When the Apollo missions were sent to the moon, we used rocket technology. When Verne wrote about a voyage to the moon in “From the Earth to the Moon,” his characters used an extremely large cannon to escape the earth’s surface. Why do you think we used rockets instead of a cannon to make the actual journey? Could we have used a large cannon?

Quick answer:

Using a cannon to propel something into space and eventually to the moon is theoretically possible, but it comes with a host of problems.

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Let's start by giving Jules Verne a bunch of credit for writing From the Earth to the Moon, because his fictional story was scientifically accurate in quite a few ways. Keep in mind that he wrote the book thirty-eight years before the Wright Brothers achieved powered flight. Verne was correct...

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that mankind could reach the moon, and his chosen launch site in the book is Florida. That's where the United States also settled on basing the space program.

Verne's choice for a spacecraft-launching mechanism was a cannon. We use liquid fueled rockets today and for the various missions that took place during the Space Race. This question asks about whether or not we could have sent something to moon using a cannon. That is conceivably possible, but there are quite a few problems with the mechanism as a whole.

Using one massive charge to accelerate the space capsule is going to create so much force in such a quick and immediate fashion that any humans on board are going to be killed from the massive G forces that are instantly applied to their bodies. You could use multiple charges along the entire length of the barrel to more slowly accelerate the craft; however, there are far larger problems to overcome. Once the spacecraft exits the barrel of the cannon, it is unlikely to be able to make any kind of course corrections. This means that the capsule must exit the barrel at the precise moment when the earth and moon are in the exact positions to allow the bullet that is the capsule to hit the moon.

Another problem deals with basic bullet mechanics. Early firearms were not very accurate, because the ball/bullet being fired was simply shoved out of the gun. A sphere flying through the air will "knuckle," as many soccer, baseball, and volleyball players can attest to. To correct bullet knuckling, gun barrels are "rifled." This means that the bullet is spun in order to get the projectile to fly straight. Doing this to a capsule with humans in it will likely cause the humans to quickly lose consciousness.

Finally, once the capsule exits the barrel, it will immediately begin decelerating. Earth's exit velocity is about 25,000 miles per hour, so the capsule will have needed to exit the cannon going far faster than that.

Rockets, like the ones used on the Apollo missions, continually accelerate up to exit velocity. This means that the rocket isn't slowing down as it gains altitude, and it means that astronauts experience much lower G forces (over a long time). A rocket can also be steered somewhat, so minor course corrections can be made after leaving the launch pad.

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