Designing a recovery system

[DJ Delorie]

Working on a 13mm Solar Sailer (BT-5 outer), starting to think about the recovery system. Since there's an inside tube and CRs, attaching a kevlar line to that is an obvious idea. However, my current inventory only includes 8lb kevlar thread, 1/4" shock cord, and some regular rubber bands. I've got 90lb kevlar on order though. Anyone have suggestions for cobbling these together in some way to build a recover system for this little rocket, or do I need to order other sized parts?

Also, suggestions on chute/streamer too. Rocksim says 1/2 oz empty, 3/4 oz with an A10 in it. Total rocket length is about 21", with about 8" of BT5 in which to stuff stuff. On a 1/4A it only goes 100ft up, so the faster it deploys the better.

 

[lalligood]

Originally posted by DJ Delorie
Also, suggestions on chute/streamer too. Rocksim says 1/2 oz empty, 3/4 oz with an A10 in it. Total rocket length is about 21", with about 8" of BT5 in which to stuff stuff. On
a 1/4A it only goes 100ft up, so the faster it deploys the better.

I'd say go wtih a streamer. Crepe paper or plastic would be more than acceptable. Anything that light would be too inclined to drift under chute & anything that small in diameter would be too difficult to pack a chute in either...

HTH,

 

[DJ Delorie]

[I'd say go wtih a streamer.

How big? Or rather, what descent speed should I target in rocksim?

 

[Fore Check]

So, this is a BT5 *outer* tube, with something smaller in the center section? Cool!

I'd personally suggest getting some 1/8" flat elastic from the sewing department at your local deparment store (Wal-Mart for instance.) Cheap, about $1 a pack.

If you don't want to wait for the 90lb kevlar, you *could* take about 11 or 12 equal sized lengths of your 8lb thread to use together as an approximate equivalent to the 90 lb stuff. You know, if you wanted a 12" piece of kevlar, cut 11 strands of 8lb thread 12" long, put them all together, and you end up with a 12" length of 88 lb kevlar made of 11 individual strands.

Alternatively, you could use the tri-fold (standard Estes/Flis shock cord mount.) But in a BT5 rocket, that could get dicey if you glue the tri-fold mount just below the nose cone because it creates a restriction that would inhibit the deployment of your streamer or whatever.

Solution: As you're building this model, you could glue the tri fold mount just inside *the bottom* of that upper tube before you glue the upper tube onto the central (smaller diameter) one. I suggest glueing that tri-fold mount in before putting the model the rest of the way together because it would be very difficult to push this mount 8" into a BT5 and glue it down successfully. You can cut a trifold paper mount out of scrap paper you have laying around.

 

[Fore Check]

I'd suggest some 1/4" wide white teflon pipe thread tape for streamer material. Heat resistant, thin, light, etc. Again, cheap ($1 or less) in the hardware department.

 

[DJ Delorie]

So, this is a BT5 *outer* tube, with something smaller in the center section? Cool!

Yeah, the Solar Sailer is a fun model because there's three BT sections involved, with lots of CRs, fins, dowels, etc. Lots of room for customizing. This particular one is BT5 outer with a BT3 inner, so the 1/4A engine presses right up against the rearmost CR-3-5. If you don't know what a Solar Sailer looks like, search for my Solar Sailer 3 thread. I've also started a 3x24 clustered SS which will be almost 9 feet tall (BT-60/70).

I'd personally suggest getting some 1/8" flat elastic from the sewing department at your local deparment store (Wal-Mart for instance.) Cheap, about $1 a pack.

Easy enough to do.

If you don't want to wait for the 90lb kevlar, you *could* take about 11 or 12 equal sized lengths of your 8lb thread to use together

The 90 is for another project, it just happens to be on the way. I was thinking I could braid three of the 8s together to make a 24lb cable, but the question is... how much strength does it NEED for that design?
And how long should it be?

Alternatively, you could use the tri-fold

The SS design kinda cries out for the kevlar option, especially in a BT5. Not much room to stuff stuff into.

Solution: As you're building this model, you could glue the tri fold mount just inside *the bottom* of that upper tube

The inner BT goes 2" into the outer BT, so I'd have to glue it about 2.5" up inside a BT5. Not going to happen

 

[DJ Delorie]

Originally posted by Fore Check
I'd suggest some 1/4" wide white teflon pipe thread tape for streamer material. Heat resistant, thin, light, etc. Again, cheap ($1 or less) in the hardware department.

Even cheaper when it's already sitting on my shelf. Why so narrow?

 

[Fore Check]

Originally posted by DJ Delorie

The 90 is for another project, it just happens to be on the way. I was thinking I could braid three of the 8s together to make a 24lb cable, but the question is... how much strength does it NEED for that design?
And how long should it be?

Hmmm....

How much strength is *necessary*? Difficult to answer. The thin thread used in Quest kits is the lightest stuff I've seen supplied in a kit for shock cord retention, but I don't know what it's strength is. I use 100# to 150# all the time, no matter what I'm building. I've never heard a discussion on the minimum.

I know Micromister uses a light kevlar with no elastic in his MMX rockets. I'd ask him what he uses there, and if you couple it with a nice length of elastic, it should work.

As far as how long - the rule of thumb on kevlar is to make it come to or just beyond the upper end of the upper tube, so you can replace the shock cord if necessary. As far as the elastic, I'd use as much as you can fit in that BT5, up to the "rule of thumb" of 3x the rocket length (63"). You could probably get away with half that pretty easily.

 

[Fore Check]

Originally posted by DJ Delorie
Even cheaper when it's already sitting on my shelf. Why so narrow?

Out of ignorance, I guess. That's just what I have. If you can get wider stuff, great!

 

[DJ Delorie]

FYI the project so far. Shown with a ruler and 13mm engine for scale, along with the drive section of the 8'9" 3x24 version.

 

[powderburner]

Originally posted by DJ Delorie
the question is... how much strength does it NEED for that design?

Good question. You would probably do just fine with a single length of your 8-lb-test kevlar, but just to make sure, a doubled- or tripled-up tether would certainly do the trick. On a 13mm rocket I just don't see any way that 90-lb kevlar is needed. (Think about it; what kind of separation velocities are required when the NC is ejected to cause that kind of loading? A 90 lb load would rip the rest of the rocket apart, even if the tether survives!)

As to length, I would suggest that a length of several (two-three?) feet would be enough. I'll bet some of these other TRF guys will give you their own rules-of-thumb?

 

[DJ Delorie]

Score! My wife happened to have a few 6 foot sections of 1/8" shock cord (er, elastic band) in her sewing kit, and let me have one of them. She also offered me some 1/4" and 1" cords, but I have plenty of 1/4" and I can't yet imagine building something that needs a 1" wide cord.

 

[lalligood]

Originally posted by DJ Delorie
How big? Or rather, what descent speed should I target in rocksim?

According to the EMRR Streamer Calculator, you'd be looking at a 1.9" x 19.3" streamer assuming a total rocket + empty motor casing weight of 1 ounce.

HTH,

 

[DJ Delorie]

Hmmm... maybe I'll mount the kevlar with a tiny swivel hook at the end, and worry about the rest of the system later? At least then I can weigh the model, rather than rely on calculated weight, and experiment with different chutes/streamers.

I found ROL's rules of thumb for this: recovery system rated for 50x model weight, shock cord 2-3x model length. That's 2.5 lbs for the kevlar (i.e. 8lbs is overkill) although the knots make it marginal. I think I'll braid three of them together for 24lbs or 12lbs with knots. I figure I'll make it long enough to have 5-10" sticking out for attachments and a couple of repairs.

48-72" of 1/8 shock cord... I'll have to see how easily it packs into the BT-5.

 

[DynaSoar]

Originally posted by DJ Delorie
Working on a 13mm Solar Sailer (BT-5 outer), starting to think about the recovery system. Since there's an inside tube and CRs, attaching a kevlar line to that is an obvious idea. However, my current inventory only includes 8lb kevlar thread, 1/4" shock cord, and some regular rubber bands. I've got 90lb kevlar on order though. Anyone have suggestions for cobbling these together in some way to build a recover system for this little rocket, or do I need to order other sized parts?

Also, suggestions on chute/streamer too. Rocksim says 1/2 oz empty, 3/4 oz with an A10 in it. Total rocket length is about 21", with about 8" of BT5 in which to stuff stuff. On a 1/4A it only goes 100ft up, so the faster it deploys the better.

Take the kevlar you've got, roll off a piece six times longer than the length you intend for the shock cord. Tie the ends together in a knot so you have a large loop. Double the loop by folding it over, and again to make it three loops. Stick your index fingers into the loop and pull it taut. Tie knots in each end (and the middle if you want). You now have a 48 pound shock cord.

If all you have is 1/4" elastic, I suggest you double that too. Just pull off twice the length, tie the ends to make a loop, and that's enough.

The nice thing about working with loops is they hook together so easily and certainly. Just stick one end of one loop through the other loop a short way, run the other end of the first loop through the part you stuck through the second loop, and pull it tight. There's no know at the joint, so it can't untie, and they way they're connected, when there's tension, it just pulls tighter

If you want to put a drop of CyA on the knots so they don't untie, that's fine. Just put it on the ends sticking out from the knot, and on the half of the knot facing them. Don't put it on the load bearing portions of the keval or elsatic, or the half of the knots facing those portions. It can make elastic fail, and if you make a piece of kevlar stiff with glue, the part right up against the stiff part will be the only part that bends, and be more likely to wear through.

I've done this many, many times, and have even used the looped kevlar embedded in a glob of epoxy on the inside of the tube as the shock cord anchor. Bury the whole end of it, with just the cords sticking out and the knot (a couple extra knots to be sure) completely embedded, and there's no way that cord will pull loose.

 

[DJ Delorie]

Followup... The 13mm got three 8lb strands braided into an 18" length, enough to have the end be a few inches outside the BT, tied a bowline on the end for the 36" 1/8" shock cord. The 3x24mm got two 90lb strands 2' long, twisted together in the middle then folded over and 4-way braided to form a loop with a 1' tail, enough to have the loop be an inch or two outside the BT.

THe 13mm is in paint now, the 3x24 is getting the dowels attached.