The Tasks of Aerothermodynamics
The aerothermodynamic design process is embedded in the vehicle design process. Aerothermodynamics has, in concert with the other disciplines, the following tasks:
1. Aerothermodynamic shape definition, which has to take into account the thermal state of the surface, Section 1.4, if it influences strongly via thermal-surface effects the drag of the vehicle (CAV, ARV), the inlet performance, and the performance of trim and control surfaces (all classes):
a) Provision of the aerodynamic data set , enabling of flyability and controllability along the whole trajectory (all vehicle classes).
b) Aerothermodynamic airframe/propulsion integration for in particular airbreathing (CAV), but also rocket propelled (RV, ARV) vehicles.
c) Aerothermodynamic integration of reaction control systems (RV, ARV, AOTV).
d) Aerothermodynamic upper stage integration and separation for TSTO space transportation systems.
2. Aerothermodynamic structural loads determination for the layout of the structure and materials concept, the sizing of the structure, and the external thermal protection system (TPS) or the internal thermal insulation system, including possible active cooling systems for the airframe:
a) Determination of mechanical loads (surface pressure, skin friction), both as static and dynamic loads, especially also acoustic loads.
b) Determination of thermal loads on both external and internal sur – faces/structures.
c) Determination of the aerothermoelastic properties of the airframe.
3. Definition of the necessary and the permissible surface properties (external and internal flow paths), see also Section 1.4:
a) The only but deciding “necessary” surface property is radiation emis – sivity in view of external surface-radiation cooling. It governs the thermal loads of structure and materials, but also the thermal-surface effects regarding viscous-flow and thermo-chemical phenomena.
b) “Permissible” surface properties are surface irregularities like roughness, waviness, steps, gaps etc. in view of laminar-turbulent transition and turbulent boundary-layer flow. For CAV’s and ARV’s they must be “sub-critical” in order to avoid unwanted increments of viscous drag, and of the thermal state of the surface.
For RV’s surface roughness can be an inherent matter of the layout of the thermal protection system. There especially unwanted increments of the thermal state of the surface are of concern on the lower part of the re-entry trajectory. In this context the problems of micro – aerothermodynamics on all trajectory segments are mentioned, which are connected to the flow, for instance, between tiles of a TPS or flow in gaps of control surfaces. All sub-critical, i. e., “permissible”, values of surface irregularities should be well known, because surface tolerances should be as large as possible in order to minimize manufacturing cost.
Another “permissible” surface property is the surface catalycity, which should be as small as possible, in order to avoid unwanted increments of the thermal state of the surface, e. g., of the wall temperature. Usually the surface catalytic behavior, together with emissivity and anti-oxidation protection are properties of the surface coating of the airframe or the TPS material.
This short consideration shows that aerothermodynamics indeed must be seen not only in the context of aerodynamic design as such. It is an element of the truly multidisciplinary design of hypersonic flight vehicles, and must give answers and inputs to a host of design issues.