I have seen that many rockets jettison their payload fairing at an altitude where the aerothermal heatflux is leass than or equal to 1135 W/m^2. What is the reason behind considering this particular number. Anymore information on this will be appreciated.
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It appears that the original value was 0.1 BTU/ft2-sec, which you can see listed in some documents, for example this is from a Boeing information sheet for a Delta II launch in September 2007:
This value can also be seen on page 23 of an SLS Mission Planners Guide from 2014:
0.1 BTU/ft2-sec converted to W/m2 is 1135.65. In another section of the Delta II document they list both units:
In theory the original calculated value could have been 1135 W/m2 and this was later converted to BTU/ft2-sec. But since 1135 W/m2 converted to BTU/ft2-sec is 0.0998 this would be quite coincidental. Also since the value seems to go back to the early days of satellite launches, the U.S. unit might be more likely as the original (obviously BTU itself originated as a British unit).
0.1 is presumably a rounded number which was based on test or calculation results that fell within a certain range, and the rounded value eventually became the standard. Perhaps comparable to the selection of 100 km as the limit for space, or NASA's designation of 400,000 feet for entry interface, both of which are based on empirical data but are also rounded standards.
Describing the Karman line as 62 miles can give an impression of preciseness that isn't actually there. The same with entry interface as 122 km, and perhaps heat flux as 1135 W/m2.
How the 0.1 value was originally arrived at might be harder to determine since it seems to have been in use as a standard for several decades. But it is apparently a pretty conservative value since it (and the 1135 equivalent) is so commonly used and considered a safe amount of heat flux for satellites.
answered Sep 29 at 8:28
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$\begingroup$ Interesting. I always assumed the number comes from the solar constant at the equator... I.e. "not getting more heat than standing in the sun at noon" $\endgroup$– asdfexSep 29 at 11:32
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1$\begingroup$ @asdfex - that seems possible, 1367 w/m2 is 0.12 Btu/ft2-sec, which in theory they might have rounded down to 0.1 and that became the standard, i.e. getting less heat than standing in the sun at noon. The standard then later converted to SI at 1135.65 which was rounded down to 1135. $\endgroup$ Sep 29 at 13:35
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1$\begingroup$ In reality it's even closer: 1367 is the value in space minus 20% losses due to a very clear atmosphere with the Sun directly overhead is very reasonable. $\endgroup$– asdfexSep 29 at 13:52
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$\begingroup$ Using the January maximum of 1412 w/m2 that would be 0.124 Btu/ft2-sec. 80% of that is 0.0994 Btu/ft2-sec. So if no more than that reaches the surface then it would be less than the maximum possible at sea level on the equator at any time during the year. $\endgroup$ Sep 29 at 14:16
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$\begingroup$ Sidenote, but "BTU/ft2-sec" makes my metric heart hurt. $\endgroup$ Sep 30 at 10:53
1135 W/m^2is360 BTU/(ft^2hr)or0.1 BTU/(ft^2s)$\endgroup$