Ejection and Chute question

I have a Maecow DX3 2.6" that I built a while back. During the building process I made two upper sections. One as the instructions indicated for single deploy with a payload bay and the other as a dual deploy with payload bay and electronics bay. I have flown it as single deploy in both configurations and it flew well. I now want to do a dual deploy flight but have a question or two.

During previous flights I used nylon screws as sheer pins but they are tearing up the tubes. Even with a grand amount of CA soaked into the tubes so I want to use friction fit with tape. How tight should the payload/nose cone interface be to overcome any force that it may encounter from the apogee charge?

This rocket is not huge and I have read elsewhere on the forum that many would suggest drogue-less. Thoughts from anyone? I was thinking if I just left my 9x9 nomex on that would be enough.

I am headed out tonight to test some ejection charges. Kind of want to get an idea of how tight the cone should be.

Here's a couple of pictures of the rocket. Dimensions are as follows;
2.6" diameter, 47.625" length. 23.875" Booster, 5" coupler length, 10.5" payload section and stock nose cone.

Nice looking rocket, Brad.

I'll throw in my $.02.

As you probably know, shear pin use helps you remain consistent, regardless of many other flight conditions. While I do use friction fit for some dual deployment flights (and lots of motor eject rockets), I find that there's a need to adjust the fit pretty regularly. Temperature and humidity change that fit, so one day you're adding tape to the coupler shoulder, and another you're removing it. My test is to be able to lift the rocket vertically by the coupler or nose cone, and not have it separate, but pulling with little additional effort, it will.

Shear pins in a cardboard tube rocket can be reinforced by inlaying a little square of brass shim stock into the surfaces (I think there are some photographic examples floating around here on TRF). The shim stock does two things: first, it becomes the shear load surface for the pin/screw, preventing elongating the holes; second, it aids in the clean cutting action to shear the pin. With a little bit of work on your rocket, I think this is the way to go. Assuming a loose fit of the coupled parts, and consistent use of the same type of shear pin material (nylon screws or styrene rod), it will always require the same amount of force to separate. Not so with friction fit.

You'll find that the drogue vs. drogueless discussion is nearly religious in nature. I prefer having a little something up there to keep the recovery geometry in order - but that's just me.

I'll look forward to seeing others' responses to your questions.

Mark

Ok, to answer your questions, you can use friction fit. I have done it many times on my arctic wolf rocket, just use enough tape to hold the nose cone on while shaking the rocket up and down with the nose cone pointed to the ground, if the nose cone falls off or slips a little you need more tape, but don't use too much tape that you have a hard time getting it on the tube.
As far as drougless versus drouge, you can do either one, the purpose of the drouge chute is to keep each part of the rocket apart so the main chute does not tangle with any part of the rocket. It's up yo you if you want to use a drouge chute or not. I hope this helps you on your endeavors.

Using a loooooong shock cord with taped bundles will help dissipate the energy from your apogee event, keeping your main in until your main event. I do this a lot, works very well.

BrAdam said:

This rocket is not huge and I have read elsewhere on the forum that many would suggest drogue-less. Thoughts from anyone? I was thinking if I just left my 9x9 nomex on that would be enough.

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I fly mine that way all the time and it works very well.

Mikec is right. I fly with a cable cutter for the main and have flown without a drogue every time until the last 1. Now I use a tiny drogue to pull the nomex away after the cut. But its so small its effectively drogueless.

chadrog said:

Using a loooooong shock cord with taped bundles will help dissipate the energy from your apogee event, keeping your main in until your main event. I do this a lot, works very well.
View attachment 169246

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I've experimented with this a bunch, and find the amount of energy dissipated questionable.

Think about it like this: work = force * distance, and the distance required to snap the tape is really short, resulting in very little dissipated energy.

Now on the other hand, it helps keep the shock cord really neat, and it certainly doesn't hurt, but unless you use some really stretchy tape I don't see it making much of a difference.

(I've done it way crazy with bundles of bundles of bundles, and they all just snap open without much force. Maybe it just isn't as effective on big rockets as on smaller ones.)

Didn't get to test tonight. Tomorrow will likely happen. I think the shim inlay is a good idea but tubes are already snug and not sure I want to rebuild that section. Taped sections also a good idea and might try. I think finger crochet of the shock cord would do the same thing. Will likely test both. Thanks for the help. Want to fly this end of the month at the WOOSH launch weather permitting. Chadrog will you be there?


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BrAdam said:

Didn't get to test tonight. Tomorrow will likely happen. I think the shim inlay is a good idea but tubes are already snug and not sure I want to rebuild that section. Taped sections also a good idea and might try. I think finger crochet of the shock cord would do the same thing. Will likely test both. Thanks for the help. Want to fly this end of the month at the WOOSH launch weather permitting. Chadrog will you be there?


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Yup sir!

BrAdam said:

Didn't get to test tonight. Tomorrow will likely happen. I think the shim inlay is a good idea but tubes are already snug and not sure I want to rebuild that section. Sent from my iPad using Rocketry Forum

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What I did was peel out a little of the cardboard tube the size of the brass insert. I then glued the inserts in. Worked so far. I did this on both the inside of the body tubes, and the outside of the couplers. The 1/2" wide brass strips at the hobby store sells work perfect, and come in different thicknesses. .015" should work fine.
Good luck.

Adrian

BrAdam said:

Didn't get to test tonight. Tomorrow will likely happen. I think the shim inlay is a good idea but tubes are already snug and not sure I want to rebuild that section. Taped sections also a good idea and might try. I think finger crochet of the shock cord would do the same thing. Will likely test both. Thanks for the help. Want to fly this end of the month at the WOOSH launch weather permitting. Chadrog will you be there?


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Another option , if you're going to use shear pins, is sharpened washers. It's a bit putzy, but works very well in saving the cardboard. Works as well in av bays as it does in nose cones.

CarVac said:

I've experimented with this a bunch, and find the amount of energy dissipated questionable.

Think about it like this: work = force * distance, and the distance required to snap the tape is really short, resulting in very little dissipated energy.

Now on the other hand, it helps keep the shock cord really neat, and it certainly doesn't hurt, but unless you use some really stretchy tape I don't see it making much of a difference.

(I've done it way crazy with bundles of bundles of bundles, and they all just snap open without much force. Maybe it just isn't as effective on big rockets as on smaller ones.)

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Chad showed me this trick and I use it on almost every rocket I fly. I believe that it does help reduce the amount of "snap" as your cord gets to Max length. I believe that resistance and weight of the bundles as well as amount of BP plays a factor. How to put a number to that is something I'm not sure of. While the "add a bit more BP" may give some a bit more of a sense of security it probably minimizes the effectiveness of the bundling.

Rrwjr said:

Chad showed me this trick and I use it on almost every rocket I fly. I believe that it does help reduce the amount of "snap" as your cord gets to Max length. I believe that resistance and weight of the bundles as well as amount of BP plays a factor. How to put a number to that is something I'm not sure of. While the "add a bit more BP" may give some a bit more of a sense of security it probably minimizes the effectiveness of the bundling.

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Nope, forget about it Russ - Carlo said it doesn't help, it doesn't help...:eyeroll:

chadrog said:

Nope, forget about it Russ - Carlo said it doesn't help, it doesn't help...:eyeroll:

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:wink: I was thinkin the same thing...
Youth... wait till his first time he is wrong professionally... the world (universe/science) has a way of humbling us all.
He's a cool guy.

Rrwjr said:

Chad showed me this trick and I use it on almost every rocket I fly. I believe that it does help reduce the amount of "snap" as your cord gets to Max length. I believe that resistance and weight of the bundles as well as amount of BP plays a factor. How to put a number to that is something I'm not sure of. While the "add a bit more BP" may give some a bit more of a sense of security it probably minimizes the effectiveness of the bundling.

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It helps if nothing else it keeps everything orderly as you pack it and helps when there is minimal space. It's worth doing. And even it its useless, it makes me fell better.

Chad, I do have brass shim in the nose cone now. I think it will help but am afraid that after several ejections the tube will still wear. Based on how the lower half is wearing I'm not sure the tube I have will stand up. You can get a first hand look at what I am dealing with at the launch. As for inlay of shim in the tube, I cannot do that now. The top of the tube is CA'd and there is no way I can peal back a layer or two. Maybe on a new tube but not on this one.
Do you just use masking tape to tape bundles?

BrAdam said:

Chad, I do have brass shim in the nose cone now. I think it will help but am afraid that after several ejections the tube will still wear. Based on how the lower half is wearing I'm not sure the tube I have will stand up. You can get a first hand look at what I am dealing with at the launch. As for inlay of shim in the tube, I cannot do that now. The top of the tube is CA'd and there is no way I can peal back a layer or two. Maybe on a new tube but not on this one.
Do you just use masking tape to tape bundles?

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I use blue tape for my bundles. Maybe duct tape could work better, though. I should test that.

BrAdam said:

Chad, I do have brass shim in the nose cone now. I think it will help but am afraid that after several ejections the tube will still wear. Based on how the lower half is wearing I'm not sure the tube I have will stand up. You can get a first hand look at what I am dealing with at the launch. As for inlay of shim in the tube, I cannot do that now. The top of the tube is CA'd and there is no way I can peal back a layer or two. Maybe on a new tube but not on this one.
Do you just use masking tape to tape bundles?

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I use masking tape, whichever color is at hand. A wrap and a half to two wraps is all I put on. I've had the unfortunate event of the main ejecting due to drogue ejection shock, doing this has stopped that from happening in situations where it used to. With a cutter of some sort installed, and flying at Bong, you'll likely lose the rocket before you wear it out - that's been my experience anyway. What time do you plan to get there Saturday?

Since it's a bit of a hike for me (Watertown to Kansasville) I try to get to the launches first thing and stay most or all of the day. Will try to get there by 10:00.

CarVac said:

I use blue tape for my bundles. Maybe duct tape could work better, though. I should test that.

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I have my doubts that Duct tape would tear at all. :sigh:
I use one wrap of any kind of masking tape I have on hand to hold the shock cord loops together and I haven't had a problem.
Now about an alternative to shear pins and friction fit etc. We had a young man that flew with us that used masking tape as shear pins. He applied strips on the outside of the rocket across the body tube and nose cone junction points. You adjust the number and width of the strips to suit the rocket and the flight profile.
It's not "pretty" but it works.
I have tried it several times with a successful outcome every time. :smile:

Sailorbill said:

I have my doubts that Duct tape would tear at all. :sigh:
I use one wrap of any kind of masking tape I have on hand to hold the shock cord loops together and I haven't had a problem.
Now about an alternative to shear pins and friction fit etc. We had a young man that flew with us that used masking tape as shear pins. He applied strips on the outside of the rocket across the body tube and nose cone junction points. You adjust the number and width of the strips to suit the rocket and the flight profile.
It's not "pretty" but it works.
I have tried it several times with a successful outcome every time. :smile:

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I'm sure duct tape would tear for a 20+ pound rocket, which would probably completely ignore blue tape.


I have also used blue tape in the manner you describe before. If you have forgotten your shear pins it works just fine on smaller rockets. And paper ones that you don't want torn up.

FWIW- I use the little rubber bands they use on dental braces for the smaller Kevlar and about 1" dia (relaxed)bands doubled over for the larger stuff. Load them on your finger and as you are wrapping up the shock cord drop one on every 2' or so. For really heavy webbing, I've used 'pony-tail' bands. Haven't blown a chute panel in 15 or so years. Just my 2 cents.

CarVac said:

I have also used blue tape in the manner you describe before. If you have forgotten your shear pins it works just fine on smaller rockets. And paper ones that you don't want torn up.

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+1 on this.

I've looked at forces at apogee (where tape held nose in place preventing an early main deployment) from a few flights and the tape can survive a good bit of force. I've seen loads of 40lbs shared across 3 tape strips. Others have pointed out to me that there are limits to this technique... a corner up in the wind might cause a tape strip to be blown off, aerodynamic heating from high speed flights can melt the tape, etc. But I like it for smaller cardboard rockets, very easy to set up. That's how I did DD with my Madcow 2.6" DX3.

Well, the good and the bad report. Tape was enough to survive the drogue event. .5 grams is enough. The bad is .5 grams was not enough to eject a tightly packed main section and 1 shear pin. In my hammer mechanic mentality the blow it up or blow it out went from .5 grams to 1 gram. That was enough to blow it out and blow it up. So…. I need a new tube with an inch or two more space to allow me to fit recovery hardware in there.

The report on the sheer pin is that the brass insert and the CA'd body tube seemed to survive the event so I will try to fly with 1 sheer pin when I get things fixed.

Anybody know how close LOC body tube and Madcow body tube are in dimensions. Might be able to get Wildman to bring a tube to the launch and retrofit the rocket at the launch for flight.

BrAdam said:

Anybody know how close LOC body tube and Madcow body tube are in dimensions.

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I'm pretty sure it's identical.

Are you using one shear pin, not two? Bad idea, because one can hang up from asymmetry. FWIW, I use about 0.8 grams on my DX3 with #2 nylon two shear pins, but my tubes are fiberglassed which helps the tube from getting ripped up.

I use one. Do it all the time and never had a problem.

BrAdam said:

I use one. Do it all the time and never had a problem.

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Didn't you just blow your rocket up in a ground test? Not saying it was related, necessarily

Perhaps not an authoritative source, but https://www.info-central.org/?article=303 says

Don't use less than two shear pins because it's possible the nosecone can cock to one side and jam if you only use one.

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Anyway, all I'm saying is that 0.8g and two pins works for me, YMMV. Good luck with your flight!

yes but two would only have made the force to eject higher and the destruction to the tube worse. One would only be a problem if you put it near the edge of the body tube and had very little engagement between the tubes. The pin would act as a pivot and cause the inside tube to bend at the center of the pin. This would cause jamming. If you have several inches of engagement and the pin in the lower section of the tube the bending cannot occur and one or two pins in likely not a problem. I blew up the rocket because I put too much powder in the ejection charge. I could have had 50 feet recovery harness in there and hit the end of it. In any event I need a new tube and probably 2 more inches in length to stuff my recovery hardware in. Then, I think .5 to .8 grams is acceptable. From taking a look at video and the pin, the pin sheered clean the the nose cone left the tube as the tube was erupting. Again one should not be a problem if the design allows it.