Dual Deploy / Shear pin questions

[PayLoad]

I've been flying for 40 years, and I'm ashamed to say I've never done a dual deploy - I've always tried to keep it simple, or I used the JLCR with great success.

So I'm building my first DD, a 54mm LOC Vulcanite with bluetube ebay & eggtimer quark.

Now what?

I found these great nylon 4/40 screws at Ace Hardware that look like they would be perfect shear pins, but how do I know? How many to use, 2? 3? what I cannot wrap my head around is, doesn't the ejection rip up the tube around the shear pin?

Man, any suggestions or pointing out a thread would be a great help. I've watched apogee videos, which are a help, but most I've seen assume you already have the charge amount (1.5 grams?) and shear pin selection. I don't know where to start

 

[waltr]

I have two LOC 54mm (2.23 in) rockets with DD.

I use ONE 2-56 nylon screw shear pin to hold the nose cone on during Drogue deployment.
I do put two 2-56 drilled and taped holes boy only use one. The second is a 'spare, in case one hole gets bugged.
How I do this in put on the nose cone and drill a hole through BT and nose. Then tap a 2-56 thread. Insert a screw and drill/tap another hole on other side. Put a mark on cone to BT for later alignment. Remove screw and nose cone. Wick some Thin CA into the tapped holes and on the inside of the BT around the holes. This hardens the BT and ensures shearing the screw without damaging the tube. After CA glue cures, re-tap the holes, with the nose cone installed. LOC tubes are pretty thick and survive with a little CA hardening.

The AV bay to Main chute tube is held together with removable rivets. These are nice but almost any metal screws would also work. These need to be removed to access the AV bay to install ematches and BP charges so should be easy to remove and install.

BP charge for main is 1.2grams to ensuring shearing the single 2-56 screw.
Do ground test to ensure the correct amount of BP.

The LOC IRIS 2.2 (54mm) has over a dozen DD flights and no damage to the tube where the shear pins go in. The LaserLoc 223 (54mm) in new and only one flight to date. DD went perfect.

I do not shear pin the booster to av bay. Simple slip fit works. Also set motor delay for a little longer than reaching apogee as a back-up to drogue deployment.

 

[JackC]

I use two 4-40 nylon screws to hold the nosecone on and two to hold my fin can to the drogue bay. Never had any failures with this set up. Long ago, I did have a failure using 2-56 nylon screws on a four inch bird that flew an O3400. The nose came off after the rocket cleared the rail.

Just make sure you calculate the amount of black powder based on your body tube length and width then increase the amount by 50%.

 

[mtnmanak]

The number and size of shear pins are a pretty well discussed topic, but, for what it is worth, I usually use 2-56 pins on rockets 4-inch and smaller. 4-40 will work fine as well.

For cardboard tubes, though, Tom (@Banzai88 ) did a pretty good write up on using metal shear plates to both protect your tube and increase the shearing effect: https://www.rocketryforum.com/threads/shear-pin-shear-plates-in-your-cardboard-tubes.136706/

I use his technique all the time - works great:

 

[PayLoad]

Do you thread both the tubing and the nosecone?

Also -

"...Just make sure you calculate the amount of black powder based on your body tube length and width then increase the amount by 50%...."

So 50% more than what this says?

https://www.insanerocketry.com/blackpowder.html

 

[Banzai88]

Do you thread both the tubing and the nosecone?

Also -

"...Just make sure you calculate the amount of black powder based on your body tube length and width then increase the amount by 50%...."

So 50% more than what this says?

https://www.insanerocketry.com/blackpowder.html

Most of us would say ground test, ground test, and ground test.

I usually start with calculating how much force I'll need to break any shear pins, work the online calculator to get to that point + about 30%, then test the first charge. From there I know if I need to go up or down. As for my backup charge (if I have one), I usually go + 30% from my final main charge.

Attached is the excel sheet that I use, never done me wrong yet.
Found it here: http://aeroconsystems. com/tips/Ejection_ChargeCalc.xls as mentioned in POF newsletter 538

 

[waltr]

Do you thread both the tubing and the nosecone?

Yes, I thread both tube and nose cone.
This does leave a piece of the nylon screw in the nose cone. To remove I use to tip of an Xacto blade, stab it into the nylon screw and turn it inward. It will fall inside to nose cone but then easy to shake out.

Since I have been running this size LOC tube and nose, start with 1gram BP. If not energetic enough than go with the 1.2gram I use.

 

[Andrew Brown]

Everyone has their own favorite BP/shear pin estimation methods. I came across this one a while ago and it really takes the guesswork out of it.

http://speedmotionrockets.com/Spreadsheets.htmlThen download the Shear Pin Calculator spreadsheet.

You will still need an estimation on how much over in BP for margin, and how much strength to hold the nose cone on during separation event. For the amount of strength to hold the nose cone on, I've seen people estimate ranging from 20-100 times the mass of the nose cone, which will dictate the number and size of the shear pins.

 

[PayLoad]

OK, so I drilled & threaded the body tube & the nosecone - I still find it amazing that they could be threaded at all. 4/40 nylon threaded pin.

Use one pin or two?

Other end that connects to ebay will be held with 3 4/40 steel screws. Good?

 

[JackC]

OK, so I drilled & threaded the body tube & the nosecone - I still find it amazing that they could be threaded at all. 4/40 nylon threaded pin.

Use one pin or two?

Other end that connects to ebay will be held with 3 4/40 steel screws. Good?

Two pins.

3 4/40 steel screws are good.

 

[PayLoad]

14 inch 54mm tube - 1 gram BP?

 

[Titan II]

14 inch 54mm tube - 1 gram BP?

https://letmegooglethat.com/?q=black+powder+calulator

 

[rharshberger]

Yes, I thread both tube and nose cone.
This does leave a piece of the nylon screw in the nose cone. To remove I use to tip of an Xacto blade, stab it into the nylon screw and turn it inward. It will fall inside to nose cone but then easy to shake out.

Since I have been running this size LOC tube and nose, start with 1gram BP. If not energetic enough than go with the 1.2gram I use.

Just drill and tap the hole to the correct size, then remove the nosecone and re-drill the hole to the screw clearance size. The screw will still fit properly and the sheared end can be pushed easily out of the clearance hole. Definitely CA the holes in the cardboard, they will still enlarge over time.

 

[JackC]

14 inch 54mm tube - 1 gram BP?

If you go to Vern Knowles website, he has a section on ejection charge sizing. On that page, a chart states 1 gram per 36 inches of a 54mm tube.

Since you are using shear pins, I would use 2 grams to be sure.

 

[AlexBruccoleri]

This probably will not go well on this forum, but on smaller rockets flying below 5000 feet, I do not think shear pins are necessary. A firm friction fit with a few vent holes beneath the shoulder of the cone has worked well for me. Shear pins are the deterministic "engineering" method, but you will need bigger ejection charges etc, and it adds complexity when prepping the rocket.

 

[tsmith1315]

This probably will not go well on this forum, but on smaller rockets flying below 5000 feet, I do not think shear pins are necessary. A firm friction fit with a few vent holes beneath the shoulder of the cone has worked well for me. Shear pins are the deterministic "engineering" method, but you will need bigger ejection charges etc, and it adds complexity when prepping the rocket.

I've always friction fit single and dual deployment. All have been 2"-4" rockets with no appreciable nose weight. Yes, main deployment section has to be fit a little tighter, and it's important that the drogue shock cord is long enough for the main section to bleed off the energy before it's fully extended. Otherwise, it's business as usual.

 

[waltr]

I found that the LOC 54mm Nose cones, even with fairly tight fit, have come off during Drogue deployment. This is when I started adding shear pins.
One 2-56 nylon has worked very well with 1.2gram BP.

Besides a long enough cord on drogue and main, I either do 3 taped bundles or braid the cord to dissipate energy to slow the seperation to prevent shock to the recovery system. Also helps prevent premature deployment of main and damage, zippers, if ejections are early or late.

YMMV

 

[AlexBruccoleri]

I found that the LOC 54mm Nose cones, even with fairly tight fit, have come off during Drogue deployment. This is when I started adding shear pins.
One 2-56 nylon has worked very well with 1.2gram BP.

Besides a long enough cord on drogue and main, I either do 3 taped bundles or braid the cord to dissipate energy to slow the seperation to prevent shock to the recovery system. Also helps prevent premature deployment of main and damage, zippers, if ejections are early or late.

YMMV

Did you have unobstructed vent-holes?

 

[waltr]

Did you have unobstructed vent-holes?

No vent holes into the Chute bays. Just into the Altimeter bay.

 

[AlexBruccoleri]

No vent holes into the Chute bays. Just into the Altimeter bay.

I strongly recommend vent holes if you use a friction fit.

 

[waltr]

I do have vent holes in my 4" LOC Goblin due to the volume to prevent lower air pressure from moving the nose. But this is motor deploy.

I've had the nose come off, and deploy main, during the Drogue ejection. I think this is for different reason. Cord pulling quickly tight pulls on the Ebay/ main chute tube and the nose flies off.

My small DD rockets (24 &34mm) only use friction fit but the nose cones are much, much lighter than the 54mm Loc nose.

There isn't a single correct method. What works, works but one may need to experiment a bit to find what does work reliably.

 

[AlexBruccoleri]

I do have vent holes in my 4" LOC Goblin due to the volume to prevent lower air pressure from moving the nose. But this is motor deploy.

I've had the nose come off, and deploy main, during the Drogue ejection. I think this is for different reason. Cord pulling quickly tight pulls on the Ebay/ main chute tube and the nose flies off.

My small DD rockets (24 &34mm) only use friction fit but the nose cones are much, much lighter than the 54mm Loc nose.

There isn't a single correct method. What works, works but one may need to experiment a bit to find what does work reliably.

I am not sure. I often use nose weight and have not had issues. If you have access to a suitable place, try a drop test where it falls and hangs free of the payload bay with nose cone, and see if the nose cone comes off from the "shock" of deceleration. I suspect you are not using enough tape but that is a weak guess.

 

[tsmith1315]

I've had the nose come off, and deploy main, during the Drogue ejection. I think this is for different reason. Cord pulling quickly tight pulls on the Ebay/ main chute tube and the nose flies off.

Yes, I learned that lesson early! And vent holes just in case.

@PayLoad do you have a decent place to ground test or will you have to do your homework and test in flight?

 

[PayLoad]

Backyard is ready for a ground test-fire this weekend. I've lost too many rockets recently to test in the air / she'll fly when I'm confident it will work. Right now screwing everything together, super-gluing the threads, re-threading, test fitting.

 

[b.wieting]

If you go to Vern Knowles website, he has a section on ejection charge sizing. On that page, a chart states 1 gram per 36 inches of a 54mm tube.

Since you are using shear pins, I would use 2 grams to be sure.

And ground test!

 

[b.wieting]

This probably will not go well on this forum, but on smaller rockets flying below 5000 feet, I do not think shear pins are necessary. A firm friction fit with a few vent holes beneath the shoulder of the cone has worked well for me. Shear pins are the deterministic "engineering" method, but you will need bigger ejection charges etc, and it adds complexity when prepping the rocket.

Probably right. It’s friction vs inertia and drag forces. Maybe as friction likely goes like D^2 whereas mass goes ~ D^3.

 

[kramer714]

i use plastic push in rivets from McMaster https://www.mcmaster.com/90136A565/ There are two different thickness and different grip length. They are pretty strong and more importantly it uses a straight hole. with the same diameter on both sides. No wiggling once installed. Do buy the correct drill bit they fit snug, and you will need to tap them in (I use the back of a screwdriver). The snug fit makes sure they cleanly shear no bending. To remove them if you dont fly, just use a small screwdriver to pry them out.

They are pretty stout, I wouldnt use them on phenolic or quantum tube, but they work well for graphite and fiberglass.

Do ground test, they need a bit more umph than #2 shear screws Depending on the size of the rocket I use 2 or 3 of them

Mike Kramer

 

[COSTransplant]

i use plastic push in rivets from McMaster https://www.mcmaster.com/90136A565/ There are two different thickness and different grip length. They are pretty strong and more importantly it uses a straight hole. with the same diameter on both sides. No wiggling once installed. Do buy the correct drill bit they fit snug, and you will need to tap them in (I use the back of a screwdriver). The snug fit makes sure they cleanly shear no bending. To remove them if you dont fly, just use a small screwdriver to pry them out.

They are pretty stout, I wouldnt use them on phenolic or quantum tube, but they work well for graphite and fiberglass.

Do ground test, they need a bit more umph than #2 shear screws Depending on the size of the rocket I use 2 or 3 of them

Mike Kramer

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I really like this idea a lot -- just the cleanliness of it -- but I am a NOOB, so what the heck do I know!

 

[PayLoad]

54mm tube, 10 inches of open space in the tube, two 4/40 nylon shear pins. Start at 1 gram of fff? Calculators say less

 

[waltr]

54mm tube, 10 inches of open space in the tube, two 4/40 nylon shear pins. Start at 1 gram of fff? Calculators say less

As I have posted, on the two LOC 54mm tubes with about 10inch length I use ONE 2-56 nylon shear pin and 1.2 gram 4F BP. I have at least a dozen flight of this configuration with full success.

4-40 are over kill and I doubt 1gm will shear them.
However, ground test to find out.