Troy,
The size of the fins was a concern for me too, especially when I
compared them to similarly sized vehicles. The long moment arm makes
them more effective but at some point I would think the boundary layer
gets thick enough that a minimum fin span would be required. I hadn't
run any aero sims in a while so I took a look at them in more detail
today. Aside from the issues mentioned below, I need to keep a
reasonable horizontal velocity at apogee so the drogue doesn't rip out.
More stable equates to more horizontal velocity at apogee. Even at just
10 mph of wind and a 10 second burn, I get a horizontal velocity of 80
ft/s at apogee for a reasonable stability margin at launch. A 15
second burn is way worse. I recently completed some dual deployment
parachute testing using my truck, driving down the street with the
rocket forebody strapped to the back. I only tested the drogue
deployment up to 50 ft/s (and that was assuming vertical velocity, not
50 ft/s into the wind) but it looks like I need to go higher based on
what I'm seeing in the simulations (and test deployment into the wind).
Launching downwind should help somewhat but it would be helpful to know
the wind profile aloft. This vehicle may have to be restricted to low
winds and shorter burns. The long-term goal is to launch it passively
then add active stabilization later, assuming I get it back in one piece.
I'll think about additional fins - thanks for the idea.
-Bob
On 7/27/20 10:34 PM, Troy Prideaux wrote:
Robert,
This is not my area of expertise so consider these suggestions to be worth what you’re paying for them.
Looking at the illustrations you sent through: the fins (to me) look a touch on the small side for a large heavy rocket that’s expecting a slow acceleration profile. The issue for me isn’t so much the stability towards the end of the burn (highest velocity) but more at the start. From my understanding, fins basically serve 2 primary purposes: (1) is to provide aerodynamic stability by shifting the Cp aft of the Cg but (2) they keep the rocket pointed in the same direction. They do that via correcting forces from aero lift on the frontal face in non zero AOA shifts and drag (normal to the face) on the opposite side in such conditions.
Obviously (for given atmospheric conditions) the correcting force provided is proportional to the fin area and speed of the rocket.
Yes, increasing the fin area will likely produce more susceptibility to flutter, but such can be overcome with fin design without significant mass penalties. It will also produce more stability margin and hence more leverage for cross-winds to weathercock the rocket into it at low speeds. Yes, there’s a trade there, but one thing you can (perhaps) do to offset that is to add some smaller fins on the opposite side of the Cg to assist offsetting the leverage the main fins have at the base from cross-winds. Obviously you would size them to achieve the desired stability margin of the rocket. You could even slide them on with sliding lugs to allow for varying fin areas for various different cross-wind conditions.
Anyway… just some ramblings from the peanut gallery.
Carry on…
Troy