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Faraday's Cage

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My most recent project is/was a 1:4 scale model of the AIM-120 AMRAAM. I made it out of aerospace grade carbon fiber/kevlar hybrid composite, it weighs 1 pound, and it uses a G80T-10. Last launch I tested a GPS logger, not tracker (available from here), which placed the apogee altitude at 2500 ft and the maximum speed at 407 mph. I have launched it three times. I will post pictures soon.

- FC
________
Honda XL600V
 
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AKPilot

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Welcome to the Forum! Hope you enjoy it here, and I look forward to seeing your pics.
 

Peartree

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I used to work as a test engineer and have thought about putting some data loggers in rockets. I would love to hear more about your GPS logger.
 

Faraday's Cage

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Actually, the GPS part is pretty simple. I bought the logger mentioned before, an antenna, an 1100 mah LiPo battery (enough for 36 hours of operation), and a bulkhead kit. I assembled the bulkhead, connected the antenna and battery to the GPS, stuck it in, and launched it. When I got it back, I connected it to a computer via a ttl to usb converter, and used the software on the sparkfun website to download the data.

Links:

Antenna
GPS
Battery
Battery Charger
TTL to USB Converter

The whole thing comes out to about $115 and it gives you data with an accuracy of +/- 5 feet.

Photos are here.
________
XT660
 
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Faraday's Cage

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Oh, and here's an Excel file containing graphs of my rocket's speed and altitude.

BTW: The x-axis represents the # of seconds since I turned on the GPS
________
CR110
 
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cjl

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Very nice :)

Interesting choice of nosecone though - it's kind of odd to see an AMRAAM with a conical. Did you break a fin on the last flight?
 

Faraday's Cage

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Yep, I broke a fin. The airframe is ROCK-SOLID (last flight my shock cord broke at apogee and it landed in a pile of wood), but the surface-mounted G10 fins snap off almost every flight. The next rocket I do will aim for Mach 1-1.5 with a G80T, so I will need to use "through the wall" construction.

Anyways, I thought the AMRAAM had a conical nosecone, and it wouldn't have mattered anyways since it was all I could find. Anyone here know of any good 29 or 38 mm nosecones that are hollow? All I could find was those super heavy solid urethane nosecones that PML sells, but I am still trying to cut down on weight. I still have to work on the paint job.

BTW: Anyone wondering why the top 6 inches of rocket aren't made of the same stuff as the rest? Here's why: carbon fiber is a Faraday Cage, so GPS won't do very well inside.

- FC
________
Ducati Mach 1
 
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cjl

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What's breaking? Is the epoxy joint failing? If so, you should scuff up the surfaces much more thoroughly prior to bonding. You can also mix a bit of filler into the epoxy to help the bond strength a bit too. Surface mounting is not inherently weak, though it is more difficult to do correctly. If you really want strength, you could always use tip to tip fiberglass or carbon fiber, but that's honestly overkill given your flight profile. You also could easily build a rocket this size of paper and plywood and it would hold up fine - I love carbon fiber as much as anyone, but there is a definite cost disadvantage to using it :)

For an example of the strength of surface mount fins though, I surface mounted the fins on my Amraam shown here:
http://i27.photobucket.com/albums/c183/chris_lapanse/L2 build/RL3_3759.jpg?t=1254114134

They've survived fine so far.
 

Faraday's Cage

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Actually, the CF I got was really cheap. $2.99/ft in a sleeve form. Here's a link. They also sell a really cool heatshrink sleeve designed for composites (in lieu of a vacuum bag). By the way, I did the CF by hand! I put the sleeve on, epoxied it, and rotated it every 15 minutes to prevent drippage.

The fin that snapped on this flight had to be repaired from last flight. I think it has a bad temper :D. I forgot to scuff when I reglued it, but I used considerable amounts of chopped fiberglas in the mix.
________
Ducati 998
 
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powderburner

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Anyone here know of any good 29 or 38 mm nosecones that are hollow?
Go get an Estes StormCaster.

The kit parts are easily adapted to make a pretty good scale AIM-120. The BT60 body tube is 41mm in diameter, the plastic nose cone is hollow, and if the NC does not provide enough space for all your gear the basic kit comes with the body tube already split in two pieces with a coupler that could be converted to a bulkhead (giving you the entire upper half of the rocket to use as an avionics bay).

If you are afraid of using cardboard BT for this task (and you shouldn't be) you can always fiberglass the outside of the airframe, or at least the bottom half.

MUCH cheaper than carbon fiber construction

PM me if you want further info on converting this kit
 

Faraday's Cage

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1. I was more asking for my next rocket, which is a performance rocket that needs a very light nosecone (look in the "plans" forum, it does mach 2 on a G80T). I guess I will just make my nosecone from scratch.

2. As I said, I did my CF very cheaply, and I did it more because I wanted to try a composite airframe than because it was necessary.

3. It was a scratchbuilt. Modifying a kit would completely defeat the purpose.
________
herbalaire reviews
 
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cjl

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Mach 2 on a G80T? I'll believe it when I see it - that seems really, really optimistic. I'll second the suggestion for Estes BT-60 plastic nosecones though - they are nearly identical in size to 38mm nosecones, and quite light.

As for the fin, it doesn't sound like the adhesive is the problem. Make sure you scuff all bonded surfaces though, and scuff them well (80 grit or rougher, and it should feel pretty rough to the touch). You can definitely surface mount without problems, but you do need to be somewhat careful about it.
 

Faraday's Cage

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Again, look in the "plans" forum and download the rocksim design. Even though its a simulation, rocksim can't be VERY far off. Perhaps it'll only do mach 1.8. I don't know.

The only concern I have about my design is that the fins could break off. I'll try scuffing next time I glue a fin.

Here's the rocksim file.
________
Suzuki GSX series
 
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cjl

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I looked (and responded) in the plans forum, and I can say with some certainty that you won't hit anywhere near mach 2. I'd also be surprised if you could match the weight rocksim predicts - your fins are lighter by far than they should be (rocksim has a bug with G10 glass - make sure the material is set to "G10" and not "G10 - PML 0.063" and you'll get the right value). Also, in general, the weight seems too light.

I'm not trying to dampen your enthusiasm or anything, and you can definitely break mach on a G80T, but that isn't the best design to do it with.
 

hardinlw

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Rocksim has two potential issues.

First is that it tends to under estimate weight. Some of the materials are wrong, like the PML G10 fin material. Some of the problem is that we usually omit the glue and paint which can be really significant.

Second, we tend to over estimate the quality of our finish and hence under estimate the drag coefficient. 0.6 is on the low side and 0.75 is maybe a bit high. Try overriding CD with those values and see how the results compare to what you got letting Rocksim estimate CD. If you really want to go fast, eliminate the launch lug and use a tower or use a pop lug.

Weight and drag are your enemies. To keep fin weight down, skip the solid G10 and use a balsa core with fiberglass skins. It's not too hard to make. The fin cores could be something like 1/8" balsa for this size and engine. Both leading and trailing edges should be sharp, and I mean paper thin. Get some mylar from a drafting supply house and wax it with a couple coats of paste wax, buffing each coat. Lay on some 2oz fiberglass cloth and roll in some laminating resin (thin epoxy, not like what you get in most hobby shops) with a small paint roller. You want the cloth wet through and through but no puddles. You need two of these and each should be big enough for all 3 fins. Now lay the fin cores on one of the skin sheets and plop the other on top. You biggest challenge is now to squeeze the skins onto the cores. I use a vacuum bagging system. If you have one of those home vacuum food sealers, it should work, but you may not want to use if for food afterwards. :D You could also put the layup between some stiff foam and pile books on top. Make the fins longer than needed and cut to length after glassing.

To laminate fiberglass (or carbon fiber) to a cardboard body tube, apply one or two layers, rolling laminating resin on with the small paint roller. Once done, wrap some peel ply around it. This is typically dacron fabric treated with a release agent so it will peel off the glassed tube. Now the trick! You need a long "airship" balloon. Blow it up, but don't tie it off. Start forcing it onto one end of the body tube, letting air out as you go. With practice and a long enough balloon, you can get it all the way onto the body tube before running out of balloon. I suggest practicing this before mixing your epoxy on a bare tube. It can be pretty frustrating. :mad:

If you doubt that this will hold up to a G80, a Der Red Max built just this way survived a G80 at LDRS. It did not survive getting stepped on during recovery, that broke one of the fins.

For max performance, you are going to want to use a body tube the saem size as the engine and surface-mount the fins. Glue on with epoxy as usual and then apply an epoxy fillet. I use the Locktite metal-concrete epoxy from Home Depot. Shape the fillet with a craft stick to get a 1/8" radius fillet. Now cut a strip of 2oz glass cloth about 1/2" wide and apply over the fillet, lapping onto the fin and body tube at each joint. Another approach is to make the reinforcing strip wide enough to go across the body tube between two fins and 1/4" up each fin. I think it's easier to handle the narrow strips.

Hope this gives you some ideas.
 

Peartree

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Now the trick! You need a long "airship" balloon. Blow it up, but don't tie it off. Start forcing it onto one end of the body tube, letting air out as you go. With practice and a long enough balloon, you can get it all the way onto the body tube before running out of balloon. I suggest practicing this before mixing your epoxy on a bare tube. It can be pretty frustrating. :mad:
Wouldn't it be easier to drop the long balloon into the BT and THEN blow it up?

Just wondering.
 

hardinlw

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John,

You are trying to get the baloon on the OUTSIDE of the body tube to squeeze the cloth onto it. In the process, you end up turning it inside out over half its length and then it is doubled back on itself. If it's not long enough, you do a second balloon from the opposite end and tape the overlap. (If not, they tend to roll up on themselves and pop off.)

Larry
 

Peartree

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John,

You are trying to get the baloon on the OUTSIDE of the body tube to squeeze the cloth onto it. In the process, you end up turning it inside out over half its length and then it is doubled back on itself. If it's not long enough, you do a second balloon from the opposite end and tape the overlap. (If not, they tend to roll up on themselves and pop off.)

Larry
OHhhh. That makes MUCH more sense. Thanks. I just wasn't picturing what you were describing.
 
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