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Of primary importance for the re-entry are initial parameters and conditions covering:
• type of space craft e.g. re-entry vehicle or uncontrolled SC after end of live
• entry from low circular orbit, elliptic orbit or from a planetary return mission
As the number and mass of uncontrolled re-entries of debris objects outbalances the controlled re-entries of return vehicles both aspects will be treated.
The main tasks of a controlled re-entry are to conduct an aero-braking under the fol-lowing restrictions:
Minimum heat transfer to the vehicle
Acceptable deceleration loads
Approach of the targeted landing area
Historically the basic flight dynamic concepts have been developed by Allen –Eggers and Chapman between 1958 and 1960.
For the flow phenomena associated with the re-entry the large variation of the vehi-cles velocity and the large variation of the ambient density are of primary importance.
The re-entry velocity range covers v = 7600 km /s to 0 m/s
The density and mean free path variation of the atmosphere covers between 160 km and the ground a span of 1,24 10-9 to 1,24 kg/m3 and the corresponding mean free path varies from 53 m to 6,0 10-8 m
Thus during re-entry the vehicles flow field changes from ‘free molecular hypersonic to continuum hypersonic .
High flow velocities and the associated high temperatures in the flow field introduce chemical gas surface and gas phase reactions.
It is accepted practice to distinguish between fluid dynamic flow regimes namely mo-lecular flow, slip flow, continuum flow and the chemical flow regimes in the flow field, namely frozen chemistry, non-equilibrium chemistry and equilibrium chemistry.
These flow regimes are usually decribed by similarity parameters like Mach number, Knudsen- or Reynolds number and Damköhler number.
These flow regimes and typical flow phenomena will be explained. As the high alti-tude phase of re-entry is closely connected to vacuum technology special attention will be put on this phase.
For the actual design or re-entry vehicles CFD analysis tools and ground simulation facilities like wind tunnels are of mayor importance. Therefore also CFD analysis methods and the ground simulation problem and its restrictions will be shortly out-lined.
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