NASA Resurrects the Capsule for Space Flight

An Artist Depiction of the SpaceX Crew Dragon. Artwork from Picryl.

Looking at old NASA photos from the ’60s got me thinking about current technology. SpaceX and Boeing are testing and operating capsules to transport astronauts to and from the International Space Station. NASA engineers continue working with Lockheed Martin and Airbus on the Orion capsule designed to take humans to the Moon and Mars.

What strikes me about these vessels is their relative simplicity. NASA chose ballistic capsules in the 1950s because they promised the cheapest, quickest way to beat the Soviet Union to the Moon.  Going up or coming down, occupants had little to do but ride it out. Capsules weren’t, and still aren’t, maneuverable once they begin their descent from orbit; they simply fall where their trajectory dictates–like a cannonball. Having lived through the Shuttle years, when reusable spaceplanes landed on runways, I couldn’t help wondering why NASA returned to capsules.

NASA’s earlier capsules. Project Mercury perfected the ability to move humans to and from orbit. Gemini taught astronauts orbital maneuvering and how to survive outside the spacecraft. Apollo put all the new skills together to land on the Moon. Illustration courtesy of NASA, via Picryl.

A capsule’s main appeal has always been its relative simplicity of design and operation. The cone shape of the American capsules was aerodynamic going up, while providing the drag necessary to slow the craft down as it dropped from orbit base forward.  The wide base actually pushed along a shock wave that helped dissipate the intense heat generated by the capsule’s descent (see illustration below). The innovative shape meant the capsule would always descend properly oriented, even if the control system failed. Ballistic capsules minimized the process of reentry, too, as it took only a burn of the craft’s retro rockets to slow it and drop it out of orbit. 

Shadowgraph images of the favorable shockwave and airflow produced by blunt body spacecraft. Image courtesy of NASA on flickr.
Not all capsules were cone-shaped. The Soviets’ Vostok, which carried Yuri Gagarin to orbit and back in April 1961, was a sphere. Photo from Wiki Commons.

When it came to simple, however, the Americans were bested by the Soviets, who adopted a sphere for their first generation (Vostok) and second generation (Voskhod) capsules.  Drag and self-orientation were excellent, although the sphere’s inherent bluntness required a stronger booster rocket than the Americans possessed.

Simplicity came with a few disadvantages. For one, ballistic capsules weren’t maneuverable during their fall to Earth.  Second, while their broad bases produced drag, descending capsules still subjected their occupants to about 8.5 times the force of gravity (G-forces, or just Gs).  Mercury’s designers mitigated this somewhat with their design of a form-fitting couch that could keep astronauts conscious at 20 Gs.  All that stress on the spacecraft meant that all early capsules were one-and-done affairs.  

There was an alternative, and the fact that NASA didn’t adopt it goes a long way to explain why nations are still launching their astronauts to space in capsules.  In the late 1950s, engineers with NACA (NASA’s precursor) discovered that a half-cone, cut in half lengthwise with the flat side up, would generate lift from the shape of its body alone.  Such “lifting bodies” could descend from orbit horizontally, producing far fewer Gs for their occupants. Once in the lower atmosphere, lifting bodies offered something ballistic capsules never could–maneuverability.  They could be flown hundreds of miles to either side of their glide path and landed on an airstrip for reuse. 

In the end, NASA chose the ballistic capsule.  With the limited thrust of the booster rockets available, and the Soviets surging ahead with their own space program, the likelihood of quicker development of a less-complex ballistic capsule made practical and economic sense.  A decade ago, as NASA retired the Shuttle, it decided to hand flying duties into low-earth orbit to commercial companies like SpaceX, Boeing, and Sierra Nevada Corporation.  The first two have designed traditional ballistic capsules, no doubt hoping to use the lessons of the past to beat the others into space.  It’s a race SpaceX has recently won.  NASA itself has chosen a capsule design to return to the Moon.  The lesson here is that ballistic capsules simply work.  They’re not pretty or romantic, but they will take astronauts to space and back safely and relatively cheaply.      

Works Cited

Burrows, William E. This New Ocean: The Story of the First Space Age. New York: Random House. 1998.

Chaikin, Andrew. “How the Spaceship Got its Shape,” Air & Space Magazine, November 2009.

Masunaga, Samantha. “U.S. astronauts are climbing back into space capsules. Here’s how they’ve improved over the past 50 years,” Los Angeles Times, 15 September 2017.

Reed, Dale and Darlene Lister. Wingless Flight: The Lifting Body Story. Lexington: The University Press of Kentucky. 2002.

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