On Secondary Gas Injection in Supersonic Nozzles

A second rendezvous scheme planned for use in Gemini missions is the semioptical technique. This method requires the flight crew to maneuver the spacecraft based on range and range rate information from the radar and visual observations of target motion with respect to star patterns. Until recently, the semioptical method was one of maneuvering laterally to drive the apparent target motion to zero relative to the star background and, at the same time, maintaining a closing range rate. Although this scheme will still be used at small ranges (3 naut miles to a few hundred feet), recent simulations have shown hat guidance to a nominal transfer trajectory is considerably more efficient when guidance must be initiated at large ranges. In the case of the first GeminiAgena rendezvous mission, the distance of closest approach is about 15 naut miles if no rendezvous maneuvers are made; from this trajectory, semioptical rendezvous by nulling the line-of-sight angular rate is very costly. Guidance to a nominal transfer trajectory is achieved by monitoring range rate as a function of range and adjusting the rate when it deviates markedly from the nominal value at that range. Fuel values for this scheme approach the low expenditure required for the closed-loop method. The final few hundred feet of the rendezvous mission and the actual docking maneuvers are strictly manually controlled. The pilots, based on viewing the target through the Gemini window, control both the attitude and maneuvering systems of the spacecraft to accomplish docking with the target. This phase of the mission, involving a controlled joining of multiton spacecraft, has introduced several design features to Gemini. The maneuvering control system provides six-directional thrust so that attitude changes are not required for performing speed or directional changes. Hence, the pilots are never required to lose sight of the target. The thrust levels for maneuvering are set at an intermediate value between the desired high thrust for orbit changes and the desired low thrust for docking. These design requirements were confirmed, again using simulation to introduce the human aspects of the system. Conversely, other proposed design features were eliminated when simulation showed the docking task could be accomplished without added complication. It is interesting to note that, although simulations of increasing sophistication were developed during the program, the demonstrated performance remained fairly constant. Apparently, the added cues, provided with more realistic simulations, compensate for the additional disturbing factors and/or system limitations. From the very first simulations, where meter presentations of range were used, until the "ultimate"