Hybrid Launch Vehicle
Second Generation Engines
The potential for improvement in mission performance by using first generation microwave engines can be seen to be very significant. However the introduction of superconducting technology in second generation engines will present quite remarkable possibilities.
At present, superconducting microwave cavities are restricted to high energy physics applications. An example is the European Tesla accelerator where 20,000 S Band niobium cavities cooled with liquid helium will form the basis of the main accelerators. Early production versions of these cavities readily achieve Q values of 5 x 109 (BAUER, S. et al ‘Production of Superconducting 9-cell Cavities for the Tesla test facility, Standford University and Forschungszentrum Rossendorf’
Accel Instruments GmBh). This would lead to a static specific thrust of 3.15 x 104 N/kW (3.2 tonnes / kW).
It is worth noting that a problem identified during the development of these symmetric cavities is the detuning caused by mechanical deformation due to internal EM radiation pressure!
A second generation engine using similar technology would however be subject to equation 2, and the effect at these high values of unloaded Q is dramatic. An average velocity of only 0.1 m/sec will reduce the specific thrust to 0.93 Tonne / kW.
Equation 2 therefore constrains the applications of second generation engines to those where the kinetic energy output is limited.