A vertical takeoff and landing aircraft (10, 110, 810) comprising: a fuselage (26, 126); a wing (42, 46, 142, 146, 842, 846) extending from the fuselage (26, 126); a rotor assembly (58, 62, 158, 160, 162, 164, 858, 860, 862, 864) extending from the wing (42, 46, 142, 146, 842, 846); a drive system (16, 116, 117, 516) coupled to the rotor assembly (58, 62, 158, 160, 162, 164, 858, 860, 862, 864); and a power system (88, 188, 888) comprising: a first power source (68, 168, 868) coupled to the drive system (16, 116, 117, 516) and operable to provide a power output for at least a forward flight mode, the first power source (68, 168, 868) comprising at least one of the following: a combustion engine, a hydraulic power source, and an electric engine with a battery; and a second power source (90, 190, 890) coupled to the drive system (16, 116, 117, 516) and configured to provide an additional high specific power output for a hover mode and a transition mode, the second power source (90, 190, 890) comprising: a plurality of gas generator cartridges; a turbine (96, 196, 508, 896) operably connected to the drive system (16, 116, 117, 516); and a plenum chamber disposed between the turbine (96, 196, 508, 896) and the plurality of gas generator cartridges, the plenum chamber in fluid communication with the turbine (96, 196, 508, 896) and the plurality of gas generator cartridges; wherein the second power source (90, 190, 890) is configured such that each of the plurality of gas generator cartridges can be activated independently to release gas into the plenum chamber to control power output of the turbine (96, 196, 508, 896). a fuselage (26, 126); a wing (42, 46, 142, 146, 842, 846) extending from the fuselage (26, 126); a rotor assembly (58, 62, 158, 160, 162, 164, 858, 860, 862, 864) extending from the wing (42, 46, 142, 146, 842, 846); a drive system (16, 116, 117, 516) coupled to the rotor assembly (58, 62, 158, 160, 162, 164, 858, 860, 862, 864); and a power system (88, 188, 888) comprising: a first power source (68, 168, 868) coupled to the drive system (16, 116, 117, 516) and operable to provide a power output for at least a forward flight mode, the first power source (68, 168, 868) comprising at least one of the following: a combustion engine, a hydraulic power source, and an electric engine with a battery; and a second power source (90, 190, 890) coupled to the drive system (16, 116, 117, 516) and configured to provide an additional high specific power output for a hover mode and a transition mode, the second power source (90, 190, 890) comprising: a plurality of gas generator cartridges; a turbine (96, 196, 508, 896) operably connected to the drive system (16, 116, 117, 516); and a plenum chamber disposed between the turbine (96, 196, 508, 896) and the plurality of gas generator cartridges, the plenum chamber in fluid communication with the turbine (96, 196, 508, 896) and the plurality of gas generator cartridges; wherein the second power source (90, 190, 890) is configured such that each of the plurality of gas generator cartridges can be activated independently to release gas into the plenum chamber to control power output of the turbine (96, 196, 508, 896). a first power source (68, 168, 868) coupled to the drive system (16, 116, 117, 516) and operable to provide a power output for at least a forward flight mode, the first power source (68, 168, 868) comprising at least one of the following: a combustion engine, a hydraulic power source, and an electric engine with a battery; and a second power source (90, 190, 890) coupled to the drive system (16, 116, 117, 516) and configured to provide an additional high specific power output for a hover mode and a transition mode, the second power source (90, 190, 890) comprising: a plurality of gas generator cartridges; a turbine (96, 196, 508, 896) operably connected to the drive system (16, 116, 117, 516); and a plenum chamber disposed between the turbine (96, 196, 508, 896) and the plurality of gas generator cartridges, the plenum chamber in fluid communication with the turbine (96, 196, 508, 896) and the plurality of gas generator cartridges; wherein the second power source (90, 190, 890) is configured such that each of the plurality of gas generator cartridges can be activated independently to release gas into the plenum chamber to control power output of the turbine (96, 196, 508, 896). a plurality of gas generator cartridges; a turbine (96, 196, 508, 896) operably connected to the drive system (16, 116, 117, 516); and a plenum chamber disposed between the turbine (96, 196, 508, 896) and the plurality of gas generator cartridges, the plenum chamber in fluid communication with the turbine (96, 196, 508, 896) and the plurality of gas generator cartridges; wherein the second power source (90, 190, 890) is configured such that each of the plurality of gas generator cartridges can be activated independently to release gas into the plenum chamber to control power output of the turbine (96, 196, 508, 896).