Version 6 (Mars Colonial Transporter-February 2016)
SpaceX developed the upper stage design further after 2015. The heaviest variant evaluated at 80.7 tonnes featured propellant tanks in the nose, crew below, and the main cargo area would have been just above the engines. The second-heaviest variant at 79.8 tonnes would have featured the crew compartment on top, main cargo area in the base, and the propellant tanks in between. The chosen variant was 73.3 tonnes and featured the crew on top, the cargo area in the middle, and the propellant tanks in the base. It sacrificed ease of landing and cargo off-loading for improved delta-v, critical to the return to Earth from Mars.
Diagram by NSF member Lamontagne.
By February 2016, the firm had evolved the MCT into an even larger ~5400 tonne, 81 m (265.7 ft.) tall, 12 m (39.4 ft.) diameter design with around 2.4 times the thrust of a Saturn V. It would be made entirely of carbon fiber and was topped by a 41 m (134.5 ft.) tall MCT with nine Raptor engines (six vacuum and three landing) fueled by 1450 tonnes of propellant. Below it was a 40 m (131.2 ft.) tall booster sporting 42 Raptor engines and carrying 3,650 tonnes of propellant at a 3.58 oxygen/fuel ratio.
The Raptor engine’s thrust was shrunk 13% to 1,961 kN (440,850 lbf), resulting in the booster developing 82,362 kN (18,515,700 lbf) of thrust, 32.4% more than it had before. 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.
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 aerocapture into Earth orbit. The MCT tanker was noticeably different from the MCT ship, with over-sized 1650 tonne capacity tanks while only 27 m (88.6 ft.). As a possible precursor to later hypersonic travel plans, SpaceX considered doing an Earth ascent tanker demonstration as a single-stage-to-orbit (SSTO) vehicle, with reduced nozzle sizes and a 540 tonne propellant load.
The vacuum version of the Raptor engine was to be 5.5 m (18 ft.) long and 3.8 m (12.5 ft.) in diameter, while the landing engine Raptor would be considerably smaller. Powering the engines would be densified liquid oxygen and methane chilled to -212 C (-350 F) and -178 C (-288 F) before launch. On the upper stage variants, all nine of the engines would fire after stage separation to minimize gravity losses, with the landing engines shutting down when no longer of benefit. Once in orbit, the tanker and ship would dock and transfer propellant by settling the tanks via rotation and pressure assist.
The ship would feature water, brine, and solid waste storage around its oxygen tank’s upper edges. Above this storage, the unpressurized cargo hold would be located. It would feature two 3 meter tall decks supported by a central column. This area would contain power systems, thermal systems, un-deployed solar arrays, radiators, life support, air handling, storage, and avionics. It would have enough room to carry 73 individuals on a two year Mars free return flight according to a minimum space free-return standard (15.1 cubic meters per person).
Render of an early MCT in Mars orbit – via Nathan Koga for NSF/L2
SpaceX would compensate for this layout’s inferior offloading characteristics with a large cargo door and a gantry crane for moving heavy cargo down to Mars. The crew would be able to get down to Mars via an egress raceway from the 1100 cubic meter pressurized compartment. The pressurized section could be entered into via an airlock attached to a docking port. It would feature the same central support column as the cargo bay. There were to be four levels, with three 2.5 m (8.2 ft.) tall floors for the crew, food and crew accommodation, and a much larger top level.
This level contained the crew mess and launch/landing seats. The vehicle’s nose contained control thrusters to control its fine movement. One element in flux was the extendable landing legs of the tanker and ship, of which there were five that were going to be heavily upgraded in size and durability.
When SpaceX committed to an all carbon fiber design, its engineers noted the huge manufacturing challenges associated with the Boeing 787’s carbon fiber fuselage sections. Other issues noted included carbon fiber’s lessened damage tolerance versus metallic designs, which SpaceX had hoped to deal with via thicker laminates. It was investigating cored versus stiffened panel composite construction. The former would allow for purging and insulation in the design, while the latter would allow a stiffened panel structure that could be co-bonded.
The plan was that the manufacturing flow of composite sections of the vehicle would start with the bare tool used to manufacture sections. Next, an inner facesheet in the shape of the end of a propellant tank cylinder would be created.
The composite manufacturing of propellant tanks would have featured automated tape laying and/or automated fiber placement. A transition ring with support would have then been added. Next, the core structure would be added, followed by the outer facesheet.
The structure would be bagged and cured before being taken off the tool. A second composite section would be stacked atop this one. A lap join would be used to bring the sections together, finishing both sets of propellant tanks.
SpaceX estimated as early as 2016 that they could build the then-MCT’s booster with just three composite sections plus an inter-stage. To build the booster alone, company engineers estimated a carbon lay down rate of 16-20 tonnes/month. An alternative four section booster plus inter-stage would require a tool with a carbon lay down rate of 8-11 tonnes in contrast. It would have needed an extra splice joint and one tonne in extra mass, however.
The ship and tanker’s TPS was being strenuously evaluated. When SpaceX decided on the initial 73.3 tonne version of the MCT, its engineers estimated that it would require ~18 tonnes of ablative PICA heat shielding. This meant roughly a quarter of the mass of the vehicle would be devoted purely to its TPS. This would curtail the mass advantages of carbon fiber.
SpaceX engineers were also concerned with finding ways to keep the costs of the TPS down. The stresses of multiple atmospheric entries during a Martian round trip would require refurbishing the entire TPS. There were similar concerns about keeping the costs of refurbishing the tankers’ TPS down between flights to improve reusability.
Version 7 (Interplanetary Transport System-September 2016)
In February 2016, SpaceX had chosen a ~5400 tonne MCT design over a larger 7000 tonne, 88 m (288.7 ft.) tall, 14 m (45.9 ft.) diameter design with three times the Saturn V’s thrust. One possible reason for this was the company found that any design more than twice a Saturn V’s mass required an impractical exclusion area.
The company apparently soon disregarded this, for the succeeding Interplanetary Transport System (ITS) announced in September 2016 tipped the scales at a jaw-dropping 10,500 tonnes! Had it launched off the pad, the rocket would have massed more than a Ticonderoga-class cruiser and was more than twice the size of the prior year’s 4724 tonne design.
Diagram by NSF member Lamontagne.
The ITS had a height of 122 m (400.3 ft.) and featured two stages. Its 77.5 m (254.3 ft.) tall booster developed an astounding 128,100 kN (28,798,000 lbf) of thrust, which would take 85 Airbus A380s to equal. This allowed the 6,975 tonne, 12 meter (39.4 ft.) diameter stage to be more than double the Saturn V’s entire mass. Topping this was the 49.5 m tall (162.4 ft.) tall, 17 m (55.8 ft.) diameter ITS spaceship with a large front window.
Its 30,890 kN (6,944,300 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. It could even deliver 450 tonnes onto Mars’ surface thanks to its three retractable landing legs. A tanker variant featured over-sized 2,500 tonne capacity tanks and 40% less dry mass.
Credit: SpaceX
The Raptor engine’s sea-level thrust had increased by 55.5% from 1,961 kN (440,850 lbf) to 3,050 kN (685,700 lbf) and its sea-level ISP was up 3.9% (334 seconds versus 321.4 seconds prior). It was even capable of throttling 20-100%, and its 300 bar chamber pressure was the highest in rocket engine history. The vacuum version would feature a higher thrust of 3,500 kN (786,800 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 modest 7% (or 469 t) of propellant required to return the ITS Booster to its pad would out-mass early Falcon 9s at 333 tonnes to put this design in perspective. Its expendable payload was more than the Statue of Liberty thrice over. Even its reusable payload was the equivalent to a Boeing 777-300. The worry amongst NSF’s experts though was that the design was simply too big to be safe, practical or economical.
Version 8 (Big Falcon Rocket-September 2017)
Within months, however, SpaceX decided to go with a smaller 9 meter (29.5 ft.) diameter design in order to avoid building an all-new factory. This would ease production at a cost of more complicated and expensive logistics for stage transportation and testing. The revised design would be called the Big Falcon Rocket (BFR).
When the 2017 BFR design was announced at the International Astronautical Congress, it stood 106 m (347.8 ft.) tall, massed a mere 4400 tonnes, and could deliver a reusable payload of 150 tonnes to orbit. Having more than doubled in size between October 2015 and September 2016, the design shrank to its smallest size.
The 58 m (190.3 ft.) tall BFR booster massed 3,065 tonnes and was powered by 31 Raptor engines producing 52,700 kN (11,847,400 lbf) of thrust. That’s equal to a “mere” 35 A380s. The Big Falcon Ship (BFS) shrank to 48 m (157.5 ft.) in length, its pressurized volume to 825 cubic meters, and mass to 1185 tonnes sans payload. Its typical return payload was up to 50 tonnes.
Diagram by NSF member Lamontagne.
The BFS ditched its three large retractable landing legs in fins for four much smaller retractable landing legs. It also featured four vacuum-optimized Raptor engines arranged around two (later three) Raptor landing engines. A pair of delta wings were added to provide pitch and roll control. This, in turn, would expand the flight envelope, allowing the BFS to land in a variety of atmospheric densities. The large front window had been divided up into two large windows arranged in rows. It also finally addressed radiation concerns by including a solar storm shelter for the inhabitants of the vehicle’s 40 cabins.
With the addition of a BFS cargo version with a huge payload bay and hinged door, the BFR gained the ability to launch and recover satellites. This, along with its smaller size, gave it a far better business case than the ITS. SpaceX said the design would be the successor to the Falcon 9 family. Possible point-to-point travel anywhere on Earth in under an hour, space station resupply, and lunar surface missions were also mentioned as additional possibilities.
Version 9 (Big Falcon Rocket/Starship-September 2018)
The next iteration of the design was evolutionary and debuted in September 2018 alongside a Moon mission announcement involving Japanese billionaire Yusaku Maezawa. The design experienced a 12 m (39.4 ft.) growth spurt to 118 m (387.1 ft.). The BFR booster grew 5 m (16.4 ft.) longer to 63 m (206.7 ft.).
The BFS grew 15% to 55 m (180.4 ft.) in length, driven primarily by an enlarged ~1100 cubic meter pressurized cabin. Powering the BFS were seven sea level-optimized Raptor engines producing 13,729 kN (3,086,500 lbf) of thrust. That’s equal to the output of nine A380s.
Diagram by NSF member Lamontagne
Helping to control the vessel were two actuated forward canards and a tri-fin layout in the back. These three fins would also replace the former wings and retractable landing legs. The new BFS would also re-feature the large front window last seen on the 2016 ITS spaceship.
Additionally, Musk hinted at the possibility of re-introducing Raptor vacuum engines to Starship, but stated, “I feel like this is the final iteration in terms of broad architectural decisions for BFR, BFS”. A renaming followed in November 2018, with the BFS becoming Starship and the BFR booster becoming Super Heavy.