Shock Cord Length, Parachutes and Shear Pins

[OccupyMars]

I am on a student team that is in the design phase of a 10 foot tall, 6 in diameter HPR. There will be one shock cord connecting the booster section to the drogue chute and the avionics bay (forward of the booster) and another shock cord connecting the avionics bay to the main chute and payload/nose cone (forward of the avionics bay). I understand that deciding on a shock cord length is pretty subjective but I would still appreciate any thoughts. Here are my questions.

1. Most people refer to their shock cord as one continuous piece when referencing the 3-5x body length standard for a shock cord, even when talking about dual deployment. Does this mean they took a 3-5x shock cord and used it for both the main and drogue chutes by cutting it into pieces or used two separate 3-5x shock cords for main and drogue chute deployment?
2. I have seen some posts about drogue vs. main parachute shock cord lengths with most people saying the main chute can have a shorter shock cord because the drogue has a higher chance of entanglement. I would have thought it was the other way around?
3. I have also seen some posts about the fact that you can oversize your drogue parachute and then your components will dangle next to each other, causing main chute deployment issues. They implied that a higher velocity is necessary to get your booster, upper airframe, and drogue chute to be separated into a "triangular" shape. Could anyone explain this phenomenon further? I am trying to decide between 50 and 75 ft/s for drogue descent.
4. I would like to place the drogue chute closer to the booster section on the shock cord so as to reduce the drogue deployment/inflation loads on the avionics bay. The idea is that if the drogue chute was closer to the avionics bay there would be a greater force pulling on the avionics bay and there could be a risk of shearing the shear pins for main chute deployment early. I got this idea from a previous post but now I am questioning if it makes sense. The drogue should be pulling with the same force on both components regardless of the shock cord length, right?

 

[David_Stack]

Regarding your item 3. Thoughts from Teddy of One Bad Hawk here: https://onebadhawk.com/why-a-3rd-loop.html (same article also touches on the questions you posed in #4...

 

[Jmhepworth]

Long shock cords are your friend. For a rocket that size either buy some tubular Kevlar from Teddy or 1” tubular nylon. I use Teddy’s harnesses in nearly every rocket I own. Either way, I would use about 50 feet in each section. You won’t have any difficulty making that much fit in either bay. Once you go dual deploy, you need sheer pins between the payload bay and the nose cone, as well as screws or rivets connecting the av bay to the payload bay.

A 24 inch drogue is sufficient. 50-75 fps is pretty slow. 100 is fine. You will need a pretty stout main anyway, so you won’t affect it at all going that fast. You want it to come down quickly so it doesn't drift. I’m not sure I understand no. 4. I put my drogue 1/3 of the way from the av bay so the booster hangs below everything. Putting the booster 1/3 of the way and the av bay 2/3 of the way seems like a recipe for tangling. You want the payload bay well away from the fin can. And I put my main 1/3 of the way from the nose cone.

 

[Hobie1dog]

Long shock cords are your friend. For a rocket that size either buy some tubular Kevlar from Teddy or 1” tubular nylon. I use Teddy’s harnesses in nearly every rocket I own. Either way, I would use about 50 feet in each section. You won’t have any difficulty making that much fit in either bay. Once you go dual deploy, you need sheer pins between the payload bay and the nose cone, as well as screws or rivets connecting the av bay to the payload bay.

A 24 inch drogue is sufficient. 50-75 fps is pretty slow. 100 is fine. You will need a pretty stout main anyway, so you won’t affect it at all going that fast. You want it to come down quickly so it doesn't drift. I’m not sure I understand no. 4. I put my drogue 1/3 of the way from the av bay so the booster hangs below everything. Putting the booster 1/3 of the way and the av bay 2/3 of the way seems like a recipe for tangling. You want the payload bay well away from the fin can. And I put my main 1/3 of the way from the nose cone.

good information, thanks.

 

[bobbyg23]

I am on a student team that is in the design phase of a 10 foot tall, 6 in diameter HPR. There will be one shock cord connecting the booster section to the drogue chute and the avionics bay (forward of the booster) and another shock cord connecting the avionics bay to the main chute and payload/nose cone (forward of the avionics bay). I understand that deciding on a shock cord length is pretty subjective but I would still appreciate any thoughts. Here are my questions.

1. Most people refer to their shock cord as one continuous piece when referencing the 3-5x body length standard for a shock cord, even when talking about dual deployment. Does this mean they took a 3-5x shock cord and used it for both the main and drogue chutes by cutting it into pieces or used two separate 3-5x shock cords for main and drogue chute deployment?
2. I have seen some posts about drogue vs. main parachute shock cord lengths with most people saying the main chute can have a shorter shock cord because the drogue has a higher chance of entanglement. I would have thought it was the other way around?
3. I have also seen some posts about the fact that you can oversize your drogue parachute and then your components will dangle next to each other, causing main chute deployment issues. They implied that a higher velocity is necessary to get your booster, upper airframe, and drogue chute to be separated into a "triangular" shape. Could anyone explain this phenomenon further? I am trying to decide between 50 and 75 ft/s for drogue descent.
4. I would like to place the drogue chute closer to the booster section on the shock cord so as to reduce the drogue deployment/inflation loads on the avionics bay. The idea is that if the drogue chute was closer to the avionics bay there would be a greater force pulling on the avionics bay and there could be a risk of shearing the shear pins for main chute deployment early. I got this idea from a previous post but now I am questioning if it makes sense. The drogue should be pulling with the same force on both components regardless of the shock cord length, right?

1. On a rocket that size I would go 25' to 30' for each section. 30' on drogue section and 25' on main.
2. Make them the same length.
3. 60fps - 70fps is kind of what I shoot for with drogue size. As long as it is not too much so the main is shooting straight down at deployment you will be fine. If both sections are next to each other then when the main deploys it will shoot straight down and the rocket will fall right through it increasing your chance of entanglement
4. Loads will be the same. With the drogue 1/3 of the way down from the avbay, it should fall in the inverted V like you want. biggest reasons for breaking the main sheer pins are too short of a drogue shock cord or too much drogue separation charge. If either of those 2 things happen, the main section could pop off when it reaches the end of the drogue shock cord at a high rate of speed.

 

[Chad]

I am trying to decide between 50 and 75 ft/s for drogue descent.

Keep in mind there's more to the function of a drogue than just a constant descent rate. This thread has some interesting information and is worth the read
https://www.rocketryforum.com/threads/drogue-sizing-thoughts.154776/

 

[waltr]

And here is more on what a good setup does:
https://onebadhawk.com/why-a-3rd-loop.html
I adjust Drogue size to get this inverted 'V' on the cord. Then Main is always ejected away from everything and inflates in the clear.

 

[Handeman]

Recovery is a system. Everything must work together to work properly. That includes ejection charge sizes, shear pins, shock cords, parachutes sizes and how they are packed, etc.
If you use a large ejection charge for the drogue side (blow it out or blow it up), it will cause a lot of inertia as the two halves separate. To compensate for that shock as the two halves hit the end of the shock cord, you can lengthen the shock cord, use more or larger shear pins on the nose cone to keep the main in place when that part hits the end of the cord, use heavier hardware and thicker bulk plates to resist the shock, use heavier and stronger shock cords, etc. Any change to one part of the system will affect the other parts.

Generally I like the drogue about 1/3 from the payload section so that section doesn't hit the booster section as it falls. Drogue size and shape will determine if you get a stable and controlled decent. The cord needs to be long enough that the 1/3 length plus the length of the payload section is less than the 2/3 length of the cord so the two parts can't contact. You can adjust that to 1/4 and 3/4 if that works better.

 

[bjphoenix]

1. Most people refer to their shock cord as one continuous piece when referencing the 3-5x body length standard for a shock cord, even when talking about dual deployment. Does this mean they took a 3-5x shock cord and used it for both the main and drogue chutes by cutting it into pieces or used two separate 3-5x shock cords for main and drogue chute deployment?

Think about this a little bit and you can see the problems with this rule of thumb- If you took your 10' long design and shortened it to 6', or lengthened it to 13', you would have a similar weight rocket with similar recovery requirement, but would it make sense to base your shock cord length to a function of 6' rocket length in one case vs. 13' rocket length in the other case? I would expect to use the same shock cord length for all 3 of these rocket designs. I contend that if you are building a 6" diameter rocket then you would have to visualize what its length would be for an average design then base your shock cord length on that rocket length.

If you are familiar with smaller rocket kits- consider something like the Estes Baby Bertha and compare it to something like the Estes Cherokee D. They will be about the same weight but the Cherokee is over twice the length of the BB. I don't think the Cherokee requires twice the shock cord length as the BB.