The Science Behind Splashdown: An Aerospace Engineer Explains How NASA and SpaceX Get Spacecraft Safely Back to Earth

For about 15 minutes on July 21, 1961, American astronaut Gus Grissom felt on top of the world – and he was.

Grissom was a pilot on the Liberty Bell 7 mission, a ballistic test flight that launched him through the atmosphere in a rocket. During the test, he was in a small capsule and reached an altitude of over 100 miles before splashing down in the Atlantic Ocean.

A Navy ship, the USS Randolph, watched the successful end of the mission from a safe distance. Everything had gone according to plan, the Cape Canaveral inspectors were thrilled, and Grissom knew he had just entered a VIP club as the second American astronaut in history.

Grissom remained in his capsule, swaying on the gentle ocean waves. As he waited for a helicopter to take him to the dry deck of the USS Randolph, he finished recording some flight data. But then things took an unexpected turn.

An incorrect command in the capsule’s explosive system caused the hatch to pop out, causing water to pour into the small space. Grissom had also forgotten to turn off a valve in his spacesuit, causing water to seep into his suit as he fought to stay afloat.

After a dramatic escape from the capsule, he struggled to keep his head above the surface as he signaled to the helicopter pilot that something had gone wrong. The helicopter managed to save him at the last minute.

Grissom’s near-death escape remains one of the most dramatic disasters in history. But splashdowns remain one of the most common ways astronauts return to Earth. I’m a professor of aerospace engineering who studies the mechanisms involved in these phenomena. Fortunately, most splashdowns aren’t all that nerve-wracking, at least on paper.

Marinepersoneel haalt de bemanning uit de Apollo 11-retourcapsule na de landing op 24 juli 1969. <a href=AP Photo/Barry Sweet” data-src=”–/YXBwaWQ9aGlnaGxhbmRlcjt3PTk2MDtoPTY0MQ–/″/>

Splashdown explained

Before a spacecraft returning to Earth can perform a safe landing, it must slow down. While traveling back to Earth, a spacecraft has a lot of kinetic energy. Friction with the atmosphere introduces drag, which slows the spacecraft. The friction converts the spacecraft’s kinetic energy into thermal energy or heat.

All this heat radiates into the surrounding air, which becomes really hot. Because the speed of return can be several times the speed of sound, the force of the air pushing back against the vehicle turns the vehicle’s surroundings into a scorching current of about 2,700 degrees Fahrenheit (1,500 degrees Celsius). In the case of SpaceX’s massive Starship rocket, this temperature even reaches 3,000 degrees Fahrenheit (almost 1,700 degrees Celsius).

Unfortunately, no matter how quickly this transfer occurs, there is still not enough time during the reentry for the vehicle to slow down to a safe enough speed not to crash. So engineers resort to other methods that can slow a spacecraft during crash.

Parachutes are the first option. NASA typically uses designs with bright colors, such as orange, making them easy to recognize. They are also huge, over 30 meters in diameter, and each reentry vehicle usually uses more than one for best stability.

The first parachutes to deploy, called drag parachutes, are ejected when the vehicle’s speed drops below about 2,300 feet per second (700 meters per second).

Even then, the rocket cannot crash into a hard surface. It has to land somewhere where the impact is absorbed. Researchers discovered early on that water is an excellent shock absorber. Thus, splashdown was born.

Why water?

Water has a relatively low viscosity – that is, it deforms quickly under pressure – and it has a density much lower than hard rock. These two properties make it ideal for landing spacecraft. But the other main reason why water works so well is because it covers 70% of the planet’s surface, so the chances of you hitting it are high if you fall from space.

The science behind water fall is complex, as its long history proves.

In 1961, the US conducted the first manned splashdowns in history. These used Mercury return capsules.

These capsules had a roughly conical shape and fell with the base toward the water. The astronaut inside sat face up. The base absorbed most of the heat, so researchers designed a heat shield that boiled away as the capsule shot through the atmosphere.

As the capsule slowed down and friction decreased, the air became cooler, allowing it to absorb the excess heat from the vehicle, thereby cooling it down. At a low enough speed, the parachutes would deploy.

The crash occurs at a speed of approximately 80 feet per second (24 meters per second). It’s not exactly a smooth impact, but that’s slow enough for the capsule to crash into the ocean and absorb the shock of the impact without damaging the structure, its cargo, or any astronauts inside.

After the loss of Challenger in 1986, when the space shuttle Challenger broke up shortly after launch, engineers began focusing their vehicle designs on the so-called crashworthiness phenomenon: the degree of damage a spacecraft sustains after it hits a surface.

Now all vehicles must prove that they can offer a chance of survival on water after returning from space. Researchers build complex models and then test them with laboratory experiments to prove that the structure is sturdy enough to meet this requirement.

On to the future

Between 2021 and June 2024, seven of SpaceX’s Dragon capsules performed flawless splashdowns upon return from the International Space Station.

On June 6, SpaceX’s most powerful rocket yet, SpaceX’s Starship, made a phenomenal vertical splash in the Indian Ocean. The rocket boosters continued to fire as they approached the surface, creating an extraordinary cloud of hissing steam around the nozzles.

SpaceX has used splashdowns to recover its boosters after launch without significant damage to their critical components, so it can recycle them for future missions. By unlocking this reusability, private companies can save millions of dollars in infrastructure and reduce mission costs.

Splashdown remains the most common tactic for spacecraft reentry, and with more and more space agencies and private companies shooting for the stars, many more are likely to occur in the future.

This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.

It was written by: Marcos Fernandez Tous, University of North Dakota.

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Marcos Fernandez Tous does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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