From: Michael T Miserendino <MTMiserendino@l...>
Date: Wed, 1 Dec 1999 15:04:00 -0500
Subject: Re: Mass Drivers (was GEV and Grav Vehicles)
A lot of interesting discussions have been going here about mass drivers and how they work. This has always been a big interest for me. Some clarification of what a mass driver is might help. A mass driver (AKA a "Rail Gun") uses magnetic attraction between electromagnets to propel a payload. The electromagnets are made up of coils conducting current which produces magetic fields. The payload is stored in a bucket coil which makes up one electromagnet. The other electromagnets are made up of a series of drive coils which form a tube. The electromagnetic fields are used to repel or attract each other. AFAIK the most common mass driver in use now is the pull type using attraction. Most people seem to think there is just one big force involved that accelerates the payload when it actually uses several smaller pushes or pulls over the length of the tube. As the bucket accelerates down the tube each drive coil turns on followed immediately by one turning off. Over time the bucket gains more and more speed from the interaction of magnetic fields converting electrical energy into physical energy. I think the first mass driver which came out in the 70's propelled objects at about 33 g's. The last one that I am familiar with could propel objects as high as 1,800 g's. They were used to study potential surface to space tranportation as well as SDI. In videos of tests that I have seen, the only noticeable transfer of energy back to the tube itself was when the controls for the driver coils were not in sync. The coils would cause a combination of push and pull and the tube would appear to shake back and forth. This was only done to demonstrate what could happen if the control was not precisem, otherwise the tube was typically mounted to a secure platform. The video greatly illustrated how the mass driver remains still during firing by showing it fire with a something like a fish tank attached to the tube mount. The water in the fish tank never moved when the controls were functioning correctly. When they did not, the water shook back and forth. I read somewhere where they are using mass drivers now for research on earthquake preventitive measures for buildings. Mike Michael Miserendino Senior Software Engineer Lincoln Re mtmiserendino@lnc.com > owner-gzg-l@CSUA.Berkeley.EDU at internet 12/01/99 02:19AM >>> more > important than the mass component. The launcher and vehicle would have Heh. Keeping in mind, of course, that Gibson himself is not exactly a techno-genius... He's a guy with a vivid imagination, but wrote all those books on a TYPEWRITER because he didn't own a computer... Someone more physics-literate than me correct me here if I'm wrong, but how do you get around the Newtonian "for every action there is an equal and opposite reaction" thing...? If a 1 kilo projectile is accellerated to 1000m/s in a combat-useful length of time (ie relatively immediate) then there's going to be a recoil whether it was fired out of a present day cannon or accelerated by a mass driver. Recoil doesn't go away just 'cause you're using electro-magnetism rather than chemical explosives, does it? (leaving aside such stuff as recoil compensators, inertial dampers, etc etc). If a tank (65 tonnes) now gets rocked back on its tracks by a shot from the main cannon, then a gev is going to suffer the same effect achieving the same result, no? If you're using a mass-driver on the gev, it will still have to deal with substantial recoil, won't it? If the GEV is of the same mass as the tank (65 tonnes), it will have to have some kind of counter-force to the recoil, or it will be shoved around... > If the vehicle can hover ala a helicopter ...then it isn't a GEV... , it would need to generate more lift > than you get just from ground effect. remember to keep in mind the difference between "ground effect" a-la GEV (pressurized cushion of air contained inside a flexible skirt which supports the mass of the GEV) and "ground effect" a-la aerodynamic-lift aircraft (such as helocopters and airplanes). Quite different... You don't really get "lift" per-se from hovercraft. The active aerodynamic force "lift" keeping the craft up off the ground is minimal (you get a bit off the lift fans, but not much... certainly not enough to keep the hovercraft up. Helocopters derive their "staying-up" power from aerodynamic lift (ie lower-pressure air on top of their wings, in this case rotor blades, and higher pressure air below the wings... the high pressure pushes into the low pressure, thereby keeping the wing up... but the pressure differences are achieved by the speed of the wing moving through the air and the shape of the aerofoil) whereas hovercraft are just creating an enclosed cushion of air to ride on. Same as how an avalanche can travel so far, so fast - riding on a cushion of compressed air which hugely reduces friction. I think then, by definition, you don't > have a GEV but a rotary wing tank. Or, perhaps, a hybrid But > the lift required would be enormous for a helicopter. I'm not sure you > [quoted text omitted] I'd say impossible. Or at least WAY unlikely. You'd need rotors so long as to be impractical for the bottom of an armoured vehicle - spinning them fast doesn't do you too much good 'cause then they go supersonic and it really starts to screw things up... That's the main limiting factor for top speeds of helicopters today - they have to avoid getting their rotors over the speed of sound or nasty things happen - especially since it happens to the advancing rotor on one side of the aircraft and not to the retreating rotor on the other side. > Oh, and the skirt... You'd need SOMETHING around blades to protect it, I like 'em too. But you aren't going to have flying tanks of the Slammers' size by using bottom mounted rotors, that's for sure... > Personally, I'm a big follower of strong AI (I know... I've been down But boring to game with.... it's the human element that makes the games fun at all:)