A spacecraft's mission objective determines what flight requirements are needed to ensure mission success. These flight requirements are
deceleration, heating, and impact and landing accuracy. A spacecraft must have a maximum value of deceleration low enough to keep the weakest points of its vehicle intact but high enough to penetrate the atmosphere without rebounding. Spacecraft structure and
payload mass affect how much maximum deceleration it can stand. This force is represented by "g's", or
Earth's gravitational acceleration. If its structure is well-designed enough and made from robust material (such as steel), then it can withstand a higher amount of g's. However, payload needs to be considered. Just because the spacecraft's structure can withstand high g's does not mean its payload can. For example, a payload of astronauts can only withstand approximately 9 g's, or 9 times their weight. Values that are more than this baseline increase risk of brain injury or death. It must also be able to withstand high temperature caused by the immense friction resulting from entering the atmosphere at
hypersonic speed. Finally, it must be able to penetrate an atmosphere and land on a terrain accurately, without missing its target. A more constricted landing area calls for more strict accuracy. In such cases, a spacecraft will be more
streamlined and possess a steeper re-entry trajectory angle. These factors combine to affect the re-entry corridor, the area in which a spacecraft must travel in order to avoid burning up or rebounding out of an atmosphere. All of these above requirements are met through the consideration, design, and adjustment of a spacecraft's structure and trajectory. Future missions, however are making use of atmospheric rebound, allowing re-entry capsules to travel further during their decent, and land in more convenient locations. The overall dynamics of aeroshells are influenced by inertial and drag forces, as defined it this equation: ß=m/CdA where m is defined as the mass of the aeroshell and its respective loads and CdA is defined as the amount of drag force an aeroshell can generate during a freestream condition. Overall, β is defined as mass divided by drag force (mass per unit drag area). A higher mass per unit drag area causes aeroshell entry, descent, and landing to happen at low and dense points of the atmosphere and also reduces the elevation capability and the timeline margin for landing. This is because a higher mass/drag area means the spacecraft does not have sufficient drag to slow down early in its decent, relying on the thicker atmosphere found at lower altitudes for the majority of its deceleration. This situation reduces the useful landed mass capability of entry, descent, and landing because an increase in thermal load leads to a heavier support structure and thermal protection system (TPS) of the aeroshell. Static stability also needs to be taken into consideration as it is necessary to maintain a high-drag altitude. This is why a swept aeroshell forebody as opposed to a blunt one is required; the previous shape ensures this factor's existence but also reduces drag area. Thus, there is a resulting tradeoff between drag and stability that affects the design of an aeroshell's shape. Lift-to-drag ratio is also another factor that needs to be considered. The ideal level for a lift-to-drag ration is at non-zero. Maintaining a non-zero L/D ratio allows for a higher parachute deployment altitude and reduced loads during deceleration. ==Planetary Entry Parachute Program==