Non-OTU: Low-G Vacuum Ballistics

[Timerover51]

Back in June of 2013, I came across the following publication put out by the Future Weapons Office of the US Army's Rock Island Arsenal, which handles R&D for new weapons: "THE MEANDERINGS OF A WEAPON ORIENTED MIND WHEN APPLIED IN A VACUUM SUCH AS ON THE MOON"(U). The publication was declassified in 1997. The guy messed up some of the math, and I am working on correcting that and retyping and reformatting it so as to post it online, hopefully for some small remuneration. However, it did get me to thinking a bit of the combined implications on projectile weapons of low gravity and vacuum conditions. Once I get more of the implications worked out, I will try for an article on Free Traveller or something.

Applying some initial thoughts to the Moon, I am using the following equations for calculations.

Moon basics:
Mean gravity is 0.1654 G. Mean Radius is 0.273 Earth.
Gravitational Acceleration is therefore, 5.32 feet ( approx. 64 inches) per second, or 1.623 meters (1623 millimeters) per second.
Distance to the horizon is:
If distance in Kilometers, d = 1.86 X sq rt (height in meters)
If distance in Statute Miles, d = 1.22 X sq rt (height in feet).

Two other useful equations are time of flight, which for theta equal to 45 degrees, t is equal to sq rt (2) X velocity divided to Gravity, and range which equals velocity (squared) divided by Gravity when theta is 45 degrees. The range does assume a flat Earth, so actual range on a planet would be farther.

Putting all of this together, look what happens with a rifle with a muzzle velocity of 3,000 feet per second is fired perpendicular to the surface. (I still think in terms of feet, inches, yards, statute and nautical miles, pounds, and gallons.) One hundred yards downrange is covered in 0.1 seconds, at which time the round drops 0.32 inches. At 500 yards, the round has dropped 8 inches. At 1,000 yards, 1 second of flight, the round has dropped 32 inches. If fired from a height of 5 feet/60 inches, or about the shoulder, the round does not hit the surface of a flat Earth until 1.389 seconds after firing, or 1369 yards downrange, actual distance further. So with your sights exactly parallel to the bore of the rifle, you aim at the base of the throat of a man and at 500 yards, you hit him somewhere near the heart.

Now fire that rifle at a 45 degree angle, remembering that the round does not slow down do to atmospheric drag, so it retains its full terminal velocity. The round hits with undiminished velocity 320 MILES downrange, on a flat Earth. On the Moon, a bit further. Gives a new meaning to the term danger space, doesn't it.

Now, look at the humble 60mm Mortar from WW2. At a 45 degree angle, and a muzzle velocity of 518 feet per second, it has a range of 1,984 yd. Under Lunar conditions, is has a range under flat Earth conditions of 16,812 yards, about that of an 8 inch howitzer. The range with the minimum charge, just the igniter, at 189 feet per second is 2238 yards, or longer than the maximum range under Earth-normal conditions. One thing though, the mortar is going to have to be rifled, fin stabilization does not work in a vacuum. Another problem, it takes about a second for the round to drop down the tube, and you are probably going to have to trigger fire it, as it hits a bit less hard on the firing pin. And one more thing to keep in mind, your MINIMUM range goes up too. Oh, and your time of flight, assuming maximum range, is 137.7 seconds. I hope that you are firing are a fixed target.

The Low-G and Vacuum conditions does very interesting things to your HE shell burst radius too, but I am still working on those. Just remember, no atmosphere to slow down the shell fragments, which does mean that they are going to travel a VERY LONG way.

One more thing to keep your minds going, the Moon's escape velocity is 2360 meters per second, or 7743 feet per second. Any round with a velocity approaching those is either a Lunar-ICBM equivalent or a low-orbiting satellite. Exactly how far behind you is your base facilities? How hard are they?

One last thing, would there be interest in my working up the distance to the horizon of the standard world diameters for Traveller, and posting that as a table?

[Vladika]

Originally Posted by timerover51

...
Gravitational Acceleration is therefore, 5.32 feet ( approx. 64 inches) per second, or 1.623 meters (1623 millimeters) per second.
...

Check your formula Acceleration is in distance unit/ time unit/ time unit

In your example it's "feet per second per second" or "meters per second per second"

[Timerover51]

Originally Posted by Vladika

Check your formula Acceleration is in distance unit/ time unit/ time unit

In your example it's "feet per second per second" or "meters per second per second"

I apologize for not adding the additional feet per second. I guess I thought that people would understand the feet per second per second or meters per second per second, since I was talking acceleration. It has now been so edited.

[Vladika]

Originally Posted by timerover51

I apologize for not adding the additional feet per second. I guess I thought that people would understand the feet per second per second or meters per second per second, since I was talking acceleration. It has now been so edited.

Just didn't know you knew and didn't want to see you correct the math with a wrong formula. You already have your work cut out for you.

[Timerover51]

The concept is interesting. I need to figure out where a couple of my resource books went when I was cleaning for more data. It is giving me a better case for distinguishing between Marine and Army troops. Training is definitely going to be different for that type of combat. If you try to switch back and forth, your combat reflexes are going to get messed up. Plus, you are going to equip a lot differently.

[flykiller]

I have always wondered why traveller high-tech is so deficient in indirect fire weapons. perhaps this is why.

[aramis]

The table would be interesting.

[Timerover51]

Originally Posted by aramis

The table would be interesting.

As I am still trying to figure out the table format, if I posted the information in basic format, could someone convert it into a standard table format for me?

[Piper]

Originally Posted by aramis

The table would be interesting.

Striker, Book 2, Rule 53. See also the sections covering planetary environmental effects on range and penetration.

[Timerover51]

Originally Posted by Piper

Striker, Book 2, Rule 53. See also the sections covering planetary environmental effects on range and penetration.

Hmm, looks like I will have to get out my copy of Striker and look at that. However, when it comes to something like penetration, that would have nothing to do with planetary environmental effects beyond how much the velocity of any projectile is reduced by atmospheric drag.

Regardless, how many members of the Forum have Striker, and would find such a table useful. I will work on compiling one, for both standard Traveller planets, and also Mercury, Venus, and Mars, plus some of the larger satellites and asteroids.