Instructables
Picture of NAR Model Rocket Payload
The National Association of Rocketry (NAR) offers sanctioned rocketry competitions, with over 25 different competition events.  One of those events is payload altitude.  In this event competitors attempt to carry a 1oz (28 gram) payload to as high of an altitude as possible.  To ensure fair and consistent competition, a standard sized payload is defined.  This instructable describes how to make your own standard payload using a 3D printer.

I made this at TechShop  www.techshop.ws
 
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Step 1: Materials and Equipment

3D drawing software (I used Autodesk 123D)
3D printer (My TechShop has a MakerBot Replicator)
About 1oz (28 grams) of fine sand
150 grit sandpaper
Something to scrape with (hobby knife, chisel, etc.)
ABS compatible adhesive (can use ABS scraps and acetone)
Small scale (with sub-gram resolution)

This instructable assumes some basic familiarity with Autodesk 123D, ReplicatorG, and a MakerBot.

Step 2: Standard Payload

The standard payload is a non-metallic cylinder filled with fine sand, at least 28 grams in weight, with a diameter of 19.1mm +/- 0.5mm, and a length of 70mm +/- 10mm.  We will build this one to the nominal size of 19.1mm in diameter and 70mm in length.  First we will design the main cylinder.  This will be a hollow 68mm long cylinder.  One end will be closed, the other will be open with a stepped shoulder.  Then we will design a 2mm long plug to mate with the open end of the cylinder.  Once the parts are printed and prepped, a precise amount of sand is added to the cylinder then sealed in with the plug.

Step 3: Main Cylinder

Picture of Main Cylinder
Create a new project in 123D.  Start by drawing a 9.55mm radius (19.1mm diameter) circle.  Extrude the circle to a length of 68mm.