From: kjenks@gothamcity.jsc.nasa.gov
Subject: Re: Railgun in Southwest US
Originator: kjenks@gothamcity
Keywords: accelleration
Organization: NASA/JSC/GM2, Space Shuttle Program Office
Distribution: sci
Date: Mon, 11 Jan 1993 20:24:52 GMT

In article <C0HtJ3.Lqo.1@cs.cmu.edu> ssi!lfa@uunet.UU.NET ("Louis F. Adornato") writes:
>Personally, I don't know that a railgun is going to be worth much for
>anything other than ASAT and ABM applications.  The problem is that a
>ground launched body can't attain orbit (at least, not an orbit that
>doesn't intersect the surface) without a circularizing burn at
>periapsis.  This means that you have to carry propulsion (motor and
>fuel), GNC hardware (star trackers or a gyro/accelerometer platform,
>momentum wheels or an RCS, guidance computer and control hardware),
>power and cooling, a shroud capable of protecting the whole shebang
>when it exits the launcher in sea level air at better than 17,000 mph
>(orbital velocity at 150 mi), aerosurfaces to prevent tumbling, and
>probably active control of same, and still retain enough mass
>capability for a payload.

You also has to survive the launch.  With accellerations up to 5
million G, temperatures in the 1000's C range, and some of the most
serious electrical and magnetic fields you can find anywhere, you
wouldn't want to be fired out of a rail gun.  Nor would you want to
try to design a payload which could be fired out of a rail gun.

But for those of you who don't know what a railgun is, I'll give you a
brief intro.  This is a VERY high level description, greatly
simplified.  A rail gun works because of the interaction between an
electrical field with a magnetic field called the Lorentz force.  If I
recall correctly, the basic equation is f = E x B.  So you want great
big E (electrical) fields and hefty B (magnetic) fields so you get a
whole lot of f = mA.

Let's see if I can explain using ASCII sketches:


         NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN    <- North end of
         NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN        magnetic field

        +++++++++++++++++++++++++++++++++++++++++P++++++++  <- Positive rail
<- Velocity vector <-                            P
        -----------------------------------------P--------  <- Negative rail

         SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS    <- South end of
         SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS        magnetic field

Where "PPP" marks the payload or payload carrier, which is a conducting
bar touching both rails.  A great big current flows through the rails,
passing through PPP.  This electrical field is perpendicular to the
magnetic field, so the PPP is subject to the Lorentz force, f = E x B.
Assuming there's little or no friction with the rails (which is very
difficult to arrange), the PPP scoots down the rails in a heckuva
hurry, accellerating as it goes.  Because there is friction with the
rails, they erode quickly and have to be replaced after every N
launches, where N varies with the friction and wear properties.

Since you need to reach orbital velocity in a short distance, you need
to have BIG accellerations.  This implies the big E fields (hence hefty
current along the rails and PPP), big B fields (hence hefty magnets),
and low mass (f = mA).

We now return you to discussions about the PEACEFUL use of space.

-- Ken Jenks, NASA/JSC/GM2, Space Shuttle Program Office
      kjenks@gothamcity.jsc.nasa.gov  (713) 483-4368

     "...Development of the space station is as inevitable as 
      the rising of the sun." -- Wernher von Braun
Newsgroups: sci.space