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

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.

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.

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.

Draw two circles (7mm and 8mm radius) centered on the top face of the cylinder.  Next, extrude the ring formed by these two circles down 5mm into the body of the cylinder.  Then, extrude the 7mm circle down 63mm into the body of the cylinder.  This completes the design of the main cylinder, so make sure you save your work.  In addition to the normal .123d file format, you also need to save the project in the .stl file format.  The .stl file will be used in a later step to generate the G Code necessary to actually print the part.

Similar to the main cylinder, start a new project in 123D.  Draw a 9.55mm radius (19.1mm diameter) circle.  Extrude the circle to a length of 2mm.

Draw an 8mm radius circle centered on the top face of the plug.  Extrude the circle out 5mm from the surface.  Next, select the circumference of the circle and add a 1mm chamfer.  This will make the plug easier to insert into the main cylinder in a later step.  This completes the plug, so save your work in both .123d and .stl format.

Note: Specifying an 8mm extrusion here and an 8mm hole in the main cylinder will likely result in an interference fit between the two.  I take advantage of that to create a tight friction fit between the two (will require some scraping and sanding in a future step).  If you don't mind adhering the plug in later, you can specify a slightly smaller circle (such as 7.75mm) to make assembly easier.

Use ReplicatorG to generate the G code files (.s3g format) required by the MakerBot from the .stl files you saved earlier.  Print both pieces on the MakerBot.

Peel off the raft (support structure to make part removal from the heated bed easier) from the bottom of both parts.  You may need to scrape off the remnants with a hobby knife or chisel.  Any remaining surface roughness or sharp edges can be sanded off with the 150 grit sandpaper.  Test fit the plug into the cylinder.  Unless you undersized the plug, they likely won't mate.  Scrape the inner shoulder of the cylinder to remove any rough parts.  Then, sand the plug wall down until a snug fit is achieved.  The two components are now done.

Place the two components on the scale.  Carefully pour fine sand into the cylinder until the total weight reaches at least 28 grams.  Due to scale to scale variation, I would recommend adding sand until you reach 29 grams or slightly higher.  You would hate to get to the competition and find that your scale read a little high and their's a little low, and they don't allow you to use your new standard payload.  If your plug is a little loose, or if you want the added security, use some ABS adhesive (or your own mixture of acetone and ABS scrap) to permanently attach the plug to the cylinder.

Good luck and fly high!