However, I would expect that the limit was structural, not thermal -- the
statement in the documentary may have been someone's misunderstanding.
The Apollo heatshield was *heavily* overbuilt, because its basic design
had to be fixed at a time when heatshield scaling laws weren't well
understood, and the designers had to err on the side of caution. (It was
also specced for a worst-case reentry plan significantly more severe than
anything that was actually used.) So it was a very conservative design,
and by the time of Apollo 13 this was well understood. I'd be very
surprised if heatshield survival was ever a constraint.
Yes, agreed for all your points. Having worked with silica/phenolic thrust
chambers over the years I found this composite to be very tolerant to
severe heating even at elevated pressures and temps including at re-entry
conditions.
And also it probably was the sustained g limits of the crew.
Ken
On Sun, Mar 19, 2023 at 11:29 PM Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Sun, 19 Mar 2023, roxanna Mason wrote:
From what I gather from watching the NASA channel covering the Apollo 13
mishap was if the re-entry angle was too steep it was the heat shield
burning up not only G forces as being the upper angle limit. 2 degrees
was the number mentioned in the documentary.
That was the flight-path angle (vertical angle between some reference,
possibly the nominal trajectory, and the actual flight path, at the start
of reentry), not the angle of attack. The flight-path angle was indeed
fairly critical: the "corridor" of acceptable flight paths was narrow in
the vertical direction -- the faster you come in, the narrower it is.
Too high and you skip off (*), probably eventually coming back but Apollo
didn't have enough consumables to wait for it. Too low and you either
exceed structural limits or exceed heatshield limits.
(* Apollo reentry was *originally* planned to use a deliberate (short)
skip in some cases, but this idea was eventually dropped. In the end it
wasn't necessary, and while the primary reentry guidance could do it,
neither of the backup systems could! Zond actually did use skip reentry,
to permit reaching a high-latitude landing site. )
However, I would expect that the limit was structural, not thermal -- the
statement in the documentary may have been someone's misunderstanding.
The Apollo heatshield was *heavily* overbuilt, because its basic design
had to be fixed at a time when heatshield scaling laws weren't well
understood, and the designers had to err on the side of caution. (It was
also specced for a worst-case reentry plan significantly more severe than
anything that was actually used.) So it was a very conservative design,
and by the time of Apollo 13 this was well understood. I'd be very
surprised if heatshield survival was ever a constraint.
Henry