Shear pin number/location

[ActingLikeAKid]

Quick question: So there seems to be a lot of information on how to size your BP charges to break your shear pins. But I haven't found much on how many shear pins to use.
For a 3" LOC-tubing rocket, I was thinking 2x2-56 pins to hold the nose cone on.
Does that sound right? Is there a formula for figuring this out?
Based upon other threads, it sounds like shear pins to hold the booster on are helpful but not necessary; for my initial foray into DD, I think I'll stick with just using pins for the NC. Then I'll use rivets to hold the bottom of the payload tube to the ebay coupler. Does that sound about right?

 

[dshmel]

Quick question: So there seems to be a lot of information on how to size your BP charges to break your shear pins. But I haven't found much on how many shear pins to use.
For a 3" LOC-tubing rocket, I was thinking 2x2-56 pins to hold the nose cone on.
Does that sound right? Is there a formula for figuring this out?
Based upon other threads, it sounds like shear pins to hold the booster on are helpful but not necessary; for my initial foray into DD, I think I'll stick with just using pins for the NC. Then I'll use rivets to hold the bottom of the payload tube to the ebay coupler. Does that sound about right?

I calculate the amount of force needed to hold the nose cone on when the shock cord reaches maximum tension by assuming a "shock force" of 40 or 50 gees when the harness snaps taught. For my L3, the nose cone weighed 3.8 pounds. I used (3) 4-40 screws which offered about 150 pounds of retention force. 40 times 3.8 pounds equals 152 pounds of "separation" force experienced between upper section of the rocket and the nose cone. However, this assumes the separation force aligns with longitudinal axis of the nose cone / body tube. If the parts are not lined up when the shock cord snaps taught, then the force occurs at an angle, which reduces the "pull out" force along the axis. The shock cord material also affect the force as tubular nylon stretches slightly to reduce the "shock" experienced by the nose cone. This 40-50 gee rule of thumb has worked for all of my DD rockets. I recommend at least 2 shear pins and not a single one, to avoid possible binding of the nose cone.

 

[blackjack2564]

. Then I'll use rivets to hold the bottom of the payload tube to the ebay coupler. Does that sound about right?

yes and harden the holes with CA.

 

[Banzai88]

Not sure if there's really a 'standard', but I observe that most people use the same number of pins as fins, since it makes for easy marking, and size ejection charges accordingly.

I've used 1 pin successfully in 2.56 inch rockets, usually use 2 pins on 3 inch or larger, and generally go with 3 pins.

You can easily do the math of 50(gees) x weight of the nose cone to get the pounds of force that the nose cone would experience, and size from there. One note on that is that if the main recovery gear slides up and down the top tube, it's settled down by the av bay upon launch, and upon drogue ejection it may slide twords the nose and act as a slide hammer with enough force to pop the pins/laundry at apogee. Check, because everyone packs the laundry burrito differently, mittigate accordingly. I've observed that "Blow it out or blow it up" for the drogue charge is one of the main reasons for popping the nose laundry at apogee on a DD bird, especially with friction fit nose cones. Size the charge appropriately, no sense in putting more stress on the system than necessary.

You can also do the same math to reveal what you "really" need as a minimum for recovery harness and attachment hardware strengh, too.

Harden your cardboard tube holes with CA or do some variation of this: https://www.rocketryforum.com/threads/shear-pin-shear-plates-in-your-cardboard-tubes.136706/

 

[Nytrunner]

+recommendation for shear strips in cardboard tubes.

 

[dshmel]

You can always ground test the shear pins by setting up the rocket in pad ready condition, then firing off the apogee charge with the rocket angled up so that the shock cord reaches full extension with the upper section in the air. I would use a shorter shock cord if your test area is limited in size.

 

[Bat-mite]

F = ma

Two different calculations needed if you are doing dual deployment. First, shear pins in the booster need to retain the coupler at motor burnout. Usually, motor burnout does not produce a huge, sudden deceleration; but to be safe, look at your simulation and find the acceleration at burnout. Subtract 1, and now you have a. m will be the weight of everything above the booster.

Let's assume acceleration at burnout is 6 G. 6G - 1 = 5. Let's assume the mass of the rocket above the booster is 3 pounds.

Now, ma = 3 * 5 = 15 pounds-force.

If you are using nylon shear pins, find out the shear point of the nylon screw. #2-56 screws have a minimum shear strength of 31 pounds. So, one shear pin in the booster is twice as strong as what you need in this example. However, as others have said, use at least two so that you don't wedge the coupler.

BTW, online BP calculators will tell you how many pins you can break at a selected charge size, and it is usually around five.

Second calculation is what others are referencing above, namely retaining the NC when the drogue charge goes off. Now you will be dealing with more acceleration due to the BP explosion.

Let's assume you get 40G from the drogue charge, minus 1, and your NC mass is 1 pound.

F = 1 * 39 = 39 pounds-force. In this case, since 39 > 31, you will need a minimum of two pins.

 

[dshmel]

On the 15th launch of my Intimidator3 I attached a tracker to the nose cone and packed the chute and shock cord differently. The (3) 2-56 shear pins holding the nose cone on yielded at apogee and deployed the main chute. First time this has happened. Must have been just the right combination of angles and forces. I will upsize to 4-40 and increase the BP charge by about a half gram.

 

[Nytrunner]

Let's assume you get 40G from the drogue charge, minus 1, and your NC mass is 1 pound.

Are you subtracting one to account for gravity? I don't think that's proper for the drogue charge event

 

[G_T]

Not sure of that math at all...

Drogue premature separation... This is generally a drag separation event if it happens prematurely. You have drag generated by the upper section, and drag generated by the lower section. The difference in these drag values determines the force trying to separate the two sections. Friction + shear pins breakage determines the resistance to that separating force.

You can run a sim such as RasAero or other methods, to determine total drag. If you are stuck with acceleration values instead of drag force, then multiply the burnout mass with the burnout deceleration to get the force.

You can treat that force as an upper bound as that value would assume all the drag is on the lower section. Design accordingly. Likely you don't need much in the way of retention to prevent premature drogue release, at least until you get to big heavy rockets. Then you can absolutely get drag separation at burnout! The heavier the front end in a relative sense, and the faster the rocket, the more likely you are to have an issue.

Separation of parts due to shock of drogue deployment... That's where the 30G or 50G design tolerance is used.

Gerald

 

[Bat-mite]

Thanks, all.