During my 35+ years as a NASA aerospace engineer I actually worked on several different Mars airplane and helicopter designs. I see that people here
are asking all the same questions I’ve heard a hundred times, so maybe I can answer all of them in one post.
First, the Martian atmosphere: The first successful Mars landers were the Viking missions in 1976. Since that time, NASA has landed the Pathfinder
mission, the MER mission, the Polar Lander mission, and the MSL mission. All of those missions carried miniaturized weather station instruments
(pressure, temperature, wind speed). Because of those in situ measurements, we have a pretty good atmospheric model for Mars (as far as density and
pressure are concerned). One important difference between Mars and Earth is the fact that the atmospheric density on Mars changes by about plus and
minus 15% every Mars year, because during Winter at the Martian south pole, a large amount of CO2 freezes out and falls to the ground as dry ice snow.
During southern Summer, it turns back to gas and builds up the atmosphere density again. That means that If you’re designing anything that depends
on the air density to operate (entry vehicles, parachutes, airplanes, helicopters) the performance of the thing you’re designing very much depends
on the time of Martian year that you arrive.
The Martian atmosphere model that NASA uses is called MARSGRAM and it can be downloaded from the internet by anyone who wants to use it. It is used
in the preliminary design of all NASA Mars missions to figure out where on Mars you can land, and with how much weight.
Another important factor in Mars mission designs is the location on Mars where you intend to land. The Southern hemisphere terrain is quite a bit
higher than the average level (referred to as the “zero datum” altitude) and the Northern hemisphere is lower than the zero datum. Since the air
density increases the lower in altitude you go, it is much easier to land Mars missions in the Northern hemisphere and that’s why all Mars landers
so far have been targeted to land there. The Mars 2020 mission that will carry the little helicopter is targeted to land in the Northern hemisphere
at the Jezero crater at an altitude about 2.5 kilometers below the zero datum shortly after the Northern Spring equinox. At that particular time and
place, the Martian atmospheric density will be about 1/50, or 2% of the Earth’s atmospheric density at sea level. If you were at a higher elevation
and a different time of the year, the air density on Mars might be only 1% of Earth’s sea level. That’s why you can’t use just a single number
for air density when thinking about Mars flight.
Notice that I have been talking about atmospheric density. That’s because the combination of air density and airspeed is what keeps airplanes and
helicopters flying. The composition of the atmosphere doesn’t really matter very much. To be more exact, it is a quantity called dynamic pressure
(often abbreviated as “q”) that creates lift and drag on a wing, a rotor, or an airframe. Mathematically, q is equal to one half the air density
multiplied by the airspeed squared. What that means is that—for a given aircraft—if the air density is lower, you have to fly faster to generate
the same amount of lift. But on Mars, where the gravity is only about .38 as much as it is on Earth, you don’t have to generate as much lift.
It might be helpful to consider a hypothetical example:
Suppose you had a 1975 Cessna 172 flying along at 2500 ft MSL on Earth. According to my operator’s manual that aircraft would weigh about 1225 kg
(2700 lb) and would require about 57 kW (76 hp) to cruise at around 49 m/s (109 mph). If you could instantaneously transport that Cessna to Mars, it
would only weigh about 465 kg (1025 lb) because of the lower gravity, but because of the lower atmospheric density, it would still have to fly about
4.65 times faster than it would on Earth, or about 228 m/s (510 mph). Because it would have to fly faster, it would also take about 77% more power to
fly on Mars—about 100 kW (134 hp). Of course, on Mars, because there is no Oxygen in the air, the power would have to be provided by an electric
motor or stored propellant.
Exactly the same principles apply to the operation of a helicopter rotor; a rotor is considered a “rotary wing”. At the time and location the
Mars mini-helicopter will operate, its rotors will have to turn about 4.65 times faster than they would on Earth and will consume about 77% more
power. I assume that’s why they couldn’t use standard RC helicopter motors, and had to make special motors to turn faster and consume more power.
Notice, that the requirement to fly a wing or to turn a rotor faster only works up to a certain point. The speed of sound on Mars is about 244 m/s
(546 mph), so that hypothetical Martian Cessna 172 would be flying around .93 Mach in order to generate enough lift. A C-172 wing couldn’t get
close to .93 Mach without shock waves forming all over it. For that reason, it is usually necessary to design a subsonic Mars airplane wing or Mars
rotor with more surface area than an equivalent Earth vehicle so that it doesn’t have to operate close to its Mach limit. The best way to do that
is usually to make the wing or rotor longer, since that will also improve the L/D ratio. That’s why most Mars airplanes tend to look more like a U-2
than a C-172.
One final point: it is often argued that since the highest a helicopter on Earth can fly is about 40,000 ft (with no payload) how could one possibly
fly in the thin air of Mars (which is equivalent to about 100,000 ft on Earth)? The answer is that a Mars helicopter only has to fly at the 100,000 ft
equivalent altitude (plus or minus a little) but in order for an Earth helicopter to fly at 100,000 ft, it has to first climb up to that altitude from
sea level. That means its rotor has to be able to deliver an amount of lift that’s greater than the weight of the vehicle at every condition
between zero and 100,000 ft. It is perfectly possible to design a helicopter that could fly on Earth at 100,000 ft (if anyone needed one) but it is
not clear that a rotor that could do that could also climb up to that altitude. On Mars, it doesn’t have to climb to that density altitude—it is
delivered there by a spacecraft.
a reply to: SpaceBoyOnEarth