The Physics of a Railgun

[Hogwylde]

You are so right. Right now there are no guided artillery shells in the current inventory. Do I really need the purple.

Not true. >>> M982 Excalibur - Wikipedia, the free encyclopedia

 

[CathyInBlue]

I'll let the defense industries do the worrying about fine-tuning targeting at range. I'm sure they can figure it out. Probably even without relying on a phase-conjugate mirror! (Sorry, couldn't help myself.)

How would the railgun system stack up against chemical lasers in solid, not gaseous, form? j/k

Not true. >>> M982 Excalibur - Wikipedia, the free encyclopedia

Apparently, he did need the purple.

As long as railguns are built into ships that are not nuclear, the fact that it takes no chemical propellant to fire them will be a wash, because it would still require massive amounts of fossil fuels to power the ship's generators to fire them. Once we're talking about railguns for the nuclear Navy, then we're talking about an effectively infinite ammo supply. Well, at least a vastly increased ammo supply versus conventional big naval guns, or even modern missile systems. And, as has been mentioned, no explosives or propellants inherent in that ammo, it becomes inherently safe to store the ammo anywhere, near food stores, near personnel, etc. If the ammo is nothing more than an exquisitely formed lump of metal, as long as it's not an out-sized lump of metal, they can be very small. It's the 9mm vs .45ACP argument. So you want 20 rnds of 9mm or 8 rnds of .45 in the space of your handgun magazine? How about 40 rnds of .50BMG in the same space? That's what this is, but again, only for the nuclear Navy where the power is essentially free until the reactor's next refit in 15 years.

And I would like to think that we have the materials science to electricly shield the electronic guidance package of a self-guided railgun projectile, but I don't know what kind of EM flux values we're talking about at launch. There must be some means of electronic signaling penetrating from the exterior to the interior of such a shell, because the projectile has to be programmed with its destination address immediately prior to launch. It doesn't matter whether that is to happen via a wire and physical contact interface or a wireless RF interface, that interface will be a chink in the EM shielding armour against the EM flux of a railgun launch. Then, in flight, if the shell is to use GPS signals to guide itself, how are the GPS RF signals to penetrate from the exterior environment to the interior electronic guidance package through the same shell that had to shield that electronic guidance package from the EM flux of launch?

One definite cure for all those ills would be optical processing. We're making strides every day in optical computing. If the delicate parts of the guidance package could be made to use photons, not electrons, then that would go miles toward hardening such a guidance package against the EM flux of railgun launch.

I think a naval railgun would be of limitted to no use in anti-satellite operations, however. First, there would be the issue of is the railgun architected such that it can be aimed almost straight up? The only reason to engineer an ostensibly ballistic weapon system as powerful as a railgun that can point straight up is for ASAT operations. When shelling surface targets inland or enemy naval targets, you're looking to reach out/sideways, not up.

Assuming you could point a given railgun system verticly, at Mach 7 (at least at the end of the barrel), you could expect to send the projectile just under 90 miles straight up in 60 seconds. Problem is, the kinds of targets of military worth are much higher than 90 miles.

That can't even take the space station by surprise. So, just to get at spy sats in sun-sync or polar orbits will take 5-15 minutes, and that's completely discounting the fact that the projectile will be decelerating the whole time. It's questionable whether the projectile could even arrive at the orbit of enemy satellites.

Could we build railgun systems specificly architected to loft satellites into orbit? Sure, eventually. Would those same railgun satellite launch systems be able to loft ASAT warhead payloads? Sure, absolutely. Would those railgun systems be mountable on naval platforms? Almost definitely not.

 

[david890]

The money spent on railgun technology may well revolutionize our whole military capacity. The navy is looking at a projectile being shot at mach 7 with a range of around 100 miles. Has anyone else figured out that this range also includes knocking satellites (or ICBM's) out of space? How about the idea that a ballistic round cannot be jammed by electronic interference, and its nastiness comes from kinetic energy instead of explosives, which means it is MUCH safer to store, handle, and fire...and it is not nearly as vulnerable to countermeasures from defensive missile technologies.

Many thought that a rifle (and the shooter) needed to be accurate out to 600-800 yards, but then we found out that most shooters waited until the target was within 200-300 yards before taking the shot. Longer range isn't always a good thing (i.e., worth the cost of design and manufacture), especially when we already have the tech to put a missile through a window at 100 miles!

As for ASAT, the US, Russians and Chinese have already done shoot-downs using chemical-based weapons. Why re-invent the tech?

Obviously, a ballistic round can't be jammed, but they CAN be intercepted quite easily (e.g., Israel and "Iron Dome").

I don't see storage as much of an issue, given that we can store Tomahawk cruise missiles well beyond the active combat area. If you're talking about storage onboard a ship, our $$$ needs to go into Point Defense weapons so that we can intercept Exocet and other types of anti-ship missiles. As for storage on a tank, the current practice of isolating the rounds from the crew compartment on the M-1 Abrams seems to work fine. Besides, enemy penetrators in development will go after the center-of-mass, and likely from directly above, which means the crew compartment is targeted instead of the ammo box. We'll have dead tanks, dead crews, but the ammo will likely be fine!

If there is research to be done on railguns, it's very likely more useful when applied peacefully, such as "Mass Drivers". These are EM-powered rails/catapults designed to hurl payloads into low-Earth orbit, or to send materials mined from the Moon or an asteroid back to the Earth. We can make them large, so the challenges of getting the mass up to the necessary velocity isn't that great. We don't need to get Mach 7 in 40', when current tech can do the same in 400' or 4000' (or 40,000' so that the acceleration is low enough to allow launching a human into orbit).

 

[eric001]

Granted, I'm no physicist. Would shooting nearly straight up give a range much greater than shooting sideways? I'm thinking that as altitude increases, air resistance drops off rapidly...and so does the pull of gravity. I would think air resistance would be the biggest factor, but either way wouldn't you get a pretty big increase in range? I do seem to remember there are some equations for escape velocity, but it's been too long for my brain to come up with them offhand, and I'll admit I'm being lazy. If the projectile exceeds escape velocity, wouldn't it basically just keep going on a predictable path though?

 

[eric001]

Many thought that a rifle (and the shooter) needed to be accurate out to 600-800 yards, but then we found out that most shooters waited until the target was within 200-300 yards before taking the shot. Longer range isn't always a good thing (i.e., worth the cost of design and manufacture), especially when we already have the tech to put a missile through a window at 100 miles!

As for ASAT, the US, Russians and Chinese have already done shoot-downs using chemical-based weapons. Why re-invent the tech? I would say options are always good. ASAT missiles can be defeated by jamming or kinetic kill weapons (that aren't SUPPOSED to be up there but I wouldn't count on that being true). A hunk of metal traveling faster than a bat out of you know where with its tailfeathers on fire may get through when a fragile missile cannot.

Obviously, a ballistic round can't be jammed, but they CAN be intercepted quite easily (e.g., Israel and "Iron Dome"). When last I read up on Israel's defense system, it suffered basically the same issues as our Patriot system did during Iraq War #1--the missiles don't always hit/explode close enough to their incoming target to destroy the warhead, and the system relies on the targets being pretty fragile/thin-skinned in order to cause enough damage to destroy them. Hitting a metallic dart going mach 7 (or 4 or 3 or whatever by the time it gets close) with an explosion or blast of shrapnel may deflect it somewhat, but definitely wouldn't stop it or even slow it down significantly.

I don't see storage as much of an issue, given that we can store Tomahawk cruise missiles well beyond the active combat area. If you're talking about storage onboard a ship, our $$$ needs to go into Point Defense weapons so that we can intercept Exocet and other types of anti-ship missiles. I am thinking modern anti-ship missiles have at least basic maneuvering capability to make missile on missile kills more difficult. I may well be wrong here. On the other hand, firing even a "small" caliber projectile at mach 7 towards an anti-ship missile wouldn't give it much time to evade--firing 10-20 or so would almost certainly nail it several miles out. Costly to create? Yup. But it'd beat the gatling gun idea ten ways til Tuesday, and you could store a whole lot more metal darts than you could missiles for ship defense.

As for storage on a tank, the current practice of isolating the rounds from the crew compartment on the M-1 Abrams seems to work fine. Besides, enemy penetrators in development will go after the center-of-mass, and likely from directly above, which means the crew compartment is targeted instead of the ammo box. We'll have dead tanks, dead crews, but the ammo will likely be fine!
All true. But in the battle between armor and armor penetration, wouldn't it be nice to be able to launch a round down a rail gun that would penetrate an earthen berm and the tank hiding behind it? Wouldn't it be nice for tanks to have a true defense against aircraft--even ones miles away? And wouldn't it be nice to be able to shoot through several enemy vehicles if you could line them up? I'd think that all it would take would be a regulator on the power input on that railgun to determine the range/penetration of the shot. With modern battlefield data integration, I'd think our forces would just LOVE to be able to use multiple targeting sources to guide a tank's shot into something the tankers couldn't even see for an insta-kill well before their target could be a danger.

If there is research to be done on railguns, it's very likely more useful when applied peacefully, such as "Mass Drivers". These are EM-powered rails/catapults designed to hurl payloads into low-Earth orbit, or to send materials mined from the Moon or an asteroid back to the Earth. We can make them large, so the challenges of getting the mass up to the necessary velocity isn't that great. We don't need to get Mach 7 in 40', when current tech can do the same in 400' or 4000' (or 40,000' so that the acceleration is low enough to allow launching a human into orbit).

Mass Drivers are certainly a valid idea. No arguments here. But let's face it--like it or not, there is a whole lot more $$ for military weapons systems research than there is for space exploration right now. I see this as one as heavily benefiting from "trickle-down" effects--military tech development solving a lot of the problems so a civilian version can go on to much greater and grander things.

 

[david890]

Originally Posted by david890

As for ASAT, the US, Russians and Chinese have already done shoot-downs using chemical-based weapons. Why re-invent the tech? I would say options are always good. ASAT missiles can be defeated by jamming or kinetic kill weapons (that aren't SUPPOSED to be up there but I wouldn't count on that being true). A hunk of metal traveling faster than a bat out of you know where with its tailfeathers on fire may get through when a fragile missile cannot.

"Kinetic kill weapons" are already in orbit! They're called "satellites"! The US, Russia or China can (depending on altitudes and relative motion) turn a functional, commercial satellite into an ASAT! Relative velocities are 17,000+ MPH, so just alter trajectory for an intercept! A slight touch from an antenna tip can potentially destroy the target sat.

Obviously, a ballistic round can't be jammed, but they CAN be intercepted quite easily (e.g., Israel and "Iron Dome"). When last I read up on Israel's defense system, it suffered basically the same issues as our Patriot system did during Iraq War #1--the missiles don't always hit/explode close enough to their incoming target to destroy the warhead, and the system relies on the targets being pretty fragile/thin-skinned in order to cause enough damage to destroy them. Hitting a metallic dart going mach 7 (or 4 or 3 or whatever by the time it gets close) with an explosion or blast of shrapnel may deflect it somewhat, but definitely wouldn't stop it or even slow it down significantly.

I have to disagree, for the same reasons ASATs work: relative velocities and the energies involved at such speeds. Mach 7 is ~2400 m/s. Let's assume a 1 kg railgun "dart" and a Vulcan Phalanx style of interceptor, using a 20mm, 100g round travelling 1100 m/s. You have 2.9 mega-Joules of energy (equivalent to a 3-ton vehicle travelling 100 MPH) in the dart and 60,000 J for the bullet. A small, glancing blow might induce enough spin (along with aerodynamic forces once the dart goes off-axis) to cause the dart to disintegrate. I doubt there's any known material that could withstand a near-head-on impact and continue on the same ballistic path.

I don't see storage as much of an issue, given that we can store Tomahawk cruise missiles well beyond the active combat area. If you're talking about storage onboard a ship, our $$$ needs to go into Point Defense weapons so that we can intercept Exocet and other types of anti-ship missiles. I am thinking modern anti-ship missiles have at least basic maneuvering capability to make missile on missile kills more difficult. I may well be wrong here. On the other hand, firing even a "small" caliber projectile at mach 7 towards an anti-ship missile wouldn't give it much time to evade--firing 10-20 or so would almost certainly nail it several miles out. Costly to create? Yup. But it'd beat the gatling gun idea ten ways til Tuesday, and you could store a whole lot more metal darts than you could missiles for ship defense.

Yes, anti-ship missiles can maneuver. IIRC, many are designed to "pop up" near the target then come straight down on the center-of-mass of the ship. Doing so ensures the kinetic energy of the missile is also used, along with any chemical warhead, to kill the ship. A dud missile might have enough KE so that a fused, explosive warhead isn't needed!

As for hoping to fire 10-20 rounds from a railgun, we can already fire 4,500-6,000 rounds from the Vulcan Phalanx! If I had to choose between 4500 rounds of Mach 3 projectiles or 20 rounds of Mach 7 darts, I'd go with 4500!!

As for storage, the space required for an operational railgun system and ammo would obviously be far greater than that for the Vulcan Phalanx and ammo.

As for storage on a tank, the current practice of isolating the rounds from the crew compartment on the M-1 Abrams seems to work fine. Besides, enemy penetrators in development will go after the center-of-mass, and likely from directly above, which means the crew compartment is targeted instead of the ammo box. We'll have dead tanks, dead crews, but the ammo will likely be fine!
All true. But in the battle between armor and armor penetration, wouldn't it be nice to be able to launch a round down a rail gun that would penetrate an earthen berm and the tank hiding behind it? Wouldn't it be nice for tanks to have a true defense against aircraft--even ones miles away? And wouldn't it be nice to be able to shoot through several enemy vehicles if you could line them up? I'd think that all it would take would be a regulator on the power input on that railgun to determine the range/penetration of the shot. With modern battlefield data integration, I'd think our forces would just LOVE to be able to use multiple targeting sources to guide a tank's shot into something the tankers couldn't even see for an insta-kill well before their target could be a danger.

At Mach 7 (~2400 m/s), I wouldn't expect a 1 kg "dart" to penetrate a berm with enough KE to continue on and kill a tank hiding behind it. If it did, rocks, obstructions, variation in soil densities, etc., would almost certainly cause the dart to tumble and slam off the target tank instead of penetrating. You'd certainly alert the enemy that you know where they are, but I suspect they would still be mobile and capable of returning fire (once they show themselves)! Keep in mind that tanks are designed to support infantry, keeping the enemy pinned down while the grunts move forward. Infantry already has the AT-4 to engage enemy tanks and other armored vehicles, and there are new man-portable, fire-and-forget weapons in the works to take out sheltered armor using the "pop-up" method mentioned previously.

I don't see the practicality in trying to adapt the main gun of the Abrams (or any future tank) into an air-defense weapon. We have our own aircraft, air-to-air missiles, ground-to-air missiles, MANPADs, airborne warning/command aircraft (AWACS), etc., to deal with enemy air forces. If enemy aircraft can get close enough to engage ground forces, our air forces have failed! I simply can't foresee a railgun of appreciable power and range fitting into the hull and turret of a main battle tank!

In fact, we need a smaller and much lighter tank that can be transported by current aircraft so that we can respond rapidly - days, not months. We needed months and a fleet of amphibious and RO-RO ships to get large quantities of Abrams tanks and other armored vehicles to Kuwait during the Gulf War! Other countries took note of this, and a future aggressor simply won't make the mistake of letting the US build up a force of appreciable size!

Again, we have existing tools and tech to handle just about any task that might be accomplished by a railgun. Let's spend the money on specific tools for specific tasks.