John:
If the fins are big enough there will be no stability problem at burnout.
If the fins are too small or nonexistent, active control is required.
Bill
On Wed, Nov 17, 2021 at 3:32 PM John Stoffel <john@xxxxxxxxxxx> wrote:
("crogers168")> writes:"crogers168" == crogers168 <<dmarc-noreply@xxxxxxxxxxxxx>
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