High Altitude Launch for a Practical SSTO

Existing engineering materials allow the constuction of towers to heights of many kilometers. Orbital launch from a high altitude has significant advantages over sea-level launch due to the reduced atmospheric pressure, resulting in lower atmospheric drag on the vehicle and allowing higher rocket engine performance. High-altitude launch sites are particularly advantageous for single-stage to orbit (SSTO) vehicles, where the payload is typically 2% of the initial launch mass. An earlier paper enumerated some of the advantages of high altitude launch of SSTO vehicles. In this paper, we calculate launch trajectories for a candidate SSTO vehicle, and calculate the advantage of launch at launch altitudes 5 to 25 kilometer altitudes above sea level. The performance increase can be directly translated into increased payload capability to orbit, ranging from 5 to 20% increase in the mass to orbit. For a candidate vehicle with an initial payload fraction of 2% of gross lift-off weight, this corresponds to 3 1% increase in payload (for 5-km launch altitude) to 122% additional payload (for 25-km launch altitude). INTRODUCTION Existing human-build structures have heights slightly less than one kilometer, however, this height is not limited by materials or construction technology, but rather is limited by the lack of a compelling application for higher towers. Towers of height fifteen to twenty-five kilometers could be easily built using present-day materials. Use of such towers could have great advantages as the launch site of a single stage to robit vehicle. As an example, table 1 shows the minimum mass required for a tower sized to hold its own weight plus that of a 2000-ton payload at the top of the tower. If the tower material is constructed from a standard construction material, cast steel, the minimum tower mass is approximately two and a half times the weight of the payload at the top. To avoid structural collapse, if made from steel, such a tower would have to be tapered slightly (area taper ratio 2.6:l) from the bottom to top. If a more advanced material with a higher strenght to weight ratio is used, graphite/epoxy composite, the tower is much lower in weight. In this case the required tower mass is only 14% of the mass of the supported payload, and no taper is needed. Even more advanced materials allow a lower mass yet to be employed. Although these simplified calculated masses do not include nonstructural beams and required auxiliary components, such as (for example) elevators required to lift the vehicle to the top of the tower, cables for bracing, and activedamping control structure for mitigating vibration and wind loads, they serve as a sanity check to show that towers considerably higher than those presently constructed are, in fact, not prohibited by the basic physics of materials. For extremely high towers, the structure would likely be constructed as a "fractal truss ,I' where the individual beams of a truss are each themselves a truss member, and so forth. An example of such a multi-level truss structure is shown in figure 1. As discussed by Landis (1998), use of the top of such a tower as the launch site of a rocket would have a long list of advantages. Single stage to orbit (SSTO) vehicles are particularly sensitive to small improvements in launch conditions. Landis (1998) estimated that the payload of a single stage to orbit vehicle could improve by approximately 60% if the vehicle was launched from fifteen kilometers altitude, instead of launching at sea level. ~~ This is a Preprint Or reprint of a paper intended for presentation at a conference. Because changes may be made before formal publication, this is made available with the understanding that it will not be cited Or reproduced without the permission of the author. https://ntrs.nasa.gov/search.jsp?R=20030022661 2018-02-25T13:08:05+00:00Z