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Ingenuity helicopter to Mars: Testing the nearly untestable

You’ve undoubtedly seen and followed the astounding and amazing technical achievement of the successful landing and functioning of NASA’s Perseverance Mars rover and its on-board, solar-powered Ingenuity helicopter which became the first aircraft in history to make a powered, controlled flight on another planet (Figure 1). While good fortune and luck play a role in the success of such missions, they are just a very small part of the intense effort performed through countless “what if” scenarios. Can you imagine that meeting where the crazy idea of a helicopter to Mars was proposed?

Figure 1 The on-board Ingenuity helicopter is truly a one-of-a-kind, custom-built unit which has successfully flown specific test patterns to test out its performance and autonomous capabilities. Source: NASA

All those possibilities and their implications on mission design lead to countless design decisions, tradeoffs, implementations, and finally, tests. The problem, of course, is that testing anything related to space vehicles and travel—whether launches, orbital missions, Mars landers, or deep-space probes—involves many decisions which can only be partially tested. Due to differences in gravity, atmosphere and temperature, it’s impossible to perform full battery of tests here on Earth that will yield 100% confidence in performance “out there.”

The issue is how close can you get, and at what cost in time and effort. An added complication is that these projects do not have the luxury of building up tens or hundreds of identical prototypes and then testing them as a group. That’s a very important test approach which you might do for a circuit board and which can uncover statistical “tail” problems; tests on a sample size of just one or two units may not reveal these subtle deficiencies.

The operating conditions for the Ingenuity helicopter as a flying machine are far different than anything on Earth. Gravity on Mars is only about one-third of the value on Earth; that makes flying easier. But the atmosphere is only about 1% of Earth’s, and that makes it much harder. Thus, the counter-rotating rotor blades, which provide lift, face extremely challenging aerodynamic conditions and must spin at several thousand revolutions per minute compared to several hundred for a design on Earth. This, in turn, brings new problems of centrifugal force, supersonic rotor-tip speed causing disruptions, and more.

To test lift performance in that low-gravity but thin atmosphere setting, engineers used a special low-pressure chamber where they semi-suspended Ingenuity from a force-balance pulley arrangement on the high ceiling. To minimize the influence of that system, they used a very thin monofilament fishing line for the suspension, but first had to evaluate various available lines to see which ones stretched the least, since stretching would affect the balancing set-up.

Caltech has published a fascinating story, detailing how engineers at JPL and Caltech set up a special wind tunnel for a forward flight test. They had short-term access to an 85-foot-tall, 25-foot-diameter vacuum chamber at JPL, but had to build a custom wind tunnel inside it to simulate the helicopter in forward flight. Unlike most wind tunnels, which create large-volume blasts of high-velocity air, this one had to create small, carefully-controlled wind patterns up to about 10 miles/hour.

The solution sounds easy: use an array of over 2,000 small fans and control them individually (Figure 2). They started with standard, widely-available small fans, but that was only the start of the solution. Each fan needed to be individually-controlled to create the desired patterns. They also needed some sort of feedback to identify if there was problem with any one of them, so the control algorithm could work around it.

Figure 2 The 2000+ small, standard fans in this wall installed in a wind tunnel built inside a vacuum chamber are individually controlled to create desired low-velocity wind patterns, simulating forward-flight conditions on Mars. Source: Caltech

Also, the large number of wires connecting the fans made it difficult to seal the vacuum chamber, so a multiplexing interface box, which went inside the chamber to reduce the wire count, wrote software that remotely monitored the fans, and directed reprogramming them if there was a problem. Even something as routine as insulation posed a problem. Wiring for the commercial fans used PVC insulation, which might outgas in the vacuum chamber and cause contamination, so they had to replace it with Teflon coverings.

It’s been my experience that when someone says “hey, that should be no big deal” or “that sounds simple enough,” I have one of two instinctive but contrary reactions. First, they have done this before, they know what they are talking about, and they’re right. Second, the complete opposite: it’s all new, they are clueless, and simple-looking and -sounding scenarios often have subtle traps and “gotchas” that only become apparent—if and when they do—when you are deep in the swamp.

At least, for the Perseverance and Ingenuity project, the teams had a fairly good idea of what conditions during the flight and after landing would be like, based on data from previous missions. I often think about the engineers and scientists working on the Apollo project and all the unknowns they didn’t even know about, let alone understand how to address them. Early rocketeers faced these problems to an even-larger extent, as they pushed ahead with hard-to-instrument hardware and systems into environments about which very little was known.

Have you ever been involved in a project where adequate testing was either very difficult or nearly impossible due to an operating environment which was poorly known or difficult to recreate? How did you work through those problems, assuming you did? What was your final level of confidence in the test process?

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1 comment on “Ingenuity helicopter to Mars: Testing the nearly untestable

  1. Steve T
    May 13, 2021

    Excellent article Bill! You always give readers deep insight to your posts!

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