Mechanical Components

Miniature Linear Actuator for Small Spacecraft

A report discusses the development of a kit of mechanisms intended for use aboard future spacecraft having masses between 10 and 100 kg. The report focuses mostly on two prototypes of one of the mechanisms: a miniature linear actuator based on a shape-memory-alloy (SMA) wire. In this actuator, as in SMA-wire actuators described previously in NASA Tech Briefs, a spring biases a moving part toward one limit of its stroke and is restrained or pulled toward the other limit of the stroke by an SMA wire, which assumes a slightly lesser or greater "remembered" length, depending on whether or not an electric current is applied to the wire to heat it above a transition temperature. Topics addressed in the report include the need to develop mechanisms like these, the general approach to be taken in designing SMA actuators, tests of the two prototypes of the miniature linear actuators,and improvements in the second prototype over the first prototype resulting in reduced mass and increased stroke. The report also presents recommendations for future development, briefly discusses problems of tolerances and working with small parts, states a need for better understanding of behaviors of SMAs, and presents conclusions.

Posted in: Mechanical Components, Briefs, TSP

Read More >>

Alternative Attitude Commanding and Control for Precise Spacecraft Landing

A report proposes an alternative method of control for precision landing on a remote planet. In the traditional method, the attitude of a spacecraft is required to track a commanded translational acceleration vector, which is generated at each time step by solving a two-point boundary value problem. No requirement of continuity is imposed on the acceleration. The translational acceleration does not necessarily vary smoothly. Tracking of a nonsmooth acceleration causes the vehicle attitude to exhibit undesirable transients and poor pointing stability behavior. In the alternative method, the two-point boundary value problem is not solved at each time step. A smooth reference position profile is computed. The profile is recomputed only when the control errors get sufficiently large. The nominal attitude is still required to track the smooth reference acceleration command. A steering logic is proposed that controls the position and velocity errors about the reference profile by perturbing the attitude slightly about the nominal attitude. The overall pointing behavior is therefore smooth, greatly reducing the degree of pointing instability.

Posted in: Mechanical Components, Briefs, TSP

Read More >>

Carbon-Fiber Brush Heat Exchangers

High thermal conductance between uneven surfaces could be achieved with low clamping force. Velvetlike and brushlike pads of carbon fibers have been proposed for use as mechanically compliant, highly thermally conductive interfaces for transferring heat. A pad of this type would be formed by attaching short carbon fibers to either or both of two objects that one desires to place in thermal contact with each other. The purpose of using a thermal-contact pad of this or any other type is to reduce the thermal resistance of an interface between a heat source (e.g., a module that contains electronic circuitry) and a heat sink (e.g., a common finned heat sink).

Posted in: Mechanical Components, Briefs

Read More >>

Thermal and Compressed-Air Storage System Provides Alternative to UPS Batteries

Three mature energy-storage technologies are combined in a new system to replace lead-acid batteries. Virtually all businesses and industries are vulnerable to electric power disturbances such as outages, sags, swells, and harmonics. These problems are less of an issue for data centers, protected behind their walls of Uninterruptible Power Supply (UPS) systems. But the typical battery-backed UPS is too fragile for use in less protected environments. UPS batteries must be maintained in a narrow temperature range and fail prematurely when subjected to a steady diet of step loads and motor drives. About six years ago, flywheel-based UPS products became commercially available. These devices store energy as rotational inertia, and are rugged enough to survive on the factory floor. However, flywheels have relatively short ride-through energy and are best-suited for use in locations with backup generators.

Posted in: Mechanical Components, Briefs

Read More >>

Stability-Augmentation Devices for Miniature Aircraft

Passive mechanical devices help miniature aircraft fly in adverse weather. Non-aerodynamic mechanical devices are under consideration as means to augment the stability of miniature autonomous and remotely controlled aircraft. Such aircraft can be used for diverse purposes, including military reconnaissance, radio communications, and safety-related monitoring of wide areas. The need for stability-augmentation devices arises because adverse meteorological conditions generally affect smaller aircraft more strongly than they affect larger aircraft: Miniature aircraft often become uncontrollable under conditions that would not be considered severe enough to warrant grounding of larger aircraft. The need for the stability augmentation devices to be non-aerodynamic arises because there is no known way to create controlled aerodynamic forces sufficient to counteract the uncontrollable meteorological forces on miniature aircraft.

Posted in: Mechanical Components, Briefs

Read More >>

Tool Measures Depths of Defects on a Case Tang Joint

Precise measurements can be made consistently. A special-purpose tool has been developed for measuring the depths of defects on an O-ring seal surface. The surface lies in a specially shaped ringlike fitting, called a “capture feature tang,” located on an end of a cylindrical segment of a case that contains a solid-fuel booster rocket motor for launching a space shuttle. The capture feature tang is a part of a tang-and-clevis, O-ring joint between the case segment and a similar, adjacent cylindrical case segment. When the segments are joined, the tang makes an interference fit with the clevis and squeezes the O-ring at the side of the gap.

Posted in: Mechanical Components, Briefs

Read More >>

Lifting Mechanism for the Mars Explorer Rover

A report discusses the design of a rover lift mechanism (RLM) — a major subsystem of each of the Mars Exploration Rover vehicles, which were landed on Mars in January 2004. The RLM had to satisfy requirements to (1) be foldable as part of an extremely dense packing arrangement and (2) be capable of unfolding itself in a complex, multistep process for disengaging the rover from its restraints in the lander, lifting the main body of the rover off its landing platform, and placing the rover wheels on the platform in preparation for driving the rover off the platform. There was also an overriding requirement to minimize the overall mass of the rover and lander. To satisfy the combination of these and other requirements, it was necessary to formulate an extremely complex design that integrated components and functions of the RLM with those of a rocker-bogie suspension system, the aspects of which have been described in several prior NASA Tech Briefs articles. In this design, suspension components also serve as parts of a 4- bar linkage in the RLM.

Posted in: Mechanical Components, Briefs, TSP

Read More >>