Some Aspects concerning the Design of Multistage Earth Orbit Launchers using Electromagnetic Acceleration

Kurzfassung

This paper reports on reflections undertaken about a possible space-launch project on the basis of the use of electromagnetic launch technology. It is assumed that the direct access to space is not possible using a single-stage system only, since for such a system muzzle velocities of approx. 10 km/s are required. Therefore, combined electrical-chemical multistage systems are considered. Multi-stage chemical rockets have been realized for LEO (Low earth Orbit) projects but it turned out that they are rather ineffective. For example, modern vertically launched rocket carrier burns more than one third of its propellant up to altitude of 15km. In contrast, the injection of small payloads in LEO by means of propelled vehicles (e.g. rockets) which are launched from electromagnetic accelerators is a concept that probably enables the access to space at reduced cost. Such a solution has serious consequences for all stages, in particular for the accelerator and payload carrier design. The paper is divided into two parts. First, some general aspects (size, forces, energy and power) of an electromagnetic launcher, which could be able to achieve the required muzzle momentum for such a mission, are discussed. More specifically, the design of such a launcher with respect to the type of electromagnetic Lorentz Rail Accelerators (known also as railgun) and to the armature technology is considered and also some of the inter-stage aspects of such a system are investigated. Special attention is paid to solutions increasing the launch efficiency of the electromagnetic launcher. The second part of the paper considers details of the rocket stages. The propelled payload carrier with very high initial velocity is exposed to harsh aerothermal loads during the ascent flight through the dense atmosphere. The analysis shows that a ground-based electromagnetic Lorentz rail accelerator (LRA) is limited to a start velocity of about 4.5 km/sec if the present state of technology is considered. At higher velocities the payload carrier will be destroyed due to the extreme heat loads. In the paper possible solutions for aerothermal protection, based on passive Thermal Protection Systems (TPS), effusion and film cooling, ablation or their combinations are analyzed and discussed. Also, the shape of the projectile has to be optimized to reduce the aerodynamic drag and to enable static stability. Some solutions to achieve this goal are here suggested. Concerning possible trajectories for such mission, a feasibility study is performed. It indicates that the injection orbit or payload mass can be increased if the initial velocity is kept constant as 4.5km/sec. Examples of such calculations for a two-stage LEO launcher are shown: the initial flight path angle the and velocity, the thrust profile of engines for both (booster and kick–off) stages influence the trajectory profile and payload mass.