Probabilities Associated with Launch-On-Time, Weather, and Rendezvous for a Space Station System 640569

A space station must be periodically resupplied with spares and vital materials, both to keep the crew alive and to maintain the station in an operable status to fulfill its mission. Furthermore, it is contemplated that crews must be rotated at specified intervals for psychophysiological reasons. Resupply and crew rotation missions must meet severe performance, logistics, and reliability constraints.

This paper concerns the probabilities associated with the many parameters that control successful rendezvous for space station logistic missions. These variables are: (1) launch-on-time (probability of completion of countdown at a precise time), (2) length of resupply interval, (3) synchronism (repetitive cycle) of station orbit, (4) launch stand turnaround time, (5) number of committed launch vehicles, (6) number of stands, (7) countdown and flight (through rendezvous) reliability, (8) length of “launch-window” time, and (9) acceptable launch weather probability during a specified time period.

All these parameters are combined in a mathematical model that provides system operational design criteria. The model involves a unique application of binomial expansions relating number of vehicles, stand turnaround time, orbit synchronism, and weather probability. These expansions generate the probability of having acceptable weather on required number of launch days. This probability is then multiplied by the functional redundancy factor of the overall probability to rendezvous model. This factor relates number of vehicles committed to achieve the overall probability of rendezvous requirement. Parametric presentation of variable reliabilities dependent upon precise and “wide-window” launch times and orbit phasing time are discussed.

The system integration of these parameters and constraints by means of a straightforward and precise model demonstrates the criticality of variable weather and the need for a wider tolerance in acceptable launch-weather criteria. The technique is a means for providing probability tradeoffs for number of vehicles and launch stands in system operational cost effectiveness studies.