[AR] Re: Dynamic stability in supersonic rockets
- From: "John Stoffel" <john@xxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 17 Nov 2021 17:32:25 -0500
"crogers168" == crogers168 <<dmarc-noreply@xxxxxxxxxxxxx>
("crogers168")> writes:
crogers168> The best example of jet damping I've ever seen was a DC-X
crogers168> scale model, a pretty big one (3-4 ft tall?), with no
crogers168> additional fins, on a high thrust L motor. I wasn't
crogers168> involved in the rocket; I just saw it fly. It turns out
crogers168> the rocket was probably unstable (statically unstable).
crogers168> But as long as that high thrust L motor was thrusting, it
crogers168> was flying straight and was having a great flight. What
crogers168> was actually happening was the rocket was statically
crogers168> unstable, and was slowly rotating, but the rotation was
crogers168> very, very slow because the jet damping was incredibly
crogers168> high, the jet damping was keeping the rocket pointy end
crogers168> forward. As soon as that motor went into tail-off, and it
crogers168> was a pretty short tall-off, the rocket immediately
crogers168> swapped ends. The jet damping contribution to the dynamic
crogers168> stability allowed a statically unstable rocket to
crogers168> successfully fly, at least until the motor shut down.
This is really interesting info, so now the question I have is how do
you handle this properly so that you don't go end over end? Or have
the rocket break due to aerodynamic loads.
I guess if you're high enough, the loads aren't a problem? But with a
small rocket, you end up losing thrust low enough that the loads are
just crazy high.
Would a lower impulse, longer burning motor be a solution? So that
you get higher up with lower aero loads before thrust goes away?
Does Falcon 9 or Atlas V have the same worries? Or are they just
staging so high up it's not a problem for them?
John
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