Shock cord material for lpr/mpr

[Tractionengines]

I extended my Goblin clone to fit the baffle. Really flew well with the JLCR.

NICE... What is the size of that "Streched Goblin"?
Here is a scratch built stretched fiberglass one I just finished. (Named GOBLINESQUE). Hasn't flown yet. 2.25" OD, ~32"long body tube.

Flight ready, and loaded with a DMS G80-14NBT it weighs 1375grams and sims right at 1000ft. (I added a few grams for extra wadding. And I still have a little to the class 1 limit, if I add an altimiter, or beeper, etc)


Mike

 

[Ronz Rocketz]

NICE... What is the size of that "Streched Goblin"?
Here is a scratch built stretched fiberglass one I just finished. (Named GOBLINESQUE). Hasn't flown yet. 2.25" OD, ~32"long body tube.
View attachment 473919
Flight ready, and loaded with a DMS G80-14NBT it weighs 1375grams and sims right at 1000ft. (I added a few grams for extra wadding. And I still have a little to the class 1 limit, if I add an altimiter, or beeper, etc)
View attachment 473920

Mike

Awesome. I added a couple inches. I’d do the same but not sure how to get a G motor here. I do know a guy…

 

[hcmbanjo]

That sounds about right. I use a lot of TB2 in constructing the baffle plus soak the joints with thin CA. Didn’t think about protecting the fire side with more adhesive but haven’t seen damage to it yet because of the tough construction.

Protecting the fire side of a baffle is a good idea.
I used a single coat of wood glue on my old Centurion.
After 15 BP launches the low side of the baffle was in great shape
and the plate was .050" thick cardstock.

http://modelrocketbuilding.blogspot.com/2013/01/centurion-autopsy.html

 

[Joshua Smith]

Protecting the fire side of a baffle is a good idea.
I used a single coat of wood glue on my old Centurion.
After 15 BP launches the low side of the baffle was in great shape
and the plate was .050" thick cardstock.

http://modelrocketbuilding.blogspot.com/2013/01/centurion-autopsy.html

That's a awesome idea and writeup @hcmbanjo ! Out of curiosity, did you generally fly with the baffle only, or did you put some wadding between the baffle and chute? How did your chutes hold up? Sounds like you were able salvage them so they must have held up well.

 

[Ronz Rocketz]

Protecting the fire side of a baffle is a good idea.
I used a single coat of wood glue on my old Centurion.
After 15 BP launches the low side of the baffle was in great shape
and the plate was .050" thick cardstock.

http://modelrocketbuilding.blogspot.com/2013/01/centurion-autopsy.html

I can definitely see using a coat of glue on the cardstock. The included baffle in the AMRAAM is quite aways from the MM tube so should be fine. The Qualman baffles are a different breed. I should use more thin CA to soak into the fire side wood.

 

[hcmbanjo]

That's a awesome idea and writeup @hcmbanjo ! Out of curiosity, did you generally fly with the baffle only, or did you put some wadding between the baffle and chute? How did your chutes hold up? Sounds like you were able salvage them so they must have held up well.

I usually used one sheet of Estes style wadding like a "peel-away" wrap on the bottom 2/3rds of the packed parachute.
The original Centurion instructions are shown below -
The Centuri Centurion was one of the first model rockets to incorporate a "plate" style baffle.

 

[Tractionengines]

Since this is the Recovery forum...here is a mockup of loading the recovery for this rocket. NOT loading for an actual flight, but just for show. Note: since I am pushing close to the 1500 gram class 1 limit; this is kind of heavy-duty comparatively.
From MMT forward. (2) 10-32 u-bolts from cable clamps. 1/8" tubular-kevlar (950#) that extends 30" past the end of airframe. With 6 inches of dual wall hi-temp heatshrink as protection over kevlar at MMT, 2" at end of airframe, and about an inch to form a loop at the end (without tying a overhand knot). Then a blanket (should be next size bigger). Then 15ft of 3/8" nylon (900#) to a swivel and quicklink. Also on the quicklink is a swivel to a 4ft Rocketman parachute, and ~18in length of cord to the nosecone (1/8" kevlar only because I had it laying around.) The taped groups for real flight, would us varying number of wraps, and varying layers of tape.
(Options: If I go big and put in a H or I205 motor and go to a HPR field; I can easily swap in a nosecone I have with a Eggfinder tracker. I can fit a JLCR. I can change to 3ft parachute to get down faster. Etc...)



Mike

 

[4regt4]

Oops. Crap. I see that my question was answered in a prior post. I'm new to the forum, and didn't see that there are more than one page to this thread......

Kevlar from the MMT up to above the BT break point and then either nylon or elastic from there. Best of both worlds with the kevlar in the area where it'll be exposed to the direct ejection charge and will be long-lasting and the shock absorbing of the nylon or elastic up to the nose and parachute. Nylon/elastic is replaceable when it becomes old. You can also right-size the kevlar to a lower WLL since most of the shock will be absorbed by something other than kevlar which doesn't stretch but snaps.

I've used steel fishing leaders but they don't hold up in the corrosive BP residue as well as the kevlar.

Noob question here. I've done a few dozen flights, but they have all been small-ish Estes kits with the stock rubber band....

How to best to join 2 different shock cord materials? Will a simple knot hold, perhaps with a drop of glue? How about a snap swivel? In particular, I'm wondering about 1/4" elastic joined to kevlar. Looks like a knot in elastic might undo itself without some help holding it together..

I'm building a bigger kit, and I'll likely use a baffle and connect the shock cord to it.

Hans.

 

[Tractionengines]

Welcome to the forum! Ask away. Lots of topics come back all the time, don't worry about it. Sometimes we even change the answers over time....

 

[neil_w]

How to best to join 2 different shock cord materials? Will a simple knot hold, perhaps with a drop of glue? How about a snap swivel? In particular, I'm wondering about 1/4" elastic joined to kevlar. Looks like a knot in elastic might undo itself without some help holding it together..

This is what I do.

First I tie a loop in the Kevlar, that gets installed into the rocket with the end of the loop just shy of the end of the body tube. Then I pull it out the back of the rocket and attach the elastic by threading the elastic through as shown. Then pull tight. This actually creates a square knot, although not in the same way one would normally tie it.

When necessary, I have attached the elastic to the Kevlar even when the Kevlar loop is still inside the body tube. This is a very easy maneuver.

In my assessment, this creates an incredibly strong join between the two. The tying of the loops on the Kevlar and elastic don't even seem to matter, the friction between the Kevlar and elastic in the square knot is so strong. It ain't going anywhere, at least for my sized rockets (LPR up to E/F motors.)

"Works for me".

 

[heada]

For low power rockets, I just tie them together with an over-hand knot. The rocket wears out or gets lost before the knot fails. But that knot connection will fail. The loops as seen above is the much better way to do it. On my high power rockets I attach them with loops and a stainless quick link.

 

[Jacktango22]

I have actually used steel trolling line from my deep sea excess gear. I bend it around the motor mount and top it off with a loop just outside the body tube. Use shrink wrap to prevent zippering. Then I can attach elastic or Kevlar to the steel loop as I desire for the launch in question.

 

[Lee]

When tying kevlar directly to elastic, inspect often. Kevlar is braided and can cut. As a fisherman, I've learned not to use braided line on a rod or reel designed for monofilament line - it will cut grooves in the guides and bail.

 

[cwbullet]

Avoid elastic unless it is in your underwear. Use kevlar and nylon cord.

 

[Jacktango22]

Avoid elastic unless it is in your underwear. Use kevlar and nylon cord.

Yup. Came to that conclusion. Thanks

 

[MidOH]

I was just about to go to a girly store to get elastic for my JLCR.

Just going to chop up someone's underwear instead.

 

[bjphoenix]

I was just about to go to a girly store to get elastic for my JLCR.

Just going to chop up someone's underwear instead.

Some years back I was using the white fabric elastic from the sewing store. It generally worked well but can get singed and will eventually degrade just like the old rubber stuff. For LPR I had good enough luck with the elastic. I used the widest elastic I could find for my level 1 rocket, maybe about 1" wide, and it worked for a half dozen flights then one day it tore upon deployment and dropped the payload section with no recovery device.

The problems with the normal rubber are that it degrades with time, and the piece provided with the kit is usually not long enough so it can cause the nose cone to snap back into the body tube. Adding kevlar to the rubber helps with this. Or replacing with a longer piece of elastic. It helps if you use some construction method that allows for the shock cord to be easily replaced when it degrades.

I've been using 10' long thin kevlar for my LPR and it works well, but being long and thin it is hard to use without getting tangles.

 

[4regt4]

Some of the MPR kit providers - Aerotech and North Coast come to mind - supply a black elastic (roughly 1/2" wide) with their kits. It seems to be better quality than any I've seen at craft or fabric stores. I wonder what it is, and if it is available at local stores.

Hans.