The "Hula-hoop" motion of a metal torus accelerates a magnetic projectile placed inside the torus. Magnetic levitation minimizes the friction. To reduce resistivity, the torus is cooled with liquid nitrogen. Cargo is exposed to the extreme acceleration. The slingatron is durable and consumes much less electric power than the coilgun or the railgun. On the other hand, it is rather heavy and complex -- the minimum mass is 104 tons.
Derek A. Tidman, "Sling Launch of a Mass Using Superconducting Levitation," IEEE Transactions on Magnetics, Vol. 32, No. 1, January 1996, pp. 240-247.
Derek A. Tidman, Rodney L. Burton, David S. Jenkins, and F. Douglas Witherspoon, "Sling Launch of Materials into Space," SSI Update, Vol. 22, Issue 1, January/February/March 1996, pp. 1-5.
Derek A. Tidman, Rodney L. Burton, David S. Jenkins, and F. Douglas Witherspoon, "Correction," SSI Update, Vol. 22, Issue 2, April/May/June 1996, p. 8.
The gas bearing is cheap but its friction coefficient is higher than that of the magnetic bearing. For example, the gas bearing utilizing nitrogen has the coefficient of about 0.006.