Crafts Entry
Posted in Garden Antiques on 01/02/2011 04:42 pm by adminIf a spacecraft re-enters the atmosphere at an angle that is too deep, why might it burn?
When a craft re-enters at an angle that is TOO DEEP, why might it burn on re-entry? Is it due to the greater speed and friction? If so, why and how is the friction produced? And why is there a need of such a great speed? I must sound really stupid. If you could help, that would be great! Thanks in advance!
It's primarily the rapid increase in air density that causes high heat in a steep entry.
To begin with, the high speed is necessary because we're talking about a spacecraft that either was in orbit (moving at 7.5 km/s or so) or is returning from a greater distance, possibly a translunar or interplanetary trip (moving even faster, roughly at escape velocity, 11.2 km/s, or more). And as billrussell said, you don't want to waste expensive propellant doing what atmospheric energy dissipation can do for free, so you enter at those speeds. As the spacecraft approaches Earth it loses gravitational potential energy and gains kinetic energy, so it speeds up. However, when it hits the atmosphere it dissipates its KE as heat, faster than it is accumulating KE, thus getting hotter and slower. A steeper descent means not only gaining KE faster but entering denser atmosphere more quickly, which means more friction, a greater rate of heat dissipation and a hotter entry with possible burnup.
A shallower descent means more gradual accumulation of KE and dissipation of thermal energy, thus less heat. Entry from a non-orbit state, i.e., approaching at more than escape velocity, can be tricky, because too shallow an entry can cause "skip-out", where the spacecraft doesn't get deep enough into the atmosphere to slow below escape velocity. In that case, it's goodbye forever. There are also deliberate multiple-skip entries, where the velocity after the 1st skip is well below escape velocity so the skips can be repeated. This may be used to moderate the entry heating. But in any case the entry angle must be tightly controlled to obtain the desired result.