Transition Issues for Atmospheric Entry

*Much has been learned about the physics underlying the transition process at supersonic and hypersonic speeds through years of analysis, experiment and computation. The application has been principally to simple shapes like plates, cones and spherical nosetips. But the shapes of the new entry vehicles are not simple. They will invariably be at angle of attack so three dimensional effects will be very important, as will roughness effects due to ablation. This paper will review the physics basis of our present understanding of the transition process. Further, because of the complex geometries, it will address the need for careful experimental work as per the guidelines enunciated years ago by the U.S. Transition Study Group. Following these guidelines is essential to obtaining reliable, usable data for use in refining transition estimation techniques . I. Introduction Entry vehicles descend rapidly through the atmosphere and decelerate as the drag forces increase with the increasing atmospheric density. The vehicle starts out fully laminar. Since Reynolds numbers increase rapidly through the des cent, transition tends to move very rapidly over the whole vehicle over a narrow range of altitudes. O ne tries to minimize the aerodynamic heating loads in entry so as to minimize the thermal protection needs of the vehicle. This means delaying transition to as low an altitude as possible . The history of high -performance entry vehicles begins with the development of the ICBM in the 1950’s. These v ehicles were essentially cone -cylinders with large nose bluntness to minimize stagnation point heating. The nose materials were often subliming ablators to take advantage of the further reduction in stagnation region heating due to the surface blowing from the subliming surface. Transition would move forward from the cylinder to the cone as the vehicle descende d through the atmosphere. If transition occurred asymmetrically on the cylinder or cone (leading to drag asymmetry), it was essential that the asymmetric effects be small enough to be corrected by the control system of the missile so as to minimize the cir cle of error about the target. In some cases, the transition unexpectedly moved forward onto the spherical nose , a phenomenon named the “blunt -body -paradox.” A study in that time period by Allen and Eggers 1 showed that for orbital and sub -orbital entry ( < 8 km/sec) , the convective aerodynamic heating rate was directly related to the vehicle’s ballistic coefficient, (W/C DA). The lower the ballistic coefficient, the lower the heating rates in entry. Hence the tendency to low weight and high drag coefficient. The highly blunted capsul e shapes of the Mercury, Gemini, Apollo and Soyuz vehicles with a blat ing heat shields are a consequence of this argument . At entry speeds above about 10 km/sec , the shock layers ahead of the entering blunt shapes become lumino us and radiate. These radiative heat transfer rates are highly density dependent. Thus the blunt body may not be the best entry shape for supercircular entry speeds. Studies by Allen et al 2,3 show that to minimize total heat transfer or total ablated mass , the optimum shape gets progressively slenderer as the entry speed is increased. Not enough was done with these vehicles to identify the major transition issues.