# Temperature of the area behind a reenty vehicle

The area in front of a vehicle re-entering the atmosphere is elevated mostly because of the high pressure of the shock wave leading it. Given the space immediately behind the vehicle is not being pressurized, it stands to reason it should be cooler.

How much cooler? Have any measurements been made? Failing that, do the software models of re-entry show a temperature for that region?

• That very much depends on the shape of the vehicle. I've always found Mach expansion/compression to be a very counter-intuitive topic. What kind of vehicle would you imagine? Oct 2, 2014 at 18:31
• @Rikki-Tikki-Tavi Let's say something like the Apollo space capsule. Oct 3, 2014 at 0:48
• Cooler than what? A back door? Prevent Columbia how? Oct 3, 2014 at 1:01
• @MarkAdler Cooler than the highest temperature in front of the vehicle. Or now I think about it, simply a temperature in degrees C or Kelvin would do fine. Oct 3, 2014 at 1:04
• A back-door speaking metaphorically (though it could perhaps be literal too) Even a means to communicate ; say, extending an antenna out the back, whilst the craft is surrounded by that ball of heat might be useful (if such a idea was not already evaluated) Oct 3, 2014 at 5:03

## 1 Answer

Of more useful physical and engineering meaning than temperature is the heat flux, which is the energy being deposited on a unit area in unit time. Even a very high temperature with very little energy behind it may do no damage. You can touch a hot pot for a short time.

These papers on detailed thermodynamic models of the MSL heatshield and the MSL backshell provide a great deal of information on the predicted heat flux on the front and back. You will see peak heat fluxes on the front on the order 100 W/cm^2, but only on the order of 10 W/cm^2 on the back.

Note that that does not mean that the temperatures have the same proportions. Temperature is complicated and depends on material characteristics, where exactly in the material (solid or fluid) it is measured, how heat is absorbed and dissipated, local geometries, etc.

In addition to heat flux, of significant engineering interest is the total heat load, the total energy deposited per unit area over the entire entry (the integral of the heat flux), measured in J/cm^2. You will find those in the papers as well. The thermal protection systems have to be designed to take both the maximum heat flux, experienced in the steepest expected trajectory, as well as the maximum heat load, experienced in the shallowest (and so longest) expected trajectory.