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Real railguns

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Wed, 2011-04-20 13:43

Arenamontanus Arenamontanus's picture

Real railguns

Just noticed this little clip about a current Mach 6-7 railgun: http://www.boingboing.net/2011/04/20/railgun-fires-hugo-a.html 32 MJ per bullet is pretty nice - if you can keep the bullet from frying. Notice all the smoke - that is part of the armature vaporizing into metal vapour. This is incidentally a fun problem for railgun projectiles in space combat. According to my calculations there are pretty tight limits here on efficiencies: you don't want the bullet to heat up so much it melts or boils off, and sacrificing a sabot means you will still have a pretty noticeable IR emission that can help the target to detect the incoming projectile. Railguns for space warfare will be far above these puny velocities (mach 6 is just 2 km/s - typical velocity differences in space are several times higher)

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Wed, 2011-04-20 18:32

The Green Slime The Green Slime's picture

Re: Real railguns

Nice fireworks show. Watching the presentation though I'm skeptical about the role of the railgun on the modern (not to mention future) battlefields. Exchanges of fusillade on the high seas isn't exactly a regular type occurrence any more. They'd probably never replace miniguns (or perhaps one day lasers) for purposes of intercepting an incoming warhead. And would they ever trump a cruise missile or a fighter jet/drone for long-range destruction? Considering a large percentage of weapons development projects seem to be bullshit vapourware for the sake of pork barreling, I wonder if one of these toys will ever see use short of the day we're building Death Stars.

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[img]http://boxall.no-ip.org/img/iso_userbar.jpg[/img] [img]http://boxall.no-ip.org/img/aliens_userbar.jpg[/img] [i]Time will perfect matter.[/i]

Thu, 2011-04-21 03:57

Arenamontanus Arenamontanus's picture

Re: Real railguns

Sure, it is likely vapourware (mainly since the majority of defence projects turn out to be vapourware). But there is apparently a good case for rail-guns for intercepting supersonic cruise missiles. In space combat you definitely want ways of launching things at several km per second, even if they are only sensor packages (they need to spread out significantly before combat to be useful).

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Fri, 2011-04-22 12:39

puke

Re: Real railguns

Sure there is value here. Those cruise missiles are exactly what the gatling guns and lasers are shooting down, even now. The rail gun will make a smaller hole, but it isnt going to be intercepted with modern countermeasures. I dont know how that translates to space combat, but the reasoning is probalby similar. Smaller, higher velocity, dumb projectiles are probably harder to stop than larger, smaller, smart ones. in the future there is nothing stopping you from making your railgun projectile smart also -- but that will always be done at some cost in size and density.

Fri, 2011-04-22 19:19

Arenamontanus Arenamontanus's picture

Re: Real railguns

I think 'smart' space combat railgun projectiles mainly have sensors and know how to rotate for best impact. Actually changing trajectory significantly would require a hefty reaction mass tank and energy source - there are some amazing benefits in impact energy if you don't have to carry those around. Missiles are more about dodging point defences and getting close before deploying an interesting payload (sabotage nano, or more likely a nuke or antimatter charge). A smart railgun projectile will not only send back info about the target from real up close (useful even if it misses) but can do clever things like rotating so it hits with a broadside (more damage against lightly armoured targets, bigger chance to hit) or straight on (better armour piercing). It can also split into several parts, so the first part breaches any outer armour such as a Whipple shield, and the subsequent parts pass through the hole digging deeper. Still working on the relative strengths of point defences, railgun projectiles and missiles - the final and defining part of my project of figuring out the physics of EP spaceship combat. A lot seems to hinge on overwhelming point defences with several targets at the same time, but the exact balance depends on things like detection ability, signal processing and how hard the defenses can strike over fairly long distances.

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Fri, 2011-04-22 23:52

puke

Re: Real railguns

Agreed. I'm not sure I've met a dedicated ship-to-ship combat game that models that level of detail, though. SFB was pretty hardcore, but their science was soft. Full Thrust was pretty abstracted. Maybe DP9's Lightning Strike? Maybe something like this (never played it personally): http://www.adastragames.com/products/adastra/av.html

Sat, 2011-04-23 05:43

Arenamontanus Arenamontanus's picture

Re: Real railguns

My current writeup is ~30 pages of rather dense physics with plenty of equations. I doubt it can be turned into a game system on its own, but it serves to tell me what can and cannot be done in space combat. For example, ships in EP can be detected at very long distances and stealth is hard (but not 100% impossible), but during battles sensors saturate or burn out at a high rate. So a lot is about clouds of wildly dodging semi-autonomous weapons and sensors trying to hit each other and the ships.

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Sat, 2011-04-30 04:50

Smokeskin

Re: Real railguns

Arenamontanus wrote:
32 MJ per bullet is pretty nice - if you can keep the bullet from frying. Notice all the smoke - that is part of the armature vaporizing into metal vapour. This is incidentally a fun problem for railgun projectiles in space combat. According to my calculations there are pretty tight limits here on efficiencies: you don't want the bullet to heat up so much it melts or boils off, and sacrificing a sabot means you will still have a pretty noticeable IR emission that can help the target to detect the incoming projectile.

Isn't most of the heat from air friction? Do you have any numbers on how much heat actually comes from the electromagnetic field? I would suspect it to be extremely little, but of course even a tiny fraction can amount to a lot at extreme energies.

Sat, 2011-04-30 04:51

Jay Dugger Jay Dugger's picture

Re: Real railguns

(CAVEAT: I used to operate naval guns and formerly had a professional interest in this sort of technoporn.) If you drill past BoingBoing, the Navy reports provide much more detail. The railguns aren't envisioned as ballistic missile defense. (That's a job for AEGIS.) They do the same job as current naval guns, but with three significant advantages. 1) Vastly improved range (more than an order-of-magnitude improvement) A railgun projectile can travel much farther inland from an off-shore target. This reduces the cost of combined operations by allowing naval gun fire support to supplement or replace organic artillery assets. 2) Vastly reduced logistical requirements (ditto) Naval guns require armored magazines with special fire suppression gear for both the propellant charges and the projectiles. Railguns don't require explosive rounds. (They'd cook-off from atmospheric heating anyway.) Railguns don't require propellant charges. This means an equivalent storage volume holds more rounds, and those rounds require no special handling. They are very close to dumb chunks of metal whose lethality derives from their speed and accuracy. 3) Much shorter time-of-flight (ditto) Faster rounds pose greater challenges to anti-artillery measures. It gets much harder to knock them from the sky as projectile speed increases. Also, tracking radars for counter-fire batteries have shorter response windows. Friendly units under fire need not endure quite so long before artillery support delivers ordnance on target. Enemy units also have shorter windows between detection and counter-fire. (I.e., once spotted, they better run fast before the response hits.) They also offer other lesser advantages, like faster rate-of-fire, fewer moving parts, and so on. I wouldn't call this one vaporware. See also the electromagnetic launch system for naval aviation being installed on the next American carrier, the Gerald Ford.

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Sometimes the delete key serves best.

Sat, 2011-04-30 04:52

#10

Jay Dugger Jay Dugger's picture

Re: Real railguns

I second the recommendation for Ad Astra Games' Attack Vector Tactical. I also suggest you look at its generalization, Squadron Strike.

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Sometimes the delete key serves best.

Sat, 2011-04-30 04:55

#11

Jay Dugger Jay Dugger's picture

Re: Real railguns

Have you done much work with electronic warfare? That seems more useful to me than kinetic kill, boarding parties (meso- or micro-scale) or directed energy weapons. Oh, and I hope to see Server Sky-style arrays as sensors and weapons, a la modern AESA sets. And nuclear-pulse rockets, too. :)

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Sometimes the delete key serves best.

Sat, 2011-04-30 05:55

#12

Arenamontanus Arenamontanus's picture

Re: Real railguns

Smokeskin wrote:
Isn't most of the heat from air friction? Do you have any numbers on how much heat actually comes from the electromagnetic field? I would suspect it to be extremely little, but of course even a tiny fraction can amount to a lot at extreme energies.

For air: "An approximate rule-of-thumb used by heat shield designers for estimating peak shock layer temperature is to assume the air temperature in kelvins to be equal to the entry speed in meters per second " (Wikipedia) In space: assume an efficiency f. Doing useful work K = 0.5 mv^2 to launch a mass m projectile at velocity v will require E=K/f J of energy, releasing K(1-f)/f J as heat. For a 1 kg 10 km/s projectile K is 0.5*10^8 J. If f is 90% (which sound far better than what we currently got) we get 5.5 megaJoule of waste heat. Assuming half of it goes into the projectile which is iron, with heat capacity 0.45 J/g K, I get a temperature of about 6000 K. Remember that in railguns the projectile has to be in contact with the rails, so there will at the very least be some friction going on which in turn releases a plasma arc.

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Sat, 2011-04-30 07:08

#13

Smokeskin

Re: Real railguns

Arenamontanus wrote:
Remember that in railguns the projectile has to be in contact with the rails, so there will at the very least be some friction going on which in turn releases a plasma arc.

It has to? I didn't know that. That'll be a problem...

Arenamontanus wrote:
In space: assume an efficiency f. Doing useful work K = 0.5 mv^2 to launch a mass m projectile at velocity v will require E=K/f J of energy, releasing K(1-f)/f J as heat. For a 1 kg 10 km/s projectile K is 0.5*10^8 J. If f is 90% (which sound far better than what we currently got) we get 5.5 megaJoule of waste heat. Assuming half of it goes into the projectile which is iron, with heat capacity 0.45 J/g K, I get a temperature of about 6000 K.

Yeah, but the factors are just guessworks In your space warfare pdf your examples are spread over 2 orders of magnitude. The idea of moving conducting materials in magnetic fields or vice versa aren't new, so I thought maybe someone somewhere had solid data on it (with respect to things might playing out quite differently at these velocities).

Sun, 2011-05-01 18:00

#14

root

Re: Real railguns

root@Real railguns [hr] There is a difference between a "railgun" and a "coilgun". The railgun runs on rails, and has a terrific amount of friction heat to deal with. The coilgun uses the magnetic field to propel the projectile, so it does not have the contact friction to deal with. However, since it does not have contact with the rails, the technical difficulty of delivering sufficient power to the projectile is considerable. There are any number of non-linear factors to deal with, and we don't really have any sort of capacitors which have the requisite "snap" to dump enough current fast enough to make them worth much. There are a few hybrid designs, such as a helical coil gun with a similarly helical projectile, but there is precious little funding going in that direction.

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