
The Space Shuttle and the Horse's Rear End (2000) - quickthrower2
http://www.astrodigital.org/space/stshorse.html
======
jcranmer
Okay, people love telling this story, but the story is really a load of stuff
coming out a horse's rear end.

Let's start with the track gauge. Standard gauge rail is 1435mm or 4'8½", but
only about 55% of the world's track is standard gauge. Russia standardized on
a 1520mm or 5' gauge, India uses a 5'6" gauge, the former British colonies in
Africa use 3'6" gauge, and there's a few other gauges kicking about.

In the US itself, the South predominantly used a 5' gauge for all operations,
until they shifted on May 31 and June 1, 1886 to the 4'9" gauge of the
Pennsylvania Railroad (yes, the US shifted ~11k miles of track in just 36
hours). Notice how I said 4'9" there--the Pennsylvania Railroad, one of the
largest railroads in US history, did not use 4'8½" gauge at the time but 4'9";
it only shifted to 4'8½" sometime in the early 20th century.

Go back in history, and the gauge proliferation was even worse, because there
was little reason to standardize when railroads mostly didn't interconnect
with each other. The notion that railroads adopted the distance between ruts
on roads is largely bullshit because those distances weren't uniform. But if
you're designing vehicles drawn by teams of horses, you're going to end up
with a harness spacing of about 5', give or take a foot. The line that the
Stephensons worked on used 4'8" (and was increased ½" for technical reasons)
for its horse-drawn operations, so Stephenson built his locomotive to the
dimension of the line, and used that dimension for later lines not directly
connected to the original line. That seed of 4'8½" lines for interconnection
eventually led to it winning out over alternative gauges, but the younger
Stephenson is reported to have personally preferred a slightly broader gauge
of perhaps 5'.

Now, the discussion of track gauge is perhaps interesting, but it's completely
irrelevant to the sizing of SRBs for the Space Shuttle. The relevant metric
there is what is known as the loading gauge--the maximum permissible envelope
that can fit within tunnels and bridges (and station platforms). Track gauge
is mostly irrelevant to loading gauge--unlike cars, where the body of the car
sits between the wheels for the most part, train bodies sit above the wheels,
and so the body can extend beyond the track gauge.

Loading gauge is far more variable than track gauge and is a more serious
constraint on railroad operations than track gauge. Most European trains can't
run in the UK, because the UK's loading gauge is absolutely tiny. Europe
generally runs on a ~10'4" wide by ~13-14' high gauge. The US uses a very
liberal gauge, with 10'8" by 14'6" the old norm but mostly replaced with a
20'2" gauge for double-stacked container freight.

A final point is that even the maximum size of a tunnel isn't determinative
for parts. You can break parts into multiple pieces and assemble them after
shipping. You can also ship oversize parts if you've got careful logistics
planning. Ever seen oversize trucks on the road? There's similar games you can
play with railroads; the oversized cargo wagons for freight are known as
Schnabel cars, and you can do things such as shift your load so that it's off
track center and get extra clearance in tunnels.

So to respond to the questions posed correctly:

> That's an exceedingly odd number. Why was that gauge used?

The ½" comes from a moderately arcane fix to an old coal railway to improve
operation. Otherwise, 4'8½" is basically "the first successful railway used
this."

> I see, but why did the English build them like that?

Because any number between ~4-6" is basically good enough, and the die in this
one case just rolled on 4'8".

> Well, why did they use that gauge in England?

It was nowhere near universal, see prior answer. The answer given here is
basically someone making stuff up. Remember, this is before interchangeable
parts was deemed to be a workable concept, and standardization was not that
precise.

> Okay! Why did their wagons use that odd wheel spacing?

As mentioned above, a team of horses two wide naturally needs you to 5±1'
gauge. The answer given is pretty much crock. (Not going to bother with the
next two questions, which are just doubling down on the crock).

> So, just what does this have to do with the exploration of space?

Nothing. The maximum dimensions of tunnels and bridges aren't as hard a
constraint as it's made out to be, since there are several ways you can
squeeze out extra space if you have to.

Just going to call out one bit that's particularly egregious.

> The tunnel is slightly wider than a railroad track

10'6" is not "slightly" wider than 4'8½", it's well over twice the size. If
you've ever ridden a train in your life, you should realize that it is
definitely far larger than just "slightly" wider than the railroad track.

P.S., for what it's worth, the SRBs on the Space Shuttle are 12'2" in
diameter, according to Wikipedia. That doesn't fit in the normal loading gauge
of even the US system.

~~~
rini17
OT: Isn't there a bot that can convert all these to metric?

------
sitharus
This comes up every so often, but is unfortunately not really true:
[https://www.snopes.com/fact-check/railroad-gauge-
chariots/](https://www.snopes.com/fact-check/railroad-gauge-chariots/)

Rail gauges are an interesting topic and there are a variety of gauges and
many were experimented with over the years. Russia and Eastern Europe mostly
runs on 1,520mm broad gauge. New Zealand and non-high-speed Japanese rails are
3'6". Wider gauges make trains more stable at high speed but require much
larger corners, and vice versa.

~~~
JackFr
Snopes rates it as partly true, and the author is a pedantic killjoy.

It is broadly true that for 2000 years European vehicles matched their roads
and adapting existing vehicles to new technology led to using axles that were
the same size. Do other gauges exist? Of course. Are the true elements of this
worth repeating? Sure.

As an unrelated aside here’s Richard Feynman explaining why train axles don’t
need differentials.

[https://youtu.be/WAwDvbIfkos](https://youtu.be/WAwDvbIfkos)

~~~
jcranmer
Honestly, rating it as partly true is giving it too much credit.

The only parts that are true are that a) the US adopted 4'8½" gauge largely
from British standardization, and b) the 4'8½" gauge was chosen by extension
of a pre-existing horse-drawn system. The moral of the story, however, is
supposed to be that "the SRB was constrained by a Roman standardized wagon
size by a long chain of events." And the first postulate is false before the
chain can start (as far as I know, the Romans never standardized wagon sizes,
not even informally--except insofar as you're going to end up picking roughly
the same size, to within a foot or two, given similar constraints), while the
last postulate is also false because _track gauge does not determine loading
gauge_.

It also appears, judging from what I can find, that the loading gauge is
actually smaller than the SRBs anyways. So even the loosest version of the
final step of the chain appears to be false. So what's the elements worth
repeating?

------
api
Modern computing analog: the first sentient AI to arrive in the Centauri
system will contain CPU cores executing x86-64 instructions, a 64-bit
extension to a 32-bit extension to a 16-bit segmented extension to an 8-bit
teletype CPU.

~~~
cheerlessbog
Surely 128 or 256 bits? Or is 64 enough for ever?

~~~
jarfil
As far as memory addressing goes, 64 bits should be good enough until the
exabyte RAM era, and there is no reason to switch the whole arquitecture
before that.

~~~
cheerlessbog
I wonder when the exabyte Era might begin, a log plot of historical desktop
memory capacities would be interesting. I can't seem to find one.

------
SteveNuts
This reminds me of a really cool video I just watched "The Day the Gauge
Changed"

[https://www.youtube.com/watch?v=4v81Gwu6BTE](https://www.youtube.com/watch?v=4v81Gwu6BTE)

Learned a lot of cool facts about the railroad days.

