China recently unveiled drawings of the capsule it plans to use to carry humans and cargo into space in the 2020s, and the design looks strikingly familiar. With a flat top, sloping sides, and a wide bottom, all of the crew capsules in development today look like candy gumdrops. Where does this shape come from?
You might recognize it from the vehicle that brought the Apollo astronauts home. With few exceptions, human spaceflight hasn't changed much since those days. The Soyuz capsule uses a similar shape today, and SpaceX, Boeing, and NASA are all designing brand new spacecraft with almost the exact same design.
All of these vehicles are similar because of three competing variables that spacecraft designers have to contend with: weight, space, and heat.
"Weight is the number one problem," says Pasquale Sforza, an aerospace engineer at the University of Florida and author of the recent book, Manned Spacecraft Design Principles. "Everything has to be really strictly fashioned for the weight."
Astronauts need as much space as possible in order to do their jobs and be comfortable. But every pound of a spacecraft's weight can add $10,000 to the cost of launching it. To maximize space, spacecraft designers like to use spherical shapes. These pack in as much internal volume as possible while minimizing the surface area--the walls.
"Weight is the number one problem."
But a perfect sphere won't do. As the spacecraft return to Earth at speeds of 17 to 25 thousand miles per hour, they're pushing through the air and creating a lot of friction. The friction generates a potentially dangerous amount of heat, and the spacecraft's shape determines how hot the vehicle gets.
In the early days of launching stuff into space and back again, engineers experimented with pointy-nosed vehicles. Those pointy nose cones were great at reducing drag and flying through the air efficiently, but unfortunately they melted when they tried to reenter the atmosphere.
It was NASA aeronautical engineer Harry Julian Allan who came up with the idea of using the blunt-nosed reentry vehicles like we use today.
Once they're built, Orion, Crew Dragon, and the rest will drop into the atmosphere bottom first. Their broad bottoms will run into air molecules on the way down. Those particles will rebound off the vehicle and crash into the incoming air. This creates a layer of air around the vehicle, so that the shockwave created by the vehicle doesn't touch it. The layer shields the spacecraft from the heat of reentry and shapes the airflow around it.
By contrast, a pointy tip does come into contact with the shockwave, exposing the front-most part of the craft to extremely high temperatures.
Similarly, a narrow sphere wouldn't be great at deflecting that heat, but a larger sphere would be too bulky to maintain a practical weight. So the result is something in between. Spacecraft designers gave these craft the rounded bottom of a very large sphere, but cut off the sides to shave off unnecessary weight.
The rounded base is important, says Orion engineer Stuart McClung. "If you made it flat it becomes very inefficient. It would be like pushing a flat plate through water--the pressure waves would probably make it very rough and choppy."
The spacecraft's sides taper into a narrowed top because as the vehicle re-enters the atmosphere bottom first, it comes in at an angle, skimming through the atmosphere instead of plunking straight down. The angled sides mean that less of the spacecraft is exposed to the friction during that skimming, and that means the sides don't need to have additional heat shields that would make the vehicle heavier.
What about the shuttle?
The retired space shuttle is an exception to the gumdrop rule. Essentially an airplane, the shuttle was designed to land horizontally on a runway so it could be reused again later. But it still followed the same rules as Apollo-inspired spacecraft. The shuttle didn't enter the atmosphere nose-first. Instead, it approached Earth at a sharp angle so that its broad underside faced the atmosphere--basically belly flopping into Earth's atmosphere.
The Mercury and Gemini vehicles were also a bit different from the gumdrop shape. These spacecraft were more funnel-shaped, with narrow, cylindrical tops broadening into that familiar cone-shaped bottom. Just like with the Apollo craft, that cone-shaped bottom entered the atmosphere first. The elongated part was just the aerospace equivalent of a car trunk.
"If you're going on a biking trip and you don't have enough room in your trunk for the bike, you might strap it on the top of your car," explains Sforza. It's the same thing with Mercury and Gemini spacecraft.
However, says Sforza, "It's not a great idea to have all the stuff hanging out the back of the spacecraft. It can give you some stability problems that are not good.... As time progressed people saw that the Apollo solution was really the best compromise."