When to use 4-40 shear pins vs 2-56

[Andrew Brown]

I know there are a lot of opinions, and I don't want to start another "glue thread." I'm just looking for a guide for when to step up from 2-56 to 4-40 sized shear pins. Is it based on nosecone mass? I'm in the process of making a 4" Alien Interceptor, my largest build by far. The nose is fairly heavy, plus GPS tracker, battery, etc. Any advice is appreciated.

 

[rfjustin]

Gut reaction/recommendation, (2) 2-56 in the booster, (3) 4-40 up top.

Ground test, ground test, ground test...

 

[Bat-mite]

Online BP charge calculators will tell you how many shear pins and what size for your rocket and BP quantity.

There's a lot of physics around this topic. The main things to remember are:

1. The NC pins need to be strong enough to hold the NC in when the booster charge rips.
2. The booster pins need to be strong enough to hold the booster in when the motor burns out, and external air pressure decreases to the point where the internal pressure in the tube will start to force the tubes apart.

Things to calculate:

1. Expected force on the booster when the motor burns out.
2. Expected force on the NC when the booster charge ignites.
3. Force required to break the pins.

Go from there, or come back with questions.

 

[Bat-mite]

More good threads.

https://www.rocketryforum.com/threads/shear-pin-retention-stregth-check-my-math.131911/
https://www.rocketryforum.com/threads/shear-pin-calculation-the-engineering-approach.132318/

 

[Bat-mite]

One more...

https://www.rocketryforum.com/threads/what-size-shear-pins-to-use-and-how-many.62529/

 

[Andrew Brown]

Justin and Bat-mite, very good replies. Thanks.

I've run the calculations on everything except the force on the nose when the booster fires. I'll read through the suggested posts and try to make some sense of it. It's trying to figure out how much force the nose goes through during deceleration after the apogee event that I'm still struggling with. Thanks again.

 

[Bat-mite]

Justin and Bat-mite, very good replies. Thanks.

I've run the calculations on everything except the force on the nose when the booster fires. I'll read through the suggested posts and try to make some sense of it. It's trying to figure out how much force the nose goes through during deceleration after the apogee event that I'm still struggling with. Thanks again.

Having a long enough harness can eliminate the shock on the NC, but the ejection force applied to the payload section can still break the NC pins if they are not strong enough. Anyway, lots of good stuff in those threads.

 

[cerving]

It depends on the mass of your nose cone (and the payload bay tube if you're doing coupler-break instead of nose-break), I've used both. If you use 2-56 on the main and they are breaking when the drogue ejects, either your shear pins are too small or you're using too much BP on the drogue. Try ground testing with a bit less BP on the drogue, if you don't think you want to go lower on the BP for the drogue then use bigger shear pins on the main.

 

[firemanup]

I use the same for 3”, 4”, 5”, 5.5”, and 6” birds fiberglass and cardboard.

Three 2-56 nylon screws at each joint, ive not had any problems on any of my flights. As always they are ground tested to figure out appropriate charges.

You can click on my flight logs below to see the variety of rockets ive flown this way.

 

[Buckeye]

The booster pins need to be strong enough to hold the booster in when the motor burns out, and external air pressure decreases to the point where the internal pressure in the tube will start to force the tubes apart.

Pins in the booster are rarely needed. The concept of drag separation is wildly over-used and misunderstood. It only occurs when the mass and drag are wildly different between the two halves of the rocket.

Tubes have natural air leakage, but a tiny vent hole in the booster is plenty to equate air pressures and keep the rocket together.

 

[dvdsnyd]

One of the linked threads had a derivation of the forces for drag separation, which would factor into shear pin sizing. This is from John Derimiggio that had gotten broken.
With his permission, here it is again.

 

[mrwalsh85]

Don't forget the length of your shock cord that is used for your booster section... Too short and you may have more inertia than expected, resulting in shearing of the top side pins.

 

[Conway Stevens]

For myself and some of the others ive flown with its all relative to the size/mass, speed and how fast it gets to speed and back out of that speed that determines the size of sheer pin. you should be able to calculate the amount of force upon the rocket. the weight and drag should also be known. Most the 38 and 54 mm rockets i use styrene rod 0.060 diameter. I always use 3 sheer pins in equal distances. Larger rockets 3 and 4 inch I use #2 some 4 inch I use #2 booster and #4 on the nose. on bigger then 4 inch stuff I use #4 up to 5 and some 6 inch. most of my 6 and 8 and larger inch use #4 on the booster and #6 on the nose. I ground test accordingly. I also reef my harnesses.

 

[Buckeye]

One of the linked threads had a derivation of the forces for drag separation, which would factor into shear pin sizing. This is from John Derimiggio that had gotten broken.
With his permission, here it is again.

Why is there no mg in the FBDs? Is it assumed to be included in a as a net acceleration? Depending on the choice of simulator or accelerometer to measure acceleration, this may not be the case.

 

[Kelly]

I think that mg does not matter here, since it's contribution to 'a' is the same for both pieces.
Mind-simming it, it seems that the equation above should be correct even if the rocket were to operate in a gravity-free (though still aerodynamic) situation.
Alternately, just use an accelerating frame of reference for the analysis, and mg disappears.

 

[Bat-mite]

Pins in the booster are rarely needed. The concept of drag separation is wildly over-used and misunderstood. It only occurs when the mass and drag are wildly different between the two halves of the rocket.

Tubes have natural air leakage, but a tiny vent hole in the booster is plenty to equate air pressures and keep the rocket together.

They are useful for keeping the rocket in one piece while you carry it to the pad. Accidental separation and laundry spillage are ... well ... embarrassing.

 

[Bat-mite]

8" FWFG ... three #8 pins in each half.

 

[dvdsnyd]

Why is there no mg in the FBDs? Is it assumed to be included in a as a net acceleration? Depending on the choice of simulator or accelerometer to measure acceleration, this may not be the case.

I think that mg does not matter here, since it's contribution to 'a' is the same for both pieces.
Mind-simming it, it seems that the equation above should be correct even if the rocket were to operate in a gravity-free (though still aerodynamic) situation.
Alternately, just use an accelerating frame of reference for the analysis, and mg disappears.

Below is clipped from the original derivation:
It looks intentional that mg from each section is not used, because the a is the deceleration from the drag only AT motor burnout.
@jderimigcould you explain this a little more please?
Thanks,
Dave

Note that the Drag Force ratio R is really the ratio of Cd*A of the top and bottom airframe sections. Usually the fin area is negligible so the Cd ratio is sometimes a good approximation, but to be totally accurate the Cd*A should be used for R. Both easily obtained from Rocksim or OR.

Also "a" is the deceleration from drag only at motor burnout. So if your sim says 5g's of deceleration, 1g of that is from gravity. So subtract that 1g out and use 4g. Note in the derivation below + is pointing "down", so deceleration is entered as a positive value in the equation. Also in the below derivation Fsep is defined as a "compression" force. So that if Fsep is positive then drag separation will not occur. Sorry for flipping the sign convention this time.....

 

[jderimig]

Below is clipped from the original derivation:
It looks intentional that mg from each section is not used, because the a is the deceleration from the drag only AT motor burnout.
@jderimigcould you explain this a little more please?
Thanks,
Dave

Since we are only interested in the Fsep, which is an internal force, we do the dynamic analysis in the rocket inertial reference frame. In that frame there is no mg force. We only observe the mg force by observing the rockets acceleration in the earth frame. So "a" in this analysis would be the acceleration measured by an on-board accelerometer. Which for a rocket after burnout is totally due to the atmospheric drag.

 

[Andrew Brown]

I'd like to thank everyone who responded to this. I'm still relatively new to "going big," so this is definitely a learning experience. I'm pretty comfortable with the concept of drag separation, and the debate around it.

At this point, my main concern is the margin of deceleration force that the nose sees when the drogue is deployed. I really don't want the main pulling out at 10k ft. Obviously, drogue should deploy at or very shortly after apogee at low vertical velocity, harness is sufficiently long (planning on 40 ft). Nose is about 2 lbs fully loaded. Assuming everything goes perfect, it sees maybe 10-20 G at the end of the shock cord. In this case, 3x 2-56 pins (or 90-100 lbs or shear strength) is just fine. My question is what sort of margin would an experienced person add to account for when things don't go "perfect?" Obviously, there are degrees of imperfection, but what would a typical margin be? Reading some of the previous posts, 100 G is used sometimes, which seems excessive, and I've seen 20 G, which seems optimistic.

Thanks,
Andy

 

[rharshberger]

I'd like to thank everyone who responded to this. I'm still relatively new to "going big," so this is definitely a learning experience. I'm pretty comfortable with the concept of drag separation, and the debate around it.

At this point, my main concern is the margin of deceleration force that the nose sees when the drogue is deployed. I really don't want the main pulling out at 10k ft. Obviously, drogue should deploy at or very shortly after apogee at low vertical velocity, harness is sufficiently long (planning on 40 ft). Nose is about 2 lbs fully loaded. Assuming everything goes perfect, it sees maybe 10-20 G at the end of the shock cord. In this case, 3x 2-56 pins (or 90-100 lbs or shear strength) is just fine. My question is what sort of margin would an experienced person add to account for when things don't go "perfect?" Obviously, there are degrees of imperfection, but what would a typical margin be? Reading some of the previous posts, 100 G is used sometimes, which seems excessive, and I've seen 20 G, which seems optimistic.

Thanks,
Andy

100G's for a less than optimal flight profile aka an oh crap! moment is easy, I usually use a minimumn 100G usually more like 150G as a worst case, and size my shock cords, eyebolts, quicklinks etc to those numbers, its not an exact figure for each rocket, but its only failed me once....in a 1.6" Fiberglass Mini-Frenzy that deployed horizontal at 500-600fps.

 

[Bat-mite]

I'd like to thank everyone who responded to this. I'm still relatively new to "going big," so this is definitely a learning experience. I'm pretty comfortable with the concept of drag separation, and the debate around it.

At this point, my main concern is the margin of deceleration force that the nose sees when the drogue is deployed. I really don't want the main pulling out at 10k ft. Obviously, drogue should deploy at or very shortly after apogee at low vertical velocity, harness is sufficiently long (planning on 40 ft). Nose is about 2 lbs fully loaded. Assuming everything goes perfect, it sees maybe 10-20 G at the end of the shock cord. In this case, 3x 2-56 pins (or 90-100 lbs or shear strength) is just fine. My question is what sort of margin would an experienced person add to account for when things don't go "perfect?" Obviously, there are degrees of imperfection, but what would a typical margin be? Reading some of the previous posts, 100 G is used sometimes, which seems excessive, and I've seen 20 G, which seems optimistic.

Thanks,
Andy

Most of this is learned through ground testing. Ground test in flight-ready configuration. Test the booster charge with the NC in place. If the booster charge also breaks the NC pins, back it off, or add more pins, or upsize pins. If the booster charge causes the NC to snap back on the harness, back it off. If the booster shear pins break but the laundry doesn't get fully out, step up the charge. Etc.

 

[jderimig]

Use other tricks to dissipate the energy of ejection. Like taping loops of the harness with masking tape.