Cone Angle Choices for Atmospheric Entry Vehicles: A Review
Wright, Michael J.
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The vast majority of past and proposed planetary atmospheric probes, landers and sample return vehicles have been blunted cones with cone angles ranging from 45 to 70 deg. The objective here is to explain the large variation of cone angle choices by examining the parameters that determine the configuration. The primary parameter is the manner in which the payload is housed, which depends on the atmospheric environment that it must withstand to meet the mission objectives. If the goal is to perform measurements under high pressures and temperatures, the instruments are housed in a spherical pressure vessel, typically. For a sphere the most compact, although not necessarily the lightest, blunted conical heat shield is a 45 deg cone. The blunted 45 deg cone also has the aerodynamic stability advantage of having a lift curve slope ( ) that is essentially zero. If a pressure vessel is not needed, the use of larger cone angles, producing higher drag, will permit using lighter heat shields by reducing the length of the heating pulse and the heat conduction time. Although it is desirable to maximize the drag, aerodynamic stability and heat transfer considerations effectively limit the cone angles to values that retain a straight shock, with the attendant thin shock layer, on the frustum. The thin shock layer requirement becomes especially important as a means of limiting radiative heating. However, when convective heating dominates, the largest practical cone angle that comes close to meeting the above requirements is used, generally, The maximum cone angle depends on the atmospheric composition. The more degrees of freedom that the dominant atmospheric gas component has, the larger the cone angle that can support a straight shock on the frustum. For Earth return (diatomic gas), cones of at least 60 deg will retain straight shocks, while in the triatomic, CO2, Martian atmosphere, the angle extends to at least 70 deg. Therefore, a cone angle of 60 deg was used for the Stardust sample Earth return capsule. To achieve the maximum deceleration in the low-density Martian atmosphere, cone angles of 70 deg have been used. In contrast, the Galileo Jupiter probe and the Pioneer-Venus probes had cone angles of 45 deg to limit shock layer radiative heating, etc., and especially for the latter, to reduce the descent time in the hot, high pressure atmosphere of Venus.