Probe and Probe Carrier Subsystem (THEMIS)

THEMIS Model in Orbit Configuration

THEMIS Mission Overview

The Time History of Events and Macroscale Interactions during Substorms (THEMIS) program is a NASA Medium Class Explorer (MIDEX) Mission. The program was selected in March 2003 with a successful launch on February 17, 2007 from Cape Canaveral Air Force Station, Florida, aboard a Delta II launch vehicle. ATK was the Prime Contractor for the Probe Bus and Probe Carrier. The University of California Berkeley was the Mission integrator and instrument provider. For information on the THEMIS mission science and instrument suite, please see the UCB web site at http:⁄⁄themis.ssl.berkeley.edu⁄

THEMIS provides answers to critical questions about the origin and phenomenology of solar and Earth magnetosphere interaction, the resultant electrical substorms, effects on space weather, disruptions in ground power grids, and communications. These affect the operation of other space satellites and the lives of humans in the sub-aurora regions on Earth.

THEMIS consists of a constellation of five microsatellites (probes) carrying identical suites of electric field, magnetic field, and particle instruments used to determine the cause of global reconfiguration and transport of explosive releases of solar wind energy into the Earth's magnetosphere — a region of the upper atmosphere that extends for thousands of miles. Each probe incorporates flight-proven instruments and subsystems reducing cost and risk while increasing system reliability. Every four days, the five probes reach the apogees of their highly elliptical Earth orbits — which line up along Earth's magnetosphere tail — providing an opportunity to measure disturbances. This data is combined with measurements of the aurora light in the upper atmosphere collected by a North American ground observatory network.

Spacecraft Bus Overview
The THEMIS Spacecraft or Probe Bus is illustrated in Figure 1a, 1b and Figure 2. The five THEMIS Probes are identical in design and are capable of being placed in any THEMIS orbit. This provides robustness to the constellation design and allows for any one of the Probes to be placed in any orbit. The Probe Bus has a number of driving requirements that dictate its design and layout. These driving requirements include:

• Minimization of mass and power usage
• Radiation hardened electronics
• Mass balance to achieve optimum spin balance for stability and fuel savings
• Maximization of fuel carrying capacity
• Magnetic cleanliness to reduce effects on instrument magnetic measurements
• Highly conductive exterior surface to minimize surface charging effects on instruments electric field requirements
• Optimized packaging of the system to reduce the overall size of the Probe on the Probe Carrier therefore maximizing the clearances between Probes for separation
• Thermal Passive Design with Multi Layer Insulation Blankets and thermostatically controlled heaters to survive early orbit maneuvers and all science orbits
• Tolerate extreme temperature swings (-115C to +105C), be thermally safe in an attitude/orientation, and operate through an eclipse of three hours.

Although a number of these requirements are encountered in many spacecraft designs, in combination, they make for an extremely challenging design. The major subsystem designs and how these subsystem designs achieve the mission objectives are described in the following sections.

Figure 1a: Rendering of THEMIS Probe - Outside View

Figure 1b: Rendering of THEMIS Probe - Top View Cutaway

Figure 2: Probe 2 Bus Fully Configured

Structure and Mechanical Subsystem
The THEMIS Probe Bus structure provides mechanical support for all other subsystems and consists of ultra lightweight panels constructed of composite graphite epoxy facesheets and an aluminum honeycomb core. In addition, there are corner panels and a center tube that houses the Axial Electrical Field Instruments (UCB provided) comprised of layers of composite materials. All panels have embedded fittings of either titanium and/or aluminum that have been machined to minimize mass. The primary structure must withstand launch loads and also the extreme temperature swings during early orbit operations and when entering the Earth eclipse. Extensive analysis and development testing was performed on the new composite elements of the structure. These environments are simulated via vibration testing and panel level thermal cycling at the subsystem level prior to delivery of the Probe structure to Integration and Test. The mass of the entire structure and mechanical subsystem including mounting hardware is 15 Kg and represents approximately 19.5% of the Probe dry mass (without fuel) and 12% of the Probe wet mass. Figure 3 shows one of the Probes populated with mass mock-ups prior to vibration testing.

Figure 3: THEMIS Probe 3 Structure

Reaction Control Subsystem (Propulsion)
The Reaction Control Subsystem (RCS) provides the actuators to change Probe velocity, inertial attitude, and spin rate, and consists of two fuel tanks, tangential and thrust engines (4), a pressurant tank, latch valves, pyro valves, and miscellaneous hardware. The RCS holds up to 49 kg of fuel in two tanks that were specially made and qualified for the THEMIS program. These tanks are made of high strength steel (inconel) and are supported by the bottom and top panels via integral polar fittings. These tanks are highly optimized for mass. The unique feature of the RCS is the combination of a pressurization system that enhances the capability of the system once on orbit. Once the fuel in the system is depleted by approximately 25%, a command is sent to the Probe initiating a pyro valve firing that repressurizes the tank system. This design feature provides more performance to the system by increasing the pressure within the fuel tanks. There are two thrust (axial) engines that provide 4.4 Newtons of thrust allowing for major orbit changes of the Probe. In addition, there are two tangential engines of the same size that provide spin control and/or lateral thrust to the Probe. The entire RCS weighs only 12 kg without fuel and is approximately 15% of the Probe dry mass. Figure 4 shows one of the Probe subsystems prior to shipment.

Figure 4: THEMIS Probe Reaction Control Subsystem (RCS)

Attitude Control Subsystem
The Attitude Control Subsystem (ACS) provides the telemetry and command capability for spin rate and attitude control in conjunction with the RCS. The THEMIS Probes, when released from the Probe Carrier and the launch vehicle third stage, are spin stabilized, which infers motion stability via spinning. The nominal rate is 16 revolutions per minute (RPM). In order to achieve spin stabilization, the Probes are configured to have their center of mass closely aligned to the geometrical thrust axis. This alignment is accomplished through painstaking placement of components and by adjusting spin balance masses prior to launch. Once the Probes are released, spinning, the ACS enables the Probes to maintain spin stability throughout the life of the mission. The ACS monitors the spin rate and attitude of the Probe once it separates from the Launch Vehicle third stage. The ACS is assisted by the onboard instrument magnetometer that provides Earth magnetic field measurements. THEMIS Probes must be stable for a wide range of configurations (multiple mass property changes) due to instrument boom deployments and fuel depletion. The ACS major bus components are the miniature sun sensor that enables estimation of spin rate from sun crossing times, and solid-state inertial reference unit (IRU) assembly that measures angular rate in the other two axes. This ACS telemetry is linked to the ground via the communication subsystem where it is processed and a set of commands is generated to be linked back up to the Probe to command the RCS as required. The ACS Bus components together (excluding the instrument magnetometer) weigh only 0.6 Kg. Figures 5 and 6 show the Miniature Sun Sensor and IRU assembly.

Figure 5: Miniature Sun Sensor Photograph courtesy of Adcole Corporation

Figure 6: IRU Assembly

Power Subsystem
The Power Subsystem provides power to all electrical components and consists of body mounted solar arrays and a Lithium-ion battery made of multiple battery cells. The THEMIS Probe is highly efficient in power usage with approximately 36.85 Watts required in full science mode for a 24-hour orbit, which includes a 3-hour eclipse and a 30-minute transmitter turn-on. That’s less than a 40-watt home light bulb. The capability for that orbit at the mission End of Life (EOL) is 40.35W. The THEMIS Probe has eight solar arrays that provide power generation for any orientation of the Probe. There are two arrays mounted on the bottom and top decks and there are four side panels. The arrays use high efficiency (>27%) cells that are bonded to the composite substrates. The side panels are also the primary structure which adds to their design complexity since they have to transfer loads between the top and bottom decks. The solar arrays are also unique: in order to reduce charging affects (minimal exposed insulators), all of the cover glass must be electrically grounded to a common ground on each panel. This is accomplished by bonding a highly-conductive grid onto the panels following cell placement. Power is stored onboard by a Lithium-ion battery that maintains the Probe power during eclipse, which can last up to three hours. The battery is lightweight and provides up to 12.0 Amp hours of power capacity. The major power subsystem components weigh approximately 10.3 kg and represent approximately 13% of the total Probe bus dry mass. Figures 7 and 8 shows one side panel solar array and the six Lithium-ion batteries.

Figure 7: Side Panel Solar Array

Figure 8: THEMIS Batteries

Communication Subsystem
The Communication Subsystem provides communication between the Probe and the Ground Stations. The Communication Subsystem consists of an S-Band transponder and S-Band Antenna mounted to the center boom structure. The transponder is lightweight and converts the Radio Signal Frequency (RF) signal from the ground into a digital signal to the Bus Avionics Unit (BAU). The transponder also performs the reverse operation where it takes the digital signals from the Bus Avionics Unit and converts it to RF. It then transmits this RF signal to the Antenna where it is radiated to the ground. The THEMIS S-Band Antenna consists of six receiver/transmit stack patch antennas and a power divider. These antennas are extremely lightweight and must have a conductive surface in order not to build up surface charge as the Probe travels through the space plasma environment. The total mass of the communication subsystem is 3.2 kg and represents 4% of the Probe dry mass. Figures 9 shows the transponder. The S-Band flight antenna is shown in Figure 10.

Figure 9: S-Band Transponder Photograph Courtesy of L3-Com Telemetry

Figure 10: S-Band Antenna

Bus Avionics Unit
The Bus Avionics Unit (BAU) provides numerous functions for the Probe Bus and contains the flight computer for the Satellite. The BAU provides for the processing of all the data handling, internal communication interface, instrument electrical interface, and power control for the Probe Bus. The BAU contains five modules with the top module containing a radiation hardened main processor (Cold Fire Processor operating at 16.78 MHz). This module performs all the onboard processing and data handling. It contains 64 MB of bulk memory and supports a 2.1 Mbps data rate interface with the instrument electronics. The BAU hosts the RTEMS real-time operating system and the application control and data handling software for the Probe Bus. The second module is a communication module that interfaces with the transponder and other modules within the BAU. The remaining modules are power modules that control the distribution of power on the Probe and provide the function of energy balance by balancing the power from the solar arrays and battery. The BAU is extremely lightweight at 3.0 kg and consumes, on average, less than 7.0 Watts. Figure 11 shows the first flight BAU.

Figure 11: Flight Bus Avionics Unit (BAU)

Thermal Control Subsystem
The THEMIS Probe Thermal Subsystem is a hot-biased design that uses solar heat input to elevate component temperatures allowing survival at all sun aspect angles as well as three hour eclipses with minimal heater power consumption (less than 12 Watts orbit average). The hot-biased thermal design includes external coatings with high solar absorbance-to-emittance ratios, such as Vapor Deposited Gold (VDG) and high-efficiency Multi-Layer Insulation blankets to minimize heat loss from the hydrazine Reaction Control System (temperature requirement of greater than 5°C). Figure 12 shows an illustration of the external Thermal interfaces on the THEMIS Probe.

Figure 12: Thermal Control Interfaces

Probe Carrier and Separation System
The THEMIS constellation of five probes was deployed from a Probe Carrier mounted to the third stage of a Delta II rocket spinning nominally at 16 rotations per minute (rpm). This is a significant engineering challenge in the design of the separation system and the Probe Carrier. However, the stability of the Probe during separation is crucial in order to avoid collisions between Probes and/or the Probe Carrier. The separation system facilitates an unobstructed and stable separation of the Probes by moving quickly away from the separation plane and imparting a low tip off rate (rotation) to the Probes. The top Probe deploys first and the lower four probes deploy simultaneously three seconds later. The deployment is triggered by the third stage of the Delta II.

The Probe Carrier is predominately aluminum alloy, is weight-optimized, and includes a patch panel that manifolds all of the umbilical electrical and control circuit cabling from the Probes to the launch vehicle. The separation system was extensively analyzed and tested to properly characterize its performance and to verify all of the mechanical parameters that drive the overall Probe and Probe Carrier system clearance verification analysis (Figure 16). Figure 13 shows the separation system and Figure 14 provides a view of the Probe Carrier structure with mass mockups set up for acoustic testing at the NASA Goddard Space Flight Center. Figure 15 shows the Probe Carrier Assembly in the launch configuration during vibration testing at the Jet Propulsion Laboratory. Figure 16 shows an illustration developed from the Probe Carrier Separation Analysis that was used to verify the satellite separations from the third stage of the launch vehicle.

Figure 13: Separation System in Test

Figure14: Probe Carrier Acoustic Testing at GSFC

Figure 15: Probes on Probe Carrier at JPL

Figure 16: Probe Separation Analysis

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