I'm using tubular nylon for shock cords. At apogee, the pyroless system
pops the nose cone and pulls the drogue out. A separate main deployment
mechanism (derived from the pyroless release) holds the main in its bag
inside the airframe and releases it at the main deployment altitude. I
think I need to beef up the strength on that mechanism though. Have you
done any load testing of your mechanism? I proof tested the mechanism
at 100 lbf but OSCALC says a drogue deployment at 120 fps will generate
168 lbf of force. When the main releases, the load will only be the
weight of the rocket (50 lbm) as it descends under the drogue. So it
just has to statically survive the drogue deployment. I was thinking of
keeping the same gear motor and adding a second stage using gears with a
larger double-D flat and bearings since the motor likely has plenty of
torque. It's shaft bending and bearing load I'm mostly concerned with.
For deploying the nosecone and drogue, the pyroless system has proved to
be very reliable as I've had no failures in 25+ ground tests.
What I'd really like is one of these but I'm running out of enthusiasm
to build new stuff:
http://www.cooperindustries.com/content/public/en/wiring_devices/interconnect/products/non_explosive_actuator1.html
https://cms.nacsemi.com/Images/FeaturedProducts/Eaton_Non-Explosive%20Actuators.pdf
An alternative of course is a pyro-actuated cable cutter to hold the
main in place.
-Bob
On 7/28/20 12:51 AM, Troy Prideaux wrote:
The more I think about my suggestion, the less enthusiastic I am about its utility. Scrub all that J
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