Combined cryogenic insulation and thermal protection systems for reusable stages

Abstract

Returning and reusing a launch vehicle stage after it has fulfilled its primary ascent mission significantly extends the range of thermal boundary conditions in which the stage has to safely operate, reaching from the cryogenic temperatures within the propellant tanks to the high external heat flux encountered during reentry to the vacuum of space. For a hydrogen-fueled stage, not only is cryogenic insulation needed but also an external thermal protection system in order to assure that the stage survives the heat loads during reentry. While usually treated as separate domains, for winged reusable launch vehicles cryogenic insulation and the thermal protection system have to be designed together since they are both integrated onto the propellant tanks external surface. This imposes new requirements on both designs. Additional complexity arises from the fact that multiple cycles of thermal and mechanical loads have to be survived without substantial refurbishment in order to assure a cost-effective design. Within the DLR this topic was first explored in the AKIRA project, which investigated selected technologies that were deemed critical for operational reusable booster stages but which are not being covered within the flight demonstrator projects of the DLR. An overview over the AKIRA experimental campaign with representative test objects, which verified the thermal functionality of the derived design, is given. Within a currently ongoing follow-on project (TRANSIENT) the design is further refined and applied to both metallic and composite tank materials. For both cases segments meant to represent a slice of an RLV propellant tank are being manufactured and equipped with the cryogenic insulation and thermal protection systems and will be tested under representative loads for up to 50 cycles. In addition to the thermal loads investigated previously, mechanical loads will also be applied to the test objects.