The Evolution of the Big Falcon Rocket

by Phillip Gaynor

In January 2015 SpaceX reversed course and suddenly revised the Raptor engine’s thrust down to 2,256 kN (507,100 lbf), though no mention was made of changes in the rocket’s overall thrust or design, other than there would be a lot of engines.

With the design’s future direction now clouded in uncertainty, a fellow team member of Vorontsov’s suggested that SpaceX was taking a radically different approach. To save on engine development costs, they believed SpaceX had simply tripled the number of Raptor engines on its launch vehicle to compensate for the drastic reduction in engine thrust.  This would later be confirmed to NSF in October 2015.

NSF experts’ evaluations showed that the new engine parameters would likely result in a 5,200 tonne launch vehicle powered by 27 Raptor engines on a 12.5 meter diameter core stage and three Raptor Vacuum engines on a 12.5 meter diameter second stage. Although the resulting rocket suffered a minor hit to performance from the increased number of engines, it would still be capable of lifting 280 tonnes to LEO.

Envisioning of Raptor Engines on BFR booster – rendered by NSF member Doesitfloat

In 2016 NSF learned that a large number of possible designs had been under consideration in the prior year for the Mars Colonial Transporter (MCT), as it was then known. There were two basic possible design choices.

The first was to make the MCT spaceship as a sort of super-sized capsule complete with its own engines for getting off Mars, a design which NSF learned SpaceX had actively considered. The second possibility was a lifting body design, also with its own engines for getting off Mars. The capsule design was more proven and conservative but offered less mass and space for cargo, crew and propellant, while the lifting body would offer greater mass and space but was less proven.

A consensus was reached amongst NSF’s experts that regardless of the design choice the MCT spaceship would enter the atmosphere on its side to maximize surface area.  It was uncertain however whether the MCT would land vertically upon its tail or land horizontally on Mars.  While a horizontal lander would enable easy off-loading and safer landings, it would also be more complicated to engineer.

One major design choice for SpaceX was whether the spaceship would be incorporated into the rocket’s second stage to save on design costs and mission complexity, or whether it would be its own dedicated stage in order to maximize performance.  Given the plethora of design variations possible, by July 2015 NSF experts had identified more than twenty possible designs for the Mars Colonial Transporter spaceship.

SpaceX meanwhile had suffered a launch failure of its Falcon 9 rocket on June 28th, 2015, that led to a 260 million dollar loss for the firm due to the inability to launch while the failure was investigated. NSF learned that as a result, the announcement of SpaceX’s Mars plans were to be pushed back until after the firm had returned to flight.

CRS-7’s launch failure – via SpaceX webcast screenshot.

The firm continued to make progress on the Raptor engine’s development however, with its oxygen pre-burner component undergoing a full power test that same month. By August 2015 Musk announced that the Raptor would feature an oxygen to methane ratio of 3.8:1.

NSF gained further insights in October 2015 about Musk’s plan. The MCT was to be 180 meters tall single stick design of 12 meters in diameter, launch off the pad with 62,239 kN (13,992,041 lbf) of thrust, and be capable of reusably lifting 236 tonnes to LEO.

It would then make a three to five month journey to Mars while carrying a full load of cargo, 100 colonists and crew. Once there, it would enter the atmosphere and slow down via a combination of atmospheric drag and retro propulsion. Early plans called for an initial Mars base to be created by landing ten spaceships in close proximity and using cranes and robots to start up preliminary propellant production.

Energy for both the base and its propellant factory would be provided by a mixture of small nuclear reactors and deployed solar panel arrays. Once up and running, plans called for sending at least ten colonists and crew on the initial manned mission to Mars to get the colony properly started. The spaceships, however, would return to Earth each synodic period so that they could be refurbished back on Earth and readied for the next launch window.

While the plan resembled Zubrin’s, it featured Earth orbit refueling of the colonization spaceships, which Zubrin’s plan did not, as SpaceX counted on fully reusing all parts of their rockets.

The plans centered on a common BFR core stage used to launch both the spaceship and the tankers used to refuel them in orbit. The booster’s listed thrust figures revealed it would have 27 Raptor engines, just as Vorontsov’s team had predicted, matching the number on SpaceX’s upcoming Falcon Heavy lifter.

It would be the greatest number of engines on any rocket stage since 30 NK-15 engines were used to propel the Soviet Union’s failed moon rocket, the N-1. The stage’s propellant mass alone would be several hundred tonnes more than the entire Saturn V or N-1 launch vehicle’s liftoff mass.

The engines on the base of the Soviet N1 rocket.

NSF learned the core stage would also have a significantly higher ideal staging velocity than the expendable version of the Falcon 9 thanks to its more efficient Raptor engines.

The plan’s summary outlined intentions to reuse both the core stage and its upper stages, with the core stage being capable of launching more than 15 times. When reusing both stages, payload to LEO would only drop to 200 tonnes.  NSF experts quickly realized this would only be accomplished if the firm attempted to land the enormous stage on either an island or autonomous drone ship downrange. This gives a far smaller hit to payload to orbit (estimates range from 4-12.5%) than the nearly 50% hit to payload caused by returning the core stage to the launch site.

Fewer details were available about the spaceship and tanker, though both were 60 meters long, vertical landing lifting body designs and would be capable of docking and transferring propellant. The thrust ratio of the core stage to the spaceship/tanker stages was a low 5:1, while most rockets have thrust ratios of 8:1 or more.

NSF experts realized this ratio meant that there would be five Raptor Vacuum engines on the spaceship. It was thought this lower thrust ratio was due to SpaceX attempting to trim the delta-V of the core stage so that it could be more easily reused. NSF later learned that three central atmosphere optimized Raptor engines would also be present for landings. The engine layout added engine redundancy in flight but had the drawback of adding more dry mass.

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