I've never heard anyone comment on the trouble STS-129 seemed to experience during the Heading Alignment Cone (HAC) phase. I was hoping someone who was involved with the program might know why it happened.

The green line represents the expected HAC spiral for this mission. The yellow line is STS-129's ground track:

STS-129 ground track

Map imagery from Google Earth

As you can see, the ground track reaches the tangent point on the HAC and then starts its right turn, but the turn is wide and goes well outside the HAC. Eventually it makes a sharp turn and cuts inside the nominal HAC. Then it follows an almost a straight line for a while before making a final turn towards the runway. You can see the orbiter roll almost level around 20K feet in the landing video.

Not only was the turn too wide, the orbiter was also low in altitude, as confirmed by the Heads-Up Display (HUD), CAPCOM call-outs at 180° and 90°, and a side view of the ground track:

STS-129 side view

The low altitude, combined with the fact that the HUD shows the speed brake closed as far as allowed, paints a picture that Atlantis was low on energy. I suspect the ground track cutting inside the HAC and the sharp final turn towards the runway are due to HAC radius shrinking:

The HAC radius is now adjustable after the HAC turn phase starts. A low energy condition of sufficient magnitude will start shrinking the radius.

Space Shuttle Entry Terminal Area Energy Management by Thomas E. Moore - Page 7 (page 13 in the PDF)

The question is, what caused the initial low energy condition? Was it high winds aloft? The commander being slow to react? Both? Some other contributing factor? Were high winds expected? I have to imagine NASA reviewed it internally, and perhaps incorporated lessons learned into training for the remaining missions. I would love to hear about it, if so.

STS-37 landed short of the runway (though still safely on the dry lake bed at Edwards) due to high winds aloft and the commander (Steven Nagel) not responding to guidance fast enough. Nagel later said:

I pass it on when I’m instructing with the STA (Shuttle Training Aircraft). I can set up an approach that looks similar to that, low energy, and say, “If you ever get here, don’t mess around. Get back on the glide path right away.” I was trying to be real smooth with it and not fully complying with the guidance command, just partway to be smooth and get it back, and that wasn’t the answer that day.

Comparison to Other Landings

Just for comparison, here's the ground track for STS-132 (also Atlantis) which targeted the exact same HAC (KSC runway 33, 20° outer glide slope, close-in aim point). It flew pretty much as expected:

STS-132 ground track

I only have ground track data for STS-127 through STS-135, so it's possible that low energy conditions were more common than I imagine, but in the limited data I have, STS-129 is the only outlier.

  • $\begingroup$ Where did you get the traj data from? $\endgroup$ Commented Jun 20, 2020 at 16:10
  • 2
    $\begingroup$ @OrganicMarble for the final missions, NASA released KML files which you can load in Google Earth. I believe nasaspaceflight.com's private forum still has them for download, and you might be able to find them elsewhere. I generated the reference HACs years ago based on the math provided in technical docs. I used to be a little obsessed with shuttle landings, perhaps. $\endgroup$ Commented Jun 20, 2020 at 23:18

1 Answer 1


This clip from the entry flight control team (at about 3:40) suggests that the low energy state was expected as a result of the wind speed. Not sure what explains the wider flight path, though.

  • $\begingroup$ Welcome to Stack Exchange! The video has closed captions available and playback can be slowed and paused, so it shouldn't be hard to write out the key sentence or two that support your answer. Without that, this becomes a link-only answer ("The answer can be found in the following off-site link...) and this is strongly discouraged in Stack Exchange. Links break/rot and the answer then loses its value. Further, people like me will not even know why the dialogue in the video answers the question without an additional sentence or two explaining why it does. Can you edit further? Thanks! $\endgroup$
    – uhoh
    Commented Nov 18, 2020 at 23:22
  • $\begingroup$ Interesting clue! Thanks! $\endgroup$ Commented Nov 19, 2020 at 20:05
  • $\begingroup$ Another thing I noticed - the pilot took CSS first, then the CDR. That's not unusual, but could be a factor. $\endgroup$ Commented Nov 20, 2020 at 14:46

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