Engineering topics
Publications and source records attributed to Mosier, Carol L..
The powerpoint presentation details the key factors that lead to the design for the Cosmic Background Explorer's (COBE) spacecraft and instrument thermal design.
The propulsion system of the Microwave Anisotropy Probe (MAP) had stringent requirements that made the thermal design unique. To meet instrument stability requirements the system had to be designed to keep temperatures of all components within acceptable limits without heater cycling. Although the spacecraft remains at a fixed 22 sun angle at L2, the variations in solar constant, property degradation, and bus voltage range all significantly affect the temperature. Large portions of the fuel lines are external to the structure and all components are mounted to non-conductive composite structure. These two facts made the sensitivity to the MLI effective emissivity and bus temperature very high. Approximately two years prior to launch the propulsion system was redesigned to meet MAP requirements. The new design utilized hardware that was already installed in order to meet schedule constraints. The spacecraft design and the thermal requirements were changed to compensate for inadequacies of the existing hardware. The propulsion system consists of fuel lines, fill and drain lines/valve, eight thrusters, a HXCM, and a propulsion tank. A voltage regulator was added to keep critical components within limits. Software was developed to control the operational heaters. Trim resistors were put in series with each operational heater circuits and the tank survival heater. A highly sophisticated test program, which included real time model correlation, was developed to determine trim resistors sizes. These trim resistors were installed during a chamber break and verified during thermal balance testing.
The Cosmic Background Explorer (COBE) is a satellite which is designed to study the 3-K cosmic background radiation. Two cryogenic instruments and a common support structure comprise the Cryogenic Optical Assembly (COA). In order to make definitive measurements of the low-energy FIR radiation, the temperature of the COA must be maintained below 3 K. Temperature control of the COA elements is achieved by the use of a superfluid helium dewar. The thermal design also minimizes power dissipation, transient operational effects, and thermal gradients within the COA. This is accomplished by isolating selected components and heat-sinking others to various locations on the cryogen tank. The thermal design was developed primarily through analysis. Analytical models were created to perform parametric design studies. Ground testing was performed to correlate the thermal models and validate the thermal design. Both test and flight data confirmed that all design requirements were either met or exceeded.
The thermal design of a calibrator used to correlate the collected data with an absolute reference onboard of the Cosmic Background Explorer is considered. Since flight conditions could not be simulated in ground testing, the design was qualified by analysis and testing. The requirements imposed on the calibrator are outlined, along with spacecraft hardware modifications aimed at reducing the thermal load on the calibrator. A geometric mathematical model created in order to obtain view factors from the cryogenic instruments to the spacecraft and dewar is discussed. It is shown that the calibrator met or exceeded all of its performance requirements, and that the modifications to such energy sources as the thermal shield and differential microwave radiometers proved effective in reducing the radiative energy the calibrator received from the spacecraft to acceptable levels.
The Cosmic Background Explorer (COBE) spacecraft, developed by Goddard Space Flight Center (GSFC), was successfully launched on November 18, 1989 aboard a Delta expendable launch vehicle. Two of the three instruments for this mission were mounted inside a liquid helium (LHe) dewar which operates at a temperature of 2 K. These two instruments are the Diffuse Infrared Background Experiment (DIRBE) and the Far Infrared Absolute Spectrophotometer (FIRAS). They are mounted to a common Instrument Interface Structure (IIS) and the entire assembly is called the Cryogenic Optical Assembly (COA). As part of the structural verification requirement, it was necessary to show that the entire COA exhibited adequate strength and would be capable of withstanding the launch environment. This requirement presented an unique challenge for COBE because the COA is built and assembled at room temperature (300 K), cooled to 2 K, and then subjected to launch loads. However, strength testing of the entire COA at 2 K could not be done because of facility limitations. Therefore, it was decided to perform the strength verification of the COA by analysis.