How many 2-56 nylon screws do you guys use !

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JP Morgan

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*LOC Magnum 3, 5.5" diameter rocket using the nose cone as the Main ejection point of separation.
So, how many 2-56 nylon screws do you guys use?
I'll install metal pieces into the nose cone for a "strong point" to shear the screws.

Thank you.

JP
 
None. I friction fit all my separation points up to K motors so far with very good results. Clear tape is my friend.
 
I'm building a clone of a LOC magnum now (almost done). I plan to friction fit as well.
 
*LOC Magnum 3, 5.5" diameter rocket using the nose cone as the Main ejection point of separation.
So, how many 2-56 nylon screws do you guys use?
I'll install metal pieces into the nose cone for a "strong point" to shear the screws.

Thank you.

JP

I would use 3 on that size airframe, I would be SURE to have an 1/8" pressure relief/bleed hole and I would ground test to make sure I was using a big enough charge. YMMV.

--Lance.
 
I would use 3, but tend to friction fit cardboard tubes. Fiberglass I shear pin.
 
Everybody is saying 3 and so am I, but why... It depends on the weight of your nosecone assembly and the length of, and material used, on your drogue shock cord. If you have a 3 lb nosecone assembly for a 5.5" dia. rocket (assuming you have some sort of tracker bay, centering ring, hard attach point) and you have a typical length cord (3X to 4X rocket length) you could experience 15 to 25 gees of shock for an end of shock cord event (nylon will be lower shock, Kevlar higher). So your shear pins then need to have a holding force of 3 lbs times 15 to 25 gees or 45 to 75 lbs shear. With 2-56 nylon screw typical shear force is ~35 lbs so 3 pins provides 105 lbs of "holding force" or 100% to 40% margin assuring that the nosecone stays in place during the drogue event. Size your main charges to give you 130 lbs of force and that provides 25% margin on separation for the main.
 
I appreciate all the responses guys!

I've got the relief holes drilled. I drilled 3/16" holes, same as I used for my LOC IV.

The idea of just using friction sort of worries me a bit. Being a paper tube and given moisture
content in the air it can vary on weather and temperature.
So that's my main concern, keeping it the same size/fit.

It's going to see one big bang before the Main is deployed and using the screws just seems
to be the best for my situation.
The consensus seems to be 3 screws then. My shock cord is Nylon, the LOC supplied cord.

I placed another order with Missile Works today for the second Altimeter, the RRC2 to back up
the RRC3. I've got their LCD Terminal too so that makes Life a little easier!

I've already tested my home made charge canisters up to 2.3 grains of FFFF Black Powder, but that's not
testing the actual charges in the rocket yet. Surely that will be enough for separation? We'll see.
I've got probably 80 E-Matches left so there will be plenty of testing.

Thanks again!

JP
 
I suggest 3 pins too, but only because the rocket has 3 fins and my OCD would freak out if I used anything else except 3 :wink:

(Seriously though, Tim's analysis is spot on-- especially with a big/light paper rocket like a Magnum, the opening shock won't be too bad.)
 
Everybody is saying 3 and so am I, but why... It depends on the weight of your nosecone assembly and the length of, and material used, on your drogue shock cord. If you have a 3 lb nosecone assembly for a 5.5" dia. rocket (assuming you have some sort of tracker bay, centering ring, hard attach point) and you have a typical length cord (3X to 4X rocket length) you could experience 15 to 25 gees of shock for an end of shock cord event (nylon will be lower shock, Kevlar higher). So your shear pins then need to have a holding force of 3 lbs times 15 to 25 gees or 45 to 75 lbs shear. With 2-56 nylon screw typical shear force is ~35 lbs so 3 pins provides 105 lbs of "holding force" or 100% to 40% margin assuring that the nosecone stays in place during the drogue event. Size your main charges to give you 130 lbs of force and that provides 25% margin on separation for the main.


I hate doing this..... but...everything I read was a tad different.

Could it be you were thinking of 4-40 pins, shear strength is closer to your calculations with those.

With 2-256 closer to 21lbs@ or 64lbs total using 3.

This is Doc D's results of a series of lab tests. Remember his motto....."In God we trust...all others bring data". LoL:grin:


SHEAR PINSHPR Strength of Materials - Recovery Materials (Data)_1260988920736 copy.png
 
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I use three. No metal to shear them, however the tube has two layers 0f 40z. glass. I typically use between 3-4G of BP.
 
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I hate doing this..... but...everything I read was a tad different.

Could it be you were thinking of 4-40 pins, shear strength is closer to your calculations with those.

With 2-256 closer to 21lbs@ or 64lbs total using 3.

This is Doc D's results of a series of lab tests. Remember his motto....."In God we trust...all others bring data". LoL:grin:


View attachment 182484

CJ,

No problem. I should have cited my source and more importantly the "why"... The source is: myself, but more importantly the why...

First, BIG CAUTIONARY NOTES:

1) I am not a mechanical, materials or structural engineer. I am an electrical and software engineer by training, but no longer practice engineering in my career. I am only an interested observer and amateur practitioner of mechanical, structural, aerospace and other disciplines as it relates to the high power rocketry hobby.

2) The following commentary may be considered heretical in certain circles of the rocketry hobby. I am not attempting to start a firestorm here, I am simply providing my observations, opinions and conclusions in the area of shear pin use in high power rocketry flights. You may agree or disagree and either is fine, as they say, your mileage may vary.

3) Also, very important is my intent—in no way am I trying to disparage any individual in rocketry, and in particular, especially not Drake Damerau and all the wonderful work his had done for our hobby. I do not know Drake personally, but over the years have done a couple of financial transactions with him on rocketry equipment and he has always been a “gentleman and scholar.”

OK, a little background is probably useful… After doing ground testing for a dozen years over numerous airframe sizes and rocket designs I began to notice a continual and consistent trend, namely, my ground tests always had to be repeated more than once as they were always under-powered. In most cases I had to add 50% more BP to the charge, sometimes even more. This began to bother me, not just because I was wasting e-matches and BP, but it consumed considerable precious time of the limited time I can devote to rocketry. Being trained as an engineer and in another life working as a quality engineer, the results really began to get under my skin. If everything was “nominal” and on a normal distribution curve, I should be seeing just as many “overcharge” events as “undercharge” events, but this just wasn’t the case. Even more disturbing is that as I spoke to, and read of experiences of, fellow rocketeers, and the vast majority experienced the same thing, i.e., their initial calculations for ground testing led to weak separations.

So given the above, I began tracing the variables back one at a time. First it was the BP pressure/volume calculations, but they seemed solid. Next it was my construction and placement of the charges. After significant testing in this second area, I did make adjustments when utilizing centrifuge charge containers, but the vast majority of my charges are in 2X to 3X length copper tubing which fired consistently and with authority (this geometry has also since been verified as effective by Jim Jarvis on his high altitude flights). At this point, I began to look at the pins, but not directly related to shear. I thought there might be a significant contribution due to bending and the particular shear material, i.e., cardboard, phenolic, fiberglass, etc. I began to look at the shears under a microscope, note any bending or elongation of holes in material (in most cases I typically enhanced cardboard and phenolic with brass inserts to aid the cut which effectively eliminated any deformation of the airframe material). I then started to consider friction in the joint, but never could come close to accounting for the under-calculation of charges due to the minor contribution from friction. That is when I consulted an old mechanical engineering friend of mine. I described the problem and my observations and what I had gone through up to that point. His view was that I had looked at most everything but the pins themselves. Showing him the chart that you referenced above, he had a bit of heartburn, in that he didn’t feel you should see the type of combinatorial effect shown, i.e., a shear for a group of pins that were arranged in a symmetrical fashion should be more additive versus the reduction in per pin force as shown. In other words, if shear for one pin is 25 lbs for instance, the result of three pins in the test should be 75 lbs. I didn’t dismiss the shear pin test chart immediately, but I just asked him how he would calculate the forces involved and it was blatantly simple. I like simple, so I set out to gather data and calculate results.

So I began referencing a number of documents from Fastenal Technical Guides to NASA Fastener Design, just to make sure my friend wasn’t “full of it” and then to gather needed data. But quickly before presenting results, just a couple comments on my reading… The first thing that I noticed was that in the screw/bolt mechanical design world the term shear is thrown around in a “loose” fashion. I’m sure it works for those in the industry, but for the uninitiated, you can get confused easily and really have to study the context when the word shear is used. Just to be clear, we are interested in single mode (versus double, ref. above docs) screw thread shear when used as a pin (i.e., not under tension with a nut on the back side). The shear force is in the direction perpendicular to the longitudinal axis of the screw and should in no way be confused with thread shear which is a completely different stress mode. The second point in reading is that, for reasons I don’t understand, screw/bolt manufacturers generally do not publish and/or guarantee shear strength on their product; tensile strength yes, shear strength no. What mechanical/structural engineers do is take the tensile strength measurement and derate that by a factor based on the material. It seems like a rule-of-thumb is to multiply the tensile strength by 60% to get shear strength. The reason I mention this is because, again, in my literature search, I saw rocketeers doing this for nylon screws, which leads to inaccurate results.

So, the punch line… The force for a single pin shear is as follows:

Shear force = stress area * shear strength​

And, shear force for an array of pins placed symmetrically and acted upon in the same force vector is:

Total shear force = shear force 1 + shear force 2 + shear force 3 + … + shear force N​

And finally there is some effect from non-perfect shear (bending, material elongation, etc. which I try to eliminate) and friction from the joint. I have found this last factor to be somewhere between 5 and 10 lbs for high power rockets and skewed to the lower end of this range. This last factor has by no means been calculated and are simply based on observations from ground testing. Often this factor is negligible and can be ignored by sizing your charges with a 20% to 25% safety factor.​

You are probably saying “That’s obvious!,” especially with respect to that first formula. And that is what I said. But to get it right you have to calculate it right using the correct inputs. First, make sure you are using the right units. Easiest for us here in the U.S. to find shear area in inches-squared and shear strength in pounds per square-inch (PSI). Next to calculate the stress area for the threaded portion of the screw you need to use the minimum pitch diameter including subtracting tolerances (see Machine Screw Thread Dimensions). Finally as mentioned earlier, we need to use the real material shear strength (versus the tensile strength * 60% estimate used for steels/alloys). The nice thing is that 6/6 Nylon material is fully characterized and shear strength is readily available (see 6/6 Nylon Resin Mechanical Properties). So, punch line #2, the shear force for a 2-56 nylon screw is:

Shear force = 0.00370 inches-squared * 10,000 PSI = 37 lbs,​

And for a 4-40 nylon screw we have:

Shear force = 0.00604 inches-squared * 10,000 PSI = 60 lbs.​

In practice I use 35 lbs for 2-56 screws and 55 lbs for 4-40 screws and then add 5 lbs for joint friction, imperfect shear, etc. I then go about calculating my charge size and then add 20% to that as a safety factor. Again, the above works for me, and in the three or so years I have been moving this direction with my ground testing, I find it much more accurate in providing “good” separation.

So, just to conclude, I am not saying that the chart posted summarizing the Damerau results is inaccurate, in fact, I’m sure it is accurate given the test setup and measurements that were done. But, based on my research, I choose not to use those values and instead have adopted what I consider a “personal best practice.”

Sorry for the rambling,
Tim

P.S. Since developing these parameters, I have found other rocketeers that have used these same, or similar values. Of note are this webpage: https://www.feretich.com/rocketry/Resources/shearPins.html and the contribution toward the end of this Info-Central article: https://www.info-central.org/?article=303.
 
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Most eloquent reply Tim.

I would have to say after reading how you tested, I would stand by those results also.

That being said.... the last batch [2500] pins I bought were aircraft grade and came rated at 25lbs from manufacturer & were moisture impervious.

After reading those links, I see there is a large variance on shear strength, depending on the supplier.
It all makes sense when thought about, guess I just was lulled into thinking there was a better standard.
Apparently not.

Which all leads back to the importance of ground testing .

Very informative read...... Thank you.
 
CJ,

No problem. I should have cited my source and more importantly the "why"... The source is: myself, but more importantly the why...

First, BIG CAUTIONARY NOTES:
1) I am not a mechanical, materials or structural engineer. I am an electrical and software engineer by training, but no longer practice engineering in my career. I am only an interested observer and amateur practitioner of mechanical, structural, aerospace and other disciplines as it relates to the high power rocketry hobby.

2) The following commentary may be considered heretical in certain circles of the rocketry hobby. I am not attempting to start a firestorm here, I am simply providing my observations, opinions and conclusions in the area of shear pin use in high power rocketry flights. You may agree or disagree and either is fine, as they say, your mileage may vary.

3) Also, very important is my intent—in no way am I trying to disparage any individual in rocketry, and in particular, especially not Drake Damerau and all the wonderful work his had done for our hobby. I do not know Drake personally, but over the years have done a couple of financial transactions with him on rocketry equipment and he has always been a “gentleman and scholar.”

OK, a little background is probably useful… After doing ground testing for a dozen years over numerous airframe sizes and rocket designs I began to notice a continual and consistent trend, namely, my ground tests always had to be repeated more than once as they were always under-powered. In most cases I had to add 50% more BP to the charge, sometimes even more. This began to bother me, not just because I was wasting e-matches and BP, but it consumed considerable precious time of the limited time I can devote to rocketry. Being trained as an engineer and in another life working as a quality engineer, the results really began to get under my skin. If everything was “nominal” and on a normal distribution curve, I should be seeing just as many “overcharge” events as “undercharge” events, but this just wasn’t the case. Even more disturbing is that as I spoke to, and read of experiences of, fellow rocketeers, and the vast majority experienced the same thing, i.e., their initial calculations for ground testing led to weak separations.

So given the above, I began tracing the variables back one at a time. First it was the BP pressure/volume calculations, but they seemed solid. Next it was my construction and placement of the charges. After significant testing in this second area, I did make adjustments when utilizing centrifuge charge containers, but the vast majority of my charges are in 2X to 3X length copper tubing which fired consistently and with authority (this geometry has also since been verified as effective by Jim Jarvis on his high altitude flights). At this point, I began to look at the pins, but not directly related to shear. I thought there might be a significant contribution due to bending and the particular shear material, i.e., cardboard, phenolic, fiberglass, etc. I began to look at the shears under a microscope, note any bending or elongation of holes in material (in most cases I typically enhanced cardboard and phenolic with brass inserts to aid the cut which effectively eliminated any deformation of the airframe material). I then started to consider friction in the joint, but never could come close to accounting for the under-calculation of charges due to the minor contribution from friction. That is when I consulted an old mechanical engineering friend of mine. I described the problem and my observations and what I had gone through up to that point. His view was that I had looked at most everything but the pins themselves. Showing him the chart that you referenced above, he had a bit of heartburn, in that he didn’t feel you should see the type of combinatorial effect shown, i.e., a shear for a group of pins that were arranged in a symmetrical fashion should be more additive versus the reduction in per pin force as shown. In other words, if shear for one pin is 25 lbs for instance, the result of three pins in the test should be 75 lbs. I didn’t dismiss the shear pin test chart immediately, but I just asked him how he would calculate the forces involved and it was blatantly simple. I like simple, so I set out to gather data and calculate results.

So I began referencing a number of documents from Fastenal Technical Guides to NASA Fastener Design, just to make sure my friend wasn’t “full of it” and then to gather needed data. But quickly before presenting results, just a couple comments on my reading… The first thing that I noticed was that in the screw/bolt mechanical design world the term shear is thrown around in a “loose” fashion. I’m sure it works for those in the industry, but for the uninitiated, you can get confused easily and really have to study the context when the word shear is used. Just to be clear, we are interested in single mode (versus double, ref. above docs) screw thread shear when used as a pin (i.e., not under tension with a nut on the back side). The shear force is in the direction perpendicular to the longitudinal axis of the screw and should in no way be confused with thread shear which is a completely different stress mode. The second point in reading is that, for reasons I don’t understand, screw/bolt manufacturers generally do not publish and/or guarantee shear strength on their product; tensile strength yes, shear strength no. What mechanical/structural engineers do is take the tensile strength measurement and derate that by a factor based on the material. It seems like a rule-of-thumb is to multiply the tensile strength by 60% to get shear strength. The reason I mention this is because, again, in my literature search, I saw rocketeers doing this for nylon screws, which leads to inaccurate results.

So, the punch line… The force for a single pin shear is as follows:
Shear force = stress area * shear strength​

And, shear force for an array of pins placed symmetrically and acted upon in the same force vector is:
Total shear force = shear force 1 + shear force 2 + shear force 3 + … + shear force N​
And finally there is some effect from non-perfect shear (bending, material elongation, etc. which I try to eliminate) and friction from the joint. I have found this last factor to be somewhere between 5 and 10 lbs for high power rockets and skewed to the lower end of this range. This last factor has by no means been calculated and are simply based on observations from ground testing. Often this factor is negligible and can be ignored by sizing your charges with a 20% to 25% safety factor.​

You are probably saying “That’s obvious!,” especially with respect to that first formula. And that is what I said. But to get it right you have to calculate it right using the correct inputs. First, make sure you are using the right units. Easiest for us here in the U.S. to find shear area in inches-squared and shear strength in pounds per square-inch (PSI). Next to calculate the stress area for the threaded portion of the screw you need to use the minimum pitch diameter including subtracting tolerances (see Machine Screw Thread Dimensions). Finally as mentioned earlier, we need to use the real material shear strength (versus the tensile strength * 60% estimate used for steels/alloys). The nice thing is that 6/6 Nylon material is fully characterized and shear strength is readily available (see 6/6 Nylon Resin Mechanical Properties). So, punch line #2, the shear force for a 2-56 nylon screw is:
Shear force = 0.00370 inches-squared * 10,000 PSI = 37 lbs,​

And for a 4-40 nylon screw we have:
Shear force = 0.00604 inches-squared * 10,000 PSI = 60 lbs.​

In practice I use 35 lbs for 2-56 screws and 55 lbs for 4-40 screws and then add 5 lbs for joint friction, imperfect shear, etc. I then go about calculating my charge size and then add 20% to that as a safety factor. Again, the above works for me, and in the three or so years I have been moving this direction with my ground testing, I find it much more accurate in providing “good” separation.

So, just to conclude, I am not saying that the chart posted summarizing the Damerau results is inaccurate, in fact, I’m sure it is accurate given the test setup and measurements that were done. But, based on my research, I choose not to use those values and instead have adopted what I consider a “personal best practice.”

Sorry for the rambling,
Tim

P.S. Since developing these parameters, I have found other rocketeers that have used these same, or similar values. Of note are this webpage: https://www.feretich.com/rocketry/Resources/shearPins.html and the contribution toward the end of this Info-Central article: https://www.info-central.org/?article=303.

Sorry but Doc's data is as accurate as you can get.
Doc works at the Scranton Army Ammunition Plant, operated by General Dynamics as the Chief Metallurgist, and the Laboratory Director at General Dynamics. Doc specializes in failure analysis, but is also responsible for heat treating processes and all material specifications and testing, and is a member of ASM, and the ASM affiliate societies of HTS and IMS, SAE Aerospace and ASTM and on the ASTM Technical Committee E28 for mechanical testing.

https://rocketmaterials.org/about/me/index.html

Bob
 
Sorry but Doc's data is as accurate as you can get.
Doc works at the Scranton Army Ammunition Plant, operated by General Dynamics as the Chief Metallurgist, and the Laboratory Director at General Dynamics. Doc specializes in failure analysis, but is also responsible for heat treating processes and all material specifications and testing, and is a member of ASM, and the ASM affiliate societies of HTS and IMS, SAE Aerospace and ASTM and on the ASTM Technical Committee E28 for mechanical testing.

https://rocketmaterials.org/about/me/index.html

Bob

Bob,

Again, maybe you didn't read my post, I was not trying to detract or disparage. I also don't want this to be a "how many alphabetical characters I can put behind my name"-type qualification (I could list off quite a bit too especially in the area of quality control and reliability). My point was just to provide my real-life observations over more than a decade of high power flying and the practical adjustments I have made to address that discrepancy.

-Tim
 
Bob,

Again, maybe you didn't read my post, I was not trying to detract or disparage. I also don't want this to be a "how many alphabetical characters I can put behind my name"-type qualification (I could list off quite a bit too especially in the area of quality control and reliability). My point was just to provide my real-life observations over more than a decade of high power flying and the practical adjustments I have made to address that discrepancy.

-Tim
That's the way I read it. Different ways of testing provide different results.
Only thing left for me to do is actual "ground testing" my charges.

There's allot of real time information here and I for one appreciate it guys!
Thank you!

JP
 
Bob,

Again, maybe you didn't read my post, I was not trying to detract or disparage. I also don't want this to be a "how many alphabetical characters I can put behind my name"-type qualification (I could list off quite a bit too especially in the area of quality control and reliability). My point was just to provide my real-life observations over more than a decade of high power flying and the practical adjustments I have made to address that discrepancy.

-Tim
Tim, no offense intended. I just wanted to point out that Doc has real test data obtained from a tensile testing machine that requires no assumptions on screw dimensions, material strengths or any other calculated parameters. The tensile machine simple pulls the test article apart and measures the force required to do shear the screws used in the test in a very specific test configuration.

Barring any test data I would make my first order estimate the same you did, however it does not include real world non-ideal behaviors that you or I would not consider. Most importantly it does not account for the zipper effect which is what the tensile test does.

Doc did not test the shear strength of a single screw and I'm pretty sure it would be quite close to your calculated single screw load requirement, but he did test the 2 screw configuration and it required an intermediate load to the single screw and the 3 or more screw asymptotic n-screw load value.

I believe the reason for this is the zipper effect. In the real world there will be a statistical variation in the strength of an individual screw, the threaded hole fastening the screw and positioning and loading of the individual screws in an n-screw array. The result is that one screw shears first, followed by another, and another until all screws shear. Since you are simply recording the total load, you most likely do not observe the step functions in loading due to instrument time constants, but as each screw breaks, there is a step increase in the loading of the remaining screws and they all shear sequentially but rapidly and as a result the average peak recorded loading is less than the single screw shear loading value.

Because of real world variations and unexpected factors, folks use a significantly larger power charge to insure a significantly greater than threshold shear load and then you, and most other experienced high power flyers, conduct a ground test to make sure the black power charge provides a sufficient load on the shear screws to insure prompt separation, and that's really the ultimate test, because regardless of what you measure or calculate, a real test in your specific configuration is the only way to confirm that your design and system work the way you want it to.

Bob
 
Tim, no offense intended. I just wanted to point out that Doc has real test data obtained from a tensile testing machine that requires no assumptions on screw dimensions, material strengths or any other calculated parameters. The tensile machine simple pulls the test article apart and measures the force required to do shear the screws used in the test in a very specific test configuration.

Barring any test data I would make my first order estimate the same you did, however it does not include real world non-ideal behaviors that you or I would not consider. Most importantly it does not account for the zipper effect which is what the tensile test does.

Doc did not test the shear strength of a single screw and I'm pretty sure it would be quite close to your calculated single screw load requirement, but he did test the 2 screw configuration and it required an intermediate load to the single screw and the 3 or more screw asymptotic n-screw load value.

I believe the reason for this is the zipper effect. In the real world there will be a statistical variation in the strength of an individual screw, the threaded hole fastening the screw and positioning and loading of the individual screws in an n-screw array. The result is that one screw shears first, followed by another, and another until all screws shear. Since you are simply recording the total load, you most likely do not observe the step functions in loading due to instrument time constants, but as each screw breaks, there is a step increase in the loading of the remaining screws and they all shear sequentially but rapidly and as a result the average peak recorded loading is less than the single screw shear loading value.

Because of real world variations and unexpected factors, folks use a significantly larger power charge to insure a significantly greater than threshold shear load and then you, and most other experienced high power flyers, conduct a ground test to make sure the black power charge provides a sufficient load on the shear screws to insure prompt separation, and that's really the ultimate test, because regardless of what you measure or calculate, a real test in your specific configuration is the only way to confirm that your design and system work the way you want it to.

Bob

Bob, I get what you are saying with respect to the cascading failure effect. And I certainly don't purport to know/understand the exact failure modes that may or may not be interacting, I'm just glad my ground testing is now much more efficient and repeatable. Probably best to just leave it at that and, like you, encourage all to ground test and do what they can to assure a successful, and more importantly, a safe flight.
 
First ground test yesterday. Using 3.8 grams of BP, the separation was aggressive and damaged one of the blankets
I used for the parachute and cord, each wrapped in a bundle separate.

It actually burned a hole in the blanket I used for the cord, it was Nomex from an old fire fighters hood. * I used to volunteer.
Apparently it was resting on the charge canister.Magnum 3 Test Hole in Kevlar.jpg
I'll have to fix this problem. Any suggestions guys?Here's a shot of the upper half spread out. I unhooked the cord from the tube in the photo. I'll do a couple of tests today using less powder. I suppose I should borrow the Kevlar blanket that goes to the lower half for my tests today, the Nomex
won't hold up any longer taking a beating like that.
Magnum 3 Test one upper half main.jpg

JP
 
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Hmmm, yeah, 3.8g is a lot. I don't think I have every used more than 3g in a LOC 5.5" or PML 6" airframe.
 
Have you sized your charge with this calculator: www.aeroconsystems.com/tips/Ejection_ChargeCalc.xls? As it may be far less than 3g, I don't know?
No I have not.
I'll be sure to check it out today.
The 3 gram charge worked out just fine. I taped the shock cord and placed it into a Kevlar blanket with tape around that.
Everything was stretched out with the 3 gram charge, not as dramatic as the 3.8 gram charge.

Thanks for the info!

JP
 
If it takes 5 pounds of force to separate and deploy your recovery system, and it takes 25 pounds to shear the pins, do you set your charge to provide 5+25=30 pounds of force, or just 25 because 25>>5 ? Is the necessary total force determined by adding all force values or just using the greatest value? Why? I've heard this done both ways but have not seen a good analysis of what is the real solution.
 
Have you sized your charge with this calculator: www.aeroconsystems.com/tips/Ejection_ChargeCalc.xls? As it may be far less than 3g, I don't know?
Question about Drogue chute; what size would you use for the Magnum 3? It's going to weigh about 20 pounds...
I'm not sure if I trust the one that came in the kit (19"). I bought one from Aerocon Systems that's 17" and supposed to
be Mach I proof.

One more test for today, it will be 2.5 grams of BP.

JP
 
Question about Drogue chute; what size would you use for the Magnum 3? It's going to weigh about 20 pounds...
I'm not sure if I trust the one that came in the kit (19"). I bought one from Aerocon Systems that's 17" and supposed to
be Mach I proof.

One more test for today, it will be 2.5 grams of BP.

JP

I simulate and shoot for ~80 fps which in real life turns out to be ~65 fps. Saying that I usually use Top Flight Ultra X chutes for my drogue. Guessing a 30" for that one, so 18" or so may be OK, I'd probably wouldn't go more than 24" with a regular chute. Do you have a picture of the Aerocon chute?
 
I simulate and shoot for ~80 fps which in real life turns out to be ~65 fps. Saying that I usually use Top Flight Ultra X chutes for my drogue. Guessing a 30" for that one, so 18" or so may be OK, I'd probably wouldn't go more than 24" with a regular chute. Do you have a picture of the Aerocon chute?
I believe that's what I read when I purchased it. So it may be a little small then.
17 inch Drogue chute.jpg
https://aeroconsystems.com/cart/all-parachutes/18-inch-hexagonal-parachute/
Web site says 18" chute but they removed the Mach I part or maybe it was another chute I was thinking about. It's
built well no doubt.

The 2.5 gram charge stretched everything out. That should work well for the Main/upper compartment as it will probably be facing downward when it fires using gravity in it's favor. Anything more will be a hard tug on the rocket, I guess.

JP
 
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I am not familiar with the Aerocon chutes although I know many on TRF use them. Hopefully they will comment. Although the fabric looks thin, the webbing over the top of the canopy is nice for a drogue application.
 
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