PPMS Credit - © Mark Wade

The Primary Propulsion Module was the definitive 1960's design for a nuclear thermal rocket stage suitable for interplanetary operations. The basic NERVA stage was modified to allow for docking and assembly in orbit, storage and reliquefaction of its liquid hydrogen propellant for periods of up to three years on long voyages to Mars and Venus, and to feed propellant to lower stages as needed.

Detailed mass breakdown was as follows:

• Liquid hydrogen propellant: 174,600 kg
• NERVA nuclear thermal engine system: 14,500 kg
• Propellant tank: 22,700 kg
• Stage equipment: 2700 kg
• Meteoroid shield: 19,000 kg
• Interstage structures: 5200 kg
• 11% growth and contingency allowance: 6800 kg

Design Life: 1100. Length: 48.20 m (158.10 ft). Basic Diameter: 10.06 m (33.00 ft). Maximum Diameter: 10.06 m (33.00 ft). Mass: 245,600 kg (541,400 lb). Main Engine: NERVA. Main Engine: 14,500 kg (31,900 lb). Main Engine Thrust: 866.900 kN (194,887 lbf). Main Engine Propellants: Nuclear/hydrogen. Main Engine Propellants: 174,600 kg (384,900 lb). Main Engine Isp: 850 sec.