The Science Behind the 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 crewed the Liberty Bell 7 mission, a ballistic test flight that launched him from a rocket through the atmosphere. During the test, it was in a small capsule and reached a summit of over 100 miles before ending up 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 stayed in his pod and swayed on the gentle waves of the ocean. While waiting 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 splashing in water remains one of the most common ways astronauts return to Earth. I am a professor of aerospace engineering and study the mechanisms involved in these phenomena. Fortunately, most splashdowns aren’t that nerve-wracking, at least on paper.

Marinepersoneel haalt de bemanning op uit de Apollo 11-retourcapsule na het neerstorten op 24 juli 1969. <a href=AP photo/Barry Sweet” data-src=”–/YXBwaWQ9aGlnaGxhbmRlcjt3PTk2MDtoPTY0MQ–/ 8e1f38a328e401d58c06f24″/>

Splashdown explained

Before it can perform a safe landing, a spacecraft returning to Earth must slow down. As it flies 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 fast this transfer occurs, there is still not enough time during reentry to slow the vehicle to a speed safe enough not to crash. So engineers resort to other methods that can slow a spacecraft during impact.

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 stress) and has a density much lower than that of hard rock. These two properties make it ideal for landing spacecraft. But the other major reason water works so well is because it covers 70% of the Earth’s surface. So the chances of you crashing into it are high if you fall from space.

The science behind splashdown is complex, as a 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 and friction decreased, the air became cooler, allowing it to absorb the vehicle’s excess heat, which also cooled it. At a sufficiently low 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 what’s called the phenomenon of crashworthiness – or the degree of damage a spacecraft sustains after hitting a touches the 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 retrieve 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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