Shock cord mount directly to airframe

[PatB]

Our kids are working on a Co2 ejection system for their SLI program. They'll be using a K motor and a 4" airframe. Their design requires that they maintain airtight sections in their parachute compartment. They'd prefer to not have to run their shock cord all the way back to the motor mount and then into their parachute compartment. They'd like to find a way to have the shock cord attachment point inside of their parachute compartment.

Their design isn't using the typical dual deployment section that comes apart at both ends. They'll be having some equipment inside their parachute compartment that they'd rather not have knocked around by a shock cord that comes into that compartment from below. Their parachute and nose cone will be the only thing ejected at deployment- no dual deployment band section of airframe.

Do you think that epoxying their 1/2" tubular nylon shock cord directly inside their fiberglass airframe would be strong enough? I don't see that as much different than epoxying the shock cord to a motor mount other than the fact that a centering ring will provide more strength.

 

[Rocketjunkie]

I use a similar method frequently, mostly for paper airframes but it works for fiberglass also. For fiberglass, wash an area on the inside of the airframe from about 6" inside to about 12" with acetone or similar solvent. Use 60-80 grit sandpaper to thoroughly roughen the washed area (about 3-4" wide). Cut a piece of tubular nylon about 24" long. Apply some epoxy to the last 4" on both ends and insert in airframe so the ends are near each other and the center (now a loop) extends slightly past the end of the airframe. Once the epoxy sets, cover the glued down ends with more epoxy and lay a piece of fiberglass cloth over the new epoxy. Once cured, make sure everything is smooth. Sand or add more epoxy as needed. The TN will break before the mount rips out. Also make sure the epoxied part is far enough down the airframe section so it doesn't interfere with the mating section or nose cone.

 

[PatB]

Thanks for the advice. It's nice to find someone with experience in this area. It's very difficult to keep their parachute compartment airtight, and not having to have another hold go through their rear bulkhead will be very beneficial.