First Scratch Build

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Roco
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First Scratch Build

Postby Roco » Sat Jul 04, 2015 5:30 pm

As the title says this is my first scratch build and I am looking for advice. This is a smaller rocket with a 24mm motor mount and I am looking to fly it on a D13 through to a F39. Building it out of a BT-56 body tube with balsa fins that I was considering of mounting through the wall. Though that might be overkill for such a small rocket.
Any advice and or critique would by helpful.
SROR.jpg

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Re: First Scratch Build

Postby CATO » Sat Jul 04, 2015 5:35 pm

Hi Roco,

Welcome to the forum, I construct 99% of my rockets TTW.
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Re: First Scratch Build

Postby OverTheTop » Sat Jul 04, 2015 7:32 pm

Hi Roco. Welcome to the forum.

Have fun with your first scratch build.

Have you considered moving the fins a little towards the front to protect them on landing a little? Could protect the balsa fins on landing. Gives you a bit more probability of survival for flights when things don't go entirely right. Just something to think about...
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Re: First Scratch Build

Postby matthew » Sat Jul 04, 2015 7:45 pm

G'day Roco. Welcome to the rubber room. :)

There's nothing wrong with through the wall mounting at any size, even if it is overkill. It will give you good skills for when you build bigger rockets.

Your rocket is a little bit over stable on the motor you have in it (presumably the D13), which means you'd be best to fly it in calm conditions or it might weathercock somewhat. I'd also suggest that at that size it's likely to vanish on an F39. You might like to think about a bright colour scheme (so you can see it in the grass/in a tree), some kind of tracking (a loud beeper), and perhaps a mylar streamer instead of a parachute (can be easier to locate in the air due to reflection of the sun).
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Re: First Scratch Build

Postby Roco » Sat Jul 04, 2015 8:02 pm

Thanks for all of the welcomes.

I've tweaked the design from the suggestions above and have ended up with this.
SROR2.jpg

matthew wrote:G'day Roco. Welcome to the rubber room. :)

There's nothing wrong with through the wall mounting at any size, even if it is overkill. It will give you good skills for when you build bigger rockets.

Your rocket is a little bit over stable on the motor you have in it (presumably the D13), which means you'd be best to fly it in calm conditions or it might weathercock somewhat. I'd also suggest that at that size it's likely to vanish on an F39. You might like to think about a bright colour scheme (so you can see it in the grass/in a tree), some kind of tracking (a loud beeper), and perhaps a mylar streamer instead of a parachute (can be easier to locate in the air due to reflection of the sun).


I plan for it to recover with a mylar streamer and adding a beeper would be nice but I don't know if it would fit in the nose cone. Still undecided on what colour the rocket should be. Might go with yellow or a lime green. The F39 puts it at around 2300 ft. It is still over stable on the D13 but ends up at a stability cal of 1.4 with the F39 in it.

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Re: First Scratch Build

Postby SpaceManMat » Sat Jul 04, 2015 8:10 pm

The build seems a bit light for an F, how fast and how many Gs on an F motor? You'll have difficulty getting balsa to stay together if it goes transonic.
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Re: First Scratch Build

Postby martymonsta » Sun Jul 05, 2015 11:24 am

Hi Roco and welcome.

Those F39's are a great little motor.

SpaceManMat wrote:The build seems a bit light for an F, how fast and how many Gs on an F motor? You'll have difficulty getting balsa to stay together if it goes transonic.


I'd be saying Mach 0.8 and 50G's on a F39, based on the BP D's proformance. I'd be less concerned about the balsa staying together and more worried about getting the gluing right and even more concerned about the body tube failing. My Apogee Aspire went Mach 1.1 on a G80 and held together with surface mount fins. After it got recovered it got blown off a table and one of the fins came off, but that was the body tube de-laminating, the fillets and balsa were fine.

Roco I would consider a very small chute instead of a streamer if you want to fly it a few times as the landings on streamers tend to beat low power body tubes up a fair bit.

Marty
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Re: First Scratch Build

Postby Roco » Sun Jul 05, 2015 12:25 pm

martymonsta wrote:Hi Roco and welcome.

Those F39's are a great little motor.

SpaceManMat wrote:The build seems a bit light for an F, how fast and how many Gs on an F motor? You'll have difficulty getting balsa to stay together if it goes transonic.


I'd be saying Mach 0.8 and 50G's on a F39, based on the BP D's proformance. I'd be less concerned about the balsa staying together and more worried about getting the gluing right and even more concerned about the body tube failing. My Apogee Aspire went Mach 1.1 on a G80 and held together with surface mount fins. After it got recovered it got blown off a table and one of the fins came off, but that was the body tube de-laminating, the fillets and balsa were fine.

Roco I would consider a very small chute instead of a streamer if you want to fly it a few times as the landings on streamers tend to beat low power body tubes up a fair bit.

Marty


You are spot on for the performance on a F39, 48g and mach 0.79. The F24 looks like it it might be a better motor for it, Mach 0.64 and a max acceleration of 31g. The F12 could also work but open rocket gave me a tumble warning but upping the rod length made it go away. Would laminating the fins with paper do any good? Saw it mentioned in Apogee's Aspire. I've ordered a streamer and a 12" parachute for it so I can switch it up. The 12' 'chute is a bit too large and I might need to cut a spill hole to increase the descent rate.

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Re: First Scratch Build

Postby OverTheTop » Sun Jul 05, 2015 2:22 pm

Would laminating the fins with paper do any good?

That is what Sirius Rocketry do on a lot of their kits. Makes for a nice smooth finish on the fins too. No sanding :D

I have built their SS Cestris and Interrogator using their prescribed method. Neither of them have suffered broken fins, and I have run them up on some composite F motors for some nice flights!

If you cut a spill hole in your chute you will find it comes down with less side-side sway. This helps with the survival of the fins also.
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Re: First Scratch Build

Postby BerowraRocketBoy » Sun Jul 05, 2015 4:54 pm

Welcome to the forum Roco,

OTT was on the money with his comments, paper laminated balsa is strong stuff!
If you're going to use it, remember to "seal" the edges with superglue in order to stop the paper from lifting up after a while. Alternatively you can "roll" the paper over the front edge (or any other edge I suppose) to save yourself some of the trouble.

In terms of fin attachment methods, TTW fins probably aren't an absolute necessity on a kit of this size and performance as long as you use some decent fillets, however, like matthew said, they won't do any harm and are good practice at larger scales.

Good luck with whatever you choose!

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Re: First Scratch Build

Postby OverTheTop » Sun Jul 05, 2015 5:40 pm

Forgot to add earlier: Make sure you don't make the beginners mistake of gluing directly onto the glossy cardboard BT. There is a "glassine" layer on them usually that needs to be sanded through to make sure you get a good bond with the tube proper.

If have seen this a couple of times, hence bringing it up here. Sorry if you already know about this :)
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Re: First Scratch Build

Postby Roco » Fri Jul 17, 2015 7:32 pm

Thanks for all of advice.
Got the parcel with the parts in it recently. However I've got a couple of questions on the assembly.
Where should I attach the shock cord? I was thinking of attaching it to the motor mount or doing a Estes-like attachment method. The only trouble with mounting it to the motor mount is that there isn't much space around the motor mount and hollowing out a grove in the centring ring might make it too thin.

Where can one buy water thin superglue for stiffing the paper on the fins or would normal superglue work fine?

Does anyone know the length of the bottom half of an aeropack 24mm retainer? I've got one currently on back order from Aus Rockets and I don't want to glue the motor tube in with too little space to glue the retainer on.

Besides Aus Rockets is there any place locally (Melbourne) that stocks 24mm reloads, specifically the F-24W?
Aus Rockets' $40 for shipping is a pain when the motors themselves cost $46 for three. I don't plan on flying too often so buying a large amount of motors and spreading the shipping cost isn't really an option.

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Re: First Scratch Build

Postby SpaceManMat » Sat Jul 18, 2015 8:54 am

Thin CRs are not likely to be an issue unless it's so thin they break before you glue them on. Once they are in there the glue is going to provide all the required support. Even more so if you are doing TTW fins.

Perhaps you can do a deal and split postage with someone else from Melbourne.
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Re: First Scratch Build

Postby Lamp » Sat Jul 18, 2015 9:02 am

G'day Roco, you have a couple of options regarding your motors but both of them will probably take time.
One option is some hobby stores can get composite motors in and although I haven't been there for years "Hearns Hobbies" in the city is one that springs to mind. They will be a fair bit more expensive than through Aus Rocketry and if you order more than a couple it will be cheaper to just get them from Blake and pay the DG charge anyway. Also, they will most likely have to specially order them in for you which may take quite some time....
The other option is to put a post up in the "Rocketry Victoria" area in the clubs part of the forum. If anyone is doing a motor order to Blake, they can be shipped together splitting the costs. If anyone is ordering bigger motors I expect they won't even worry about splitting the shipping costs. You will then just have to arrange to meet so you can pick them up. How long it takes just depends on when someone needs to place an order...
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Re: First Scratch Build

Postby martymonsta » Sat Jul 18, 2015 12:50 pm

As Pete said. I've found it's hard to find motors in shops cheaper then AR, because AR imports and sells, where as Hobby Shops get a 3rd party to import and then they need HZ shipping to get it to the hobby shop and then everyone wants a piece of the profit. For instance it is cheaper for me to buy a Bulk pack of C6 BP motors (12 motors) from AR and pay HZ shipping, then it is to buy 4x 3 packs from my local hobby store.
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