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The NPR.org news item Meet John Houbolt: He Figured Out How To Go To The Moon, But Few Were Listening shows the iconic photo of John Houbolt next to a black board showing a free return scenario.

I have always wondered why arrow #5 (top one near the Moon) appears to be pointed the wrong way, and recently I realized that #6 does as well. Why is this?

If it doesn't indicate the direction of spacecraft motion, what does it point towards?

Click image for full size

American aerospace engineer John Houbolt as he stands at a chalkboard in July 1962

American aerospace engineer John Houbolt as he stands at a chalkboard in July 1962 showing his lunar orbit rendezvous plan for landing astronauts on the moon.

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    $\begingroup$ Aren't they burns? $\endgroup$ – Organic Marble Jul 18 at 15:06
  • $\begingroup$ @OrganicMarble oh, the arrow indicating the (approximate) direction of thrust does seem to fit, with the skew of #4 and #10 indicating a correction maneuver. It's that simple? $\endgroup$ – uhoh Jul 18 at 15:20
  • $\begingroup$ Do you happen to know what "escape weights" refers to? $\endgroup$ – Magic Octopus Urn Jul 18 at 17:42
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    $\begingroup$ @MagicOctopusUrn I don't know for a fact but looking at the drawings and the graph below that label, it appears to be the mass of the craft that leaves the moon for the Earth. $\endgroup$ – Organic Marble Jul 18 at 19:22
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    $\begingroup$ @MagicOctopusUrn The plot at bottom right shows $w_E/W_E$ as a function of $w_L/w_O$. What the plot (and the diagrams near the top) mean and approximate numbers that go into it might make an excellent follow-up question. $\endgroup$ – uhoh Jul 18 at 21:58
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They are burns, with the direction of the arrow roughly indicating the direction of thrust.

enter image description here

  1. Ascent
  2. Earth orbit insertion
  3. Trans-lunar injection
  4. Midcourse correction
  5. Lunar orbit insertion
  6. Burn to drop out of lunar orbit for landing (LM)
  7. Ascent from lunar surface (LM)
  8. Lunar orbit insertion (LM)
  9. Trans-Earth injection
  10. Midcourse correction

Whew! Found a reference. Here's a diagram by Houbolt with the same nomenclature (sans vectoring).

enter image description here

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    $\begingroup$ Ooh, buuuuuurn. $\endgroup$ – David Richerby Jul 19 at 14:45

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