Rail Buttons

Rail buttons are special posts which mount to the outside of your rocket and replace the traditional soda-straw looking tube on the rocket known as a launch lug. The buttons (shown in the image to the right) are designed to side down a special launch pad called a "rail launcher" (see the picture below). By putting two buttons on your rocket, they will guide the rocket up the long rail when the rocket takes off.

Why not just use a bigger launch lug? That is a really good question and shows that you're thinking things through like a real aerospace engineer. But here's the situation...

In the past, high-power rocketeers used really large diameter launch rods to support their big rockets. But the lug on the side of the rocket became so big in diameter, that it added a lot of drag and was really ugly to look at. So rocketeers tried to use smaller lugs and the smaller pads.

And you've probably seen what happens when a big rocket sits on a small launch pad. The rocket blows around in the wind, and actually bends the skinny rod. You have to wait until there is a lull in the wind for the rocket to stop swaying around before it is safe to launch the rocket. Isn't that your experience?

Round launch rods work good for small and mid-power rockets, but they are not so great for big heavy rockets. What is needed is a long guide that is very stiff, sturdy, and relatively inexpensive. One device, know as a tower launcher would work, but they are not very cheap and are hard to transport to and from the launch site.

NASA faced this same problem decades ago and came up with a special rail pad. The rocket used a similar device that performed the same function as the rail button. NASA called these the launch shoes.

With a rail launch pad, the rocket is supported very securely. It doesn't sway in the wind at all. In addition, the buttons are very small and hardly add any amount of drag to the rocket compared to a big honking launch lug.

Incidentally, all the rocket clubs around the world that fly high power rockets are now using rail launch pads. So by getting rail buttons on your rocket now, you'll be ready to launch when you visit one of the clubs.

What is great about a rail launcher is that because they are so long, the rocket has a lot of time to build up speed before they reach the end of the rail. That means when the rocket clears the rail, it will be going so fast that the wind will have a lesser effect on the trajectory. The rocket will fly straighter, and therefore a lot higher in the sky.

In addition, because the rocket comes off the rail straighter, that actually makes it a safer flight because you'll be able to predict with higher certainty where it is going to go. You can almost always equate a straighter flight with added safety.

If you like those slow-realistic lift-offs, then you'll want to use a rail launcher. While it looks like the rocket is accelerating slowly, it is doing so on a long rail, and will probably still be a safe flight. But always make sure that the rocket will leave the rail with a minimum speed of 30 mph. You can use the RockSim software to estimate the lift-off speed and to find out how long of a rail launcher you're going to need.

Q. How do you mount rail buttons on your rocket?

A. For the standard-size rail buttons, drill a small hole for the screw and flanged weld-nut backing. Insert the flanged nut through the hole from the inside of your rocket. If your rocket is too narrow to fit your hand into, you can use a magnetic stick or similar device to insert the flanged nut.

Assemble the rail button as seen in the diagram to the right. There are two sides to the rail button, one is routed to compensate for the screwhead, allowing it to fall flush with the edge. Put a dab of epoxy or thread-lock glue in the nut to keep the screw from coming out and you losing your rail buttons. You can also put epoxy clay on the back of the flanged nut to prevent shock cords from catching.

To use the small rail button without the flanged nut, you will need to purchase a longer screw. #10 (3/16"), 1/2" minimum length.

For the large rail buttons, there is no flanged nut included. Just drill a small hole for the screw that goes through the plastic button. Put a little bit of thick CyA (super glue) on the threads and slide it into the hole. There is very little forces trying to pull the button straight out. All the forces are in a side-to-side direction, which it absorbed by the steel screw in the hole. In other words, the CyA glue will hold the button on the rocket. But try to prevent glue from getting on the plastic button. Ideally, you want the button to spin on the screw so that it has less friction as it slides along the launch rail.

On large diameter rockets (where you can get your fist inside the body tube), you can put a little dab of Fix-It Epoxy clay on threads on the inside of the rocket. This is a lot stronger and will make a nice streamlined contour over the threads so that the parachute or shock cord is less likely to snag on the threaded portion. We highly recommended this if you can do it.

For more information and a demonstration on how to install your rail buttons, please watch the video below, part of a series on building high-power rockets. Both methods of using a flanged nut and just a screw (into a centering ring) are explained.

Building Your First High Power Rocket - Part 4, Rail Button & Retainer Installation

Putting together a high power rocket is actually easier than you might imagine. In this fourth video, you'll see how install rail buttons and the Aeropack engine retainer. 

Q. Where on the rocket should the rail buttons be positioned?

A. First of all, radially, they should be placed equal-distant between any two fins. Remember, the rail is 1-inch wide, so you can't put them too close to any single fin or the fin itself will prevent the button from engaging the rail.

Forward-and-Aft Location: You would position them just like you would two small launch lugs. Ideally, you should put one button in front of the CG point, and one button behind. Most modelers place one at the very rear of the rocket so that it is easier to see it as it is being loaded onto the launch rail. Once you have the bottom button on the launch rail, it is easier to get the top button to slide on without scraping all the paint off the side of the vehicle.

The forward button's location is not too critical. As mentioned, ideally it should be positioned ahead of the CG point on the rocket. But if you have anything inside the rocket that needs to slide forward (like a parachute ejection-piston device), you'll have to put the button behind it so that the threaded screw on the inside of the tube doesn't prevent it from operating properly.

The nice thing about rail buttons is that they are more forgiving than traditional launch lugs. They can be slightly misaligned, and they will still slip easily along the rail (because they are circular).

Q. How small of a rocket can the rail buttons be used on?

A. Because of the extra length of the launch rail, there is not a bad reason to use a rail launcher for any size rocket. We always recommend a longer launcher for the extra benefits that a straighter lift-off have. You can probably put rail buttons on small rockets. But because they are wider than a launch lug, they will produce more drag on a small rocket than a smaller lug. A good starting point for using rail buttons is a 2.0 inch diameter rocket. If you want to use the long launch rail on smaller rockets, see our alternate suggestion of the "Fly-Apart Rail Guides" in Peak-of-Flight Newsletter 247.

Q. How do I input a Rail Button into RockSim?

A. Use a launch lug to simulate the rail button. Make the length and the diameter to match the dimensions listed on this page. Then use the mass-override to tweak the mass to 2.1 grams.