The spaceship itself would feature a large habitat section complete with suites of crew cabins mounted on top. To carry cargo, the spaceship would feature a lower cargo bay mounted above the Raptor engines, with the engines fed via a pair of central tubes that would cut through the cargo bay. This layout would allow the easy offloading of heavy cargo like vehicles or nuclear reactors.
The spaceship and its tanker counterpart would be supported by five extendable landing legs that would not fold down like the Falcon 9’s legs. There were no plans for a launch abort system (LAS) on the spaceship, which surprised many of the NSF experts, as the vehicle would be transporting 100 colonists at a time. A LAS is required for NASA’s Commercial Crew Program’s vehicles in contrast.
Although the launch vehicle would emulate the Falcon 9 v1.2’s design in its use of chilled propellant and multiple versions of one engine to power everything, there were a number of key design differences. Its propellant tanks would be made out of carbon fiber, a radical design change in the industry that would allow for a more efficient rocket.
They would also not be pressurized by gaseous helium, as is done with many US rockets and which was involved in the disintegration of two Falcon 9 upper stages. Instead, SpaceX would use gaseous forms of both propellants to autogenously pressurize the tanks, eliminating a major failure point at an approximate mass penalty of just less than 1% of payload.
NSF learned that other fuel and oxidizer combinations were under consideration, with perhaps the most plausible alternative being C2H4/O2 (Ethylene/Oxygen).
Regardless of the chosen diameter, the enormous BFR mentioned in the plans would be the most efficient launch vehicle to LEO ever, besting the latest record holder, the expendable version of the Falcon Heavy launch vehicle (4.49% of launch mass), by over half a percent. On a relative percentage basis, the payload as a percentage of launch mass would be some 11% greater than the next most efficient rocket. The rocket would, however, have a thrust to weight ratio of 1.36 to 1, which meant the launch vehicle could potentially lift even more with larger propellant tanks.
NSF assembled a design team of top experts, who would be advised by rocket designers Dmitry Vorontsov and Chuck Longton (co-founder of the DIRECT Project), to make sense of the figures and design elements (more workings in this L2 Evaluation Thread).
The NSF design team quickly settled on the most likely design being a 105 meter tall, 15 meter diameter two stage launch vehicle massing some 5,500 tonnes. As they ran the figures and filled in various holes in the documentation, they came to several realizations.
The plan’s summary mentioned a height of 180 meters was likely outdated, as the rocket’s thrust dictated a much shorter vehicle. It was also likely that the performance figures were extremely conservative, as simulations showed the vehicle with the given diameter lifting more than 280 tonnes to LEO with Falcon 9 style mass optimization.
NSF experts also noted that the design would need to produce approximately 100 kW of power from its solar arrays at Mars to support 100 colonists and crew. Given Mars receives 37-51% of the sunlight Earth receives depending on its distance, the spaceship’s solar panel arrays would need to produce 270 kW near Earth in order to meet minimum Mars power requirements.
The nuclear reactor type thought most likely by NSF experts to power the colony would be a Thorium fueled molten salt reactor, which would be able to take advantage of Mars’ extensive thorium reserves.
The NSF design team also noted some areas of concern that SpaceX would need to work on, including the lack of a mentioned launch abort system and possible debris strikes on the engines or spacecraft at Mars landing and liftoff. Simulations showed the spaceship’s Raptor engines would unleash a force equivalent to that of a Category 5 hurricane underneath the vehicle at Mars liftoff, which would increase the risk of a debris strike.
NSF confirmed new details about the design’s evolution in February 2016. The firm had selected an 81 m tall, 12 m diameter design with around two times the thrust of a Saturn V. A larger 88 m tall, 14 m diameter design with three times the Saturn V’s thrust lost out.
One possible reason for this was that evaluations showed that the 12 m design would require a smaller exclusion area of approximately 8 km (5 miles) in the event of a vehicle failure on the pad. SpaceX had evaluated this area by looking at over-pressurization, which would fall to safe levels at this distance. Any design more than twice as large as a Saturn V would face restrictions on launch site selection due to an enlarged exclusion area.
The Raptor engine’s thrust was shrunk 13% to 1,961 kN (440,850 lbf), forcing a change in design architecture. The booster would feature 42 Raptor engines, giving the rocket 82,362 kN (18,515,700 lbf) of thrust, 23.4% more than it had before. It carried 3,650 tonnes of propellant at a 3.58 Oxygen/Fuel ratio. It was to land near the launch pad instead of at sea via a combination of thrusters powered by hot ullage gas and grid fins. This change would give SpaceX easier launch operations at a very significant cost in payload capacity.