My first build.

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Re: My first build.

Postby Digit » Fri Mar 04, 2016 5:40 pm

Thanks guys I'll pick some up tomorrow and give that a go. Can anyone tell me what an empty B / C casing weighs? I'm trying to size my parachute now the body is assembled.
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Re: My first build.

Postby Lister » Fri Mar 04, 2016 5:46 pm

12-13g from memory.

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Re: My first build.

Postby OverTheTop » Fri Mar 04, 2016 5:55 pm

Just measured. Dead C6-7 weighs 10.0g.
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Re: My first build.

Postby Digit » Sat Mar 05, 2016 7:37 pm

Parachute has been cut to size and all the lines attached, plastic primed the nose cone ready for filling that grove. Going to prime body tube assembly tomorrow however for tonight what's the go with sizing shock cords? I realise it's probably more of a precise art for HPR but this kit came with a meter and a half long cord, which seems long to me?
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Re: My first build.

Postby Lister » Sat Mar 05, 2016 8:16 pm

It comes with what I needs to absorb the shock of the deployment charge.. its like a pop gun.. dont shorten it

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Re: My first build.

Postby Lamp » Sat Mar 05, 2016 10:44 pm

With low power rockets often the shock chord is elastic and for these a shock chord length of about the length of the rocket is fine.
With bigger rockets, elastic will not hold the strain so tubular nylon or Kevlar is generally used. Because these materials have little stretch you need a longer length. Tubular nylon does stretch a little and is therefore a bit "softer" than Kevlar which has almost no stretch at all.

Contary to what you think, there are no calculations or hard and fast rules for shock chord length in HPR. Personally I like to use at least 3 times the length of the rocket as a minimum (preferably more) using tubular nylon and at least 4 times the length of the rocket (preferably more) for Kevlar.

You are more likely to zipper your rocket using Kevlar however it is much stronger than nylon so you can use a thinner chord and pack it into a smaller space.
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Re: My first build.

Postby kopius » Sun Mar 06, 2016 7:29 am

SCR rockets all use high quality Kevlar, even the smallest of LPR. Cheap elastic is often weak and deteriorates quickly from ejection gases so whilst it can be shorter, it won't last anywhere near as long.

The rule of thumb for shock cord length is 3-5 times the length of the rocket. Just like Lamp said, often 4:1 is a good ratio right in the middle, however it can be dependant on a few variables. Generally all off the shelf kits have a suitable length that should not be modified (we'll definitely not shortened). This is why it is called 'shock' cord, to absorb the shock.

p.s. Your build thread has been great and you have done a very good job with the assembly, well done! Looking forward to seeing the finished product ;-)


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Re: My first build.

Postby OverTheTop » Sun Mar 06, 2016 8:34 am

If you use the very thin kevlar it can floss the airframe and zipper it. My personal preference is to change the thin stuff up to 1/8" braided to give a bit more thickness and less chance of zipper.

A couple of other discussions worth a look are here:

www.ausrocketry.com/forum/viewtopic.php?f=9&t=5248

You could try z-folding like some people (me in this case) use on larger rockets:
viewtopic.php?f=6&t=4612&start=75
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Re: My first build.

Postby Digit » Mon Mar 07, 2016 6:04 pm

Thanks for the kind words folks, it came with good looking kevlar so that's what went in, didn't shorten it at all. Happy with how the nose cone turned out, and priming the body really brought out the spirals so i gave it a few light coats of the putty too which has got it looking nice. Another coat of primer for body tube then I'm ready some colour. :)

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Plastic primer on.

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Spray putty applied then sanded back to fill molding marks.

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Undercoat over putty, gotta be happy with that.

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How body tube looks at the moment.
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Re: My first build.

Postby OverTheTop » Mon Mar 07, 2016 6:16 pm

Looking good Digit :)

A word of warning: The bigger the rockets you build, the more sanding is involved (and fiberglass gets itchy from time to time) :lol:
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Re: My first build.

Postby Lister » Mon Mar 07, 2016 6:28 pm

That looks awesome mate.. looks a million times better than my first rocket.. im guessing you have experience building model aircraft or something as you mentioned you were using dope on the balsa?

Your not wrong OTT, I have spent the past week sanding and polishing these FG mini nuke kits.. have to use hot water and a scrubbing brush to get the FG out of my skin, maybe I should have gone with Itchy and Scratchy Rockets instead of Lukes Nukes :)

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Re: My first build.

Postby OverTheTop » Mon Mar 07, 2016 6:54 pm

have to use hot water and a scrubbing brush to get the FG out of my skin

Try no soap and cold water first and see if that helps.
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Re: My first build.

Postby Digit » Mon Mar 07, 2016 7:08 pm

Nope don't build models just winging all this so far, asked guy at hobby shop what 'sanding sealer' is and he sold me a tin of the dope. I do however have experience with FG, I used to work in ceilings with that nasty sh*t, I'll be donning the paper space suits once I've advanced to large rockets. I'd rather sweat than itch for days. :mrgreen:
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Re: My first build.

Postby Lister » Mon Mar 07, 2016 8:31 pm

OverTheTop wrote:
have to use hot water and a scrubbing brush to get the FG out of my skin

Try no soap and cold water first and see if that helps.

Have tried that, didnt seem to work as well as the hot water.. cant remember If I used soap though so will try without. I usually just turn the hot tap to the hottest I can stand and scrub away and that gets rid of 90% of the itch.. am going to try barrier cream when I get some, you tried that for FG?

Digit wrote:Nope don't build models just winging all this so far, asked guy at hobby shop what 'sanding sealer' is and he sold me a tin of the dope. I do however have experience with FG, I used to work in ceilings with that nasty sh*t, I'll be donning the paper space suits once I've advanced to large rockets. I'd rather sweat than itch for days. :mrgreen:

In that case you have done a bloody brilliant job. I only ask because my mates with planes are dope fiends :lol: you can also use a thin CA (super glue) in place of dope in a pinch but I havent seen many people using dope on rockets.. might have to give it a go on some balsa.

If you catch the bug anywhere near as severely as i have you will be making Fibreglass rockets in no time :) Just make sure you wear eye protection and a respirator when sanding/cutting FG.. I had a 3mm splinter in my eye a couple of weeks back, two hospital visits and a heap of needles and burrs going into my eye was enough for me :wink:

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Re: My first build.

Postby Digit » Wed Mar 09, 2016 7:02 pm

Well she's ready for top coating (so is my pocket rocket :lol: )! I'm not sure what to use is there anything specific? enamel, acrylic, do I use clear coats etc.

I don't do much painting, any advice would be appreciated thanks.

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