The two Kennedy Space Center crawler-transporters, used to transport Saturn V, Shuttle and SLS rockets to the launchpad, move on caterpillar treads:

enter image description here

Here's a SpaceFlight Insider video clip about current work on modifying the crawlers for SLS:

The much lighter Soyuz is instead transported to the launchpad on tracks, a standard Russian gauge railway, it seems.

Rail tracks seem to me an obvious choice for moving super-heavy cargo a short distance between few fixed points. Caterpillar treads are more useful to traverse rough terrain, and is mostly used on construction and military vehicles. Why do the crawler-transporters use caterpillar treads instead of running on rail tracks?

Added: It seems as if Buran and the N1 launcher also had a crawler transporter, the same one, but towed by locomotives on rail tracks (or do I misunderstand that?) Hmm, the book linked to says it was towed by four 100 horsepower locomotive. Hardly.

  • $\begingroup$ That's a great video. Not seen that before. $\endgroup$ Jul 12 '15 at 9:56
  • $\begingroup$ Buran and N1 were transported on rails: 2 sets of standard-gauge rails running parallel. Locomotives are more likely to be 1000 hp than 100. $\endgroup$
    – Hobbes
    Jul 12 '15 at 14:08
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    $\begingroup$ Soyuz is a bit odd comparison, it's assembled horizontally and then also transported such to the pad. Crawler-transporters are on the other hand used to transport a mobile launch platform together with the launch vehicle from a vertical assembly building to the launchpad. Better comparison would then be with Vandenberg AFB Launch Complex 6 that uses mobile VAB and service tower that both move on rails to and fro the launch tower, and the site was also a designated Space Shuttle launch site (tho that never happened). $\endgroup$
    – TildalWave
    Jul 12 '15 at 20:02
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    $\begingroup$ Ariane V uses railway and a truck to pull the launcher onto the railway. $\endgroup$
    – Manu H
    Jul 22 '19 at 8:22

Weight distribution would be be the main reason.

The Shuttle stack (or Saturn V stack) empty, weighed an immense amount. Shuttle more so, since the SRB's were full during movement. (Can't fuel a solid rocket on the pad). The SRB's weighed 1.3 million pounds each ready for flight. That is really an immense amount, over a small area.

The tracks of the crawlers are huge, which better distributes the mass over a larger area of the ground.

There was talk that for SLS with its 5 segment SRB's the ramp up to the pad will have to be rebuilt/reinforced to handle the extra weight.

A rail line can distribute the weight over more wheels, but they are very narrow. While a mile long train might weigh as much as the Shuttle stack, remember it is distributed over a mile of track. The crawler is fairly compact in foot print.

A Soyuz or Proton which are taken to the launch pad by rail are much much smaller in comparison.

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    $\begingroup$ The crawler also has to carry the launch platform and (for Saturn) the tower. $\endgroup$
    – Hobbes
    Jul 12 '15 at 11:07
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    $\begingroup$ Also Soyuz is taken in a horizontal position, which distributes its weight a bit more then a vertical setup used by NASA. $\endgroup$
    – Suma
    Jul 15 '15 at 8:24
  • $\begingroup$ And then what would happen when a rail vehicle arrives at the launch pad? Would need special wide-gauge to straddle the flame pit? The crawler approach makes a lot of sense in that context. $\endgroup$
    – Anthony X
    Feb 13 '18 at 22:08
  • $\begingroup$ @AnthonyX You come it an 90 degrees to the flame trench. I.e. You build it different than you build it for a crawler. $\endgroup$
    – geoffc
    Feb 13 '18 at 22:20
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    $\begingroup$ To an extent each of the crawler-transporter's 8 tracks can be likened to an 11-axle rail vehicle that lays out sleepers for weight distribution in front of itself and picks them up for reuse in rear. However, in contrast to an adhesion railway, the powered axles need not be weight bearing and vice versa, which probably simplifies the engineering. (So it is more like a self-laying rack railway, only without the complexities of pointwork). $\endgroup$ Jul 20 '19 at 19:55

The Kennedy Space Center Story, written by NASA in June 1970, NTRS document 19710024295, p. 29 describes the options that were considered, and the reason for the final choice:

The scheme by which to transport launchers and assembled Saturn V vehicles was carefully explored by NASA engineers. A barge canal system was investigated. Models were tested in the Navy's David Taylor Model Basin on the Potomac near Washington, D.C. They revealed that hydrodynamic problems caused by a barge large enough to carry the rocket in upright position would be extremely difficult or costly to solve. Also, an elaborate launch pad would be needed.

Other potential solutions proved impractical, or in the case of a rail­ road, too costly for moving the tremendous loads involved. Pneumatic­ tired transporters, ground effects machines, and other ideas were discarded. The final choice was a track-mounted crawler so big that after partial assembly at the plant of Marion Power Shovel Company, Marion, Ohio, it was taken apart in moveable sections, shipped to the launch center and there assembled. This solution was derived from the strip mining industry and involved the use of hydraulic power for jacking, leveling and steering.

It sounds like cost was the ultimate factor.

(A "ground effects machine" is a hovercraft. That would have been awesome.)

  • $\begingroup$ The book "Moonport - A History of Apollo Launch Facilities and Operations" also mentions that using rails to transport the launch vehicle and platform to the pad was possible, but that the switching arrangements allowing them to route the vehicles to one of several different pads would bee too complicated. It also has some comparison of using barge vs rail to transfer the launch vehicle, unfortunately it doesn't describe the crawler-based concept (hq.nasa.gov/office/pao/History/SP-4204/ch4-5.html) $\endgroup$ Jul 26 '19 at 10:13

Another thing to consider is that (most of) Kennedy Space Center launch pads are significantly elevated above average ground level, whereas Baikonur Cosmodrome is flat, instead having large trenches dug underneath the pad.
Compare KSC: Kennedy Space Center elevated pad with Baikonur: Baikonur Cosmodrome trench Where the Russians ride over smooth, level terrain, the Americans have to overcome this elevation as the final part of the journey to the launchpad. Pushing a train uphill is difficult (citation needed), as you risk losing grip and sliding backwards. However, friction between the caterpillar tread and the ground is immense - so it's easy to go uphill or stop halfway there. This could have been one of the secondary reasons (alongside those already mentioned in other answers) of choosing them instead.

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    $\begingroup$ Adhesion is an issue, but could be overcome with a rack railway (en.wikipedia.org/wiki/Rack_railway) or winch. $\endgroup$
    – Hobbes
    Jul 22 '19 at 10:15
  • $\begingroup$ See this page: "A ramp with a 5% slope leads from the crawlerway to the top of the launch structure." Trains may climb 7 up to 10 % slope, so 5 % should be possible. $\endgroup$
    – Uwe
    Jul 22 '19 at 10:38
  • $\begingroup$ @Uwe Standard adhesion freight trains don't do 7%. The most concrete number I have is from this page, it's "preferably below 1.5%". 5% seems doable with a rack railway, like Hobbes suggests, but then I'd say we're no longer in the "standard railway" regime, more or less engineering a custom solution anyway. $\endgroup$
    – Przemek D
    Jul 22 '19 at 10:45
  • $\begingroup$ There are a lot of steep adhesion railways, see. $\endgroup$
    – Uwe
    Jul 22 '19 at 10:59
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    $\begingroup$ 24 railways over 5% is not that much (given how many railways are out there in total). Also, notice that many of these are either closed or lightweight tourist railways. Remember that we're talking about hauling well over 1000 metric tons of space-grade hardware up this slope. I'm not saying it's completely impossible, out-of-the-question kind of endeavor - just that this is a technological difficulty that might have contributed to ruling out a launchpad railway. $\endgroup$
    – Przemek D
    Jul 22 '19 at 11:09

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