The six successful Apollo missions to the moon in the 1970s explored different parts of the lunar surface. In contrast, NASA’s plan for a return to the Moon involves repeated missions to the same location, and building up infrastructure there, like research stations on the South Pole. If they need to build an outpost, reason its engineers, then they are going to need a crane to help do it.
Since President Bush announced ambitious plans for space exploration in January 2004, work has begun developing detailed logistics plans for one key plank of the platform, returning to the Moon by 2020.
In June, a NASA engineering team tested their first full scale prototypes of lunar equipment, including rovers, robots, trucks and a crane, in the lunar environment of Moses Lake, Washington.
Actually, the device is as much manipulator as it is crane, says task leader John Dorsey, senior aerospace engineer at NASA’s Langley Research Centre structural mechanics and concepts branch.
“It turns out that when you are looking at the requirements of lunar operations, you need something that could do crane-type operations as well as manipulator operations. The device we have designed and built is a hybrid,” he says. “It is a three-degree-of freedom robot arm, with a waist joint, a shoulder and an elbow, and this articulation allows us to do more than standard crane architecture can.”
“On the lunar surface, we are trying to minimise the number of devices and the amount of mass transported to the lunar surface because it is so expensive. We are aiming to make one device that is very versatile,” Dorsey says.
More than anything, the Lunar Surface Manipulator System device resembles a large, modern self-erecting tower crane–which is no surprise, because a future version of the crane is designed to unfold by itself, with the help of cable actuators.
When erected, a central mast or king post rises from a base section. The mast supports a two-part jib (called the arm, and the forearm), and a counterjib.
An off-the-shelf worm gear motor at the bottom slews the structure. Out the back of the base, a winch pulls on a cable connected to the counterjib to luff the entire jib at a joint where the mast reaches the jib (‘the shoulder’). A second winch mounted at the top of the mast luffs the farthest part of the jib (‘the forearm’) relative to the jib at a second joint (‘the elbow’). The winches can pull both shoulder and elbow joints up or down 45 degrees. Guy wires stabilise the mast to the base section, counterjib to mast top, and mast to jib.
The crane is designed to lift a maximum of 2,000kg (4,400 lbs) on the moon. “A number of payloads fall into that mass: scientific instrument boxes, power boxes, battery boxes,” Dorsey says.
The Moon’s force of gravity is only a sixth of the Earth’s, so lunar loads weigh less than loads of equivalent mass on earth. In earthbound tests, the crane only needs to be able to load 300kg at the elbow, and 150kg at jib-end.
The arm and forearm each measure 3.75m long, and the mast measures the same. When luffing at maximum angle, the crane’s tip height is 9.6m high at a radius of 5.3m.
Dorsey explains how the team came up with these dimensions. “All of the infrastructure components are landed on the lunar surface, and the payload deck of the lander is 5m–6m from the surface. If our device is on the surface next to the lander, it has sufficient reach to lift payloads off the lander deck, slew, and lower them. That established our reach requirements.”
Next year the team starts work on a second version of the crane, which will include extra features. A self-unloading feature will enable the crane to jump from a lander on to the Athlete remote-control moon buggy. It puts its jib-end tip in a grapple, releases the base, and swings off the lander and inserts the bottom of the mast into a mobile base. The Athlete vehicle has a payload bed with automatic levelling, which would keep the crane stable.
Dorsey also says he has plans to build a dragline attachment to move regolith—moon dirt—to build berms or scoop into hoppers for oxygen plants. He is also expecting to mount a winch at the jib-end with a hook, so it further resembles a crane, or to mount another manipulator arm on it to perform different functions, such as grappling, or operating cameras for external inspection.
The engineers made the 225kg (500lb) crane prototype out of stock 4in aluminium extrusion; these will be replaced with graphite-epoxy composite tubes for the mission. It currently uses off-the-shelf winches, which will be replaced with lower-weight alternatives. The hoists had factors of safety of about five times maximum load; it used a safety factor of three for compression tubes and rigid links. The entire design has a safety factor of four for buckling, which is the first failure mode; when overloaded the crane buckles out of plane. The engineers tested the crane with a 118% proof load in the laboratory.
Right now the engineers use laptop computers to run all of the motors with velocity controls. The engineering team are planning to develop a similar kind of automated positioning programming that robots use. This is called inverse kinematics path planning, and works backwards from the geometry of the final destination position to work out a path to get there.
The project team consists of seven people working part-time on this project, including NASA engineers William Doggett and Thomas Jones; Lockheed Martin designer Bruce King, working on a contract basis; Michael Grimes, a graduate student at the National Institute of Aerospace; and IT specialist David Mercer from Aerospace Computing. Dorsey estimates the entire project spend so far as between $300,000–$400,000, including salaries, materials and shop time.
“When I describe this to people who have never seen it before, I see the wonder in their eyes, that’s when I remember, I’m really doing about the coolest job I can imagine. When I am working 14 hours a day building it to get it ready for a demonstration, I tend to forget that,” Dorsey says.
