The United States is developing more spacecraft to carry people into space than any time in history. You might think the whole rocket full of explosive propellant is the biggest engineering challenge, but right now, it’s the soft landing that’s tricky—because designing parachutes, it turns out, isn’t so easy.
Earlier this year, auditors identified parachute problems as a top risk [pdf] to NASA’s commercial crew program, which has hired SpaceX and Boeing to design and operate vehicles to bring astronauts to the International Space Station.
Boeing successfully tested out the emergency escape system on its new Starliner spacecraft on Nov. 4, but one of its three parachutes failed to deploy—a survivable anomaly, yet not ideal. Its rival, Elon Musk’s SpaceX, decided to re-design the parachutes on its Dragon spacecraft in September, and now reports 13 successful tests with hopes to qualify the design by the end of the year.
Earlier this year, a failed parachute test for a planned Russo-European moon mission threatened to delay its launch. And it took NASA 12 years to finish designing and testing the parachutes for the Orion spacecraft that will take NASA astronauts to the moon.
The issue, as you might imagine, is physics. When a parachute is deployed to generate resistance in the atmosphere and slow a falling body, the flow of gas against and around its structure is complex and turbulent, especially at supersonic speeds. The study of that kind of turbulence is one of the most difficult phenomena for physicists to characterize and predict. There is even a $1 million prize available for solving one of its defining equations.
It was certainly a challenge for the Apollo program. “A major difficulty in design and development was the lack of adequate analytical methods for properly predicting dynamic behavior, loads and stresses,” a 1968 NASA paper on the Apollo parachutes concluded. “Development of these prediction methods must precede any major improvements…of future spacecraft systems.”
That’s still true today. “Parachutes are a pain in the ass,” Miguel San Martin, a NASA engineer who has done extensive work landing robots on other planets, told Quartz in 2014. “[They are] convenient to analyze, slow the spacecraft dramatically, a very efficient machine, but we don’t understand the physics of how the parachute works, it’s a difficult thing to model. We are always holding our breath.”
There’s also the issue of precision when it comes to parachute landings, which require ships or helicopters to patrol landing zones of hundreds of square miles. That’s another reason why the Space Shuttle relied on its aerodynamic shape to glide to a landing on a runway, and that newer spacecraft-makers like SpaceX and Blue Origin are focused on rocket-powered landings for their futuristic spacecraft.
But those plans are more expensive and untried, not to mention equally demanding on the fluid dynamics front. Landing a capsule with parachutes is a more proven technology—used regularly by the Russian Soyuz spacecraft, for example. But the newer American spacecraft are much larger than than the Apollo capsules or the Soyuz, increasing the velocities and loads faced by their parachutes.