Blown Away by Jules Verne's Space Cannon G Forces

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Yesterday we celebrated the birth (in 1828) of legendary science fiction novelist Jules Verne, who penned such classic works as Twenty Thousand Leagues Under the Sea and From the Earth to the Moon.

The latter is one of the earliest depictions of space travel in popular fiction, featuring a capsule shot out of a cannon. It seems absurd by modern standards, but there's a hint of solid science underneath the fiction.

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Set a few years after the resolution of the American Civil War, the novel centers on the disaffected members of the Baltimore Gun Club. The men are bored and restless; there are no new artillery weapons to construct and test, nothing left to blow to smithereens.

So their leader, Impey Barbicane (!), proposes they build a gigantic cannon to fire a projectile to the moon. Ultimately the plucky Gun Club succeeds in launching not just a projectile, but three men into space.

Verne's notion of launching men to the moon with a cannon seems implausible at best, but some of the underlying principles are quite sound, given how little was known at the time.

Newton's third law of motion says that for every action there is an equal and opposite reaction. Firing a projectile from a cannon produces a powerful recoil force in the opposite direction, capable of propelling an object a considerable distance.

That's the principle behind jet propulsion. Rockets are subject to the same aerodynamic forces of weight (gravity), thrust, drag and lift.

Verne knew his capsule would need to counter inertia by generating sufficient thrust to overcome the pull of gravity. Once airborne, it would be subject to drag, which increases proportionally with acceleration, so greater and greater amounts of thrust would be needed to keep it in the air.

Hence his idea of using a cannon to fire a capsule into space. Verne even attempted some rough calculations for the cannon that were quite close to what would be required in reality.

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The biggest sticking point would have been the cannon's muzzle: In Verne's design, it was much too short. In order to achieve escape velocity, that short a muzzle would need to generate acceleration beyond what a human being could survive.

That's because generating that kind of power results in some pretty extreme G forces. "G" is a unit for measuring acceleration in terms of gravity, and G force describes the force experienced by an astronaut when the craft accelerates or decelerates rapidly.

For example, 1G is the force of Earth’s gravity: what Verne's "astronauts" would feel when the capsule is stationary or moving at a constant speed. Acceleration causes a corresponding increase in weight, so at 4 Gs, the travelers would experience a force equal to four times their respective weights.

At higher speeds, those G forces can be bone-shattering, as John F. Ptak points out:

Verne's 20,000-lb projectile to the Moon would sit in a cannon-hole in the Earth that was 280 meters deep with a diameter of 2.7 meters, which would sit on the bottom of the hole capping off  200 feet of guncotton (weighing 400,000 pounds!).  Somehow this mass would be ignited, and as Verne (or his brother) calculated, would produce an initial velocity of 12,000 yards per second, which is 36,000'/sec, or 129,000,000 mph, fps [NOTE: reflects correction sent by John Ptak] which is a big enough number to calculate against the speed of light, which is 670,616,629 mph, all this to attain the (more or less correct) escape velocity V of 11.2 km/sec. (Very high-velocity shells fired by tanks fitted for kinetic energy penetrator ammo attain a muzzle velocity of 5700 ft/sec.) And somewhere in there would be a crew greeting a rather-incredible-to-write-down 22,000 gs.

Ptak compares this to the G forces that race car drivers experience when they go from 0 to 100 mph in less than a second: around 5.4g. As for the space shuttle, NASA astronaut Koichi Wakata estimated the G forces would be about 1.7g at launch, increasing to just over 2g at the separation of the solid rocket boosters. For the last minute of the ascent, he estimated they experienced 3g.

In other words, modern-day astronauts, clad in specially designed space suits to better withstand G forces, can't endure anything close to 22,000 gs.

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But that's why it's called science fiction, right? Verne's novel inspired many young boys who went on to create the earliest rockets for the space program, most notably J. Robert Goddard and Werner von Braun. And his vision was immortalized in George Melies' silent film classic, La Voyage Dans La Lune:

Image credit: NASA

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