Note: Launch Lugs refers to hollow cylinders to constrain a rocket to a launch rod, only used in low-power rockets.

To constrain a rocket during liftoff, it is connected to a rail using two guides, that keeps it flying in the right direction as it builds up the speed required for stable flight.

Rail Types

Launch rails are usually a traditional 8020 aluminum extrusion, either the 1010 or 1515 style. It is important to know what the slot size of the launch rails at your launch site are, so that your guides will fit.

Open

Example: 1010 vs 1515 nominal dimensions

Rail Button vs Rail Guide

Buttons and guides accomplish the same thing but are used for different applications. Somewhat confusingly, referring in general to guides can include both rail buttons and rail guides.

Rail Button

A rail button is a wheel attached to the outside of the rocket that can ride inside the launch rail.

Rail buttons are usually used on smaller rockets. They are more forgiving with their alignment than guides, but are a bit heavier.

Rail buttons are usually made of a plastic exterior like PEEK or Delrin, with an aluminum core to hold a bolt, which is threaded into the rocket and constrained with epoxy and a nut on the inside.

Rail Guides

A rail guide slides inside the launch rail and keep the rocket pointed vertically

Rail guides provide more guidance than a rail button and can be lighter, but they require precise alignment to prevent binding, unlike buttons. 

Rail guides are usually flush-mounted to the outside of the rocket with epoxy. This also provides the benefit that you do not need to drill holes into the rocket.

Rail Guide Positioning

A rocket must have two rail guides. Generally, one is placed on or near the CG of the rocket, and the other is placed as far down the rocket as possible. While this is considered the best practice, you can place them anywhere on the rocket that ensures they meet the threshold for strength and provide stable flight speeds. Keep in mind that the effective length of the rail used to determine if a flight is stable is the distance from the top of the rail to the first guide, so guides lower down gives more time to reach stable flight speeds. 

Material Selection for Rail Guides

Rail guides can be purchased or easily designed and manufactured locally. The most common way to manufacture rail guides is with 3D printing. This allows you to create complex geometry that reduces aerodynamic drag and allows the guides to perfectly match the contour of the body tube.

For larger and more complex rockets, aluminum rail guides are often used. There are not many manufacturers for these, so they usually have to be machined on campus.

Rail Guide Strength

Rail guides must be strong enough to resist "tear out" during launch forces. This is commonly achieved by using a stout, solid design and reinforcing the underside of the guides with a wood or plastic block. For surface-mounted guides, a thick bead of epoxy is applied under, around, and even on top of the flange of the guide to prevent it from separating from the rocket. To ensure there is no tear-out, rockets should be designed to comply with Section 6.2.5 of the IREC Design, Test, and Evaluation Guidelines issues by the Experimental Sounding Rocket Association. This section can be found below, quoted from Revision C, Effective Nov 2019.