[AR] Re: Dynamic stability in supersonic rockets
- From: "John Stoffel" <john@xxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 18 Nov 2021 10:23:11 -0500
"Henry" == Henry Spencer <hspencer@xxxxxxxxxxxxx> writes:
Henry> On Wed, 17 Nov 2021, John Stoffel wrote:
crogers168> ...The jet damping contribution to the dynamic stability
allowed a statically unstable rocket to successfully fly...
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.
Henry> It doesn't really change the situation much, except insofar as it can
Henry> mislead you about how stable your design is, because it helps the
Henry> situation until burnout. After burnout, the fins need to be big enough
to
Henry> do the whole job, perhaps helped by spin stabilization. (Or else the
Henry> rocket needs elevons etc. run by an active control system.)
So larger fins would impart more drag on the way up when you don't
really need them. It would be an interesting experiment to try and
have some way to keep the fins retracted until near burnout, then have
them extend out to increase stability. Sorta like an F-111 or B-1,
but probably more so that the fins are nested as much as possible, or
hidden in slots in the body.
This would be tough in a small rocket where the motor takes up most of
the body, since the fins would ideally be towards the rear I guess.
Maybe a three motor rocket so that fins could slot in between the
motor tubes.
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?
Henry> Big rockets are often aerodynamically unstable over much of
Henry> their flight envelope. They have active control systems and
Henry> use thrust vectoring during powered ascent, and they usually
Henry> stage high enough that aerodynamics are not a big consideration
Henry> by that point.
So active control is the real answer. Whats the smallest actively
controlled HPR these days that people have built?
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