Solar thermal power / propulsion systems (PPS) are an alternative to the nuclear thermal PPS as the upper stage launch vehicle (LV) to reduce the cost of launching to geosynchronous orbit (GEO).
However in this case due to limitations on the amount of heat provided to the solar PPS, the time of the interorbital flight is sharply increased (from a week to a month).
Therefore if the nuclear PPS allows overcooling of the cryogenic components (usually hydrogen and oxygen) on the start position, the solar PPS demands the active thermocontrolling.
It is shown the optimal decision in this case is the closed Brayton energy conversion system (CBECS). The advantage of this system is high efficiency in GEO and also the possibility to increase significantly its efficiency during the interorbital flight due to CBECS working gas cooling by cryogenic fuel components of the PPS propulsion system.
Two versions of the solar closed Brayton power / propulsion system (CBPPS) were considered: first, the thermo-controlling of the cryogenic fuel tanks is realized by the thermodynamic vent system; second, this thermocontrol is provided by the cryogenic machine.
The cycles and schemes of this systems are analyzed, the data for their main elements are discussed. Mass-size of these 5 kWe and low constant thrust CBPPS were estimated for a high latitude launch site using a middle class LV. The comparison of these versions showed the significant advantages of the closed Brayton cryogenic/ power/propulsion system (CBCPPS) for the long term storage of the cryogenic fuel in the GEO.
Some results of the development of the CBCPPS key element, turbogenerator-compressor (TGC) neon centrifugal compressor (CC), are discussed.