SpaceX – Launch Vehicle Concepts & Designs

SpaceX Launch Vehicle Concepts (2010) - Image: SpaceX
SpaceX Launch Vehicle Concepts (2010) – Image: SpaceX

While focusing on the evolution of its Falcon 9 and Falcon Heavy launch system and the implementation of re-usable rocket technology, SpaceX has also been working on concepts and designs for future heavy-lift and super-heavy lift rockets. SpaceX and its CEO Elon Musk are aiming at manned flights to Mars in the next two decades which will require powerful launch vehicles that can send payloads of up to 100 metric tons to Mars.

A number of concepts and ideas have been presented by SpaceX over the years, starting with the evolution of its Merlin engine to become the most-powerful single-chamber engine ever built in order to power Falcon X and Falcon XX rockets that could launch 140 metric tons into orbit. In recent years, SpaceX turned its back on the Merlin 2 engine design in favor of a methane-fueled engine called Raptor. This engine will be the workhorse used on SpaceX’s transportation infrastructure known as Mars Colonial Transporter MCT that will eventually deliver humans and cargo to Mars.

Merlin 2 Engine

Image: SpaceX
Image: SpaceX

As part of the evolution of the Merlin Rocket engine series, SpaceX performed concept studies of scaling up Merlin 1 design components to build a large single chamber LOX/RP-1 engine that became known as Merlin 2. The engine was conceptualized for use on future heavy and super-heavy launchers (Falcon X & XX). Details on the design of the Merlin 2 were provided in 2010 at the AIAA Joint Propulsion conference.

Merlin 2 would keep the basic design of the Merlin 1C and 1D engines, sticking to the gas-generator cycle also known as open cycle. In this design, a small portion of the LOX and RP-1 flow is directed to a pre-burner which burns these propellants to generate the hot gas that is needed to power a single-shaft dual-impeller turbine that is shared by the LOX and RP-1 turbopumps. The gas-generator exhaust is then either discarded via an exhaust nozzle (which could be used for roll control) or in case of Merlin 2, the hot gas is injected into the engine bell to slightly improve the engine’s performance. With the turbopumps at flight speed, LOX is directly delivered to the combustion chamber while the RP-1 fuel is first passed through the engine heat exchanger to provide cooling before reaching the combustion chamber.

Merlin 2 was conceptualized for two operational scenarios – the operation at a 70% throttle point and burns at 100% to lift a heavier vehicle.

At the 70% Throttle Setting, Merlin 2 would provide 5,340 Kilonewtons of sea level thrust with an impulse of 271 seconds, climbing to a vacuum thrust of 6,320 Kilonewtons and a vacuum impulse of 320 seconds. The engine would operate at a chamber pressure of 101.5 bar and a propellant flow of 2,010 Kilograms per second.

SpaceX ConceptFiring at full throttle, Merlin 2 would reach a sea level thrust of 7,560 Kilonewtons with a specific impulse of 285 seconds, increasing to 8,540 Kilonewtons and 321 seconds in vacuum. At 100%, Merlin 2 operates at an increased propellant flowrate of 2,750 Kilograms per second and a chamber pressure of 137.9 bar. Burning at full thrust, Merlin 2 would have been more powerful than the F-1 engine used on the Saturn V rocket.

The Merlin 2 engine was designed to deliver a thrust to weight ratio of 150 which was first realized by the smaller Merlin 1D version used on Falcon 9.

Laying out a possible future plan for Merlin 2, SpaceX indicated in a presentation given at the 2010 at the AIAA Joint Propulsion conference that the engine could be designed in three years time at a cost of about one billion US Dollars with a production cost of ~$50M per engine.

Although development of Merlin 2 was was not officially discontinued, the switch to the methane-fueled Raptor engine (see below) likely marked the end of the Merlin 2.

SpaceX Concept1
Image: SpaceX
Image: SpaceX (edited)
Image: SpaceX (edited)
Image: SpaceX (edited)
Image: SpaceX (edited)

Falcon 9 & Falcon Heavy with Merlin 2

Once operational, a single Merlin 2 engine could have been used to replace the nine Merlin 1D engines on the first stage of the Falcon 9 rocket. Replacing the Merlin 1D engines with Merlin 2 would have actually increased the liftoff thrust and payload capability of Falcon 9 and Falcon Heavy at that time (M1D was still in development), but the vehicle’s engine-out capability would have been eliminated.

Falcon X

At the 2010 at the AIAA Joint Propulsion conference, SpaceX presented potential designs for future heavy and super-heavy lift vehicles. These presented designs were no firmed up configurations of future launchers, and merely represented conceptual ideas, however, it is likely that SpaceX would take a path similar to the presented designs for their future launch vehicles.

Falcon X is a two-stage launch vehicle standing more than 75 meters tall with a core diameter of approximately 6 meters. Its first stage would have used three Merlin 2 engines operating at 70% for a total liftoff thrust of 16,000 Kilonewtons (1,630 metric tons). With three Merlin engines powering the first stage, the launcher would provide engine-out capability.

Details on the vehicle’s upper stage were not given, however, SpaceX was working on a 667-Kilonewton LOX/LH2 version of the Raptor engine at the time for use on upper stages making it a likely candidate.

Falcon X could deliver payloads of up to 38 metric tons into Low Earth Orbit.

Falcon X Heavy

As with Falcon 9 Heavy, the Falcon X launch system would include a heavy version using three identical cores, each sporting three Merlin 2 engines for a liftoff thrust of 48 Meganewtons (4,890 metric tons). The vehicle would likely use the same upper stage as Falcon X. This configuration would be capable of launching payloads of up to 125 metric tons to Low Earth Orbit.

Image: SpaceX (edited)
Image: SpaceX (edited)

Falcon XX

Falcon XX is a single core launch vehicle with a diameter of 10 meters and an estimated length of about 100 meters using six Merlin 2 engines operating at the 100% thrust setting on its first stage creating a liftoff thrust of 45,360 Kilonewtons (4,625 metric tons).

Again, the upper stage of the vehicle was not specified at the time, but a one or two-engine Raptor design would have been one likely way to go. Falcon XX would have engine-out capability for a portion of its ascent. Low Earth orbit payload capability has been estimated at 140 metric tons.

A heavy-version of the Falcon XX with three cores has not been presented by SpaceX, but would be an option to boost the vehicle’s payload capability to more than 400mt.


BFR or Big Falcon Rocket (or Big F… Rocket) was a phrase coined around 2005 as a title for future heavy-lift SpaceX rockets including the Falcon X/XX and other potential designs.

Raptor Engine Family

Raptor Diagram (2010 Design) - Image: SpaceX
Raptor Diagram (2010 Design) – Image: SpaceX

First presented in 2009 by SpaceX, the Raptor engine started out as a LOX/LH2 powered engine design for upper stages. The engine was proposed to have a vacuum impulse of 470 seconds achieving a thrust of 667 Kilonewtons (68,000kgf) with a throttle range of 50 to 100%. Optimized for operation in vacuum, Raptor would have flown with an extended nozzle with an area ratio of 250:1.

Unlike the Merlin rocket family, the Raptor was initially conceptualized to be a cryogenic staged combustion engine with a closed cycle. The majority of LH2 fuel and a portion of the oxidizer flow is fed at high pressure to the pre-burner that burns the propellants and supplies hot gas to two separate turbines that are used to power the LH2 and LOX turbopumps.

The exhausted fuel-rich gas is then injected into the combustion chamber with the rest of the oxidizer to complete the combustion. A portion of the fuel is used for engine cooling (regenerative cooling) before being injected into the engine as well.

Raptor was designed to operate at a chamber pressure of 117 bar. The Raptor engine characteristics as of 2010 are given in the table at the bottom of this page.

In 2012, the Raptor development program got underway after a change in direction as SpaceX’s Elon Musk announced that Raptor would become a family of methane-fueled engines. No longer designed for exclusive use on upper stages, different versions of Raptor would be used as first stage and upper stage engines, not unlike the Merlin 1 engines that are used on Falcon 9’s first stage and as a vacuum-optimized version on the second stage.

Image: Purdue University/Spaceflight101
Image: Purdue University/Spaceflight101

With the switch from LH2 to methane, Raptor’s design thrust was drastically increased for it to power large launch vehicles. This re-designed version of Raptor will keep its staged-combustion design, but use a Full-Flow cycle which has not been used for the 2010 Raptor design shown above.

A Full-Flow Staged Combustion Engine is a variation of the Staged Combustion Cycle (shown above) in which all of the oxidizer and fuel pass through their respective turbopump turbines. The fuel is first directed through the nozzle heat exchanger to provide regenerative cooling before being passed to the fuel turbine; the oxidizer flows directly from its turbopump to the LOX turbine. To power the turbines, a small amount of fuel & oxidizer is exchanged between the lines which then is then burned in two pre-burners (one oxidizer-rich, one fuel-rich) to deliver the hot gas to power the turbines that drive the turbopumps. The propellants are then fed to the combustion chamber where the combustion process is completed.

The advantage of the full-flow cycle is that the turbines operate at lower temperatures since more mass passes through them leading to increased reliability and a longer engine life which is particularly important to potential re-use of the engine. In addition, this engine design can deliver higher chamber pressures and improve the efficiency of the engine.

Methane has a slight advantage over Rocket Propellant-1 in terms of specific impulse, but can not reach that of Hydrogen. However, there are other advantages over LH2 such as easier handling and storage, no concerns associated with Hydrogen embrittlement and a much lower production cost. In addition, liquid methane has a higher density than LH2 which has obvious implications for tank and vehicle dimensions. Compared to RP-1, methane does not lead to coking of the engines which is a common problem with RP-1 that requires oxygen-rich combustion to limit coking, but creates a more corrosive environment.

RaptorIn October 2013, SpaceX officially confirmed that the Raptor engine would be tested at NASA’s Stennis Space Center. SpaceX personnel started working at test complex E at Stennis in mid/late 2013 to implement modifications needed to support methane engine tests. The E-2 complex can only facilitate engines up to 500kN which is sufficient for testing the individual components of the Raptor engine such as the Pre-Burner.

For testing of the complete Raptor, a bigger test stand is needed. As of late 2013, testing at Stennis was expected to commence in 2014.

In February 2014, Tom Mueller, SpaceX head of rocket engine development, elaborated on the design of future SpaceX vehicles and engines at an event in Santa Barbara. He stated that the Raptor engine that is currently being worked on would have a vacuum thrust of 4,400 Kilonewtons (448,700kgf) and achieve a vacuum impulse exceeding 360 seconds. Raptor would have an estimated and Sea Level thrust of around 3,800 Kilonewtons (362,800kgf) or slightly less. With these figures, Raptor would reach a higher specific impulse than the Russian-designed RD-0162 methane-fueled engine.


MCT, the Mars Colonial Transporter, is a SpaceX concept for the company’s and Elon Musk’s goal of building a space transportation system to deliver humans and cargo to Mars and sustain a permanent settlement on the Red Planet.

Exactly what the MCT is has not been specified (whether it is a launch vehicle, a spacecraft or the complete infrastructure needed to fly humans to Mars). Musk has often said that his plan was to send a manned mission to Mars within two decades with the vision of a future colony of several thousand people.

Details on the MCT have not been given except for a few hints that indicate the the MCT spacecraft would be ‘100 times the size of an SUV’ and launch on top of a launch vehicle based on, but much bigger than the existing Falcon 9. It has also been confirmed that this launch vehicle would use the Raptor engine.

Image: SpaceX
Image: SpaceX


Falcon – Raptor Designs

Falcon 9 Octaweb - Photo: SpaceX
Falcon 9 Octaweb – Photo: SpaceX

With Merlin 2 being replaced by the revised Raptor design using LOX/Methane propellants, the Falcon X and XX concepts as presented in 2010 are no longer of any use.

As specified in February 2014 by Tom Mueller, the future heavy lift rocket that will be used to transport crew and cargo to Mars will be an upscaled version of Falcon 9. The launcher will have a core diameter of ten meters (like the Falcon XX) and use nine Raptor engines, likely arranged in an Octaweb pattern developed for the upgraded Falcon 9. With nine Raptor engines, this launch vehicle would have a launch thrust of 34.2 Meganewtons (3,490 metric tons).

The second stage of that vehicle would most likely use one or two Raptor engines optimized for operation in vacuum to send the MCT spacecraft on its way to Mars. This single-core vehicle would be capable of delivering significantly more than 100 metric tons to Low Earth Orbit, but that figure would be reduced due to the implementation of first stage boost-back.

SpaceX has also stressed that their future launch vehicles and would be designed to be re-usable utilizing the same methods that are being developed for the Falcon 9.

A three-core version of this launcher would be capable of sending payloads of around 100 metric tons to Mars.

SpaceX Launch Vehicle Concepts

SpaceX Concepts2


Raptor 2010