What is a gimbal -- and what does it have to do with NASA?

While a gimbal can be any support that can pivot around an axis, most gimbal systems look like a series of concentric rings. The outermost ring mounts to a larger surface, like a boat's instrument panel. The next largest ring connects to the outermost ring at two points that are perpendicular to the outer ring's surface mount. Then, the third largest ring mounts to the second largest one at two points perpendicular to the connection between the first and second ring, and so on. Sound confusing? Take a look at the following illustration.

Each ring can pivot around one axis. How is this useful? On its own, it's just interesting to look at. But by mounting an object to the center of the system, you can make sure the object can face any particular direction at any time.

Well, almost any direction at any time. One problem with gimbal systems is gimbal lock. Gimbal lock occurs when two axes in a three-gimbal system align. When that happens, the object's movement is limited. An entire range of motion becomes impossible. This is what you see on the right in the above illustration.

Gimbal lock is a serious problem. There are two ways to avoid gimbal lock. One is to adjust the gimbals, either by maneuvering the surface so that the gimbals swing another way or by physically resetting the gimbals themselves. If gimbal lock does occur, the gimbals must be reset to work again. Another solution is to add more gimbals to the system. Adding a fourth gimbal helps eliminate gimbal lock, but it also makes the system bulkier and more complicated. Since most gimbals are part of electronic systems, adding more complexity is not always the best choice.

Gimbals allow designers to create devices that are more flexible than a fixed, stationary device. It's also possible to orient a device so that it's facing a specific direction independently of how its surrounding environment moves or changes. Such an application has dozens of uses, ranging from a cup holder that adjusts so that you don't have to worry about spilling your coffee to an array of satellite antennae that can turn to face incoming signals.

So what does this have to do with NASA? Find out in the next section.

­What do gimbals have to do with NASA? The answer boils down to this: almost everything. Not only does NASA use gimbals when designing navigational systems and instrument panels, but also for building training simulators and other terrestrial components. Without gimbals, it would have been very difficult for NASA to find a way to send the first astronauts up into space safely.

In training missions, NASA uses gimbals to simulate situations astronauts will encounter while in space. Some early training simulations required astronauts to put on a harness and dangle from a suspended, gimbaled system to simulate a spacewalk. Because the astronauts were in a set of gimbals, they could reorient themselves to different directions, just as they could in space. Gimbals also played an important role in motion simulators, giving simulator cabins a higher degree of freedom of movement.

NASA used gimbals in early spacecraft for everything from instruments to propulsion systems. In navigational systems, gimbals are useful for determining and changing the orientation of a spacecraft in relation to something else, such as the Earth or a space station. Gimbals are also useful for components like solar panels. Mounted on a gimbal system, the panels can tilt and rotate to face the sun even as the spacecraft's orientation changes.

One of NASA's most important spacecraft instruments is the inertial measurement unit (IMU). An IMU measures changes in pitch, roll and yaw as well as acceleration. The IMU contains accelerometers and gyroscopes to monitor changes in spacecraft velocity and attitude. For the Gemini missions, NASA used a four-gimbal system. But for the Apollo missions, NASA decided to go with a three-gimbal system. That's because engineers worried that they would miss their goal of landing a man on the moon before 1970 if they waited to perfect a four-gimbal system. Because the Apollo spacecraft IMU only used three gimbals, astronauts had to stay alert and realign the spacecraft to avoid gimbal lock.

NASA also used gimbals when building the propulsion systems for spacecraft. A fixed rocket engine or thruster would only be able to provide thrust in a single direction. Mounted on gimbals, the same propulsion unit could tilt to provide thrust in different directions. This is critical whenever a spacecraft must align itself with another body, whether it's another spacecraft, a planet or the moon.

It's pretty amazing to think that a simple series of interconnected rings made it possible for NASA to send a manned spacecraft to the moon. Without gimbals, we couldn't navigate or travel in space with any precision.

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