Friction fit dual deploy - fools errand?

Okay, I admit it: Maybe this wasn't such a good idea after all...

Last year I decided to take a crack at building a dual-deploy configured rocket. I mod'ed an Estes Leviathan by following some of the "zipperless" mods that other folks have done (to implement a break-point at the bottom of the main section of body tube) and then built an e-bay that sits half-way down the main body tube. It's a pretty standard config beyond that with the main chute in the upper section above the e-bay. All-in-all it's come together well, but the first time I flew in the DD config the parachute deployed at apogee instead of on the way back down when it was supposed to. (The chute eject charge was heard on the way down, the nose cone just didn't hold the laundry in when the apogee charge went off.) No damage was done, and the rocket actually landed really close to the pad despite the early deployment, but that wasn't how it was supposed to work...

My plan from day one was to friction-fit the nosecone, but having read many posts on this forum about DD it seems that there's a definite preference to using shear pins. (For obvious reasons I'm sure.) Is friction-fitting the nosecone on a DD config a fools errand, or is there a reasonable chance of getting it right? I'm not opposed to exploring a more conventional (shear pin) solution, after all I'm sure there's a preference to doing it that way for a reason, but I'd like to try to stick with what I've done if I can.

Has anyone used shear-pins with medium-thickness cardboard tubing? (I'm not sure how best to describe the thickness of Leviathan's tube - it's definitely thicker than standard Estes tubing, but still not as thick as my LOC Weasel.) If I soak the cardboard in CA will it give it enough hardness that a couple of 2-56 nylon pins will shear cleanly, or am I still as likely to tear the tube as I am to shear the pin?

Any thoughts?

KevinM said:

Is friction-fitting the nosecone on a DD config a fools errand, or is there a reasonable chance of getting it right?

Has anyone used shear-pins with medium-thickness cardboard tubing? (I'm not sure how best to describe the thickness of Leviathan's tube - it's definitely thicker than standard Estes tubing, but still not as thick as my LOC Weasel.) If I soak the cardboard in CA will it give it enough hardness that a couple of 2-56 nylon pins will shear cleanly, or am I still as likely to tear the tube as I am to shear the pin?

Any thoughts?

Click to expand...

Yes, friction can work just fine. Give yourself plenty of drogue cord, and don't over-do the drogue charge. Ground test.

The problem with friction is that it is inconsistent. Eventually I got tired of adding and removing masking tape depending on the weather, trying to get the fit to "feel right." So, I put shear pins on my cardboard rockets. LOC 1.52" and 2.2" airframes are plenty beefy to withstand a shearing force. Wimpy LOC 2.6" and Estes cardboard, probably not so much. My 2.6" rocket held up fine for a few flights, but the last one tore open the shear pin hole.

With the fairly lightweight tubing used on the Leviathan you are better off finessing it without the pins.

What about using little pieces of tape externally (axially applied)? I think I saw someone here mention this technique, but I haven't tried it- idea is that the thin width of masking tape shears. Ground testing is obviously required, but if friction isn't sufficient to keep your laundry in, I'm not sure this will be either.

I do it exclusively.

I do friction fit DD on some of my cardboard rockets. Quite a few times with my LOC Magnum.

When using shear pins w/ card board, people sometimes remove a square of cardboard (1 layer) and glue in a thin square of brass sheeting with a hole drilled in it to do the cutting.

soopirV said:

What about using little pieces of tape externally (axially applied)? I think I saw someone here mention this technique, but I haven't tried it- idea is that the thin width of masking tape shears. Ground testing is obviously required, but if friction isn't sufficient to keep your laundry in, I'm not sure this will be either.

Click to expand...

I do it with my Fantazia Crayon bank conversions as the nosecones are fairly loose, I put two strips of masking tape across the joint, perpendicular to the joint not parallel.

soopirV said:

What about using little pieces of tape externally (axially applied)? I think I saw someone here mention this technique, but I haven't tried it- idea is that the thin width of masking tape shears. Ground testing is obviously required, but if friction isn't sufficient to keep your laundry in, I'm not sure this will be either.

Click to expand...

I've used Scotch tape for that, it breaks easier than masking tape. Two pieces of tape about 3/8" wide usually does the trick. Doesn't work if the nose is too heavy, or has a tracker in it, though.

Thanks to all, I'm glad to hear my approach wasn't out in left field.

I spent some time yesterday tweaking/testing and I think I'm ready to give it another go. One thing I changed was the type of tape used to adjust the nosecone friction: Where I had previously used masking tape I've changed out for scotch tape because it's thinner and I'm hoping it will give me better control over the amount of friction achieved. I've also significantly increased the friction over what was there on the last flight, but confirmed that my deployment charge has no trouble blowing the NC out.

In hindsight I'm also wondering if the apogee charge on my last flight was a little too energetic so I'll be looking to tweak that a little as well.

My next launch window is Saturday, I'll report back on how it goes.

I've been using friction fit on all of my 4" and smaller rockets. I use masking tape and use an X pattern on each of four sides. This gives you four points with the thicker tape and a fixed area where the friction is applied. If I need more friction, I apply an additional piece over the top of the existing Xes. I use the same test that most use for nose cones on standard motor eject rockets. I want to be able to pick the rocket up by the nose cone and give it a gentle shake and not have it open up. I small slip is OK, but you don't want it to come apart or close to it, especially on the nose cone. The drogue joint can be a little looser.

The key here is what other have said, do NOT over do the apogee ejection charge. In my case, I built the bottom section as zipperless. (I won't bother with that on a DD again) My drogue when stretched from top of canopy to the bottom of the shrouds is only 30" long and is attached 36" from the upper section. Since I Z-fold the rest of the apogee cord, I only need to have the two halves separate about 4 - 5 ft to ensure the drogue chute gets pulled out of the upper drogue tube and into the the air stream and opens. Once that happens, the rest of the apogee cord will open the way it is supposed to. In my 4" rocket with 24" of apogee tubing, I only use 1.5g of 4F BP.

Friction fitting works just fine, but don't overdo the ejection charges. If it's going very high make sure and vent the body tubes so the pressure difference between inside the tubes and outside doesn't force the nosecone off.

If you do go the way of shear pins, you may want to epoxy a little square of metal shim to the tube to assist with breakage.

Bat-mite said:

If you do go the way of shear pins, you may want to epoxy a little square of metal shim to the tube to assist with breakage.

Click to expand...

I do this with .005 brass shim sheet. Easy, quick, and works like a charm, even in the dreaded 1 pin configuration.

Banzai88 said:

I do this with .005 brass shim sheet. Easy, quick, and works like a charm, even in the dreaded 1 pin configuration.

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Would you be willing to post a tutorial on this? I get the concept, but execution has me a bit doubtful...

Handeman said:

I've been using friction fit on all of my 4" and smaller rockets. I use masking tape and use an X pattern on each of four sides. This gives you four points with the thicker tape and a fixed area where the friction is applied. If I need more friction, I apply an additional piece over the top of the existing Xes. I use the same test that most use for nose cones on standard motor eject rockets. I want to be able to pick the rocket up by the nose cone and give it a gentle shake and not have it open up. I small slip is OK, but you don't want it to come apart or close to it, especially on the nose cone. The drogue joint can be a little looser.

The key here is what other have said, do NOT over do the apogee ejection charge. In my case, I built the bottom section as zipperless. (I won't bother with that on a DD again) My drogue when stretched from top of canopy to the bottom of the shrouds is only 30" long and is attached 36" from the upper section. Since I Z-fold the rest of the apogee cord, I only need to have the two halves separate about 4 - 5 ft to ensure the drogue chute gets pulled out of the upper drogue tube and into the the air stream and opens. Once that happens, the rest of the apogee cord will open the way it is supposed to. In my 4" rocket with 24" of apogee tubing, I only use 1.5g of 4F BP.

Click to expand...

A picture is worth a thousand words... Any chance you could photograph your setup for us?

Thanks!
Jim

soopirV said:

Would you be willing to post a tutorial on this? I get the concept, but execution has me a bit doubtful...

Click to expand...

I've had a couple of the student groups that I advise reinforce shear pin holes on the shoulders of plastic nose cones using strips of .010" brass sheet (K & S metals display at the hardware and hobby stores). Clean both surfaces, epoxy, drill appropriate holes for the shear pins (in this case, 4-40 nylon screws). Clean cuts, no elongated holes or bending the shear pins. I don't have photos handy, but I'll get some to post as soon as I have a chance.

Backing up a bit, I've used friction fit on an old 4" cardboard airframe / phenolic av-bay / plastic nose cone dual deploy rocket for years, and it's always worked just fine...but, I find that I've got to mess with the fit (removing/adding tape) at nearly every launch. I blame that on temperature and humidity fluctuations here in the Midwest. I hope to refurbish it soon, and when I do, I'll be using shear pins: figure it out once, and it's consistent, without the fuss, thereafter. Just my $0.02.

Mark

Another option is to create loops in your shock cord with masking tape. This will act like a shock absorber.

On a recent launch I put in six tape loops in the Kevlar shock cord on my Loc Iris. She went to 2714 feet on an I211 and the dual deploy worked perfectly. Four of the six loops tore open, the cord slipped out of one loop and the last loop was still intact after landing. This was the second launch that I used the loops and both times they worked well. On several previous launches I popped the nose cone off at apogee using only friction fit. I tried shear pins, but they ripped up the tube too much during ground testing, so I never tried shear pins on an actual launch. I really like the tape loop method.

Here is the thread where I read about them from a forum member called mikec.

https://www.rocketryforum.com/showthread.php?124685-Shear-Pin-Failure-Video-amp-Need-Advice&highlight=tape+shock+cord+shear+pins

Zeus-cat said:

Another option is to create loops in your shock cord with masking tape. This will act like a shock absorber.

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I've done this as well. Even is not doing the shock absorber, making 1 or 2 shock cord bundles helps keep things tidy and reduces the change of a tangled chute.

K'Tesh said:

A picture is worth a thousand words... Any chance you could photograph your setup for us?

Thanks!
Jim

Click to expand...

I don't really have any pics of the recovery prepped for flight, but these should give you an idea of how it's set up.


On this one, the center purple band is the altimeter bay (right at the horizon). The fincan separates just above the bottom chrome strip and about 24" of the lower payload tube stays with the altimeter bay.



Here is the whole recovery system stretched out. The red & white drogue is attached where the stretched shock cord will have to pull it out of the lower payload tube. With the lower shock cord z-folded, the drogue always gets pulled from the lower payload before the z-folds come apart. The chute protectors is also attached where the drogue is attached. I think the altimeter bay band was still gold on this pic.

Handeman said:

I don't really have any pics of the recovery prepped for flight, but these should give you an idea of how it's set up.

View attachment 291099
On this one, the center purple band is the altimeter bay (right at the horizon). The fincan separates just above the bottom chrome strip and about 24" of the lower payload tube stays with the altimeter bay.


View attachment 291098
Here is the whole recovery system stretched out. The red & white drogue is attached where the stretched shock cord will have to pull it out of the lower payload tube. With the lower shock cord z-folded, the drogue always gets pulled from the lower payload before the z-folds come apart. The chute protectors is also attached where the drogue is attached. I think the altimeter bay band was still gold on this pic.

Click to expand...

Nice work, I like zipper-less fin cans, its usually how I build most of my HPR rockets.

rharshberger said:

Nice work, I like zipper-less fin cans, its usually how I build most of my HPR rockets.

Click to expand...

I built that one in '09 for my L2. It's has been a great rocket, but I haven't built any more zipperless designs for DD since. It just seems easier to prep when you only have one BT to attach to the av-bay instead of two. I will say, it was very easy to add the longer BT to the lower section to accommodate the Pro54 6GXL motor.

Buckeye said:

Yes, friction can work just fine. Give yourself plenty of drogue cord, and don't over-do the drogue charge. Ground test.

The problem with friction is that it is inconsistent. Eventually I got tired of adding and removing masking tape depending on the weather, trying to get the fit to "feel right." So, I put shear pins on my cardboard rockets. LOC 1.52" and 2.2" airframes are plenty beefy to withstand a shearing force. Wimpy LOC 2.6" and Estes cardboard, probably not so much. My 2.6" rocket held up fine for a few flights, but the last one tore open the shear pin hole.

Click to expand...

Yes.. And Z-Fold

Thanks to all who have responded to this thread - I had a stellar flight today with my Leviathan. Aerotech G79W sent her up to 900ft, and the AIM USB 3 altimeter brought her back. With some tweaking of the nose-cone friction and minimizing the apogee charge the nosecone remained in place this time, the main chute deployed about 300ft as planned, and everything was recovered as it was supposed to be. Thanks all for the shared wisdom!