Space tug spacecraft/module exchanger (on-orbit servicer)

On-orbit exchange of spacecraft modules containing experiment or subsystem hardware or consumables performed at goesynchronous altitudes with the space tug and at lower altitudes with the shuttle is described. One promising concept for such a tug-mounted spacecraft module exchange is the McDonnell Douglas Direct-Access Servicer. This servicer can also be used in low orbit, mounted on the shuttle. In this concept, push-pull injector/retractor devices are combined with a rotating indexer to provide a minimum-complexity mechanism with a high capacity for coincident exchange of many modules.