US Navy's Railgun Now Undergoing Tests in New Mexico • Hacker News Board

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#261

Post by imtringued » Fri, Jun 28, 2019, 2:05 PM UTC

ijpoijpoihpiuoh wrote: post #94

derefr wrote: post #49

Earlier quoted context omitted.

It's not like the technology behind railguns hasn't been stable and ready to use for decades now. It's just that, until the more recent invention of ancillary technologies like supercapacitors, it's been so costly to fire them that there was no logical place for them on the battlefield, so we never bothered to operationalize them.

To me, it's not entirely clear why we're bothering. Do we actually envision shore bombardment as a useful combat modality going forward? IIRC the types of ships that can mount this weapon are the same types that are currently vulnerable to hypersonic anti-ship missiles. All our potential equiv-tech adversaries have those, to my understanding. So wouldn't the railgun only be useful against lower tech opponents? And ag…

The railguns can be used to shoot down missiles. Interception is a matter of speed. If railguns are faster they win and we are back to fighting with bullets.

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#262

Post by mabbo » Fri, Jun 28, 2019, 2:08 PM UTC

ChuckMcM wrote: post #228

mabbo wrote: post #135

Earlier quoted context omitted.

The concept is the same for any projectile weapon: I'm going to convert stored energy into kinetic energy. Bow and arrow? Stored energy in the bow turned into kinetic energy in the arrow. Handgun/cannon/musket? Stored chemical energy in some compound, quickly converted into kinetic energy in the bullet via exploding. A rail gun isn't much different- it's just a different means to store the energy (in electrical stora…

I don't disagree, at 100,000 feet all weapons are mass gets propelled by force, where I part ways with you is here: "A rail gun isn't much different" While I completely agree that in concept rail gun isn't different, making one is extremely different. That is why I would have little trouble making a bow and arrow over the weekend that would serve as a respectable weapon[1], but I would not be able to create a similar…

Oh, we're definitely in agreement- conceptually the same, implementation details of a few orders of magnitude different.

Mostly I was replying to the idea that "hey, we've known how to do this for 100 years". Sure we have- conceptually. We knew that batteries/electricity + magnets could implement the same pattern as "chemicals exploding". But we sure as heck didn't know how to make it practical.

That's why it's 100 years later and very few of us have either electric cars or rail guns.

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#263

Post by imtringued » Fri, Jun 28, 2019, 2:20 PM UTC

mikeash wrote: post #215

aeneasmackenzie wrote: post #212

Earlier quoted context omitted.

You use an extremely long track, not high acceleration.

Hard to do if you’re aiming straight up.

No we aren't aiming straight up because we need to get into orbit. We need to go sideways as fast as possible. As long as you're not aiming at the ground you will always aim "up" but not straight up.

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#264

Post by homonculus1 » Fri, Jun 28, 2019, 2:28 PM UTC

killjoywashere wrote: post #252

ses1984 wrote: post #245

Earlier quoted context omitted.

I think kinetic or chemical refers to the type of energy that puts the project in motion, from something like a bowstring to rocket propellant.

I fully understand what OP was trying to say. The problem is words. You can think whatever you want. If you want to deal with other people in a shared reality and influence the arc of history, I suggest learning to identify, adopt, and use rigorously defined words as intended, wherever possible, because that frees up your very limited intellectual capabilities to focus on the things you actually want to change.

This is pedantry. OP's context was clear, and a comment thread on HN is not "the arc of history".

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#265

Post by SEJeff » Fri, Jun 28, 2019, 2:41 PM UTC

nradov wrote: post #238

SEJeff wrote: post #150

Earlier quoted context omitted.

Yes, but not both. You have ground to air, ground to ground, ship to ground, ship to ship. For the most part (with some standouts that do multi-role such as the Tomahawk) each munition is specialized for a single role. The railgun is being heralded as being capable of doing all 3 for limited use cases, which is fairly unique.

The SM-6 is effective against both air and surface targets.

The SM-6 is anti-air and anti-ship. It has the option to add GPS guidance to it (SM-6 Block IA and the newer IIA), but it is much more expensive as a Tomahawk and is generally not used for this case. A Tomahawk is around $1.4 million per whereas the SM-6 Block I was $5.6 million per and the newer full rate production version (Block IA and newer) is $4.8 million per.

For reference, the Hypervelocity Projectile (made by BAE Systems) that Railgun fires comes down to a unit cost of $86,000-$90,000 per shot depending on what source you read. So ~$90,000 per railgun shot gives you ~53 shots from the railgun for every single SM-6 when it comes to cost. This is entirely game changing from an economics perspective with similar capabilities. By lacking rocket motors or being explosive, it is also much safer to transport so you can likely pack more ammunition onto a ship.

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#266

Post by SiempreViernes » Fri, Jun 28, 2019, 3:24 PM UTC

imtringued wrote: post #261

ijpoijpoihpiuoh wrote: post #94

Earlier quoted context omitted.

To me, it's not entirely clear why we're bothering. Do we actually envision shore bombardment as a useful combat modality going forward? IIRC the types of ships that can mount this weapon are the same types that are currently vulnerable to hypersonic anti-ship missiles. All our potential equiv-tech adversaries have those, to my understanding. So wouldn't the railgun only be useful against lower tech opponents? And ag…

The railguns can be used to shoot down missiles. Interception is a matter of speed. If railguns are faster they win and we are back to fighting with bullets.

And what is the advantage over lasers in this "shoot down small fast things" role?

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#267

Post by mikeash » Fri, Jun 28, 2019, 3:34 PM UTC

imtringued wrote: post #263

mikeash wrote: post #215

Earlier quoted context omitted.

Hard to do if you’re aiming straight up.

No we aren't aiming straight up because we need to get into orbit. We need to go sideways as fast as possible. As long as you're not aiming at the ground you will always aim "up" but not straight up.

“Imagine putting one or many of those new Space X internet satellites into a rail gun that goes straight up.”

Firing sideways could save you much more fuel but also means you’re in the lower atmosphere at much higher speed and for much longer.

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#268

Post by Balgair » Fri, Jun 28, 2019, 3:42 PM UTC

steve19 wrote: post #200

mabbo wrote: post #135

Earlier quoted context omitted.

The concept is the same for any projectile weapon: I'm going to convert stored energy into kinetic energy. Bow and arrow? Stored energy in the bow turned into kinetic energy in the arrow. Handgun/cannon/musket? Stored chemical energy in some compound, quickly converted into kinetic energy in the bullet via exploding. A rail gun isn't much different- it's just a different means to store the energy (in electrical stora…

You put it well. The problem of of putting energy into projectiles caused a move from kenetic to chemical weapons (missiles). But delivering chemicals to a target is expensive and at some point scaling it up becomes politically dangerous (you cannot just lauch even a conventional icbm without all sorts of risks). Scaling up kenetic weapons ended up with huge battleship guns that did almost as much damage to the ship…

One important difference is heat management.

Electric weapons are super neat. Their specs can put the projectile very kenetic, they can uitilize more exotic energy sources, etc.

However, in terms of a man portable gun, they just really don't have the specs needed. Sure, we can make them more ruggeded and more effective at lower KE output. But a major issue for a man portable gun is heat. Heat causes expansions and affects ductility, ductility and expansion cause jams. Jams mean you're dead.

It turns out that the brass cartridge is really really good at getting hot and then taking that heat with it as it is ejected. The HK-G11 is a good example of the issues that the lack of heat ejection can cause [0].

As we do not have super-conducting materials that can be man portable and at room temperature, there is significant heat build up in the copper rails of the rail gun. With catridges you can throw that heat away easily, with rails, you have to have a cooling system in them to continue to use the gun. Either that or you have to stop the warfighter from using the gun until it cools. This, especially when lives are on the line, is non ideal.

Hence, railguns are mostly used in much larger applications like ships. There you can have the cooling equipment to keep the gun functional and operating and you can use the ocean as a heat sink. Your heat issues are still present though. More exotic materials than cooper are used for the rails, things that need to be at LN2 temperatures. But on a ship, you have the room and weight to carry the LN2 cooling equipment. Currently, railgun tech is still being developed. Reliability is a big issue, one that was solved in chemically launched projectiles through decades of tial and error. Rail guns are likewise going to need those decades of learning to occur.

[0] https://www.youtube.com/watch?v=QGKcvM2Hh4g

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#269

Post by petra » Fri, Jun 28, 2019, 3:47 PM UTC

Balgair wrote: post #268

steve19 wrote: post #200

Earlier quoted context omitted.

You put it well. The problem of of putting energy into projectiles caused a move from kenetic to chemical weapons (missiles). But delivering chemicals to a target is expensive and at some point scaling it up becomes politically dangerous (you cannot just lauch even a conventional icbm without all sorts of risks). Scaling up kenetic weapons ended up with huge battleship guns that did almost as much damage to the ship…

One important difference is heat management. Electric weapons are super neat. Their specs can put the projectile very kenetic, they can uitilize more exotic energy sources, etc. However, in terms of a man portable gun, they just really don't have the specs needed. Sure, we can make them more ruggeded and more effective at lower KE output. But a major issue for a man portable gun is heat. Heat causes expansions and af…

We have much better engineering tools today, so why would it take decades ?

Re: US Navy's Railgun Now Undergoing Tests in New Mexico

#270

Post by logfromblammo » Fri, Jun 28, 2019, 4:19 PM UTC

ars wrote: post #193

logfromblammo wrote: post #133

Earlier quoted context omitted.

You need about 1200km to reach LEO from the equator with max acceleration of about 3G. I did the math once, and a track from the Galapagos Islands to the peak of Chimborazo on the continent in Ecuador would do it.

I was a little shocked at that number, so I ran the math again, and wow. Quite a bit longer than I expected. Maybe a circular track till you build up speed, then switch over to the mountain track? Of course the circular track would have g-forces due to that..... (Do you feel like doing more math?) I guess it's more practical to use the track just for the initial boost above the atmosphere, then a rocket to add speed.…

A circular track means you'd have to accelerate the vehicle inward proportional to the square of the velocity.

If target exit velocity is 11 km/s, and the acceleration of the straight portion of the track is insignificant compared to the circular portion, in order for the combined linear and centripetal accelerations to stay under 3G, I'd make each component 20.8 m/s^2.

So if 20.8 m/s^2 = (11000 m/s)^2/r, then r has to be 5800 km. Closing the ring at that radius makes the circumference larger than the distance needed for a linear track.

Untrained humans could not ride it up to Earth-escape velocity. LEO only needs about 8 km/s. So that requires a radius of 20.8 m/s^2 = (8000 m/s)^2/r, 3000 km. Still too big.

What if we bump the G threshold to 5G? Then centripetal portion of the acceleration should top out at 34.7 m/s^2, and required radius drops to 1850 km.

Okay, cargo only then, just to LEO. 25G max acceleration, 8000 m/s exit velocity. Radius 370 km, circumference 2300 km. Still more track than needed by a linear launcher.

It's always cheapest to put all the acceleration towards moving the vehicle in the direction you want it to go, instead of wasting some energy pushing it in different directions that ultimately cancel out.

You could use a spiral track, but the final leg of the spiral is going to have an effective radius so large it might as well just be straight the whole way.