Zipper Repair

Bad things can happen if a parachute deploys when a rocket is moving at a high rate of speed. This could be the result of many factors - like maybe your delay was too short or too long, or your rocket had a flight trajectory that was way off perpendicular, or maybe your drogue chute failed to deploy. But whatever the reason, the result is often the same: the dreaded zipper. A zipper is a ragged gash caused by the recovery harness ripping through your airframe. If you have ever opened a Band-Aid wrapper with the little red string that tears its way through the paper packaging, then you know how a zipper can happen at high speed. And lets be honest most rocket builders know their way around a box of Band-Aids.

There are ways you can ruggedize your rocket design to help prevent a zipper if you plan ahead. The construction article located [HERE!] dealt with this topic. But what can you do with a rocket that has suffered a significant zipper?

A little off the top, please

If you can stand the idea of shortening your rocket by a few inches, the easiest way to get that bird back to the launch pad is to cut off the damaged bit of tubing. But before you pick this quick and easy fix, lets consider how this affects the stability of the rocket if you need to shorten it by more than just a few inches.

Whenever you shorten your rocket, both the center of pressure (CP) and the Center of Gravity (CG) will move aft -- and not necessarily at the same ratio. A previously stable rocket would become unstable if the Center of Gravity is no longer forward of the Center of Pressure, or marginally stable if the two are about even. The rule of thumb is that, in a stable rocket, the CG should be at least one caliber (airframe diameter) forward of the CP -- youll want to make sure that you havent created a hazard when you reduce the length of your rocket. You may need to add nose weight to balance your rocket if the airframe is shortened too dramatically.

You make a cute coupler

The alternative at the opposite end of the repair spectrum is to replace your damaged airframe. Once again, cut away the damaged portion of the tube; but the next step would be to join a replacement section of tubing to the shortened rocket using a suitable coupler. You may even choose to use this opportunity to modify your original design you could stretch the rocket, add a payload section, or even convert it to an anti-zipper design per the article mentioned above.

There are aerodynamic considerations for this modification as well. Adding too much length can over-stabilize your rocket. The result can be that your rocket can react to a cross wind much like a weather vane. The flight trajectory will divert upwind once the rocket leaves the launch rod, and the flight path will more closely parallel the ground plane. As a result the rocket will reach apogee sooner and will not slow as it would if it flew vertically, leading to yet another high-speed parachute deployment and the potential for repeat airframe damage.

Another clever subtitle would go here if I could think one up

There may be situations where the solutions above are not practical. Its possible to repair the damaged tube opening without shortening the rocket or replacing the damaged section. All you need is masking tape, some waxed paper, some epoxy finishing resin, a small piece of fiberglass cloth or fiberglass tape, and one or two hose clamps.

First, wrap the shoulder of your nose cone (or a section of coupler tube) in one layer of waxed paper and secure in place with masking tape. Slide this inside the damaged end of the tube and apply masking across the tear caused by the zipper. Cover the void completely, but dont stick the masking tape to the waxed paper and dont try to tape the tube too tightly at this point. Remove the wrapped nose cone (or coupler) from the tube.

Cut a piece of the fiberglass cloth large enough to cover the zipper and then some. Mix a suitable amount of epoxy resin and hardener, and wet out the cloth with it. Apply the wetted cloth to the inside of the tube across the zipper. Leave just a bit of the cloth hanging out past the end of the tube and use this to hold the cloth in place while you slide the wrapped nose cone (or coupler) back into place. Place the hose clamps around the tube at the repair point and tighten them down gently (you can substitute several tight wraps of masking tape if you prefer).

After the resin has cured, remove the masking tape from the outside of the tube. Add some lightweight spackle or spot putty to any surface imperfections and sand smooth. Remove the nose cone (or coupler) from inside the waxed paper. Peel out the waxed paper and sand smooth the inside the repaired tube; remember that a parachute will be ejecting from this opening, so you dont want to leave any edges that it can get caught on. Then prep and paint -- youre ready to go!