Active Image Stabilization for Shuttle-Based Payloads
This paper presents a pointing performance analysis of the Space Infrared Telescope Facility (SIRTF) as a generic example illustrating the fundamental dynamics and control problems related to optical instruments that are, or will be, flown mounted on the Space Shuttle. The disturbance level on the Shuttle, and slew requirements of the payload itself, along with the limitations of the present instrument pointing system (IPS) is found to seriously limit the pointing accuracy of optical payloads. The implementation of an active image stabilization (AIS) system, using an active secondary mirror, is shown to provide substantial improvement in performance. Two control strategies are presented, using IPS-mounted rate integrating gyros only, or in conjunction with a focal plane sensor feedback loop.
Orbital Operations with the Shuttle Infrared Telescope Facility (SIRTF)
The Shuttle Infrared Telescope Facility (SIRTF) is a cryogenically-cooled, 1-m-class telescope that will be operated from the Space Shuttle as an observatory for infrared astronomy. Over the 2- to 200-μm band, SIRTF will be 100 to 1000 times more sensitive than existing infrared facilities. This paper discusses the scientific constraints on and the requirements for pointing and controlling SIRTF as well as several aspects of SIRTF orbital operations. The basic pointing requirement is for an rms stability of 0.25 arcsec, which is necessary to realize the full angular resolution of the 5-μm diffraction-limited SIRTF. Achieving this stability requires the use of hardware and software integral to SIRTF working interactively with the gyrostabilized Shuttle pointing-mount. The higher sensitivity of SIRTF, together with orbital and time constraints, puts a premium on rapid target acquisition and on efficient operational and observational procedures. Several possible acquisition modes are discussed, and the importance of source acquisition by maximizing the output of an infrared detector is emphasized. The ability of the pointing-mount to slew the telescope over a range of angles is discussed, including its capability for executing raster scans over limited areas. Both minimum-time scans and constant-rate scans, which may take significantly longer in some cases, are discussed.
Intensification of Bubble Disintegration and Dispersion by Mechanical Stirring in Gas Injection Refining
Water model experiments were performed for establishing highly efficient gas injection refining processes. Mechanical stirring was applied to disintegrate the injected bubbles and to disperse them widely in the bath. The bubble disintegration and dispersion were investigated by changing rotation mode (direction of rotation), rotation speed, blade size of the impeller and gas flow rate. Forward rotation of the impeller induced a stable tangential flow and could not disperse bubbles in the bath due to formation of a vortex around the impeller shaft. The tangential flow could be suppressed by forward–interrupt rotation, which could reduce the vortex formation to some degree. However, forward–interrupt rotation could not disperse the bubbles widely in the bath. Forward–reverse rotation could prevent the vortex formation completely and create a strong shear stress field, which intensified the bubble disintegration and dispersion in the bath. Higher impeller rotation speed and larger blade length in forward–reverse rotation could enhance the bubble disintegration and make the dispersed bubbles smaller. The bubble dispersion zone became wider with larger blade length. The bubble size tended to be larger at higher gas flow rates. However, its dependence on the gas flow rate became smaller at higher impeller rotation speed.