Shear Pins With Cardboard

[lovingia]

Hello friends. I have a concern I'd like some help with. I've built several dual deploy rockets, all fiberglass. I'm fairly familiar with the ins and outs from that perspective. My latest build is a LOC VII. I got my level 1 on a LOC IV and thought this would be nostalgic and fun. My worry is the shear pins on a cardboard airframe. For example, BP Estimator recommends a start of 5.61g of powder with either 11 #2 pins or 7 #4 pins. I understand hardening the airframe around the pin holes with CA....I'll probably use West Systems thin epoxy. My concern is won't that many pins stand a good chance of damaging the airframe? Would I be better off with the #2 or #4 pins?
Thanks for any help you can offer.

 

[Banzai88]

That's.....a lot of pins. I would question the need for so many.

I have a tutorial here where I show how to put brass plates in the cardboard. Simple, strong, and fast and foolproof.

https://www.rocketryforum.com/threa...-in-your-cardboard-tubes.136706/#post-1627875

 

[David Schwantz]

I also have used brass plates. You can soak the cardboard in thin CA to harden it. You can add a metal plate on the outside. This works very well on a scale bird as it simulates a hatch. But if I get your post correct, you are talking about adding 11 pins in a #2 size?? That is a lot of pins. Most of mine have 2-56 holes drilled and tapped, with four going around the airframe. But, I normally only use one screw per flight. This has been plenty for most rockets. If you have a NC with lots of weight this can take more pins to retain the force at ejection, or burn out.
Whatever you do, always ground test first.

 

[Scott_650]

Or just friction fit and keep it simple. And, as already suggested, ground test to make sure.

 

[Rschub]

 

[tsmith1315]

Or just friction fit and keep it simple. And, as already suggested, ground test to make sure.

This would be my solution on a cardboard/plastic kit without excessive weight up front.

 

[Buckeye]

Hello friends. I have a concern I'd like some help with. I've built several dual deploy rockets, all fiberglass. I'm fairly familiar with the ins and outs from that perspective. My latest build is a LOC VII. I got my level 1 on a LOC IV and thought this would be nostalgic and fun. My worry is the shear pins on a cardboard airframe. For example, BP Estimator recommends a start of 5.61g of powder with either 11 #2 pins or 7 #4 pins. I understand hardening the airframe around the pin holes with CA....I'll probably use West Systems thin epoxy. My concern is won't that many pins stand a good chance of damaging the airframe? Would I be better off with the #2 or #4 pins?
Thanks for any help you can offer.

5.61g with 11 pins sounds crazy. See this LOC VII thread:

https://www.rocketryforum.com/threads/loc-vii-build-thread.150312/post-1849391

 

[Zeus-cat]

I use a snug fit by adding tape where needed. I also Z-fold the shock cord deployed at apogee to reduce the sudden stop forces on the nose cone. Just fold over the shock cord an inch or two, wrap tape around it and repeat until you run out of shock cord.

 

[Hobie1dog]

5.61g with 11 pins sounds crazy.

+1...... someone at LOC must be into numerology

 

[cerving]

Something is not right with your BP Estimator data, because 5.61 grams of BP and 11 shear pins will result in your cardboard body tube rupturing most spectacularly. I would recommend about 2 grams of BP at most, use friction-fit on the drogue section, and maybe two #2 shear pins on the top. Harden the holes with CA glue, and a brass shim to help them shear would be a good idea too.

 

[Buckeye]

Something is not right with your BP Estimator data, because 5.61 grams of BP and 11 shear pins will result in your cardboard body tube rupturing most spectacularly. I would recommend about 2 grams of BP at most, use friction-fit on the drogue section, and maybe two #2 shear pins on the top. Harden the holes with CA glue, and a brass shim to help them shear would be a good idea too.

Depends on the weight of cone itself and any payload inside of it. This is a 7.5" rocket. In the link above, he used 2.3 grams of powder and three #2 shear pins which were not enough to hold the nose at apogee. ~2.5 grams sound about right, but I would start with 4 pins.

 

[ep29030]

Instead of shear pins, use masking tape as “shear tape”. I have a 54mm rocket made of Quantum tube and it shears perfectly with two pieces of 1/4” green tape across the nosecone/body tube junction. No holes to drill, no brass plates to add. Ground test it to assure that your tape setup works. Might want to try 1/2” tape instead of 1/4”. Peel tape off rocket post-flight and ready for next launch.

 

[SolarYellow]

Color it with a Sharpie if it makes you feel better about how it looks on your launch rod photos.

 

[pathtouch]

As a data point I use (2) 2-56 nylon shear pins and 3.0 grams of Triple 7 in my 7.5” LOC Doorknob. My nose cone has no weight though and is incredibly light. Survived a J381 and L1090 so far.

 

[bad_idea]

Instead of shear pins, use masking tape as “shear tape”. I have a 54mm rocket made of Quantum tube and it shears perfectly with two pieces of 1/4” green tape across the nosecone/body tube junction. No holes to drill, no brass plates to add. Ground test it to assure that your tape setup works. Might want to try 1/2” tape instead of 1/4”. Peel tape off rocket post-flight and ready for next launch.

It sounds like you're able to get the tape to break instead of having adhesion to one or the other part fail first. What tape do you use?

I've been curious about shear tape since seeing it used unsuccessfully a few months ago. It seems like it should provide consistently repeatable results if adhesion and the width of the tape are consistent, but if adhesion fails before the tape breaks, I could see where surface, temperature, etc. could make consistency more difficult to achieve. I think that is what happened in the case of the failure I saw. (Pretty sure I saw the same rocket fly again yesterday without deploying main at apogee, but I didn't get a chance to see if it was using tape or something else this time.)

 

[SolarYellow]

One could do a wider piece of tape and perforate it at the seam, so there is more adhesive area and tearing at the desired location is favored.

I have had tape fail on a C6-7 motor retention in BT-20. Wrapped 1/4-inch masking tape around the back of the motor for a thrust ring, and then blue masking tape from there to the body tube. When I found the rocket, the blue tape had let go of the plain tape on the motor and the motor was nowhere to be found. Still managed to get the streamers out, somehow. It always worked on the A8 and B6 motors. I'm thinking maybe it was the greater heat of the C6 that made it let go. Shouldn't be an issue on an ebay/airframe joint.

 

[ep29030]

It sounds like you're able to get the tape to break instead of having adhesion to one or the other part fail first. What tape do you use?

I've been curious about shear tape since seeing it used unsuccessfully a few months ago. It seems like it should provide consistently repeatable results if adhesion and the width of the tape are consistent, but if adhesion fails before the tape breaks, I could see where surface, temperature, etc. could make consistency more difficult to achieve. I think that is what happened in the case of the failure I saw. (Pretty sure I saw the same rocket fly again yesterday without deploying main at apogee, but I didn't get a chance to see if it was using tape or something else this time.)

I use 3M green "lacquer tape”. I like the 3M brand—good adhesion. I use a piece about 4-5 inches long. I’ve never had a failure with this approach. I’m sure you would get different performance with other tape brands. I have become a tape snob for rocketry and I use 3M tapes for about everything. You get what you pay for in tape.

 

[ep29030]

One could do a wider piece of tape and perforate it at the seam, so there is more adhesive area and tearing at the desired location is favored.

I have had tape fail on a C6-7 motor retention in BT-20. Wrapped 1/4-inch masking tape around the back of the motor for a thrust ring, and then blue masking tape from there to the body tube. When I found the rocket, the blue tape had let go of the plain tape on the motor and the motor was nowhere to be found. Still managed to get the streamers out, somehow. It always worked on the A8 and B6 motors. I'm thinking maybe it was the greater heat of the C6 that made it let go. Shouldn't be an issue on an ebay/airframe joint.

I use 1/4” 3M green tape for thrust rings in low and mid-power motors. Never had a failure there. Good tape and worth the price. I buy mine 3-5 rolls at a time on eBay to get a better price.

 

[SolarYellow]

It wasn't the thrust ring that failed, it was the adhesion of the blue 3M masking tape to the thrust ring that failed to hold the motor in the rocket at ejection.

I looked on fleabay and found some 3M 1/4 inch/6mm green tape, but it was crazy expensive. I've been using some ordinary beige stuff, 1/4 inch wide. Has worked fine so far. I'm sure I paid much less than I see the 3M green on fleabay for.

 

[bad_idea]

I have become a tape snob for rocketry and I use 3M tapes for about everything. You get what you pay for in tape.

I concur.

It wasn't the thrust ring that failed, it was the adhesion of the blue 3M masking tape to the thrust ring that failed to hold the motor in the rocket at ejection.

I used 3M purple high-temp tape for both thrust rings and taping the thrust ring to a motor tube.

 

[Kelly]

A big vote for tape on cardboard (though, I even use it on fiberglass now). 3M tape has a strength of about 23 lbf per inch of width. I use about a 2 inches of length across the joint, with as much width as necessary (determined by the BP calculator). Use a total of 3 pieces, separated by 120°. Never had a prblem with the tape sticking when it needed to, or coming off when it needed to.
Other tape strengths can be found here.

 

[thequick]

I’ve never had problems with #2 nylon shear pins in LOC or Madcow cardboard.l with CA. 25+ flights.

I didn’t have a good feel for tape so my experience was hit and miss. Typically the tape would let the main out at apogee (fail at in a good way)

 

[MikeT]

Brass Plates for the win.

 

[Mark F]

Instead of shear pins, use masking tape as “shear tape”. I have a 54mm rocket made of Quantum tube and it shears perfectly with two pieces of 1/4” green tape across the nosecone/body tube junction. No holes to drill, no brass plates to add. Ground test it to assure that your tape setup works. Might want to try 1/2” tape instead of 1/4”. Peel tape off rocket post-flight and ready for next launch.

I like tiny dual deploy rockets and have used this method successfully for some time.

 

[TheTank]

Wow... that's a whole lot of pins for a cardboard tube and the stock 7.5" plastic LOC cone that isnt that heavy to begin with. I use max 3 4-40 pins on 6" fiberglass cones that weigh a ton more and just set my charges and my shock-cord length appropriately to ensure they don't prematurely sheer.... and that is with 4 -6 grams of powder. I made the mistake once of blindly following a BP calculator on a 7.5" inch cardboard rocket and damaged the body tube where I then went in and glassed the inside. I would definitely start lighter and increase through successive tests if needed to get it right.

 

[kswing]

On my 7.5" Bruiser EXP I used 3 4-40 shear pins with brass plates as backing for the NC. I ended up using about 4.25g of powder and it worked well. If I recall correctly, the BP calculator estimated much more, but, through testing I found that less was plenty.

 

[kjhambrick]

TLAR

 

[b.wieting]

I fly my 7.5" MegaMagg (and MiniMagg too) dual deploy with three 2-56, just threaded into CA-strengthened body tube. Satisfactory in ground tests and flights.

 

[Rschub]

I fly my 7.5" MegaMagg (and MiniMagg too) dual deploy with three 2-56, just threaded into CA-strengthened body tube. Satisfactory in ground tests and flights.

I need to work on my recovery configuration then because I got a zipper and popped the pins on my last flight at apogee, probably due to using kevlar for the first time.

 

[b.wieting]

Maybe kevlar, tho I've used it without incident on the Maggs. How about drogue shock cord length?

Couldn't figure out why Wildman Blackhawk38, with two 2-56 on the 38mm airframe, popped the main in addition to the drogue (streamer) at or shortly after apogee, two flights in a row.
I discovered that I'd accidentally swapped the (shorter) main and (longer) drogue shock cords, so apparently the abrupt forces when the short shock cord stretched out sheared the pins.
After swapping the cords to the correct (main-short, drogue-long), had a picture perfect flight, streamer at apogee, main at 300'.