Parachutes are plaguing space programs. SpaceX doesn’t like Parachutes. They are difficult to design, hard to package, and easy to damage. The larger the mass of the spacecraft, the more effort to slow down. Larger, more efficient, complex parachute systems are needed. Several failures have hit the industry over the last few years, including SpaceX Crew Dragon, ESA ExoMars, Boeing CST-100, and the NASA Orion to name a few.
How do parachutes work and why are they hard?
The idea of a parachute is simple. All falling objects fall the same when under the same conditions… that is so long as no outside force is exerted on it. So two objects dropped from the same altitude, one a feather and hammer will fall equally. Don’t believe me? NASA tested it on the Moon. During Apollo 15 moon walk, Commander David Scott performed a live demonstration for the television cameras. Commander Scott did the Apollo 15 Hammer and Feather test. He held out a geologic hammer and a Falcon feather and dropped them at the same time. Because there is not an atmosphere on the Moon, they were essentially in a vacuum. With no air resistance force, the feather fell at the same rate as the hammer. Ironically, Apollo 15 had a second demonstration of falling objects when one of the parachutes failed to function as planned.
On Earth, and any other planet with an atmosphere, air acts as a resistance force for an object moving through it. We can get more air resistance force by increasing the surface area. Depending on the shape of the object, it’s orientation, and the amount of resistance will increase, and therefore slow the object down. Unbalanced and uncorrected resistance can cause the object to start to turn, twist and tumble. A parachute system is deployed to generate air resistance from the atmosphere. (note that the thicker the atmosphere the more resistance) Parachutes designed for use on Earth will not be the same as a parachute designed for Mars.
Beyond density of air, the resistance depends on the speed of the and slow a falling body, the flow of gas against and around its structure(Shape). This airflow not easily to calculate as it is complex and turbulent, especially at supersonic speeds. The Apollo program did, as Elon Musk mention, find challenge in the development and testing of the parachutes. It was described as a major difficulty in design and development due to a lack of adequate analytical methods for properly predicting dynamic behavior, loads and stresses. Fast forward fifty years, Apollo mission recommendation of continued development of prediction methods for parachutes still plagues spacecraft.
Some recent challenges with parachutes
In 2019, Boeing successfully tested out the emergency escape system on its new Starliner spacecraft, but one of its three parachutes failed to deploy. Since the parachutes were designed with redundancy, the situation could be survivable, although likely to damage the craft and possibly injury the occupants.
On September 21, 2019, Elon Musk Tweeted: “Great work by SpaceX parachute engineering! The Crew Dragon parachutes are way more difficult than they may seem. The Apollo program found them to be so hard that it became a notable morale problem!” And he is right, parachutes are not easy. SpaceX has completed more than 30 drop tests of their parachute system, including the successful Demo-1 mission flight test to help prove out the parachute system safety.
SpaceX initially tried to use the parachutes only as a backup, but then needed to rely on the parachutes when they found difficulty in proving propulsive landing of the Crew Dragon spacecraft was safe enough to meet NASA standards. SpaceX opted to re-design the parachutes on its Dragon spacecraft, and go through a grueling new set of testing with NASA. In March of 2020, those same dynamic loads that make design of parachutes a challenge surfaced in a different way. The test article for SpaceX for a parachute test was damaged when the helicopter carrying it needed to make an emergency drop prior to the craft being ready. This most recent setback just two months before SpaceX planned to launch two people aboard a Falcon 9 rocket with the Crew Dragon spacecraft on top.
The Orion spacecraft has a complex parachute system as well. The system has 11 parachutes, a series of cannon-like mortars, pyrotechnic bolt cutters, more than 30 miles of Kevlar lines connected to 36,000 square feet of parachute canopy material. The parachute system is attached on the top of the spacecraft. After 10 minutes of descent through Earth’s atmosphere, a precise deployment sequence must be started in order to slow Orion and its crew from about 300 mph to a 20 mph for splashdown in the Pacific Ocean. For reference, car frontal air bags are generally designed to deploy in “moderate to severe” frontal or near-frontal crashes of around 8 to 14 mph.
Parachutes are unreliable. There is a significant uncertainty with the use of a parachute system as to where you will end up. The effective area of landing is much greater than a propulsive landing. Factors include weather in the lower atmosphere interacting with the geometry of the spacecraft. A propulsive landed spacecraft would be able to offset a great deal of that uncertainty as we have seen with the Falcon 9 first stage booster landings.
The joint ESA Russian ExoMars mission has been having issues as well.
For ExoMars Rosalind Franklin rover to explore the planet for signs of life it needs to land safely. A two-parachute system was designed , each with its own pilot chute for extraction. The first main parachute has a diameter of 15 m and will be deployed while the descent module is still traveling at supersonic speeds, while the second main parachute has a 35 m diameter, which will be the largest to ever be used on a Mars mission.
Originally published at https://westeastspace.com on March 26, 2020.