Space Vehicle Mechanisms

Brian Gore, Steve Didziulis, and Michael Hilton

Mission success requires the precise and reliable operation of numerous mechanisms that secure, deploy, move, and release space and launch vehicle components. Aerospace has developed particular expertise geared toward optimizing the design and analysis of these moving mechanical assemblies and mechanisms.

All space vehicles contain mechanisms or moving mechanical assemblies that must move by some combination of sliding, rolling, rotating, or spinning—and their successful operation is usually mission-critical. For example, solar arrays are often stowed for launch to survive the ascent environments and to reduce their envelope, but once in space, they are deployed and must be continually rotated to maximize exposure to the sun. Antennas are sometimes mounted on rotating gimbals to maintain sufficient signal strength. Remote-sensing optical payloads track a scene of interest or examine new targets as the space vehicle orbits. The internal lenses and mirrors of optical sensors are often mounted on adjustable mechanisms to maintain or adjust focus or to reject undesirable signals. Space vehicles must maintain attitude either by spinning or by the use of flywheels or gyroscopes. All of these devices, and many others, depend upon the successful and long-term operation of moving mechanical assemblies.

Unlike many other space vehicle subsystems, moving mechanical assemblies generally are not redundant and therefore represent potential single point failure modes. They therefore require stringent design practices and thorough analysis to ensure proper operation. Aerospace has developed specialized tools for characterizing the motion of rigid and flexible components to verify that mechanisms will perform as intended. In many cases, Aerospace has gone beyond its traditional role in modeling and validation to develop novel practical approaches to prevent mechanism failure.

Restraint and Release Design

Restraint-and-release mechanisms, or launch locks, support and restrain deployables and other movable elements during ground processing and launch. They are actuated on orbit to release the installation preload and to allow separation. The load-carrying elements normally counter the compressive preload and the external loads acting on the deployable structure. Pin pullers, separation nuts, bolt or cable cutters, and frangible bolts are most commonly used for releasing the launch locks. These elements use either pyrotechnic devices or nonexplosive devices to initiate release. Nonexplosive devices can reduce the shock caused during release.

Aerospace analyses have helped to prevent failures of restraint and release mechanisms and to identify failure modes. For example, the DMSP F5 spacecraft was ready to launch when the locking mechanism for the Optical Line Scanner (OLS) failed to release during final testing. The scanner needed to be locked for launch to protect the bearings against the high launch loads. If the lock did not release on orbit, the primary mission of the satellite would have been lost. As part of the team investigating ways to unbind the mechanism, Aerospace recommended a fluorocarbon lubricant of the type that is widely used in the hard-disk industry. Aerospace also helped develop a procedure to use a boroscope to find the problematic surfaces and a catheter tube to apply the lubricant to the bound surfaces without destacking the space vehicle from the launch vehicle. This fix resulted in a successful release of the launch-lock, and the launch proceeded without costly delay.

A flexure pivot. The lower, fixed portion of the hinge is attached to perpendicular blade flexures. The upper, rotating arm is attached to the other end of the blades. As the arm rotates, the blades flex to allow the motion.

Other programs were not so fortunate. A well-known example of an antenna that failed to deploy on orbit was the primary antenna of the NASA Galileo satellite. Despite numerous attempts and various strategies, mission controllers were unable to fully deploy the high-gain antenna and had to rely instead on a secondary low-gain antenna for communications. Aerospace contributed to an analysis of the failure and concluded that some of the pins designed to hold the antenna ribs during launch had seized in a phenomenon known as taper-lock. This prevented the antenna ribs from disengaging from the central tower, hindering the opening of the large umbrella-like antenna dish.

Hinges and Pivots

Structures that need to deploy on orbit are most often articulated by a system of hinges that incorporates drive actuators and rate controllers (dampers). The hinges employ either journal bearings or rolling element bearings to reduce friction. In the case of journal bearings, redundancy can be achieved by means of bushings that can rotate on the outside diameter as well as the inside diameter thus providing two independent sliding surfaces. Hinges can be combined in a universal joint to provide rotation about two axes. Rotation in three planes can be provided by a ball and socket or spherical pivot, a configuration that has been used for pointing mechanisms.

If the range of motion is relatively small, elastic flexure pivots can be employed. The advantage of flexure pivots is the elimination of free-play and moving parts. However, they require stringent design efforts to prevent fatigue failure, concerns for wear, and the need for lubrication. The fatigue aspect must be carefully evaluated, especially for flexure pivots consisting of blades that are brazed or welded.

A common example of a hinged structure is a solar array, which typically consists of a number of rectangular panels that are stacked together for launch. In orbit, the hinged panels are released and rotate to the final configuration. A hinged boom or yoke may be used to extend the solar panels farther from the space vehicle to reduce shadowing or sensor fields-of-view obscuration. Testing these deployment mechanisms on Earth can be difficult. For example, in one instance, Aerospace helped analyze and flight-qualify an innovative solar-array hinge mechanism. The hinges were not stiff enough to support the weight of the entire wing in Earth's gravitational field, so the hinge testing required a combination of iterative contractor lab testing and Aerospace simulations. Results from single- and double-hinge testing, in conjunction with corresponding validated models, exposed several deployment effects in a wing-level simulation that would not have been detected using standard qualification-test techniques, because of the deployment's complex nature.

Complex deployment sequence for a spacecraft solar array with 12 degrees of freedom, in constantly changing orientations. The hinge mechanisms that were used to deploy the panels could not support the weight of the panels under Earth's gravity, necessitating a multi-iterative approach of testing and simulation to ensure the panels would not collide with each other or the spacecraft during deployment.

Aerospace's analysis uncovered a panel-to-panel recontact during deployment, as well as the ineffectuality of a set of deployment articulation fingers, and served to support the value of a time delay programmed into the software to release the two solar wings. Those deficiencies were fixed, and the spacecraft solar arrays deployed successfully on orbit.

Latches

Latches are used to lock movable elements in position after they have deployed or have reached their final configuration on orbit. A positive latch provides a rigid configuration and provides more resistance to backdriving of the movable element. This characteristic is especially useful for precision pointing devices, where a high degree of stiffness is needed in the deployed configuration to reduce pointing errors. Most latch mechanisms are required to operate just once, to latch a deployable component at the end of travel; however, some latches must be capable of multiple latch-and-release cycles—for example, if a deployable needs to be restowed prior to orbital maneuvering. One of the simplest latch mechanisms is a spring-loaded pin that drops into a hole at the final deployment position. Over-center mechanisms, such as four-bar linkages, are also commonly used for latch mechanisms. Motor-driven latches can be used, particularly when the application requires multiple latch-and-release operations.

Typical four-bar linkage diagram. In the configuration on the right, the linkage has been driven to its over-center position (the orange and green arms are co-linear) and can no longer rotate regardless of the direction of applied motion to the red crank. The blue bodies with hash marks represent fixed, "ground" points.

On the Titan program, approximately 30 latches were used to attach an exit closure to the main engine nozzle extension; the latches were designed to release prior to main engine firing. At the functional test of the latch/release mechanisms, almost half failed to open because of excessive friction between the latch pins and catch surfaces. Aerospace recommended component testing to investigate sensitivities to lubrication between the surfaces, release-spring forces, and other mechanical design parameters. Aerospace obtained several latch mechanisms from the contractor and performed the needed tests. Results directly led to a new lubrication combination on the latch pin and catch, stiffer torsion springs, a larger step on which the pin slides, and a preflight run-in procedure to minimize the friction coefficient. Releases on all subsequent tests and flights were successful.

Continuously Rotating Elements

Some space vehicle mechanisms must operate continuously for the life of the mission. Such mechanisms can be found in attitude-control subsystems. Control moment gyros and momentum wheels provide vehicle stability by generating a large angular momentum through high-speed spinning (typically 3000 to 8000 rpm). Reaction wheels operate at lower speeds. Solar-array drives are used to continuously orient solar panels toward the sun. Antennas are continuously rotated by gimbals to provide proper pointing. Rotating mechanisms are also used in sensors that are operated in continuously rotating or scanning modes.

Aerospace has worked to ensure optimal operation of attitude-control reaction and momentum wheels, solar-array drives, and rotating sensors for programs such as DSP, DMSP, DSCS III, Milstar, SBIRS, and numerous classified programs and NASA missions. For example, Aerospace analyzed the lubricants from wheels removed from the Hubble Space Telescope during one of the refurbishment missions and performed detailed examinations of components from a failed control moment gyroscope bearing recently returned from the International Space Station. The analyses, coupled with laboratory studies, have indicated that synthetic lubricants could increase the life and reliability of these components and decrease the frequency of anomalous behavior. As a result, synthetic oils have replaced mineral oils in some programs, such as the Operational Line Scanner on DMSP, reaction wheels on GPS, and control moment gyroscopes for many classified programs.

Ball Bearings

A ball bearing consists of grooved tracks within two rings, or races, that rotate relative to each other and are supported and separated by balls that roll along the grooves. A cage or retainer keeps the balls from colliding or bunching up while rolling. Many people think a ball bearing is the sphere or ball that rotates within the raceways, but the term "bearing" actually refers to the entire component. One type of ball bearing commonly found in space mechanisms is called an angular contact bearing. In this design, the balls contact the raceways at an intermediate angle between the radial and axial directions. When mounted in pairs, these compact bearings provide high stiffness in both radial and axial directions.

The mechanisms used in space vehicles impose special challenges for the design of bearings. All bearings develop stresses at the ball-raceway contact regions in response to the applied external load. As a general rule, it is desirable to use the smallest bearing possible that will support the applied loads to minimize size, weight, and friction torque (resistance to rolling). In almost all cases, launch loads (as opposed to operational loads) determine the size of bearings in space mechanisms. Bearings need to be preloaded to prevent ball skidding by applying an axial force to one ring relative to the other to maintain the contact angle. This preload can be "soft" (applied by springs)—or "hard" (created by machining the mating surfaces of bearing pairs such that a preload is created when the pairs are compressed together). Hard preload is often required to provide maximum stiffness (rigidity) of the rotating system. The challenge of hard preload is that variations in temperature can drastically change the preload, affecting both friction torque and lubricant life. Bearings in space vehicle mechanisms are passively cooled, so the preload must be carefully chosen.

A surface profilometer is used to measure the inner race of a ball bearing to determine mechanical part geometries, surface finish, and wear patterns. The device has been used to investigate anomalous bearing wear during ground tests and to evaluate components from the failed control moment gyroscope on the International Space Station.

Over the years, Aerospace has developed internationally respected expertise in ball bearing design, analysis, and testing. For example, Aerospace has developed several software tools to analyze bearing designs. One such program, called BRGS, enables the assessment of preload, stresses developed, and nominal torque as a function of launch and orbital conditions. This program has been the primary tool Aerospace uses to evaluate bearings and to aid in redesign.

Another program, Motion, works with BRGS to assess the effect of manufacturing imperfections on the vibration disturbances generated by bearings. Originally written to determine the sources of noise in momentum-wheel bearings, Motion was used successfully to identify a once-per-revolution defect of a control moment gyroscope bearing ring that was caused by improper stress relief after a machining operation. Recent updates of Motion also feature Monte Carlo analyses of permitted geometric variations in bearing balls and rings. Combined with knowledge of the space vehicle structure, bearing-generated vibration levels at payload sensors can be assessed. Because these programs also require knowledge of the bearing temperature and its environment, another program called DYBA (Dynamic Bearing Analysis) was developed. It also uses BRGS as an engine and is currently being upgraded to extend its use throughout the industry.

Drives and Pointing Mechanisms

Drive mechanisms provide the motive force or torque for space vehicle mechanisms. There are two common types of drive mechanisms: those using springs or other stored-energy devices, and those using electric motors. Both can be used for rotational and translational movement. Compression and tension/extension springs can be used to impart energy for short linear travel. Torsion springs are often used for rotational motion.

For applications needing a more uniform torque or force throughout the range of travel, constant-force or constant-torque springs can be used. These springs have a force or torque output that is nearly constant throughout the range of motion. Electric motors are typically used in deployment systems where greater control is required or where repeated usage is planned, such as deployment, restowing, and redeployment. Motors can be used in direct-drive or geared configurations, and in axial or rotary applications. Spring-driven mechanisms often incorporate rate-limiting devices to avoid potentially harmful velocity buildup toward the end of travel. Common rate-limiting devices include viscous and eddy current dampers.

Viscous dampers generally consist of a sealed housing containing a high-viscosity fluid. Also inside the housing is a rotating vane, with very small clearances to the housing. As the vane is moved through the fluid, the friction of the fluid passing between the vane and housing, or sometimes another control orifice, provides resistance to the motion. The fluid resistance is proportional to the speed of rotation, and thus the damper provides more resisting torque as deployment speed increases. Eddy-current dampers use the reverse principle of an electric motor. Motion of a rotating or sliding shaft of conductive material through a magnetic field generates eddy currents. These currents create a magnetic field of their own, due to the shaft motion, but in the opposite direction, causing a repulsive force. The eddy currents dissipate into heat through the resistance in the conductive material, causing the system to function as a viscous damper equivalent.

Some devices, such as antennas, telescopes, or scanning mirrors, must be able to slew, in some cases rapidly, from one point to another and to stop at a specific location with great accuracy. These devices employ precise, robust drive systems comprising precision bearings, motors, position sensors, and feedback control loops. Antennas are usually required to track a ground station or other orbiting satellites (such as the Tracking and Data Relay Satellite System, TDRSS) to maintain communications. This mode can require fine, continuous adjustments to the antenna gimbals. In addition, the antenna may be required to slew quickly over large angles to move from one ground station or space vehicle to another as the line of sight changes. Telescopes are usually not required to slew quickly but have tight pointing requirements. Two-axis gimbals are commonly used, with their orientation and configuration selected according to the telescope or antenna requirements. Sensor mirrors may have the highest slew rates and precision tracking requirements. These, in turn, impose severe requirements on drive system stiffness, position knowledge, and control. In contrast to one-time deployment mechanisms, pointing mechanisms must operate for the life of the space vehicle. This longevity requirement places a high emphasis on adequate lubrication and properly selected and installed bearings. Aerospace has often assisted contractors by running life tests of the proper duration and in the correct environment when they did not possess sufficient resources to do so. In addition, proper balancing is important for high-speed or high-inertia devices to avoid jitter, as is eliminating backlash in precision pointing devices.


James Kirsch operates the slip-ring test facility for a vacuum performance test (top). This slip-ring test facility (closeup, right) measures the properties of sliding electrical contacts to gain insight into materials' performance and the impacts of storage and operational environments. Slip rings are used to transmit power and signals through rotating interfaces.

Best Design Practices

Throughout the years, Aerospace has contributed to, and often coauthored, many of the documents that have captured evolving design knowledge and best practices to ensure reliable operation of space vehicle mechanisms. The most important of these was the military specification MIL-A-83577, Moving Mechanical Assemblies for Space and Launch Vehicles, which was canceled by the Department of Defense in 1996 as part of the trend of "acquisition reform." In the years to follow, it became clear that this created a situation in which the government and Aerospace encountered increasing difficulty in judging adequate compliance to best engineering practices during the acquisition of new systems. Actual hardware failures occurred with increasing regularity. Under the direction of the Air Force Space and Missile Systems Center, Aerospace composed a Technical Operating Report (TOR) in 2004, based heavily on the canceled military specification, to serve as a stopgap guidance document for several upcoming acquisitions. This TOR became the starting point for a new AIAA standard developed by a committee of industry experts led by Aerospace. The new standard, AIAA S-114-2005, Moving Mechanical Assemblies for Space and Launch Vehicles, was issued in 2005.

The AIAA standard, though based on the older military specification, contains new information that reflects the development of technology since the 1980s. One example is life testing of moving mechanical assemblies. The original standard required that new mechanisms be qualified by a ground life test for twice the operation cycles expected in orbit. One difficulty with that requirement is the lack of an acceptable method to accelerate the life testing of lubricated mechanisms operated continuously, such as a momentum wheel, because operational speed affects the efficacy of the lubricant. In the 1980s, the typical design life of a space vehicle was three to five years; however, thanks to improvements in space vehicle technologies, the design goal of most space vehicles is now ten years (and even longer for some interplanetary missions). To acknowledge this new trend, the new standard recommends testing at 1.5 times the design life for high-cycle mechanisms whose testing cannot easily be accelerated, though twice the design life is preferred. Of course, many satellites—such as DMSP, DSP, and GPS—that were originally designed for a service life of three years have often operated successfully for ten years and beyond; if design life goes to ten years, testing will become impractical, and engineering judgment will be even more critical. The new standard also has guidance for newer restraint and release mechanisms and bearing materials that have entered into practice since the late 1980s and that were not addressed in earlier standards.

Conclusion

Moving mechanical assemblies are not the only enabling technology for space and launch applications, but most of the functions performed by spacecraft and their boosters depend on mechanical assemblies to work. As technology advances and the push for space performance grows, new and innovative means to package, release, deploy, and latch payloads will be required. On the other hand, the longer missions made possible by these technological advances will render impractical real-time life tests of moving mechanical assemblies. Mission success will depend more heavily on accumulated knowledge and the ability to use computer simulations to model system loads, performance, and life. Understanding the potential system limitations related to the design and materials, coupled to the expected operational conditions, is key to ensuring that spacecraft mechanisms are not the life-limiting systems on spacecraft.

Acknowledgement

The authors wish to acknowledge Alan Leveille, who developed and continuously upgraded all of the key bearing analysis software used by Aerospace, and for his numerous contributions in solving bearing problems on space and launch vehicles.

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