Air transport breaks down

Question

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).

Answer 1

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.

Answer 2

Short answer for air travel
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.

Short answer for space travel
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:

1. Nuclear Pulse Propulsion
2. Laser Launch/Light Craft
3. Ram Accelerator
4. Light Gas Gun
5. Coilgun

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.

Answer 3

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.

Answer 4

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.