# The Naked Scientists Forum

### Author Topic: will a bullet go into Orbiting of the moon  (Read 7571 times)

#### C-guy

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• Posts: 28
##### will a bullet go into Orbiting of the moon
« on: 18/09/2004 22:01:10 »
I was wondering, assuming the a gun can work in the absence of oxygen, can you shoot a bullet into orbit on the moon?

Moving at the speed of light.
« Last Edit: 26/05/2007 04:06:08 by ukmicky »

#### gsmollin

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##### Re: will a bullet go into Orbiting of the moon
« Reply #1 on: 19/09/2004 03:16:06 »
Only if the muzzle velocity is 4300 feet/sec. (1300 m/s)

#### chris

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##### Re: will a bullet go into Orbiting of the moon
« Reply #2 on: 20/09/2004 18:04:38 »
gsmollin - can explain how you arrived at that answer please ?

Cheers, Chris

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#### gsmollin

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##### Re: will a bullet go into Orbiting of the moon
« Reply #3 on: 20/09/2004 18:32:02 »
Yes, pretty crude calculation...
LEO velocity = 25,700 fps.
moon g = earth g/6, to 1 significant figure.
LMO velocity= 25700/6= 4300 fps.

Obviously there are factors not considered, such as comparative heights of orbits above the center of the moon and earth. On the surface of the moon, one is closer to the center than on the earth. A better estimate, taking that into account and scaling the orbital altitude down is about 5300 fps. The other issue is whether we are firing the gun at altitude, or from the surface of the moon.

I'm still not digging out the orbital mechanics book here, since I think the curiosity is idle. If C-guy needed exact answers, he would specify an apilune and perilune, for instance.

#### C-guy

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##### Re: will a bullet go into Orbiting of the moon
« Reply #4 on: 21/09/2004 02:46:08 »
an exactly circular orbit

Moving at the speed of light.

#### tweener

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##### Re: will a bullet go into Orbiting of the moon
« Reply #5 on: 21/09/2004 20:24:48 »
5300 fps muzzle velocity is mighty fast for any rifle I've heard of, but is not out of the realm of possibility.  4300 fps is probably achievable in some existing rifles.  And, a rifle will fire in the absence of oxygen because the powder has all the components to react (i.e. the reaction is not using O2 from the air).

----
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#### OmnipotentOne

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• Posts: 146
##### Re: will a bullet go into Orbiting of the moon
« Reply #6 on: 23/09/2004 21:21:32 »
All you would need is a Gattling gun, 6000 rounds a minute!   So the orbiting bullet if fired at the right FPS and altitude would never be pulled into the moons orbit?  I mean if it was sligtly over the escape velocity wouldnt it travel off toward uranus?

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#### gsmollin

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##### Re: will a bullet go into Orbiting of the moon
« Reply #7 on: 24/09/2004 02:19:58 »
Well, if you stood your 6000 rounds/minute, 5300 fps velocity Gatling gun on top of a tall lunar mountain, and fired it, the bullets would travel around the moon in a circular orbit, and then they would be traveling towards URanus!

#### aks_physics

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##### Re: will a bullet go into Orbiting of the moon
« Reply #8 on: 24/09/2004 11:33:59 »
hey gsmolin please tell me how you come to these calculation because  as i think you left something for to be calculated

aksphy

#### C-guy

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##### Re: will a bullet go into Orbiting of the moon
« Reply #9 on: 25/09/2004 06:24:20 »
I need the bullet to stay in orbit!

Moving at the speed of light.

#### tweener

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##### Re: will a bullet go into Orbiting of the moon
« Reply #10 on: 28/09/2004 03:59:38 »
Basically, if you shoot a projectile at less than the escape velocity and more than the minimum orbital velocity, it will describe some sort of ellipse around the moon and come back to the same point it started from.  The calculations let you determine how long it will take and what the shape of the ellipse will be.

If you shoot it with less than orbital velocity, it will crash into the moon somewhere before getting all the way around.  If you shoot it faster than escape velocity, it'll never come back.  In between there are lots of ellipses that can work.  If there are other bodies around, like the earth, Sun, other planets etc. the calculations become more important because a long orbit could be hijacked into some other path, depending on relative positions.

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#### gsmollin

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##### Re: will a bullet go into Orbiting of the moon
« Reply #11 on: 28/09/2004 04:24:06 »
There are scores of books that give simple Newtonian orbital mechanics. I have several, but right now I'm using one called "Vector Mechanics for Engineers: Dynamics", by Beer and Johnston. In Chapter 12, the development for orbital mechanics is found. I used this book because it makes the simplifying assumption that the orbiting particle's mass is negligible compared to the orbited body, which is true in our case. Equation 12.27 gives the orbital tangential velocity for a circular orbit:
vcirc = sqroot(GM/r0), where:
G is the gravitational constant 6.673x10 raised to the -11 nm2/kg2,
M is the mass of the body, 7.349x10 raised to the 22 kg for the moon,
and r0 is the altitude above the center of mass of the body.

Soving for r0, and subtracting the equatorial radius of the moon (1737.4 km) we get an orbital altitude of 141.87 km. If we want to know the orbital velocity at the equatorial surface of the moon, we use 1737.4 km for r0 and solve for vcirc = 1.6801 km/s = 5512 fps.

Note that these results are for orbital tangential velocity, not for a case where the gun is aimed above the horizon.

#### syhprum

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##### will a bullet go into Orbiting of the moon
« Reply #12 on: 02/06/2007 21:27:21 »
I find it difficult to imagine Americans going anywhere without guns, did the Apollo astronauts take guns to the Moon?.

#### lyner

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##### will a bullet go into Orbiting of the moon
« Reply #13 on: 02/06/2007 23:48:23 »
To go into a 'proper' orbit, once you have reached a suitable altitude, you need to fire  your engine again and change direction (more or less tangential) or you will crash back into the moon / earth (etc.) after having followed an ellipse.
The idea of an orbital launch from a mountain top is only quoted to help understand the basics. If there is any atmosphere, such an orbit would not be sustained.

#### The Naked Scientists Forum

##### will a bullet go into Orbiting of the moon
« Reply #13 on: 02/06/2007 23:48:23 »