Status: Study 2008. Payload: 4,500 kg (9,900 lb). Gross mass: 15,000 kg (33,000 lb). Height: 23.00 m (75.00 ft). Diameter: 6.00 m (19.60 ft).
A conical crew module attached to the top of the rocket, carrying up to five people, would undock, turn 180 degrees, and dock nose-first with the orbital station module. The module's two LOX tanks, each spheres six meters in diameter, would be purged of any remaining propellant and then pressurized to serve as habitation modules.
The concept harkened back to Krafft Ehricke's plans for an Atlas spent-stage space station in the late 1950's. IOS planned to use the Neptune to serve the orbital space tourism market, selling week-long trips for $2 million per person starting in 2008 - however the source of development funding for such a large launch vehicle, not to mention a new crewed space capsule - was unclear. The Neptune booster used the same stage-and-a-half design of the Sea Star technology demonstrator, with a large engine in a booster module and four smaller vernier engines attached to the main vehicle structure. The engines were larger versions of the LOX/LNG engines developed for Sea Star, generating a total of 2500 kN of thrust at liftoff.
Crew Size: 5.