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Dual Deployment Main Deployment Descent Rate Question

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Ccolvin968

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I guess this might be a similar post to my question about chutes and deployment with my JLCR.

I have a calculated descent rate of between 65 ft/s - 70 ft/s for my drogue chute to slow the rocket down before the pilot pulls the main out.
Is it too fast to deploy a 68 inch chute or will I be okay?
It will be pulled out by an 18 inch drogue chute from a deployment bag at whatever altitude I set it to.
I have repurposed a 68 inch military pilot chute that are used to pull large airdrop loads out of the back of the aircraft.
The descent rate for that chute is 23.8 fps with a 15 lb payload underneath of it.
Any info helps.
Just want to be sure I get my DD right!
 

Lowpuller

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It's all about how you pack the main to open. You need a nice slow, controlled opening with enough altitude to let it happen. Proper deployment sequence is critical.
 

Ccolvin968

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I was going to pack it how we pack it to do cargo extraction.
"S" fold it into the deployment bag.
It'll take some experimenting, but I will start extra high to be safe.
 

cbrarick

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Wow! That's a lot of moving parts for your first DD, especially for a light rocket. Why not just starting out the way jolly logic wants you to, and eliminate pilots and d-bags? They are just two more failure points...
 

Ccolvin968

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It's a 15 pound rocket ready to fly. Not sure what is considered heavy or light yet.
I don't like the thought of using a JLCR for HPR flights.
Also, I know it's very different, but I pack pilot chutes and deployment bags every day in my job.
I have yet to have one fail. *Knocks on wood.*
I know with more moving parts, there is greater chance of failure, but in this case I believe two extra parts are acceptable risk.
 

timbucktoo

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I fly a lot of DD droguless and have yet to have any deployment issues with main opening or shredding. Altimeter data indicates anything from 50-80 fps when main deploys with that configuration.
 

Lowpuller

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To clarify:

Drogue (What size?), deployed at apogee

Pilot Chute (using an 18" drogue) deployed at your set altitude

Main 68", deployed by pilot chute from a d bag

Did I read that correctly? You will have three canopies?


If so I would ditch the pilot chute, design the ejection charge and harness to extract the main. As you know the most important thing is line stretch before the canopy comes out of the dbag.
 

rharshberger

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65 to 75 fps is where I try and set my drogue descent rate. A d-bag and pilot work very well, as long as you have a good quality main chute there shouldn't be an issue with with your planned setup.
 

timbucktoo

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Lowpuller - dbags are basically designed to be used with a pilot chute and I wouldn't use a dbag without one myself. I used one with my L3 project and was surprised how well it worked.
 

Lowpuller

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Timbucktoo

I'm a parachute rigger. Dbags (aka deployment bags) are used to control deployment sequences with or without a pilot chute. For example most static line drops use a dbag but no pilot chute. The primary purpose of the dbag is to contain the canopy until you reach line stretch.

Think of it like this:

Line stretch then canopy opening, is like slowly applying the brakes in your car, the seat belt pressure against your body equates to the load the rocket and canopy feel.

Canopy opening then line stretch, is the equivalent of going 125 mph (in the case of a skydiver) and slamming the brakes on as hard as you can, this equates to huge load on both the rocket and the canopy resulting in blown canopies or failed eye bolts or both, basically whatever is the weak link.

I think a pilot chute is ok to use just not necessary, it's sole job is to extract the canopy from the dbag. In some cases it may also act as an additional drogue and reduce opening force as well, but becomes another potential entanglement point hence the use of free bags.
 

Ccolvin968

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Yes, that is correct.

I'm going to go with another 18" drogue at apogee. 65 ft/s descent rate.
That makes sense though.
I just wouldn't have a backup charge from the motor if I put the main on the aft end deployment.
 

Lowpuller

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I've not tried this yet but here is what I have been toying with.

Standard DD with the drogue in the booster section, take advantage of the motor eject as a backup. I had tertiary drogue deploy on my Level 2. Two altimeters and motor eject.

Main in the payload section attached to the nose. Dbag attached to the top of the AV Bay. When the ejection charge fires it draws the lines off of the dbag to line stretch then extracts the main. This would guarantee line stretch before canopy opening. However it is very dependent on the size of the ejection charge to ensure deployment.

I've always been concerned that if you use a pilot chute and don't adequately load it, you might not extract the main.

I just bought a JLCR not sure how I'm gonna pack the canopy yet. Lots of the postings on line including the JL video show the lines packed inside the canopy, in my opinion this is a recipe for line burn on the canopy and does not guarantee line stretch.

Im curious to see what you come up with.
 

Ccolvin968

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Actually, that's how I pack my chute all the time.

Flat fold to a long fold kind of thing.
From there, I fold the lines 3/4 the way to the apex.
I leave about 3 inches out with the swivel and shock cord at the bottom.
I have yet to have a chute burn or chute fail to open.
 

timbucktoo

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There's no reason shroud lines should burn unless your not wrapped in nomex. There's some good threads about a burrito wrap.
 

Ccolvin968

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Rich,

I'm using a 68 inch pilot chute that we use in the military for container drop extraction and small 15 lb training bundles.
The loadmasters regularly throw them out if the back of a C130 going 140 mph ish.
That's a lot faster than my 47mph deployment. So it hould be fine! **Should be** haha!
.
 

rharshberger

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Rich,

I'm using a 68 inch pilot chute that we use in the military for container drop extraction and small 15 lb training bundles.
The loadmasters regularly throw them out if the back of a C130 going 140 mph ish.
That's a lot faster than my 47mph deployment. So it hould be fine! **Should be** haha!
.
You know more about the capabilities of your chutes than I do, of course you have a professional need to know them. I think the set up you are planning will work great. I use a 36" drogue, 24" pilot, and a Cert 3XL main with a nomex d-bag my wife made for me, all on my 5.38" 35 lb Cherokee-D upscale. Everytime I have flown it the system has worked perfectly.
 

Lowpuller

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The "line burn" I am speaking of occurs to the canopy, not the lines. Basically a friction burn from the lines being pulled across the canopy. I always try to put my entire canopy including lines inside the burrito but again I am talking about friction burn, not heat burn.
 

timbucktoo

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The "line burn" I am speaking of occurs to the canopy, not the lines. Basically a friction burn from the lines being pulled across the canopy. I always try to put my entire canopy including lines inside the burrito but again I am talking about friction burn, not heat burn.
Gotcha! That's a new one on me.
 

Lowpuller

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Most of my experience is with skydiving rigs, lots of weight and fast deployment speeds. I'm not sure it always correlates to rockets, especially the smaller ones, but I try to apply the same principles.

I've got a 68" Top Flight canopy another TRFer sent me that I might repair. All but one line ripped off on deployment, and it has some evidence of line burn. However I think he tried to deploy it on the way up versus the way down!!
 

rharshberger

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Most of my experience is with skydiving rigs, lots of weight and fast deployment speeds. I'm not sure it always correlates to rockets, especially the smaller ones, but I try to apply the same principles.

I've got a 68" Top Flight canopy another TRFer sent me that I might repair. All but one line ripped off on deployment, and it has some evidence of line burn. However I think he tried to deploy it on the way up versus the way down!!

Skydivers if I understand correctly have free fall speeds of about 120mph (176 ft/s) and a lot of shroud line length so line burn could definitely be an issue. Whereas in rocketry we generally aim for drogue descent rates of half what a skydiver experiences in free fall, and generally much lighter weights and shorter shroud line lengths which under normal deployments is not to stressful on our gear, but like the chute you have demonstrates a abnormal deployment can be hard on the equipment. My usual goal with all of my rockets is drogue descent rates of approximately
75 fps and main descent rates of 20 fps or less.
I dont think you are wrong for trying to apply skydiving concepts to rocketry at all, basically I think the part after the chute leaves the skydivers pack or the rocket airframe should be very similar. The important part in rocketry is just getting the laundry out of the airframe and into open air, once that occurs there is a very good chance of the chute opening. Most rocketry recovery failures I have had or observed have been the chute not getting out of the airframe or the chute was insufficiently protected from the ejection charge and hot gases/particles sealed parts of the chute together creating a para-wad or the effect of a badly reefed chute.
 

JohnCoker

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The surface you land is matters for main descent rate. For heavy rockets landing on desert, I prefer 15fps. If you're landing a light rocket on grass, faster is better.
 

rharshberger

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The surface you land is matters for main descent rate. For heavy rockets landing on desert, I prefer 15fps. If you're landing a light rocket on grass, faster is better.
True statement John, consideration of the landing surface is something we do have to contend with, thank goodness concrete and rocks are not common in the areas I fly.
 

Ccolvin968

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We fly on sod fields, so we are very lucky.
I just read my reply to you earlier Rich.
Sorry if I came off as cocky or a know it all.
Just sharing my experience.
No offense was intended.
 

rharshberger

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We fly on sod fields, so we are very lucky.
I just read my reply to you earlier Rich.
Sorry if I came off as cocky or a know it all.
Just sharing my experience.
No offense was intended.
No offense taken, we all have other hobbies and they sometimes even have some form of application to our hobby. Without our experiences being shared we don't learn new things. I fly on sod mainly as well. You and Lowpuller both have experience with other types of parachute systems, and I have learned a bit more about larger chute system by reading your posts in other threads, and from Lowpuller a simple technique for fingertrap loops in tubular kevlar.
 

Coop

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I guess this might be a similar post to my question about chutes and deployment with my JLCR.

I have a calculated descent rate of between 65 ft/s - 70 ft/s for my drogue chute to slow the rocket down before the pilot pulls the main out.
Is it too fast to deploy a 68 inch chute or will I be okay?
It will be pulled out by an 18 inch drogue chute from a deployment bag at whatever altitude I set it to.
I have repurposed a 68 inch military pilot chute that are used to pull large airdrop loads out of the back of the aircraft.
The descent rate for that chute is 23.8 fps with a 15 lb payload underneath of it.
Any info helps.
Just want to be sure I get my DD right!

Sounds like a solid plan to me, provided you and your RSO are both okay with the 23.8 FPS landing speed.

I'd keep the pilot and freebag the main, recover the nose separately. This gives you the proper deployment sequence and will lighten the load under main --which might bring you down a smidge closer to 20 to make the guy at the table --and the club's underwriters-- happy. Size the pilot right for what it will be recovering (nose, line, bag), and the two assemblies will descend at roughly the same rate and your spread between the two will be surprisingly tight. That's me. Your flight.. do it how you like, as long as it's safe.

Re: your drogue descent rate... I've blown mains at 100-110 FPS without issue. That was a 50-pounder on a 24" x 24' streamer, and using the above method on the main (n.b.: the pilot, which opened rapidly with a mere Z-fold, was also without damage). I don't make a habit out of deploying at that speed--but others I know do (Teddy from One Bad Hawk comes screaming to mind). The descent rates we see in rocketry aren't anywhere near freefall openings (usually), and the rockets are (usually) much lighter than garden-variety hairless ape with skydiving gear. Then again, the chutes are (usually) of lighter construction, too --A, AA, and B thread on the canopies.

Good to have a plan, and consider options... here's wishing you the best with it!


Later!

--Coop
 
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