Why they made the Space Shuttle's booster rockets the size of two horses' hindquarters.

If you’ve ever seen a picture of a US Space Shuttle on its launch platform, you may have noticed two white rocket things, one on either side of the main body. Seeing them, a number of questions may have crossed your mind, like what they’re for, exactly, and what happens to them once they’re done with, and, if you live under the flight-path, what the likelihood might be of one of the things landing on your head, or your house; but of all the things you might have wanted to know, the relationship of these rockets to the dimensions of a horse’s backside may not have been quite top of your list.


But just in case you did want to know the answer, I’ll explain it here. The rocket things are Solid Rocket Boosters, or SRBs, and they work very much like giant fireworks, except without a giant explosion at the end (usually).

These SRBs are made by a company called Thiokol at a factory in Utah, and are delivered to the launch site by train. The railway from the factory runs through a tunnel in the mountains. This puts an upper limit on the thickness of the SRBs: they have to be narrow enough to fit through the tunnel.

The diameter of the tunnel, in turn, is determined by the width of the track; and the width of the track is just over fourteen hands, or the size of two horses’ behinds.

The reason that the track is this size is because the USA, like most countries in the world, use a standard guage for their railways, which is known as Stephenson’s Guage.

This was the wheel spacing used by the first locomotives, developed by George and Robert Stephenson in the early nineteenth century, and it came about because before the Stephensons there were no such things as trains, and no such things as train factories, so the wheels and axles for first ones had to be made by the people who made horse-drawn trams.

These tram-builders used the same jigs and tools that they used for building carts and coaches, which used that wheel spacing; and they used that spacing because if they tried to use any other spacing the coach wheels would get damaged on some of the old, long distance roads, because that was the spacing of the old wheel ruts.

These old wheel ruts were the result of generations of wheeled traffic being designed around a maximum width of two horses, side by side; and the optimum width for two horses, side by side, is roughly the same as the height of a single average-sized horse, or fourteen hands.

And that, ultimately, is what determined the size of the Space Shuttle’s booster rockets.