Attachment of parachute.

[Frausing]

I am building the small AEROTECH INITIATOR.

I have built the engine tube now.

When I glue the motor tube to the rocket, it is not possible to get to the eye / mounting point at the top of the motor tube any more.
So I want to tie the right knot on the elastic cord, but which one should i use. ?
After that I have to find the optimal length where the parachute should go. the elastic cord is 2.5 meters (100 inches), Do I just ties it approx in the middle ?
and after that I have to tie it firmly in the nose cone, but there I am sure to I just use the same knot as for the motor tube.
How will you attach the parachute to the elastic string?

Hope some of you can help.

 

[techrat]

As you likely want a loop around the mount point atop the Motor Mount, I would suggest a Surgeon's Loop (knot). Since you can't get your hands into the body tube to tie such a knot, you've got to attach it to the MMT *before* you glue that into the airframe, and thread the shock cord through the airframe before glue gets applied to hold the MMT in place. At least, that's what I would do under those circumstances, but hey, I'm no expert.

 

[jmasterj]

Bowline, doubled figure-8, doubled overhand knot, even two half hitches will work fine. A dab of white glue will make sure it doesn't come loose.

For the chute, I don't know what optimal is but I would put it maybe 25cm down from the nose cone. You want those two parts relatively close together so that the nose cone will pull the chute out at ejection.

 

[techrat]

For the chute, I don't know what optimal is but I would put it maybe 25cm down from the nose cone. You want those two parts relatively close together so that the nose cone will pull the chute out at ejection.

Now would be a good time to mention adding a fishing swivel, so the cute stays as tangle-free as possible.

 

[Frausing]

Thanks guys
I will look at the 2 knots
Yes the shock cord shall through the airframe before.
A fishing swivel also sound like a real good idea, it will do so it must be easyer to handle the parachute after it lands an repack it.
I will also set the chute closer to the nose cone (25cm), it sound right to have it closer as you say.
Thanks

 

[waltr]

What I do is use heavy Kevlar through the Eyebolt. Then bring it up and tie it with a figure-8 knot above the BT. This allows pulling the loop around to inspect the entire length. Also allows replacement. Untie and tie/tape the end to the new piece of Kevlar. then pull it down and through the eyebolt and back to the top of the BT. Remove old piece and tie ends together.

Shock cord, nylon, Kevlar, elastic, gets tied to this loop.

I like the chute attached 1/3 way from nose to BT on the shock cord.

 

[Frausing]

I like the way your thinking, so I can change the cord going through the eye in the body tube.
and 1/3 is a good way to remember it, to other rockets when I build more.

 

[David Schwantz]

A good simple overhand knot to form a loop that you can pull thru the eyebolt and back into itself. Can change it at anytime as it is simple to loosen at the eyebolt. Chute needs to be tied in place on cord near enough the NC so that it will not hit airframe during descent.

 

[Frausing]

I just tried tying a Figure 8 Double Loop overhand knot as suggested earlier, it won't work for this.
And the Surgeon's Loop knot there I will have to get the motortube through the knot.
Just if anyone else need to try this.
I will make a kevlar cord through the Eyebolt. Then bring it up and tie it above the bodytube. so I can inspect it.
Then I will make sure that the nose cone can't hit the bodytube. I will make the elastic cord a little longer. I did read about a Zippered Tube here Https://www.rocketryforum.com/threads/zippered-tube-time-to-lengthen-shock-cord.173144/
I am grateful to you all. Thanks

 

[dr-ws]

I just tried tying a Figure 8 Double Loop overhand knot as suggested earlier, it won't work for this.
And the Surgeon's Loop knot there I will have to get the motortube through the knot.
Just if anyone else need to try this.
I will make a kevlar cord through the Eyebolt. Then bring it up and tie it above the bodytube. so I can inspect it.
Then I will make sure that the nose cone can't hit the bodytube. I will make the elastic cord a little longer. I did read about a Zippered Tube here Https://www.rocketryforum.com/threads/zippered-tube-time-to-lengthen-shock-cord.173144/
I am grateful to you all. Thanks

Posted above are several good suggestions to ponder and effectuate, but one additional consideration: at the top of the Kevlar [from the eyebolt atop the fore motor mount centering ring] cord (yes, I would try one of the anti-zippering methods here, too), I would use some sort of light metal ring (a smaller quick-link? a small keyring? even a rounded washer?) to which to connect dissimilar stringlike materials. I'm a Kevlar guy (John Coker: "Elastic is for underwear, not rockets."), but Kevlar cuts [eventually] everything, so join to it using metal, I say.
Also, I agree with the thought to connect your parachute at less-than-halfway along the shock cord joining the body tube/airframe and nose cone, because: 1) you don't want your nose cone and airframe bouncing off one another all the way to the ground; 2) less chance to get tangled up together; 3) depending on your rocket, your airframe and nose cone will tend to descend at differing velocities.
Denmark, huh? Cool! I hope to meet you someday! Best of luck!

 

[techrat]

I'm using keyrings -- because I have so many keychains. Go to any trade show, and vendors give away keychains with their company logo on some plastic bit, I have a drawer full of these things. So, they came at the right price: free. I cut away or otherwise remove the company logo plastic bit, leaving me just the metal ring, and use that as my connector between the nose cone (or Kevlar) and what I then use to attach my chute (with a swivel). It all stays pretty lightweight, and even so, a little extra nose weight never hurt anybody.

 

[David Schwantz]

Dude, you are doing it all wrong. Send those logos on the keychain up in the rocket. When you recover it, take pics and send them to the company. And of course, ask for sponsorship

 

[techrat]

Dude, you are doing it all wrong. Send those logos on the keychain up in the rocket. When you recover it, take pics and send them to the company. And of course, ask for sponsorship

Now you're making me think we should decorate rockets like NASCAR, and try and get sponsors!

 

[David Schwantz]

I think a guy did that already. He flew a disc golf thing.

 

[waltr]

As to not zipping the BT.
What I have been doing and works very well is either make three bundles of the Kevlar shock cord and tape them. The force of ejecting the nose pulls these bundles apart absorbing energy.
Second method is to braid the shock cord. Tim at Apogee Rockets has a video on how to do this with a small (18mm) rocket cord. This works well for much larger rockets. Two antidotes:
Buddy in our local club Braids the 1" wide nylon cord on his 43 pound L3 rocket. He learned this from the TAPS when building for L3 cert.

I use this on my LOC 4" Goblin (3.2 pound). Last flight the delay should have been 5 seconds but went to 10 second (I must have done something wrong drilling the delay). Apogee to about 600 feet, nosed over and ejection fired about 150 feet up.
Chute deployed and there was NO Damage to chute harness or BT. This has the loop of heavy Kevlar out the BT as I described above then 1/2" nylon that was braided.

 

[bjphoenix]

I built a 2.6" LOC kit many years ago and I was trying to figure out how to get the cord through the eyebolt. My young daughter grabbed the cord, stuck her arm down the tube and threaded it through.
Many years later I cut that cord out and replaced it. I taped a hobby knife on the end of a stick to cut through the cord, then I got a couple of bent coathangers. I used one to reach in there and feed the loop through the eyebolt, then I used the other one to reach down and grab the look and pull it back to the end of the tube. I put the free end of the cord through the loop and then pulled the free end out pulling the loop back to the eyebolt.

 

[Dane Ronnow]

Here's how I did the motor tube attachment on my Aerotech HV Arcas:



The loop above the yellow heat shrink extends about 4" outside the body tube. The main shock cord (20' of braided Kevlar on mine) ties to that loop. This lower tether can be pulled through the screw eye to inspect or replace. Inside the white heat shrink is a square knot to join the ends. Inside the yellow, a single overhand knot to form the loop.

 

[waltr]

Looks good and basically what I do.

 

[Frausing]

So are the rocket done.
Thanks to you all for the great help.
Now I will try to find a program (OpenRocket or RockSim) and try to find center og pressure.
And then I still have to buy the motor
Thanks to you all

 

[jmasterj]

Looks great!