# Let's say a ship mounted railgun is feasible what kind of damage could it really do?

Railguns have a whole cartload of technical problems to overcome but let’s say with dogged persistence a ship mounted railgun is feasible in the next 20 years or so. What is the real world envelope of damage that a weapon like that could do against naval or land targets?

Per this article lets say the velocity is Mach 3 to Mach 6 and you can launch a 25 lb tungsten projectile (per below) at that speed. what kind of real world kinetic damage is that going to do on the receiving end vs a conventional explosive shell of the same weight?

(The 25-pound projectile is a non-explosive bullet filled with tungsten pellets inside an aluminum alloy casing, or sabot, that falls away after the projectile leaves the barrel)

What’s the range? The projectile will lose velocity the whole way so the velocity at target needs to be calculated before you can calculate the energy delivered.

ETA: Explosives deliver a different kind of damage compared to a kinetic projectile.

Per the article assume a range of up to 50 to 100 miles. If it’s leaving the gun at Mach 3 to Mach 6 and it weighs 25 lbs what’s the impact force at

Mach 3
1 mile
5 miles
10 miles
25 miles
50 miles
100 miles

Mach 6
1 mile
5 miles
10 miles
25 miles
50 miles
100 miles

This a handy online mass/velocity/energy conversion calculator I found that will even give me energy in terms of explosive power but I need to know how fast the projectile will be travelling at impact and I don’t have the math to calculate that.

The article says that the projectile is fired out of a 5" barrel. Lets say that without the sabot it’s 3" in diameter.

The drag equation is F = 0.5rhoCAV^2. Using the density of air at sea level and C=0.04 for a long aerodynamic body, this gives the drag force as 0.00011 kg/m*V^2.

Assuming only horizontal travel, and using a pretty coarse numerical integration:

The projectile reaches 50 miles in ~ 40 seconds with a velocity of 1998 m/s and an energy of ~22MJ.

Thanks!

This is not the way I envisioned a rail gun. They were supposed to use lightweight projectiles at extremely high velocities. Mach 6 is about 6700 ft/sec, the Abrams tank main gun uses projectiles that reach 5500 ft/sec today. No, they don’t weigh 50 pounds, but this rail gun is just an expensive way to get a 20% increase in velocity. A single 120mm sabot-cased, fin-stabilized round costs about \$10,000.

The energy is proportional to the velocity squared. Now the rail gun Arnold used in “Eraser” used a pea sized aluminum pellet at a significant fraction of light speed. That’s a rail gun!

Heck, we send groups of men to the moon in minivan sized vehicles at 5 times that velocity. I know it wouldn’t work in the atmosphere. No, I don’t know how to make one. But I’m not in the slightest bit impressed.

Dennis

[Bolding mine]

Recoil’d be a bit of a bitch, tho’.

Here’s what would happen if you actually could accelerate a small mass to near the speed of light with a rail gun. It doesn’t end well for anything nearby.

https://what-if.xkcd.com/1/

USS Iowa class battleships of WWII had nine 16" guns, which would lob 2,700 lb armor-piercing shells 23 miles.

The USS Nimitz carries around 90 F-18 Hornets, which can range out 600 miles with a couple AIM-9 Sidewinders with 25 lbs warhead each.

These two examples are old old technologies … not sure how this rail gun is any better … looks like the cost of one shot is about the same as a dozen cruise missiles …

You’ve mixed apples and oranges as the Sidewinder is intended to attack aircraft in flight.

A better comparison would be the F-18s can each carry 2x2000# bombs with some small degree of armor peircingness. The CEP of the Iowa is measured in hundreds of meters. The CEP of the F-18 is measured in the high single digit meters. Unless you attach the JDAM guidance kit to the bombs. In which case it’s measured in onesy-twosy’s of meters.

Given that modern ships are mostly unarmored, for anti-ship use there’s not much difference. Shore fortifications are a different story.

I think the theory is that rail guns use cheaper rounds - essentially just a well-made steel bar - and have room for a few decades of improvement, while other current arms are extremely expensive rounds delivered by phenomenally expensive systems - and are at essentially unimprovable levels of development.

That, and the ship doesn’t need to carry tons of propellant and explosives. Whether or not that’s practical is another matter. I talked to a couple of people working on a Navy railgun project a few years ago. I don’t know what they’ve achieved since then but at the time they were still trying to reached goals for projectile size and velocity and were a long way from actually hitting anything.

I’m comparing the two weapons systems by using an equal weight of ordinance … the rail gun delivers 25# 100 miles … the F-18 delivers 25# 600 miles … I don’t know the range of the F-18 carrying 2x2000# … but let’s say 100 miles, we’re still have 160 times the ordinance for the same range …

To be honest, 25 MW of power seems a little high, and expensive … and what would be the CEP of the rail gun?

The F-18 has a bit more versatility … gliding one in and tail-dancing over an enemy position has been known to have certain diplomatic benefit …

Ahh, now I see. I totally skipped over the 25# as being a railgun comparison and was thinking just of Iowa vs. Sidewinder. Shoulda doublechecked my reading comprehension when that seemed an odd comparison for you to make. Oops.

FWIW, F-18 range carrying 2x2000# is not much shorter than with the pure air to air armament. Typically you’d carry more external fuel tankage with the bomb load; with air to air you’re more concerned about external fuel tanks limiting manueverability.
Railgun CEP is an interesting question. We can now build guided artillery shells, albeit not cheaply. So electronics surviving insane accelerations is not inconceivable. Whether they could be built to survive the EMI environment inside a railgun is a different matter.

Absent guidance the railgun will have the same problem the Iowa or any naval rifle has: good = small errors in azimuth and large = bad errors in range. The splash pattern looks more like a comet’s orbit than the Earth’s orbit. Which renders the term *Circular *Error Probable a bit inapt.

When shooting at relatively close range at a target with a lot of vertical relief like a ship, range errors are partly covered by target shape. If the target ship is 100 feet tall, any shot that passes through the ship’s range and azimuth at any height between 1 inch and 99’11" is a hit. Even though absent the ship, that 100 foot high shot might have carried another half mile before splashing.

All this is inherent in the flat trajectory of naval shooting. Unlike the high angle plunging fire commonly used by land artillery. So I’d bet an unguided railgun slug would have the same issues. Amplified by the speed and the simple fact that at twice the distance you need half the angular error to achieve the same positional error.

All in all, it’s not obvious to me what you gain with a railgun other than less HE on the ship for safety and perhaps the ability to carry many more shots if the slugs are smaller and/or lighter. IF the launcher can survive that many extra shots.

Bottom line: IMO they’re worth a look, but they’re not the slam-dunk that directed energy weapons are.

I’m not sure why this is a relevant comparison.

Do they cost the same? Can you fit the same amount on a ship? Do they require the same amount of maintenance? Training to use? Support equipment? Same time to impact?

A truck bomb has more ordinance and larger range than the F-18, but those aren’t the only relevant parameters.

The OP asks what kind of damage a rail gun could do … 25# slug at 700 mph compared to 4,000# of TNT … back-of-the-\$100-bill calculation gives me 1 MJ vs. 8.4 GJ of energy released … mind you, the F-18 can carry a couple B61’s … that’s 680,000 tons of TNT equivalent … not sure it’s wise launching one them suckers with a rail gun …

I guess it depends on the rate-of-fire for the rail gun … and I’m pretty sure the USS Nimitz can have in the air more than just one F-18 at a time …

… did I mention the nose-mounted Vulcan on the F-18?

One aspect of the potential damage I wonder about is if the projectile is travelling at (say) 700- 1500 mph at impact and it’s this solid, super strong slug of metal weighing 25 lbs. If I’m the bad guy sitting in an office building and it does not hit me dead on isn’t it simply going to blow through the building until it loses enough energy to embed itself in the ground. I can imagine it will do some serious kinetic damage to whatever it hits but if I’m 20 to 30 feet away from where it hits how dangerous is it to me?

Shoulder-mounted actually, but *definitely *forward facing.

Previous was @watchwolf49.

@astro: Like old fashioned cannonballs you’re going to get a certain amount of shrapnel, etc. What the Navy calls “splinters” for tradition’s sake.

An office building is about the limit case of a flimsy skin stretched over a strong but sparse skeleton. That’d be just about the worst-case target for a KE penetrator. As you say. Even taking out a single support beam is unlikely to do much.

Against harder targets or ones, such as ships and aircraft, that can’t take much damage without collateral collapse problems you’ll get better results.