# Air transport breaks down

How much would the gravitational force (on earth) have to increase to make air-travel impractical (not impossible as escape by space flight to other planets would be a nice option). But to a point where the use of fixed winged aircraft becomes impractical as the energy required by propeller/jet engines becomes so costly (with current technology) that the amount required becomes impractical for commercial use.

If we know the gravitational force required to achieve the above. What would be the smallest asteroid strike to achieve this and could it be done with causing global collapse of civilization (maybe some local collapse). i.e strike at the center of Antarctica (melts ice but no dust cloud or tidal wave).

• The becomes impractical for commercial use is a quite undefined "definition". As a result of the decreasing utility laws, you will find that increasing prices will progressively move away more users, but up until the very end some applications may be worth enough to keep them until the very end. Which is the point? When local distance flights are replaced by roads/rail? When trasantlantic flight is only affordable to half its current users? or a tenth? – SJuan76 Apr 12 '15 at 19:48
• Is increasing gravity important? Or is stopping air travel the important part? Increasing gravity would be difficult without massive amount of handwavium, and would cause other problems like buildings falling down. An easier way would be a massive solar flare taking out GPS, computers and maybe messing with the magnetic field so even compasses would be unreliable. – AndyD273 Apr 12 '15 at 20:05
• Messing with the fuel supply might be good enough, at least if you are OK with continued use of lighter than air aircraft. – Doug McClean Apr 13 '15 at 3:01
• @AndyD273: Even taking out all those technical aids, it would be perfectly possible to fly VFR. I've done so for many years, just by looking at the terrain rather than navigation instruments. – jamesqf Apr 13 '15 at 4:08
• I'm not sure, but wouldn't reaching escape velocity with conventional (chemical) rockets become impractical before air travel does with increased gravity? – I'm with Monica Apr 13 '15 at 9:43

Commercial flight would be the least of concerns on Earth if the gravity rose that much.

Surely for current planes, where the rise would need to be of order of some 30% to even make "lightweight" flights (mail, passengers) not viable for airplanes that can lift 40+% of their mass as heavy cargo, the effects of such gravitational increase would be on extinction level of problems.

First, the atmospheric pressure would increase. Making flight easier, but suddenly oxygen concentration becomes dangerous to health. The climate is in ruin as water evaporation point would change - enormous droughs. Ecological disaster, as many airborne but less "powerful" species would lose ability to fly. Gas solubility in water would change, leading to much more acidic water with carbon dioxide dissolving easier. All satellites would crash. Winds would get much more dangerous - not only due to the climate change which would surely cause hurricanes, but simply because higher air pressure carries more energy at the same speed. Lots of chemical processes (probably including fuel combustion in car engines) would be affected. The shift of load on earth crust would lead to powerful earthquakes.

Seriously, if you want to get the airplanes out of the air, think of some less drastic way than increasing the planet gravity.

• A volcanic eruption which fills the air with ash globally would do the job. The ash cloud should be thin enough to not cause climate changes, but stay long enough in the air to hinder air traffic. – vsz Apr 13 '15 at 6:13
• @vsz Limited amounts of fine ash would ground the planes only shortly, until engines are upgraded to hold the ash back. More heavier ash would again be an extinction level event, "nuclear winter" style. – SF. Apr 13 '15 at 6:44
• @SF. Holding the ash back is not as simple as you seem to think....you've got air entering the engines at 500 mph... – Tim B Apr 13 '15 at 9:08
• @TimB: The finer particles will just get through, marginally reducing engine efficiency - it's not like military airplanes can't fly through clouds of dust (also, these with propellers and "classic" internal combustion engines do have normal air filters.) Only particles big enough or dense enough to clog up / damage the engine are a problem - and if we have these worldwide, we're in much more trouble than just "air transport crippled". – SF. Apr 13 '15 at 10:29
• @SF. the problem when the Icelandic volcano put large quantities of dust in the atmosphere, grounding flights, was that the tiny particles caused ablation effects on the engines, thus any plane that flew trough it would fly ok, until the damage caused to the very-high-precision blades in the jets degraded to the point where they would fail. Obviously no airline would allow this, but neither would they want the bill for repairs. (also, tanks in the Iraqi desert found that the very fine dust kicked up would clog air filters to the engines very quickly - small particles are still a problem) – gbjbaanb Apr 13 '15 at 12:14

The practicality does not depend upon the mass or gravity of a body. It is the ratio of surface gravity to air density that makes it practical or impractical.

As long as the ratio of gravitational acceleration to air density remains constant, air travel remains practical.

Increase surface gravity while keeping air density constant will eventually make air travel impractical. When it becomes impractical depends upon how efficient your engines are and what you deem to be the minimum sized payload worth your while.

If you doubled planetary gravitation while keeping air density constant, commercial air travel would be impractical (though some special planes might still be possible). For example, take the numbers from a Boeing 747. If you doubled gravitational acceleration, the aircraft could take off if it was empty but it could carry no cargo.

Take off gross weight: 333,390 kg
1 g empty weight:      162,400 kg
2 g empty weight:      324,800 kg ~ 333,390 kg


In order to double the Earth's gravity, you'd have to double the mass of the planet while keeping the radius the same OR keep the Earth's mass the same and decrease its radius to 70% of current.

It would take the collision of two bodies of mass of Earth or larger to do it and that would liquify the Earth - No survivors. Plus the atmosphere and hydrosphere would be permanently lost.

There is no scenario that I can envision that could do this and leave any survivors.

IMO, chemical rockets are on the verge of being impractical now. Even with staging (which makes the performance better) they aren't widely used now except as a specialty transportation mechanism for very high value transportation.

So if you doubled surface gravity, your only practical method of space launch might be one of these that I documented on The Case for Space section of my blog:

Basically only engines with very high specific power (e.g. nuclear bombs) or don't have to carry their propellant would work for space launch for a 2g planet - air density doesn't affect this much except to make it more difficult.

Lift - Weight
From a first order analysis, lift is the force required to lift the aircraft off the ground. Lift must equal the mass of the aircraft in order to lift off.

$$L = \frac{m_aM_pG}{r^2} \rightarrow L = m_a a_p$$

$m_a$ - mass air vehicle
$M_p$ - Mass of planet
$a_p$ - Planet's gravitational acceleration G - Universal gravitational constant
r - radius of the surface of the planet

The lift equation is:

$$L = \frac {1}{2} C_L \rho V^2$$

L - Lift force
$C_L$ - Coefficient of lift (dependent upon aircraft & wing shape)
$\rho$ - Density of air
$V^2$ - Velocity of vehicle squared

So putting them together we get:

$$m_a a_p = \frac {1}{2} C_L \rho V^2 \rightarrow a_p = \rho \frac {C_LV^2}{2m_a}$$

Simplifying we get

$$\frac{a_p}{\rho} = \frac {C_LV^2}{2m_a}$$

This equation shows that $C_L$, V, $m_a$ remain constant if the ratio of $\frac{a_p}{\rho}$ remains constant.

Drag - Thrust
In addition to weight issues, you must also pay a drag penalty.

The drag equation is identical to the lift equation but uses a different constant. You can approximate the drag coefficient as 1/10 of the lift equation.

$C_D$ ~ $\frac{C_L}{10}$

So

$$D = \frac{1}{20}C_L \rho V^2$$

The turbine engine thrust equation is:

$$D = T = \left(\dot{m_a} + \dot{m_f} \right)v_e - \dot{m_a}v_i$$

$\dot{m_a}$ - Mass flow rate of air, which can also be expressed as $\dot{m_a} = \rho A v$
$\dot{m_f}$ - Mass flow rate of fuel
$v_e$ - Engine exhaust velocity
$v_i$ - Velocity of air at the inlet (when multiplied by $\dot{m_a}$, this is also known as ram pressure
A - Area at the inlet or exhaust (depending upon where you're doing the calculation)

But it is usually approximated with the following (the fuel's contribution to thrust is mostly through heating):

$$D = \dot{m_a} \left(v_e - v_i \right)$$

I am not going to go through all the gyrations to do this exactly. I'm assuming the inlet and exhaust are the same size (they almost never are) but I simply want the feel of the equations and for this purpose it works.

$$D = \rho A v \left(v_e - v_i \right)$$

Combining with the Drag equation and we get

$$\rho A \left(v_e^2 - v_i^2 \right) = \frac{1}{10}\frac{1}{2}C_L \rho V^2 \rightarrow \frac{1}{10} m_a a_p = \rho A \left(v_e^2 - v_i^2 \right)$$

Substituting in the Lift equation equivalence to aircraft mass times surface gravity, I get: $$\frac{a_p}{\rho} = 10 \frac{A \left(v_e^2 - v_i^2 \right)}{m_a}$$

Anyway long story short, it looks like its Drag remains the same as long as the ratio of surface gravity to atmospheric density remains constant.

• This leads to a better alternative idea: greatly reduced atmospheric pressure/density. If the sea level pressure were reduced to what's presently the pressure at 8,000' or 10,000', say 21 inHg, that wouldn't be enough to keep us from flying but it would force payloads to be much smaller and for commercial flight to be generally less practical. There are some commercial airports that high, but operations to them are more complicated. The ecological side effects would be dramatic, though. – Doug McClean Apr 13 '15 at 3:06
• @DougMcClean: It would also suffocate everyone - airplanes can fly at altitudes where humans die from lack of air. You might increase oxygen content to offset it (humans can survive in ~30% atmospheric pressure but breathing nearly pure air) but this would lead to another slew of problems, flash fires burning tar off the roads and setting steel on fire being quite common. – SF. Apr 13 '15 at 14:06
• Absolutely they can fly way higher than we can live. Taking off and landing is a different story. Taking off and landing gets very impractical at similar altitudes to where living requires lots of work but is still doable. Taking off and landing carrying heavy payloads even moreso. When airplanes fly at altitudes where humans die (quickly) from lack of air, they do so at airspeeds that are much too high for landing. – Doug McClean Apr 13 '15 at 14:46
• @DougMcClean: I believe the speeds necessary for start and landing are directly proportional to air density. 50% pressure drop = double the starting speed. Still within range. (they fly so fast at these altitudes to reduce travel time and fuel usage; they could fly much slower - not as slow as at sea level but still not much faster than that - they just don't, because - what for?) – SF. Apr 13 '15 at 17:45
• @DougMcClean, I'm not trying to make an argument here and I don't disagree with most of the things you said, I just corrected your misconception about a $\rho$ being directly proportional to v. But if we're picking nits, C-130s don't carry tanks. In the US only C-17 and C-5 can. – Jim2B Apr 14 '15 at 1:06

A better way to prevent general air travel is not so much to make it physically implausible, but to make it impractically costly.

When the Icelandic volcano erupted and delivered a lot of dust into the atmosphere flights were grounded for ages. The dust was not so bad that it affected people of the ground - I recall my car had a layer of dust over it so I was obviously breathing it in and I never noticed anything untoward, but the dust would have affected planes flying through it at speed, the dust would sucked into the very high-precision jet engines and would damage them, if not causing them to fail eventually, landing the airline with a huge bill for repairs. Note that some planes did fly, particularly turboprop ones that were used to measure the density of ash in the atmosphere.

Incredibly fine dust will clog air filters, particularly if they have a lot of air sucked into them, in a way that will not affect something that works slower like your lungs.

So put something in the air that is not good for high-pressure or high-speed machinery. Pollen, dust, pollution will all do.

As others have pointed out, changing gravity is not really going to bother air travel much, and is going to create major problems elsewhere. As for the idea of an asteroid strike changing gravity... Well, any asteroid big enough to change Earth's gravity enough to be measurable even by sensitive instruments is going to turn the Earth into a ball of molten magma.

If you want an idea of what modern society would be like without air travel, remember that we have a real-life example: the days after the 9/11 attacks, when commercial air travel was shut down in the US. Research that, and extrapolate.

• I don't really see what this adds to earlier answers in terms of actually answering the question. The first paragraph does address the question as asked but has mostly been covered by the earlier answers; the second paragraph is about the after-effects rather than the cause. Could you edit to improve, perhaps by adding something that has not already been said elsewhere in response to this question? – a CVn Apr 13 '15 at 14:12
• @Michael Kjörling: What do you think the question is, how to cause air travel to not be available, or the effects of not having air travel? And what have I said here that has been said elsewhere? (Other than my agreement with previous answers re gravity not being a problem.) – jamesqf Apr 13 '15 at 16:51
• Well, the OP wrote in the question "How much would the gravitational force (on earth) have to increase to make air-travel impractical" and "What would be the smallest asteroid strike to achieve this". Seems clear enough to me that those questions focus on how to make air travel impractical, rather than what the effects would be on a modern society once air travel has become impractical. – a CVn Apr 13 '15 at 17:54
• You make two main points that address the question as asked, as I see it, and those are both in your first paragraph. You yourself start out by saying "As others have pointed out" (that is right at the beginning of this answer) and Jim2B summarized the reasoning about the asteroid strike by concluding "There is no scenario that I can envision that could do this and leave any survivors." Just to be absolutely clear: I am not saying you can't post an answer of your own, I am simply suggesting that you write it in such a way that it adds something significant over answers previously posted. – a CVn Apr 13 '15 at 17:54