How fast would a bullet need to leave a gun in order to get into space?

22 February 2009

Question

How fast would a bullet need to leave a gun in order to get into space?

Answer

Chris - The forum has come to our rescue to answer this one: www.thenakedscientists.com/forum. There's a terrific answer.

RD points out that anything leaving the Earth into space needs to be travelling at escape velocity. That means 11.2km/s.

To put that into perspective, even a high-powered assault rifle probably fires things at 1km/s. It's running significantly too slow in order to achieve escape velocity. It also doesn't take into account air resistance.

There's a very good answer from BoredChemist. One thing to consider here is what about if we just build a powerful rocket and fire something from Earth? He says there's a limit to how far you can get something to go and how fast you can get it to go using a gun. The projectile is driven by hot gases producing the explosion but the gas is made of molecules and they have a range of velocities. The hotter the gas, the higher the average velocity.

To a fair approximation, the average speed of the molecules is the speed of sound in that gas. A projectile is moving faster than that the gas molecules will get left behind so they can no longer push on the projectile to make it go faster. By fudging the issue and using hot light gases like helium you could make it go a bit faster. There's no way that's you'll get to escape velocity.

Sorry about that!

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