First HPR build

[redsox15]

I have about 6ft of shock cord that came in the kit I just got and I don't think I need to use all of it. What is a safe length to go with, without getting too short? The rocket I am building is the Fantom (4"kit) by LOC Precision

Thanks,
Matt

 

[judo]

3 x body tube length. I say, use it all.

 

[FROB]

I have about 6ft of shock cord that came in the kit I just got and I don't think I need to use all of it. What is a safe length to go with, without getting too short? The rocket I am building is the Fantom (4"kit) by LOC Precision

Thanks,
Matt

Matt,
you're right, in fact you don't need any of it, since it's waaay too short to begin with!
If this is your first HPR build, it's hard to appreciate just how forceful the ejection charge can (or should) be.
Opinions vary on this subject of course, but most people doing high power will use much longer and non-elastic shock cords- or recovery harnesses as we tend to call'em. We also use various tricks like sliding chain knots to help dissipate the shock and avoid them snapping back and smashing the nose cone into the booster and dinging them or worse.
I have several scratch and kit built models including a LOC bullet, with which i will use no less than 25 feet of 3/8" military nylon strapping for a harness. It coils up into a nice neat 3" disk which insures it doesn't tangle on ejection too-
If that doesn't sell you, you will agree the first time it lands in a tree- even with the chute tangled high in the branches, chances are the rocket will be hanging within reach by the time you get there.
A rule of thumb i use is, 5x diameter in inches is how many feet you should have as a bare minimum.

 

[DAllen]

Some peoples answer to "How much shock cord should I use?" is "How much can you fit?" Definitely use it all. In fact, only 6'? Man, I thought they'd pack more than that in there.

I have a 4" scratch built bird with 18' of 5/8" tubular cord which I thought was REALLY overkill. That was until I had an extra long delay on my L1 flight with an Ellis H275. Sucker was coming hot from 2600' and ejected at probably 200'. Not a bit of damage or even a hint of a crinkle in the body tube. Did manage to rub a bunch of paint off the leading edge of my fin onto the cord.

-DAllen

 

[Handeman]

Echo, Echo, Echo.....

6 feet is way too little shock cord for a 4 foot long 38mm MMT rocket.

 

[redsox15]

I was off by about 6'. I have a 12' shock cord, which from what I have seen from the replies, I should use every inch of it. (I have no idea how I missed by that much)

Thanks for the input, im sure I will have some more questions as the build goes on

Matt

 

[DAllen]

Fire away with the questions because that's what this forum is for. If it weren't for this forum I would not have learned as much as I have so I don't mind helping out where I can.

-DAllen

 

[pyrovette20]

You didnt mention what type shock cord also. If its a nylon or kevlar strap use as much as possibe it compacts down easily. The nylon has a little stretch the kevlar almost none. I use kevlar only now. A trick I learned is to fold the strap like an accordian about 3'-4' wrap a layer of masking tape around it. Repeat till done. As the parachute comes out it will cause tension on the line and start breaking the tape. It just helps absord some of the shock.

 

[Viperfixr]

If I've learned anything in the one year I've done HPR, it's that you almost cannot have enough recovery harness. I had two 20' sections for my recent level 2 cert flight, and the first part of it probably should have been longer. My level 1 rocket had 25' of kevlar harness and it was enough.

From what I've seen, not only does a longer harness give you less of a "shock" to the recovery system when it comes out & inflates, it lessens the likelihood that your two body tubes will smash into each other on the way down.

 

[redsox15]

The shock cord is an elastic nylon shock cord. It is definitely not kevlar.

Any suggestions on motor retention? I want to make my own because I don't think that wrapping the motor in masking tape and making a snug fit in the mount is the most sure-fire way of keeping a motor in the rocket.

Thanks
Matt

 

[DMcCauley]

I'd use about 20 feet.

PML sells some really nice 3/8" nylon shock cord which is perfect for small rockets.

 

[BsSmith]

This may be the wrong thread to tell you this, but don't tie your shock cord to the eyelet on the nose cone. Especially if this is your L1 rocket. I was not allowed to fly my L1 rocket until the shock cord was mounted to my nose cone differently.

 

[ben_ullman]

This may be the wrong thread to tell you this, but don't tie your shock cord to the eyelet on the nose cone. Especially if this is your L1 rocket. I was not allowed to fly my L1 rocket until the shock cord was mounted to my nose cone differently.

I don't think its a rule that it needs to be attached a different way. It is good to use another way.

Ben

 

[bobkrech]

The shock cord is an elastic nylon shock cord. It is definitely not kevlar.

Any suggestions on motor retention? I want to make my own because I don't think that wrapping the motor in masking tape and making a snug fit in the mount is the most sure-fire way of keeping a motor in the rocket.

Thanks
Matt

You want positive motor retention.

Check out https://www.info-central.org/construction_retention.shtml for some different ways to do it.

The simplest way is to use a pair of tee nuts mounted in the the lower motor mount ring and use screws and washers to prevent the motor from kicking out on ejection.

At CMASS you can tie the shock chord to the NC, however you want to tie the chute to the shock chord, not the NC. Make sure that you tie it so the the parachute is fully pulled out of the airframe by the NC/shock chord ejection otherwise you might find that the chute gets stuck in the airframe.

Bob

 

[kandsrockets]

The shock cord is an elastic nylon shock cord. It is definitely not kevlar.

Any suggestions on motor retention? I want to make my own because I don't think that wrapping the motor in masking tape and making a snug fit in the mount is the most sure-fire way of keeping a motor in the rocket.

Thanks
Matt

Throw that elastic in the trash and atleast get some tubular nylon. Elastic is for holding up boxers not for your rocket.

 

[DAllen]

Echoing everyone elses comments here. Elastic in HPR works - but sucks. You have many options for attaching a cord to a cone other than the eyelet. The eyelets work but also suck.

One way I have done with success is to cut a 2' piece of the cord and fold over on itself. Really rough up the inside of the tip of the nose cone. Tie a knot in each end and place the ends in the inside of the tip of the nose cone and pour enough epoxy to cover the knots with the nose pointed down of course. You may find it a lot easier to cut the bottom of the cone off - but not the shoulder - for better access. Attach the cord to this loop once said epoxy is set.

Another way is to scrounge a hardware store for a matching eye-bolt and wall anchor combination to epoxy into the bottom hole of the nose cone. Attach the eyebolt to the shock cord. There are probably a dozen different products that would work.

If a store clerk asks if you need help...the correct answer is "no" because answering "yes" inevitably leads to, "What's this for?" Which leads to really confused looks if you answer honestly. The longer you are in HPR the more you will get used to this.

-DAllen

 

[Viperfixr]

I know they cost $20 or so, but I like the Giant Leap Slimline retainers for 29-38mm. Not much weight, simple, and very positive retention. I just put a 54mm Aeropack retainer on a new rocket, and that's very nice too--little more $ up front. The Aeropack customization option for the retaining cap is a great looking touch.

 

[Handeman]

This may be the wrong thread to tell you this, but don't tie your shock cord to the eyelet on the nose cone. Especially if this is your L1 rocket. I was not allowed to fly my L1 rocket until the shock cord was mounted to my nose cone differently.

I didn't see your nose cone attachment, but I really don't agree with that. I have several L1 rockets where the shock cord attach's to the molded loop on the nose cone. I've never had a problem.

There are a couple of conditions that should be meet if you are going to use the molded loop on the nose cone to attach the shock cord.
1. Don't use nose cone weights. The loop will hold when just the weight of the nose cone is there, but if you add weight, then the inertia can break loop.
2. Don't use quick links or other metal loops to attach the shock cord to the nose cone. I've seen a nose cone that had just a small section of the loop broken out. It was just the width of the metal quick link. The loose quick link slamming into the plastic loop isn't a good thing. Tie the shock cord to the loop.

 

[Gabe Osborn]

A good friend of mine who has mentored me through my L1, L2 and soon to be L3 flights has given me some excellent suggestions for this situation. First and foremost, be concerned with the safety of others on the range. Then build your rocket with that thought in mind. A few extra minutes on a project is better than cutting a corner and having someone get hurt or have a close call.

As for the shock cord, dont use the elastic.. ever. The plastic loop in the nose cone can and will break sooner or later. Dont let your nose cone come in hot. Its easiest drill 2 holes in the bottom of the nose cone and run your tubular nylon or kevlar strapping through that and tie it off. I do not use tubular nylon more than 3 flights without replacing it.

Make sure your chute is connected as closely to the nose cone as possible. It is the first thing coming off when your ejection charge goes off and you want your chute out as quickly as possible. I connect my chutes to the same quicklink/u-bolt connection as the nose cone. Chute comes out every time since the nose drags it out.

As for retention, all the suggestions will work. I personally have an Aeropack retainer on all my rockets. They are easy to use and work flawlessly every time.

 

[redsox15]

You would think that the materials given in the kit would work just fine and not break the first time I launch it. I think I will switch out the shock cords and find a better way of attaching the shock cord mount. The instructions are telling me to expoy the mount (which is a piece of nylon string with a loop knot in it) to the inside of the airframe. As of right now I haven't started assembling anything because I am still clarifying some things. Im taking it very slow on this once since it is my first build

Matt

 

[DAllen]

Oh...those items will very likely work the first time right out of the bag. It's just that I've seen too many people have to go hunting for a nose cone that just fell from 2000'+ feet after the 4th or 5th flight. The parts will work I just wouldn't expect any longevity out of them.

-DAllen

 

[jj94]

I use around 15-20 feet for my larger MPR rockets, and it is worth the little extra cost. I had an early deployment while the rocket was still going what I would estimate to be 150-250 MPH from a G71 Redline (great motor by the way). The 20 foot shock cord unraveled and the only damage was a .75 inch deep zipper in the edge of the body tube. I'd be scared to know what the damage would've been with a 6 foot shock cord.

 

[daveyfire]

I have a stock 4" Expediter that's at least ten years old and uses the LOC elastic shock cord. It still works. I even used the stock epoxy-nylon rope shock cord mount and tied it straight to the plastic eyelet on the base of the payload section

Make sure the cord is well protected from ejection gases and use plenty (read: all) of it, and it should survive without a problem. Upgrading to nylon or Kevlar will also work (and is a wise choice for scratch builds or heavier vehicles), but is not a requirement if the rest of the model is built to specifications.

 

[redsox15]

I am thinking about cutting a piece of plywood out and using it as a divider from the ejection gases and the parachute because there is nothing separating the two.
My plan is to use the plywood not only as a divider but also a way of mounting my shock cord. There will be holes in the middle of the divider to the the gases through so my rocket wont explode.

Matt

 

[Luv2launch]

I am thinking about cutting a piece of plywood out and using it as a divider from the ejection gases and the parachute because there is nothing separating the two.
My plan is to use the plywood not only as a divider but also a way of mounting my shock cord. There will be holes in the middle of the divider to the the gases through so my rocket wont explode.

Matt

You don't really need to use a divider you can always put an eye bolt onto the forward centering ring mount your shock cord to that and get a nomex cord protector and nomex blanket to put between the chute and ejection charge they work really good too.

 

[bobkrech]

I am thinking about cutting a piece of plywood out and using it as a divider from the ejection gases and the parachute because there is nothing separating the two.
My plan is to use the plywood not only as a divider but also a way of mounting my shock cord. There will be holes in the middle of the divider to the the gases through so my rocket wont explode.

Matt

Matt

Here's what you can do. https://www.info-central.org/construction_antizipper2.shtml I would drill (6-8) 1/2 near the circumference of the bulkhead (1/8" 5-ply minimum to 3/16-1/4" maximum) thick is enough for a 4" rocket), and use a second 1/8" bulkhead ~ 1" lower in the airframe with a 1.5" central vent hole. This makes a baffle that has no direct path for burning powder particles to impact the chute.

BTW The LOC method of securing a shock line with epoxy to the airframe does work and it's extremely strong. I treed a stock VB 2.6" Raven (35' up) and yanked directly down with a lineman's pole on the parachute/shock chord that was attached to the airframe hoping the attachment would break but it didn't budge. Since myself and another person was hanging on the pole, we were applying a 400 pound load to the attachment point. We never did get the rocket out of the tree, but we did prove the recovery system was good for a 200G shock load!

Bob

 

[Pantherjon]

The shock cord is an elastic nylon shock cord. It is definitely not kevlar.

Any suggestions on motor retention? I want to make my own because I don't think that wrapping the motor in masking tape and making a snug fit in the mount is the most sure-fire way of keeping a motor in the rocket.

Thanks
Matt

Here is how I do motor retention on my HPR rockets

2 threaded inserts, 2 bolts, a few nuts as spacers and a couple washers...Works like a champ and best part? Total cost for ALL the parts , about $6!

 

[jj94]

I use t-nuts and install them in the aft centering ring. Then I use short screws and mirror clips to hold the motor in. It's cheap, strong, quick, and hidden.

Here is what a t-nut looks like. Drill a hole in the centering ring, and then install this in through the back of the centering ring. When the centering ring is glued into the rocket, the side where you can see the t-nut goes in first. The exposed side of the centering ring should only show two holes.
https://picasaweb.google.com/joshuajoung/MotorRetention#5305740232885607458

So here is what it looks like when the t-nuts are installed in the centering ring. It's a very clean look.
https://picasaweb.google.com/joshuajoung/MotorRetention#5305740226538622194

Finally, here is what the entire setup looks like. The mirror clips are secured down via the screws which are screwed into the t-nut. The clips, in turn, secure the motor. In this case, the motor is a single use, so I would have wrapped a few layers of narrow masking tape in the back of the motor to create a thrust ring.
https://picasaweb.google.com/joshuajoung/MotorRetention#5305740221860748338

The images are really big, because I couldn't get the link Picasa gives me for embedding pictures, so I had to copy the image URL and use '

' when I was posting. If they are too big, I'll change the images to just a link for the image.

 

[Peartree]

I can't get the pictures to show. Is it just me?

 

[bobkrech]

I had the same problem and looked at the file but didn't see anything wrong with it.

Folks might want to download a program like the FastStone Image viewer to reduce the size of the image file. It's availabe for download here.

https://www.faststone.org/FSViewerDetail.htm

Bob Krech, TRF Moderator