Quantcast

Parachute Testing

The Rocketry Forum

Help Support The Rocketry Forum:

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
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
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
The Sumo under chute, approximately 30 seconds before landing...

ImageUploadedByRocketry Forum1474425357.308348.jpg

Later!

--Coop
 

Tonimus

Well-Known Member
Joined
Jul 9, 2014
Messages
1,511
Reaction score
4
Nice. That 'chute does look mighty flat. Very neat.
 

TangoJuliet

Well-Known Member
Joined
Jun 26, 2016
Messages
1,239
Reaction score
9
I'm not sure what Cd stands for in this case, and I'm not an HPR guy, but I would like to see more about these 'chutes. Can they be scaled down for LPR/MPR?
 

blackbrandt

That Darn College Student
Joined
Mar 18, 2012
Messages
9,281
Reaction score
41
Cd: Coefficient of drag. A unitless number that describes the drag characteristics of an object. The higher the drag coefficient, the "draggier" an object is.
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
Cd is coefficient of drag. It is used to represent the effectiveness of a parachute, pretty much. For example, a parachute with a CD of 1.2 I will recover more weight at a given speed than one of the same size with a CD of 0.75.

These chutes could, theoretically, be scaled to any size, up or down (same as any other parachute).

Later!

--Coop
 
Last edited:

Banzai88

Lvl 1,Wallet....Destroyed
TRF Supporter
Joined
Jul 15, 2015
Messages
2,561
Reaction score
726
Remember that with different test platforms, what you're getting is 'effective Cd', which is a combination of the drag created by the parachute and the rocket, along with whatever side slip lift is created by the cross wind. It'll get you within a pretty tight range of what your actual parachute Cd is, at least close enough to estimate, but a little less than what you're actually computing (put a lead ball under each parachute and drop it from a known height a statistically significant number of times and you'll get a number lower than what you compute based on simple raw numbers and a recovering rocket for a weight).
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
Remember that with different test platforms, what you're getting is 'effective Cd', which is a combination of the drag created by the parachute and the rocket, along with whatever side slip lift is created by the cross wind. It'll get you within a pretty tight range of what your actual parachute Cd is, at least close enough to estimate, but a little less than what you're actually computing (put a lead ball under each parachute and drop it from a known height a statistically significant number of times and you'll get a number lower than what you compute based on simple raw numbers and a recovering rocket for a weight).
I absolutely agree--but such are the limitations of the hobbyist. More flights on multiple rockets are clearly required if #1 were to be pursued. As it is, though, it serves as a good basis for general comparison for future efforts when I can ask: "is this redesign better or worse than #1? Am I moving in the right direction?"

Later!

--Coop
 

Banzai88

Lvl 1,Wallet....Destroyed
TRF Supporter
Joined
Jul 15, 2015
Messages
2,561
Reaction score
726
I absolutely agree--but such are the limitations of the hobbyist. More flights on multiple rockets are clearly required if #1 were to be pursued. As it is, though, it serves as a good basis for general comparison for future efforts when I can ask: "is this redesign better or worse than #1? Am I moving in the right direction?"

Later!

--Coop
Yep, I do the same thing. In fact, this weekend I'm flying my first home brew Fruity style toroidal on one of my test mules to see if I'm on the right track!
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
Best of luck to you with that! It's a fascinating pursuit --at least it has been for me.

To this end, thus far, I've acquired 3 sewing machines and a serger. Heh.

Me: "No, seriously, hon... THIS machine will do stuff the others won't..."
Her: "Like have a reverse?"
Me: "Shut up."


Later!

--Coop
 

Banzai88

Lvl 1,Wallet....Destroyed
TRF Supporter
Joined
Jul 15, 2015
Messages
2,561
Reaction score
726
Best of luck to you with that! It's a fascinating pursuit --at least it has been for me.

To this end, thus far, I've acquired 3 sewing machines and a serger. Heh.

Me: "No, seriously, hon... THIS machine will do stuff the others won't..."
Her: "Like have a reverse?"
Me: "Shut up."


Later!

--Coop
I have a similar t-shirt!
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
I tried to rationalize out why I needed a double-needle machine... she wasn't having it. Making do with my 80 year-old Singer, instead.

Later!

--Coop
 

Buckeye

Well-Known Member
TRF Supporter
Joined
Sep 6, 2009
Messages
2,627
Reaction score
477
Remember that with different test platforms, what you're getting is 'effective Cd', which is a combination of the drag created by the parachute and the rocket, along with whatever side slip lift is created by the cross wind. It'll get you within a pretty tight range of what your actual parachute Cd is, at least close enough to estimate, but a little less than what you're actually computing (put a lead ball under each parachute and drop it from a known height a statistically significant number of times and you'll get a number lower than what you compute based on simple raw numbers and a recovering rocket for a weight).
All true.

How about ejecting the parachute with a very small payload that holds just the altimeter (analogous to your lead ball)? Maybe add a little more weight so the thing doesn't drift a country mile. Then, you can neglect the rocket from the drag considerations while still running the experiment with rockets. Or, just drop it from a 10 story building ;)
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
I had an offer to do something very much along these lines... dropping a fixed weight under chute from a doubly-verified altitude with an attached altimeter, not involving the rocket at all other than the trip up to apogee. I've not yet taken the man offering up on it because... well... I kinda like having an excuse to launch my own stuff and justify my brand of nerdery as "research." I may, though, if I have a few that I'd like to cross-compare, or just for giggles, later on...

I also considered asking one of the fire companies if I could maybe borrow their tower for a little bit. I'm fairly certain I could have that done without too much trouble. Maybe cost me some burgers or something...


Later!

--Coop
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
Continuing on in this pursuit... GT Mark V.

ImageUploadedByRocketry Forum1477171250.899110.jpg

Later!

--Coop
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
Thanks! I'm planning on another variant --wider band with a slightly smaller gap-- which I'd like to complete in time to test against this side-by-side. I just finished drawing the gore pattern about 1 cup of coffee ago.


Later!

--Coop
 

TOM C

Well-Known Member
Joined
Mar 27, 2014
Messages
127
Reaction score
0
Chute looks real nice.
Will watch for the side by side testing..
 

Tonimus

Well-Known Member
Joined
Jul 9, 2014
Messages
1,511
Reaction score
4
It would be interesting to use a fixed mode to test these. Like a test stand in the back of a truck doing 15mph with a force gauge and then calculate your Cd from that.
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
6,993
Reaction score
1,445
It would be interesting to use a fixed mode to test these. Like a test stand in the back of a truck doing 15mph with a force gauge and then calculate your Cd from that.
I have routine access to a ~100ft drop. Is there particular testing which would be useful?

I was thinking of modified stock chutes ( standard, pulldown, spill hole, pulldown w/ spillhole ) on a few sizes. If it would be useful I could expand that, I suppose.
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
Chute looks real nice.
Will watch for the side by side testing..
Thanks, Tom! It'll likely be the Sunday of the next MDRA launch... If my calcs are close, my Phoenix might be a good weight for the test bed...


It would be interesting to use a fixed mode to test these. Like a test stand in the back of a truck doing 15mph with a force gauge and then calculate your Cd from that.
I may or may not (mostly may) have been spotted in a co-worker's truck in Medic 7's parking lot doing exactly that...


I have routine access to a ~100ft drop. Is there particular testing which would be useful?

I was thinking of modified stock chutes ( standard, pulldown, spill hole, pulldown w/ spillhole ) on a few sizes. If it would be useful I could expand that, I suppose.

Hmm.... check your PMs....





Later!

--Coop
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
This project continues...

Today, the intent was to do a side by side comparison of my two newest parachutes... Mark V and VI. Both these are similar to the previous 4 or 5 versions in overall design, with evolutionary improvements to gore geometry and alterations to the gap width and the band.

It's rocketry. That things do not go as intended is kinda part of the gig.

So test bed is a scratch-built 4" Phoenix with a 54mm MMT, with a mid-mounted av-bay (between the forward fins), into which a Raven III has been installed. In the nose, there was the Jolly Logic Altimeter Three--a unit which really gave me fits at the beginning, but today has proved its worth. More on that later.

Phoenix is 6.1875 lb dry. I tried to select a motor that wouldn't send it too high, as we would be looking at the recovery more than the flight, which led me to the CTI 6XL white motor. Extra advantage of being a bit cheaper than 54 or 38mm was, admittedly, attractive as well.

Range opened, I RSO'd and weighed her in. She came to 7 lb loaded with motor, 54-38 adapter and the 38-29. She flew well, straight on through the crosswinds without weathercocking to an apogee this side of 1,500 feet. Chute opened, and she began drifting down...

... A bit more on the "drift," than she did the "down," if you dig. Across the field... Over the flightline... The far field... Aaaaand into the trees.

Mordor, if you will.

So off I went to try and locate her.

Some time later, I found her, WAY up in a tree. The telescoping pole the club has was too short by a long way. But I did manage to drop a pin on the GPS location with hopes of a later recovery.

I did, however, manage to download the data off of the Jolly Logic unit. I had to fully extend my arm and stand on my tiptoes to keep the bluetooth connection, but I did manage it.

Descent at 15 FPS.

I'm still reeling.

That's far better than I had hoped. I obviously do not have a recovery weight --guessing about 6.5-6.75, once you figure in the loss of propellant and dog barf. For the preliminary calculation, I'll be using the average of these two --6.625lb-- with revisions later on as necessary.

Our formula, again is:--
Cd *S *0.5 *rho *V^2 = M
Cd * ((24^2*3.141)/144) *0.5 *0.002377 * 15^2= 6.625
Cd * (576*3.141)/144) *0.5 *0.002377 * 225=6.625
Cd * (1809.216/144) *0.5 *0.002377 *225= 6.625
Cd *12.564 *0.5 *0.002377* 225= 6.625
Cd * 3.360=6.625

6.625/3.360= 1.971

Cd= 1.971

I'm aware this is a family friendly forum, and thus, issue a warning to those sensitive to the occasional vulgarity which is forthcoming...


....


....


....



Holy ****!

1.971! Far better than anticipated... I'd very very much like to get that rocket back in-hand so I can weigh the thing and get more accurate data from the on-board Raven 3.

I'm disappointed in that I did not get to recover the rocket immediately, but am very thankful that I was able to download the data from the Jolly Logic unit whilst still treed. The initial results are very encouraging. I do believe I'm on the right track, here. I'd like to see what the Mark VI does vs the Mark V. I believe it will be comparable, perhaps slightly better, as far as Cd. Where I expect a notable difference will be between the opening force at dual-deploy speeds... which will be the next phase of testing.

Photos attached shortly...


Later!

--Coop
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
ImageUploadedByRocketry Forum1479614267.696020.jpg
CTI 29mm 6GXL I-243 White

ImageUploadedByRocketry Forum1479614288.042057.jpg
Got a good handle on the "up" part.

ImageUploadedByRocketry Forum1479614315.681548.jpg
"No... no, no, no, no, no, no... that's where Gollum lives! GAH!"

ImageUploadedByRocketry Forum1479614339.244678.jpg
Tantalizingly out of reach.

ImageUploadedByRocketry Forum1479614355.515018.jpg
"Hrrrmph."

ImageUploadedByRocketry Forum1479614367.082532.jpg
At least we have this...


Later!

--Coop
 
Last edited:

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
Bean bag, fishing line, cable saw.
I was trying to borrow Fred's bow... but he was out of line at the moment. I'll get it back. Just not today. The pole MDRA has is 25 or 30' long... nowhere even close enough to reach it. It's easy 70-80' up. (photo is zoomed in) I tried throwing sticks up there to try and knock it down, but wasn't getting close. Oh, well... If I were jock enough to throw that well, I never would have wound up in the nerdery building rockets in the first place...


Later!

--Coop
 

tightwad

Well-Known Member
Joined
Jul 8, 2011
Messages
1,087
Reaction score
27
Looks like well placed shotgun blast would bring it down. ER...I mean a blast to the limbs should free it.
 
Top