3

How much do you need to decelerate in order to be captured into a stable lunar orbit from a Trans-Lunar Ejection or TLI-type trajectory?

Russell Borogove
  • 168,364
  • 13
  • 593
  • 699
TeslaK20
  • 693
  • 1
  • 5
  • 8
  • Are you asking about the minimum needed for lunar capture, or more typical insertions into low lunar orbit? – Russell Borogove Jul 01 '18 at 13:44
  • I suspect the theoretical minimum may be zero. – peterh Jul 01 '18 at 19:40
  • The minimum won't be zero, because there's no third body to provide a braking assist to change the almost-parabolic trajectory into ellipse, but the difference between these is minimal; with the transfer maneuver performed perfectly, it would be infinitesimal; with the error common to this type of maneuvers - a couple meters per second. – SF. Jul 02 '18 at 15:36
  • It’s zero; the sun’s influence can be used. – Russell Borogove Jul 02 '18 at 15:46
  • Let me put it this way - assuming you could be put onto a lunar flyby by piggybacking on a probe, like the 4M mission did, what would be the minimum needed to enter orbit? – TeslaK20 Jul 07 '18 at 11:18

1 Answers1

8

Strictly speaking, the minimum ∆v is zero: from a ballistic capture trajectory, marginally stable lunar orbits at about 20,000 km altitude can be reached for zero or nearly-zero ∆v cost; an orbiter can then descend to a lower orbit at leisure using low-thrust high-efficiency ion engines. More generally, low energy transfers allow a tradeoff between travel time and insertion ∆v.

From an Earth-Moon Hohmann transfer, it takes about 680 m/s to reach a low lunar orbit at ~100 km altitude.

The Apollo missions started from a faster transfer orbit, so they required a little more ∆v; Apollo 11's initial LOI burn was 889 m/s to enter a 113 km pericynthion, 313.4 km apocynthion orbit.

Russell Borogove
  • 168,364
  • 13
  • 593
  • 699