We must meet a set of requirements to achieve mission objectives that mark the beginning of a space-mission planning. The re-entry phase of a mission is no different. The three most competitive requirements that must be delicately balanced are
• Accuracy of landing or impact
The vehicle’s structure and payload limit the maximum deceleration or “g’s” it can withstand. (One “g” is the gravitational acceleration at Earth’s surface—9.798 m/s2. The amount of deceleration is so high that even steel and aluminum can crumple like paper. Fortunately, the structural g limits for a well-designed vehicle can be quite high, perhaps hundreds of g’s. But in case of a fragile human ...view middle of the document...
If we were to get out our calculators and punch in the numbers for the Space Shuttle, we'd find that its total mechanical energy is
E = 3.23x1012 Joules
The Shuttle has kinetic energy due to its speed of 7700 m/s and potential energy due to its altitude. It needs to lose all this energy in only about one-half hour to come to a full stop on the runway (at Earth’s surface). But we know that energy is conserved, so the question that would come up is- where does all the “lost” energy go? It converts to heat (from friction) caused by the atmosphere’s molecules striking its leading edges. This heat makes the Shuttle’s surfaces reach temperatures of up to 1477° C (2691° F). We have to contend with the total heating and the peak heating rate to withstand such high temperatures.
The third mission requirement is accuracy. Beginning its descent from more than 6440 km (4000 mi.) away, the Space Shuttle must land on a runway only 91 m (300 ft.) wide. To meet these constraints, we must adjust the trajectory and vehicle design. On the other hand, if a vehicle can land in a larger area, the accuracy constraint becomes less important. In all cases, designers adjust the trajectory and vehicle shape to match the accuracy requirement.
As it is seen from all these constraints, a re-entry vehicle must walk a tightrope between being squashed and skipping out, between fire and ice, and between hitting and missing the target. This tightrope is actually a three-dimensional re-entry corridor through which a re-entry vehicle must pass to avoid skipping out or burning up.
The size of the corridor depends on the three competing constraints-deceleration, heating, and accuracy. For example, if the vehicle strays below the lower boundary (undershoots), it will experience too much drag, slowing down rapidly and heating up too quickly. On the other hand, if the vehicle enters above the upper boundary (overshoots), it won’t experience enough drag and may literally skip off the atmosphere, back into space. If designers aren’t careful, these competing requirements may lead to a reentry corridor that’s too narrow for the vehicle to steer through! Whereas the above three constraints determine the re-entry corridor’s size, the vehicle’s control system determines its ability to steer through the re-entry corridor.
The amount of lift that will be generated by the reentry vehicle and the ballistic coefficient are determined by the vehicle’s size. The drag coefficient is the toughest and most difficult component of the ballistic coefficient to be determined for the reentry vehicle, which depends mainly on the vehicle’s shape. At low speeds, we could just stick a model of the vehicle in a wind tunnel and take specific measurements to determine the coefficient of drag. But reentry occurs at hypersonic speed, which can be as much as 25 times of the speed of the sound where wind tunnel testing doesn’t prove to be a practical method of measurement of the coefficient of drag, because no...