The Crawler-Transporter that brought Ares 1X and its Mobile Launcher (MLP) to the pad last week is one of two that were originally built in the mid-60s for the Saturn series of flights, and have been in use since, for Saturn, Shuttle and now Ares.
Crawler-Transporter near the VAB
There are really two alternatives to transporting rockets vertically to their launch position. The first is to build the rocket horizontally, move it horizontally and hoist it erect at the pad. The massive scale of the Saturn boosters really rules this out. The second option is to build the rocket at the pad. Several factors are against this: weather at the Cape is pretty volatile – hurricane winds, rain, lightning – holding up work; the rate of launches required to meet the end of ’69 deadline meant that having a rocket under construction at the pad would lead to unacceptable congestion.
So the solution was to build a massive Vehicle (initially Vertical) Assembly Building (VAB), construct each rocket inside, out of the weather and transport the rocket to the pad from there. But, again, the massive scale of the Saturn boosters meant this was no trivial problem.
Barges on canals (problems with stability and wind) and railways (difficulties with rail stability and cornering) were both considered as options, but the solution settled on was to transport the rocket, MLP and Launch Umbilical Tower (LUT) by crawlers.
The coal miners of Ohio had been using massive tracked excavators to strip mine coal.
Bucyrus-Erie steam shovel crawler, Kentucky. Note man on top for scale.
These remain some of the biggest vehicles in the world. In February 1962 NASA engineers from the Launch Facilities and Support Equipment Office (LFSEO) were contacted by the Bucyrus-Erie Company who realised the potential of their massive machines to move launch vehicles. The Marion Power Shovel Company won the tender, bidding 8 million dollars (competing with Bucyrus-Erie Company, its Ohio competitor, which has since bought Marion, who bid 11 million dollars) and built two wonders. Philip Koehring worked on the Bucyrus bid, and was immediately recruited by Marion to project-manage the massive engineering work.
The first test of the C-T took place in (July?) 1964, attended by several dignitaries and managers from NASA.
First test of the Crawler-Transporter
The image shows, from L: Richard L. Drollinger, Director of Engineering, Marion Power Shovel Co.; Theodor A. Poppel and Donald D. Buchanan, both of Kennedy Space Center; S. J. Fruin, Executive Vice President and Philip Koehring, Project Engineer, both of Marion; and Kurt H. Debus, Center Director at Kennedy Space Center
Each of the two crawlers weighs in at 2700 tonnes, and is supported by four pairs of enormous caterpilar tracks, one at each corner. Each of these tracks has 57 shoes, 0.3 x 2.3m, each weighing nearly a tonne in themselves. The tracks are driven by 16 electric motors. Steering seems to be via three hydraulic rams which push the truck around on its guide tube. The turning radius is 152 metres, only four times the vehicle’s length.
Crawler truck assembly
The platform which supports the MLP and vehicle has an area of 726 square metres or so, and can rise from 6m above the ground to 8m above ground level. This platform can be kept horizontal, even during the ride up the 5% slope to each pad.
The massive power needed to move such loads is supplied by a diesel-electric system. Generators with a combined power of 5500 kW supply motors, steering and hydraulic pumps. Despite this massive output, the speed of the C-T is limited to about 2mph (0.9m/s) when unloaded, and 1mph when loaded. Bearing in mind the cumbersome nature of its loads, that seems fast enough.
Bob Myers, Crawler Systems Engineer, in one of the cabs
The monster is driven from one of two cabs. Controls seem very simple, belying the precision achievable: the C-T must deliver the vehicle and MLP to the pad and lay it down gently within very tight limits. It is said that the C-T can be moved forward and back in increments as small as 1/8 of an inch (about 3mm)!
So large is the crawler it has a control room inside, under the main deck. Here engineers monitor and control the motors and generators that supply the electrical power for C-T and MLP systems. The whole thing is surrounded by catwalks allowing engineers to access any part of the C-T and gain access to the MLP.
Schematic drawing of the Crawler Transporter
Control room windows
The first Crawler transporter ready for service, Jan 1966
A while ago I enjoyed building a Saturn V scale model (and here). While researching this I’ve found there is a model of the C-T you can build as well. It looks fantastic, although it is 1:144, so not compatible with my original Saturn V. However, if this modeller ever finishes this and gets the plans out there I’ll be occupied for ever, but happily!
1:96 Crawler model detail
Sources: All the sources are referenced in text links or image links. Most useful, and recommended reading for an understanding of the development of Cape Kennedy Space Center and its hardware is ‘Gateway to the Moon’ by Benson and Faherty. This book is the first 14 chapters of Moonport: A History of Apollo Launch Facilities and Operations, 1978, part of the NASA History Series.
Hammer and Feather 
