Shear pins for heavy nose cone

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kruland

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Hi all,

My L3 build has run into a question I'd like opinions on. The PML 7.5" fiberglass nose cone has been weighted to a total mass of 8.5lbs. I had wanted to use 4 shear pins but I'm quite a bit concerned that #2 will be too small. Any help is appreciated.

Kevin
 
I use the same # of sheer pins as fins so I can keep them in alignment
so I have clean airflow between them for av bay vents...
There are 2 things you can do...
You can go to a heavier sheer pin,,, 4/40....
But it's more important to fix the problem
rather then a bandaid on the cause of the problem. ...
If your boosters harness is long enough for the energy of separation
to dissipate before the 2 separating sections come to the end of the boosters harness
you shouldn't need extra sheer pins in the nose...

Teddy
 
7.5" FG NC? Sounds like a #4-40 is reasonable. I suppose you could drill and insert #2-56 and ground test. If the apogee charge pops the NC, then you need to step up. Or, just go with #4-40 and ground test.
 
I use 4 ......4-40 pins on my 7.5 projects. With the same PML NC.

With a 4ft payload section, 4.5 grams of BP for primary and 5.5 for secondary altimeter.
Main alt. set at 800 for primary & 700 for second.
This works for me....at least a starting point for you.
 
Thanks everyone. Jim, any concerns with all that extra mass I put in there? 4x 4-40 is where I'm leaning right now. My payload sections are only 2ft, so I'm looking at 2.5g BP charges right now. Of course I can verify that with ground test.

Kevin

I use 4 ......4-40 pins on my 7.5 projects. With the same PML NC.

With a 4ft payload section, 4.5 grams of BP for primary and 5.5 for secondary altimeter.
Main alt. set at 800 for primary & 700 for second.
This works for me....at least a starting point for you.
 
On a small 2.5" rocket I had to use 3 x 6-32 to keep it from deploying during descent. Nosecone had a GPS unit in it, weighed a little over 10 ounces. Rocket went to 27k, dry weight was around 20 pounds.

Edward
 
Thanks everyone. Jim, any concerns with all that extra mass I put in there? 4x 4-40 is where I'm leaning right now.

Kevin

Mine weighs 6lbs.

"any concerns?" That depends entirely on YOUR fit.

Mine... I can lift the payload/av-bay/parachute/NC assembly.... by the NC...[no pins installed] and shake it. The NC fit is snug & smooth enough it barely moves. I can wrap my legs around the payload & "pull" the NC out with a bit of force, but it will come.

With the pins installed, I'm good to go. I do have the front 1/3 of cone filled with foam as in all my large projects [4in. & up,molded NC's] to provide extra stiffness to prevent flexing/damage upon landing. Why my cone also weighs more than stock ones.

I use 50 ft of 1in. tubular nylon for apogee, which allows the payload to scrub off any excess speed upon deployment, preventing the possible "jerk", that might cause issues ,if shorter cords were used and the payload came to end of cord.

The whole issue of proper deployment is a delicate balance of BP, cord length, NC fit, & number of pins.
Then add in the dynamics of actual flight ,& there are too many variables for "one size fits all".

After you fly enough, you just get a good "feel" for how things should fit.
Good luck with your attempt.
 
Jim,

That was exactly the validation I needed. NC is foamed and shot pinned with rod. The fit is currently a little sloppy, but that's easy enough to fix with a little tape. Got 50' of apogee cord and I was going to do the taped loop trick. I'm feeling much better about 4 4-40 pins now.

Kevin

Mine weighs 6lbs.

"any concerns?" That depends entirely on YOUR fit.

Mine... I can lift the payload/av-bay/parachute/NC assembly.... by the NC...[no pins installed] and shake it. The NC fit is snug & smooth enough it barely moves. I can wrap my legs around the payload & "pull" the NC out with a bit of force, but it will come.

With the pins installed, I'm good to go. I do have the front 1/3 of cone filled with foam as in all my large projects [4in. & up,molded NC's] to provide extra stiffness to prevent flexing/damage upon landing. Why my cone also weighs more than stock ones.

I use 50 ft of 1in. tubular nylon for apogee, which allows the payload to scrub off any excess speed upon deployment, preventing the possible "jerk", that might cause issues ,if shorter cords were used and the payload came to end of cord.

The whole issue of proper deployment is a delicate balance of BP, cord length, NC fit, & number of pins.
Then add in the dynamics of actual flight ,& there are too many variables for "one size fits all".

After you fly enough, you just get a good "feel" for how things should fit.
Good luck with your attempt.
 
Kevin,

I think 4 X 4-40 will be fine as others have suggested. Don't forget to include sufficient pressure relief holes in each of your airframe sections. You don't need any early help pushing everything apart with internal ground-level atmospheric pressure.

Thanks, --Lance.
 
Do the math.

Don't listen to "I use 4 so you should be OK" - total crap....

Figure the acceleration on your NC and figure what is needed to retain it.

THIS IS AFTERALL, AN L3 ATTEMPT - YOU SHOULD KNOW HOW TO DO THIS.
 
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Fred,

Would love to do the math. Can you suggest how to get started? I'll just wing some things here and maybe you could point me in the right direction.

Assume worst case, which would be rocket at apogee and nose down. Velocity at deployment = 0, acceleration at deployment is gravity = 10m/s/s.

The apogee deployment charge of 2.5g will pressurize the payload to 6psi. (7.5" diameter x 18" volume) Somehow this pressure will exert an impulse on both the aft section and the forward section. I'm going to need a little bit of help with this. Presumably, I'll need to have the individual masses of the two pieces - I'll measure and estimate those when I get home.

The impulse of the apogee charge will result in an instantaneous velocity of the forward section. This combined with the force of gravity acting on it (over the length of the shock cord) will give me final velocity of the forward section when the cord is fully extended. From this velocity and the mass of the NC, I can compute the momentum of the NC at deployment.

For worst case, I'll assume the shock cord and drogue do not absorb any of the force. The impulse force on the forward section will be computed as if it were hung from an immovable object. The change in momentum of the nose cone is the impulse it experiences. I'm going to have to assume some small amount of time over which this impulse is distributed in order to compute the force it experiences. Would 0.01s be a reasonable number?

Finally using this force, I can then use the chart from shear pin strength which shows that 4 4-40 nylon screws have a combined strength of 150lbs.

Does this sound about right? There's lots of gaps in there I'd appreciate some help with.

Kevin

Do the math.

Don't listen to "I use 4 so you should be OK" - total crap....

Figure the acceleration on your NC and figure what is needed to retain it.

THIS IS AFTERALL, AN L3 ATTEMPT - YOU SHOULD KNOW HOW TO DO THIS.
 
First, let me say, IMHO, this is one of the key skills to be mastered at L2
There shouldn't be any questioning or second guessing from the crowd needed.
This is YOUR L3 - the gateway to "unlimited flying" and a "merit badge" that indicates YOU know what YOU are doing when it comes to flying rockets.
So why are you asking here? How is your L3CC/TAP letting you past this stage.

You touched a pet peeve of mine......sorry for the rant.

But - for all those listening and not also willing to search, here's the basics....

For DD, you need to keep the main chute bay retained while you blow the drogue bay.

From your experience of studying a multitude of altimeter logs from your previous DD flights [you did that, right] you have learned an understanding of the magnitude of the "shock" accelerations that occur at apogee.

Use that approximate magnitude, with some margin, to calculate the forces on the retained mass in your rocket.
Use that calculated force to size your retention devices.
Use your retention device plan to estimate the needed charge size to remove said retention given your rocket volume. Do not skip ahead to this step like most attempt.

None of this is hard. But I'm not going to spoon feed it to you. You are an experienced L2.
But notice that ALL three steps above are about YOUR rocket, not those flown by others, similar or not.
This is why I say pay no attention to those who chime in and say "I use 3, so 3 is the count."

My question is why you have not done this from the start for all your DD flights?
L3 attempts are not the time to learn. You are there to DEMONSTRATE COMPITENCE!

Again - sorry to rant at you in particular - this is a big red flag that L3 candidates are not really ready.
 
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I'd like to suggest a consideration here, in general, for those designing rocket recovery systems. Many who consider this problem seem to do so from the perspective of everything working nominally. That is a dangerous assumption when it comes to rockets and rocket flight. Assume the anomalous, assess the probability, and then make the design decisions.

Suppose the not all that uncommon situation of a rocket which does not end up with a perfect trajectory relative to the air mass. That is, it has some horizontal component of velocity compared to the air. At apogee such a flight will achieve zero vertical speed but the horizontal speed will not be zero. The higher the rocket goes, the less stable the rocket, and the greater the wind at launch, the greater the EXPECTED horizontal velocity at apogee.

So, there are TWO mechanisms promoting premature deployment of main at apogee. The first, which is what has been considered a little bit in this thread, has to do with the relative masses of the nosecone (or connected forward section) and the motor section of the rocket, their relative velocities at ejection, dissipation of that energy, and the resultant shock transmitted to the retention mechanism for the main chute.

The second is the shock generated upon opening of the drogue at non-zero airspeed. This is dependent on the speed, the size of the drogue, the drogue's opening speed, the drogue's drag coefficient, etc.

So, an engineering approach would be comprised of two parts.

First, to decide on a maximum expected horizontal velocity component at apogee and then apply a safety factor. Simulation for nominal flight in max expected crosswind with a variety of motors and a few degrees for non-vertical launch rail can give the horizontal speed range at apogee. The largest of the simulation results is still in the EXPECTED range. So throw a safety factor in there. 1.5x? 2.0x? Decide. Now throw in a couple second late deployment, a second or two for drogue opening, do a bit of vector math, and find the velocity. From the speed and whatever info you have on the drogue, determine how much force the drogue can then generate.

From there, determine the number of appropriately sized shear pins it would take to not have a premature deployment.

Second, Determine the drag force max on the rocket trying to pull the nosecone out early for a premature drogue deployment at motor shutdown. Simulations can help there. For simplicity, assume the nosecone drag is zero and then apply a small safety factor for assurance. Determine the number of appropriately sized shear pins it would take to not have a premature deployment, for both sections.

For the pins retaining the main, you now have an answer.

For the pins retaining the drogue, take the maximum of these two answers.

Now determine how much ejection charge is required to reliably shear these pins for each section.

Etc.

Gerald

PS - Nothing in this approach handles anomalous situations. This is just to handle EXPECTED situations. If you want overkill, scale up from there. Overkill is not a bad plan. Someday you might need it. After all, the up part is optional. The down part is not.
 
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So, there are TWO mechanisms promoting premature deployment of main at apogee. The first, which is what has been considered a little bit in this thread, has to do with the relative masses of the nosecone (or connected forward section) and the motor section of the rocket, their relative velocities at ejection, dissipation of that energy, and the resultant shock transmitted to the retention mechanism for the main chute.

The second is the shock generated upon opening of the drogue at non-zero airspeed. This is dependent on the speed, the size of the drogue, the drogue's opening speed, the drogue's drag coefficient, etc.


To add a third, in an un-vented compartment there will be an atmospheric pressure differential from ground level to apogee. While this force may be less than the other two, calculations will verify that.
 
Drogue bays (all bays, actually) need to be retained during burnout and for any pressure build up that isn't vented.

Look at your data from past flights to learn about deployment forces....you have the data.....because you've done a multitude of L2's with electronics.
What do those accelerations look like? 10G's....20G's.....50G's.....more?

Devise a plan to ride though them.
Devise a plan to eliminate the retention when no longer needed.
 
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To add a third, in an un-vented compartment there will be an atmospheric pressure differential from ground level to apogee. While this force may be less than the other two, calculations will verify that.

RGR
 
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