The more I think about my suggestion, the less enthusiastic I am about its
utility. Scrub all that :)
Interesting analysis on the horizontal velocity issues. I’ve never done that.
What kind of material are you looking at using for shock cords?
Troy
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Robert Watzlavick
Sent: Tuesday, 28 July 2020 3:18 PM
To: arocket@xxxxxxxxxxxxx; Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket
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