A compressed hydrogen gun fires a powerful magnet into the reverse coilgun, which converts kinetic energy of the magnet into electric energy. The magnet is a solenoid. It is immersed in a static magnetic field at the moment of firing. When it leaves the field, a powerful current is generated in the solenoid. Chemical rockets are expensive but they can carry fragile cargo including people.
Guns are cheap but they subject cargo to extreme acceleration.
The following idea combines gentle acceleration of a rocket with the low cost of a gun. Soon after the launch, a transverse gas gun fires a projectile filled with solid oxidizer. Far above the atmosphere the rocket docks with the projectile and accelerates cargo to orbital velocity. K. It is impractical to carry liquid fuel or liquid oxidizer in the projectile because the extreme acceleration would generate excessive pressure. Probably the best gun design is a tube filled with hydrogen at a pressure of 10 bars.