The "BT-70" (Apogee P/N 10160) is a great size body tube to make egglofter rockets. An egg will fit nicely inside it, with a little bit of extra room to allow some padding to protect the egg from breaking.

To go with the BT-70 size tube, Apogee Components is also stocking some tube couplers. This allows you to take the BT-70 size tubes, and join them together to make longer size rockets. It's fun to make long rockets, isn't it?

Additionally, I have created a new die-cut centering ring for the BT-70 tube. This will allow you to take a BT-50 (24mm) tube (Apogee P/N 10099), and use it as an engine mount inside the BT-70 size tube. The rings are made from a thick cardboard, which is very strong but it is still fairly lightweight. With the new centering rings, you'll be able to fly your BT-70 size rockets on "D", "E" or "F21" size motors. This is the mid-power range that is more impressive than beginner kits, but still are under the 1 pound weight limit, so you don't need a permit to fly your rockets. But wait. There's more. When you order a set of rings, you don't get just one or two rings, you get a "bulkhead disk plate" too. What is it good for? Great question.

Imagine you're building a rocket with a payload bay. As I've already mentioned, the BT-70 size tube is perfect for large diameter payloads, like eggs or electronic altimeters. You want to house the payload in a section of the rocket and protect it from the heat of the ejection charge.

What most modelers do is create a compartment near the front of the rocket, just below the nose cone. To seal off the base of this compartment, you need a solid disk. But at the same time, you also have to attach the payload compartment to the rest of the rocket. To do that, you use a tube coupler. What this new disk does is fit inside the tube coupler.

Why inside? Why not just size the disk so that it seals of the inside of the the larger BT-70 tube? The reason is for extra strength. Instead of positioning the disk at the end of the tube coupler, you recess it slightly into one end. By doing this, you can put a glue fillet on both sides of the bulkhead; effectively doubling its strength!

Another use for the bulkhead disk would be in a "sliding piston assembly," like used on a lot of high power rockets. The piston is used to pop the parachute out of the rocket, so you don't have to use wadding. Again, you put it on the inside of the coupler.

For more information on making payload bays, or pistons to eject your parachutes out of the rocket, see the book: Model Rocket Design and Construction.

When you order the new BT-70 size centering rings, you'll automatically receive the bulkhead disk plates at no extra charge!