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Comment by simion314 | Hacker News Reader

We will use more helicopters instead of rovers for new missions ? Or larger crafts would be much harder or riskier to use ?

Or maybe have a helicopter that can move the rover with the equipment to different locations.

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Surfacing a comment: Here's a paper that describes what the next gen could/should be. The lead author is the head of Mars heli, IIRC.

https://ieeexplore.ieee.org/abstract/document/9843501

In short, future designs target ~30kg heli, 5kg payloads. Other designs by collaborators are closer to 20kg. It's probably possible to transport a few of these on the existing lander technology, which would be awesome.

The scholar.google.com keywords you want are "Mars Science Helicopter" and a good touchpoint author is T. Tzanetos or S. Withrow-Maser

> Other designs by collaborators are closer to 20kg. It's probably possible to transport a few of these on the existing lander technology, which would be awesome.

Actually it could be like 50 of them. Plus some ground robots to put together solar farm. And wooh... we get the first extra terrestrial permanent base

Well size is the limiting factor for fliers, since they like to have broad surfaces with low weight. But I think you're referring to some, ahem,

untested possible landing vehicles ...

in which case, yeah, you have a lot of robots.

For the solar farm assembly case, It's actually a lot easier to have a teleoperated robot doing the work, a few astronauts in orbit doing the operation / construction. In the case of building things, you want as much space / weight landed to be the thing being built, not the builders, per se.

Non-paywalled article about the concept, including renderings of the hexacopter design:

https://spectrum.ieee.org/the-next-mars-helicopter

Ingenuity was just a technology demonstrator. I think it demonstrated the technology splendidly, so we are likely to see more helicopters on Mars in the future.

Not sure if Nasa has said yet which roles they see for future Mars helicopters. The initial idea behind Ingenuity was to use them as scouting vehicles for rovers. Of course rovers improved a lot too, with better autonomous driving. But with a Mars rover driving about 100 meters/yards per day scouting helicopters are still useful.

Maybe we will also see Helicopters carrying more instruments themselves. But I imagine in the beginning that's mostly better imaging instruments. Weight is still an issue for flying things, no matter the planet. But maybe we will see some future missions that instead of a car-sized rover and one tiny helicopter have a fleet of helicopters with a small support-rover for exploring wider areas.

The existing helicopter is extremely small and light, IIRC. less than one kg. So it definitely won't be picking up a 900kg rover, even if you tried to scale it up somehow. The atmosphere is just too thin to support anything but a minimal payload.

But yeah having more helicopters might be feasible - for surveying the surface.

I was surprised to learn that it’s actually a fair bit heavier. I was lucky enough to get to attend a talk by the head of the Ingenuity program, and he mentioned how the mass ballooned a bit to something under 5 pounds.

(Listed as 4 pounds on this official fact sheet) https://mars.nasa.gov/files/mars2020/MarsHelicopterIngenuity...

Is there a problem of scaling this up to say a 20kg payload?

I’m not an aeronautical engineer, so I guess what I’m asking is if there is some problem scaling up flying machines in an extremely thin atmosphere?

One issue might be rotor span. Ingenuity has pretty big rotors to counter the thin atmosphere (about 4 feet top-to-tip).

On earth rotor sizes are limited by the speed at the wing-tip. Once you make the rotor too long the tips start approaching supersonic speeds, giving you all kinds of weird mach effects. To make matters worse, the speed of sound is about 30% lower on Mars compared to near earth's surface.

Yea good point. Apparently the blade tip speed on Ingenuity is Mach 0.6-0.7!

Interesting note about this: the speed of sound on Mars is only ~70% of that on Earth, due to less atmospheric density. Might change your Mach numbers!

It's not due to less density, but rather a different gas composition (CO2 vs. N2+O2).

Let's run the numbers!

The speed of sound in an ideal (calorically perfect) gas is given by

  a = sqrt( gamma * R * T )

where gamma is the ratio of specific heats (thermodynamic property of a gas, which may vary with temperature), R is the individual gas constant, and T the temperature of the fluid. All of these are going to be different on Mars versus on Earth:

  Earth:
  R = R_atm = 287 J / (kg * K)
  gamma = 1.44
  T = 293 K (taking room temperature as an average temperature)

  Mars:
  R = R_CO2 = 188 J / (kg * K)
  gamma = 1.37
  T = 210 K (from a quick google, about -60 deg C)

If the Martian and Earth atmospheres were at the same temperature, then the speed of sound on Mars would be 80% that of the speed of sound on Earth. Given the temperature difference, the speeds of sound are

  a_mars = 232 m/s
  a_earth = 347 m/s

So yes, much of the difference is due to the composition: the Martian atmosphere has a higher atomic weight, which leads to a lower individual gas constant, and decreases the speed of sound. However, a substantial amount of the difference is simply due to the different temperatures on the surfaces of the two planets.

I included it

Oh interesting! I can see how that would be a harder problem (although not insurmountable since some planes on earth have supersonic propellers).

Just a quick edit - wow, u didn’t realize the span was already 4ft! Anything much larger could definitely be hard to pack inside a fairing!

What about multiple smaller rotors? Or would that cause weird turbulence effects? Could we use some kind of jet engine?

Here's a paper that describes what the next gen could/should be. The lead author is the head of Mars heli, IIRC.

https://ieeexplore.ieee.org/abstract/document/9843501

In short: 30kg heli, 5kg payloads. Other designs by collaborators are closer to 20kg. It's probably possible to transport a few of these on the existing lander technology, which would be awesome.

The scholar.google.com keywords you want are "Mars Science Helicopter" and a good touchpoint author is T. Tzanetos or S. Withrow-Maser

Ames and JPL were still collaborating on this when I worked there.

I don't know the answer to your question, but for context here are the weights of Mars rovers:

Sojourner (1997): 11 kg

Spirit & Opportunity (2004): 185 kg

Curiosity (2011): 899 kg

Perseverance (2020): 1,025 kg

I know these are much larger - I’m just really curious about the dynamics of scaling up rotorcraft & why it is problematic. ie - do rotor physics become impracticality large or fast at some point for materials science, or is it purely a space problem for rocket launches.

Oh for sure, it's a great question.

I have led efforts building Chinook style tandem rotors with 2 sets of blades from a size like the Trex 800, powered by a 2 stroke engine, as well as 40kg max takeoff quad-planes with both electric quadrotor and 2 stroke engine (for forward motion).

But because I was the main lead and pushing the pace so fast, I wished I did it with a more rigorous aero-engineering to it. I started both projects with barely any experience developing aircraft.

Thinking about your question, here are my 2 cents:

The biggest thing I stugged with is how the vibrations and the accompanying harmonics on the sytem as the rotors spin up and down. I could see it on the logs as the rotors spin through certain Hz, there's would spikes in virbational ampiltudes at predictable frequencies. As the blades get bigger the forces (probably) goes up. Sometimes, these frequecies (especially the lower ones) are at the range where its very hard to find the right materials to damp it out of the control and sensing electronics. Ingenuity probably deals with a virbration range that well into the hundreds/thousand of Hz and I do remember that renge is not a difficult range to damp out, vis a vis the low tens of hertz.

Also, the harmonics is related to ground resonance. I had built my tandem with "skids" that are rigidly attached to the rest of the frame. When the system made contact with the ground on just one skid, that one skid becomes a pivot, the vibration has no where to go and I witnessed first hand, first time, what ground resonance can do to mechanical systems. I can never forget seeing M4 through to M8 hex holt beads being sheared clean off after the resonance event. Only later did I find out that in full scale systems, they have dampeners between the main body and the skids of the aircraft. See https://m.youtube.com/watch?v=IIC-oBzLYhQ ;

Staying in flight is not as hard. But getting the ssytem to land and spin down proerly was big pain without understanding ground resonance and its effect on mechanical design. When I saw the little puny legs of Ingenuity, this experience of mine came into mind and I was glad they had legs like to damp out vibrations as it came down to land.

Then there is the relation between the mechanical vibration regimes of the system, the polilng rates of the foundational flight sensors and the freqency of the main flight stability and movement control loop itself.

With bigger systems, the cables (for signal and power) could run longer too (becoming long long antennas), which means you can run into problems with noise of various origins. If I'd do it again, something like CAN bus would probably be something I look at seriously. Bigger systems also draws more power, and that can have an impact on how much management is needed for noise. Bigger power draw usually also means heavy power store & delivery system, which affects CG management, when then means you can't move things around to management noise. At some point, I felt like I was doing dancing a multi-factorial show.

I wished I could be clearer. Perhaps someone more qualified can chime in.

These must be the masses of the Mars rovers, the weights would be measured in newtons (or pounds in the USA) and would differ between mars and earth.

In a thin atmosphere, lifting a heavier payload needs bigger rotors or increased RPM, which increases power demands and structural stress. The challenge is to keep the vehicle light enough to fly while also making it sturdy enough to carry the payload and survive environments.

Why can’t “more rotors” be another solution?

For sure, that’s another way to decompose “bigger rotors”. It would probably be appropriate to dive into a conversation about Ingenuity’s design goals, requirements, and the trades performed to end up with what they got.

This is what I'm thinking too. A number of posts above talked about physical limits based on speed of sound and rotor length. Cool, so add two rotors, or four.

Ok - this makes sense to me. Also taken into account the context that anything going to space needs to be light at this point in time. Hopefully we don’t have that restriction forever :)

It scales up to 20kg, yes.

I think the current plan is that helicopters will be very light with minimal instrumentation and will be used mostly to scout ahead for rovers. The rovers will be much heavier and include many instruments.

(Of course, all of NASA's long-term plans for Mars would be completely disrupted if Starship lowers the cost-per-kg of delivering equipment by two orders of magnitude, which arguably is likely.)

Making it more economical to escape Earth's gravity well isn't going to alter the physics of the Martian atmosphere or the relative utility of copters vs rovers for Martian exploration. Which is to say, even if you stationed humans on Mars, they'd still be exploring remotely via copter/rover pairs, just no longer with a human-to-robot latency measured in tens of minutes.

However we might be willing to drive a lot more agressively if we know we can get a mechanic out to a stuck rover. Similarly, cheaper delivery might make a large number of smaller more disposable vehicles more appealing for many missions, just like what happened to satellites in the last decade.

If you increase the mass and/or number of vehicles by 100x, a bajillion things will change.

A heli that can move a rover is basically worse than a heli that has rover instruments and a few wheels. You have extra weight and parts and complexity for hitching and carrying that you can just avoid by giving a small rover flying ability.

Even the combo is probably too much complexity. A heli with good imagers, spectrometers, and the ability to cart soil samples would be fantastic.

That's the goal, yes. Depending on the destination, naturally. Here's what's planned for Titan: https://en.wikipedia.org/wiki/Dragonfly_(spacecraft)

Was going to post this.

Titan is such a wonderful place for a nuclear powered helicopter. Much better than rover/submarines/floaters, IMHO. A balloon would also have been excellent, but the extra mobility from helis is going to be amazing.

Minimizing moving parts, so much as possible, when dealing with hardware tens of millions of miles away, let alone with a 13 minute delay in 1-way messaging, is generally smart. And Mars' atmosphere is so thin that these rovers will never be moving any meaningful payload, so their only real use case is as a scouting type system. But they also add very little value there given the existence of orbiting satellites. Even the Mars Recon Orbiter (from 2005) captures images in the < 1m resolution range.

IMO NASA wanted to try to deal with the sort of 'oh boy... another rover' fatigue and saw the drone as a way to spice things up with some passable science arguments behind it, and a relatively minimal cost. Further supporting this is that the helicopter wasn't an initial part of the plan - it was strapped on at the 'last minute', speaking in government time. In any case, I would comfortably wager against us seeing more drones in future missions, at least to Mars.

In the shorter term, I see the helicopter as making the rover more capable, by finding routes and destinations of interest. And the rover makes the helicopter more capable by providing a recharging station. So they're both at their best when working as a system. Maybe a rover can support multiple choppers.

> the rover makes the helicopter more capable by providing a recharging station

Does it? I thought the helicopter was just solar powered.

That's cool. Thinking about it, there must be a tradeoff. The rover could support more massive batteries, but then "docking" between the two machines would have to be utterly reliable.

Why not some helium balloon type craft that could float along with low power for longer periods of times? Could cover vast distances? Descend into fertile plains looking for samples?

Helium's lifting capability is proportional to the density of the atmosphere, which is very low on Mars.

Is it viable to use vacuum instead of helium?

Wouldn't you need something with some amount of pressure to stop the "shell" you plan to float from collapsing?

What if we use hydrogen, which is halfway between helium and vacuum?

Hydrogen it is.

Hydrogen leaks.

IIRC as a result of Ingenuity's success, one of the proposals for the Mars sample return mission architecture involved several helicopters to retrieve Perseverance's sample canisters (it drops them as it goes along, so that there's no worry about how to get the samples out of the rover in the future).

I should add though that the prospects of the parasites in Congress properly funding such a complex mission seem pretty low for now.

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