What was Apollo 13?

Apollo 13 is the third manned mission to the moon in the Apollo program and was performed in April 1970. Two days after the launch, the oxygen tank of the service cabin exploded, the spacecraft was severely damaged, and a large amount of oxygen and electricity was lost; the three astronauts used the spacecraft's lunar module as a lifeboat. The navigation and control systems were not damaged, but in order to save electricity, they were shut down before returning to the Earth's atmosphere. The three astronauts faced various crises caused by the damage to the living system in space, but they finally returned to Earth.

At the beginning of the Apollo 13 mission, there was also a less famous but equally dangerous accident. When the second-stage rocket burned, the middle propeller No. 5 was shut down ahead of time, so the other four propellers had to extend the burning time. Engineers later discovered that the cause of the problem was longitudinally coupled vibration, which was enough to tear the second-stage rocket. At that time, the thruster was subjected to a frequency of 16 Hz and a gravity of 68G, and the engine frame was stretched about 7.6 cm (3 inches). Fortunately, the vibration caused the thruster pressure to drop, and the control computer turned it off automatically. Small amplitude longitudinal coupling vibrations have also appeared in previous Apollo missions (even considered the potential problem when the earliest Giant-Gemini unmanned mission), but in the Apollo 13 turbopump The abnormal amplitude of cavitation and longitudinal coupling increases the vibration amplitude. In subsequent missions, the rocket made anti-longitudinal coupling modifications (already underway before Apollo 13) to solve this problem. The modification is to add a helium storage tank to the liquid oxygen line of the middle thruster to reduce the chance of vibration, and to ensure that when this method does not work, the 5th thruster will automatically shut down and propel on all five second-stage rockets. Fuel valve on the valve.

Apollo 13 explosion

During the voyage of Apollo 13 to the Moon, 321,860 kilometers from Earth, the No. 2 oxygen tank of the service cabin exploded. ^[1-2] At that time, the command center required three astronauts to stir the oxygen tank to ensure that the oxygen was evenly distributed. After Swift agitated the oxygen tank, the damaged oxygen tank Teflon insulated wire caught fire, which increased the air pressure inside the oxygen tank (standard pressure is 7 million Pa) and caused an explosion. The exact cause of the explosion is unknown. One argument is that the asteroid hit the service module or even the lunar module.

The explosion also damaged the rest of the service cabin, especially the No. 1 oxygen tank. After the explosion, the command / service capsule lost all oxygen in the two oxygen tanks. Oxygen in the service cabin is a necessary part of the command / service cabin power supply system. In other words, the spacecraft has little electricity left after the explosion. There are batteries in the command cabin to return to the atmosphere, but they can only be used for about ten hours. Because electricity had to be reserved for returning to the atmosphere, the three astronauts had to use the lunar module as a "lifeboat." The steps of the lunar module as a "lifeboat" had just started simulation training shortly before the departure of Apollo 13. ^[3]

Apollo 13 returns

The damage to the spacecraft forced the cancellation of the lunar landing mission originally in the Fra Mauro Heights. Some at the command center proposed that the spacecraft should immediately turn around and accelerate its return to Earth. However, the only service propulsion system (SPS) on the entire spacecraft that has the direct thrust required for the U-turn is located at the rear of the service cabin where the accident occurred. Due to the unknown damage status, the ignition propulsion system has the risk of causing another explosion. For safety reasons, the command center decided not to Use a propulsion system. They chose to use the moon's gravitational return, bypass the back of the moon and let the spacecraft enter free return orbit. In order to enter the free return trajectory, a flight trajectory must be corrected. Normally, the service propulsion system is used to correct the trajectory. However, due to the impact of the accident, after a long period of study by the engineer team, the command center decided to use the landing rocket of the lunar module. After bypassing the moon, the landing lunar rocket was ignited and burned with PC + 2 (Peri Cynthion, near moon point + 2 hours) to accelerate the return to Earth. On the way back to Earth, the landing module landing rocket ignited again to complete a simple orbital correction.

How to maneuver the severely damaged spacecraft to return to earth safely is the biggest problem for the three astronauts and ground commanders. The lunar module was originally designed for two astronauts to use for two days, but now three astronauts rely on it for four days. The lithium hydroxide filter used to filter carbon dioxide on the lunar module cannot meet this demand. Although the command module has additional filters, they have a different shape from the interface on the lunar module. With the constant increase of carbon dioxide concentration, the ground commander immediately came up with a solution. They instructed the astronauts to connect two different shapes of filtering devices with the only materials on the spacecraft, and successfully reduced the carbon dioxide concentration of the lunar module. .

As the spacecraft gradually approached the atmosphere, NASA made a special decision: take pictures of the service capsule to analyze the cause of the accident, and discard the service capsule first instead of the lunar capsule that should normally be abandoned first. When the three astronauts first saw the service bay, they were surprised to find that the entire panel on the fuel cell and oxygen tank had been blown up.

The ground command center is also worried that the command cabin that has lost its temperature in order to save power and shut down the life support system during the return flight will cause a short circuit in the electronic control system due to water condensation. This cannot be prevented. Only when the system is restarted will it be known whether the system is running smoothly. Fortunately in the end everything worked fine.

The three astronauts eventually returned to Earth safely, although Hayes must be hospitalized because of a urinary tract infection due to lack of drinking water and difficulty urinating. On the way back, the three astronauts were told not to discharge urine or other liquids out of the cabin, as this would affect the spacecraft's orbit without the thruster's corrective orbit.

Although the accident itself was very unfortunate, the three astronauts should still feel lucky that the spacecraft had a problem on the way to the moon, not a return trip; otherwise, the remaining resources they could mobilize in an emergency would be greatly reduced. If the explosion of the service cabin occurred around the moon or on the way back, the chances of survival for the three astronauts are very slim. (If the lunar mission is completed normally, the lunar module will be abandoned and the three astronauts will not have a rescue capsule)

Paradoxically, another failure of the oxygen tank before the explosion may have just saved the lives of the three astronauts of Apollo 13. After 46 hours and 40 minutes after the start of the mission, the pointer of the No. 2 oxygen tank read abnormally, and once exceeded 100%. To find out, Swift was asked to stir the oxygen tank: this additional stir would have been scheduled after the moon landing. If this is the case, Lowell and Hayes may never have the opportunity to return to Earth after landing on the moon.

NASA conducted a detailed investigation afterwards and modified the spacecraft design to avoid similar incidents.