Upper stage / space tug - studied by Boeing in 1967. Standard S-IVB but with structural strength increased from 78% to 217% depending on station, resulting in 11.8% increase in empty weight.

Length: 17.79 m (58.36 ft). Basic Diameter: 6.61 m (21.68 ft). Span: 6.61 m (21.68 ft). Mass: 118,400 kg (261,000 lb). Main Engine: J-2. Main Engine: 1,438 kg (3,170 lb). Main Engine Thrust: 1,031.600 kN (231,913 lbf). Main Engine Propellants: Lox/LH2. Main Engine Propellants: 104,300 kg (229,900 lb). Main Engine Isp: 421 sec.