NewtonPulsifer wrote:

Explosives are 5 megajoules or less per kg. Forum estimates for batteries are 15.54MJ/kg+ for Eclipse Phase. So you could easily replace create a gas pressure gun with superconducting batteries. Just flash vaporize something to create the gas pressure with the electric power. In addition, there could be better gun propellants available compared to early 21st century gunpowders. That would be way more efficient than a rail/coil/gauss gun.

Depends on how we define "efficient" A two-stage light gas gun requires both a gunpowder ignition and a piston chamber full of helium or hydrogen. That means your light gas gun weapon is going to need gunpowder cartridges separate from the bullet (along with magazines for each), and a storage housing for holding the light gases (more than a mere cartridge; light gas guns tend to use a decent amount of gas per discharge). I don't think this makes for a very feasible man-portable weapon.

NewtonPulsifer wrote:

I guess railguns could catch up in this arms race if by 10 AF somehow it was figured out how to make them much more efficient. I'm trying to think up sci-fi uses for a very efficent rail/gauss/coil effect.

Besides weaponry, railguns are excellent for material shipment, eliminating the need for a propulsion system or cargo hauler. Why haul it when you can just launch it?

Arenamontanus wrote:

We had a thread last year about riding them.

Thread found and read. Fantastic. Expanding the concept of using a railgun as a "coach" version of personal travel, how practical would it be use them to travel between habitats? You wouldn't have to worry about reaching escape velocity, and there's no deceleration in vacuum. You would have to have A) extraordinarily precise information about the future position of the destination, B) a means of slowing down once you got there, and C) a habitat crazy enough to let you shoot people at it with a giant space gun. A) The computational power in the EP setting is probably enough to work things out to several thousand decimal places, and presumably the two end points would share telemetry. B) is a problem given the velocities we're dealing with. Is it possible to run a railgun in reverse, i.e. decelerate something traveling absurdly fast by turning the kinetic energy into electricity (similar to regenerative braking)? C) There are several habitats that are, to put it charitably, not in their right minds.

Railguns have a theoretical efficiency of 50%, far higher than that of coilguns or any chemically-propelled system...

Arenamontanus wrote:

NewAgeOfPower wrote:

Railguns have a theoretical efficiency of 50%, far higher than that of coilguns or any chemically-propelled system...

Do you have any good reference for this? I really need it for a paper I am writing, and I would be most obliged.

I do not. A quick Google search turns Lp^2*E/(E*Lp^2+4*R^2*m+2*R*2^(1/2)*(m*(2*R^2*m+E*Lp^2))^(1/2)) as an ideal railgun efficiency equation, where there is no friction, and the capacitors are fully discharged by an ideal switch, Lp = Effective Rail Inductance, (H/m) E = Total Energy stored in Capacitors (Joules) M = Projectile Mass (Kilograms) R = Resistance (Ohms) This was taken from the 4hv forums.

You could aerobrake in the upper atmospheres of planets/moons, though this obviously has the downside of a catastrophic failure killing everyone, it isn't much worse than sailing off into the void if you missed the receiver array. Also you could simply use less fuel. A mass driver launched ship with very small engines for maneuvering and for gradually slowing down as it approached the destination would need about half the fuel of a regular ship. You could even use one time use rockets etc. if it wasn't a normal destination. In any case small maneuver engines would probably be necessary just in case something wandered into the path of the launch vehicle or its path was otherwise disturbed, gas leak etc.

Arenamontanus wrote:

For EP, this is an important aspect: you need a lot of cooling for big railguns and coilguns. There will be heating proportional to the total energy put into the projectile.

How exactly does one get rid of heat in a vacuum? The only option available is radiation, and heatsinks or other methods of increasing the surface aread are fairly inefficient with nothing around the fins. Do you just sink it it into a water ice asteroid and hope for the best?

Laser and doppler cooling cannot remove large amounts of heat: they are great for achieving low temperatures, but the energies are really low. Despite Brin's "Sundiver" lasers are not very good at removing heat (they have a lousy thermal capacity per joule). The best approach is likely to just have a lot of mass that has good thermal capacity and put the heat in there. In vacuum it can then slowly re-radiate the heat as IR, or you could eject the mass - or vaporize it.

Arenamontanus wrote:

The best approach is likely to just have a lot of mass that has good thermal capacity and put the heat in there. In vacuum it can then slowly re-radiate the heat as IR, or you could eject the mass - or vaporize it.

Alternatively, you can find methods of using that waste heat. A military railgun, for instance, can potentially pump the waste heat from one round into the next, converting ammunition into superheated rounds (excellent for armor piercing, I imagine). In fact, that might be a handy way for a military ship to radiate heat all around… dumping heat into any weaponized projectiles or jettisoned waste is a great way to make lemonade out of lemons.

Chevre wrote:

I seem to remember some description from one of the Mass Effect games about spraying microdroplets of some kind of liquid (some kind of metal, I think) into space and then recollecting the droplets once they had cooled. Would this actually work?

Liquid droplet radiators have been investigated. They seem to make sense for spacecraft and preusmably big installations, but I am not sure they make much sense for smaller weapons. http://www.projectrho.com/public_html/rocket/basicdesign.php#id--Heat_Ra... Some papers: http://courses.ucsd.edu/rherz/mae221a/reports/Nelson_221A_F07.pdf http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900002490_1990002... One big minus is that this way you will be a bright IR source, easy to target. Pre-heating other railgun bullets: I doubt this is very useful. First, the damage is done by kinetic energy and not thermal energy. Second, hot bullets might be harder to accelerate - because they might be above the Curie temperature (where iron loses ferromagnetism) and because they might melt. And their total thermal capacity is not going to be big.

Chevre wrote:

The Enemy wrote:

Could be used to launch larger-than-normal sized objects, as well.

Wouldn't that necessitate an even bigger deceleration tunnel on the other end? Unless, of course, you were launching large objects that you didn't want to land so much as impact with massive destructive force. Asteroids, for instance.

Would it be possible to make it modular? Say 4 rails or similar that one can space apart depending on the incoming/outgoing load? It would need to be in space tho, rather on a surface somewhere.

Chevre wrote:

750 wrote:

Would it be possible to make it modular? Say 4 rails or similar that one can space apart depending on the incoming/outgoing load? It would need to be in space tho, rather on a surface somewhere.

That's an interesting solution to the (incredibly dangerous) habitat-to-habitat transit system.

Borrowed it form Babylon 5 jump gates.

Arenamontanus wrote:

Consider a 10 km/s railgun bullet. It has kinetic energy 0.5*10^8 J/kg. If we have a 50% efficient railgun it has to use 10^8 J/kg. Let's assume half goes into the bullet - then it would have a thermal energy equal to its kinetic energy. For iron that would be about 11,000 degrees (using thermal capacity of 0.45 kJ/K kg). A bit too hot to remain even liquid. If we say the max heating allowed is 2750 K (assuming absolute zero initial temperature) the bullet heating has to be about 40 times less. The problem is melting/vaporising, not Curie.

Well… damn.

Arenamontanus wrote:

I think you cannot maintain ferromagnetism in liquids (liquid magnets are cheating by having an emulsion of nanoparticles suspended in them). Of course, a railgun that sends a largely molten metal slug can still be useful - it is just that as the above calculation shows you can punch more energy into something by upping the velocity rather than the temperature.

Oh I agree. But heat is an important issue to address in space combat. I figured that finding a means to weaponize your heat might be a bit better than simply radiating it to space, especially since targeted weapons are likely going to be heat-seeking. Ooh, there's a thought. Radiated slugs also double as an excellent flare in space, potentially calling off simple thermal guidance systems.

Decivre wrote:

Oh I agree. But heat is an important issue to address in space combat. I figured that finding a means to weaponize your heat might be a bit better than simply radiating it to space, especially since targeted weapons are likely going to be heat-seeking. Ooh, there's a thought. Radiated slugs also double as an excellent flare in space, potentially calling off simple thermal guidance systems.

I would expect that there's no such thing as a 'simple' thermal guidance system by the time of Eclipse Phase. I'm not up to date on the latest and greatest in flare rejection technology, admittedly, but even as of 5+ years ago, you had IR seekers that examined multiple portions of the spectrum (typically UV in addition to IR), that compared the shape and the intensity of the target signature with any detected signatures, and that considered factors like speed and direction of travel. Just firing a heated slug would do next to nothing to fool even a modern IR missile, and the tech has doubtless improved substantially by the time of EP. I would expect that even if a guided weapon was IR primary, given the miniaturization of tech and the incredible processing power available, it would also be carrying at least one other type of secondary guidance. This would be especially true for the inner system, where the sun always has to be accounted for (I'm less sure how this would work in space, but putting the sun in the background is very much a valid part of a defensive strategy against IR missiles for aircraft, and I'd assume a very high risk of saturation for any thermal sensor.) This isn't to say that you couldn't potentially use some of your excess heat as part of an overall masking strategy for the ship, but any projectiles will have to be emitting in an extremely specific manner in order to have a chance of fooling the seeker -- you're not just firing heated projectiles, you're firing drones that fly like your spacecraft and emit like it across multiple spectra.

In the case of aircraft and modern IR missiles, it's not so much that they're going to go after the sun as that it makes it more difficult for the seeker to differentiate between the aircraft and the background -- it increases background clutter, essentially. It's not a defense that a pilot could rely on by itself, but it can still help.

Prophet710 wrote:

Even feasible? or already dead horse?

Feasable but have some defects: extra rail are payload, if you cool aboard don't dump heart only move it to another place, if you dump rail for recovery solve heat problem but have some manouvre problem.

750 wrote:

And i thought that IR missiles stopped going after the sun (a common issue during the Vietnam war) because they now ignore anything hotter than some value or other.

Very bright obyecr overflow sensor blinding them, Dazzlers in EP rulebook do this and can be used even today. [url=http://en.wikipedia.org/wiki/Dazzler_%28weapon%29]dazlers[/url] My idea is use very hot (or bottled heat) railgun shots (for thermal dump) mixed with regular cool shots and active ECM (dazlers, radar noise, chaff...) to create a confusion, for point defence you can reduce fire to unhampered trajectory and knowing what you have done this display don't hamper your own scans. PS space missile in EP can be piloted by beta forks so bordering human intelligence (alpha can be hampered by self preservation ;) )!

NewAgeOfPower wrote:

I believe you cannot exceed 50% efficiency, due to Newton's Second Law. As much energy is driving you backwards as is driving the projectile forwards.

Remember that you can make the railgun assembly far, far heavier than the projectile: while momentum is conserved, the backwards velocity is v*(m_projectile / m_railgun). If m_railgun is an asteroid mass, the velocity becomes pretty small no matter how fast the projectile is. And the kinetic energy is the square of this. (Fully worked out: recoil efficiency loss K_recoil / (K_recoil + K_projectile) = m_projectile / (m_projectile + m_railgun). So if the projectile is 1 kg and the gun assembly is a thousand tons, the loss is one millionth of the total energy. ) I have been trying to dig up proper papers on railgun and coilgun efficiency, and found a few. Quenched superonductor coilguns look like they are most effective - no resisitivity losses and very small inductive losses. Probably what is used in EP. Unfortunately few has been built in reality, so most studies and look at railguns, which are much messier.

Trekkin wrote:

How are the coils quenched? I can't conceive of the heat from the projectile passing being enough, or in the right place, to heat the coils enough without the friction slowing the muzzle velocity to near zero.

You can do it by sending a strong magnetic field across them. Remember that superconductors have an upper allowed current and magnetic field.

Quote:

And how would one cool them back down quickly enough to enable fully automatic fire?

I don't think the authors have worked on it, since they were mainly thinking about lunar mass drivers and space launches rather than weapons. If quenching is done without heating autofire should be doable, the big limitation is how fast you can charge up the coils.

Arenamontanus wrote:

You can do it by sending a strong magnetic field across them. Remember that superconductors have an upper allowed current and magnetic field. I don't think the authors have worked on it, since they were mainly thinking about lunar mass drivers and space launches rather than weapons. If quenching is done without heating autofire should be doable, the big limitation is how fast you can charge up the coils.

Do you mean like this paper? http://www.askmar.com/Massdrivers/Superconducting%20Quenchgun.pdf It seems to suggest quenching/current cessation happens automatically in the case of a moving ferromagnetic projectile, or do I have that misunderstood? I'm afraid I lack the physics to figure out the limitations on charge time for something hand weapon-sized, but it doesn't look impossible to me. I suppose as a brute-force solution multiple barrels might be connected to a single power source as a sort of minigun, with each barrel discharging in turn.

Arenamontanus wrote:

Decivre wrote:

How will entropy be affected if the railgun barrel (rail? whatever it's called) is coated in a frictionless material? Wouldn't that improve efficiency?

Sure, but there is no truly frictionless materials. The problem with a railgun is that you have an object moving across the rails either touching them while accelerating to multi-kilometers per second speed (ouch, friction) or a plasma discharge connecting it to the rails (ouch, heat). Real railguns tend to wear out quite dramatically - note all the smoke in the videos, a lot of that is burned railgun.

Maybe you could use liquid helium 3 or some other superfluid as a lubricant? That might be so close to frictionless it would be effectively the same. I've heard that helium 3 liquid has some really bizarre properties, and it can even flow uphill. In Haldeman's 'forever war' it was said that you can't stand on a helium 3 surface.

NewtonPulsifer wrote:

Explosives are 5 megajoules or less per kg. Forum estimates for batteries are 15.54MJ/kg+ for Eclipse Phase. So you could easily replace create a gas pressure gun with superconducting batteries. Just flash vaporize something to create the gas pressure with the electric power. In addition, there could be better gun propellants available compared to early 21st century gunpowders. That would be way more efficient than a rail/coil/gauss gun. I guess railguns could catch up in this arms race if by 10 AF somehow it was figured out how to make them much more efficient. I'm trying to think up sci-fi uses for a very efficent rail/gauss/coil effect.

Sorry, took me a while to dig this out. The Fire Fusion & Steel supplement for Traveller:New Era was a spectacularly anal look at Weapon and Vehicle design. Everything is designed with real equations of motion where possible and there are a number of essays about the evolution of the relevant technologies up to the present day and into the theorised future. One of the subjects covered is ETC (Electrothermal Chemical) propelled projectile weapons. The theory goes that regardless of what you do the muzzle velocity of any projectile is limited by the gas expansion rate of the explosive used. This gives a maximum velocity of somewhere in the vicinity of 2000 m/s. The way around this is exactly what you proposed, you take a fuel that you vaporize into a plasma with a massive electric discharge. This has the advantage of permitting much higher muzzle velocities and not having massive initial gas pressure that drops off as the projectile moves along the barrel. Gas pressure can be built more slowly and maintained until the projectile leaves the barrel. This allows the chamber and barrel to be lighter while being more efficient. Muzzle velocity was theoretically doubled and maximum impulse of the recoil was greatly reduced. This was technology that was in development at the time, about 15 years ago, which I don't think has turned out the way the authors predicted but it certainly seems feasible that it might prove worthwhile at a future date. On the subject of recoil and efficiency I'm afraid I'm in disagreement with Arenamontanus. It doesn't matter a damn how massive the gun effectively is, 50% of the energy is going into the gun. Obviously this applies equally to gas pressure and railguns. ---- ps. We were throwing around figures of up to 45MJ/kg for EP batteries. The figure of 14MJ/kg was an absolute minimum figure.

Blergh, puke. Do you know what, I won't tell you how many years I studied Physics at the college level, but I want a bleeding refund. It's taken me an hour to bash through those equations and work out who was wrong and why. Don't know where I got the idea that Conservation of Momentum meant both sides ended up with equal Kinetic Energies but that's twenty years of my life I'll never get back. Apologies, carry on.... edit You know on further analysis I can only conclude that I learned the basic equations of motion far too young. I analyze the crap out of everything I see these days and I obviously didn't back then. F=ma therefore, if every action has an equal and opposite reaction, an object with half the mass accelerates twice as much and ends up with twice the velocity. Therefore an object with half the mass ends up with twice the Energy. edit II deleted more drooling nonsense.

Is that not what I just said?

The way I see this, as the scales fall from my eyes, is that the velocity of the projectile from any form of gas expansion weapon is limited to the expansion rate of the gas. Say, between 2000m/s and 3000m/s. This limit puts you in a bind. You want to reduce recoil and deliver as much Energy to the target as possible. With an upper limit of 4.5MJ/kg of projectile you are going to be using rather large projectiles and consequently suffering quite a lot of recoil. Railguns on the other hand might easily achieve 7000m/s which is nearly 25MJ/kg. You could therefore use much smaller projectiles to deliver the same energy to a target. This results in only 42% of the momentum of the larger projectile and you can carry a lot more ammo. To take that to its logical idiocy, if you were to 'Railgun' an M16 so that it fired a projectile with the same energy at 7000m/s, the projectile would weigh 0.072g and the recoil energy of each shot would be less than a thousandth of a modern M16. The impulse would go up a bit but you would probably feel less than two hundredths of the kick. And a magazine would hold over a thousand rounds.