Scratch AMRAAM AIM-120 Original Design / Scratch Built

Scratch - AMRAAM AIM-120 {Scratch}

Contributed by Marlin Meyer

Published: 2010-07-01
Manufacturer: Scratch
(Contributed - by Marlin Meyer - 07/01/10) Scratch - Amraam Aim

Brief:
Based on Estes design, a single stage, 18mm motors, parachute recovery, scale-like missile.

Construction:
Here are the parts from Semroc:

  • BNC-55 nose cone
  • SE-2 screw eye
  • BT-55 body tubes (2) each 11"
  • JT-55C tube coupler
  • BT-20 motor tube
  • RA-2055 centering rings (2)
  • EH-28 engine hook
  • LL-110 launch lugs (2)
  • 15" parachute from TopFlight
  • Fins are balsa covered with paper

Very much an Estes build. Quite simple. I like to paper the fins, so that took some time. Surface mounted fins. I use Titebond II. I didn't get enough glue on the coupler when putting the two tubes together and they got stuck with a bit of a gap. That took some time to fix. I installed the motor mount before I thought about the shock cord - DUH, so I used a body tube scrap to make a short coupler and attached the shock cord by wrapping around the coupler and glueing to the top of the upper body tube. It's almost like Estes, but I think a little more secure. If you look at the Rocksim file, you will see that the forward fins attach partly over where the two body tubes join together. Not a big deal, but if you want to make one of these, I thought that might help.

Finishing:
For finishing I did the usual Rustoleum sandable primer, sanded, made some repairs, primed again, sanded again. The nose cone was sealed with Fill n Finish and received the same treatment as the rest of the rocket. I did the nose cone separate from the rest. The paint was as close as I could come to photos I've seen on the web. All done with Krylon.

Flight:
I flew on a clear day with very light winds 0-5 mph. My first motor choice was C6-7. Altitude wasn't great (approx. 350 ft. and the chute was very late, but no damage. Launched again with C6-3. Not much difference in altitude, but the chute ejected much closer to apogee.

15 inch diameter chute from TopFlight brings it down nice and slow. It landed about 150 feet from the launch pad. You might want to go a little smaller, cut a hole, or shorten the shroud lines if it's windy.

The best flight was AT D13W-7. Altitude around 1000 ft.

Recovery:
If I built another one, I would attach the shock cord at the motor mount.

Summary:
The paint came out great on this one, so I decided to do a little research and found photos. Made some decals that probably aren't too scale, but they look good. The blue stripes were done with tape.

Other:
As far as tips that this build taught me I'd have to say DON"T FORGET THE SHOCK CORD!

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