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

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

#121

Post by xyzzy_plugh » Thu, Jun 27, 2019, 9:06 PM UTC

WhyKill wrote: post #118

openasocket wrote: post #116

I've never quite understood why we need to change technologies so completely to get the projectile moving faster. As in, why couldn't we do the same thing by just making a bigger explosion in the barrel? I assume there's some point of diminishing returns or inefficiency that makes it impractical, but I haven't been able to find a source that gives the math to explain it.

Gas pressure just can't linearly accelerate something as efficiently as magnetic force can. You can get so much more velocity for the same barrel diameter instead of scaling barrel diameter and thickness because you have thrust being added through the whole length of the barrel instead of gas pressure being used to propel a slug at the end and then dissipating.

Yep, this is really a "why not both?" scenario.

Chemical propellant in first stage, electromagnetic acceleration in the second stage.

I'm curious if they want plasma projectiles explicitly, but I guess it's all kinetic at the end of the day.

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

#122

Post by sharpneli » Thu, Jun 27, 2019, 9:06 PM UTC

SEJeff wrote: post #38

clarkmoody wrote: post #22

Earlier quoted context omitted.

Someone a little better with aerodynamic drag computations could do a quick back-of-the-napkin to figure out the maximum range of something fired at sea-level going Mach 3,4,5,6,etc. The biggest issue I see is just vaporizing the projectile on launch, so there would be an upper bound on muzzle velocity based on projectile material properties.

But the US Military uses depleted uranium (the hardest material) for tank rounds, so they could make pretty hardy railgun projectiles.

Not the hardest. Bit less than Tungsten so pretty hard but far from hardest. It is used because it is really really dense and hard enough. Perfect for piercing armor.

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

#123

Post by shereadsthenews » Thu, Jun 27, 2019, 9:08 PM UTC

leetcrew wrote: post #86

Fej wrote: post #73

Why have coilguns not gotten the same amount of attention as railguns?

I'd guess the main reason is that it's an inherently more complicated design; if you don't sync the electromagnets perfectly you spend energy decelerating the round. it does have the advantage that the projectile doesn't need physical contact with an electified rail, so maybe it could be less damaging to the device itself.

These railguns also have synchronized pulsed power systems that are quite complex. The University of Texas has been researching this stuff for decades. Here's a simple paper I Googled up https://apps.dtic.mil/dtic/tr/fulltext/u2/a639371.pdf

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

#124

Post by xyzzy_plugh » Thu, Jun 27, 2019, 9:10 PM UTC

mcguire wrote: post #114

1) " The system charges for two minutes... " One shot per two minutes is pretty bad; the current 5" gun shoots 16-20 rounds per minute. 2) "...at speeds up to Mach 6." One of the issues with old school, large caliber naval guns was that the blast from the guns would damage or destroy equipment near the muzzle. (I was just reading Pursuit by Ludovic Kennedy, about the British pursuit of the Bismark, and (IIRC) one shi…

No, beyond the initial propellant. The projectile accelerates constantly (hand waving here) so it's a constant force on the origin (relative mass should make this a wash), whereas the projectile enjoys exponential velocity.

Two minutes is pretty short, I'm actually blown away. If only we could all charge our electric vehicles that quickly.

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

#125

Post by logfromblammo » Thu, Jun 27, 2019, 9:11 PM UTC

ars wrote: post #107

antpls wrote: post #63

Could the same technology be used to help the lift off of Falcon9-sized space rockets from a deep vertical tunnel ?

See here: https://en.wikipedia.org/wiki/Mass_driver You actually want horizontal velocity, not vertical. Getting "up" is easy (it's only a few miles), it's the horizontal speed that keeps you there. The only reason rockets launch straight up is to avoid the thick air as quickly as possible, then they turn and accelerate sideways the rest of the way.

Or they do a gravity turn to round off the corner, largely in the same manner as that one math problem you probably did in school that involved finding the fastest path that includes running on the beach for some distance, then swimming at slower speed to a point in the water.

Also, different rocket designs have different efficiencies at different outside pressures due to overexpansion or underexpansion of the exhaust plume. Kicking a rocket out of the atmosphere--or at least up to a lower pressure altitude--with a big gun first stage changes the requirements a bit for the second stage rocket engine. It's slightly more efficient.

But the biggest advantage is that you aren't ever accelerating the fuel for the first stage along with the rest of the vehicle. The disadvantage is that you need a very long track if you don't want to reduce vehicle occupants to thin red paste with excessive acceleration. Also, you can evacuate the gun barrel and put one or more plasma windows at the end, so your initial acceleration doesn't have so much atmospheric drag.

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

#126

Post by zw123456 » Thu, Jun 27, 2019, 9:11 PM UTC

You could rule a punkin chunkin contest with that if you could find a way not to pie :)

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

#127

Post by xyzzy_plugh » Thu, Jun 27, 2019, 9:12 PM UTC

coder543 wrote: post #58

pneill wrote: post #51

Does anyone know why there is a flash of light and a puff of smoke when the gun fires? Since it's electromagnetic, you'd think you wouldn't see anything.

Railguns are not coilguns. Railguns maintain physical contact between the projectile and the rails for the entire length of the barrel. That's a lot of friction. Friction leads to heat, light, and a puff of smoke from material that has been ablated during the launch, if I were to guess. This leads to one of the big problems with railguns, which is that you have to replace the rails frequently due to wear and tear.

They don't need to maintain physical contact. You can have an air gap, as long as the circuit shorts through the projectile.

The rails still wear down.

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

#128

Post by semi-extrinsic » Thu, Jun 27, 2019, 9:13 PM UTC

JDulin wrote: post #70

pneill wrote: post #51

Does anyone know why there is a flash of light and a puff of smoke when the gun fires? Since it's electromagnetic, you'd think you wouldn't see anything.

A combination of three things: 1) The flash is air in contact with the super-hot rail ionizing instantaneously from the heat, and becoming plasma. 2) Friction (From the projectile against both the rail itself, and air) producing vast amounts of heat, which then a) causes water vapor in the air to flash to steam b) produces smoke from burning metal I am not sure how much of the smoke is from 2)b), because I'm assuming…

Nitpick on 2 a): you can't turn water vapor into steam, water vapor is steam.

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

#129

Post by layoutIfNeeded » Thu, Jun 27, 2019, 9:13 PM UTC

openasocket wrote: post #116

I've never quite understood why we need to change technologies so completely to get the projectile moving faster. As in, why couldn't we do the same thing by just making a bigger explosion in the barrel? I assume there's some point of diminishing returns or inefficiency that makes it impractical, but I haven't been able to find a source that gives the math to explain it.

Germans already took conventional artillery to its limits:

https://en.m.wikipedia.org/wiki/Schwerer_Gustav

https://en.m.wikipedia.org/wiki/V-3_cannon

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

#130

Post by notacoward » Thu, Jun 27, 2019, 9:13 PM UTC

deweller wrote: post #16

alistproducer2 wrote: post #10

Am I supposed to celebrate the spending of untold billions on an unnecessary weapon of death while my Bridges and roads crumble, my health care system sucks and absolutely nothing is being done about, or to prepare for, climate change?? I'm not impressed

Is there a widespread lack of funding for bridges and roads infrastructure in the US? The bridges and roads around me in middle TN are well maintained. These are funded by local government resources, right?

> Is there a widespread lack of funding for bridges and roads infrastructure in the US?

Yes.

https://www.nbcnews.com/news/us-news/more-50-000-american-br...

https://www.statista.com/chart/12737/thousands-of-american-b...

Generally towns and states rely on federal funding for a significant part of road and bridge maintenance. Generally, they've been falling behind. Most states have long lists of bridges that they know are not really safe to drive on any more.