1. Feb 5, 2014

### GTOM

http://en.wikipedia.org/wiki/Coilgun

" In 1978, Bondaletov in the USSR achieved record acceleration with a single stage by sending a 2-gram ring to 5000 m/s in 1 cm of length"

Based on formulas v=a*t s = a/2*t^2 does that mean that a m long cannon could accelerate a small slug to 50km/s in space?

Could any armor offer significant protection against a row of theese slugs?

2. Feb 6, 2014

### CWatters

If we assume constant acceleration we can use the SUVAT equation of motion..

http://en.wikipedia.org/wiki/Equations_of_motion#SUVAT_equations

V2 = U2+2as

where

V = final velocity
U = initial velocity
a = acceleration
s = distance

If you solve for "a" and plug in the numbers you get a = 125,000,000 m/s2 which is quite impressive.

If you assume that acceleration could also be achieved and maintained for a distance of 1m and run the numbers again you get a velocity of 15,811 m/s which is 15km/s or 35,368mph.

That's slightly slower than the velocity achieved by the New Horizons spacecraft (37,000mph) so it's not total impossible to achieve such speeds.
http://en.wikipedia.org/wiki/New_Horizons

However the amount of energy required is quite high.

3. Feb 6, 2014

### GTOM

"If you solve for "a" and plug in the numbers you get a = 125,000,000 m/s2 which is quite impressive."

I got ten times bigger a, 5000x5000/2 x 100. It talked about a cm, not a dm (tenth m), or am i missed something?

The energy requirement is equal to the projetile's kinetic energy + energy loss, isnt it?

4. Feb 6, 2014

### CWatters

Oops yes my mistake. I divided by 0.1 instead of 0.01

That will also give you a higher velocity for the 1m gun.

5. Feb 6, 2014

### HallsofIvy

Of course, it does not automatically follow that an experiment done in 1 cm can be scaled up to 1 meter without losses

6. Feb 6, 2014

### CWatters

Indeed. There was a thread on this forum recently about the fastest gun/bullet. As I recall to achieve high velocities you can't just keep adding more explosives to the bullet because that increases the mass of the expanding gas that also has to be accelerated...or something like that.

7. Feb 7, 2014

### GTOM

I read that with a light gas gun, speed of sound in the hot gas is a limiting factor.
With electromagnetic acceleration, i would think the ultimate barrier is speed of light. (Of course building a 100.000km accelerator is quite prohibitve.)
Well energy demands and waste heat are definitally problems, what others can be?

8. Feb 12, 2014

### caz

The devil is in the engineering details. The two main areas are
1) Materials/structural design so that the projectile can survive launch and that the gun is reusable. Also remember that flight aerodynamics matter which put limitations on projectile shape/velocity. Plates do not fly very well. Also, at extreme velocities spaced plate arrays like whipple shields for satellites become effective protection systems. I believe that the national labs built an electromagnetic gun in the 80's that shot around 10 km/s using flux compression.
2) Pulsed power system design. you have to store a lot of energy and then release it in a short period of time. At this time, you probably cannot build something small enough to fit on a ground vehicle which is why the navy is probably the best bet for adopters of this technology.

As a side note, gas guns with tricks max out in the upper teens. The Sandia Z machine has launched plates around 50 km/s for EOS studies.

9. Feb 12, 2014

### bahamagreen

Operation Plumbbob... 1957, old school approach; ton of steel goes 50,400mph

from Wiki:

Propulsion of steel plate cap

During the Pascal-B nuclear test, a 900-kilogram (2,000 lb) steel plate cap (a piece of armor plate) was blasted off the top of a test shaft at a speed of more than 66 kilometres per second (41 mi/s). Before the test, experimental designer Dr. Brownlee had performed a highly approximate calculation that suggested that the nuclear explosion, combined with the specific design of the shaft, would accelerate the plate to six times escape velocity.[7] The plate was never found, but Dr. Brownlee believes that the plate never left the atmosphere, as it may even have been vaporized by compression heating of the atmosphere due to its high speed. The calculated velocity was sufficiently interesting that the crew trained a high-speed camera on the plate, which unfortunately only appeared in one frame, but this nevertheless gave a very high lower bound for the speed. After the event, Dr. Robert R. Brownlee described the best estimate of the cover's speed from the photographic evidence as "going like a bat out of hell!"[7][8] The use of a subterranean shaft and nuclear device to propel an object to escape velocity has since been termed a "thunder well".

10. Mar 6, 2014

### GTOM

" Also remember that flight aerodynamics matter which put limitations on projectile shape/velocity. Plates do not fly very well. Also, at extreme velocities spaced plate arrays like whipple shields for satellites become effective protection systems."

Well i imagined, that theese things can be used in space, so aerodynamics wont matter. If they fire multiple small slugs in a row, can even Whipple shields protect the attacked spacecraft?
(Fire n+1 slugs if they have n layers of spaced armor.)

11. Mar 11, 2014

### GTOM

"Pulsed power system design. you have to store a lot of energy and then release it in a short period of time."

Can it be possible, that the explosion of a bomb converted to electrical energy, to accelerate lesser mass, but to a bigger speed?

12. Mar 11, 2014

### caz

In space, hitting the target is issue. Satellites are relatively fragile. Explosives have been used for flux compression (and acceleration - google shaped charge), but the issue is still how to couple the energy in a manner that does not destroy the projectile.

BTW, Whipple shield performance increases with velocity. A projectile that penetrates at velocity v might not penetrate at 2v.

13. Mar 14, 2014

### GTOM

"but the issue is still how to couple the energy in a manner that does not destroy the projectile."

Just to make sure i understand well, in case of accelerating a solid homogenic metallic projectile in space, the problem is friction with the coils?

"BTW, Whipple shield performance increases with velocity."

That is why i asked, whether it can be countered by multiple projectiles after each other, one is shattered by first layer, but makes a hole, second hits the second layer... ? (something similar to a tandem charge meant to counter reactive armor)

Could be sheer impact damage also useful? (Of course, Newton's third law, the cannon has to release all that energy, but it isnt the same, whether the fist or the stomach gets the same.)

14. Mar 14, 2014

### caz

You could use multiple impacts to get through a whipple shield, but you could also shoot slower.

15. Mar 14, 2014

### caz

I do not know what you mean by shear impact damage.

16. Mar 14, 2014

### Drakkith

Staff Emeritus
It means impact damage. The "sheer" is just an adjective in this context and is used to emphasize that impact damage is the only thing being considered in the question.

17. Mar 14, 2014

### Staff: Mentor

Sheer versus shear...

EDIT -- beat out by the Storm Trooper again!

18. Mar 14, 2014

### Drakkith

Staff Emeritus
Space Marines. First in, last out. Here, there, everywhere.