A propulsion system includes multiple solid rocket motors that are activated by a single initiator. The rocket motors act in parallel, providing thrust in a single direction. The initiator activates an ignition charge that is in or operatively coupled to a non-annular plenum that transports hot gasses from the ignition charge to the rockets to be ignited. The plenum may be an unchoked plenum, allowing flow of hot gasses without choking. The plenum may be lined with an insulator material. A cover may be used to cover the plenum, and also to receive the rocket motors. The rocket motors may be solid-fuel rocket motors, with propellant grains and nozzles. The individual rocket motors may have separate ignition booster charges coupled to the plenum, which are ignited by the ignition charge and which in turn ignite the propellant grains.