Orbital transfer vehicle requirements and concepts

The requirements, definitions, and current planning for cost effective transportation from low earth orbit to higher energy earth orbits and other orbital transfers, to complement the Space Shuttle, are discussed. The issues that arise from the uncertainties of future priorities and funding as well as the maintenance of a development capacity for a future new space engine are examined. Attention is given to mission requirements including orbiter transfer requirements and power module specifications and also to candidate and cryogenic OTV concepts. The configurations for the solar electric propulsion system are presented along with the program and space transportation system evolution.