In the area of hypervelocity aeroshells, conflicts may be created by the parallel requirements of high aero force braking and stability.
This thesis presents a theoretical and experimental study of aerodynamic force coefficients in hypervelocity reacting flows, at conditions of limited static stability.
Planetary entry vehicles with sphere-cone shaped aeroshells have been observed in the past to suffer static instability or a reduction of static stability at hypersonic speeds and low angles of attack. This, in many cases, contradicts predictions based on Newtonian theory, may be counter-intuitive, and is an important consideration in vehicle design. Regions of instability along a trajectory can be identified with extensive CFD analysis or experiments, but it is time consimiing and expensive to do so for a complete flight path. Methods for rapidly finding conditions which are predisposed to potential instability are of great value, and enable computational and experimental efforts to be used most effectively.
The phenomenon has been found to regularly occur in association with a transition in the sonic line, so that identification of sonic line transition forms a useful indicator of possible areas of concern.
Sonic line transition may be initiated by thermo-chemical effects, and the change in the postshock ratio of specific heats has been used successfully in the past to flag potential regions of concern.
An improved analytical method is presented here which predicts the location of the sonic line in nonequilibrium flows, taking into account both the chemical composition and effective specific heat of the reacting gas. The approach taken is to model the primary factors that determine the location of the sonic line. Where a transition in the sonic line is seen, the condition can then be targeted with CFD and experimentation to accurately assess stability, as sonic line transition alone does not necessarily imply reduced stability.
As validation, the theoretical model was used for a study of the Mars Pathfinder reenti-y path, and correctiy picked up the sonic line transitions known to occur for that trajectory. In addition, a potential instability on the Beagle2 ti-ajectory was also identified. The analytical model was then used to predict a likely transition within conditions typical of the present capabilities of the X3 superorbital expansion tube in a hypersonic flow of Carbon Dioxide.
A one piece, 3 component stress wave force balance with a short sting and filleted stress-bars was designed. The ability of the balance to measure small moments in the presence of large drag forces was modelled using Finite Element Analysis, and demonstrated through appropriate calibration.
The stress wave force balance was constructed and calibrated, and the sensitivity of the balance to load distribution at appropriate levels and load distributions was quantified. The balance was then used to measure forces on a scale model of the Pathfinder in X3, the largest expansion tube in the world at conditions shown by the analysis to be ones where potential static instabilities may exist for this geometry. The experiments were the first to be conducted in X3. The experiments also are the first measurement of simultaneous axial and normal forces as well as pitching moments in an expansion tube. The measured aerodynamic forces and moments were then used to assess the static stability of the geometry at the conditions tested. This was also the first assessment of static stability in an expansion tube.
Results of the experiments indicate that the static stability of the capsule is significantly less than Newtonian theory would predict at the tested conditions.
The stress wave balance and the theoretical technique both have applications beyond the sphere-cone geometries on which they have been demonstrated and proven. For instance, any mission where hypersonic stability is important, and where accurate aerodynamic force coefficients are needed may beneficially be studied in impulse facilities using the stress wave force balance. Possible examples are aerocapture vehicles, waveriders and inflatable ballute reentry technology. Some of these applications should be easier to study than the test case used in this work, as the larger values of lift to drag ratio mean none of the measured properties are near zero at the points of interest.