An Overview of Advanced Concepts for Near Space Systems

A brief review of both near-term and far-term platforms proposed for near-space operations is given. The primary focus of the paper is, however, a review of potential advanced propulsion systems for such long-duration near-space platforms. The basic requirements for near-space propulsion systems are defined. Low Reynolds number propellers, the current workhorse, are used as a baseline for comparison. Two broad classifications are identified as potential sources of force in near space: rarefied gas and electric propulsion. Radiometric force propulsion systems, the only candidate propulsion systems found in the open literature, suffer from both significant uncertainty in their underlying physics and from significant operational difficulties. Thermal transpiration propulsion systems were shown fundamentally incapable of providing the required performance. Air-breathing electric propulsion systems for long-duration near-space missions will be significantly different than their in-space counterparts with specific impulses likely under 100s. Electrohydrodynamics propulsion systems show some promise, but have thus far shown limited thrust efficiency at sea level operation, and the efficiency is only predicted to get lower at higher altitudes. The potential effects of systems based on breakthrough physics are also qualitatively discussed. All of the identified potential advanced propulsion concepts for long-duration near-space operations suffer from major technological challenges with significant advancements required for any of them to be viable.