Ground testing with deployment bag

[ericm541]

So i'm doing something new which is having a deployment bag setup, as i've been ground testing the deployment charges the bag just doesn't want to come out. The bag fills the diameter of the airframe, i can move it easily inside but it will not fall out. I've been wondering if i didn't size the bag right, 6" inside, about 5 3/4" bag.

 

[rharshberger]

Unless it gets pulled out by the energetic force of the harness and nosecone yanking it out the gasses from the deployment charge just go right around it, as long as the pilot chute can come out freely the bag will be extracted. Can you turn the airframe upside down and the d-bag slide out without assistance? Is this a freebag setup or are the nosecone and upper payload bay tethered together?

 

[ericm541]

It's not a free bag setup, everything is connected, the bag doesn't fall out on it's own.

 

[rharshberger]

It's not a free bag setup, everything is connected, the bag doesn't fall out on it's own.

Does the pilot chute make it out into the simulated "airstream"?

 

[ericm541]

It did come out and the end of the bag was at the end of the airframe, but i would guess that it's not enough to pull the bag out the rest of the way.

 

[rharshberger]

It did come out and the end of the bag was at the end of the airframe, but i would guess that it's not enough to pull the bag out the rest of the way.

Can you post a picture of your setup stretched out, or a diagram?

 

[Bat-mite]

I had the same concerns the first time I used a D-bag. There is absolutely no way to simulate on the ground what happens in the air -- the chute protector unwrapping, the pilot chute jerking the bag off the chute, etc.

But I think that if you can get the pilot chute out, as Rick indicated, the rest will be "automagic."

There are two reasons why the bundle might not be coming out: 1) it is packed too tightly into the airframe and requires too much force to budge; 2) it is so loose in the airframe that the gas is going by it, and there is not enough force to pull it out.

Agree with Rich that a diagram, picture or video would help. Also, make sure that the D-bag is as far front in the payload tube as possible. If the harness is in front of the D-bag, then maybe the NC only has enough oomph to pull out the harness.

Again, in the air, gravity does a LOT of the work, almost all of it. And during ground testing, as soon as the NC lands, gravity becomes insignificant. Remember that once the nose cone is out (in the air), it will start to fall, whereas the booster is being slowed by the drogue. There is no way to simulate that on the ground.

 

[Handeman]

The pilot chute is what will pull the bag out and off the main, not your deployment charge.

Note: gravity has almost nothing to do with the deployment in the air. All the parts are falling at the same speed so gravity doesn't affect one part any more then the other and can't make the d-bag drop out of the rocket since its pulling everything down at the same rate.

It is the air flow as the rocket falls that will make your deployment work. The deployment will probably be much more violent than you expect. As was mentioned, on the ground, once the parts hit the ground, all similarity to the events in the air ends. In the air, that nose cone with all of it's momentum is going to keep going until the cord or the d-bag provide enough force to stop it. But even if the nose cone doesn't pull the d-bag out of the rocket, the pilot chute will. You might be surprised at the force even a small pilot exerts when it inflates at 75 - 100 ft/sec. I've had a d-bag tear severely along the nylon straps sewn on it because of the force the pilot chute exerted.

 

[cwbullet]

I have deployment bags but have not used one on a cert flight. As indicated, Ground testing is key to get it outside the body tube. It still may get fouled in the air. There is no way to test for that in my book.

 

[mpitfield]

Can you turn the airframe upside down and the d-bag slide out without assistance?

This is important.

The bag should slide out freely, if not then you need to pack it so it does or get a narrower but longer bag, if appropriate.

I pack mine in the following manner.

First in is recovery harness, then bag then pilot.

When she pops the drag from pilot pulls the d bag out and chute from the d bag. The drag from nosecone also pulls the harness out which helps to pull the d bag out because typically the nosecone is attached closer to the d bag. Once the d bag is out things happen quickly and reliably, if packed properly.

Gene at Fruity Chutes said it well, "neatness counts". If you pack your bag and it looks messy then repack it until it looks clean. He has videos online on how to pack a D bag.

 

[ericm541]

Great advice, I can take some photos of my setup, I also take a quick video when i do ground test so i can review if needed.

 

[Bat-mite]

Main bundle on the right: D-bag and pilot inside Nomex, Nomex secured to the harness, main on a BB swivel, swivel and harness will be attached to NC via quick-link. Sorry that it's out of focus.

NC pretty much can't come out w/out pulling out the bundle.

 

[mpitfield]

Here is a thread that posted last January which includes an attempt to illustrate how I pack mine and how it deploys.

https://www.rocketryforum.com/threads/demystifying-deployment-bags.144212/

 

[Bat-mite]

Here is a thread that posted last January which includes an attempt to illustrate how I pack mine and how it deploys.

https://www.rocketryforum.com/threads/demystifying-deployment-bags.144212/

Yup, that was a cool thread.

 

[OverTheTop]

I use a Dbag for main deployment. It is a slight friction fit (won't fall out) in the air frame and has the main, shroud lines and pilot under the flap. It has it's own harness to the airframe.
NC has a separate harness to the airframe and gets blown off at apogee.
Main deployment involves blowing the bag out of the airframe. Then the flap opens, pilot inflates and pulls the main out of the bag. Works pretty well and I have never had main at apogee.

 

[ericm541]

There's great threads here on deployment bags, prior to posting i read a few including Mike's.Looking at everything I really think my bag is too big, it fills out too much and makes it too tight, i can pack the parachute by making it longer however, it will expand then fill out the bag. Other than that it's same typical setup, everything neat, i'm using a cert-3 XL for main, i'll still post pics.

 

[Bat-mite]

There's great threads here on deployment bags, prior to posting i read a few including Mike's.Looking at everything I really think my bag is too big, it fills out too much and makes it too tight, i can pack the parachute by making it longer however, it will expand then fill out the bag. Other than that it's same typical setup, everything neat, i'm using a cert-3 XL for main, i'll still post pics.

How did you arrive at what size bag? I had no idea what size bag I needed for my 15' silk military chute. I contacted Gene at Fruity Chutes, and he wanted the weight of the chute and the material (silk), and from there he did a calculation. I ended up with a 6" diameter bag, which can be wrapped in Nomex and fit nicely in my 8" and 7.5" payload bays.

 

[ericm541]

I guessed lol, and was wrong it seems, assuming a bag description of 5.75" would be good for a 6" airframe. I'm thinking about turning the bag inside out and sewing additional seams up 2 sides, think that could work?

 

[OverTheTop]

How about a couple of wraps of something like Kapton tape at judicious intervals along the bag to pull it in a little?

Sewing would work as well I think. Not an expert on that though.

 

[Handeman]

I guessed lol, and was wrong it seems, assuming a bag description of 5.75" would be good for a 6" airframe. I'm thinking about turning the bag inside out and sewing additional seams up 2 sides, think that could work?

I think that would absolutely work. I make my D-Bag for my L3 and make it 5" diameter for a 6" ID tube. There is also elastic bands on the outside to hold the shroud lines in place during the process. Overall, with the bag stuffed and the shroud line under the elastic, it fits snug in the 6" tube.
The pilot chute is right under the nose cone and I don't expect the ejection charge to push the d-bag out of the tube. The pilot chute is more than enough to pull it out. Considering how much the bag was tore from the first deployment (and repaired), I think the pilot could pull it you no matter how hard and tight I could stuff the bag in the tube.

 

[mpitfield]

I guessed lol, and was wrong it seems, assuming a bag description of 5.75" would be good for a 6" airframe. I'm thinking about turning the bag inside out and sewing additional seams up 2 sides, think that could work?

In my case I had some experimental D-bags made and we used nomex and kevlar thread due to it's heat resistant properties. I know that Fruity chute D-bags are nomex, not sure about the thread but a quick email to Gene for his advice is likely the your best option.

 

[ericm541]

This is made of kevlar, it's from GLR a TAC-9 bag, I have a good sewing machine setup I can always remove the threads later. I really want the weight of the bag,chute,pilot,etc to move easily inside I don't need the weight stuck close to the nose cone. Next option would be to have something specific made, or i could make a burrito, figuring this out is part of the fun.

 

[rharshberger]

putting an additional line of stitching or two should not be an issue as long as afterwards the chute still fits into the bag properly. FWIW I make my own d-bags out of nomex (well, I cut them out and my wife sews them), the reduction in diameter should help with your issue. Are you folding the Cert3 and rolling it like a conventional chute? I usually just sort of fold mine lengthwise until its a long narrow triangle, the sort of z-fold/stuff it into the bag, and if I pick the bag up by the pilot chute loop the chute very easily just slides out of the bag even though the bag feels like its hard as a rock.

 

[ericm541]

I tried to just stuff the Cert-3 in the bag, it was horrible lol, looked like a sausage gone wrong. Then i folded it like i did burrito style for my 5" rocket (i'm using the same chute) fit better in the bag and came out easy

 

[OverTheTop]

Personally I just stuff it in, apex first. Works for me. YMMV.

 

[Bat-mite]

When I got my bag from Gene and looked at the size of my chute, I said to myself, "No way this chute can ever fit in there."

But it does! I just have to keep stuffing it in and compressing it until it is all in there.

 

[NateLowrie]

When Tom Cohen showed me how he does deployment bags, he’s folding the chute enough to line up the gores and then stuffing in the bag. So, what I’ve been doing is my normal folding up to the point in which I would start to roll it up burrito style and then stuff it into the bag. Tom said that the right size/fit is when the chute barely fits in the bag and will spill out without holding the flap shut. After doing this a few times I agree with him.

 

[rharshberger]

When Tom Cohen showed me how he does deployment bags, he’s folding the chute enough to line up the gores and then stuffing in the bag. So, what I’ve been doing is my normal folding up to the point in which I would start to roll it up burrito style and then stuff it into the bag. Tom said that the right size/fit is when the chute barely fits in the bag and will spill out without holding the flap shut. After doing this a few times I agree with him.

Great description, same method I use.

 

[OverTheTop]

Tom said that the right size/fit is when the chute barely fits in the bag and will spill out without holding the flap shut. After doing this a few times I agree with him.

That sounds about what I have too. The main is actually well and truly inside the bag, with its shroud lines, but the pilot freely falls out.

 

[ericm541]

I was able to work on sewing this weekend lol, I sewn a good 2" in from the original seam and it worked out pretty well. I was able to stuff the parachute in apex first and it fit great in the bag, it didn't drop out on it's own however, slight tug on the lines and it came out smoothly.




The orange TN is Z folded inside the black nomex cord protector, it opens up with velcro making it easy to setup.

Not sure if i'll use that drogue as a pilot, the test today was much better than before, everything came out better and nothing was damaged. Thank you everyone for the tips and advice.