# What's the deal with the disc the NRL "projected" at 334,800 mph (538,800 kph)? **URL:** \ **Category:** Factual Questions\ **Created:** [January 4, 2006, 9:51am UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379 "2006-01-04T09:51:09Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![proghead7772003](https://avatars.discourse-cdn.com/v4/letter/p/7ab992/32.png) [@proghead7772003](https://boards.straightdope.com/u/proghead7772003)\ **Post date:** [January 4, 2006, 9:51am UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/1 "2006-01-04T09:51:09Z")
This comes from the 1996 edition (most recent one I have) of the Guiness Book of Records: > [@](#): > > _ **Highest velocity** _ The highest velocity at which any solid visible object has been projected is 93 miles per second (334,800 MPH) in the case of a plastic disc at the Naval Research Laboratory, Washington, D.C., in August 1980. 93 miles per second. That’s 149.7 kilometers per second. Mach 451.5! 0.0005 _c_!! :eek: Am I the only one who thinks this is amazing? My questions are: How did they do it? Why did they do it? Does that record still stand? What about the disc? How big was it? Where did they fire it _to_ and how far did it go? And finally, what happened to it? My guess on that last part is that it was vaporized by atmospheric friction… assuming it wasn’t fired in a vacuum, that is. I’ve been looking for answers to these questions for a long time. That tiny entry in the GBR tells you nothing other than the event took place. Google is no help. The only mention of it I could find via google ([here](http://www.google.com/search?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial_s&hl=en&q=%22334%2C800+mph%22&btnG=Google+Search) - other search terms turned up nothing at all) contained no more info than the GBR entry. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [January 4, 2006, 4:40pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/2 "2006-01-04T16:40:16Z")
I believe this projectile would have been fired from a rail gun. The barrel is a set of parallel conducting rods, and the projectile is conductive, and completes the circuit. A huge amount of current is forced through the rails/projectile, and the induced magnetic field pushes the projectile down the rails. I took a college course on pulsed power, and one of my classmates built a rail gun as his project. Try googling “rail gun” or looking on Wikipedia. ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [January 4, 2006, 4:48pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/3 "2006-01-04T16:48:24Z")
Ah, they’re 1980’s-style death frisbees. ---
**Author:** ![Mindfield](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mindfield/32/19759_2.png) [@Mindfield](https://boards.straightdope.com/u/Mindfield)\ **Post date:** [January 4, 2006, 5:21pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/4 "2006-01-04T17:21:48Z")
How much current would be required to fling a projectile _that_ quickly? And how long would the rails have to be? (I realize the two are related; more current can mean shoter rails) I have a hard time envisioning something accellerating to that velocity without a really long set of rails and an incredible current to induce the kind of magnetic field necessary to propel a projectile at such speed. Plus, how would this be done with a plastic disc? I can only assume it would have two metal or magnetic rings on either side and would be propelled by four sets of rails (two top, top bottom) in order to maintain the least amount of friction and wind resistence to be capable of even reaching such speeds without flipping or twisting. I did read once about how to make a home-made rail gun using an ordinary wooden ruler, some scotch tape, a small ball bearing, and an evenly-spaced series of neodymium magnets. A standard 12" ruler was able to propel the bearing at an impressive speed for such a short run. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [January 4, 2006, 5:30pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/5 "2006-01-04T17:30:01Z")
Some background (pdf file) [Naval Electromagnetic Gun Technology Assessment](http://www.onr.navy.mil/nrac/docs/2004_rpt_em_gun.pdf) [NOT YOUR GRANDPA’S SHOOTIN’ IRON: Rail Guns -](http://www.military.com/soldiertech/0,14632,Soldiertech_RailGuns,,00.html) > [@](#): > > Obviously, it would be fitting for a weapon with such potential power to be housed in the most powerful vessel around. The Navy, which has been at the forefront of rail gun technology since the early 1980s, plans to incorporate rail gun technology with its next-generation surface fleet, which will include ships such as the hefty DD(X) destroyer. Ranges of up to 200 nautical miles for rail gun projectiles are envisioned, with GPS-guided projectiles traveling at six times the speed of sound. The fact that rail guns and directed-energy weapons do not require powders or explosives will free magazine space for strike and other mission areas (the trade-off is that surface ships will need to generate massive amounts of electric power to support them). A proper-sized round could provide missile-like capability. Take your standard Tomahawk land-attack cruise missile – for the same amount of time it takes a Tomahawk to reach a target, an EM gun can deliver twice the destructive power to the same target, while operating at about 6-12 rounds per minute. At a fraction of the cost per round, tremendous volume fires could be delivered. The DD(X) destroyer, with its all-electric drive and Integrated Power System (IPS), is the first step towards full electromagentic weapon capability. The IPS can scale up to provide additional electrical power as demand grows – the key to surface fire support capabilities ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [January 4, 2006, 5:35pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/6 "2006-01-04T17:35:13Z")
Interesting article re physical engineering limitations in building and firing rail guns. Real world use of rail guns apparently tends to beat the shit out the mechanism every time it’s fired. [PowerLabs Rail Gun 2.0 Research](http://www.powerlabs.org/railgun2.htm) ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [January 4, 2006, 5:35pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/7 "2006-01-04T17:35:58Z")
> [@Mindfield](#): > > Plus, how would this be done with a plastic disc? I can only assume it would have two metal or magnetic rings on either side and would be propelled by four sets of rails (two top, top bottom) in order to maintain the least amount of friction and wind resistence to be capable of even reaching such speeds without flipping or twisting. Been a while since I read about this, but the design I saw had a polymer projectile coated with aluminium. It had two conducting rails either side of it, and magnets top and bottom. A _big_ current ran between the rails through the disc, so high that it turned the aluminium to a high-density plasma. This served both to carry the current from the rails across the disc without friction, and to conduct the current with virtually no resistance. > [@Mindfield](#): > > I did read once about how to make a home-made rail gun using an ordinary wooden ruler, some scotch tape, a small ball bearing, and an evenly-spaced series of neodymium magnets. A standard 12" ruler was able to propel the bearing at an impressive speed for such a short run. Possibly this one, or a variant? [Chapter 1: Magnetism -- Build a Gauss Rifle magnetic linear accelerator](http://sci-toys.com/scitoys/scitoys/magnets/gauss.html) ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [January 4, 2006, 7:01pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/8 "2006-01-04T19:01:06Z")
> [@](#): > > Ranges of up to 200 nautical miles for rail gun projectiles are envisioned, with GPS-guided projectiles traveling at six times the speed of sound. 200 nautical miles is over the horizon. In order to travel to a target that’s beyond line of sight the projectile will have to slow down enough to drop below the horizon due to gravity. And if these projectiles are relying on their velocity to damage their target when they strike, this means they will be useless by the time they reach their targets. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [January 4, 2006, 7:10pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/9 "2006-01-04T19:10:07Z")
> [@Little Nemo](#): > > 200 nautical miles is over the horizon. In order to travel to a target that’s beyond line of sight the projectile will have to slow down enough to drop below the horizon due to gravity. Sorry, but what the hell does _this_ mean? You know the Earth is round and the horizon is not a fixed line, right? Or am I misunderstanding you? As written, this makes no sense at all. Explain, please. ---
**Author:** ![RandomLetters](https://avatars.discourse-cdn.com/v4/letter/r/8edcca/32.png) [@RandomLetters](https://boards.straightdope.com/u/RandomLetters)\ **Post date:** [January 4, 2006, 7:41pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/10 "2006-01-04T19:41:23Z")
> [@Little Nemo](#): > > 200 nautical miles is over the horizon. In order to travel to a target that’s beyond line of sight the projectile will have to slow down enough to drop below the horizon due to gravity. And if these projectiles are relying on their velocity to damage their target when they strike, this means they will be useless by the time they reach their targets. The projectiles (like all guns) will be fired in a parabolic arc. Ignoring that pesky atmosphere for a minute, that means the projectile will come down at the same speed it is launched at. The fact that the earth’s surface is curved or that the shot is over the horizon doesn’t bother that arc at all. ---
**Author:** ![Cerowyn](https://avatars.discourse-cdn.com/v4/letter/c/82dd89/32.png) [@Cerowyn](https://boards.straightdope.com/u/Cerowyn)\ **Post date:** [January 4, 2006, 7:57pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/11 "2006-01-04T19:57:42Z")
And, of course, there’s that whole gravity thing. A projectile’s path can be bent by gravity without [appreciable] loss of energy. In fact, in order _not_ to have the path bent by gravity over a 200 nm trajectory, the projectile would have to be accelerating. ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [January 4, 2006, 8:04pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/12 "2006-01-04T20:04:10Z")
I believe **Little Nemo** is referring to the “cannon model” of orbital velocity. If you fire a cannon sideways, the cannonball will fly in a parabolic arc until it hits the ground. Fire it faster, the ball flies further. Fire it fast enough, and the Earth curves away from the cannonball faster than the ball falls back to the Earth. You’ve now achieved orbit. [http://en.wikipedia.org/wiki/Orbit#Understanding\_orbits](http://en.wikipedia.org/wiki/Orbit#Understanding_orbits) Low Earth orbit is about 8 km/s, well under the 149.7 km/s attained by the death frisbee. ---
**Author:** ![Mindfield](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mindfield/32/19759_2.png) [@Mindfield](https://boards.straightdope.com/u/Mindfield)\ **Post date:** [January 4, 2006, 8:30pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/13 "2006-01-04T20:30:07Z")
> [@matt](#): > > Been a while since I read about this, but the design I saw had a polymer projectile coated with aluminium. It had two conducting rails either side of it, and magnets top and bottom. A _big_ current ran between the rails through the disc, so high that it turned the aluminium to a high-density plasma. This served both to carry the current from the rails across the disc without friction, and to conduct the current with virtually no resistance. Okay, now my understanding is that plasma is essentially when a material becomes superheated to the point of becoming gaseous. Based on this, wouldn’t the aluminum coating just sort of disappear at high velocity? Or are there other physics at work that make it stick to the surface it’s bonded to? > [@](#): > > Possibly this one, or a variant? [Chapter 1: Magnetism -- Build a Gauss Rifle magnetic linear accelerator](http://sci-toys.com/scitoys/scitoys/magnets/gauss.html) That’s one of the ones I saw, yes. I’m certain there was another design that was similar, but instead of using that kinetic method, it featured sets of two evenly spaced neodymium magnets on either side of the ruler channel, placed at even intervals along the ruler. The projectile (ball bearing) was drawn to the first magnet and slung on to the next, where the magnetic field of the second set would propel it even faster on to the next in the series, and so on. Without the loss of momentum caused by the kinetic method it was able to travel faster in twelve inches than the above was. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [January 4, 2006, 9:10pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/14 "2006-01-04T21:10:06Z")
> [@Mindfield](#): > > Okay, now my understanding is that plasma is essentially when a material becomes superheated to the point of becoming gaseous. Based on this, wouldn’t the aluminum coating just sort of disappear at high velocity? Or are there other physics at work that make it stick to the surface it’s bonded to? Excellent question! Don’t know the answer, but I’ll take a wild-assed guess. At the current levels we’re talking about, the aluminium coating is going to _explode_ into plasma. Wires and conductive films can explode with overpressures comparable to detonating explosives if the power is high enough. This is used in certain designs of detonator, notably the exploding foil detonators used in the Fat Man Nagasaki A-bomb. At the instant the current is turned on in the railgun, the aluminium coating, which may be on only the rear surface of the disc, is virtually instantaneously converted to plasma by simple resistive heating. It will initially have the dimensions and density of its solid form. As it explodes its density decreases but it will exert tremendous pressure. The polymer projectile can therefore be thought of as “surfing” on an accelerating disc of explosively expanding plasma. Depending how fast the railgun accelerates the projectile, the aluminium plasma may only have expanded by a small degree before the maximum projectile velocity is increased. The polymer and plasma part company well after the railgun has done its work. Like I say, it’s only my WAG. If **Stranger On A Train** spots this thread you’ll probably get a better one. ---
**Author:** ![proghead7772003](https://avatars.discourse-cdn.com/v4/letter/p/7ab992/32.png) [@proghead7772003](https://boards.straightdope.com/u/proghead7772003)\ **Post date:** [January 4, 2006, 9:32pm UTC](https://boards.straightdope.com/t/whats-the-deal-with-the-disc-the-nrl-projected-at-334-800-mph-538-800-kph/338379/15 "2006-01-04T21:32:18Z")
> [@astro](#): > > Some background (pdf file) > [Naval Electromagnetic Gun Technology Assessment](http://www.onr.navy.mil/nrac/docs/2004_rpt_em_gun.pdf) > > [NOT YOUR GRANDPA’S SHOOTIN’ IRON: Rail Guns -](http://www.military.com/soldiertech/0,14632,Soldiertech_RailGuns,,00.html) Thanks everyone, for your input. I had considered rail guns but until I read the above article I wasn’t aware they were quite so far along in developement. I was always under the impression that no one had managed to get them to work very well but, apparently, that’s not the case. Still, I haven’t seen anything that suggests a railgun capable of anything near 150 km/s.