Need Help on my Graduator

[DynaSoar]

Originally posted by Mad Rocketeer
Question: How should I attach the shock cord and parachute? I think the stock shock cord is quite likely sufficient, and I'm convinced that the stock attachment is fine. I usually use fishing swivel clips to attach the cord and/or chute to make them interchangable, replacable, externally storable, etc., and to help prevent tangles. This one's too big for those lightweight, somewhat flimsy clips though. I'd like to use quick links if they're not too heavy. What size, metal, weight, etc. should I be looking for for this model? The smallest ones I see at Lowes still seem too large to me. I'd probably loop the shock cord attachment line through a small ring of some kind, like a key ring, before attaching it. Then I'd tie the shock cord to that (so it would be less likely to be cut by the attachment, which looks like Kevlar maybe) and to the nose cone. Then I'd tie a butterfy knot a foot or so from the nose and use the quick link to attach the parachute to that. Thoughts?

Thanks for mentoring me through this one, guys!

I like the LOC shock cord anchor method. I've gone to using that on my scratch birds, except I use 240 lbs. test kevlar instead of nylon cord.

I use quick links on pretty much everything these days. The smallest ones at Lowe's (1/8") are fine. They're rated at 220 lbs. They do seem a little large, but that just means they're easy to work with.

I tie a loop in the end of the shock cord and loop that through the quick link so it's double strength, tightens whoen pulled on, but tightens on the quick link rather than crimping itself causing it to cut through. See the thread in Support & Recovery: "Attaching shroud lines to a swivel" for pictures how to. I either use the quick link to attach shock cord, chute and nose all together, or I use the loop method to attach the shock cord to the nose, tie a loop into the shock cord at an appropriate distance from the nose, and use the same method to attach the chute there.

BTW, quick link make it real easy to unravel tangled shroud lines. Just open it and let them loose. If you let the shrouds slide through the link, they'll tend to balance themselves and be less likely to spin. However, they still tangle A heavy duty swivel somewhere between the shrouds and the shock cord is still a good idea.

 

[Mad Rocketeer]

Originally posted by DynaSoar
BTW, quick link make it real easy to unravel tangled shroud lines. Just open it and let them loose. If you let the shrouds slide through the link, they'll tend to balance themselves and be less likely to spin. However, they still tangle A heavy duty swivel somewhere between the shrouds and the shock cord is still a good idea.

I've seen those as barrel swivels in nickle or ball bearing swivels in stainless steel on rocketry sites, but can you get acceptable ones at Lowes (or similar)? If so, I may do that with the Graduator. Otherwise, I'll just use a quick link for now and add a swivel when I can.

I've not looked on Wal-Mart, Lowes, or hobby shops for Kevlar twine or fishing line. Is it easy to find, or am I better off looking on-line to get the right stuff? Is it worth waiting and buying some on-line, or is the nylon provided in the kit good enough to go with for now?

 

[DynaSoar]

Originally posted by Mad Rocketeer
I've seen those as barrel swivels in nickle or ball bearing swivels in stainless steel on rocketry sites, but can you get acceptable ones at Lowes (or similar)? If so, I may do that with the Graduator. Otherwise, I'll just use a quick link for now and add a swivel when I can.

I've not looked on Wal-Mart, Lowes, or hobby shops for Kevlar twine or fishing line. Is it easy to find, or am I better off looking on-line to get the right stuff? Is it worth waiting and buying some on-line, or is the nylon provided in the kit good enough to go with for now?

Lowe's has some that'd work, but I've never used them. Until recently I've only used the quick links and untangled afterwards. I'm going to start using them though -- I had a shock cord get real twisted recently.

You can get real heavy duty swivels that'd handle the shock of a high speed deployment. That doesn't matter so much with an elastic shock cord. Even when I use tubular nylon, I use an eleastic bungee in it for shock absorbtion. I haven't needed anything yet stronger than 200 lbs or so rating.

I've only bought kevlar thread from eBay. There was a vendor selling an assortment of about 6 or 7 different weights for a reasonable price.

Since you've got the nylon already, I'd use it. I used it on my Weasel and Vulcanite. There's no reason to switch to kevlar on a long rocket. The anchor is far enough from the ejection charge that there's no heat problem.

 

[Mad Rocketeer]

Originally posted by DynaSoar
Lowe's has some that'd work, but I've never used them. Until recently I've only used the quick links and untangled afterwards. I'm going to start using them though -- I had a shock cord get real twisted recently.

I think I'm going to go the same way. I'd considered adding a simple spit ring, like a keychain ring, to the shock cord mount loop to hook the shock cord to so it wouldn't be cut, but given the fact that the mount twine is nylon rather than Kevlar and the shock cord seems to be of a good length, I think I'll just tie a bowline in the end of the shock cord and hook that loop into the shock cord mount loop with a lark's head. That way, I can put a dab of wood glue on the bowline to keep if from coming loose (though the bowline is known for being a knot that doesn't slip) and still have an easy way to remove the shock cord from that end. The bowline would be glued, and the lark's head couldn't come free without having the other end of the shock cord loose. Yet, if I did unhook that end from the nose, the lark's head comes loose easily. I could just tie a sheet bend (good for dissimilar sizes like the twine and the wide shock cord), but it would lack some of the advantages of the first method.

I could either tie the other end to the nose cone or hook a split ring through the nose cone loop, tie another bowline at that end (through the split ring), and glue the knot. That way I could work the cord through the ring if I needed that end free. I'd then go about 3 or 4 inches from the nose end of the shock cord and tie a butterfly knot, making a loop that I could hook the parachute to via a quick link. The split ring (whose strength I don't really trust too far) would be holding only the nose.

Originally posted by DynaSoar
I've only bought kevlar thread from eBay. There was a vendor selling an assortment of about 6 or 7 different weights for a reasonable price.

Yeah, I saw that thread too at the time and looked at the Kevlar. I didn't buy any yet though. I may go back or buy some at pogee, etc.

Originally posted by DynaSoar
Since you've got the nylon already, I'd use it. I used it on my Weasel and Vulcanite. There's no reason to switch to kevlar on a long rocket. The anchor is far enough from the ejection charge that there's no heat problem.

Yep. When I got a closer look at the stock twine, it looked plenty strong. I used it.

The day after finishing the fin can, I found this in a stack of rocketry stuff I'd printed off ti file and refer back to. It's an article someone wrote on Building The Graduator. I had liked most of the ideas and had originally planned to build it pretty much this way, except no third CR or baffle. I wish I hadn't let it get burried.

I put a pair of 6-32 blind nuts in as the very first step. Good for positive retention, and good to give you something to pull on to remove the rear CR after the front CR and MMT have been glued in. (I friction fit the rear CR in to keep everything aligned for that part of the process, then removed it again until I had my internal fillets done. I'd planned to use Kaplow Klips but bought too narrow a brass strip. My current thought is to replace the screws with wider-headed and longer-shafted ones and use washers instead of clips to hold the motor in. I'll still need brass tubing for spacers. Making the clips would be easy enough too, once I have a motor or motors that will fit this beast. (Well, I do have some C11's that I could use to measure the clip reach needed for 24 mm motors in the adapeter.) Any reason to prefer one method over the other?

I added more balloons for the launch lug fillets, and lo and behold I was able to do both at the same time, and they retained the curve I originally gave them rather than flattening out like the fin fillets did. They'll need no filing for the curve, but I'll do just a little at the leading and trailing edges of the fillet to streamline them a bit.

 

[Mad Rocketeer]

Originally posted by DynaSoar
Lowe's has some that'd work, but I've never used them. Until recently I've only used the quick links and untangled afterwards. I'm going to start using them though -- I had a shock cord get real twisted recently.

I think I'm going to go quick link only. I'd considered adding a simple spit ring, like a keychain ring, to the shock cord mount loop to hook the shock cord to so it wouldn't be cut, but given the fact that the mount twine is nylon rather than Kevlar and the shock cord seems to be of a good length, I think I'll just tie a bowline in the end of the shock cord and hook that loop into the shock cord mount loop with a lark's head. That way, I can put a dab of wood glue on the bowline to keep if from coming loose (though the bowline is known for being a knot that doesn't slip) and still have an easy way to remove the shock cord from that end. The bowline would be glued, and the lark's head couldn't come free without having the other end of the shock cord loose. Yet, if I did unhook that end from the nose, the lark's head comes loose easily. I could just tie a sheet bend (good for dissimilar sizes like the twine and the wide shock cord), but it would lack some of the advantages of the first method.

I would then either tie the other end of the shock cord to the nose cone loop or add a medium split ring (like a keychain ring) to the nose cone loop, tie another bowline in the shock cord (through the split ring), and put a bit of glue on the knot. I could work the shock cord through the split ring if I ever needed it free. Then I'd tie a butterfly knot 3 or 4 inches from the nose cone end of the shock cord and use quick link to hook the parachute shrouds to it. That way, the parachute lines could move freely through the link, helping to prevent tangles, as you say. Also, the split ring (the strength of which I don't trust 100%) would be holding only the nose cone's weight and momentum.

Originally posted by DynaSoar
I've only bought kevlar thread from eBay. There was a vendor selling an assortment of about 6 or 7 different weights for a reasonable price.

Yeah, I saw that thread at the time too and looked at the Kevlar. I didn't buy any yet though. I may go back or buy some at pogee, etc.

Originally posted by DynaSoar
Since you've got the nylon already, I'd use it. I used it on my Weasel and Vulcanite. There's no reason to switch to kevlar on a long rocket. The anchor is far enough from the ejection charge that there's no heat problem.

Yep. When I got a closer look at the twine, it looked plenty strong. I used it.

The day after finishing the fin can, I found this in a stack of rocketry stuff I'd printed off to file and refer back to. It's an article someone wrote on Building The Graduator. I had liked most of the ideas and had originally planned to build it pretty much this way, except no third CR or baffle. I wish I hadnt let it get buried.

I put a pair of 6-32 blind nuts in as the very first step. Good for positive retention, and good to give you something to pull on to remove the rear CR after the front CR and MMT have been glued in. (I friction fit the rear CR in to keep everything aligned for that part of the process, then removed it until I had my internal fillets done. I'd planned to use Kaplow Klips but bought too narrow a brass strip. My current thought is to replace the screws with wider-headed and longer-shafted ones and use washers instead of clips to hold the motor in. I'll still need brass tubing for spacers. Any reason to prefer one method over the other?

I added more balloons for the launch lug fillets, and lo and behold I was able to do both at the same time, and they retained the curve I originally gave them rather than flattening out like the fin fillets did. They'll need no filing for the curve, but I'll do just a little at the leading and trailing edges of the fillet to streamline them a bit.