losing shear pins at altitude

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watermelonman

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While I have certainly had my successes, I would like to get dual deployment to the point where it is as reliable as model rockets for me. Flight computers, wiring, blowing charges, and getting the main out have been solid. My latest concern, however, is losing the shear pins and letting the main out early. While it is clearly the safest failure mode, it kills the value of dual deployment.

I saw this happen at least twice between XPRS and BALLS. I suspect it was made worse by high altitude winds, though a particularly powerful drogue blast or weak pins could be at fault.

How much of a problem has this been for others over time? Right now the only solution I can think if is to use more pins and more charge.
 
If they are falling out, then they are in too loose. It should take some effort to pry them out with your fingernails. I had a main come out when the drogue charge fired, but it was because I left my pins out. I have never had my main come out at apogee if I had my pins in.
 
What are you using for shear pins? How many pins at each joint? How much weight do they have to hold together? Are you fan folding and taping your shock cords?
 
I use 3x 2 56 pins, not sure what the XPRS failure had. Also I run electrical tape around the airframe at that position so they definitely do not fall out.

I have not taped and folded cords, is that to help them absorb force as they go?
 
:confused: "Right now the only solution I can think if is to use more pins and more charge." "Also I run electrical tape around the airframe at that position so they definitely do not fall out."

So what's the problem ... ?
 
Size the hole such that you have to use a hammer (AKA "Shear Pin Insertion Tool") to tap the pins in. That way you know they aren't coming out and you can safely cut the tops off for high velocity flights.
 
Did the pins fail or did they fall out? I have had my pins fail a time or two due to too much force on the drogue deployment charge. At drogue separation, the booster and main section reached the limit of the shock cord causing excessive force on the main section and broke the shear pins. I have since learned to use slightly less charge to have a less powerful separation.
 
I put a piece of tape over my shear pins to make sure they don't come out.

I've done about a dozen dual deploys and only had the main come out early once. I believe my drogue shock cord was too short and the force at deployment ejected the nose cone with main.
 
:confused: "Right now the only solution I can think if is to use more pins and more charge." "Also I run electrical tape around the airframe at that position so they definitely do not fall out."

So what's the problem ... ?

Hah, the problem was that I watched a much more experienced flyer pop main at a much higher altitude I have reached and I want to avoid that! Also I had a partially deployed main earlier that day, but I have solved that problem.

Sorry if many of my questions are speculative or hypothetical. I try to learn as much as I can from flights of others, as well as my own. Mostly, I wanted to see how common this problem is. Sounds like, not that common.
 
Not enough good things can be said about having more than enough shock cord. Don't skimp! At least five times the length of the rocket at each end.
 
Sheesh. I wish you guys understood the definition of long shock cords... :rofl:

I use at MINIMUM 40 feet drogue and 30 feet main on a medium L1 sized rocket....
 
Not enough good things can be said about having more than enough shock cord. Don't skimp! At least five times the length of the rocket at each end.

Sheesh. I wish you guys understood the definition of long shock cords... :rofl:

I use at MINIMUM 40 feet drogue and 30 feet main on a medium L1 sized rocket....

Both excellent points!

Question for Mellonman - Did the shear pins fall out or were they sheared?
 
It's hard to answer without seeing the rocket or at least knowing the weight of the sections and the length of the shock cords. Maybe the drogue shock cord is too short. A longer cord that is fan folded and taped can absorb some of the shock. Is the main in the middle of the rocket between the fin can and the drogue or is the main in the top section?

My guess is that you need more or heavier shear pins. FWIW: On my 3" Saab I have 2 nylon rivets on the drogue break and 4 on the main. They need a lot more pressure to shear than a 2-56 screw. The nose had about 24oz of lead in it though too. I'd be willing to be that if you replaced those screws with rivets and resized your main deployment charge your problems would be solved.
 
I have been 3/3 on my personal Dual Deploy flights.. so far there has never been a single failure in the recovery system. Only on one of those did i use sheer pins but those worked as well. I have been witness to several DD failures at launches, though probably not near as much as others on this forum. The ones I know the issue: 1 had drogue charge pop out the main, 2 caused the payload to separate due to insufficient tape used to hold nosecone on (no sheer pins), and another one failed becuase the battery fell out of it's holder during acceleration or some other point during flight.
 
I had a premature separation at or near apogee on my most recent Tomach launch. It was not due to the pins falling out as they were cleanly sheared, which leads to the possibility that they can fail multiple ways.

Based on the logs I will never know the exact cause, however on this nosecone I used the normal drill a hole and insert method for my 3 x 2-56 shear-pins. The holes were tight, not overly, however if you need to remove the shear-pins for any reason, and reuse them, then you can potentially compromise their integrity to the point that an over-energetic deployment charge may cause them to fail. If this was a possibility then the obvious would be to not reuse them however their is always another way.

To back up a bit. My dual altimeter logs reveal that the charges were at least close, to the point that my primary and backup altimeter may have popped at or very near the same time which could of caused the rocket to split at apogee with quite a bit of velocity, possibly enough to shear the already compromised shear-pins...strictly one logical deduction.

My new strategy to mitigate this failure is the following:

- use a 2 second delay on the back-up altimeter instead of a 1 second.
- the mid-section break point on this rocket, as well as other DD rockets, I have had good success in over-drilling the holes and epoxying in a self clinching nuts. This allows me to screw my shear pins in and out as well as making it a lot easier to remove the shear-pins in-case I need to disassemble last min, which happens.
- backed off my over-charge from 1.5g to 1.2g BP. My ground testing showed that 1g was the right amount but I added a bit more for margin, likely too much.
- changed from a 20' Kevlar harness to a 25'. For what it is worth my reasoning was that the extra 5' would help to dissipate the energy that much more.

Bottom line is I believe that shear-pin failures have multiple modes.

BTW my failure resulted in a 2.4 mile recovery, which is the distance it drifted from just under 16,000 feet, a great test for my TeleGPS and Compsec AT-2B.
 
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I use 20 or 30 ft shock cords and fan fold and tape them each flight. I drill and tap the holes for my shear pins. the inside tube is then drilled out to a tight press fit. that way the screw stays in and can be un screwed if needed. the inner part just pushes out after it is sheared off so it is easy to remove.
 
I've got most of my rockets drilled for 4-40 pins. I've been pushing in inch and a half long 2-56 pins, then twisting the cone just a bit to lock them in place. If the nosecone is tight enough, no worries.

My issue seems to be wiring the drogue charge into the main payload bay. ><
 
If I understand your question..it sounds like your main is also being 'deployed' during your apogee event(s)? If so, I agree with Chris. We need more info. weight of parts, typical alitiudes, shock cord lengths now, current shear pin size and numbers and if your using a drogue chute. The bigger the rocket and the higher you go the harder it is to keep your main from being shaken out at apogee. Bigger apogee charge and bigger shear pins sounds like the same issue will happen.

Properly sized, spaced and number of shear pins, apogee charge sizing, shock cord lengths, drogue sizing (if used) and folding (folded to minimize shock of rapid opening) and shock cord absorbing methods are the keys.

Tony
 
If I understand your question..it sounds like your main is also being 'deployed' during your apogee event(s)?

Actually, if I'm reading it right... he saw two other people do it, and is worried about problems he's not having.
 
my bad..I had read his posts then came back hours later and read the replies prior to answering. The replies got me off track.

Eric..if you're not loosing your main at apogee. you're doing it right! Go give advise to those that are having the trouble.

Tony
 
You are crazy.
Proper design and recovery technique trumps super long cordage any day.

The early deployment may have been from improper venting of recovery bay.
under sized sheer pins can be another.
I use just one 2/56 on a 4" fg rocket flying on L motors.

Proper programming of your flight computer can do odd things also...

JD

Sheesh. I wish you guys understood the definition of long shock cords... :rofl:

I use at MINIMUM 40 feet drogue and 30 feet main on a medium L1 sized rocket....
 
I thread all shear pin holes so they do not come out. Then it is ground testing to verify charge sizing.

4" phenolic rocket is using 3 2x56 screws. 4" fiberglass is using 4 4x40.
 
something that I don't think has been mentioned is the possibility of the primary charge setting off the backup charge. Dizwolf had this happen during ground testing and got a good video of it. if the charges aren't sealed up well, they can both go off at once. that will definitely put some stress on those pins at the end of the shock cord!
 
something that I don't think has been mentioned is the possibility of the primary charge setting off the backup charge. Dizwolf had this happen during ground testing and got a good video of it. if the charges aren't sealed up well, they can both go off at once. that will definitely put some stress on those pins at the end of the shock cord!

Good point Riley. That is one of the possibilities I had considered for my failure, although I seal my end caps very well something could still happen.
 
It's ironic that adding redundancy adds another potential failure mode.
 
I use 3x 2 56 pins, not sure what the XPRS failure had. Also I run electrical tape around the airframe at that position so they definitely do not fall out.

I have not taped and folded cords, is that to help them absorb force as they go?
There's nothing magical about dual deployment. It's done all the time. If something happened in your rocket flight, it should be pretty easy top figure out if you supply us with enough information to figure it out. At a minimum we need to know:

  1. What was the simmed apogee of the rocket?
  2. What was the diameter and weight of your rocket?
  3. Was the compartment that failed vented or not, and what altitude the failure occur at?
  4. You imply that you used 2-56 shear screws, so how did you thread the airframe, and what was the shear surface for the screws? Why did you need to use tape to keep the screws in place?
  5. How long were the shock cords, and what was the rocket velocity at the time of the deployment?

Bob
 
It's ironic that adding redundancy adds another potential failure mode.

True. But the reason for the redundancy is to ensure the laundry comes out rather than a ballistic impact.

So using redundant systems reduces the risk for the impact it can increase the risk of a premature ejection. But having proper delays, cord length/Z fold, and isolation (keep one charge from activating the other) can mitigate this risk.

While the rocket may drift it is the safer alternative
 
My issue seems to be wiring the drogue charge into the main payload bay. ><

Many years ago this was a common reason for failures on TRA level 2 certification flights. (Then they changed the rules.) I noticed this and made a change to how I used my altimeters.

I took a polarized four pin connector and attached wires to it. One connector was attached, more or less permanently (I haven't removed the screw terminals and soldered the wires in their place, but I ought to.), to the altimeter while the other went to the avionics bay. Now I only have to make the connections correctly once. On launch day I don't have to worry about it at all. Just mate the connectors and go.

I would prefer that altimeters use polarized connectors rather than the usual screw terminals for just this reason.
 
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