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Is Starship aerodynamically stable when traveling nose first, or does it require constant gimbaling from the main engines to keep it pointing nose first? Is this part of the explanation for why Space X kept Starship under power for the entire ascent of the SN8 flight?

If a rocket has fins then those fins are usually at the back. These rear fins put the center of drag behind the center of mass, making the rocket aerodynamically stable. Starship has big fins at the front and looks stable in the bellyflop position, traveling belly first.

If Space X had attempted an ascent where the Starship accelerated rapidly from the pad, then coasted to it's apogee, would Starship naturally move to a belly up position as it coasts up to it's apogee?

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  • $\begingroup$ "Most rockets have fins at the back": how many currently operational rockets with fins can you list? $\endgroup$ – Christopher James Huff Dec 10 '20 at 16:44
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    $\begingroup$ @ChristopherJamesHuff Soyuz, of course, and I'm gonna call F9's landing legs really stumpy fins. So that's most current launches... $\endgroup$ – user20636 Dec 10 '20 at 21:20
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    $\begingroup$ "I'm gonna call F9's landing legs really stumpy fins." How many legs does a dog have if you call its tail a leg? $\endgroup$ – Russell Borogove Dec 10 '20 at 21:48
  • $\begingroup$ I don't understand the last sentence of the first paragraph. You were expecting part of the ascent to occur without powered flight? How? How fast did you think it was going to go for this test? $\endgroup$ – Harabeck Dec 10 '20 at 22:08
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    $\begingroup$ @Harabeck I for one expected a flight more like New Shepard's, with a shorter burn and unpowered coast to an apogee of 12.5 km. This approach reduced aero stresses and demands on the RCS system, but also left a lot more time for things to go wrong due to vibration, overheating, etc. It also shows their confidence in Raptor itself...the last one to cut out on ascent burned about as long as a booster engine would in a real launch. $\endgroup$ – Christopher James Huff Dec 11 '20 at 17:19
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Is Starship aerodynamically stable when traveling nose first, or does it require constant gimbaling from the main engines to keep it pointing nose first? ... Starship has big fins at the front and looks stable in the bellyflop position, traveling belly first.

Starship also has larger fins at the back, with what looks to me like about ~2-3 times the area of the fore-fins, so I expect it's aerodynamically stable on ascent, though with a smaller stability margin than if the fins weren't present. Compare with air-to-air guided missiles, which often have large fins at the back, smaller fins at the front, and non-vectoring solid rocket motors -- these are designed for stable flight, but with a small stability margin to keep them maneuverable.

Is this part of the explanation for why Space X kept Starship under power for the entire ascent of the SN8 flight?

I don't think so. In vertical flight in low-altitude dense atmosphere, you have to maintain power to maintain ascent. Once the engines were shut off, Starship slowed rapidly and shortly began its descent. With enough thrust at liftoff, you could theoretically accelerate to high speed immediately and coast to your target altitude, but it's not efficient due to atmospheric resistance, which increases with the square of velocity.

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    $\begingroup$ It's more likely that the slow ascent was intended to reduce aerodynamic forces, and that they paid a stiff penalty in gravity losses for it. This short flight wasn't at all limited by efficiency: it actually slowed to a hover for a time on one engine just before the dive (you could see smoke/vapor blowing away horizontally), possibly burning off propellant until it reached the load desired for landing. $\endgroup$ – Christopher James Huff Dec 10 '20 at 18:34
  • $\begingroup$ @ChristopherJamesHuff It wasn't actually a hover, they were jogging sideways, probably so that get away from the pad and test steering the craft with just the fins on the way down. $\endgroup$ – Harabeck Dec 10 '20 at 22:10
  • $\begingroup$ @Harabeck can't think of a reason why they wouldn't have done the translation while ascending. The vehicle was tilted while balancing on one engine, and there were likely non-zero winds at that altitude, both of which could give the impression of horizontal movement. $\endgroup$ – Christopher James Huff Dec 10 '20 at 22:43
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    $\begingroup$ Starship's fins were straight out on ascent, meaning in the best case they would still only provide stability in one axis. $\endgroup$ – Austin Dec 11 '20 at 5:25
  • $\begingroup$ @ChristopherJamesHuff I'm no rocket scientist, but it could be that the maximum deflection they could safely achieve at full power would not allow them to get the full horizontal distance they wanted over such a short ascent. I'm also seeing discussion that they need to be going very slowly anyway for the start of the belly flop. $\endgroup$ – Harabeck Dec 11 '20 at 16:51

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