To Join or Not to Join?

Before we start looking at different joining methods, it is important to understand some of the basic driving force behind the need for joints in aerospace structures. As was stated before, some joints are always necessary as we need to make things such as doors to allow passengers and cargo to enter the vehicle. These separate structures will need to be joined in order to meet this functional requirement. But what are some other reasons for needing a joint? The following list is not exhaustive, but it provides some insight to what may be the reasoning behind a particular joint you need to design.

Maintenance and Inspectability

In order to perform maintenance and/or inspect a particular part of an aircraft or spacecraft, you must have access to that part. Joints are thus necessary in order to allow disassembly of complex structures, or to enable an access panel or door to be included in the structure. It also has a large effect on how you perform maintenance. Consider for example the Boeing 787 vs. the Airbus A350. Both planes have composite fuselages, however, the 787 is comprised of large integral structures and thus has fewer joints while the A350 is comprised of smaller fuselage skin panels all joined together. The 787 has less joints, which has positive implications on assembly cost and structural efficiency, while the A350 will have more joints, which can introduce a weight, cost, and efficiency penalty. So what is the benefit of more joints in this case for the A350? Well, consider if a major damage occurred in service to both aircraft. For the 787, replacing the entire fuselage barrel would likely be a cost-prohibitive option, while for the A350 it could more easily be replaced. Again, all designs are compromises, so this does not mean either design choice is better than the other.

Politics

Yes, even in the aerospace industry, there are politics! Not all decisions related to the design, manufacture, and assembly of aircraft and spacecraft are made on engineering and performance grounds. An aircraft or spacecraft may be subdivided in a way to ensure that some of the design and engineering work goes to different parts of a country or even different countries for political reasons. For example, the International Space Station is an international project that requires additional joints to allow collaborating countries to make their own portion of the station in their own country.

Design Optimization

Different portions of a structure will have different requirements, often leading to the selection of different materials and/or manufacturing methods for regions of a structure. Joints are needed to connect these different regions.

Logistics

As we discussed for the reason of politics, sometimes parts will be manufactured in different locations in a country, or in different countries all together. However, this is not always motivated by political reasons. Sometimes there are real engineering reasons for parts to be manufactured in different places, but ultimately, these parts need to be brought together and joined.

Damage Tolerance

We already discussed the importance of the Damage Tolerance concept to the safety of aircraft structures, and briefly saw how structures such as skin panels with riveted stiffeners have a higher damage tolerance than integrally machined stiffened skin panels. As a result, many joints in aircraft facilitate this design philosophy.