Oooo, a rocket question!

I know, it's *yawn* to most of you. :) But it's not often I get to act like Horschack.

Victor left a comment on my post about the upcoming Bowling Ball Loft contest. He was referencing part of the rules:

I took a look at the rules, and this intrigued me: Use a launch rail, tube or tower. Rods are prohibited due to past bad experience.

And he asked:

Can you explain the difference between the four launch pad configurations (apologies if my terminology is not correct) and what kind of bad experience they may have had with a launch rod? (I realize that might be speculation on your part.)

No speculation needed, I know exactly why they don't allow the use of launch rods in this situation. First a little background:

An unguided rocket (like we fly) has to be moving at a certain speed for the fins to have a stabilizing effect. Usually it’s around 40mph, although a lot of different factors can make a difference one way or another. Since hobby rockets are launched very nearly vertical, we use different ways of making sure that the rocket stays pointed straight up until it’s moving fast enough for the fins to take over.

All of these assume that the launch pad itself is stable. Good wide legs, low center of gravity, anchored to the ground or hefty construction; all of these combine to ensure that the launch pad won’t tip or tilt when the thrust of the motor kicks it. Attached to the launch pad itself will be the rod, rail, tube or tower.

The oldest method is the launch rod. Most commonly used for the smallest model rockets (1/8” x 36” long), it doesn’t scale up well but is still used - up to 1” diameter rods around 12 feet long. The problem is that when more power and weight are used, the rod tends to ‘whip’ which can fling a rocket off vertical. Not a biggie with a nine ounce model rocket, but it can be very dangerous with a nine pound rocket. A ‘launch lug’ is used, which is just a length of tubing glued to the rocket that slides loosely over the rail. On smaller rockets, the lug looks like a short piece of soda straw.

The launch rail is quickly becoming the standard method of launching bigger rockets. Made of extruded aluminum, the extra mass and shape of the rail makes for a much stiffer guide, which ensures that the rocket stays vertical as it launches. Instead of lugs, ‘rail buttons’ are used, which slide into the channel of the rail to provide the guidance. There's a picture of a typical rail in the extended entry.

A launch tower is primarily used in altitude contest launches. Instead of a lug or buttons attached to the rocket, the tower provides the guidance for a rocket by using three rods or rails spaced around the rocket body (between the fins). In its simplest form, a launch tower can be three parallel rods sticking up out of a coffee can full of cement. The main advantage is that since the rocket doesn’t have lugs or buttons, there is significantly less drag, which makes for higher altitudes. The main disadvantage is that a tower is only good for one diameter of rocket, unless some way of adjusting the guide rods is included, which adds to the complexity and cost. This elegant design here – by another Ted – allows for the three most common diameters of model rockets.

A launch tube is similar to the tower, except that the guidance is provided by the walls of the tube against the tips of the fins. Unlike a gun barrel, there is no back pressure assisting the liftoff. There are ways to use the ‘cannon’ method of launching as well, but it’s difficult enough that it’s not usually worth the effort and extremely rare to see it done.

Professional rockets use a variety of these methods, usually for the same reasons we do. The Super Loki Dart sounding rocket (this picture is of a scale model) is launched from an 12’ long tower (picture here along with some specs) that is spiraled like a gun barrel to provide spin and extra stability. The Loki reaches Mach 5 in a little less than a second, so staying straight is critical or the rocket will break into pieces.

Personally, I use launch rods up to about ¼” diameter – on anything up to about 2 pound rockets. I have rail buttons mounted on our larger rockets, and a lot of our rockets are rigged to use either, just in case a rail isn’t available. Given a choice, I’ll use the rail any time, because I’ve seen some scary flights caused by rod whip.

In this picture, a hex nut is shown (red arrows) as part of the mounting process. Ignore that and notice the shape of the rail and how the rail buttons will fit into any of the other slots.

Posted by Ted at December 22, 2003 03:16 PM
Category: Rocketry