By watching the live feed of IFT-4, its pretty obvious that Starship does not actually re-enter in the "skydiver" position, but rather at an AoA of around 45°. There is no doubt that this is a lifting trajectory. But how much lift is produced?
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asked Jun 29 at 15:51
Abdullah is not an AmalekiteAbdullah is not an Amalekite
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$\begingroup$ 89°AoA would also be lifting $\endgroup$– user721108Commented Jun 29 at 16:51
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1$\begingroup$ What surprised me was how it seemed to fly almost level for nearly three minutes at 68 km which either was some incredible gliding or else the telemetry was not correct. I never heard or read anyone mentioning this during or since. It took 33 seconds for it to drop from 69 km to 68 km, but then starting at T+50:41 it stayed at 68 km for 2 minutes 53 seconds before reaching 67 km. It then took only 23 seconds to go from 67 km to 66 km. I thought maybe the telemetry was frozen but during its time at 68 km the speed dropped from 24,900 to 22,200 km/h. On the coms it said it was experiencing 0.5 g $\endgroup$– Steve PembertonCommented Jun 29 at 16:53
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$\begingroup$ @user721108 - I never said it couldn't, I said I have not heard any mention of it which is somewhat surprising if it glided nearly level for nearly three minutes that this hasn't been mentioned somewhere, at least not that I have come across. Thus my theory that maybe the telemetry was incorrect. $\endgroup$– Steve PembertonCommented Jun 29 at 16:56
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$\begingroup$ imo telemetry was correct. No clue why that skip entry event wasn't talked about live, since it should be something interesting to tell about while happening $\endgroup$– user721108Commented Jun 29 at 17:10
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$\begingroup$ We can discuss all day the reasons and possibilities for a 3 minute level glide during reentry, which I would be all for. But it's surprising that Starship glided nearly level for three minutes and no one is talking about it, when every nut and bolt and every movement of Starship, even lifts on and off the OLM is discussed in minutia. Would love to hear some confirmation from SpaceX, or find some analysis somewhere of the livestream telemetry. My quick observations are that as it approached 70 km the AOA increased from 61% to 67%. Then early in its 68 km glide the nose steadily lowered to 54%. $\endgroup$– Steve PembertonCommented Jun 29 at 17:34
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One reference here gives 1.0 L/D at 45 degrees, although I don't know why, because the max is 1.5 from the source:
https://forum.nasaspaceflight.com/index.php?topic=56619.0
The source is independent analysis, and I don't have the full text. Looking for similar sources, this is a more proper publication from another Chinese institution
That gives a max value of 3 at the optimum of 40 degrees.
For reference the Shuttle is often quoted at 1.5, and the X-41 might be as much as 3. However, Starship looks much much more like the Shuttle than the X-41, and because of that I trust the 1.5 number much more. What's also likely is that the paper giving the 3 did a less-than-hypersonic or low-hypersonic analysis.
There are reasons you might not even want the maximum L/D. If it were too high, the craft could go into a skip, which might lead to even worse heating. Assuming the telemetry was correct, there is a meaningful chance they actively changed angle in order to hold altitude constant, which means running at L/D lower than optimum for most of the constant-altitude coasting time.
Numbers from the company are not obviously available, so everything you read will be informed speculation, at best.
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$\begingroup$ 1.5 is not a very good number for the shuttle orbiter. aviation.stackexchange.com/a/105672/31936 $\endgroup$ Commented Jul 3 at 13:56