With the spaceflight community still in awe at the unveiling of the Interplanetary Transport System (ITS), SpaceX has set itself some lofty long-term goals. However, while the realization of the plan is up for debate, the presentation has at least revealed some of the key specifications of the system, not least the Raptor engine.
The Raptor Engine:
Since the 2010 disclosure of the Raptor code name for its next-generation engine, it has been through different combustion cycles, propellants and thrust levels. This began with notional plans for 667kN of power, once reaching as high as 6,670kN, before now settling at 3,000kN of sea level thrust.
In all launch vehicle designs that require new engines, the long pole has always been main rocket propulsion – with engine development a key stage for any space project.
Having a full array of seasoned test stand capabilities is a basic requirement, as NPO Energomash found out with the excruciating development of the RD-170 that resulted in the destruction of its test stand.
Full flow engines, like the Raptor, are usually considered to be impossible to test in the subsystems evaluation stage. The main problem is the highly integrated nature of the cycle.
In the full flow staged combustion cycle, almost all the fuel is first burned with a tiny bit of oxidizer in the fuel preburner to generate hot gas. Then, all that gas mass is put through the turbines, to drive the turbopumps, which are used to feed the preburners.
Rocket pumps have to increase the pressure to enormous levels. This is required because even after going through the cooling passages – the preburner and the turbine – it still needs to be injected into the main combustion chamber at the required pressure level.
In the Raptor’s case, this is expected to be a record-breaking 30MPa – more than 300 atmospheres of pressure. For comparison, the previous record holder for chamber pressure, the RD-170 derivative RD-191, was 25.74MPa.
In the full flow case, the same process of converting to hot gas and driving the turbopumps also needs to be done with the oxidizer.
While complex, this cycle allows for the use of all available mass to power the turbomachinery, thus enabling the highest possible chamber pressure.
The higher the chamber pressure, the higher the efficiency of the engine, as measured by specific impulse. It also reduces the physical size of the engine hardware, since it required less plumbing/piping, while the throat of the engine is also physically smaller.
However, the problem for this type of cycle is that to test the preburner in the full flow case, you need both the fuel and oxidizer pumps to feed it. However, to drive the turbopumps you need the output of both the preburners.
Also, to test the combustion chamber and its injectors, you need the turbopumps and the preburners.
As such, individual testing of each subsystem is almost impossible for full flow engines, unless you have a test stand capable of supplying extremely high-pressure liquid and hot gas of both the fuel and oxidizer at the same time.
Enter the famous Stennis Space Center and its critical E2 test stand.
While incapable of handling the full size of the expected Raptor engine unit, the Stennis test stand enabled the individual testing of each subcomponent of the 1MN scaled prototype that SpaceX currently has at its test facility in McGregor, Texas.
During late 2013, the SpaceX engineers arrived at Stennis to help upgrade its unique E2 test stand, enabling it to supply liquid and hot gaseous methane.
On April 21, 2014 the upgraded test stand was officially inaugurated and a full test program of each individual subsystem was tested and validated.
Since the final thrust level of the Raptor had not been settled, it was decided that the first integrated test engine would be a 1MN sub-scale engine.
It enabled the full testing at Stennis E2 and allowed for the development of robust startup and shutdown sequences, characterize hardware durability and anchor analytical models that would be used for future designs.
The physics of mixing gasses in the combustion chamber is not as well developed as the more traditional gas-liquid or liquid-liquid forms in the West.
SpaceX engineers developed specialized software to compute just the
mixing process in detail while adding approximations for the rest of the
flow. This still allowed for very high fidelity simulations to be
carried with reasonable computing resources.
However, this modeling breakthrough still requires validation with real results first, which will be realized by the demonstrator engine.
Once the final engine thrust was defined, the engine could be scaled up with relative ease. The full flow cycle is very helpful in that sense and the 1MN thrust level would already be considered a big engine.
With the production engines – as currently envisioned – it would need to triple its thrust. Not trivial, but still within what could be considered highly representative as a demonstrator.
On August 8, 2016, the first integrated Raptor demonstrator left its Hawthorne base for SpaceX’s very own Raptor test stand in its McGregor testing facility.
With a thrust of 1MN (225klbf) at sea level, this was to be the first methane full flow engine to ever reach a test stand. In fact, it was the second full flow engine for any propellant.
Glushko’s RD-270 was the pioneer in 1967 – interestingly also designed for a Mars bounded rocket, the UR-700.
However, that engine never solved its combustion instability issues and its hypergolic propellant made it an environmental tragedy waiting to happen. The launch vehicle was never provided with a green light.
The Raptor demonstrator turbomachinery is capable of producing 27MW of power, with more power “per unit of thrust” of any hydrocarbon engine.
It also demonstrates the use of 3D printing, with 40 percent of the engine utilizing this technology when measured by mass.
Experts analysis via L2 also implied that the integrated nature of the design would be difficult to construct and assemble if built by the traditional method.
Then came the recent – and timely – milestone. On September 26, at around 3am UTC, the first ignition test of the demonstrator was performed.
Via the standard procedure for the first ignition test of a new engine – called a “burp test” – a short firing tests the ignition phase, usually without the engine reaching full power.
However, this first test was more than just a burp test, with SpaceX’s CTO and founder, Elon Musk, showing a video – during his overview at the IAC forum in Guadalajara – of the Raptor igniting and firing for approximately nine seconds before shutting down.
In L2 McGregor photos, the engine has been observed without the sporting of an extension. Mr. Musk has since confirmed that the development engine will eventually have a nozzle with an expansion ratio of 150, the maximum possible within Earth’s atmosphere.
Should all go to plan with the development, the Raptor will provide the required thrust to launch the massive ITS off the launch pad.
Raptor will come in two versions, one optimized for the first stage – and thus with a short nozzle – and another one for the spacecraft – with a long nozzle optimized for vacuum and Mars’ weak atmosphere.
As is traditional for SpaceX, both engines variations would have the same powerpack and thrust chamber. The only difference would be on the nozzle.
They will also have a version with – and without – Thrust Vector Control (TVC). The use of fixed engines on the outer rings of the ITS allows for more tight packing and reduced dry mass.
The vectored inner engines would supply control authority. Differential throttling may be an option in case of emergency.
The first stage model is documented as sporting a sea level thrust of 3,050 kN with an isp of 334 seconds, and 3,285 kN and 361 seconds in vacuum. Its diameter will be less than two meters.
The vacuum version of the Raptor will produce 3.5MN of thrust, with a specific impulse of 382 seconds, thanks to its 200 expansion ratio nozzle. It will have an estimated diameter of four meters.
With an estimated 80MW of power on the turbopumps, both engines would use subcooled liquid methane and oxygen as propellant. This improves the specific impulse, the thrust and reduces the risk of cavitation on the turbopump.
Also, Raptor will have some changes that are critical to reducing the fluids from five to two, eliminating the issues related to hypergolics.
The engine will have heat exchangers to heat methane and oxygen so the ITS can self-pressurize. The helium pressurization system has provided quite a few headaches to SpaceX on its current rockets and as such is not surprising its engineers saw the option to eliminate it entirely as preferable.
Additionally, it will eliminate the TEA-TEB hypergolic cartridges currently used by Merlin engines. Instead, Raptor has implemented a new spark ignition system that, at least theoretically, would allow for unlimited re-ignitions.
Raptor will also be able to throttle from 100 percent to 20 percent of thrust, providing additional options during its powered phases.
Having a 30MPa chamber pressure not only allows for the best specific impulse – short of the much heavier hydrogen-fuelled engines – but it also reduces size and mass significantly.
In fact, Mr. Musk stated that he expects Raptor to actually have the highest thrust to weight ratio ever for a production rocket engine. A record currently held by the company’s other engine, the Merlin 1D.
The small size, which Elon characterized as similar to the Merlin 1D, makes SpaceX confident that it will be able to produce them in mass. Currently, SpaceX is producing Merlins at a rate of 300 per year.
With 51 engines per each ITS, SpaceX will need quite a number of them for its initial fleet.
The first stage will sport 42 engines on the first stage, and nine on the second. Interestingly, the second stage – as currently envisioned – would use six vacuum and three atmospheric optimized Raptors.
However, for each trip at least two upper stages would be needed, one spacecraft and at least one tanker. Numerous tankers are likely to be required for each ITS heading out to Mars.
While the ITS sports numerous technological breakthroughs – such as the fully composite tanks and methane-oxygen thrusters – Raptor is the most critical technology in enabling a design that SpaceX currently expect could reduce the cost per passenger (plus cargo) to Mars, from around 10 billion to just 140,000 dollars.
The reality of that goal is open to debate, but SpaceX has shown a path towards that ambitious future.
(Images via SpaceX, NASA and L2 McGregor (Gary Blair). To join L2, click here: https://www.nasaspaceflight.com/l2/)