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On the NASA website, it says that the propeller on Ingenuity will need to spin faster than what has been done before during all of the previous tests to accommodate for the rarefied atmosphere during the martian winter.

they will have to spin faster than we have ever attempted with Ingenuity or any of our test helicopters on Earth.

Why weren't any high rotation speed tests made on Ingenuity (or on an engineering model) done on Earth? It seems very unlikely that the engineers would not anticipate the effect of the atmospheric density variations.

I understand that the atmosphere isn't the same on Earth, but there are chambers that can reproduce the martian atmosphere where spacecraft can be tested (I remember reading about one such chamber in Germany, but I can't recall the exact place). So why didn't we test Ingenuity at 2800 rpm in a martian-like atmosphere before sending it to Mars?

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    $\begingroup$ I think you are reading too much into that article, I'd be very surprised to hear they hadn't tested it in a near-vacuum chamber. $\endgroup$
    – GdD
    Sep 20 at 20:04
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    $\begingroup$ Video of the vacuum test chamber: youtube.com/watch?v=FxGSPH8-p14 (don't have time to write up a proper answer) $\endgroup$
    – Harabeck
    Sep 20 at 20:16
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Ingenuity is a low cost technology demonstrator. As a technology demonstrator, NASA and JPL cut lots of corners to keep the cost low (low by NASA standards, only \$80 million US compared to the \$2.2 billion cost for the rover). Ingenuity was planned to make just five flights during the first few months of the mission. If it had fallen short of this goal of five flights by a flight or two it probably would still have been deemed a success. It successfully completed those first five flights, with one incident from which recovery was possible. It has since made eight additional successful flights, far exceeding expectations.

It transitioned from being a mere technology demonstrator to being an operational aid after that fifth successful flight. "Operational aid" means it can be used in a non-mission critical sense to help scout paths for the rover and to scout targets of interest. It will never be made mission-critical; it was not designed for that. It was designed as a technology demonstrator.

One of the corners that was cut was to design for the benign atmospheric conditions that were expected in the few months after landing. Designing and testing for the not so benign atmospheric conditions many months after landing would not have been consistent with the technology demonstrator nature of the project. Doing so would have increased the costs by quite a bit, and it wouldn't have made sense.

Since Ingenuity has proved the viability and usefulness of a Mars helicopter, possibly sacrificing it to see if can survive making the rotor spin so fast that the blade tips are moving close to speed of sound makes sense. The helicopter will be essentially dead if the change isn't made, and may well be dead if the change results in a failure.

But if it succeeds, the helicopter can be continued to be used as an operational aid -- at least until the helicopter fails, possibly for some other reason. That failure (and it will fail eventually, almost certainly well before Perseverance fails) will not detract from what Ingenuity has already done.

Why not wait?

A comment suggested to save Ingenuity for better weather in the future. That would be a very long wait. The graph below shows why waiting is not an option.

Graph of local pressure at Gale Crater as a function of time
Source: Trainer, Melissa G., et al. "Seasonal variations in atmospheric composition as measured in Gale Crater, Mars." License: CC BY-NC 4.0

Perseverance landed on Mars on February 15 2021, about a week after the northern hemisphere spring equinox. The atmospheric pressure on Mars rises for about 100 days after the spring equinox. That made for ideal flying conditions.

But then atmospheric pressure began to fall. By the northern hemisphere summer equinox on August 25, 2021, the fall was in full decline, and will continue to decline for the next 100 days or so after August 25. The atmospheric pressure will remain below the August 25 level for another 100 days after the late summer minimum.

Since transmission of commands to and data from Ingenuity requires relativity close proximity to Perseverance, waiting would require the rover to remain near its current location for the next five months. That would be highly counterproductive to the overall goals of the rover's mission. Waiting is not an option.

References

Trainer, Melissa G., et al. "Seasonal variations in atmospheric composition as measured in Gale Crater, Mars." Journal of Geophysical Research: Planets 124.11 (2019): 3000-3024.

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  • $\begingroup$ Well, the cost (of additional testings) vs the benefits (of extending the horizon) is not a simple decision. If Ingenuity doesn't help much Perseverance in its main mission (compared to Curiosity for ex.), then making a final "crash test" is plausible. OTOH, despite it's a POC by design, if it enhances Perseverance's capability, perhaps a more forward-looking strategy is to save it for better weather in the future. Let's not forget that Ingenuity has a camera and a "huge" storage capacity too. $\endgroup$
    – Ng Ph
    Sep 21 at 19:59
  • $\begingroup$ @NgPh That would be a many month long wait. The pressure on Mars is dropping because it is deep winter at Mars' south pole. The solstice was less than a month ago, on 25 August 2021. Mars' weak atmosphere is currently freezing onto the southern icecap and will continue to do so for many months. Winter lasts a long time in Mars' southern hemisphere due to Mars' elliptical orbit. Waiting for many months for the weather to get better is not an option with regard to the primary mission, which is the Perseverance rover. The rover soon will move on, with or without Ingenuity. $\endgroup$ Sep 21 at 21:12
  • $\begingroup$ Thks for a great edit. But I find the argument "waiting would require the rover to remain near its current location for the next 5 months" disputable. It does IF Ingenuity is necessary for the rover's mission, which you showed to be negative. Why not leave the helicopter safely on ground for 5 months and come back to resume joint operation after a couple of months? $\endgroup$
    – Ng Ph
    Sep 23 at 14:22
  • $\begingroup$ @NgPh? You should flip that around and ask why? Without the rover nearby, there is no commanding of or receiving telemetry from the helicopter. There is no way of knowing if the helicopter will still be alive on the rover's return. There's a real possibility that it will be dead. For example, a dust storm could have flipped it over. Its solar arrays could be covered with dust. Its non-hardened computers and avionics could have been killed by a cosmic ray. That would be five months of wasted time by a very expensive rover. $\endgroup$ Sep 23 at 19:14
  • $\begingroup$ I don't see it as "5 months wasted", because by default the rover can operate w/o Ingenuity. That's the bottom line. The choice is between (i) taking the risk that after x months in stand-by it may be no longer operational or (ii) continuing to use it outside it's safety envelop, now, in adverse weather. Only with the following, I can see that there is no other alternative than risking to kill it now:(a) it doesn't have a deep sleep mode (battery conserving); (b) it is very useful (albeit, not essential) to continue operating with it's help in unforeseen adverse weather . $\endgroup$
    – Ng Ph
    Sep 23 at 22:08
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The answer is easy, it was given in your link to the NASA website:

When we designed and tested Ingenuity on Earth, we expected Ingenuity’s five-flight mission to be completed within the first few months after Perseverance’s landing in February 2021. We therefore prepared for flights at atmospheric densities between 0.0145 and 0.0185 kg/m3, which is equivalent to 1.2-1.5% of Earth’s atmospheric density at sea level. With Ingenuity in its sixth month of operation, however, we have entered a season where the densities in Jezero Crater are dropping to even lower levels. In the coming months we may see densities as low as 0.012 kg/m3 (1.0% of Earth’s density) during the afternoon hours that are preferable for flight.

The difference may seem small, but it has a significant impact on Ingenuity’s ability to fly. At our lower design limit for atmospheric density (0.0145 kg/m3), we know that Ingenuity has a thrust margin of at least 30%. Thrust margin refers to the excess thrust that Ingenuity can produce above and beyond what is required to hover. That additional thrust is needed on takeoffs and climbs, during maneuvers, and also when tracking terrain with varying height. But if the atmospheric density were to drop to 0.012 kg/m3 in the coming months, our helicopter’s thrust margin could drop to as low as 8%, which means that Ingenuity would be operating close to aerodynamic stall (a condition where further increases in the blade’s angle of attack does not produce more lift, only more drag).

We will begin by performing a high-speed spin of the rotor without leaving the ground, reaching a peak rotor speed of 2,800 rpm (more than a 10% increase relative to our prior Mars experience of 2,537 rpm). If all goes well, we will follow this with a short test flight at a slightly lower rotor speed of 2,700 rpm.

This is the test strategy for Mars. There is no information about the tests made on Earth before. No information about the maximum rotor speed used on Earth. There is no indication that all tests on Earth were done with speeds below 2,800 rpm or 2,700 rpm.

So ingenuity was designed and tested for flights at atmospheric densities between 0.0145 and 0.0185 kg/m3. It needs a thrust margin of at least 30 %. Without that margin, ingenuity might be able to hover some centimeters above ground, but the margin is needed to do a real takeoff and to climb at least some meters.

A helicopter rotor is exposed to very strong centrifugal forces. These forces increase with speed. The rotor is designed for a certain maximum speed. If you test it above this maximum speed it is a destructive test. Of course there should be a safety margin between theoretical maximum speed and nominal speed limit.

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    $\begingroup$ I think the OP focussed on this extract from NASA: "Thankfully, there is a way to tackle this issue – but it involves spinning the rotors even faster than we have been doing up to now. In fact, they will have to spin faster than we [NASA] have ever attempted with Ingenuity or any of our test helicopters on Earth". So, can the rotors spin faster, but at speeds not tested? If yes, what prevents NASA to test it then (before sending it to Mars), or now (on an engineering model). $\endgroup$
    – Ng Ph
    Sep 20 at 21:17
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    $\begingroup$ "So why didn't we test Ingenuity at 2800 rpm in a martian-like atmosphere before sending it to Mars?" Did they test at 2800 rpm or not? It looks like they didn't, so the answer is not "easy". $\endgroup$
    – uhoh
    Sep 21 at 0:56
  • $\begingroup$ @NgPh The answer is simple: Ingenuity is a low cost technology demonstrator. As a technology demonstrator, NASA and JPL cut lots of corners to keep the cost low (low by NASA standards). Ingenuity was planned to make just five flights during the first few months of the mission. If it had fallen short of this goal of five flights by a flight or two it probably would still have been deemed a success. It has made thirteen successful flights, far exceeding expectations. $\endgroup$ Sep 21 at 6:42
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    $\begingroup$ One of the corners that was cut was to design for the benign atmospheric conditions that were expected in the few months after landing. Designing and testing for the not so benign atmospheric conditions many months after landing would not have been consistent with the technology demonstrator nature of the project. Doing so would have increased the costs by quite a bit. $\endgroup$ Sep 21 at 6:46
  • $\begingroup$ So TL;DR : No tests were done at 2800 rpm. I figured that much out from the NASA article. But why weren't any tests done at 2800 rpm before launch? I guess the answer is in the comments. $\endgroup$
    – usernumber
    Sep 21 at 14:49

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