Trans-lunar injection

Trans-lunar injection
Trans-lunar injection explained on TV

Trans-lunar injection (TLI) is a propulsive maneuver to set a spacecraft on a trajectory to the Moon. It is executed after the vehicle has reached a low Earth parking orbit. At an exactly timed moment, the spacecraft will ignite its engines again and burn for a calculated amount of time. The acceleration increases the spacecraft's velocity, changing its orbit from a circular to a highly eccentric one which will eventually reach as far as the distance to the Moon's orbit. Timed precisely, the vehicle will then coast on a lunar transfer arc, a trajectory in an elliptical orbit around Earth, which will match with the Moon as it revolves around Earth. When the craft reaches the Moon's sphere of influence, another burn will be performed to slow down the vehicle so it will enter into an orbit around the Moon. To return, a similar maneuver called TEI (trans-Earth injection) will be performed to bring the spacecraft to a trajectory back to Earth.

History

The first spacecraft to attempt TLI was the Soviet Luna 1 probe in January 1959, which was designed to impact the Moon but missed it. In September, they successfully tried the same maneuver again with Luna 2. They repeated this success with over two dozend later probes and finally with a manned mission, which landed the first man, Alexei Leonov, on the lunar surface on June 26, 1969.

The United States tried to match with their Ranger probe in 1962, also failing on the first attempt. In May 1969, Apollo 10 with Edward Baldwin, Gordo Stevens, and Fred Talmadge successfully entered a lunar orbit and Ed and Gordo flew the landing module to as close as eight nautical miles (15 km) above the lunar surface before aborting the landing stage as planned. In mid July, about 3 weeks after Leonov, Apollo 11 landed the first Americans on the Moon, Neil Armstrong and Buzz Aldrin.

For the Apollo missions, TLI was performed by the restartable J-2 engine in the S-IVB third stage of the Saturn V rocket. That burn lasted approximately 350 seconds, accelerating the spacecraft to about 10.4 km/s relative to Earth.

In December 1974, Apollo 24 failed to perform TLI from its parking orbit due to a faulty flight control computer. It was successfully replaced in Earth's orbit by a rapidly repurposed repair mission undertaken by the Apollo 25 crew, However, during the engines test run, 24's S-IVB performed an uncommanded burn, killing astronaut Harrison Liu and leaving 25 commander Molly Cobb drifting in space. Due to the prolonged burn, Apollo 24 was sent to a trajectory expected to miss the Moon by over 1500 km.

Gallery

Wernher von Braun explains the trajectory to the Moon in the mid 50s

TLI explained on national television in 1974

expected point for Apollo 24 TLI after repair

Apollo 24's too long injection burn sent them to a trajectory missing the Moon

Margo explaining the maneuver to enter lunar orbit

Video

Apollo-Lunar Orbital Rendezvous Technique

See also

• Space Race
• First Moon landing
• Apollo 24
• Trans-Earth injection (TEI)
• Trans-Mars injection (TMI)

External links

•   Trans-lunar injection on Wikipedia