Thorsten
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We all know that if you slam a spacecraft into the atmosphere on the wrong trajectory, it's going to break up in flames.
What I'm interested in here is modeling the 'when' of that, and I'm curious as to how people solve the problem in Orbiter, or whether anyone is aware of a documented professionally used model.
Assume I have a reasonably good model which gives me a figure of merit temperature on the heat shield (there's different temperatures at different points of the heat shield in reality, and to some degree I can estimate what they are as well, but let's keep it simple).
What I've used so far is a condition that the figure of merit temperature may not be larger than some maximum, otherwise the thermal protection system (TPS) fails. I believe however that is unrealistic and physically not correct (especially after studying temperature time curves for some off-nominal Shuttle trajectories which ought to be possible in reality but generate high temperature peaks for short times).
The actual failure mode that occurs when the heat shield fails or is thermally overstressed is that structure behind the heat shield comes to a temperature where it fails under stress.
Now, that condition is a line in a two-parameter plot - at sufficiently high stress the structure will fail even at room temperature, and at sufficiently high temperature it will melt, i.e. fail without any structural stresses. Somehow there's a line connecting the two conditions - beyond some (temperature/stress) the structure fails. But I don't really know what that line is.
The stress is fairly easy to get from the aerodynamics simulation, but the temperature less so.
The TPS works by (drastically) reducing the heat conduction, so most of the heat reaching the TPS is radiated away rather than conducted inward to the structure. A TPS failure hence means the heat flux to the structure is higher than normal.
Now, the structure itself is a heat sink (i.e. it will take some heat flux, increasing in temperature in the process) - but it will also conduct heat to other parts of the structure, and even potentially radiate heat when it can be conducted to a reasonably cool part.
Thus, even knowing the heat flux through the TPS, getting a temperature on the structure behind isn't easy because it again depends on factors which I do not know and find difficult to estimate.
So I can figure out easily how not to do it, but I don't really see an efficient way to do it closer to reality without guessing a lot of numbers (which never is a good idea...).
How do you do it for your Orbiter spacecraft? And how do professional spacecraft designers estimate it before going into the full numerics?
What I'm interested in here is modeling the 'when' of that, and I'm curious as to how people solve the problem in Orbiter, or whether anyone is aware of a documented professionally used model.
Assume I have a reasonably good model which gives me a figure of merit temperature on the heat shield (there's different temperatures at different points of the heat shield in reality, and to some degree I can estimate what they are as well, but let's keep it simple).
What I've used so far is a condition that the figure of merit temperature may not be larger than some maximum, otherwise the thermal protection system (TPS) fails. I believe however that is unrealistic and physically not correct (especially after studying temperature time curves for some off-nominal Shuttle trajectories which ought to be possible in reality but generate high temperature peaks for short times).
The actual failure mode that occurs when the heat shield fails or is thermally overstressed is that structure behind the heat shield comes to a temperature where it fails under stress.
Now, that condition is a line in a two-parameter plot - at sufficiently high stress the structure will fail even at room temperature, and at sufficiently high temperature it will melt, i.e. fail without any structural stresses. Somehow there's a line connecting the two conditions - beyond some (temperature/stress) the structure fails. But I don't really know what that line is.
The stress is fairly easy to get from the aerodynamics simulation, but the temperature less so.
The TPS works by (drastically) reducing the heat conduction, so most of the heat reaching the TPS is radiated away rather than conducted inward to the structure. A TPS failure hence means the heat flux to the structure is higher than normal.
Now, the structure itself is a heat sink (i.e. it will take some heat flux, increasing in temperature in the process) - but it will also conduct heat to other parts of the structure, and even potentially radiate heat when it can be conducted to a reasonably cool part.
Thus, even knowing the heat flux through the TPS, getting a temperature on the structure behind isn't easy because it again depends on factors which I do not know and find difficult to estimate.
So I can figure out easily how not to do it, but I don't really see an efficient way to do it closer to reality without guessing a lot of numbers (which never is a good idea...).
How do you do it for your Orbiter spacecraft? And how do professional spacecraft designers estimate it before going into the full numerics?