# What are the three peaks in this Apollo reentry heating plot?

The question Challenging Karman line from above includes the following plots and cites an answer in Quora by Robert Frost, Instructor and Flight Controller at NASA who has provided many authoritative answers about spaceflight (as well as other goodies) and so I think they cary some validity and authority.

Question: I see three distinct peaks here. The biggest one centered at about 150 seconds, with two more at 450 and 750 seconds respectively. Is it possible to understand why there are three distinct times that the spacecraft received major heating events?

Exploring the plots further:

The absolute maximum heating shown is about 2.5E+06 watts/m^2 and eyballing the first peak I estimate an integrated heat of about 6E+08 Joule/m^2. Note that this is of course a tiny fraction of the energy dissipated in reentry, and that's what makes reentry survivable.

Just for comparison, from Wikipedia the diameter of the command module is 3.9 and the mass seems to be roughly 5,000 kg though I don't know it may change during the mission. That would be about 470 kg/m^2, and with an estimated velocity of 11,000 m/s that's 3E+10 Joules/m^2.

• There should be a relation to the skip reentry trajectory of the Apollo spacecraft. See a NASA page.
– Uwe
Oct 24, 2018 at 9:36
• I had never really looked at this plot before. I find "time from 121920 m" to be absolutely hilarious. "121920 m" is of course equivalent to 400K feet, the definition of entry interface, and this is a sterling example of when mindlessly converting historical English unit numbers to metric units makes things more confusing. Mar 30, 2023 at 20:43