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The Secret of Moon Travel: Why TLI Burns Are Critical to Space Missions
In the history of space exploration, the Trans-Lunar Injection (TLI) burn is considered one of the key technologies in the process of heading towards the moon. This propulsion measure is not only the first step in sending a spacecraft to the moon, but also a combination of the art and science of celestial motion. From the high-profile TLI burns of the Apollo missions in the 1960s to today's emerging missions, their importance remains unchanged.
TLI is a propulsion maneuver that increases the spacecraft's speed to a point where it can turn from the Earth to the Moon.
The TLI burn will begin once the spacecraft reaches a low circular orbit around the Earth. A typical TLI burn increases the spacecraft's speed, causing it to transition from a circular, low Earth orbit to a highly eccentric orbit. The process involves precisely timing burns and thrust to ensure the spacecraft approaches the moon at just the right time. 」
Once the burn begins, the spacecraft will move along a nearly elliptical orbit aimed at the Moon's location during its orbit.
As the spacecraft enter the Moon's sphere of influence, they will perform a flyby beyond the Moon, a process that effectively reduces the number of propulsion burns required. Going a step further, in some cases, TLI could even be designed to have a free return orbit, meaning the spacecraft could autonomously return to Earth without the need for further propulsion. This design adds safety to manned space missions.
For the Apollo 8, 10, and 11 missions, the spacecraft were designed to return to a free return orbit.
Mathematical Modeling of TLI
To optimize the design and implementation of TLI, different mathematical models are usually used to predict the orbit. Therefore, the Patch Conics method is widely used, making the effects of the Earth and the Moon the main considerations. Each model has its advantages and disadvantages, depending on the actual task requirements.
In a more realistic scenario, a spacecraft is subject to the gravitational pull of multiple celestial bodies, which makes the operational model more complex, but also more accurate.
In the first lunar exploration mission, the Soviet Union's Luna 1 attempted a TLI burn on January 2, 1959. Although it failed to achieve the expected goal, the subsequent Luna 2 successfully impacted the moon, paving the way for subsequent space exploration. The mission laid the foundation. Since then, the United States and other countries have launched multiple probes to the moon to achieve their respective space exploration goals.
History Review
During the Apollo mission, the successful launch of TLI became a glorious history. Using the restartable J-2 engine, the Apollo spacecraft could perform a TLI burn for 350 seconds at a time, allowing the ship to reach a speed of 10.4 kilometers per second. During this process, observers were able to perceive this spectacular burning moment from the ground and became witnesses of history.
Apollo 8's TLI was observed at dawn in Hawaii and became the subject of much awe and was widely reported.
With the advancement of technology, lunar missions have become more diverse and innovative in recent years. Missions ranging from Japan's Hiten satellite to China's Chang'e program rely on efficient TLI burns to ensure their successful orbit placement.
Whether it is a historical mission or future exploration, the design and execution of TLI combustion is a crucial step. Can the journey of exploring the moon, a dream that has lasted for thousands of years, open a new chapter for humanity in the future?
