The Wiki Man | 12 July 2008

Alongside the vast fuel tank which powers the Space Shuttle into orbit are two spindly tubes known as Solid Rocket Boosters (or SRBs). Their shape is not ideal: their manufacturer, a firm called Thiokol, had intended them to be fatter, but was constrained by the width of a horse’s rear end.

It appears that Roman chariots arrived at a standard axle width of 4’ 81/2” for the simple reason that this width could accommodate two horses’ bums between the shafts. Standardisation of axle length was vital, as on muddy roads your wheels formed ruts that set solid in dry weather. A vehicle with a non-standard axle can become fatally cross-rutted, a danger for off-road drivers even today. So the 4’ 81/2” standard became literally entrenched.

Axle-makers maintained this length for the next couple of millennia, even into the railway age (helped by the fact that many early rail carriages were adapted road vehicles). A few years later, Britain’s pioneering role in railway development saw the gauge gain hold in the United States, where it influenced the dimensions of railway tunnels. And since SRBs need to be ferried to the launch site by rail, their diameter had to be reduced to allow them to pass through tunnels between Thiokol’s plant in Brigham City, Utah and Florida. So NASA rocketry found itself subject to a standard first adopted in the age of the horse.

This story is regularly used to illustrate how adopted standards, however arbitrarily devised, may have massive consequences long after the original reasons for their adoption have become irrelevant. 

There are many more such cases, the most famous being the sub-optimal QWERTY layout of a keyboard originally intended to avoid key-jamming in mechanical typewriters. Similarly, the size of a CD was chosen simply so that CD cases two abreast could sit on retail shelves recently vacated by LPs.