mach 1.2-1.4 if the best case inlet area calculations were used. So that's an important flight regime, i.e, MFR at Mach .9 and when breaking the Mach barrier, so that's a flight regime we should be concerned about.
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And TBH Trident, I was more concerned about puncturing your claims regarding speed being able to unconditionally overcome air density issues at altitude. If you look at real thrust / altitude diagrams for engines, as with the one you showed me, it's hard to pinpoint MFR as the only factor limiting engine performance at altitude. Likely, there's also factors of total pressure recovery at different altitudes and speeds that limit the airframe. I'll point out that with your Al-31 chart, at quite a few altitudes the Al-31 thrust just flatlines despite increasing Mach. Another possible factor is that the charts we've seen discuss pressure recovery as a function of speed. But does pressure recovery also vary as a factor of freestream air density? That could be an interesting factor, but quite likely dynamic pressure overwhelms static pressure when it comes to inlet TPR, even if dynamic pressure is supposed to be near-constant if the inlet does it work correctly at the compressor.
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I will also note that I'm very happy that we're having a serious conversation about inlets. This is, as overscan's book discussed, an oft-overlooked element of fighter design. Stuff like wing designs, wing loading, engine designs, and so on, these are all way sexier for enthusiasts than the inlet. I didn't quite manage to catch your insulting statement, so I'd like to point out I enjoy our conversation.