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I started getting curious about this question during my Flight Test Engineering class. My professor was talking about a part of the International Standard Atmosphere that is a similar pressure to that of Mars's surface. So I was wondering how a supersonic aircraft would perform in such a colder, emptier atmosphere. On one hand, because Mars's atmosphere is less dense than Earth's, there would be less pressure drag to slow it down, and the lower gravity could reduce the amount of lift the aircraft needs to fly. Hence, an aircraft would be able to fly faster and more efficiently than it could on Earth. But on the other hand, a lower temperature and a lower density would mean that the speed of sound is lower on Mars than it is on Earth. Thus supersonic effects, like wave drag and shockwaves, would start to form at lower speeds than they do on Earth; which could possibly negate the benefits of flying in a lower pressure environment.

So, assuming it is a rocket propelled spaceplane to neglect the effects on the propulsion system, how well would a supersonic aircraft perform when it is flying in the Martian atmosphere compared to Earth's? And how would it be possible to optimize such an aircraft for Martian supersonic flight?

Edit: I also posted this question onto r/AerospaceEngineering earlier in case anyone is interested in some of the answers there too.

Mattias
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    Welcome, Mattias. Flying at supersonic speed over Mars would be fun... until it was time to land. The low air density means high stall speed. Really high. Existing Martian runways would be too short for you, so you would need to "land" by returning to orbit. – Woody Jan 18 '22 at 02:33
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    @Woody "Existing Martian runways" ... are there many? – Organic Marble Jan 18 '22 at 02:38
  • @Woody Well, I guess its a good thing that retrorockets exist! – Mattias Jan 18 '22 at 02:42
  • @OrganicMarble ---- Not sure how many, but all the ones I've landed on would be too short. – Woody Jan 18 '22 at 03:30
  • Related: https://space.stackexchange.com/questions/19124/what-would-be-a-few-challenges-to-build-a-landing-strip-on-mars/19129#19129 – GdD Jan 18 '22 at 09:00
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    You may be misunderstanding stall speed @Woody. Airspeed is a measure of dynamic pressure, I think what you are trying to say is that because of the low air pressure the ground speed at which the airplane would need for approach and landing is extremely high. – GdD Jan 18 '22 at 10:20
  • I’m voting to close this question because it's about flight in an atmosphere and would be better suited to Aviation.SE. – GdD Jan 18 '22 at 10:21
  • @GdD, Re, "stall speed / ground speed," the traditional way to land a fixed-wing airplane is to stall the wings when the wheels are just inches above the runway. One can also just fly it on to the runway (Don't ask me how I know!!), but the ground speed at touchdown will be higher that way, and depending on the length of the runway and/or the strength of the landing gear, the outcome could be less than desirable. – Solomon Slow Jan 18 '22 at 17:45
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    @SolomonSlow, that's true for many light airplanes but by no means all of them, and I've flown both types. You certainly do not stall a jetliner or fighter onto the runway, although you do land close to stall speed. – GdD Jan 18 '22 at 19:30
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    @GdD I think aviation SE is Earth-centric, but not sure. Possibly the question would be accepted there. On the Space SE, aerodynamics is not off-topic. Afaik lift increases quadratically by the velocity in rare atmosphere. So I think, maybe a 13 times higher velocity would be needed in the 160 times more rare Martian atmosphere. The typical landing speed is some hundred km/h on the Earth, so maybe 1-2 km/s landing speed would be required on the Mars, which is unimaginable today on any land vehicle. However, plane optimized to the Mars might have a much lesser landing speed. – peterh Jan 18 '22 at 22:24
  • @GdD Some fighter aircrafts are vertical landing capable, particularly USA loves to burn ten billions for such projects. What is with the opposite of what the F-35 can do? Instead short takeoff-vertical landing, what about a short takeoff-short landing? I am thinking on some solution which turns the engines down to allow stable, controlled flight much below stall speed. – peterh Jan 18 '22 at 22:28
  • @GdD Btw, another problem what I can see: Martian atmosphere is mostly carbon dioxide, it is totally unusable as oxidizer. Thus, beside the fuel, also the oxidizer must be provided by the plane itself. However, the atmosphere can be still used for aerodynamical stabilization and as ballast mass. I am thinking on a turbofan where both fuel (some hydrocarbon) and oxidizer (liquid O2 or hydrogen-peroxide) is injected into the engine, but it sucks the carbon dioxide from the atmosphere and emits a CO2 + H2O mixture. It could be lesser efficient than turbofans, but still much better than rockets. – peterh Jan 18 '22 at 22:39
  • @peterh That's one of the reasons why I was assuming it's a rocket propelled spaceplane. I was mostly focusing on the aerodynamics aspect of the spaceplane rather than the propulsion system. If it's rocket propelled, it doesn't need a breathable atmosphere to break Mach 1. Plus, assuming a colony has a source of water to work with, it's possible to use the atmosphere to generate methane as a fuel for methalox engines. – Mattias Jan 21 '22 at 17:29
  • @Matthias Drives generate force by accelerating things backwardly. In rockets, these things are the combustion products of the fuel + oxidizer. However, E=1/2mv^2, but I=mv. If you accelerate not only the combustion products, but also what you have, you get more force. For example, quadrupling the mass halves the exhaust speed (if the energy is the same), but with 4x more mass you end up with double impulse (force). Rockets can not do that because they work in vacuum. – peterh Jan 23 '22 at 10:23
  • There is a Russian project about an Earth aircraft capable to fly in the mesosphere: https://en.wikipedia.org/wiki/Ayaks Probably it is in a really early phase, and doing it without oxidizer in the atmosphere is likely much harder. It will likely need to be supersonic, even if it is possible. Now I think, it must be really a rocket. – peterh Jan 24 '22 at 04:09

1 Answers1

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There are a few factors to consider here:

  1. The air is thinner,
  2. The speed of sound is different,
  3. The gravity is lower,
  4. The aircraft will not be able to "breathe" oxygen in the atmosphere, and
  5. Weather

The speed of sound can be estimated using the formula for the speed of sound in an ideal gas on Wikipedia's page on the speed of sound

$$v = \sqrt{\gamma k T / m} $$ where

$\gamma$ is 7/5 for diatomic gasses (a co-linear triatomic gas such as CO2 is equivalent to a diatomic gas for our purposes here),

$k$ is the Boltzmann constant (1.38e-23 // J/K).

$T$ is the temperature in Kelvin, and

$m$ is the mass of a molecule of the gas (7.311e-26 kg).

So, if the temperature is 273.3 K (0˚C), then the speed of sound will be roughly 269 m/s, or 968 km/hr. If the temperature is 210.3 K (-63˚C) then the speed of sound is 236 m/s (849 km/hr). (Note: [A NASA article][2] reports the speed of sound at -63C on Mars as being 240 m/s and [another source][3] says that it varies with pitch due to the way the carbon molecule vibrates. On Earth, the speed of sound is 343 m/s, which is only around 30% faster. The assumption that an aircraft would have to travel at supersonic speeds to be practical may be incorrect.

On the plus side, since there's 1/3rd the gravity on Mars, the plane's wings will not need to generate as much lift. Because the weather on Mars is more benign, the aircraft probably doesn't need to be over-engineered as much as on Earth, so it can be made lighter.

On the minus side, the energy density of the plane's propulsion system will be lower because it will need to carry its oxygen supply. Dust storms might be too much of a challenge for navigation systems, in which case flights might be suspended for months at a time. Wings will need to be bigger and runways will need to be longer.

Austin Meyer, author of X-Plane, wrote [an article][4] about his efforts to simulate the physics of the Mars atmosphere in X-Plane. Some of his observations were:

  • Take-off in a well-designed airplane can occur at 400 knots (206 m/s) which is close to, but still below, the speed of sound on Mars
  • While inertia remains the same, lift diminishes, so it becomes more difficult to turn or flare
  • You would likely need arresting gear on runways to stop, since parachutes, breaks, and reverse thrust will not work well on Mars

To summarize, it will probably be harder to engineer airplanes (and runways) for Mars and they will not be as nimble as airplanes on Earth. However, it seems likely that we will be able to engineer airplanes for Mars that will work well enough to be useful for travel. If the cost of fuel (or electricity) were the same as on Earth, then they are likely to perform better on the "Cost per Available Seat Kilometer" metric due to the thinner atmosphere, lower gravity, and more benign weather on Mars.

[2]: https://mars.nasa.gov/mars2020/participate/sounds/#:~:text=Speed%20of%20Sound,meters%20per%20second)%20on%20Earth [3]:

[4]: https://www.x-plane.com/adventures/mars.html
phil1008
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    There should be a mistake in speed of sound calculation. NASA tells about 240 m/s https://mars.nasa.gov/mars2020/participate/sounds/#:~:text=Speed%20of%20Sound,meters%20per%20second)%20on%20Earth. Also, carbon dioxyde is 3-atomic molecule. – Heopps Dec 27 '23 at 09:36
  • According to the Wikipedia page, "a co-linear triatomic gas such as CO2 is equivalent to a diatomic gas for our purposes here". The temperature that NASA used is -63C, but that doesn't account for the difference. Not sure yet what the reason is for the large discrepancy... – phil1008 Dec 27 '23 at 19:44
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    Somehow, I had the wrong number for the mass of a CO2 molecule. Thanks for catching my mistake! – phil1008 Dec 27 '23 at 21:51
  • While the speed of sound is highly relevant, the question is asking how the aircraft would perform, adding some stuff from here: https://www.x-plane.com/adventures/mars.html (or playing with it yourself if you have software) would enhance the answer. – GremlinWranger Dec 27 '23 at 23:27
  • @GremlinWranger Thanks - that was a very relevant article! – phil1008 Dec 28 '23 at 00:38