Why does Atlas V have so many different types of payload fairings?

Question

So I was curious about why the Atlas V rocket has so many versions, with three-digit designators like "401" and "551". The obvious differentiator is the number of solid rocket boosters you can add to the core rocket. Also, there's an option for extra engine for the second stage (though that hasn't flown yet AFAICT).

One thing I don't get though is how many different payload fairing options they offer. They have a 4-meter and a 5.4m fairing types, with very different designs, but both have 3 different length versions. And while, say, 5-m Long is a lot longer than 5-m Short (by almost 5 meters), the three different versions of the 4-meter fairing have almost negligible differences.

My question is why. Wouldn't it be better to just have maybe two versions, one standard and one extra long? Perhaps this is a stupid, opinion-based question. But is there an obviously good reason for this? Do the 4m and 5m fairing types feature a different design because of some important trade-off? Is the weight difference between the length options significant enough to warrant making them?

Answer 1

You can find lots of information in the User Guide (420-page PDF).

Page 57 shows the performance effects:

Atlas V 400 series performance is based on the use of the 4-m EPF. GTO performance with the LPF is approximately 35 kg (77.2 lb) more than the EPF with the 401 configuration and 44 kg (97 lb) more with the 431 configuration. GTO performance with the XEPF is approximately 35 kg (77.2 lb) less than the EPF with the 401 configuration and 44 kg (97 lb) less with the 431 configuration.

abbreviations are: Large Payload Fairing (LPF), Extended Payload Fairing (EPF), and Extra Extended Payload Fairing (XEPF).

They've also included the reason for the 3 fairing sizes (same PDF, page 267):

Atlas V developed the EPF to support launches of larger-volume spacecraft by adding a 0.9-m (36-in.) high cylindrical plug atop the cylindrical section of the LPF. The XEPF is a modified version of the EPF that incorporates an additional 0.9-m (36-in.) high cylindrical plug to further increase the available payload volume.

So they started with the shortest fairing, then developed larger versions as the need arose. I suspect they saw no sense in throwing away a proven design, so all 3 were kept around.

Answer 2

You're forgetting that on top of their mass being different, larger ø also means larger cross sectional area, and with it increase in aerodynamic drag. So larger ø ones are heavier, subject to stronger atmospheric resistance, have larger contact area subjected to ascent heating, and with a more contoured launch vehicle's body deviate from ideal aerodynamic shape and are subject to more vibration due to pressure differential and transonic wave drag. It makes sense to have many different ø and length payload fairings to choose from so they fit requirements of different payloads and can achieve best performance and mass to orbit.

Answer 3

While other answers are perfect from an engineering point of view, there is another aspect. The 4m and 5m fairings have different designs and are produced at different sites.

5m fairings have aluminum honeycomb core and graphite epoxy face panels and are made in Emmen, Switzerland (RUAG), the same manufacturer that makes Ariane 5 PLFs.

Basically, this is a business decision to outsource production to the producer which has the experience, technology, and frequent enough orders to justify keeping the line open. Mind you, it is only 23 employees, and mostly skilled manual labor (not a Chinese consumer plastics sweatshop, and not a high-volume auto assembly line either).