Why NASA's Orion Spacecraft Looks So Familiar

For space travelers, the physics of entering the earth's atmosphere at hypersonic speed dictates a spherical-cone design with high drag, which best manages heat and maintains stability

NASA's Orion prototype capsule of the Multi-Purpose Crew Vehicle

Photograph by Bill Ingalls/NASA via Getty Images

(Updates Orion launch date to Friday morning in paragraph 5)

As NASA's Orion craft makes its first flight in the decades-long journey to land humans on Mars, the craft will strike many people as a throwback. Squint an eye and the Orion crew module bears a remarkable resemblance to the Apollo craft that ferried astronauts to the moon more than 40 years ago.

NASA's choice of a blunt-bottomed conical capsule design on both spacecraft isn't an accident. The blunt cone shape causes high drag to help slow the craft as it descends. When it returns to earth from its farther-flung missions, Orion will be traveling at nearly 7 miles per second, a hypersonic speed multiple times faster than the speed of sound. At that speed, managing the enormous temperatures the craft will generate—which exceed 5,000 degrees Fahrenheit—will require an effective design with intense shielding. That's one reason the initial test flight's prime job is to assess the updated shield built for Orion.

"The 'capsule shape' happens to be good aerodynamically for slowing down the vehicle without it burning up like a meteor," Kelly Smith, a NASA guidance engineer, wrote last month in a public discussion the agency held on Reddit. "Sharp shapes tend to heat up too much and melt/vaporize. A blunt shape works well hypersonically for keeping the heating to more manageable levels. If you look at ballistic missiles, all of their nose cones are 'blunt' as well (spheres, sphere-cones, etc) to deal with the extreme heating environment."

apollo-vs-orion
Courtesy NASA

 

 

 

 

 

 

The cone's imbalanced shape also causes the craft to fly "crooked" as it hurtles toward earth at a rate of about five times the speed of sound, Smith explained. "This angle of attack causes Orion to have a little bit of lift; we can use this lift to steer the vehicle and control the entry trajectory by banking Orion like a glider," he wrote. Russia uses a similar shape for its Soyuz capsules, as does SpaceX's Dragon spacecraft.

Weather permitting, Orion has been rescheduled to launch on Friday morning on a 4.5-hour, two-orbit cycle around earth, reaching an altitude 15 times higher than that of the International Space Station. The 3,600-mile altitude will help NASA test the craft's response to the higher radiation levels found outside low-earth orbit—from which the space shuttle never strayed—and to reach a 20,000 miles per hour entry speed, about 84 percent of the speed generated by a return from the moon. "Although our computers have gotten a lot more powerful, the physics of atmospheric entry hasn't changed since Apollo," says NASA spokeswoman Stephanie Schierholz.

The Orion program's homage to Apollo doesn't end with the basic look. NASA also consulted with several retired employees who worked on the Apollo program; at least one will be on hand to observe the flight at Mission Control in Texas. Up close, the similarities end abruptly. Orion is roughly three times larger than the Apollo crew module, built to carry four astronauts as far as Mars, a 70-million mile round-trip journey that could take as many as 23 months. On shorter trips, the craft can fit six. 

 

 

Inside, Orion's "glass cockpit" would not look particularly foreign to any iPad user, with touchscreens similar to those used in modern jet cockpits. Eliminating physical switches and their associated cabling saves enormously on weight. Only about 60 physical switches remain.