Unsteady Flight and Steady Aerothermodynamics?

The aerothermodynamic flow fields and flow phenomena considered in this book result in mechanical loads (pressure and skin friction) and thermal loads (temperatures and heat fluxes) at the surface of the flight vehicle under consideration. The propulsion system of a CAV or an ARV adds to both of them. We have to ask ourselves whether the flow fields and phenomena found at RV’s, CAV’s and ARV’s can be considered as steady phenomena. We follow closely the considerations in this regards which can be found in

[7].

The flight of a RV is entirely an unsteady flight. A CAV nominally may fly in a steady mode, but actually, as also does the ARV, it flies more or less in an unsteady mode. For an introduction to flight trajectories of RV’s and CAV’s, see, e. g., [5].

The mechanical loads summated over the vehicle surface show up as the aerodynamic properties of the vehicle: lift, drag, pitching moment etc., called all together the aerodynamic data set or model of the vehicle [7].[12]

The well-proven approach of the aircraft designers is also employed for RV’s and CAV’s/ARV’s: the aerodynamic data of a vehicle are—with one exception—steady motion data. This approach is permitted as long as the flight of the vehicle can be considered as quasi-steady flight. The actual flight path—with steady and/or unsteady flight—then is described with the help of three-, six-, and even more degrees of freedom trajectory determinations, see, e. g., [25], with appropriate systems and operational constraints and control variables [5].

A reliable criterium which defines when the flight can be considered as being quasi-steady is not known. Nevertheless, the experience indicates that one can assume RV and CAV/ARV flight to be quasi-steady, see also Section

4.1. This is the reason why the aerodynamic data are always obtained in a steady-state mode (steady motion), experimentally and/or computationally. Hence it is permitted, too, to consider aerothermodynamic flow fields and flow phenomena to be steady flow fields and phenomena.

The mentioned exception is an aerodynamic vehicle property which is truly time-dependent: the dynamic stability, see, e. g., [26]. The dynamic sta­bility actually is the damping behavior once a disturbance of the vehicle attitude has happened, for instance an angle-of-attack disturbance, or a dis­turbance around one of the other two vehicle axes. The time dependence indicates, whether the unsteady—in general oscillatory—motion induced by the disturbance is damped or not. Although very important, the dynamic stability can be considered as being not a primary aerodynamic data set item.

There are other phenomena which are truly unsteady. One is due to the thermal inertia of a thermal protection system. The wall temperature distri­bution will not always adapt the value which belongs to the instant state of the flight. An example is mentioned on page 40 in Chapter 3. Other examples are (localized) unsteady shock/shock and shock/boundary-layer interactions, processes in propulsion systems, in gauges and also in ground-simulation fa­cilities. We do not further pursue this topic.