If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #567
Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147
I came across some interesting comments recently. The gist is that when a railgun fires, the recoil momentum is not in fact imparted on the rails (the only forces on them are friction and repulsion against each other), and is in fact distributed along the entirety of the circuit. I understand this vaguely in quantitative terms, but does anyone have any more substantial information? Google didn't turn up much beyond a research paper or two I can't access.
Registered Member #567
Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147
Yes. But the conflict here is where the equal and opposite reaction force is exerted.
What causes the projectile to accelerate is the magnetic field, correct? But the magnetic field isn't 'anchored' to the rails. My vague understanding is that the momentum is spread out over the entire system which is conducting, but exactly how eludes me, which is what I'm asking.
Registered Member #289
Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154
This is indeed a matter of current debate, but having seen several rail guns fire in person and having built one myself I can say that the gun does recoil. What percentage of the recoil goes in what direction I can not say, and if you Google it, apparently neither can anyone else, at least not with any meaningful data behind it.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You don't need arithmetic, as much as topology
A circuit through the projectile and rails is completed by the lead-in wires and the power supply. All components of this circuit will feel the force that is trying to expand the loop. Just do Biot-Savart.
The rails themselves will therefore not feel any "recoil" as such, their force is outwards and they will have been built with structural stuff to resist this seperating force.
The rest of the circuit tends to be difficult to conceptualise, as the wires and power supply are not built to any pre-conceived geometry, like the rails and projectiles. Not only that, they are heavy, whereas the projectile is light, so the forces won't be so apparent. Any attempt to seperate these components to measure the forces directly, whilst still keeping them fixed to each other with stiff large cross section wires will be doomed to large experimental error.
The most important thing is that, just because it is hard to do the maths, don't assume that therefore it breaks well established rules of physics and new and exotic things happen. First of all, see whether the existing rules work. Then, and only when you have done the maths and come to a contradiction start looking for new rules of physics.
Fortunately, there are some fairly easy ways to see that the situation does obey Newton's, Maxwell's, Lorentz's and a few others' Laws, without resorting to mathematics with numbers (just with concepts).
1) Draw a boundary round the entire system (physicsts like this approach). Put the table, the power supply, the rail gun on a fractionless sled. Fire the gun, and see the whole table recoil back so that its momentum is equal and opposite to the projectile. Or instead of the frictionless sled, hang it from a long pendulum - that is an experiment you *can* do in your yard. Only when you've done that and found it wanting should you start jibbering about new physics.
2) Draw the electrical circuit in 3D and identify the direction of the force of each element of it, see Biot Savart if you need help with that. You know that as the current is uniform, the force per unit length of conductor is uniform. Do the line integral of force to determine the force on each element of the circuit - so the projectile, the rails, the discharge capacitors. The neat thing about the line integral of this sort of force is that you can take any path from A to B and get the same result. So if we take A and B as the points at each end of the projectile, then do the integral from A to B and we get the force on the projectile. Do the integral from B to A, through the rails, cables, supply caps and switch, and we get an equal magnitude oppositely directed force, which is the recoil. It's plain and simple maths, well, plain and simple 3D vector calculus.
What percentage of the recoil goes in what direct I can not say, and if you Google it, apparently neither can anyone else,
Exactly 100% of the recoil goes in the direction opposite the projectile. What of this is made up of force on the cables, the switch and the supply caps depends on where they are and in which direction they are pointing. The point about flexible cables is that they are flexible. If you extend the rails back, and switch the current on by the rather dodgy mechanism of holding a charged cap and dabbing it on the rails, then 100% of the recoil will be felt by the cap. If we bolt the cap along the direction of the rail, and put a shorting jumper from the cap across to the other rail, then it's felt by the shorting jumper, the force felt by the cap is pushing it away from the other rail, not from the projectile. The force felt in the cap is more or less the same magnitude, but it's not directed in the anti-projectile direction, so it's not called recoil.
Which bits of the circuit feel "recoil" then is based on the strict defintion of recoil as being in the opposite direction to the projectile. All parts of the cirucit feel a force tending to expand the area of the loop.
There is no debate, at least between people who know what they are talking about.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
As said the seat of the recoil is not currently known, and there is debate. A paper I read tried using flexible metal sheets that gave and crumpled to determine the recoil seat, but the results were inconclusive.
Registered Member #289
Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154
Perhaps I should clarify the area of debate which I was commenting on. Rail guns are strange, mystical devices that work on black magic, the energies of the universe, and pull lots of medium electricity out of the ether, kind of like Tesla coils and Women, and therefor the laws of physics do not apply....
Ah No; In all seriousness I did not mean the laws of physics do not apply here, the same amount of energy that moves the projectile forward will push the gun back (perhaps recoil is the wrong term though), I don't think anyone is arguing that. The question is how. See, if we where to fire a rail gun with no stop behind the rails, they would not just go flying out the back. Instead, the repulsive force between the two (which is also acting on the projectile) pushes them up against the mounting structure with more than enough force to hold them there against the forward motion of the projectile. So, how is that force distributed along the rails? This depends on pulse shape, projectile weight, velocity, etc. so there is debate as to where, when and how hard this force is applied to different parts of the rail gun system. Remember, the intra-gun forces are complex and grater than the projectile motion force, just look at how poor rail gun efficiencies are. Most of their power is wasted as heat, but some of it is wasted pushing cables and G-10 around as well. I'm sure the navy has a CRAY in the corner of a lab somewhere that spits out a 3D model of this force every time the gun fires, but it does seem to be something that comes up often in the hobbyist community.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Perhaps I should clarify the area of debate which I was commenting on. Rail guns are strange, mystical devices that work on black magic, the energies of the universe, and pull lots of medium electricity out of the ether, kind of like Tesla coils and Women, and therefor the laws of physics do not apply....
Ah No; In all seriousness I did not mean the laws of physics do not apply here,
phew!, you had me for a moment there!
the same amount of energy that moves the projectile forward will push the gun back
no, the same impulse, recoil is momentum, not energy,
(perhaps recoil is the wrong term though), I don't think anyone is arguing that. The question is how. See, if we where to fire a rail gun with no stop behind the rails,
ahh!, perhaps that's your problem. There *is* a "stop" attached to, if not behind the rails. You have to get the current into and out of the rails, through a wire and a switch, from your power supply. They all experience forces too.
they would not just go flying out the back. Instead, the repulsive force between the two (which is also acting on the projectile) pushes them up against the mounting structure with more than enough force to hold them there
that's true, but irrelevant
against the forward motion of the projectile.
no. The force due to a current in a magnetic field acts at right angles to the current. So the force acts sideways to, not along the rail. The rails try to fly apart, but there is no force in a direction that could be called recoil.
So, how is that force distributed along the rails? This depends on pulse shape, projectile weight, velocity, etc. so there is debate as to where, when and how hard this force is applied to different parts of the rail gun system. Remember, the intra-gun forces are complex and grater than the projectile motion force, just look at how poor rail gun efficiencies are. Most of their power is wasted as heat, but some of it is wasted pushing cables and G-10 around as well. I'm sure the navy has a CRAY in the corner of a lab somewhere that spits out a 3D model of this force every time the gun fires, but it does seem to be something that comes up often in the hobbyist community.
some of that's true, but it's all irrelevant.
Consider that the power supply and its wires connecting it to the rails all take the form of an ideal (zero dimension), charged co-axial cable. Because the conductors are co-axial, there is no net force on them taken as a pair due to magnetic fields, whether external or generated by the current carried. How do we connect that source to the rails? We connect it to the mid-point of a conductor, at the "breach" end of the rails. When the gun is fired, the projectile experiences a force in the west direction, one rail tries to go north (that's exactly north, not east-by-nor-nor-east), the other south, and the conductor that is distributing the power to the rails tries to go east. I think we've found our recoil! This conductor is the same length and carries the same current as the projectile, but goes in the opposite direction.
If you make the gun more complex, using seperated wires from a power supply of finite size, the vector addition gets more tedious, but the basic topolgy remains, current flows in a circuit, and some part of that circuit will be feeling a force equal and opposite to the projectile. However, in all cases, that part is not going to be the rails, because that bit is built accurately to be perpendicular to the firing direction, and thus unable to feel any force due to current along the boresight direction. How the recoiling part conducts its impulse to the whole structure of the gun depends on how it is strapped down. A "loose" bit of cable may thrash around when fired, but it is still constrained by its electrical connections, and the structure of the gun has to tug that bit of cable to stop it leaving the machine, with an impulse that is given, in magnitude and direction, by Newton's Laws.
Is it worth repeating? There is no recoil force on the rails.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
It's similar to recoil force in ordinary guns, which acts on the breech and not directly on the barrel.
I started this post after seeing a close analogy between "location of recoil force" in railgun and in a pneumatic mortar I made many years ago. A short piece of 1/8"-wall steel irrigation pipe has breech closed by a thick metal disk. Empty 2 liter plastic soda bottles (loaded from muzzle) serve as pressure chambers and burst diaphragms. 17 pound projectiles fly more than 5 seconds to confirm thermodynamic models.
So what is the load on screws that secure the breech plate, under 300 psi design pressure (8000 lb recoil force)? The answer is: practically none, if the supporting trunnions are fixed to the back of the plate!
Chamber pressure pushes the projectile upward and the breech plate downward. The barrel wants to expand radially. But there's no vertical force on the barrel except from friction, gravity, and its own small inertia. (It is momentarily dragged downward by breech plate, as the supporting structure and foundation deflect). Firing pressure does not tend to eject the breech plate from barrel. The mortar would behave about the same if barrel were slip fit around the plate, unless the projectile got stuck.
-Rich p.s. I agree with Dr. Slack. In railgun, forces parallel to direction of fire (i.e. recoil) develop only in conductor places where the current is not in that direction.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Railgun recoil
Ah No; In all seriousness I did not mean the laws of physics do not apply here, the same amount of energy that moves the projectile forward will push the gun back (perhaps recoil is the wrong term though), I don't think anyone is arguing that. The question is how. See, if we where to fire a rail gun with no stop behind the rails, they would not just go flying out the back. Instead, the repulsive force between the two (which is also acting on the projectile) pushes them up against the mounting structure with more than enough force to hold them there against the forward motion of the projectile. So, how is that force distributed along the rails? This depends on pulse shape, projectile weight, velocity, etc. so there is debate as to where, when and how hard this force is applied to different parts of the rail gun system. Remember, the intra-gun forces are complex and grater than the projectile motion force, just look at how poor rail gun efficiencies are. Most of their power is wasted as heat, but some of it is wasted pushing cables and G-10 around as well. I'm sure the navy has a CRAY in the corner of a lab somewhere that spits out a 3D model of this force every time the gun fires, but it does seem to be something that comes up often in the hobbyist community.
