G Whiz

Space tourism will be here in a flash, with SpaceShipTwo from Sir Richard Branson's Virgin Galactic ready to whisk passengers beyond Earth's atmosphere as early as 2009. Save your money, however, because the suborbital excursion will cost $200,000. And it will require some prep work. Like fighter pilots and astronauts, space tourists will need instruction on how to handle the g forces they'll face during launch and re-entry. That's where a human centrifuge can come in handy, like Environmental Tectonics Corporation's (ETC) STS-400 at the National AeroSpace Training and Research (NASTAR) Center in Southampton, Pa.

Back story

In the early days of aerospace development, human centrifuges gauged the body's capacity to endure acceleration forces higher than 1 g. Initial devices were primitive, sometimes driven by hand. As aircraft performance skyrocketed throughout the 20^th century, centrifuges became training grounds where pilots learned to handle the unpleasant effects of g force, including brownout (loss of color vision), tunnel vision (loss of peripheral vision), blackout (loss of vision while conscious), and the dreaded g-LOC, or g-force induced loss of consciousness. In aircraft, g forces pull blood away from the head, causing visual and cognitive disturbances.

With the increasing sophistication of military aircraft, centrifuge design pushed onward to higher g levels and onset rates, the rate of change of g. Especially after the advent of the F-16 Fighting Falcon, which when flying in a turn can create 9 g in 1.5 seconds (6 g/sec onset rate), older training centrifuges reached their limit to match the anticipated onset rates. ETC's first centrifuge, built in 1987, had an onset rate of 6 g/sec and a peak force of 15 g, meeting the needs of that era. Since then, ETC has exceeded its initial mark, building several higher performance human centrifuges, five single degree of freedom (DOF) and five dual gimbal systems.

Design dilemmas

The greatest challenge in centrifuge design and construction is to build a device that is both light and stiff. It must be built like an aircraft, but is bolted to the floor. Like an aircraft, the system has a tendency to grow in weight during the design phase. To avoid that, every piece and part drawing has the weight indicated; with the newest system, frequent checks with the weight budget lead to the redesign of a few pieces, shaving off a few ounces. While the rotating mass of early centrifuges was around 150 tons, the current generation centrifuge has a rotating mass of just 11 tons. An iterative process using finite element analysis and 3-D CAD was incorporated to design the arm and gimbal system.

To accelerate a 25-ft arm and gondola within three-quarters of a revolution from 1.4 to 9 g demands an extremely light arm to avoid a huge power draw — the kind of power that's rarely available on most military training bases. In fact, another design surprise was that the power grid at ETC (at 11 kV) couldn't handle the current inrush without dimming the lights in the local town of Southampton. They resolved the problem by connecting to the 33 kV power grid in cooperation with the local power company.

How it works

A human centrifuge is a device with a relatively long arm that rotates around its main shaft to create centrifugal forces in a radial direction. The faster the rotation, the higher the g forces. Trainees are secured into seats positioned within a gondola, mounted on a dual axis attached to the end of the arm. This provides for pitch and roll control, allowing precise manipulation of the g vectors. Older centrifuges allow only one axis of motion for the gondola and are attached through a single roll axis to the arm, while newer designs have two DOF, as the gondola is suspended in its pitch and roll axes. The pitch axis is suspended within a roll ring, which attaches to the end of the arm. These two axes are controlled to align the g vectors, so trainees feel the primary gz (vertical force) pressing them into the seat.

The drive system is a direct driving gear reducer/motor combination connected to the centrifuge shaft; its components include a helical bevel gear reducer, flexible couplings, torque limiting device, centrifuge drive shaft, electrical drive system, dc electric drive motor, pedestal with two tapered roller bearings, and an emergency braking system. The all-steel, 25-ft. arm's low weight and high stiffness allow it to be controlled by instructors to “fly” like an aircraft. The entire rotational system, including the arm, is designed for 25 g maximum acceleration and hours of continuous operation.

The Gondola Positioning System (GPS) is a high torque mechanism that precisely controls pitch and roll; it consists of backlash-free cycloidal gear reducers and dc motors. A slip ring mounted on the opposite side of each axis allows ± 360° rotation for the gondola. Each pitch and roll drive unit incorporates brushless servomotors, backlash-free cycloidal gear reducers, a stiff coupling between motors and gear reducers, and electromechanical brakes.