Question: Why does it take so long to move the Artemis Moon Rocket to the launch pad? And why do they even have to move it? Couldn’t they just assemble it on the launch pad? Doesn’t seem very efficient to me. —SS, Space Coast, FL
Answer: You ask several good questions, so let me start with your “assemble it on the launch pad” idea. The photo above shows the Artemis II moon rocket being moved from the Vehicle Assembly Building (VAB) to launch complex 39B. The rocket needs to be assembled in the VAB because that facility has the powerful overhead cranes needed to stack the multiple components of the rocket. The VAB is weatherproof and climate controlled, facilitating assembly. The launch complex has its own specialized hardware, but it’s designed for launching rockets — not assembling them.
FYI, those tall (594 ft) structures visible to either side of the rocket are part of the lightning protection system surrounding the launch complex. There are three towers in that system, and they have successfully diverted lightning strikes away from the rockets on several occasions.
Now, regarding the time it takes to move the rocket, let me toss out a few numbers:
- The Artemis II stands 322 feet (98 meters) tall, weighs roughly 3.5 million pounds (1.6 million kg) “dry” and more than 5.75 million pounds (2.61 million kg) when fully fueled with its liquid hydrogen and liquid oxygen propellants.
- NASA’s crawler-transporter 2 (CT-2) is the size of a baseball infield and weighs more than 6 million pounds (2.7 million kg). That includes the mobile launch assembly (ML1) that interfaces CT-2 with the rocket.
- CT-2 is powered by two Cummins 1500 kW AC diesel generators that drive the electric motors powering its eight separate treads. Given the total weight load, friction, and power, CT-2 can hit around 1 mph. It burns through around 165 gallons of diesel for every mile it travels.
Here’s a close-up look at CT-2 with some humans included for a sense of scale. What looks to be a separate crawler following close behind is actually the rear end of CT-2:
With the roughly 4 mile trek between VAB and 39B, and the 1 mph speed, it would seem the travel time should be around 4 hours. But it takes way longer than that because CT-2’s not going 1 mph over the entire 4 miles. It spends the first hour slowly accelerating from zero to 1 mph, and the last hour decelerating from 1 mph to zero at the launch complex. If it has to deal with winds, that can further extend the travel time. There’s a lot of air drag on a structure that large. In practice, travel times range from 8–12 hours.
Physics imposes constraints on moving this large a mass. When you exert a horizontal force on a vertical object you create a torque around its center of mass (COM). That torque could topple it. With an acceleration of: a = Δv/Δt = 1 mph / 60 seconds = 0.0075 m/s2 you would barely feel it. But the Artemis rocket will feel a reaction force of: F = ma = (1.6 x 106 kg)(0.0075 m/s2) = 1,200 N = 275 lb. That’s not enough to topple the rocket, but they also need to allow for wind gusts, so they take it slow. The accompanying ML1 provides additional structural support and further increases the safety margin.
CT-2 is one of two crawlers built in 1965 to support NASA’s Apollo program. They carried the Saturn V rockets that got us to the Moon. The crawlers are some of the largest machines ever constructed. Like tanks, the crawlers drive on treads rather than wheels. Each crawler has eight treads, and each tread consists of 57 “shoes.” Each shoe weighs 2,100 pounds (953 kg). As CT-2 treks between the VAB and 39B, it runs along a roadway of imported river rocks. Due to the combined weight of CT-2, SLS and ML1, those rocks are crushed into sand as the treads pass over them, dispersing vibrational energy. As the rocks crumble, they actually contribute to a smoother ride — almost like driving on bubble-wrap. The rocks need to be replenished regularly, as that sand is eroded away by wind and rain.
CT-2 is designed to roll underneath ML1, pick it up, and carry it smoothly to 39B. Because each launch complex is built atop a sloping hill, the crawler uses its hydraulic suspension to raise and lower the corners of the platform and keep it level all the way to the pad. There, it sets ML1 in place so the rocket can be integrated with the launch complex hardware.
So to answer your question SS, NASA moves moon rockets about as fast as they can. When you think about the physics involved, it’s amazing they can do a roll-out in 12 hours.
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