2/56 shear pin ejection pressure

There is that nervous pressure that the upcoming launch could not go well. The specific issue is a first time user (me) using shear pins twice for a dual deployment flight. A new Wildman Jr is about to go up on a mild load to test systems and it is not clear how much Oomph is required to separate the 2.6 inch x 18 inch booster section and payload bay with (currently) one 2/56 shear pin.

The question is, how much black powder will it take to assure the shear pin will break free?

Please feel free to point toward existing information and formulas for this - I just couldn't find them.

The project can be found at https://www.rocketryforum.com/showt...ild-Thang-Jr-build-challenge&highlight=scarab.

https://www.feretich.com/rocketry/Resources/shearPins.html

Black powder calculator:
https://www.rimworld.com/nassarocketry/tools/chargecalc/index.html

I would start with one gram of 4f. That might actually be a little much. As suggested by Timbucktoo in the next post, ground test.
I usually don't use a single shear pin for fear of cocking. For a small diameter rocket like 2.6 inch, I would use three styrene shear pins instead, which shear easier than nylon, or I might just use tape on the shoulder.

Formulas give you a good starting point but ground testing with all your laundry will give you a more definitive answer.

The Juniors are 54mm/2.125". It is fiberglass and pretty sturdy. 1 gram should get it done, but even 1.25 is not going to hurt assuming you recovery harness has some length to it.

I have one that I fly with two 2x56 nylon screws as shear pins. I use long bridles and 1.25 to make sure everything comes out. No ill effects.

timbucktoo said:

Formulas give you a good starting point but ground testing with all your laundry will give you a more definitive answer.

Click to expand...

Stated another way, never put anything into the air that you are guessing/hoping/praying will be successful. However, you will note that most BP calculators give you a result that is good for up to five #2-56 pins or three #4-40 pins.

markkoelsch said:

The Juniors are 54mm/2.125". It is fiberglass and pretty sturdy. 1 gram should get it done, but even 1.25 is not going to hurt assuming you recovery harness has some length to it.

I have one that I fly with two 2x56 nylon screws as shear pins. I use long bridles and 1.25 to make sure everything comes out. No ill effects.

Click to expand...

Yep. Ground testing should get the chute completely out without stressing the harness (i.e., no "snap back").

From material and parts strengths (found online, so a few grains of salt required), It probably takes about 25-30 lbs to shear one 2-56 nylon screw.

For your 1 pin, use a charge calculated to pressurize your bay volume so you have enough force to throw the nose and chute/harness Plus break the pin (+30 lbs)

But Ground Test! More than once!

I think the 25-30# strength of the screws is reasonable because when I was on my school's IREC team, a buddy and I pulled apart a 3 pin set-up by hand. Definitely wasn't over 45 pounds of force on my end.

I have been flying the Wildman Jr and other variants for the last 6 years. I always use 3 2x56 nylon pins and 1 to 1.25 grams. Works fine all the time.

Pat

I think I read on your other build thread that you were using two flight computers for redundancy? I would for sure ground test if you can. Regardless of whether you ground test or not, I have the opinion that the second "back*** ejection charge should be more than enough to separate your rocket. In other words, make that second charge, no matter what, separates the thing. I'd like to see mine come down in some pieces rather than screaming towards the ground not separated at all.

Anyone know of any instance where someone put absolutely way too much of an ejection charge in a fiberglass rocket, had a properly timed deployment, and still separated a harness from its anchor point?

I'll +1 for ground testing. Calculators are great to give you a starting point, but nothing compares to the empirical evidence of a ground test. Maybe the calculator is right, but maybe your coupler is a bit tighter and might need more BP to get good deployment. Only ground testing with all the chutes, shock cords, nomex shields, etc. in place will give you the peace of mind to know that you have a sufficient charge setup.

I usually put +20% charge from what ground testing proved was optimal in my backup to make sure I get deployment if the primary fails.

Ground test. Your nerves will thank you for it.

I use two or 3 pins on a 54mm and use .8g for 2 pins and 1.2g for 3. BUT, what size are your vent holes in the two sections? I use a 3/32 hole in each section. the vent size can make a big difference. If I was you, I'd try ground testing with 1g. Use an old shirt or rag that is about the same size as your chute so you don't risk burning it up. if the charge doesn't pop the nose off it can toast your chute.

Nytrunner said:

It probably takes about 25-30 lbs to shear one 2-56 nylon screw.

For your 1 pin, use a charge calculated to pressurize your bay volume so you have enough force to throw the nose and chute/harness Plus break the pin (+30 lbs)

But Ground Test! More than once!

I think the 25-30# strength of the screws is reasonable because when I was on my school's IREC team, a buddy and I pulled apart a 3 pin set-up by hand. Definitely wasn't over 45 pounds of force on my end.

Click to expand...

timbucktoo said:

Formulas give you a good starting point but ground testing with all your laundry will give you a more definitive answer.

Click to expand...

+1. I use 30-ish pounds per 2-56 nylon screw, (well, actually 100 pounds of force to shear three of them. I never use just one.) Some BP calculators give you a pressure (as a function of bay volume and quantity of BP). A problem with using that alone is that you really care more about Force than Pressure. A long skinny payload bay with the same volume as a short fat one will give the same pressure for the same charge. But the pressure required for a larger diameter nosecone is less (IMHO). Remember from High School Physics (thank you, Mr, Sorensen), Pressure = Force / Area, which gives Force = Area * Pressure. Since the pressure acts over a larger flat surface area (bulkhead area=cross section of body tube diameter), which goes up as the square of the radius of the body tube (A=pi*r*r), less pressure is needed to impart a given force to shear the pins, push off the nose cone, and push out the laundry. For a big rocket, the laundry generally has more mass to move. For a small rocket, the laundry generally is a tighter fit. So, the minimum force to shear the pins needs to be increased (think at least double) to get everything else out.

As others have recommended, ALWAYS ground test your ejection charges.

And let us know how it goes.

The BP calculator I use (RocketCalc on iOS) shows that 1 gram of BP will produce about 100 lbf in your payload bay. That's enough to shear three 2-56 nylon screws. Plenty to deploy your chute.

I don't ground test any of my charges as none of my several million neighbors in the DFW area are really comfortable with me setting off charges that sound like a bomb. I use shear pins that require a known force and use a comfortable margin above that. In 15 years its never failed me.


Tony