Coop
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- Joined
- Dec 15, 2011
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This past weekend, at MDRA, I finally got down to the field late Sunday morning. I would have been there an hour and a half earlier, however, the EMS gods did not find it necessary to let me go just yet... had to stay late to finish the paperwork.
So after working 25 hrs of my 24-hr shift, I arrived, all strung out on black coffee and having a wicked ammonium perchlorate jones.
First order of business was to get my Jolly Logic Alt 3 verified. My trusty ol' Kraken did that job... 550' on a D12-5, with its standard recovery profile of helicoptering around on its small chute at a leisurely 16 FPS.
Once data was verified from the on-board JL, we got to work.
I've been making a bunch of streamers for my dual-deploy rockets. Last time, I recovered a number of MPR and smaller HPR rockets with a streamer as primary recovery, just to prove it could be done. This time, I came with actual parachutes.
Sort of.
I'd brought three I wanted to test, yet, failed to bring the one MPR rocket I'd intended to use as a test bed, so I was reduced to two. No worries.
First, was a 31" chute, a modification of a disk-gap-band type loosely based upon the Viking probes' mains. The object of the test is to determine the Cd. It is a 12-gore canopy, with a slightly lower gore to width ratio than the Viking specs, and features a pull-down apex to a toroid shape. This was in my Pem-Tech Bucky Jones... a rocket reminiscent of the 50's science-fiction serials, complete with gaudy gold and silver coloring, art deco swoopiness, and a dorsal cockpit. She comes in horizontal to protect her swoopy, pointed fins by means of a kevlar tie-off to the shock cord.
Bucky flew to 2,215 feet on a CTI H135-7. Ejection was slightly before apogee, and she descended at 18 FPS. At recovery, she weighed 2.6 lbs.
Cd *S *0.5 *rho *V^2 = M where Cd = Coefficient of Drag, S= reference area in square feet, rho = 0.002377, V = velocity in FPS and M = lbs at recovery.
Cd * (3.141 *15.5^2 /144) * 0.5 * 0.002377 * 18^2 = 2.6
Cd * 5.240 *0.5 *0.002377 * 324= 2.6
Cd * 2.017 = 2.6
From here, I just divide the weight by the calculated figure to get the Cd.
2.6/2.017 = Cd
2.6 /2.017= 1.293
Cd= 1.293
I'm pretty pleased with that, as my first attempts yielded a Cd of about 0.43, if memory serves.
Chute #2 is an evolution of the 31". This is another 12-gore canopy, measuring 40", and is much, much flatter in profile, with a decreased gap, band width, and apex vent. The object of this test was to verify stability of the design in such a profile, figuring that the venting characteristics, if they work on this pancake, will translate to stability across any gore: width ratio. Also, to give a base point of Cd for future efforts.
Chute #2 flew in a slightly modified AT Sumo. A 3-grain H163 was chosen for propulsion because this would allow better visual tracking of the entire flight. Flight was to 1299 feet, with ejection slightly before apogee after cutting the delay to 7 seconds. Descent was at 18 FPS also, and the rocket at touchdown weighed 2.7 lb. There were no noted oscillations, helicoptering, or inflation problems--descent was stable, and due to weathercocking during ascent and drift during descent, I got to see the chute approach my position, pass in front of me, then drift off to its landing position.
I'm calling it stable as all get-out.
Applying the same formula for Cd:
Cd *S *0.5 *rho *V^2 = M
Cd * (3.141 *20^2 /144) *0.5 *0.002377 *18^2 = 2.7
Cd *8.726 *0.5 *0.002377 *324=2.7
Cd *3.360=2.7
2.7/3.360 = 0.803
Cd = 0.803
So, it was all a success for me: I got to see some friends, satisfy the AP spike in my brain, get some data, and --even better-- verify the design is stable. Now to see if I can't get the Cd up where I think it should be...
Later!
--Coop
So after working 25 hrs of my 24-hr shift, I arrived, all strung out on black coffee and having a wicked ammonium perchlorate jones.
First order of business was to get my Jolly Logic Alt 3 verified. My trusty ol' Kraken did that job... 550' on a D12-5, with its standard recovery profile of helicoptering around on its small chute at a leisurely 16 FPS.
Once data was verified from the on-board JL, we got to work.
I've been making a bunch of streamers for my dual-deploy rockets. Last time, I recovered a number of MPR and smaller HPR rockets with a streamer as primary recovery, just to prove it could be done. This time, I came with actual parachutes.
Sort of.
I'd brought three I wanted to test, yet, failed to bring the one MPR rocket I'd intended to use as a test bed, so I was reduced to two. No worries.
First, was a 31" chute, a modification of a disk-gap-band type loosely based upon the Viking probes' mains. The object of the test is to determine the Cd. It is a 12-gore canopy, with a slightly lower gore to width ratio than the Viking specs, and features a pull-down apex to a toroid shape. This was in my Pem-Tech Bucky Jones... a rocket reminiscent of the 50's science-fiction serials, complete with gaudy gold and silver coloring, art deco swoopiness, and a dorsal cockpit. She comes in horizontal to protect her swoopy, pointed fins by means of a kevlar tie-off to the shock cord.
Bucky flew to 2,215 feet on a CTI H135-7. Ejection was slightly before apogee, and she descended at 18 FPS. At recovery, she weighed 2.6 lbs.
Cd *S *0.5 *rho *V^2 = M where Cd = Coefficient of Drag, S= reference area in square feet, rho = 0.002377, V = velocity in FPS and M = lbs at recovery.
Cd * (3.141 *15.5^2 /144) * 0.5 * 0.002377 * 18^2 = 2.6
Cd * 5.240 *0.5 *0.002377 * 324= 2.6
Cd * 2.017 = 2.6
From here, I just divide the weight by the calculated figure to get the Cd.
2.6/2.017 = Cd
2.6 /2.017= 1.293
Cd= 1.293
I'm pretty pleased with that, as my first attempts yielded a Cd of about 0.43, if memory serves.
Chute #2 is an evolution of the 31". This is another 12-gore canopy, measuring 40", and is much, much flatter in profile, with a decreased gap, band width, and apex vent. The object of this test was to verify stability of the design in such a profile, figuring that the venting characteristics, if they work on this pancake, will translate to stability across any gore: width ratio. Also, to give a base point of Cd for future efforts.
Chute #2 flew in a slightly modified AT Sumo. A 3-grain H163 was chosen for propulsion because this would allow better visual tracking of the entire flight. Flight was to 1299 feet, with ejection slightly before apogee after cutting the delay to 7 seconds. Descent was at 18 FPS also, and the rocket at touchdown weighed 2.7 lb. There were no noted oscillations, helicoptering, or inflation problems--descent was stable, and due to weathercocking during ascent and drift during descent, I got to see the chute approach my position, pass in front of me, then drift off to its landing position.
I'm calling it stable as all get-out.
Applying the same formula for Cd:
Cd *S *0.5 *rho *V^2 = M
Cd * (3.141 *20^2 /144) *0.5 *0.002377 *18^2 = 2.7
Cd *8.726 *0.5 *0.002377 *324=2.7
Cd *3.360=2.7
2.7/3.360 = 0.803
Cd = 0.803
So, it was all a success for me: I got to see some friends, satisfy the AP spike in my brain, get some data, and --even better-- verify the design is stable. Now to see if I can't get the Cd up where I think it should be...
Later!
--Coop