Ballistic Lunar Transfers vs. Direct Transfers

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The conventional "Apollo-type" transfers take between three and five days to get to the Moon. To execute an Apollo-type transfer, the spacecraft must first perform a large maneuver (3.14 km/s) to leave the Earth. Then the spacecraft must perform another large maneuver (819 m/s) to insert into a lunar orbit. Ballistic Lunar Transfers (BLT's) take 3-4 months to get to the Moon, but can insert into a lunar halo orbit without any maneuver at all. The only maneuvers required after leaving the Earth (3.24 km/s) would be trajectory correction maneuvers to compensate for launch vehicle errors and small maneuvers needed to stay on the correct trajectory.
The animation above shows how a Ballistic lunar transfer looks in the rotating coordinate system aligned with the Sun-Earth line. The large blue sphere is the Earth, the small green spheres are the Sun-Earth Lagrange points EL1 (left) and EL2 (right). The gray sphere orbiting the Earth is the Moon and the small pink spheres are the five Lagrange points in the Earth-Moon Three-body system. The spacecraft requires just a little extra ΔV to travel out past the Moon, where the Sun's gravity helps boost the spacecraft's energy so that it neatly arrives onto a lunar halo orbit with no insertion ΔV. Taking a BLT to a halo orbit would enable the payload to be about 30% larger than if a direct transfer were used. The mass savings are not as pronounced if the spacecraft is headed for a lunar orbit instead of the halo orbit.

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