William J. Biter

Development of the variable emittance thermal suite for the space technology 5 microsatellite

The advent of very small satellites, such as nano and microsatellites, logically leads to a requirement for smaller thermal control subsystems. In addition, the thermal control needs of the smaller spacecraft/instrument may well be different from more traditional situations. For example, power for traditional heaters may be very limited or unavailable, mass allocations may be severely limited, and fleets of nano/microsatellites will require a generic thermal design as the cost of unique designs will be prohibitive. Some applications may require significantly increased power levels while others may require extremely low heat loss for extended periods. Small spacecraft will have low thermal capacitance thus subjecting them to large temperature swings when either the heat generation rate changes or the thermal sink temperature changes. This situation, combined with the need for tighter temperature control, will present a challenging situation during transient operation. The use of “off-the-shelf” commercial spacecraft buses for science instruments will also present challenges. Older thermal technology, such as heaters, thermostats, and heat pipes, will almost certainly not be sufficient to meet the requirements of these new spacecraft/instruments. They are generally too heavy, not scalable to very small sizes, and may consume inordinate amounts of power. Hence there is a strong driver to develop new technology to meet these emerging needs. Variable emittance coatings offer an exciting alternative to traditional control methodologies and are one of the technologies that will be flown on Space Technology 5, a mission of three microsatellites designed to validate “enabling” technologies. Several studies have identified variable emittance coatings as applicable to a wide range of spacecraft, and to potentially offer substantial savings in mass and/or power over traditional approaches. This paper discusses the development of the variable emittance thermal suite for ST-5. More specifically, it provides a description of and the infusion and validation plans for the variable emittance coatings.

An evaluation method for application of condition-based maintenance technologies

In this paper we describe a new and efficient means of selecting appropriate condition based maintenance (CBM) technologies to manage equipment and system health cost effectively. The method accounts for both the technological capabilities and organizational factors to assess the expected costs and benefits of implementation. By analyzing both the expected benefits and costs, decisions as to the most cost effective application of CBM technologies (including combinations) can be made; thus ensuring scarce facility resources are directed where they will provide the most benefit. Because uncertainty for the cost and effectiveness estimates is considered, decision makers can make informed judgements of the business risk associated with each particular alternative and thus develop more robust decisions, Also, by developing both qualitative and quantitative estimates, the method supports effective monitoring of decisions to ensure desired outcomes are achieved.

Electrostatic Appliqué for Spacecraft Temperature Control

The electrostatically controlled radiator (ESR) uses electrostatic hold‐down of a high emissivity composite film to control spacecraft skin temperature. It functions as a thermal switch and changes the mode of heat transfer between the spacecraft skin and the radiator film from conduction to radiation and has demonstrated large changes in apparent emissivity. The present device operates at high DC voltages and is designed with a rigid backing. An improved version, termed a micro‐ESR, is being fabricated as an applique. Since the size has been reduced, much lower operating voltages are possible. In addition, the system is conformal, allowing it to be applied to complex surfaces. This paper discusses the results from vacuum testing of the existing ESR devices. It also describes the process to form the applique.

Electrostatic radiator for satellite temperature control

An objective for advanced satellites and spacecraft is to continually reduce both their size and mass. This reduction can place severe constraints on the thermal control systems. In addition, mission requirements also may dictate the need to alter the spacecraft energy profile during the course of the mission. To facilitate these advances, significant research has been conducted to develop variable emittance coatings and devices to provide active spacecraft thermal control. Several of these technologies have matured to a level where space based testing is feasible and will be performed as part of the ST5 new millennium spacecraft mission. One of these technologies utilizes electrostatic hold-down of a high emissivity composite film to actively control spacecraft skin temperature. This electrostatic radiator (ESR) device functions as a thermal switch and changes the mode of heat transfer between the spacecraft skin and the radiator film from conduction to radiation. This device has demonstrated large changes in effective emissivity in laboratory cold thermal vacuum testing. In this paper, the theory of operation of the ESR and the construction of the device as configured for demonstration on the ST5 mission is presented. The role of the ST5 mission in the maturation process for VEC technologies is discussed. This paper also describes test results for the ESR through flight qualification testing for the ST5 mission. Finally, anticipated operational characteristics and mission reliability estimates are provided.

Electrostatic Radiator for Spacecraft Temperature Control

This paper describes development of and test results for an electrostatically switched radiator (ESR). This is a device that can control the radiation emitted from a surface by controlling the position of a thin membrane. The present structure has been fabricated for flight testing on NASA’s ST5 New Millennium program. It consists of 4 separately controlled radiator sections with a total active area of 57.6 cm2. As opposed to the original approach, this structure has the outer membrane at ground potential and is constructed onto a printed circuit board. In this paper we discuss the current state of development of the ESR, including device fabrication and test results.

Development Status of Electrostatic Switched Radiator

In this paper, we describe the current status of the development and testing of the electrostatic switched radiator (ESR) variable emissivity control devices. This paper will discuss the current state of the first generation ESR which has been integrated onto NASA ST5 satellites for flight demonstration. It also will describe the current state of development of the second generation ESR applique. Plans for maturing the technology readiness levels of each device and potential applications to thermal control systems are presented.

Development of an Inductively Coupled Magnetoelastic Torque Sensor

Application of an inductively coupled torque sensor is described. The active element consists of an electrically conductive wire on which a magnetoelastic material is electrodeposited. This element exhibits a nonlinear impedance with current and also with stress. When mounted onto a shaft, these variations can be processed to provide a measurement of the applied torque. The physical characteristics of the sensor (i.e. small size, low mass, and rugged construction) suggest its use in demanding applications or harsh environments. Because of the magnetic properties of the sensor, inductive coupling can be utilized to provide both excitation and signal detection; thus permitting a contactless method of measuring torque. Additionally, because both the sensor and electronics are not overly complex, the system seems suitable for wide-spread application. The fabrication and electrical characteristics of the stress sensitive wires as a surface mounted torque sensor are described, as are the external magnetic field excitation, detection system, and signal processing techniques used to determine the applied torque. Test results from an experimental system are presented.

Electrostatic Switchable Appliqué

In this paper, the development of a second generation electrostatic radiator (ESR) is described. This device consists of a different operational structure and materials than the first generation ESR. The second generation ESR employs a less insulating membrane than the previous device. Additionally, the insulator has been moved from the membrane to the rigid substrate. This construction results in significantly reduced operating voltages compared to the first generation device. The construction also permits easier fabrication which permits the device to be applied to the spacecraft as an applique. This paper discusses the current state of development of the ESR applique, its construction and results from laboratory thermal vacuum performance testing.

Current controlled variable inductance wire

This paper describes development of a magnetic wire whose impedance varies with the current. The wire is a copper wire coated with a magnetic film, deposited via electrodeposition. The high permeability of the coating, coupled with the small geometry, results in a relatively large inductance. In addition, as the current is increased, the loss increases, thus increasing the impedance by an order of magnitude. Due to these properties, these wires may be potentially useful as a passive current limiting and surge protection device.