Aft Main Chute Configuration

[AllDigital]

I've got a 100 lb 8" rocket (14 feet tall) that I'm using to test an experimental HED deployment system. As a back-up, I've got an 11 foot chute in the fincan, below the avBay, that is triggered by radio controlled pyros (a poor mans FTS if the HED system fails).

In a flight this weekend the HED system worked perfect, but after the drogue deployed out the nose the three 4-40 shear screws gave out and the fincan slowly slide off, the rear "backup" chute slide out and up and did a full wrap of the avBay (a tangled mess). I suspect if we had fired the pyros (a whopping 4G of FFFF) the unfurling of the chute would have been different, but it has made us second guess the harness layout for the backup chute.

See below... our current configuration is A. We have 50 feet of kevlar 7/16 kevlar tubing, the chute is 20 feet long stretched out, and the shrouds are anchored to the avBay end (6' third loop), while a chute bag is anchored to the fincan. In this configuration, the chute is always going to fully pull-out if the line is 100% extended, but it has to re-orient itself 180 degrees if the rocket is pointy end up (e.g., drogue is out on top, but the HED main fails to deploy). Configuration B would pull out the chute in the right orientation, but anchoring the chute so close to the fincan would risk the chute staying in the can and only having the bag rip free (low, but still a risk). The other option would be "C" (no picture) which is B with an extra 20' of line between the chute anchor and the fincan. That would ensure the chute is fully extracted out of the fincan, BUT having to pull 60-70 feet of line out and tight is also a risk when this is meant to be an "FTS" backup to be used in the final seconds of flight -- if the primary main does not open by 1500 feet. That said, it has to work just as well if the nose/drogue doesn't come off and it is coming down ballistic.

So, what are your thoughts? how best to prevent the "avBay wrap" tangled mess we got this weekend?

Thanks,

Mike

 

[rharshberger]

B is better IMO

 

[Handeman]

Put a pilot chute on the d-bag. That way, it is always pulled in the "right" direction no matter what orientation it starts in. You also don't have to worry about putting a huge ejection charge in to ensure sufficient stretch of the shock cord.
About 25 - 50 ft. of a small line attached between the pilot/inside of d-bag, and the top of the main will keep the pilot from drifting off after deployment.

 

[AllDigital]

Put a pilot chute on the d-bag. That way, it is always pulled in the "right" direction no matter what orientation it starts in.

So, don't anchor the d-bag, instead let it float but with a pilot chute to pull it off? That makes a lot of sense. Assuming the pilot chute does its job, that could result in a more controlled (properly oriented) deployment of the chute.

The big BP charges are not specifically for extending the line. We had three 4-40 shear screws holding the fincan to the avBay (about 180 pounds of retention). It turns out that wasn't enough, so we'll add another pin (240 pounds). Four grams gives us about 290 pounds of force to separate. Also, since this is a radio back-up pyro, we also want it to separate in a ballistic (near) terminal velocity situation.

Thanks,

Mike

 

[RocketScientistAustralia]

A shows the rear harness as long enough to get the chute fully out of the rear section. A drogue attached to the top connection point will assist. The chute will inflate no problems once out. If you think you're going to be deploying at higher than normal velocities make sure you use a reefing ring. This prevents your chute from snapping open and shredding at faster than normal deployment velocities.
Good luck with the flight.

Norm

 

[tfish]

maybe some z folds with electrical tape to absorb the ejection charge and chute inflation ..,,,process.



Tony

 

[RocketScientistAustralia]

maybe some z folds with electrical tape to absorb the ejection charge and chute inflation ..,,,process.



Tony

Z folds in your main shock cord control the shock and are there to prevent fuselage zippering and shock cord breaking. However may not all fully release. You should still ensure there is a sufficient amount of free shock cord to allow your chute to be fully pulled from the rear section and deployed far away enough from the fuselage bits in case some z folds do not fully open. As happened shown in the video. Chute deployment reefing rings control the rate at which the canopy opens. Also called slider rings. https://shop.fruitychutes.com/products/stainless-steel-slider-ring-1-566-id Both methods reduce the shock, The reefing ring gives the parachute more protection from shock.

Norm

 

[AllDigital]

A shows the rear harness as long enough to get the chute fully out of the rear section. A drogue attached to the top connection point will assist. The chute will inflate no problems once out.

A is what we are using today and failed last weekend. With the rocket pointing up, under drogue out the nose, the aft section opened, the backup chute traveled straight up and wrapped the avBay and upper airframe (see photo below). So, the question is how best to prevent that scenario from repeating? I like the idea of detaching the d-bag and putting a pilot chute on the bag to to first pull the orientation up and away from the avBay to unfurl the chute.


(this shot is after the HED main deployed, but you can see the backup chute wrapped around the avBay and upper airframe, along with the d-bag at the bottom near the fincan)

 

[RocketScientistAustralia]

so you have 3 chutes. A drogue,a main and a backup main?
Drogue, no issues? main, no issues? backup main got tangled in main deployment bag? Have you got a full sketch of what you have with harness lengths?

 

[AllDigital]

so you have 3 chutes. A drogue,a main and a backup main?

Correct

Drogue, no issues? main, no issues? backup main got tangled in main deployment bag?

Close, but not quite. Drogue, no issues. Main, no issues. Backup main (in fincan) accidentally came out, due to shear screw failure, and wrapped around the avBay-Upper airframe above.

The "backup chute" would normally only be fired (manually) if the nose failed to separate (no HED anything) or if the drogue deployed, but the main didn't deploy. In the photo above, all three chutes are out, but the backup made the avBay into a burrito.

Here is the full configuration...

 

[RocketScientistAustralia]

Correct

Close, but not quite. Drogue, no issues. Main, no issues. Backup main (in fincan) accidentally came out, due to shear screw failure, and wrapped around the avBay-Upper airframe above.

The "backup chute" would normally only be fired (manually) if the nose failed to separate (no HED anything) or if the drogue deployed, but the main didn't deploy. In the photo above, all three chutes are out, but the backup made the avBay into a burrito.

Here is the full configuration...
View attachment 505959
[/QUOKOTE]
So 2 issues I see. First. Fix your shear pins and the rate at which your primary main opens. This is the shock load on the shear pins from the main opening. Z folds in the harnesses. Your 6ft line on the backup needs to be longer. At least the 20ft of the chute plus a safety margin. That way if the chute gets deployed it only has the harness to catch on not a lumpy body part. The 50ft on the backup only needs to be say 25 ft. but now you would have the av bay and the rear body section whacking each other. So make the 6ft section 35ft and the 50ft section 25ft. The AV bay now drops below the rear body section. I'll leave you to thing about what occurs if both mains open and the drogue deploys........

So 2 issues I see. First. Fix your shear pins and the rate at which your primary main opens. This is the shock load on the shear pins from the main opening. Z folds in the harnesses. Your 6ft line on the backup needs to be longer. At least the 20ft of the chute plus a safety margin. That way if the chute gets deployed it only has the harness to catch on not a lumpy body part. The 50ft on the backup only needs to be say 25 ft. but now you would have the av bay and the rear body section whacking each other. So make the 6ft section 35ft and the 50ft section 25ft. The AV bay now drops below the rear body section. I'll leave you to thing about what occurs if both mains open and the drogue deploys........

 

[AllDigital]

Well... we had a second test flight last weekend of this rocket and we needed to fire the "FTS" backup chute. We went with @Handeman's suggestion to stick with configuration A above, but not secure the d-bag, instead attach a pilot chute to the d-bag, so when it comes out it orients properly and the pilot chute carries it away.

Once again, the backup chute tangled into a hot mess, but fortunately, the pyro charge forced the primary main out the top, so the rocket was slowed and landed at 24.5fps. In this latest situation, the vehicle was coming in ballistic at 280fps when we deployed the backup pyros. It appears that the bag stayed in the fin can, while the chute was pulled out. I assume the blast pushed the bag down further into the can, so the path of least resistance was for the chute to pull out without the bag. Maybe a few wraps of line around the bag would have ensured the bag came out first (?). There is no doubt that any chute would be challenged to deploy at 280fps, but that isn't an uncommon use case for the design of this use case. I have a 3' Rocketman ballistic Mach 2 chute that I am thinking should be the pilot chute and/or just switch the whole chute out to a mach2 chute, especially after seeing what happened to our primary main on this flight.

The real root issue on our last flight was an under-sized or worn/damaged drogue line. A new respect for how loops and sewing reduce strength. You can see all the details in the video below.

https://vimeo.com/manage/videos/685342633

Here are a few photos of how the backup chute deployed out of the fin can into a balled mess...


The black shroud lines were all packed tight in the d-bag, so you can see them coming out while the bag is still in the can.


Now the chute is coming out of the can, while the d-bag is still in the can.



The chute does not get above the six foot anchor point before it tangles into a hot mess (e.g., it didn't tangle on the avbay)



here it is tangled for the remainder of the trip with the d-bag and pilot chute finally out. "go little pilot chute, go!"

 

[David Schwantz]

IMO it is how you are packing your back up main. The shroud lines should not have come out before chute if done correctly. Lines should be inside a gore going up. and then another gore going back down. This way they are all tucked away inside the chute and will not be able to deploy before chute is beginning to inflate. And as far as your shear pins go, I am using 4 6-32 nylon screws on a 70 pound V 2 with only 3 grams to shear them.

 

[AllDigital]

IMO it is how you are packing your back up main.

Thanks. I've always packed my d-bags like Ky explains here in his video. I've never really had issues in a traditional dual-deploy configuration with over 100 chutes packed. To pack the d-bag, I put the chute apex in first, stuffing the chute into mini z folds, and then the shroud lines last. The flap gets folded over. In this configuration, I put the bag "flap end down", thinking the pull pressure from the bottom would bring the whole bag out, but it didn't.

When I don't use a d-bag, I always pack like you describe above. Would a ballistic situation justify a different way of packing? How do others pack their deployment bags?

We added a fourth 4-40 shear screw on this flight and had no issues, but on the first flight the premature shear was caused by the drogue whipping the full length rocket around on end five times, like a lengthwise centrifuge, with unexpected rotational force on the fin can. Likely too much snap force -- the same thing that severed our drogue line in flight #2.

 

[sharkbait]

IMO it is how you are packing your back up main. The shroud lines should not have come out before chute if done correctly. Lines should be inside a gore going up. and then another gore going back down. This way they are all tucked away inside the chute and will not be able to deploy before chute is beginning to inflate. And as far as your shear pins go, I am using 4 6-32 nylon screws on a 70 pound V 2 with only 3 grams to shear them.

I need a little more clarity, from the look of your sketches, you are using HED to deploy a 5’ drogue that is attached to the top of your 14’ main. What is keeping the 14’ main inside the airframe after the NC blows off and the drogue deploys ?

 

[RocketScientistAustralia]

Have you done a simple walk out test. Go to a park on a lightly breezy day, test by quickly walking out all the bits. Everything should deploy one at a time with no tugs or stuff ups.
There are many ways to pack a chute. For my main I use stuff packing like a sleeping bag. That does not mean I just randomly stuff it in. Well sort of , but carefully stuffed in so it deploys freely with no tangles.
However you pack it,
The method you use should be repeatable and the full deployment should be consistent.
There was a lot of metalwork in your nosecone and top section. Are you sure there are no wear points for the harnesses to get frayed on. If there is anything it can get caught on, it will.
For your back up main, there is only 6 ft of harness between your AV bay and your backup chute connection point. This should be at a minimum the full length that your chute stretches out. You show 20' in your sketch. So roughly 25 ft min for harness from chute to AV bay. With having your ejection charge underneath the backup chute, you may be throwing the chute into the deployed lines. So I know there has been some discussion saying it doesn't matter whether your ejection charge is on top or underneath, but it may be part of the issue.
Lastly, this is your rocket. None of us are there and all we can do is give you the best advice we have. It's up to you to make it work reliably. I agree with David that the way the chute deployed could be a packing issue. But it could be beacuse the ejection charge is behind it and compresses up all the nice packing into a ball of poop before it gets out.
Good luck and thanks for allowing all to learn from this.
Norm

 

[AllDigital]

Thank you for the feedback. Here are some quick answers to your questions.

I need a little more clarity, from the look of your sketches, you are using HED to deploy a 5’ drogue that is attached to the top of your 14’ main. What is keeping the 14’ main inside the airframe after the NC blows off and the drogue deploys ?

The HEDD is an electric latch-spring system that does not use pyros. The nose is under spring tension and held in by latch #1. When latch #1 opens it releases the nose, along with the drogue, and 50' of drogue line. The drogue line is held in by latch #2, which is bolted to the bulkhead at the bottom of the payload section. That line is protected by a 2" pipe, so it doesn't compress or have friction when the main chute is packed. A custom chute bag is incorporated and secured into the payload section that holds the main chute in, until latch number two is released and the drogue line (attached to the apex of the main) pulls out. The drogue line, pulls a string that releases the top of the built-in bag freeing the main chute so it can come out. This is all very experimental, but worked successfully in Test #1 and worked as designed, except for the drogue line snapping, in test #2. Much more testing is needed before publishing the specifics. It still has multiple failure modes.

Have you done a simple walk out test. Go to a park on a lightly breezy day, test by quickly walking out all the bits. Everything should deploy one at a time with no tugs or stuff ups.

Yep. We ground test released and pulled the HEDD deployment system over 50 times in every possible orientation. We did not do as much testing on the back-up system, but we did do a walk-out pull test and least 3-4 times on the last configuration (d-bag with pilot). I suspect the difference is 1) the separation blast packs the d-bag deep into the fin can and 2) the vehicle going 280 fps. Those variables are harder to test for at a park

There was a lot of metalwork in your nosecone and top section. Are you sure there are no wear points for the harnesses to get frayed on. If there is anything it can get caught on, it will.

It is possible that the line snagged on the latch, which is at the top of the airframe, but I don't think that is our culprit that cut the line. The line comes up out of its protection tube and immediately does a 180 over the lip of the airframe (see photos below). Unlike a pyro separation, where the nose goes straight out, with this system the nose and drogue go aft out the back. By the time 50' of line is tightened up the snap force is absorbed at the lip of the upper airframe at 180 degrees. To protect against line damage, we sewed in a padded protector (see photo). Likely the line was worn from the first flight and/or the sewn stitches weakened the line. This kevlar line was rated at 1500 lbs (suspect), but probably not strong enough -- although, it did perform fine in test #1 with an incredible drogue shock force.

For your back up main, there is only 6 ft of harness between your AV bay and your backup chute connection point. This should be at a minimum the full length that your chute stretches out.

I would agree with that and will extend it for the next flight, but in this case the tangling never went above the six foot attach point. In other words, it never reached the avBay like it did in test #1. Nevertheless, anything we can do to minimize the failure points is worth it.


This is an early photo of the configuration looking down the upper airframe. You can see how the line exits and goes 180 degrees over the airframe edge. You can also see the latch holding it in at the bottom. To minimize the wear we sewed in a padded protector on the line. Also, the notch to the right was all filled in with glass/epoxy, so was not a snag point. The custom chute bag inside the airframe also provided snag protection across 90% of the latch.


i don't have a good picture of the line protector, but this one captured a little bit of it. The line snapped at the stitching you can see in the photo, so I suspect the stitches weakened the line, along with wear from the previous flight.

 

[RocketScientistAustralia]

Kevlar is great, but has no real stretch. You need to have some form of stretch to take out the shock when it reaches the end. This can be taped bunches of 5-7 loops. This seems to be the sweet spot for bunches.
Something caused your lines to come out of the primary main deployment bag. Was there harness underneath it that the drogue harness caught and got dragged past the deployment bag? Partially opening it. It kind of looks like there is a knot that formed between the black harness and kevlar in the footage and pictures.

 

[AllDigital]

Something caused your lines to come out of the primary main deployment bag. Was there harness underneath it that the drogue harness caught and got dragged past the deployment bag?

the blue/orange chute with black shrouds (above photos) is the backup chute (under remote pyro control). It was packed in the d-bag with shrouds on top then the flap was folded over. Then it was inserted upside down into the fincan (shroud lines on bottom), so when the two halves separated it would get pulled out of the can and then open. That is the way it worked in the pull test, but not in flight. As you point out, in flight the shroud lines pulled out and the bag stayed in.