Introduction

Today, several concepts of reusable space transportation systems are under investigation. The main purpose of these concepts is to offer a cost-efficient manned and unmanned transport into space. Horizontal take-off and landing (HTOL) launch vehicles could be accelerated on ground by rocket assisted take-off sledges which results in the size reduction of the reusable launch vehicle's landing gear and wings. Thus, the payload capacity can be increased. As against much more expensive magnetic sledges, hot water propulsion systems were already recommended by Eugen Sänger, a famous German aerospace scientist. This propulsion system is environmentally friendly, safe and cost-saving.

Fig. 1:  Rocket assisted take-off system with hot water propulsion for future reusable space transportation systems, developed at the Institute of Aeronautics and Astronautics, Berlin (left), and for the investigated European RLV HOPPER in an artist's impression (right)

Since the beginning of the 1990s, hot water rocket propulsion systems are investigated theoretically and practically at the Institute of Aeronautics and Astronautics at Berlin University of Technology. Particularly in the context of study works and diploma theses as well as the course "Project Space Transportation Systems", supervised by the AQUARIUS group, the development of technology demonstrators for horizontal take-off systems with hot water propulsion for future reusable space transportation systems has been advanced intensively since 1998. This includes the so-called X-RATOS family, where the acronym X-RATOS stands for „eXperimental Rocket Assisted Take-Off System“.