The Flyin' Stovepipe

by Roger Deran, LUNAR #534

At the February launch I brought a recent incarnation of the Flyin' Stovepipe, a tubular glider that I submitted to Model Rocket News in 1968. (The plans for it are still available at "JimZ" which is http://www.dars.org/jimz/eirp_56.htm) The first flight was a failure, the second a success, with a long slow glide that curved in a large semicircle around the field. The first flight was observed by the entire club, the second by only the few diehards that stayed on after the rainout, using Brett Buck's personal launcher. (Note that he was given explicit permission from LUNAR staff to do so - you can't just set up there any time you want.) Jack Hagerty's comments about the first flight were I think typical of the opinion some of you must have right now about the Stovepipe. Before the launch: "A tube glider?" and after the launch: "Needs work!" But the glider actually works well. I will explain the failure below, but first I will have to explain the principle of operation.

The glider is a lightweight tube approximately square in sideview that has a weight ring at the leading edge to move the Cg forwards to about the 1/3 position from the front. It is lofted by a concentric tumble-recovery booster which ejects itself rearwards. The booster looks like an Estes "Scout" from the 60's, or an 18mm mosquito with a motor hook.

You can make a glider tube yourself out of a sheet of typing paper. Fold over the long edge of the typing paper about 3/4" at a time, for about five folds. Take the short edges and bring them together forming a tube. Tape the short edges together and make sure the tube is relatively round. The glider requires a football type of throw, so that it spins while moving in the direction of its axis. The flight will be about 20' or so before the glider runs out of gas and falls. The glider can also fly without the spin stabilization, but is not so easy to throw. The rocket version spends most of its time gliding without the spin stabilization.

The fins of the booster are square on the end and slide inside the glider body, but are stopped at the front by the weight ring. The body tube of the booster is motor-diameter and -length. The motor hook is much longer than the motor. The nosecone is glued in place, and there is a small hole in the body tube next to it, through which ejection gasses go. On recovery, the motor slides rearwards but is stopped before leaving the body tube by the hook. The ejection charge propels the motor backwards and the body tube forwards at the same time (due to conservation of momentum). It is the momentary forwards motion of the body tube that throws the glider forwards off the booster (or the booster backwards out of the glider, depending on your point of view.) Once the booster is free-falling, it tumbles because it is unstable with the motor having been moved so far aft.

So why the initial failure? Simple: it spit the motor! The motor hook, which is rather flexible due to its extra length, was pushed completely out of the way by the ferocious ejection charge. The fix? Equally simple: curve the end of the motor hook forwards so that it actually points into the aft end of the motor. My motor hook is made of 55 mil music wire.

Here are some Stovepipe construction notes that are not noted in the instructions at JimZ:

You cannot get BT101K any more as far as I know. I wrapped a light glider tube using two layers of 2" brown packaging tape over a 98mm EZI-65 payload section "mandrel". Wrap the inner layer sticky-out, and the outer sticky-in. Using the kind of tape that adheres when wet makes the alignment easy. Just wrap the first layer and tape its ends down, adjusting to get a perfect spiral wind. Do the same for the second layer, placing it at a one inch offset from the first layer. Then undo 6" of one end of the second layer, wet, and replace. After that dries, undo the unglued part of the second layer, wet, and re-wrap, smoothing from the already-glued end. Cut the ends of the glider tube square by cutting against the square end of the mandrel. The weight ring is fashioned out of nested 1/2" sections of spare glider tube. Make the glider tube light. My latest glider is 8 grams. The original BT101K glider was 25 grams.

The motor hook must not tear loose. To fix it firmly, the forward bent end should be longer than the forward end of a normal motor hook; about 1/2" is fine. Press it into the side of the solid-balsa nosecone. Epoxy a lengthwise slice of 18mm body tube over the hook to fix it to the main tube. The aft end should be longer than normal too. Bend it into a "fishhook" shape so that the end actually points forwards.

Forget about the "saddles" shown in the JimZ picture. They are very hard to align to the fins and are really not necessary.

A good first flight motor is an A8-3. This gets you to about 150', and ejection will be right at apogee.

Several of us have already flown or are experimenting with Stovepipes. I hope to see some of them at the next launch.

Information date: Mar. 16, 1999 lk