The flight altitude of satellites in low Earth orbits is continuously reduced by the action of the atmospheric air drag. Without correcting measures they will finally fall back to ground. For the design, the operation, and the calculation of the atmospheric re-entry it is therefore important to know the aerodynamic properties of such satellites.

  • During the design phase of large structures the expected aerodynamic drag and torques drive the dimensioning of the propulsion system and the required amount of fuel for drag compensation and attitude stabilisation.
  • During operation the aerodynamic properties are of special interest for the orbit calculation, the mission planning, and the orbital lifetime prediction.

  • For a controlled re-entry the knowledge of the aerodynamic forces and torques helps planning maneuvers for trajectory and attitude control. For uncontrolled re-entries the trajectory and attitude motion can be estimated.


The software RAMSES ("Rarified Aerodynamics Modeling System for Earth Satellites") has been developed to calculate the aerodynamic properties of arbitrarily shaped satellites in orbit and during the early re-entry phase. A thorough calculation of the real flow conditions around a satellite would require the solution of the general equations of fluid mechanics (Boltzmann, Navier-Stokes) including chemical reactions. This requires even for aerodynamically shaped bodies a high computational effort. But at least the aerodynamic properties needed for the calculation of the re-entry trajectory can be approximated with sufficient accuracy by semi-empirical approximative method. Such methods are used in the RAMSES software.

The RAMSES software consists of several subsystems:

  • Interactive construction of the satellite geometry
  • Gas-surface modelling
  • Computation of the aerodynamic coefficients
  • Graphical evaluation and derivation of fitting functions

The satellite geometry is constructed from elementary geometric bodies. In this way very complex satellite geometries can be modelled and calculated. For the aerodynamic calculations the surfaces are partitioned into small triangular 'panels'. The physical properties can be defined separately for each geometric part. The aerodynamic coefficients are computed with free-molecular methods (Integral method or Monte-Carlo), or by bridging between the limiting cases of free-molecular flow and continuum flow (computed with Newton's method). The computed data arrays can be approximated by using one- two-, or threedimensional fitting functions.

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