93-181

Exploiting Orbital Effects For Short-Range Extravehicular Transfers

Trevor Williams*and David Baughman**

Abstract

If extravehicular transfers between two nearby spacecraft are to be performed using as little propellant as possible, the effects of orbital mechanics must be exploited fully. This problem has been studied recently for transfers of several thousand feet using the Manned Maneuvering Unit, which has a ~v of about 45 fps. Orbital effects are not so evident in short-range transfers unless the relative velocities involved are quite small, as would be the case if a system with a small propulsive capability were used. The new Simplified Aid for EVA Rescue (SAFER) backpack, with a ~v of 9 - 12 fps, is an example of such a system. The problem studied in this paper is that of using SAFER to carry out efficient short-range transfers from the payhad bay of the Space Shuttle Orbiter to the vicinity of the underside of the vehicle, for instance for inspection and repair of thermal tiles or umbilical doors. Trajectories are shown to exist, for the shuttle flying nose forward and belly down, that take the astronaut to the vicinity of the underside with no thrusting after the initial push-off. However, these trajectories are too slow to be of practical interest, as they take roughly an hour to execute. Addtionally, they are quite sensitive to errors in the initial push-off rates. To overcome both of these difficulties, trajectories are then studied which include a single in-flight impulse of small magnitude (in the range 0.1 -0.4 fps). For operational simplicity, this impulse is applied towards the Orbiter at the moment when the line-of-sight of the EVA crewmember is tangential to the underside of the vehicle. These trajectories are considerably faster than the non-impulsive ones: transit times of less than 10 minutes are achievable. Furthermore, the man-in-the-loop feedback scheme used for impulse timing greatly reduces the sensitivity to initial velocity errors. Finally, similar one-impulse trajectories are also shown to exist for the Orbiter in a gravity-gradient attitude. 

*Associate Professor, Department ot Aerospace Engineering, University ot Cincinnati, 797 Rhodes Hall, Cincinnati. Ohio 45221. Senior Member AIAA and AAS.

**Graduate Student, Department of Aerospace Engineering, University of Cincinnati, Cincinnati, Ohio 45221.Student Member AIAA