In the 1980s and 1990s, NASA’s Jet Propulsion Laboratory (Pasadena, CA) began developing advanced microsystems and microelectronics technologies and components for future space applications. One of these experimental designs, the Micro-Inspector Spacecraft, is capable of visual inspection of a host space vehicle with support from NASA’s Exploration Systems Mission Directorate. The piezoelectric actuator in the Micro-Inspector’s propulsion system is from Dynamic Structures and Materials (DSM, Franklin, TN), and controls the many valves of the satellite’s engines.
Using low-pressure iso-butane as a propellant and low-power piezoelectric valves as a thruster, the propulsion system, designed by VACCO Industries (South El Monte, CA), uses DSM’s FPA-180E titanium-framed Flextensional Piezoelectric Actuator, flexure-guided and specifically designed for space- and weight-sensitive applications, weighing 6 g and measuring 7.6 × 9 × 25.4 mm. The actuator provides mechanical motion to move the valving element; the valving element controls the flow to the valve. The thruster’s utilization of the actuator in its valve is two-fold: either fully open or fully closed. Eight DSM FPA-180E actuators are in the VACCO/JPL system: fuel in the fuel tank flows through a conduit through various other valves and filters to a plenum tank, where the butane vaporizes before being expelled via a series of eight VACCO thruster valves, each of which uses the FPA-180E actuator.
The actuator was designed as a custom device for VACCO/JPL. The materials were selected by being able to meet space-compatible requirements such as outgassing in a vacuum, temperature, and exposure to radiation (gamma radiation up to 100 kRad in tests). Through the arrangement of the flexures, the design of the frame provides an amplification of the displacement changes of the piezoelectric ceramic as the voltage field applied to the ceramic changes. The use of flexure guidance in the device’s “frame” (the metal structure surrounding the piezo ceramic) enables motion in the satellite’s valves that is free of backlash and friction. In the case of the Micro-Inspector, the applied voltage range of the actuator was reduced to 0-150V, which corresponded to a reduced displacement range of 135 microns.
The Micro-Inspector, also referred to as a “SmallSat,” was to remain small and light, and measures only 8 × 8 × 2" with a mass of <5 kg. Despite its size, the rectangular Micro-Inspector contains all the subsystems of a typical spacecraft, including propulsion, thermal, power, telecommunications, command, data handling, and attitude determination and control. The Micro-Inspector also has onboard cameras, a sun sensor, solar arrays, collision avoidance systems, and a star tracker. All operations are optimized to have minimal impact on spacecraft consumables including propellant and power.
The spacecraft is designed to be attached to its host vehicle during launch. At a desired time during the mission, the Micro-Inspector would separate from its host and become independent, communicating with the host in real time. The Micro-Inspector is capable of receiving commands for additional utility. The inspection vehicle is capable of circumnavigating the host for a full inspection or to investigate specific spacecraft anomalies to enhance mission safety. The Micro-Inspector is meant to provide external visual inspection for any host vehicle. This could be for routine inspections similar to what the space shuttle currently does immediately following launch and prior to re-entry. It is capable of monitoring dynamic events like deployments of solar arrays or other large structures, the separation or rendezvous of two vehicles in orbit, or inspecting stuck mechanisms.
Of particular concern, and the primary mission of the Micro-Inspector, is monitoring for impact damage. Micrometeors and space junk (the remains of launch material, explosions, and even defunct satellites) have become a serious issue for NASA. Most debris is about the size of a grain of sand, and capable of erosive, but nevertheless minor, damage. Some other objects, however, are fairly large and travel at several thousand miles per hour. An impact would not necessarily destroy a manned space vehicle outright, but the damage done could cause depressurization or prove catastrophic upon reentry. The Micro-Inspector can almost continuously monitor the external hull of a vehicle for any damage at any time in orbit. After operations are complete, the Micro-Inspector would go through an end-of-life disposal, ensuring it is at a safe distance from and of minimal risk to the host vehicle.