Math Mutation 72: Shuttle Butt
Have you heard about how the width of the solid rocket boosters on
the space shuttle can be directly traced to the width of an ancient
Roman horse's rear end? Here's how the story goes.
Apparently the solid rocket boosters for the shuttle are
transported by rail, and need to be correlated with the standard
railroad gauge, or track width, in order to fit through tunnels.
This standard width is about 4 feet 8.5 inches. This width comes from
standards set in England. The English apparently set this standard
because early trains were designed based on similar wagons, and that
was a standard spacing for wagon wheels. The wagon wheels have that
spacing because they had been built for centuries to correspond to
ruts in old roads. And the major roads in England and Europe were
first built by the ancient Romans, who had a standard wheel width on
their war chariots. And finally, the chariots were designed to match
the width of the back ends of a pair of warhorses. Thus, the solid
rocket boosters on the space shuttle were directly based on the width
of a Roman horse's behind!
It's a cute story, but sounded a bit suspicious, so I checked out
a few urban legend websites, like Snopes and The Straight Dope. My
suspicions were confirmed-- there are quite a few holes in this
story. To start with, railroad tunnels are significantly wider than
the tracks, though perhaps some tunnel-related limitation reduces the
size of the shuttle's boosters. As for the standard gauge-- there
were a lot of creative American designers who experimented with
different railroad track widths in the nineteenth century. During the
Civil War, the Confederacy had three different sizes of tracks, and
that may have contributed to their loss. The North had maximized
efficiency by using the most popular width as a standard. After the
war, destroyed Southern infrastructure was rebuilt (with central
organization) to match the North, and that's how the U.S. gained one
standard track width.
But more fundamentally, I think this story is a classic example of
how correlation does not mean causation. In other words, just because
one event occurs before another, it doesn't mean that the first one is
the cause. It's true that at the dawn of the railroad era wagon
wheels were about the same width as ancient Roman chariots-- but
that's because from ancient times through the Industrial Revolution,
transportation technology was a constant. Whether an ancient Roman
charioteer or a 19th-century settler, you were using wheeled vehicles
pulled by pairs of horses. So, the common cause of the width of a
pair of horse's behinds explains the widths of ancient Roman chariots,
medieval English wagons, and the first railroads.
By the way, before I go, I'd like to thank listeners Matthew and
Julie from Canberra, Austrailia, who emailed me to offer a donation to
the podcast! I told them that I actually don't spend an appreciable
amount of money to produce the cast, so suggest that they instead make
a donation in honor of Math Mutation to their favorite charity.
They did so, donating to Doctors Without Borders. If any of you like
this podcast enough that you are motivated to spend money, please do
likewise, donating to your favorite charity in honor of Math Mutation
and telling me about it in email. However, if you think the podcast
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steal from any charities, as that wouldn't be very nice.
And this has been your Math Mutation for today.
References:
Snopes
page on the Railroad Gauge urban legend
Straight
Dope page on the Railroad Gauge urban legend