Epoxy is the most common building tool in rocketry. It provides a lightweight, strong bond between parts to make sure they do not come apart under flight forces. Different epoxies have different requirements and use cases, so make sure you understand what you need before choosing an epoxy.

When selecting an epoxy, there are four main things to consider. Cure time, cost, material compatibility, and strength. 

Cure time refers to how long it takes for the epoxy to go from just mixed to full strength. This is different than the working time, which is what is on the front of packages or in the name of epoxies. Working time is just how long it is in a liquid state for application, but once working time is complete, epoxy may only be at 20% strength. Most 5-minute (working time) epoxies have an 8- to 24-hour cure time. 

Cost is obviously how much epoxy costs. Epoxy is a very expensive material that many people overlook when designing a rocket. Make sure to take this into account when budgeting. Expensive epoxies may be stronger or easier to work with, but often times something cheaper is just as good.

Material compatibility refers to what materials the epoxy can be used on. Some epoxies will react violently with some materials (PVC is a common one, epoxy can sometimes melt it), while for other materials the surfaces may require special coatings or care (often metal needs to be scuffed or sanded first). Material compatibility information can be found on the epoxy datasheet. 

Strength refers to how much force the epoxy can withstand before breaking. It is usually represented as a modulus, in PSI. This is especially relevant when dealing with things like applying charge caps, or connecting the thrust plate to the rest of the rocket. 

Commonly Used Epoxies

For most applications a general-purpose 5-minute epoxy like JB ClearWeld is more than sufficient. It is a low-cost epoxy that can cure quickly and is compatible with a wide range of materials. 

For more advanced or high-strength applications, G5000 is commonly used. It is more expensive and harder to with with than JB Weld, but has a strength that rivals solid aluminum. 

For fixing together motor parts, sometimes a high-temperature epoxy must be used. An example of this is Duralco 4703, an epoxy that can withstand sustained temperatures of up to 650 degrees Fahrenheit. High temperature epoxies are often very hard to work with, sometimes requiring active heating to cure properly. 

Epoxy Testing

John Coker has conducted testing on a number of different epoxies, covered in an article here.