Sending a vehicle into space is one thing, getting it safely back is a total different story. Dynamic stability in particular is a critical issue, in particular in the so called low speed regime (supersonic/transonic/subsonic regime). One can distinguish two types of stability, static and dynamic. Any moving vehicle is subject to minor changes in the forces acting on it. When such a change will lead to further changes bringing the vehicle back to it’s original position the vehicle is called statically stable. Dynamic stability is related to a perturbation of the steady state (flight and motion) of a vehicle and how the oscillations generated by this perturbation are damped out. CFS Engineering routinely performs steady CFD simulations to assess static stability and unsteady CFD simulations to analyze the dynamic stability of aircraft and re-entry vehicles.
Separation – 6DoF
Moving body simulations are a challenging task including moving grids, rigid body movement and complex flow features that need to be resolved. The NSMB CFD solver includes a 6-DoF (Degrees of Freedom) module that recomputes the position of the moving body at each time step. The chimera method is employed in which each body has its own grid, that are sliding across each other. In the example on the right the apex cover has its own grid around it, and is moving upwards. The capsule also has its own grids, and moves slowly downwards. A back ground grid is used on which both the apex cover and capsules are moving.
Re-entry vehicles enter the atmosphere (Earth or Mars) at very high speed. This speed is reduced to the atmosphere, generating heat. For this reason re-entry vehicles have a Thermal Protection System (TPS) protecting the vehicle. CFS Engineering performs routine CFD simulations to determine the heat flux on re-entry vehicles. The value of this heat flux strongly depends on the properties of TPS, and this is translated in different boundary conditions for the temperature and species that can be used in the CFD solver (adiabatic wall, temperature imposed, radiative equilibrium, fully, partial or non-catalytic wall).
The earliest rockets employed solid propellants, and today solid propellant rockets are among the most powerful on the world. To increase the specific impulse Aluminum particles are added in the solid fuel grain. There particles radiate, and the NSMB CFD solver includes a particle tracking module that permits to track the particles leaving the rocket nozzle.