RyanRising Posted March 16 Share Posted March 16 (edited) 3 hours ago, sevenperforce said: I think you're making an assumption here about what the "regime of typical LEO spacecraft entries" constitutes. It's not an either/or distinction between radiative and convective heating. Rather, re-entries start with radiative heating as the dominant factor, and then the radiative heating decreases while the convective heating increases. You're correct that certain vehicles are designed to accept convective heat and then radiate it away. However, the image you provided above (Figure 10) isn't applicable to LEO entries generally. The source you cite says they "apply only to single-pass, nonlifting, parabolic-velocity entries." The real numbers are more complicated. The actual numbers can be found in this presentation, which explains the relationship between convective heating and radiative heating based on multiple factors. Importantly, one of the factors is the effective radius of the vehicle: Qconv ∝ v3(ρ/R)0.5 but Qrad ∝ v8ρ1.2R0.5 As the effective vehicle radius increases, convective heating decreases, but radiation heating increases. For something like the Shuttle, which had a very deep atmospheric entry (to allow a lifting entry) and a very high effective radius, it would make sense that radiative heating was a major component. It is true that I’m making several assumptions about “what the typical LEO entry is.” Those assumptions are relatively high altitude, >~30km, and speeds of less than 8 km/s. Anything above those speeds wouldn’t be a low earth orbit and anything reaching those speeds at lower altitudes is… well it isn’t a crewed spacecraft like any we’ve seen before. I think these are reasonable assumptions. The paper you reference does not support your claim that LEO entires are dominated by radiative heating at first and then convective heating as they slow down. It does show that this is the case for higher-velocity entires, and the paper is clearly concerned more with those than LEO ones. These graphs start from 10 km/s and only go up from there. However, even in the larger 5m radius case the heat fluxes at 10 km/s are roughly equal, and it’s reasonable to extrapolate that it becomes very much lower at LEO entry speeds of 8km/s. (edit: to be clear, this doesn’t mean there is no radiative heating whatsoever in these regimes, only that it is significantly lower i than the convective heating) The equations you show are proportional, and describe the growth of each. However, this still leaves room for constant scale factors, and the examples they give show that for the size of entry vehicle they consider, those scale factors work out so that convective heating will dominate radiative heating at lower entry speeds near 8 km/s. The presentation says as much: reflective TPS is applicable for high-velocity, interplanetary missions. It doesn’t claim it’s very useful for LEO missions. The scales that this presentation considers include crewed space capsules that have been fielded. For those, it’s pretty clear that convective heating is higher than radiative at speeds < 8 km/s. However, it’s still possible that the flatter areas of the Space Shuttle experienced much more significant radiative than convective heating. I don’t have data on the type of heat flux experienced in those areas, but maybe I just need to look a little more. Edited March 16 by RyanRising Quote Link to comment Share on other sites More sharing options...
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