According to an old story that’s circulated the internet for years, the dimensions of the space shuttle’s solid rocket boosters were prescribed by the width of a horse’s rear end.
The story goes from Roman chariots that were made wide enough to accommodate the back end of two war horses to British roads that were built for those chariots and ended up with ruts where their wheels were to Engish wagons that were built wide enough to fit those ruts to trains that were built from the jigs and toolings for those wagon and thus U.S. railroads were all built to the width of a Roman chariot and thus based on the width of two horse’s butts. And then it takes it a step farther to the fact that the shuttle solid rocket boosters were designed to be transported via rail and thus had to fit through a railway tunnel determined by the width of a train and thus, yadda yadda, horses.
The particulars of the story get some stuff wrong. U.S. railways didn’t originally have a standard gauge, and were built to a variety of widths before being standardized, so there was no particular magic number that they had to be. On the other hand, it’s also worth noting that the original railroad cars were horse-drawn, so there was a more direct connection between the widths of train tracks and horses, so there is some basic truth to the story, even if the particulars aren’t exactly right.
I was thinking about this story again recently because of interesting fact I learned about the Space Launch System rocket I’m honored to support.
The core stage of SLS is 27.6 feet in diameter, because it’s designed to have the same diameter as the space shuttle’s external tank in order to more effectively take advantage of existing manufacturing and launch facilities. We were talking about that at work, and the question came up as to why the shuttle’s external tank had that diameter. We suspected at first it, in turn, had something to do with the facilities left over from the Saturn days, but weren’t able to find the answer anyway.
So I called someone I know who worked on the external tank, and asked him. And the answer has to do with the fact that the shuttle’s solid rocket boosters were to be mounted to the side of the external tank. Given the volatility of the fuels inside the tank, you wanted the attach points to be somewhere on the structure off of the fuel tanks inside it. The length of the solid rocket booster had already been established, and that determined what the length of the external tank would need to be to properly accommodate the attach points. The engineers knew what the volume of the tank had to be in order to hold enough fuel for launch, so once the length was established, the diameter was just a question of division.
Which means that the next time astronauts fly around the moon, they’ll be launched on a vehicle with a diameter determined loosely by the width of horses’ butts.