The Evolution of the Big Falcon Rocket

by Phillip Gaynor

The Falcon 9 v1.0 was successfully flown five times between 2010 and 2013 before being replaced by the higher lift capacity Falcon 9 v1.1, which essentially was a new launch vehicle.  Although it had the same diameter as its predecessor, v1.1 featured a new octaweb thrust structure, enlarged propellant tanks and 19% higher thrust and more efficient Merlin 1D engines.  Later versions were even equipped with legs and fins for landing attempts.  The first flew on September 29th, 2013, with a total of 14 successful launches out of 15 attempts.  SpaceX’s initial landing attempts were onto barges positioned out at sea, however all three landing attempts with the v1.1 failed.

The design rapidly evolved further into the Falcon 9 v1.2, the first launch of which saw a Falcon 9 core stage safely landed near its launch pad on December 22nd, 2015. To enable the landing, the Falcon 9 v1.2 had added extendable landing legs onto its core stage, upgraded cold gas thrusters, improved guidance programming for retro propulsion, and accuracy improving grid fins.

First Falcon 9 landing – Credit SpaceX

The rocket also set a record as the most efficient launch vehicle ever launched to Low Earth Orbit (LEO), which provided the performance margin necessary for reuse. Although the rocket featured typical upgrades like higher thrust engines, enlarged propellant tanks and a strengthened structure, it also featured a new design element. It used chilled propellant to increase its propellant density, which improved performance by allowing the Falcon 9 to carry more propellant at almost no cost in dry mass. Almost all of the Falcon 9’s design architecture would later be used in the design of the BFR.

SpaceX’s Raptor engine went almost entirely unmentioned from August 2010 until June 2011, when engineer Jeff Thornburg was placed in charge of development and given a small team to work on the engine. Its development proceeded slowly due to its low level of priority.

By October 2012, with SpaceX’s engine expertise growing and finances improving, the Raptor had transformed into an engine several times as powerful as the Merlin 1 engine. Multiple Raptor engines were to power a future rocket capable of lifting 150-200 tonnes to LEO.

The next month, Musk announced that the Raptor was to be a methane fueled engine, which was the preferred fuel for SpaceX’s Mars colonization ambitions. The engine’s cycle was changed from gas generator to a staged combustion cycle, which was more complicated but allowed higher efficiency and thrust.

Methane was chosen over kerosene due to it being cleaner, being possible to synthesize on Mars, and being easier to use in a multi-start engine. Additionally, Methane won out over hydrogen, which can also be produced on Mars, because liquid methane is much more energy dense, which allowed for a lighter and cheaper BFR. Lastly, methane does not cause metal embrittlement and has higher melting and boiling points, allowing it to be stored for much longer periods without heavy propellant boil off. The Raptor’s first thrust figure of at least 2,942 kN (661,400 lbf) was announced in October 2013.

Nasaspaceflight.com (NSF) member experts became intrigued with the possibilities suggested by these early revelations.  Within months, NSF member expert Dmitry Vorontsov created the very first images, mass estimates and launch simulations of the SpaceX Mars launch vehicle in a dedicated L2 evaluation section.  Vorontsov’s intuition was that that the initial design would resemble an enlarged Falcon 9 with 9 Raptor engines on the first stage and a single Raptor engine on the upper stage.  He also evaluated a triple core version of the design, which SpaceX was understood to be evaluating at that time.

NSF’s early work showed that a single 9.8 meter core version of this rocket was only capable of lifting 120 tonnes, less than the 150-200 tonnes SpaceX claimed. A triple core version massing 5,440 tonnes and powered by twenty eight Raptor engines would have been capable of lifting more than 286 tonnes to LEO.

In March 2014, SpaceX Vice President of Propulsion Tom Mueller confirmed Vorontsov’s choice of a Falcon 9 style engine configuration of nine Raptor engines on the core stage and one on the upper stage.

Early BFR conceptions – rendered by Dmitry Vorontsov for NSF/L2

Mueller also announced an increase in the Raptor engine’s thrust to 4,448 kN (1,000,000 lbf), and the choice of a 10 meter diameter for the rocket. Also, the Raptor engine would be employing a rarely used but highly efficient combustion cycle called full flow staged combustion.

New NSF evaluations showed this would improve the lift capability of the single core version to 174 tonnes to LEO. Although record-breaking, such a rocket by itself was not capable enough to allow the colonization of Mars, let alone a return journey.

NSF’s experts at the time believed the likely course was to employ a “dual launch” approach, with the Mars colonization spacecraft being launched with crew into orbit and then refueled by a tanker spacecraft.  A similar approach but with multiple tanker launches would later be confirmed in October 2015 as SpaceX’s preferred launch architecture.

In this approach, following the final refueling the tanker would return to Earth while the spaceship would make the burn for Mars. Once on Mars, the spaceship would be refueled with specialized equipment and local resources, much like was envisioned in Robert Zubrin’s Mars Direct project.

In May 2014, further details of the Raptor engine were given at the Space Propulsion conference in Cologne, Germany. Raptor’s sea level thrust had increased to 6,914 kN (1,554,300 lbf). The vacuum version of Raptor had a thrust of 8,238 kN (1,851,900 lbf) and an ISP of 380 seconds.

The result was the vehicle’s thrust had jumped to 66,223 kN (13,988,300 lbf). In June 2014, as SpaceX started Raptor component testing, Mueller revised the Raptor engine’s thrust to 7,414 kN (1,666,700 lbf), which rivaled the most powerful rocket engines ever built. The vehicle’s total thrust as a result increased a further 7.23% to 66,723 kN (15,000,000 lbf).

Furthermore, details emerged that SpaceX was moving away from considering a multi core design and was instead concentrating on a single large core, with diameters ranging from 10 to 15 meters under consideration.

The major reason for this shift likely was due to the much smaller performance penalty for reuse allowed by a single core rocket versus a multi core rocket. Following the upgrade in thrust, NSF evaluations showed that any of the single core variants under consideration would have been capable of lifting more than 300 tonnes to LEO.

Further NSF evaluations also showed that due to its sheer size and the high ISP of its engines, barge landing reuse would only cost about 4% of the rocket’s maximum capability.

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