Dual Deploy Rocket Set Up Questions

[Bruiser]

I am looking at building my first DD rocket from a 2.6 diameter Aerotech kit. Basically I will turn the coupler into an Av bay but I have a few questions about sheer pins and harnesses.

Referring to the drawing, I have the harness going to the drogue chute then to the bottom of the Av bay. The upper harness goes from the top of the Av bay to the nose cone and the chute attaches to the nose cone. I think that is a pretty standard set up but I'd just like to verify.

Sheer pins and Screws:
It seems standard practice to have sheer pins holding the lower BT to the lower part of the Av bay. Would three 2-56 plastic sheer pins be good for that? I also saw some people imbed a brass plate in the body tube. Does that help the pin to sheer and increase longevity of the BT?

Then there are the screws to hold the Av bay to the bottom of the upper BT. I think some people use push pins for this but I was thinking of going with three 10-32 machine screws going into weld nuts. I was also thinking of adding a brass plate to the outside of the body tube to prevent the screws from crushing the BT each time they are reinstalled.

Last there is the nose cone. Do I need sheer pins there or does the nose cone need to fit snuggly so it won't come off while flaying about on the drogue? If I need sheer pins would the same set up as on the lower BT/Av bay work here as well?




Thanks,
-Bob

 

[Azamiryou]

I think standard practice is sheer pins at the nose cone (or both).

Sheer pins at the booster prevent drag separation, which might or might not be an issue depending on design an speed.

Sheer pins at the nose keep the nose from popping off at the apogee ejection event. That's much more likely, due to reaching the end of the shock cord with a snap.

 

[Buckeye]

It is more common to have the shear pins on the nose cone (for the reason mentioned by @Azamiryou) but not the booster section. Drag separation rarely happens, and you want to ensure that the drogue event produces separation to prevent a ballistic trajectory.

Two (2) #2-56 nylons screws are plenty on the nosecone. Brass shear plates glued to the body tube would be a good idea since the 2.6" tube is thin and wimpy. Ground test.

Lastly, three (3) #10 screws is way overkill to hold the av-bay. This is a MPR rocket. Two (2) #6 or #8 machine screws are plenty, or the push-pins.

 

[Art Upton]

FYI, AeroTech and BMS BT-80HD tubes I use are not wimpy.
I have not used brass shear plate on them. Others have, but I did not bother.

Now Estes 3" and Estes BT-80 tubes are thin, and would be a good choice for a shear plate on them.

I just doubled checked to make sure I remembered correctly about AeroTech 2.6 tube.

Take an AT 2.6 tube and try to gently squeeze it, No joy.
Now take an Estes BT80 tube and gently squeeze it, better stop quick or you will permeant distort it.

 

[Bruiser]

Like Art wrote, the AT 2.6 tubes are not wimpy BT-80 Estes tubes. According to their website, Aerotech 2.6 tubes have .040 thick walls. I just measured one and they are indeed .040 thick walls. This is one of the reasons why I choose to use one of their kits for parts to build a 2.6 Black Brant III. I may scrap the kit bash if I can find a BF sale on the Aerotech Astrobee-D.

Oh, and I didn't mention. This is going to be mostly a G and baby H (29/180 case) rocket.

So good to go on the harness routing.

So no sheer pins on the lower body tube?

10-32 screws are overkill to hold the Av bay to the upper body tube: step them down a bit and go to two screws.

Probably should go with 2-56 shear pins on the nose cone.

Good?

-Bob

 

[Buckeye]

OK, the Aerotech 2.6 tubes are not "wimpy" for flight, but I think any cardboard tube can potentially tear/elongate from shear pins. Even after hardening the holes with CA, I occasionally had issues. So, I default to brass plates.

 

[Bruiser]

OK, the Aerotech 2.6 tubes are not "wimpy" for flight, but I think any cardboard tube can potentially tear/elongate from shear pins. Even after hardening the holes with CA, I occasionally had issues. So, I default to brass plates.

I'm good with the brass plates. I like to build rockets thinking they will be able to be launched again and again. Do you inset the brass on the inside of the BT or the outside?

-Bob

 

[David_Stack]

Inspired by Nytrunner's "The road to L2 is paved with practice. PSII trainer fleet" thread (https://www.rocketryforum.com/threa...aved-with-practice-psii-trainer-fleet.140897/), I 'cloned' a 2.6" Darkstar using Estes PSII parts so that I too could gain familiarity with dual deployment.

https://www.rocketryforum.com/threa...a-project-ddt-dual-deployment-trainer.172528/

For my model I fly without shear pins, relying solely on friction fit (nose cone to payload bay, and avionics bay to booster). The weight of the airframe components/recovery gear is such that I have not found shear pins to be necessary to secure the nose cone to the payload tube (plus I bundle my recovery harness into taped bundles to dissipate energy upon deployment). The payload tube is secured to the avionics bay with three (3) 2-56 screws.

Largest motor flow to date in that airframe is a G80, 1700+ feet at apogee.

 

[David_Stack]

OK, the Aerotech 2.6 tubes are not "wimpy" for flight, but I think any cardboard tube can potentially tear/elongate from shear pins. Even after hardening the holes with CA, I occasionally had issues. So, I default to brass plates.

I'm good with the brass plates. I like to build rockets thinking they will be able to be launched again and again. Do you inset the brass on the inside of the BT or the outside?

-Bob

Tom (Banzai88) wrote a good instruction on how to fabricate/install shear plates:

https://www.rocketryforum.com/threads/shear-pin-shear-plates-in-your-cardboard-tubes.136706/

 

[thzero]

Scotch tape to hold nose cone ..

 

[thzero]

10-32 screws are overkill to hold the Av bay to the upper body tube: step them down a bit and go to two screws.

3mm, nylon even, is what I use with my bt-70 and bt-80 fleet.

I've got #10-32 holding together a 4" fg rocket, so way overkill.

 

[Capt. Eric]

For my model I fly without shear pins, relying solely on friction fit (nose cone to payload bay, and avionics bay to booster). The weight of the airframe components/recovery gear is such that I have not found shear pins to be necessary to secure the nose cone to the payload tube (plus I bundle my recovery harness into taped bundles to dissipate energy upon deployment). The payload tube is secured to the avionics bay with three (3) 2-56 screws.

Largest motor flow to date in that airframe is a G80, 1700+ feet at apogee.

FWIW, I also went without any sheer pins. This on a 4" dia Madcow Frenzy with a J244 that went to 3500ft.

I had a slightly different arrangement where the nose cone is fixed/screwed to the body tube, and the AV bay separates from the upper body tube as well as the lower bt (where the drogue is). Either way is fine. Friction fit worked ok, but I will admit that the upper bt separated just before the ejection charge popped. At that point, it was low enough anyway, but next time I'll make it slightly tighter. On your smaller rocket, you should be fine without them, but ground testing is a good idea.

 

[b.wieting]

John Coker usually has good tips: https://www.jcrocket.com/

 

[Onebadhawk]

I have some helpful reads on my site..
Check them out..

Teddy

https://onebadhawk.com/why-a-3rd-loop.html#/

 

[tfischer]

I built and flew my first dual deploy rocket this year. It is a 2.6" dia. cardboard LOC Hi-Tech. It's first 3 flights have been successful.

Some build approaches and learnings from my build that might help with your questions:

1. I added a blue tube ebay with 1" airframe switchband, to make it DD

2. I used the brass shear plate method from @Banzai88
https://www.rocketryforum.com/threads/shear-pin-shear-plates-in-your-cardboard-tubes.136706/
Found the 0.005" brass at a local Hobbytown store. It has worked very well.

3. During the build I found I could not fit my main chute plus shock cord into the 10" payload bay (upper chute bay). My solution was to move the main chute to the booster tube and the drogue to the upper chute bay, and shear-pin the booster-ebay joint. The nose cone/upper chute bay joint was friction fit (noose cone is very light, and the upper chute bay contents fit very tight and do not slide a lot (which could add to the separation forces there) so friction there has worked okay.

The ebay upper end gets riveted to the lower end of the upper airframe (upper chute bay) using plastic removable rivets from Apogee Rocketry, they are long enought to fit thru the airframe and blue tube ebay.

4. During ground testing of charges, 3 shear pins seemed very hard to break with charge sizes reasonable for the small chute bay. After re-calculating separation forces I convinced myself that 2 pins were enough. This has worked well in ground testing and flights.

5. I started using centrifuge tubes for ejection squibs (I could not tie the tiny knots on latex glove finger based method). These are the same plastic tubes that ejection charge comes in with AT motors. I drill a hole thru the bottom, insert the ematch, and tape around the hole with blue mask tape to keep the charge from leaking out around the ematch. These hold up to 2g of BP, and I pack any empty space with dog barf for smaller charges. I got this method from tips on this forum and from John Coker videos (thanks everyone for those tips).

6. This rocket uses a single blue raven with magnetic power switch. During final flight prep, I align the ebay sled so that when the ebay gets riveted to the upper airframe, the sled aligns with a stripe on the rocket airframe. This stripe helps show approximately where to pass the magnet to activate the blue raven when rocket is vertical on the pad.

 

[Bruiser]

I'm wondering if a LOC electronic bay will fit in the Aerotech kit. Aerotech calls out a inside diameter of 2.558 and the LOC e-bay is to fit their tubes which are 2.56 ID. Seems like it should work but I thought I'd asked if anyone has tried it.

-Bob

 

[Bruiser]

I'm going to find out. I ordered an Aerotech Astrobee-D and The LOC 2.56 ebay. Hopefully it fits.

-Bob

 

[Bruiser]

It turns out the LOC 2.56 ebay does fit the Aerotech 2.6 body tube very well.

-Bob