Ground testing vs actual deployment differences

[AP aroma]

I am having trouble because my ground tests and actual flights are reacting very differently. I thought i understood ground testing: set up everything as you would for the actual flight then work your way up in the quantity of BP that gives you the desired results for ejection. Maybe there is something i am not understanding because I am getting totally different results with the flight.

The rocket:
- 38 mm with a 29 mm motor tube
- 29 mm G-79 motor
- 41 inches long
- 1.6 lbs.
- dual deploy, altimeter based ejection
- ejection charge in motor removed
- 12" drogue chute
- 24" main chute
- 100 # test Kevlar shock cord
- 15 feet of cord for the drogue and main chutes
- shock cord z-folded with tape at both ends of fold
- one 2-56 nylon shear screw for each parachute bay

I consulted a online charge calculator and started testing with .20 grams of 4-F black powder The charges just made a chuff sound and the rocket didn't separate at all. I eventually worked my way up to .5 grams for the drogue bay and .8 grams for the main. This seemed to get the parachutes out and about 1/2 of the z- folds broke. When I flew this rocket the shock cord for the main broke and it came down in two pieces. I thought this must be a fluke so I flew it again and this time the drogue shock cord broke. I went through the ground testing again and came up with the same results. I flew the rocket again ( I'm stubborn) this time both the drogue and main shock cords broke and the rocket came down in 3 pieces with the AV-bay free falling. The obvious thing to do is go to a heavier shock cord but this rocket is so tight I don't know if there is room. My main question is how can the ground tests and actual flight have such different results.

 

[dvdsnyd]

How is the rocket positioned for your ground tests?
Laying flat on the ground? Or is one end propped up slightly, so it can fly out at a bit of an angle?

 

[AP aroma]

I prop the rocket up on a box with the fins on the ground and the nose elevated at maybe 30 degrees.

 

[vcp]

Just to be clear, you're saying these cords broke before their respective 'chutes opened?

 

[AP aroma]

I have no way of knowing when the broke. The rocket was too high to get a visual.

 

[SDramstad]

How long are your shock cords and how much does each section weigh?

EDIT. Maybe I should read the whole post first...... I would lengthen the cords a bit and go to around half a gram of 4F black powder for each charge.

 

[SDramstad]

Also I have never used a sheer pin on a 38 mm rocket. Just shim the coupler with tape to get a snug fit. Also just using 1 sheer pin can cause the coupler to jam. 0 or 2 pins would be better and my preference would be no sheer pin.

 

[AP aroma]

It seems to me that 100 # test cord should be adequate on a 1.6 lb. rocket. Am I thinking right?

 

[SDramstad]

Kevlar has no real "give" to it so you can easily exceed 100 lbs on it even with a light weight rocket.

 

[afadeev]

I am having trouble because my ground tests and actual flights are reacting very differently. [...] When I flew this rocket the shock cord for the main broke and it came down in two pieces. I thought this must be a fluke so I flew it again and this time the drogue shock cord broke. [...] I flew the rocket again ( I'm stubborn) this time both the drogue and main shock cords broke and the rocket came down in 3 pieces with the AV-bay free falling. The obvious thing to do is go to a heavier shock cord but this rocket is so tight I don't know if there is room.

It seams that your challenge is not around ground testing BP amounts, but the strength / size of the shock cord.

BTW, repeatedly flying the same airframe configuration that comes back ballistic, is the opposite of safe!

- 100 # test Kevlar shock cord
- 15 feet of cord for the drogue and main chutes

That's a really puny Kevlar cord!
I use 150# cord on my low-power rockets, never mind mid-/HP-ones!
Mid-power ones, similar to what you are flying, get 700# Kevlar cords:
https://www.amazon.com/gp/product/B086ZNCP5K/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I eventually worked my way up to .5 grams for the drogue bay and .8 grams for the main. This seemed to get the parachutes out and about 1/2 of the z- folds broke.
[....]My main question is how can the ground tests and actual flight have such different results.

Sounds like your ground testing resulted in the nose digging into the ground (not all shock cord folds extended), so you never tested the strength of the cord on the ground.
In the air, the rocket sections were free to extend fully, and exceed the strength limits of the 100# shock cord. Thus the observed results.

The obvious thing to do is go to a heavier shock cord but this rocket is so tight I don't know if there is room.

Yes, the answer is obvious.
Please upgrade the strength of your shock cords on mid/HP-airframes.

If you don't have enough room in the airframe for the laundry and proper shock cords, you have two choices:
1). Fly with smaller, or smaller packing chutes, or streamers.
2). Extend the airframe to give you more room for the appropriate safety gear to fit in.

HTH,
a

 

[les]

Where does the rocket break? Need to understand the volume for the 0.5g for the drogue vs the 0.8g for the main.
Also, 100# kevlar is not that thick (or strong). It may tear through you tape too easily. And if you have knots that further weakens the cord.

 

[David Schwantz]

I have 100 lb kevlar and mine is like sewing thread. I use it only on 13mm stuff. Is this what you are using? If so, it is way to light for a 38mm rocket. I buy mine from Emma Kites, The 2000 lb stuff is about 3/16" dia. post a pic of your kevlar.

 

[AP aroma]

 

[thequick]

Could abrasion be causing the cord failure? If the rocket is fiberglass or carbon fiber the cord might be getting cut through by the body. I’ve been adding tape the the cord where it rubs on the edges of the body.

 

[David Schwantz]

well that is much larger than the 100 lb stuff I have. Looks more like my 1500 lb cord. But the abrasion thing is a good thought. If it is a glass tube I like to sand the inside and outside of all edges. This also saves on bandaids

 

[mikec]

I use kevlar cord like that all the time. If it's breaking I suspect it must be defective in some way or there's something else going on.

I also use single shear pins on fiberglass rockets this size and have never had an issue.

 

[AP aroma]

Thanks for your interest/replies guys. I'm at a loss to explain this, that how I ended up here. I suppose abrasion cold be the source of the problem though I have never had that issue before.

 

[HHaase]

There's a hell of a lot of fraying on that cord, and it just doesn't look right to me.

Are you sure it's kevlar? Where did you source it from? Almost looks like cotton or dyneema by the way its fraying.

 

[AP aroma]

It was purchased from Apogee Components.

 

[afadeev]

It was purchased from Apogee Components.

OK, but what exactly was "it" ?

 

[HHaase]

Their 100# kevlar, part number 30325, shows as a bright yellow on the apogee website. This is consistent with all the kevlar I've purchased in the past. The material you have does not visually appear to be the same. Does it have a stiff and rubbery feel to it? Kevlar is easily distinguished by feel from cotton or nylon. What you have does seem to match their heavy cotton string. I wonder if maybe you got the wrong stuff sent to you by mistake. Maybe check your order history to confirm it was part number 30325 and not 30320.

Either way, I'd say your issue is incorrect or defective shock cord material. 100# cord on a 1.6lb rocket shouldn't have failed. I'd replace the entire length of it with new replacement material.

 

[David Schwantz]

Here is the stuff from Emma's Kites.

 

[AP aroma]

I checked my order and it was actually the 300 # Kevlar cord, part # 30326.

 

[AP aroma]

David, you feel that Emna's kites is a good source for shock cord?

 

[Walter Longburn]

you feel that Emna's kites is a good source for shock cord?

I use the 500 lb test for low/mid power. I also use UHMWPE from them for parachute shroud lines.

 

[David Schwantz]

I have always had good service from them. amazon carries their stuff also. Seems like quality cord. Make a lot of sizes.

 

[manixFan]

The amount of fraying on that Kevlar is worrisome, I would not fly anything with that much surface damage.

There is another issue with Kevlar/aramid fibers that is not well known. Once kevlar/aramid cords have been subjected to a shock load, their rated strength drops considerably. So if it routinely experiences 'hard stops' during ejections, it will lose strength and fail under a much lower load than would be normal. If you do a google search for 'kevlar aramid shock load weakness' you'll find comments like:

"In climbing ropes, it was found that kevlar could be seriously compromised after stopping a fall, which induces a shock load. No sign of damage can be seen but the strength of the rope could be seriously compromized."

Which is exactly what can happen when too short of a shock cord is used with an overly energetic ejection charge, or really just under normal circumstances.


Tony

 

[JoePfeiffer]

That looks a lot like my used Kevlar (also purchased from Apogee); my new, not-yet-flown stuff is a brighter yellow.
Back when we thought it was 100# line it seemed really odd a one pound rocket could break it under any circumstances whatever; 300# really doesn't seem possible.

At this point I'd reach out to Apogee and see what they say. In spite of what it looks like to me, that almost has to be cotton sent by mistake. I have, on rare occasions, had an error in an order from them -- they've always made it right immediately.

 

[AP aroma]

That is a good thought about contacting Apogee. If nothing else they may have an opinion about what is going on. I'm also thinking about taking the broken cord and seeing how much weight it will support by slowly adding items of a known weight to a platform supported by the cord until it breaks.

 

[FMarvinS]

If the Apogee call is not helpful, then in summary, as stated above, go to 20 feet of kevlar on booster to eye nut attached to lower AV bay bulk head, where abrasion occurs (e.g. at break in booster tube) use heat shrink tubing (2 layers) for abrasion protection, as state above sand all sharp tube openings, if feasible use a higher rated kevlar or better, a tubular nylon cord. If no room, as stated use a smaller chute or get a coupler and segment of body tube to extend the current body tube to ensure more volume for chute packing.
Also, during ground testing, did you seal off motor tube so that no BP gases could escape via the motor mount/nozzle? If not, put the motor/casing in the motor tube and fully cover forward end with nomax to assess the right amount of BP. Also, the chutes/cord should be practically almost fully extended for a positive test result.

Good luck,
Fred