The SUMS experiment will obtain measurements of free-stream density during atmospheric entry in the hypersonic, rarefied flow regime. These measurements, combined with acceleration measurements from the companion high-resolution accelerometer package experiment, will allow calculation of orbiter aerodynamic coefficients in the flow regime previously inaccessible to experimental and analytic techniques. SUMS complements SEADS by providing data at higher altitudes. The resultant flight data base will aid in future development of analysis techniques and laboratory facilities for predicting winged-entry-vehicle performance in hypersonic rarefied flow. Furthermore, SUMS will measure equilibrium gas composition at the inlet port, making the experiment a pathfinder for future mass spectrometer application in the study of aerothermodynamic properties of the transition flow field.

The SUMS experiment system consists of a sample orifice, an inlet system and a mass spectrometer. The sample orifice penetrates a thermal tile just aft of the fuselage stagnation point and just forward of the orbiter nose wheel well. The orifice is connected to the inlet system by a short tube through the forward nose wheel well bulkhead. The inlet system is connected through a longer tube to the mass spectrometer, which is mounted above the inlet system on the forward nose wheel well bulkhead. SUMS is designed for easy removal and reinstallation between flights to accommodate modification or repair.

The mass spectrometer is a flight spare unit from the Viking project's upper atmosphere mass spectrometer system. The unit has been modified to be compatible with the orbiter's mechanical, electrical and data systems. The mass spectrometer measures gases from hydrogen through carbon dioxide at a five-second rate. The inlet system contains two switchable flow restrictors that expand the measurement range of the mass spectrometer and position its measurement interval over the desired altitude range. Data from SUMS are output to the OEX data system for recording during flight operation.

SUMS is controlled by stored commands that are transmitted to the orbiter during flight and by internal software logic. Application of power for vacuum maintenance or for normal operation is controlled by stored commands; while internal control of system operation, such as opening and closing valves, is performed by preprogrammed logic. SUMS will be powered on shortly before deorbit burn initiation and will sample the inlet gases down to an altitude of 40 nautical miles. At an altitude of about 59 nautical miles, the range valve will close to switch between the two flow restrictors. At 59 nautical miles, the inlet valve and protection valve will close; but the mass spectrometer will continue to operate until landing, observing the pump-down and background signals after entry.

Operation of SUMS on repeated shuttle flights will not only build a substantial body of aerothermodynamic data for future winged-entry-vehicle design applications, but also add to the knowledge of mass spectrometer applications in aerothermodynamic research. As a further benefit, data will be obtained on atmospheric properties in the altitude range where experimental data are, to date, extremely sparse.