Steve Pisani
One of my favorite bits of Internet lore is the connection between the design of the rocket boosters on the space shuttle and the size of Roman war chariots. Here is the rough retelling:
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The US standard railroad gauge (distance between the rails) is 4ft, 8.5in. This gauge is used because the English built railroads to that gauge and US railroads were built by English expatriates.
Why did the English build railroads to that gauge?Because the first rail lines were built by the same people who built the pre-railroad tramways, and that’s the gauge they used.
Why did those wheelwrights use that gauge then?Because the people who built the horse-drawn trams used the same jigs and tools that they used for building wagons, which used that wheel spacing.
Why did the wagons use that odd wheel spacing?For the practical reason that any other spacing would break an axle on some of the old, long distance roads, because this is the measure of the old wheel ruts.
So who built these old rutted roads?The first long distance roads in Europe were built by Imperial Rome for their legions and used ever since. The initial ruts were first made by Roman war chariots, which were of uniform military issue. The Imperial Roman chariots were made to be just wide enough to accommodate the back-ends of two war horses.
This story does not end there, however.Look at a NASA Space Shuttle and the two big booster rockets attached to the sides of the main fuel tank. These are solid rocket boosters or SRBs. The SRBs are made by Thiokol at their factory at Utah. The engineers who designed the SRBs might have preferred to make them a bit fatter, but the SRBs had to be shipped by train from the factory to the launch site in Florida. The railroad line from the factory runs through a tunnel in the mountains and the SRBs have to fit through that tunnel. The tunnel is slightly wider than the railroad track.
So, the major design feature of what is arguably the world’s most advanced transportation system was determined by the width of a horse’s ass.
Now, this story may not be 100% factual but, I like to believe it is pretty damn close and provides plenty of food for thought.
The headline in my mind is that design decisions propagate.
The initial constrain of “accommodate the back-ends of two war horses” got passed along over and over simply because the next step relied on the previous. So by the time NASA was designing their rockets it had to be taken into account.
At a macro level, we (humanity) have been building on top of what came before us for all of history. At each stage in the design tree of horse’s ass to space ship, no one said “welp gotta keep this to the original ass spec”. They stayed pretty close to it because it was already built. No need to re-do the work. What gets my attention though is that the rocket’s size could have been larger if the constraint wasn’t there. Would that have made them more effective? I don’t know but it doesn’t sit well with me that the design was limited based on the needs of the inital design.
Back down on a micro level, the decisions we make can propagate farther than we anticipated. Writing new backend code for your company’s web app? Well whatever comes next is probably going to build on top of it. Designing a database structure? Whatever comes next is probably going to build on top of it.
All of this is my long-winded way of saying: when you’re making something, take the time to understand constraints that might be piggy-backing in your work and if they are holding you back.