The Evolution of the Big Falcon Rocket

by Phillip Gaynor

SpaceX planned on reusing the Booster, Tanker and Ship 1000, 100 and 12 times respectively, with the Ship’s reuse heavily limited by the massive heating it would endure during its aero capture into Earth Orbit. NSF learned that two launch sites were being considered: KSC/Cape Canaveral, Florida and Brownsville, Texas, with the former having more than twice as many launch opportunities as the latter. This was due to its superior geographic location for Mars launches.

The design continued to evolve behind the scenes between February 2016 and the time of Musk’s first announcement about the system to the September 2016 meeting of the International Astronautical Congress in Guadalajara, Mexico. It was announced the day before Musk’s address that SpaceX had conducted the first test firing of a sub-scale Raptor engine.

Raptor test firing at McGregor – Credit SpaceX

The following presentation by Musk first noted the very aggressive goal of improving the cost of transport to Mars by five million percent to around the median cost of an American house. To enable this, Musk said, orbital refueling was necessary, as it would spread development costs over more launches, compress the schedule, make performance shortfalls less problematic, and allow for a launcher 10-20% as costly as the alternative.

Refueling on Mars, much like in Zubrin’s plan, was also included to allow reuse of the ship and leverage the resources of Mars. According to Musk, Methane was selected as the fuel because it allowed a smaller vehicle size than hydrogen, had the lowest costs, allowed easy reuse, could be produced on Mars, and was just as transferable in orbit as kerosene.

The system architecture showed the system would have a common booster stage and two upper stage versions. The first upper stage would be the ITS (Interplanetary Transport System) Spaceship itself, while the second would be the ITS Tanker. The reuse goals were the same as in February, with the Booster, Tanker and Spaceship being rated for 1000, 100 and 12 reuses (over 12 Mars synods totaling 26 years). The system would also use autogenous pressurization of propellant tanks and a carbon fiber primary structure as expected.

SpaceX’s grand ambitions finally became fully apparent when the launch vehicle’s expendable payload capacity of 550 tonnes was announced. The rocket would be 133% more capable than the version SpaceX had planned just the year before.

This incredible performance was enabled by the Raptor engine. Its sea level thrust had increased by 55.5% from 1,961 kN (440,850 lbf) to 3,050 kN (685,667 lbf), its sea level ISP was up 3.9% (334 seconds versus 321.4 seconds prior), it was now capable of throttling 20-100%, and its 300 bar chamber pressure was the highest in rocket engine history.

The Raptor Engine via the 2016 IAC presentation

Its vacuum version would feature an even higher thrust of 3,500 kN (786,831 lbf) and an ISP of 382 seconds, the highest hydrocarbon ISP engine on record, thanks to a very large 200 to one expansion ratio nozzle. The Booster’s engine count of 42 Raptor engines remained the same.

Thanks to the engine’s large increase in thrust, the Booster developed an astounding 128.1 MN (28,798,026 lbf) of thrust, which allowed the 6,700 tonne, 77.5 meter (254.27 ft) tall, 12 meter (39.37 ft) diameter stage to more than double the Saturn V’s entire mass. Similar to the Falcon 9, it would use a combination of rocket retro-propulsion and grid fins to guide itself back to Earth. Despite featuring an even higher separation velocity than the Falcon 9, it would require just 7% of its propellant to return to its launch pad for reuse.

The ITS Spaceship’s height of 49.5 meters (162.4 ft) was 21% longer than the previously planned 41 meters (134.51 ft). However, it had the same long term goal of carrying more than 100 crew and passengers, although the vehicle massed more and had a wider diameter of 17 meters (55.77 ft) while using the same engine configuration. Its 31 MN (6,969,077 lbf) of thrust would boost the 2,100 tonne vehicle and up to 300 tonnes of cargo into LEO, where it would unfurl its solar panel arrays.

Comparison to the Saturn V graphic – via SpaceX

It would land vertically on Earth and Mars via three centrally located Raptor engines, and could deliver up to 450 tonnes of cargo to Mars. Unlike the plans detailed in February however, it would use three massive landing legs rather than five smaller legs. It maintained enlarged versions of the crew and cargo compartments on top from the February 2016 design.

Its Tanker stablemate differed from its February design by being the same length and shape as the Spaceship. However, it used enlarged tanks and swapped functionality for 40% less dry mass, which allowed it to carry 27% more propellant to orbit. Altogether, this system would allow Mars journeys averaging 115 days, with the ITS Spaceship either aerocapturing into orbit or proceeding directly to landing upon arrival at Mars.

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