David Compton asked:
How fast would a bullet need to leave a gun in order to get into space?
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!
David Compton asked the Naked Scientists: In an earlier Podcast Dr. Dave mentioned that 11.2 kilometres / second was the value of escape velocity. I was wondering how that applies in the real world? The discussion was about firing a bullet into space. If the speed of the bullet was 11.2 kilometres / second when it left the barrel of the gun, would that be fast enough to reach space? That bullet will immediately begin to slow down due to air resistance and gravity. Would that 11.2 kilometres / second have to be increased to account for these forces? I figure that a small .50 calibre bullet weighs 647gr (41.9g), which typically has a velocity of 928 meters/second. So my question is this: How fast would a 41.9g projectile have to going when it leaves the barrel to make it into space? Thanks! What do you think? David Compton , Sat, 25th Oct 2008
Escape velocities do not take air resistance into account.
Take a look here
This was the big gun thing that some people though Saddam Hussein was trying to build at one time.
There's a limit to how fast you can get soemthing using a gun. The projectile is driven by the hot gasses produced by 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 aproximation the average speed of the molecules is the speed of sound in the gas. Once the projectile is moving faster than that, the gas molecules get left behind they can no longer push on the projectile and make it go faster.
I think you can get to about 3x the velocity of sound, BC, but not much more with conventional explosive type propulsion. There is no such limit with the "rail gun" electromagnetic propulsion though. It is not exactly a "muzzle" you would have then, but I guess the idea is the same in that all the propelling force equipment stays on the ground. graham.d, Sun, 26th Oct 2008
Ummm there's a big difference between 'reaching space' and escape velocity.
Very interesting wolfekeeper.