Shear Pins vs Friction Fit

So I am currently building a Binder Design Excel DD (4in) and was wondering what the real benefits of using shear pins vs using masking tape or simmilar for a friction fit. I have friends who swear by shear pins and use them for everything and I have friends that absolutely love friction fit and never use shear pins.

Beyond the obvious what are some pros and cons of using one vs the other

Advantages of Shear Pins
You don't have to wrap and strip tape on your tubes.
You don't have to get it to fit "just right."

Advantages of Friction Fit
You don't have to buy pins.
You don't have to worry about pins ripping your airframe when they break.

IMO, it's taster's choice. On an FG airframe, no reason not to use pins. They're cheap, do the job very well, and break cleanly. But on a paper airframe, you usually need to use little strips of copper to get a clean break w/out tearing your tube.

For a BD kit, I'd probably go with friction. I use friction on my Vertical Assault, and it is seven feet tall with a 54mm MMT.

Taster's choice.

I agree with Bat-mite. I only use friction fit with all of my <=4" rockets, including the 10 lb L2 rocket. With my FG 6" L3 rocket, it's shear pins all the way.

With shear pins, it's insert and go. With friction fit, you have to spend a little more time making sure things are right. For the most part, I don't add or remove tape very often, but you do need to check the fit every time.

I have found that putting the tape on in an X pattern is a very good way to build up tape. It allows the tubes to slide together past the ends of the tape before encountering the thicker/built up layers. Depending on how much space you need to build up, you can add one, two, three or more tape Xs.

One thing you have to watch is if you CA the ends of the cardboard tubes. Make sure the inside edges of the tubes are flush and even with the rests of the BT. If the CA forms a ridge at the end of the tube, you'll have problems getting tubes together, adjusting the fit, and getting things to come apart right.

Cardboard or similar materials with some give - friction fit. Fiberglass and other hard rigid materials - shear pins. I use both depending on the material. With LOC parts, friction fit works well and once fitted rarely needs tape added or removed. With rigid tubing, it's much harder to get the right fit and the tape tends to roll up when inserting or at separation necessitating replacement of the tape. Much easier and more consistant using shear pins.

I prefer shear pins on rockets >3" diameter or when the nose cone is heavy (noseweight). For cardboard rockets I drill #51 holes for the pins, then harden them with 3-4 soaks with thin CA. Then I redrill and tap the BT and NC. I find that tapping, while a bit more trouble, results in a cleaner shear. Also, if your NC has ridges in it (like Madcow), then make sure you drill into one of the ridges. Any gap between the BT and NC will reduce the cleanliness of the shear and may lead to some tearing. I have a couple cardboard rockets with 10+ flights and no distortion on the shear pin holes.

David

I'm old school and friction fit my rockets up to six inch dia. 6" is the biggest rocket I have at 20 pounds ready to fly. I see some advantage of shear pins but it is more work to glue in some sort of metal to keep the pins from ripping the tube. CA would be another fix for that as well. With a lot of glass tube rockets on the market there will be no problem with pins. I just like friction fit with 2" wide clear tape.

I once went to a cutlery dealer with the intent to purchase a set of high quality kitchen knives.

The proprietor set a knife before me and began extolling the many virtues of &#8220;Hollow Ground&#8221; blades.
He ended this by saying that &#8220;Many of the finest knives made use Hollow Ground blades&#8221;.

He then laid another knife before me and began extolling the many virtues of &#8220;Flat Ground&#8221; blades.
He ended this by saying that, you guessed it, &#8220;Many of the finest knives made use Flat Ground blades&#8221;.

Shear pins vs friction? See above.

You meant "flat" in the last one, right?

My *opinion* is that the primary distinction between friction fit and shear pin use (in HPR) is this:

Friction fit (at least in this part of the world, where there's a wide range of possible temperature and humidity) means checking and perhaps adjusting the fit with every flight, and chances are, it will never be exactly the same from one day to the next.

Shear pin use allows for a known, repeatable necessary breaking force. As long as the tubes and couplers fit loosely without the pins, it's the same every time.

That said, I do use both methods. I know that on a 4" cardboard tube rocket with friction fit, I'm going to have to fiddle with it before flight to get that "just right" feel. That's okay, but on it's next significant repair, I'll probably adapt it to use shear pins. Yep, that's easier with a fiberglass rocket, and doing so on cardboard requires inlaying some shim stock, or other reinforcement.

My $0.02

Mark

markjos said:

My *opinion* is that the primary distinction between friction fit and shear pin use (in HPR) is this:

Friction fit (at least in this part of the world, where there's a wide range of possible temperature and humidity) means checking and perhaps adjusting the fit with every flight, and chances are, it will never be exactly the same from one day to the next.

Shear pin use allows for a known, repeatable necessary breaking force. As long as the tubes and couplers fit loosely without the pins, it's the same every time.

That said, I do use both methods. I know that on a 4" cardboard tube rocket with friction fit, I'm going to have to fiddle with it before flight to get that "just right" feel. That's okay, but on it's next significant repair, I'll probably adapt it to use shear pins. Yep, that's easier with a fiberglass rocket, and doing so on cardboard requires inlaying some shim stock, or other reinforcement.

My $0.02

Mark

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I agree with Mark on this. Even within a day you can have changes. You can get your friction fit "just right", take it to RSO, then wait for a pad all while the rocket can be cooking in the sun or getting really cold. Especially on fiberglass rockets there is no reason to use tape. Still, always test your ejection charge/shear pin combo before your flight.

I agree with Mark. Weather, Motor thrust/ burn rate all determine my choice.
I have had motors like V max pop the nose at burnout even with pins. Also on some rockets
Remember to put a small vent hole for pressure change on ascent.

markjos said:

My *opinion* is that the primary distinction between friction fit and shear pin use (in HPR) is this:

Friction fit (at least in this part of the world, where there's a wide range of possible temperature and humidity) means checking and perhaps adjusting the fit with every flight, and chances are, it will never be exactly the same from one day to the next.

Shear pin use allows for a known, repeatable necessary breaking force. As long as the tubes and couplers fit loosely without the pins, it's the same every time.

That said, I do use both methods. I know that on a 4" cardboard tube rocket with friction fit, I'm going to have to fiddle with it before flight to get that "just right" feel. That's okay, but on it's next significant repair, I'll probably adapt it to use shear pins. Yep, that's easier with a fiberglass rocket, and doing so on cardboard requires inlaying some shim stock, or other reinforcement.

My $0.02

Mark

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I'd give you more like $0.25 for that advice!

Give this a try; https://www.arsabq.org/Tools/Drag_Separation_1997-2003.xls

Cheers

That's awesome. Thanks.

Rocketry can be made as simple or complicated as you want. Both ways work and both have their plusses and minuses. I do all my dual deploy flights with tape because I like cardboard rockets. In the end it boils down to what you will have more fun doing and little else.

I've been using tape but on a recent DD flight, the nose cone popped off shortly after apogee. I double checked the fit before launch and was confirmed by safety officer so we both scratched our heads when we saw the main a little earlier than expected. That was a 4" frame. Probably gonna put a couple of shear pins on it next time and forget about the tape.

timbucktoo said:

I've been using tape but on a recent DD flight, the nose cone popped off shortly after apogee. I double checked the fit before launch and was confirmed by safety officer so we both scratched our heads when we saw the main a little earlier than expected. That was a 4" frame. Probably gonna put a couple of shear pins on it next time and forget about the tape.

Click to expand...

My question would be, how strong was your apogee charge? My DD rockets only need to separate about 1/4 of the shock cord length to make sure the drogue is out. I will z-fold the cord and many times there are still some taped z-folds left after it lands. I've never had a problem with friction fit nose cones coming off at apogee. Don't over do your apogee charges.

I'm using about 0.8 grams BP for both main & drogue. Using a PML CPR3000 with piston deployment for both main & drogue. No motor ejection backup due to bulk head.