**1. Introduction**

To achieve practical reentries of flight vehicles, speeds in excess of the earth's escape velocity (≥ 11 km/s) are needed [1]. On reentry, this hypersonic speed is slowed down by viscous drag: firstly, thermal energy is created through the deceleration and compression of the high velocity incoming flow to a high pressure and high temperature in the hypersonic boundary layer close to the surface (heatshield); and secondly the convective and radiative heating associated with high temperatures accelerates the wall reactions [2]. The blunt shape of heatshields consisting of carbon-based materials [3] are effective structures that enable most of the generated heat to be carried away from the vehicle [4]. Computational models of the heat loads [5, 6] experienced during atmospheric reentries are continually being updated [7, 8]. The validation of these models is critical for the safe and economic design of future flight vehicles [9]. Proper analysis of heat flux [10], gas/surface interactions [11], and properties of heatshield materials [12] are needed for ablation performance and evaluations [13, 14]. Various thermochemical processes during reentry [15], descent, and landing also support the ablation of heatshields [16]. Flow parameters [17] such as enthalpy, stagnation pressure, velocity and heat flux are identified in **Figure 1**. It explains how the velocity of a reentry Stardust is slowed from 12.8 km/s to almost 2 km/s. A significant reduction in velocity is experienced

**Figure 1.** *Stardust trajectory parameter [18].*

between 70 to 40 km altitude due to higher atmospheric density resulting in more effective drag. At the same time, the heat-flux rises to a peak of 11 MW/m<sup>2</sup> at 62 km altitude. All space vehicles like Apollo, Huygens, Stardust, Hayabusa and Orion generate heat at reentry as a consequence of the drag used to reduce speed [19].
