The wild physics of Raptor: SpaceX’s methane-guzzling rocket engine


SpaceX’s Merlin engines – shown during the launch of the Arabsat-6A satellite mission – will be superseded by the firm’s Raptor engine

SpaceX

SpaceX CEO Elon Musk has his mind set on going to Mars. To do so will require a vast new spacecraft, powered by a new rocket engine unlike anything that’s been seen before. Step forward Raptor, the future of SpaceX’s endeavours.

Raptor is designed to power the new reusable vehicles SpaceX’s is building, the Starship spacecraft and the Super Heavy rocket. Combined, these vast machines are designed to take up to 100 people into space, possibly to the Moon and Mars, with a tentative launch of humans around the Moon planned in 2023. SpaceX tested the first flight-ready version of Raptor in February 2019 and, last week, it flew a Raptor for the first time on a “hop” test of a prototype of Starship, called Starhopper.

The engine has been in development for the better part of a decade, going through a number of iterations. At its core it’s like other engines, burning chemical fuel to produce thrust. But its use of liquid oxygen and methane – something largely unprecedented in the rocket industry – and its innovative design means that it just might be SpaceX’s ace in the hole when it comes to exploring the Solar System.

“There is no other rocket engine capable of producing as much energy out of liquid methane and liquid oxygen as the Raptor engine,” says Tim Dodd, an industry expert who runs the popular Everyday Astronaut YouTube channel. “And making it more reusable with little to no refurbishment is the idea. That’s definitely going to help their business case if they can just fly over and over.”

The reusability is a key aspect, as Musk has said each engine needs to be capable of flying up to 1,000 times to support the ambitious operations of Starship. That’s a major challenge; the most re-used engines in space exploration history was the main engines on each Space Shuttle, which flew up to only a few dozen times each. “It’s quite ambitious,” says Dodd. “I don’t know if 1,000 flights is necessarily going to be achievable in the near future. If it lives up to its potential, maybe 1,000 is within the realm of possibility one day.”

SpaceX’s existing engine is called Merlin, which is used on its operational Falcon 9 and Falcon Heavy rockets, but Raptor heralds a significant improvement. One is that it has double the thrust of its predecessor thanks to a much higher pressure, 380,000 pounds of thrust at sea level versus 190,000 pounds, despite being a similar size.

The other is the use of methane. No methane-powered rocket has ever made it to orbit, with Starhopper’s test hop the other day being the first time a methane engine had actually taken flight. Methane prevents a build-up of deposits in the engine compared to other fuels like kerosene, a process known as coking, while its higher density than other fuels allows for higher performance at a lower cost.

“The cost of propellant for liquid rockets is such a trivial proportion of the total launch costs,” says space consultant Rand Simberg. “With reusable vehicles, we want to get to the point at which we care what the propellant costs. In airlines typically 35 per cent of the total operating costs is fuel. With a rocket it’s less than one per cent traditionally.”

Raptor also uses what’s known as a full-flow staged combustion engine, only the third engine in history to employ this technique, whereas Merlin uses the more common open cycle system. The previous two attempts at such an engine, one in the Soviet Union in the 1960s and another in the US in the early 2000s, never made it beyond testing.

A full-flow stage combustion engine refers to how a pump spins a turbine to drive the engine, using what’s called a preburner to get this process going by injecting a small amount of fuel. Normally some of the propellant is expended in a traditional open cycle engine to start this process, but Raptor will use every drop of propellant available, making it one of the most efficient rocket engines ever built.

“Raptor burns that fuel at a high enough pressure that can then steer the fire from preburner back into the combustion chamber and completely burn that propellant with the rest of the propellants,” says space consultant Charlie Garcia from the Massachusetts Institute of Technology (MIT). “And it does this in a very clever way that only the Russians have done previously… by putting all the propellant in the engine through the preburners.”

The end result is that Raptor has a much higher pressure than Merlin, about three times greater, making it the highest pressure rocket engine in existence and leading to its aforementioned larger thrust than Merlin despite its similar size. In 2016, Musk referred to the “insane pressure” inside the main chamber of the engine, 300 bars, which required the development of a new metal alloy.

Of course, Raptor’s other major innovation over its predecessor is the use of methane, which harkens back to SpaceX’s ultimate goal. SpaceX isn’t the only company moving to methane, as rival firm Blue Origin – headed by Amazon CEO Jeff Bezos – is also developing its own methane engine, called the Blue Engine 4 (BE-4). With numerous benefits, it’s a switch that almost seems long overdue.

Most previous rocket engines have relied on using fuels like kerosene in place of methane. But the main benefit of using methane is that it is much denser than other fuels, meaning the rocket can be smaller. Its lower cost, too, means the total cost of launching can be brought down.

This could be crucial, because the number of Raptor engines SpaceX is looking to build is immense, to say the least. Each Starship vehicle is designed to fly with six Raptor engines, along with 35 on the Super Heavy rocket – a total of 41 per launch. SpaceX’s biggest rocket so far, the Falcon Heavy, launches with 28 Merlin engines (27 on the bottom and one on the upper stage). With the number of launches SpaceX is planning for Starship and Super Heavy, their rate of production will need to increase significantly.

“Since they’re using so many of them on the Super Heavy vehicle, they’re going to have to be ramping up manufacturing to an absolutely insane pace,” says Dodd. “Elon’s talked about making one in 12 hours, which would be unheard of in the industry. Even if they make one a week that’s pretty impressive.”

It’s also believed that methane could be available on the surface of Mars or elsewhere in space to be used as fuel – perhaps by mining water and carbon dioxide on the surface – known as in-situ resource utilisation (ISRU). “You could reasonably easily extract methane from the Martian surface, and potentially the lunar surface,” says industry analyst Caleb Williams from the consulting firm SpaceWorks.

This is vital to SpaceX’s goal of regular trips to and from Mars with Starship, allowing it to be almost self-sufficient in terms of fuel. “This has been the holy grail of Solar System access for humanity,” says Garcia. “Because when you can refuel in space, now all of a sudden your propellant doesn’t have to come from Earth.”

Alongside these lofty claims, Raptor represents a large shift in SpaceX’s ambitions from modest Earth-based launches to, potentially, exploration of the Solar System. Starhopper’s recent test was a key step in the right direction, and if Raptor even partially lives up to some of its promises, the future could be very bright indeed. If SpaceX really do reach Mars, Raptor will have a huge part to play.

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