Shear pins in the booster?

[smoon]

I just finished a RW AGM-58. It is built for dual deploy, but I will be flying it single deploy, motor eject with a chute release for a while. The slip fit of the payload section into the booster is so smooth that I am worried about possible separation at motor burnout, so I was wondering if it would be a good idea to use shear pins at the booster/payload joint to make sure this does not happen.

The fit is not loose, by any means, so I cannot use tape. Even scotch tape is too thick to use. The payload assembly just slides so smoothly into the booster that premature separation concerns me.

Thanks in advance,

Steve

 

[djs]

If you pick up the upper end of the rocket and shake it, does the booster separate, or does it stay together?

Some people do put shear pins in the booster, but you may have to up the ejection charge on the motor to give it enough force to break the pins and separate the rocket.

 

[smoon]

If you pick up the upper end of the rocket and shake it, does the booster separate, or does it stay together?

Some people do put shear pins in the booster, but you may have to up the ejection charge on the motor to give it enough force to break the pins and separate the rocket.

That is exactly why I asked. The booster slips right off the payload section when I pick it up. The fit is close enough that any kind of tape for a friction fit is not going to happen, which is why I am looking at shear pins.

Also, the booster weighs 8 pounds dry and without parachute, while the payload section weighs just 4 pounds. I figure the deceleration of the booster stage would be such that the payload section might slip out at motor burnout and destroy the rocket, which I want to avoid at all costs.

Steve

 

[Bat-mite]

You could indeed get drag separation at motor burnout, especially with an aggressive motor like a VMAX.

Find out the size of the ejection charge for your motor. Go to a BP calculator site and see what size charge and how many shear pins they recommend.

 

[smoon]

Thanks John. I will use the calculator I have and see what it says.

It appears that the Aerotech J450 doesn't come with the ejection charge, so I will need to provide my own. I have not opened my motor, but instructions for this motor size on the AT Web site don't show the ejection charge as part of the kit, but tell you to fill the ejection charge well with black powder.

Steve

 

[djkingsley]

Check out this thread, post number 10. Calculating-Nose-Cone-Drag-Separation

 

[Bat-mite]

Even better. Then you can size it to your liking.

Most calculators I've seen will give you a charge size, and also tell you how many shear pins it can break and what size.

 

[smoon]

Thanks for the link Dennis. If I can get my head around all that, and where all the numbers come from, I can do the calculations.

I am certainly no rocket scientist, but I can solve equations, given where the variables come from.

Steve

 

[smoon]

Even better. Then you can size it to your liking.

Most calculators I've seen will give you a charge size, and also tell you how many shear pins it can break and what size.

I use the spreadsheet calculator by Chuck Pierce. It calculates all of that and more.

Steve

 

[djs]

It appears that the Aerotech J450 doesn't come with the ejection charge, so I will need to provide my own. I have not opened my motor, but instructions for this motor size on the AT Web site don't show the ejection charge as part of the kit, but tell you to fill the ejection charge well with black powder.

I'd be pretty surprised if it didn't come with an ejection charge.

 

[djkingsley]

Thanks for the link Dennis. If I can get my head around all that, and where all the numbers come from, I can do the calculations.

I am certainly no rocket scientist, but I can solve equations, given where the variables come from.

Steve

It takes a minute to get your head around the equations, but the required inputs can come from an OpenRocket model and the simulation with your motor. If you need help, let me know, I worked through this for my L3 rocket, so it's still pretty fresh in my mind

 

[rcktnut]

That is exactly why I asked. The booster slips right off the payload section when I pick it up. The fit is close enough that any kind of tape for a friction fit is not going to happen, which is why I am looking at shear pins.

Also, the booster weighs 8 pounds dry and without parachute, while the payload section weighs just 4 pounds. I figure the deceleration of the booster stage would be such that the payload section might slip out at motor burnout and destroy the rocket, which I want to avoid at all costs.

Steve

Did you pick your rocket up with the motor installed, chute, etc. in place.? Makes a big difference with the mmt plugged. The fit sounds good enough to provide the "sucktion factor" with the motor in place.

 

[smoon]

I did pick it up with the chute installed, but not the motor. I have a vent hole in the booster, but I will try with the motor installed, too.

I also received some advice on using a small piece of packing tape to increase the friction, so I can try that, too.

 

[rms]

Rather than tape I have used thin cyanoacrylate glue on the coupler where it slides into the body tube and once cured sanded it back down for a tighter fit. Bear in mind, a little goes a long way though. Better to add a little at a time rather than have to sand away a lot to much.

Greg

 

[smoon]

Last night, I threaded the forward closure on my 54/1706 case, stuffed a towel in the case, and secured the case in the rocket. When I picked it up by the payload section, it still separated, but the separation was delayed and much slower than without the motor installed.

I will add the recovery gear today and re-do the test to see what effect filling the empty space in the booster will have.

Steve

 

[smoon]

Rather than tape I have used thin cyanoacrylate glue on the coupler where it slides into the body tube and once cured sanded it back down for a tighter fit. Bear in mind, a little goes a long way though. Better to add a little at a time rather than have to sand away a lot to much.

Greg

I have read about people doing this, but did not think about it for my own rocket. If there is still some question about the rocket prematurely separating after I check it with the parachute installed, I will give this a shot.

Thanks for the tip Greg.

Steve

 

[jderimig]

What is the mass ratio of the upper to lower section in flight configuration?

 

[soopirV]

I use the spreadsheet calculator by Chuck Pierce. It calculates all of that and more.

Steve

Have a link for that? A search here didn't turn up much...

 

[tim cubbedge]

Have a link for that? A search here didn't turn up much...

Scroll down about 2/3 of the page. Listed under "calulators & software"

https://aeroconsystems.com/cart/hot-tips-resources

Here is the file if you can't find it:
View attachment Ejection_ChargeCalc.xls

 

[soopirV]

Scroll down about 2/3 of the page. Listed under "calulators & software"

https://aeroconsystems.com/cart/hot-tips-resources

Here is the file if you can't find it:
View attachment 312030

Thanks! Bookmarked that page for future reference...I found the calculator, but it only does BP charges, I though from the OPs comment that I quoted that it calculated drag separation. Anyone know of a calculator for that? The equations linked to by Dennis in post 6 above are daunting and bringing me back to dark times in algebra!

 

[tim cubbedge]

Thanks! Bookmarked that page for future reference...I found the calculator, but it only does BP charges, I though from the OPs comment that I quoted that it calculated drag separation. Anyone know of a calculator for that? The equations linked to by Dennis in post 6 above are daunting and bringing me back to dark times in algebra!

Really don't remember where I found it but it was authored by Tim Dixon!

View attachment Separation Analysis.xlsx

 

[soopirV]

Really don't remember where I found it but it was authored by Tim Dixon!

View attachment 312036

amazing! Thanks!!

 

[Nathan]

I would recommend shear pins for both nosecone and booster for all filament wound fiberglass rockets like your AGM-58. The fiberglass is so smooth that the sections can easily separate prematurely if shear pins are not used.

 

[smoon]

thanks for providing the link to the BP calculator, Tim. You beat me to it.

The nice thing about it is that it is a spreadsheet. I have it sitting on a flash drive for use any time.

 

[smoon]

Thanks! Bookmarked that page for future reference...I found the calculator, but it only does BP charges, I though from the OPs comment that I quoted that it calculated drag separation. Anyone know of a calculator for that? The equations linked to by Dennis in post 6 above are daunting and bringing me back to dark times in algebra!

My apologies if there was any confusion. I wish there was a drag separation calculator. Maybe I can come up with one, once I get my head around all the equations.

Steve

 

[smoon]

Really don't remember where I found it but it was authored by Tim Dixon!

View attachment 312036

Thanks Tim. I'll have to give this a go.

Steve

 

[smoon]

I would recommend shear pins for both nosecone and booster for all filament wound fiberglass rockets like your AGM-58. The fiberglass is so smooth that the sections can easily separate prematurely if shear pins are not used.

I definitely see your point, and this is my first big, dual deployment capable FWFG rocket, so this part is new to me, and I want to make every flight possible successful, which is why I started this thread.

I may do the shear pins, even if drag separation does not look to be an issue. I can be a belt and suspenders kind of person at times.

Steve

 

[Bat-mite]

On one of my flights of a FWFG rocket, I had to really shove to get the NC into the payload tube. Once it was in, I realized I didn't line up the shear pin holes. NC was in there so tight that I couldn't turn it.

Finally, I said, "Fine. It's in there so tight that I don't need shear pins."

Nose cone and main came out at apogee.

 

[soopirV]

On one of my flights of a FWFG rocket, I had to really shove to get the NC into the payload tube. Once it was in, I realized I didn't line up the shear pin holes. NC was in there so tight that I couldn't turn it.

Finally, I said, "Fine. It's in there so tight that I don't need shear pins."

Nose cone and main came out at apogee. [/]

I'd almost always rather have too many parachutes than too few! We've all had eject failures I'm sure! Hope the retrieval walk wasn't too long?

 

[smoon]

On one of my flights of a FWFG rocket, I had to really shove to get the NC into the payload tube. Once it was in, I realized I didn't line up the shear pin holes. NC was in there so tight that I couldn't turn it.

Finally, I said, "Fine. It's in there so tight that I don't need shear pins."

Nose cone and main came out at apogee.

With this rocket being single deploy, motor eject for now, that is what I want to happen. Well, not the nose cone. It has shear pins, anyway. I am looking to make sure that the first separation is at apogee, not before, or just as bad, after.

Steve