How is the Falcon 9 aerodynamically stable while ascending? The rocket doesn't look like it should be able to fly straight. With the large, largely empty fairing at the top and the heavy engines at the bottom and no fins, it looks like it would want to flip around. Is it simply really well controlled by computers to not stray from the prograde vector, or are the engines really light, or what?
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asked Jun 22, 2017 at 20:06
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$\begingroup$ I'll leave it to the aero engineers to give a complete answer, but I would note that it has enough control systems to fly backwards in order to land. $\endgroup$– Cort AmmonCommented Jun 22, 2017 at 20:15
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3$\begingroup$ Would Space Exploration be a better home for this question? $\endgroup$– Qmechanic ♦Commented Jun 22, 2017 at 20:20
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1$\begingroup$ I might have thought that a large heavy fairing/payload on top and light engines at the bottom would be much less stable. $\endgroup$– Jon CusterCommented Jun 22, 2017 at 21:40
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3$\begingroup$ @JonCuster, if I remember correctly from my model rocket adventures decades ago, the center of pressure needs to be behind the center of gravity for stability, i.e., moving the center of gravity towards the top increases stability on ascent. I seem to recall (from experience) that longer rockets needed less fin area than shorter rockets. $\endgroup$– Alfred CentauriCommented Jun 22, 2017 at 22:24
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$\begingroup$ I suppose the sheer length is helpful. (thanks Alfred) When landing, it has grid fins deployed and most weight in the bottom so it is easily stable. While ascending, I suppose it simply relies on computer guidance systems, and the fact that it isn't too terribly unstable. $\endgroup$– Nathanael VettersCommented Jun 23, 2017 at 17:11
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1 Answer
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It isn't.
Stabilisation is achieved by gyros which command the rocket nozzles to swivel so a sideways component of thrust can be used to balance the rocket. This has been true since the days of the A-4, which used moveable graphite fins sticking into the rocket exhaust to stabilise the device.
Before, rockets used much larger fins and a long ramp for stabilisation, and the rocket had to gain enough speed for the fins to become effective. Only by adding artificial stabilisation did bigger rockets become possible.
answered Jun 26, 2017 at 8:57