Btukfyl

NBC News' Mars InSight's landing team leader is all 'nerves and excitement' ahead of risky touchdown includes a conversation with Rob Grover, "who leads the team in charge of InSight's landing at NASA's Jet Propulsion Laboratory in Pasadena, California."

Question: What does Grover mean by InSight's nylon parachute being a "soft good"?

NBC: What’s the riskiest part of the landing?

Grover: There’s a couple of times in the control room where we’ll be particularly happy as the landing is unfolding. One of them is parachute deployment. The parachute is not inherently extra-risky. But rather than being a rigid design like the metal lander, which is very predictable, it’s made of nylon. It’s what we call a soft good. And when we deploy that, it’s a little less predictable. While we’ve had great success deploying parachutes on Mars, and we expect to again, it’s one of those moments when we’ll be happy when it’s out and we know we’re descending under the parachute.

I think another one is, we have to have the radar working correctly in order to land. We have radar antennas underneath the lander. While the lander does navigation from the top of the atmosphere, it’s not accurate enough to know precisely where the ground is, so we need the radar to give us fresh information on how high we are above the ground and what velocity we’re descending at. Without the radar, we don’t have good enough information to land successfully. Once the radar successfully acquires the ground, then that’ll be another moment where we’ll kind of breathe a sigh of relief.

I'm pretty sure that by "soft good" he means it's an element of the system that can't be completely constrained from a simulation or engineering standpoint, in this case fabric. If you're working with static systems or systems with a limited amount of degrees of freedom, it's possible to calculate and analyse every possible state the system can be in. For example a simple hinge only has one degree of freedom and essentially two moving parts. You can figure out every possible angle that it can be in. Even with more complex mechanical linkages such as a robot arms, you can use reverse kinematics to find the angles of all the joints in the system and then run stress or strain calculations on that.

Grover says the parachute deployment is stressful because this type of analysis can't be done with the parachute (or cloth in general) and there's a functionally infinite amount of states the parachute can be in during it's deployment. They can preform a thousand tests with the chute packed identically each time yet the actual deployment is still pretty chaotic--even if it works 1000 times, they can't calculate that it will work the 1001th time. This unpredictable behavior of cloth or other "soft goods" is also why Automation hasn't completely replaced humans in the clothing industry. It is still very hard for computers to manipulate, simulate, and predict fabrics compared to rigid parts.