"It came with the kit!" is a phrase I hear often on the rocket range. Sometimes I hear it when telling a high-power applicant that I have concerns with the eye screw the shock cord is attached to, and sometimes I hear it after the flight has failed because that eye screw gave out. It's important to remember that rocket kit designers fly exclusively on windless sod farms where deployment is always at 0 velocity with no horizontal movement, and the rocket is delicately set down in the soft unmown grass. Someday, I want to launch there too, but in the meantime, I fly in the desert where wind varies greatly in both speed and direction as your rocket increases in altitude, and deployment always involves horizontal movement even if your delay is perfect. To combat these conditions, you need to either decrease the force applied to your shock cord attachment points, or increase the strength of those points.
Many high-power rocket kits come with small wire eye screws to attach the shock cord to either a bulkhead or centering ring. Some kits through caution to the wind and come with downright tiny eye screws. Do yourself a favor and throw those out before you get started. Instead, look for a forged eye-bolt, welded eye bolt, a solid hanger bolt, or a u-bolt. There are other options too, but those are the most common. The strength you need and the space you have available are going to guide your choice here.
Figure 1: A bulkhead and centering ring from a high power kit with the provided wire eye screws shown with the hanger bolt and forged eye bolt replacing them.
To get started, it's helpful to know how much strength your options have. This data is almost always available online just a quick search away. If your selected hardware does not have a strength rating, you can assume it's not meant to take much strain at all. Here are some examples:
|#8 wire eye screw||40 lbs|
|#10 wire eye screw||30 lbs|
|1/4" wire eye screw||50 lbs|
|1/4" forged eye bolt||500 lbs|
|3/8" forged eye bolt||1200 lbs|
|1/4" hanger bolt||50,000 psi (using typical kevlar line that works out to at least 400 lbs)|
Keep in mind that these strength ratings are for gradual application of force directly in line with the fastener. Those two conditions are not likely to happen during a deployment event. It's easy to see looking at the table, that the change from wire eye screws to anything solid is a jump of at least 10 times the strength, and for a few cents to a few dollars, depending on your choice, it's absolutely worthwhile to make that upgrade.
Figure 2: The installed hardware. Always remember to use epoxy, locknuts, thread locker, or some other option to secure the hardware from loosening.
Do yourself a favor, and upgrade your attachment points before you start your kit assembly. Even if that provided eye screw holds on the first launch, it could still give out later on. You can also extend the length of your shock cord to give the parts of your rocket more time to slow down before yanking on your attachment point. This can make weaker hardware more reliable as well as decrease the chances of a zipper happening. I like to do both, and while I have seen many wire eye screws fail in the field, I have never seen an upgraded fastener fail.