Shear Pin Placement and Usage Question

[Ccolvin968]

So I just received my 4" Tomach from Madcow today.
I have heard of many people using shear pins in their DD rockets
Are these required for a DD setup?
Also, is it possible to switch from a DD setup to a Single deploy on the same rocket?
Sorry. Complete noobie questions.
Just want to make sure I'm not messing anything up.
Thanks!

 

[djs]

So- several questions here:
1. Yes you can do DD and single deploy on the same rocket. For DD, you have the main parachute in the payload bay, and a drogue parachute in the booster (bottom end). For motor eject, you just move the main to the section between the av bay and booster.
2. Shear pins are used to keep the main from shaking out of the payload bay at apogee- you want the main to deploy lower. Some people do this with a friction fit, but shear pins make it more reliable. However, if you have shear pins, you'll need to test your ejection charges to make sure it pushes the laundry out and breaks the shear pins.
3. Shear pins usually go into the top of the payload bay, between the payload bay and the nose cone shoulder. I usually use 3 of them, spaced evenly around the body.
4. Some people put shear pins between the av bay and the booster tube to prevent drag separation before apogee. I've never done this, but it is fairly common.

hope this helps!

 

[Nathan]

I would definitely recommend also having shear pins to hold the av bay to the booster tube. I once had a rocket without shear pins on the booster drag separate, causing the main to deploy at about 300 mph. Drag separation can occur right at motor burnout, when the rocket is at max velocity. Here is the video:

[video=youtube;2KDEqTTXIUk]https://www.youtube.com/watch?v=2KDEqTTXIUk[/video]

 

[rharshberger]

I would definitely recommend also having shear pins to hold the av bay to the booster tube. I once had a rocket without shear pins on the booster drag separate, causing the main to deploy at about 300 mph. Drag separation can occur right at motor burnout, when the rocket is at max velocity. Here is the video:

[video=youtube;2KDEqTTXIUk]https://www.youtube.com/watch?v=2KDEqTTXIUk[/video]

Its also a good idea to shear pin zipperless fin cans as they have a habit of drag separating as well.

 

[OverTheTop]

Shear pins are used to keep the main from shaking out of the payload bay at apogee

The use of shear pins and breather holes in the bays prevent the pressure differential blowing the NC off as the rocket quickly ascends. It is surprising how much pressure can be built up in a 4" rocket internals as it ascends, just because the air inside wants to get out.

I use shear pins wherever there is a separation point on the rocket (HPR). More controllable than friction or luck.

Good luck with the 4" Tomach build. I have a 54mm version that is sitting in a field (18 months) currently . It has flown to 21500' and Mach 1.8. Really happy with that bird. I will be even happier when I get it back this time!

 

[Ccolvin968]

Good info guys!
I really appreciate it. It definitely answers the questions I had.

 

[Nytrunner]

Just to tag onto the question; for chute-bay vent holes, one should be sufficient right?
Its not like an altimeter where you need a uniform pressure in the AV bay cavity.

 

[OverTheTop]

Just to tag onto the question; for chute-bay vent holes, one should be sufficient right?

One would suffice. Sometimes I use one, sometimes the same as the number of fins. Symmetry is nice sometimes, but not always. It is a personal choice I believe.

 

[djs]

Just to tag onto the question; for chute-bay vent holes, one should be sufficient right?

I do one hole per airframe "cavity"- so one on the booster, and one on the payload bay. AV bay obviously already has several holes for air sampling for the altimeters.

 

[crossfire]

That video almost looked like early ejection. Or forward closure delay problem?

 

[rharshberger]

Looked like a classic drag separation to me, separated immediately after motor burnout.

 

[Lowpuller]

I don't want to hijack the thread but I have a question about drag separation.

Why does it occurred between the nosecone and payload, hence the use of rivets, but less often, if at all between the booster section and AV Bay????

 

[manixFan]

Looked like a classic drag separation to me, separated immediately after motor burnout.

Not to hijack the thread either, but watch the animated GIF and come to your own conclusion.

I see a coupler failure. You can see a big burst at the nozzle just before things go sideways and then it sure looks like the rocket folds in half. I've got video of several drag separations and I've always been able to see several frames where the fincan trails the upper section and pulls the chute out. This does not seem to fit that pattern. It sure looks like the motor is still under thrust during the event.

Not trying to start an argument, I just like doing video forensics on rocket 'anomolies'.


Tony

 

[sharkbait]

Not to hijack the thread either, but watch the animated GIF and come to your own conclusion.

I see a coupler failure. You can see a big burst at the nozzle just before things go sideways and then it sure looks like the rocket folds in half. I've got video of several drag separations and I've always been able to see several frames where the fincan trails the upper section and pulls the chute out. This does not seem to fit that pattern. It sure looks like the motor is still under thrust during the event.

Not trying to start an argument, I just like doing video forensics on rocket 'anomolies'.




Tony

I'm seeing what you're seeing.

 

[rharshberger]

Not to hijack the thread either, but watch the animated GIF and come to your own conclusion.

I see a coupler failure. You can see a big burst at the nozzle just before things go sideways and then it sure looks like the rocket folds in half. I've got video of several drag separations and I've always been able to see several frames where the fincan trails the upper section and pulls the chute out. This does not seem to fit that pattern. It sure looks like the motor is still under thrust during the event.

Not trying to start an argument, I just like doing video forensics on rocket 'anomolies'.


Tony

You are correct sir, thank you!

 

[Nathan]

Not to hijack the thread either, but watch the animated GIF and come to your own conclusion.

I see a coupler failure. You can see a big burst at the nozzle just before things go sideways and then it sure looks like the rocket folds in half. I've got video of several drag separations and I've always been able to see several frames where the fincan trails the upper section and pulls the chute out. This does not seem to fit that pattern. It sure looks like the motor is still under thrust during the event.

Not trying to start an argument, I just like doing video forensics on rocket 'anomolies'.


Tony

Sorry, wrong. The coupler (which was the av bay) was undamaged. The only damage to the rocket was a zipper about a foot long in the BlueTube airframe of the payload section from when the chute opened. I cut off the zippered part of the payload tube and continued to fly that rocket for about another year, until it was totally destroyed this spring by a J425 CATO that blew the 6 grain 38mm motor casing wide open.

 

[manixFan]

Sorry, wrong. The coupler (which was the av bay) was undamaged. The only damage to the rocket was a zipper about a foot long in the BlueTube airframe of the payload section from when the chute opened. I cut off the zippered part of the payload tube and continued to fly that rocket for about another year, until it was totally destroyed this spring by a J425 CATO that blew the 6 grain 38mm motor casing wide open.

Bummer that the rocket survived that only to meet its demise later.

I'm not trying to argue, just trying to understand. As I've mentioned I've seen a number of drag separations via video and they've always been pretty easy to identify. In this case the motor was still clearly burning for a few more frames after the separation. Going frame by frame it's clear that the something happened in the motor just before the rocket separated and then it continued to burn for a bit after that. As I mentioned in nearly all the drag separations I've looked at the two pieces 'coasted' for several frames together and the fincan trails the upper airframe. This video looks a lot different than that.

Now that I know that the coupler didn't fail, and looking at the frames more closely it *almost* looks like there is smoke from an ejection charge at the point of separation. It's to the left of the fincan and flame plume. It's visible in frames 6-8. Is it possible the altimeter fired early?

I've been doing video analysis of flights for many years and try to learn how to properly interpret what I'm seeing. It's an interesting video. Please don't take offense at my suggestions, as I said I'm just trying to understand what I'm seeing. When I'm wrong I want to try and figure out what really happened to add to my ability to get it right next time.

And just to be clear, I am certainly not arguing against shear pins for the booster - I use them for the majority of my dual deploy rockets. I am in complete agreement with your suggestion that they can prevent against drag separation of the fincan at motor burnout.

Thanks,


Tony

 

[NateLowrie]

The use of shear pins and breather holes in the bays prevent the pressure differential blowing the NC off as the rocket quickly ascends. It is surprising how much pressure can be built up in a 4" rocket internals as it ascends, just because the air inside wants to get out.

I use shear pins wherever there is a separation point on the rocket (HPR). More controllable than friction or luck.

Good luck with the 4" Tomach build. I have a 54mm version that is sitting in a field (18 months) currently . It has flown to 21500' and Mach 1.8. Really happy with that bird. I will be even happier when I get it back this time!

Good post.

To sum things up, the reason to use shear pins is to prevent a separation joint on the rocket from separating before you want it. This can happen due to any of the following:

• Drag separation on the boost up.
• The pressure differential that results from the outside pressure decreasing while the inside stays the same or reduces at a slow rate (if you have a vent hole)
• The separating of one section at apogee can "shake off" a friction fitted section with the motion that occurs.

Regarding the pressure differential: You can calculate the differential dispersal through the flight based off of NOAA pressure data at atmosphere, your Rocksim/OR sim and the equalization rate through the vent hole. Based on that and the size of the body tube you can figure out how much upward force the max differential exerts on the nosecone. You can know take that force and divide it by the break force for the shear pin size you are using and get the minimum number of pins you need to contain the pressure differential. I have a small command line program I am working on converting into a web page utility for this.

 

[Bat-mite]

I don't want to hijack the thread but I have a question about drag separation.

Why does it occurred between the nosecone and payload, hence the use of rivets, but less often, if at all between the booster section and AV Bay????

I think you are a little mixed up.

Imagine the entire rocket under force going upward, against gravity. The only thing making it go upward is the force of the motor.

Now, imagine a really high thrust motor suddenly burning out, no longer providing any upward force. The rocket, if it were one piece, or if the various pieces were held together by something (i.e., shear pins or tape), then it would continue to coast upward until the force of gravity overwhelmed the inertia.

But, if the rocket is in multiple pieces, and if one piece (i.e., the booster) is much draggier than the others, then the rocket will separate at the first available breakaway point, namely the booster-to-coupler joint.

Drag is applied to the booster, but the finless payload and NC are carried upward by inertia. So the rocket separates at the booster-to-coupler joint, the booster starts to fall down, and the upper half continues upward.

But ... the upper half has no fins, and thus is highly unstable. So rather than just continue upward, which wouldn't be so bad, it goes unstable, start flipping and spinning, and usually has terrible results.

Shear pins to hold the booster and coupler together prevent this. Or friction fit with tape. However, you can calculate how many shear pins you need, and ground test that configuration, whereas your amount of tape varies from launch to launch based on temperature and humidity. So with tape, there is a degree of luck involved.

The other problem is when the shear pins holding the NC to the payload tube aren't sufficient to withstand the shock cord shock when the apogee (drogue) charge fires. The payload tube, coupler, and NC blow out of the booster; then, the the shock cord stretched all the way to it limit, and jerks back on the NC. The inertia on the NC may cause it to separate from the payload if it is not correctly secured with shear pins. This can lead to zippers (at worst), or deployment of the main at apogee (which nullifies the whole DD thing).

So, two different problems, two different results. Knowing how many shear pins to use is the singular solution to both.

 

[claytonbirchenough]

Not to hijack the thread either, but watch the animated GIF and come to your own conclusion.

I see a coupler failure. You can see a big burst at the nozzle just before things go sideways and then it sure looks like the rocket folds in half. I've got video of several drag separations and I've always been able to see several frames where the fincan trails the upper section and pulls the chute out. This does not seem to fit that pattern. It sure looks like the motor is still under thrust during the event.

Not trying to start an argument, I just like doing video forensics on rocket 'anomolies'.


Tony

Looks like something else went wrong to me. Would need to compare the time under thrust to the thrust-time curve of the motor used. Also would need to know if there were vent holes in the booster section. Drag separation can still happen under a little thrust, but generally not.

 

[Nathan]

Bummer that the rocket survived that only to meet its demise later.

I'm not trying to argue, just trying to understand. As I've mentioned I've seen a number of drag separations via video and they've always been pretty easy to identify. In this case the motor was still clearly burning for a few more frames after the separation. Going frame by frame it's clear that the something happened in the motor just before the rocket separated and then it continued to burn for a bit after that. As I mentioned in nearly all the drag separations I've looked at the two pieces 'coasted' for several frames together and the fincan trails the upper airframe. This video looks a lot different than that.

Now that I know that the coupler didn't fail, and looking at the frames more closely it *almost* looks like there is smoke from an ejection charge at the point of separation. It's to the left of the fincan and flame plume. It's visible in frames 6-8. Is it possible the altimeter fired early?

I've been doing video analysis of flights for many years and try to learn how to properly interpret what I'm seeing. It's an interesting video. Please don't take offense at my suggestions, as I said I'm just trying to understand what I'm seeing. When I'm wrong I want to try and figure out what really happened to add to my ability to get it right next time.

And just to be clear, I am certainly not arguing against shear pins for the booster - I use them for the majority of my dual deploy rockets. I am in complete agreement with your suggestion that they can prevent against drag separation of the fincan at motor burnout.

Thanks,


Tony

No offense taken. I assumed that it was drag separation because it appeared that the separation occurred at about the time of motor burnout. After looking at it frame by frame it does appear that the motor was still making thrust when the separation occurred. So I guess it is possible that the altimeter fired the apogee charge prematurely. That was a Raven 3 which I have continued to use with no other problems.

Looks like something else went wrong to me. Would need to compare the time under thrust to the thrust-time curve of the motor used. Also would need to know if there were vent holes in the booster section. Drag separation can still happen under a little thrust, but generally not.

Yes it did have vent holes in both the booster and payload sections. The motor was a CTI 6 grain 38mm J381 Skidmark. According to Openrocket it would have been going 357 mph at motor burnout.

That was my first dual deploy rocket (every one I have built since has shear pins). The av bay had a single vent hole which was only about 1/4". From what I know now I think that was not enough av bay venting; it probably should have had either three vent holes, or one larger vent hole. I don't know if that may have caused premature firing of the apogee charge. That rocket was flown many times and that problem only occurred that once.

 

[Handeman]

Did you remove the ejection charge powder from the CTI motor before you flew it? Could it have been a burn through of the delay grain that set off a premature ejection charge? Would that also account of the burst at the nozzle just before things went south?

I had a couple of rockets that did the same sort of thing, but the last one was a Leviathan. No DD and no weight in the plastic nose cone. I really doubt the nose cone could have drag separated, but the main deployed right about motor burnout. I'm suspecting a fast/burn through of the delay grain and early ejection on that one.

 

[ksaves2]

Not to hijack the thread either, but watch the animated GIF and come to your own conclusion.

I see a coupler failure. You can see a big burst at the nozzle just before things go sideways and then it sure looks like the rocket folds in half. I've got video of several drag separations and I've always been able to see several frames where the fincan trails the upper section and pulls the chute out. This does not seem to fit that pattern. It sure looks like the motor is still under thrust during the event.

Not trying to start an argument, I just like doing video forensics on rocket 'anomolies'.


Tony

Yeah, coupler failure "or" perhaps the sustainer wasn't vented and the expanding interior air popped off the upper bay? Or a combination of both? As interior
pressure pushed the sustainer up, there wasn't enough "tube" to support and the upper bay twisted off. Nonetheless Tony I agree it's not likely drag separation.
Cool pickup there. Kurt

 

[HyperSonic]

Good post.

To sum things up, the reason to use shear pins is to prevent a separation joint on the rocket from separating before you want it. This can happen due to any of the following:

• Drag separation on the boost up.
• The pressure differential that results from the outside pressure decreasing while the inside stays the same or reduces at a slow rate (if you have a vent hole)
• The separating of one section at apogee can "shake off" a friction fitted section with the motion that occurs.

Regarding the pressure differential: You can calculate the differential dispersal through the flight based off of NOAA pressure data at atmosphere, your Rocksim/OR sim and the equalization rate through the vent hole. Based on that and the size of the body tube you can figure out how much upward force the max differential exerts on the nosecone. You can know take that force and divide it by the break force for the shear pin size you are using and get the minimum number of pins you need to contain the pressure differential. I have a small command line program I am working on converting into a web page utility for this.

Would the diameter of the vent holes (or number of) need to be changed with a change in velocity? Say a mach 3+ rocket compared to only mach 1. Also if vent holes were enlarged, would the shear pins need tweeked in any fashion? Because of pressure exiting the vent holes. Maybe even bumping up the BP charge?