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The Kerbol density is set to 0. https://imgur.com/a/zJEqWU2

Often, you might want to take advantage of an extraterrestrial atmosphere by flying a plane or using parachutes to slow down a craft. This can be more or less effective depending on the conditions there: a parachute that lowers a capsule down to Earth safely will crash at high speed into Mars, while a propeller plane that fails during testing on Kerbin might actually work just fine in Eve's thicker atmosphere. In this sense, Eve is better for atmospheric flight, while Mars is worse. But by how much? I've seen this concept quantified before in specific instances, but not in the general case, and as far as I know, it has not been given a name. I propose a new metric to measure it: the flight effectiveness index. (If this name is already taken, or if the concept actually does already have a name I don't know about, I'd love to hear about it so that I can correct this post.) Unfortunately, air is complicated, and many simplifying assumptions are often made. One unfortunately common one is the lack of distinction between atmospheric density (how much mass the air in a given volume has) and atmospheric pressure (roughly proportional to how much energy the air in a given volume has). While clearly related, these concepts can end up being very different because of the fact that air molecules move at different speeds. KSP does maintain the distinction between pressure and density internally, but fails to convey it in situations where it is relevant, such as when parachutes open. They open at a minimum pressure, when perhaps a minimum density would be more consistent with the actual performance of the parachute. It's not just a problem in KSP: pressure and density are both measures for "how much air there is", so the distinction between them is not intuitive and can easily lead to confusion. Even xkcd, a webcomic now known for its very good scientific accuracy, at one point got pressure and density mixed up. (source: https://xkcd.com/620/) The 9% figure would be correct except for the fact that Titan's atmosphere is not 50% denser than Earth's; it has 50% more pressure (and consequently over four times the density because the air is so cold). The relationship between density and pressure can be derived from the ideal gas law. (The ideal gas law isn't perfect, but it's close enough under normal conditions.) PV = nRT, where P is the pressure, V is the volume, n is the number of moles of air, R is a constant, and T is the temperature. Wait, this equation doesn't have density in it! We need to relate the density to other things that are in the equation. The density ρ, by definition, is the mass m divided by the volume V. The total mass m is the molar mass μ times the number of moles n. Putting this all together, we find: PV = nRT P = (n / V) RT P = (m / (μV)) RT P = (ρ / μ) RT Pμ / RT = ρ R is a constant, so the density is proportional to the pressure times the molar mass divided by the temperature. The rest of the comic is basically accurate: the amount of mass held up by a given wing or parachute (at a given velocity) is proportional to the density divided by the surface gravity. (Explanation in the spoiler box for why this works.) This gives us our final definition for the flight effectiveness index: FEI = (Pμ) / (Tg), where P is the pressure (in kPa), μ is the mean molar mass (in amu = g/mol), T is the temperature (in K), and g is the local surface gravity (in m/s2). Importantly, this is dependent only on the local conditions, not on any characteristics of the vessel. (For maximum accuracy, the local surface gravity should include the effect of the body's rotation; that is, it should be reduced for rapidly spinning bodies. However, this does not make much of a difference unless the body is rotating quite quickly.) In KSP, the pressure, temperature, and local gravity can easily be obtained in the above units from the barometer, thermometer, and gravioli detector respectively. The molar mass is not obtainable in-game, but it is globally constant across each body. For stock atmospheres, it can be found on the body's KSP wiki page. For most modded atmospheres, it can be found in the body's .cfg file as atmosphereMolarMass (in kg/mol, which must be multiplied by 1000 to obtain it in g/mol). While this definition is based on SI units, it would be nice if there was an easy-to-remember "Earth/Kerbin standard" value. (Earth and Kerbin have basically identical conditions.) Fortunately, there is! Plugging in standard thermodynamic conditions, we find: P = 101.325 kPa (1 atm) T = 298.15 K (SATP standard) μ = 28.9644 amu (US standard for Earth; Kerbin global constant) g = 9.80665 m/s2 (1 g) FEI = (101.325 * 28.9644) / (298.15 * 9.80665) ≈ 1.00375, which is basically 1 (to within the actual variation on Earth of any of pressure, temperature, or even gravity). Of course, actual conditions on Earth and Kerbin vary considerably. In the thinner atmosphere at high altitude (0.8 atm, for example), the FEI falls to 0.8, meaning that a plane can only hold 0.8 times as much mass. In the cold polar weather (temperature: roughly 240 K), the FEI rises to 1.25. The flight effectiveness index can now be calculated on other celestial bodies. Let's take Eve as an example of a popular destination for planes. On the equator at sea level at noon, the conditions are: P = 506.625 kPa T = 423.7 K μ = 43 amu g = 16.67 m/s2 FEI = (506.625 * 43) / (423.7 / 16.67) ≈ 3.08. This means that a parachute can hold up 3.08 times as much mass as would be safe on Kerbin, and a propeller plane can carry 3.08 times the mass (including the mass of the plane, so you get a lot more than 3.08 times as much payload). While this definition is designed to be as easy as possible to calculate accurately, it still requires some work. Alternatively, I've included tables of typical FEI values on atmospheric planets and moons from a variety of systems. The FEI varies across the surface of any body, mainly due to variations in altitude (i.e. mountains, which you probably already knew to look out for) and temperature (which doesn't change all that much unless the body is a tidally locked planet). The given values are at the datum level at a latitude of 17 degrees, with a temperature averaged across all daily and seasonal variation. Stock system: Real Solar System: Outer Planets Mod: Galileo's Planet Pack: Grannus Expansion Pack: JNSQ: Whirligig World: Strange New Worlds: Edge of Eternity:

While exploring the ksp atmosphere, found out that it's important to know drag/lift/thrust profile for each height. So what really affects those profiles, pressure or density?