Before Elon Musk walked on to the stage at the 67th annual International Astronautical Congress to declare exactly how humans are going to become an interplanetary species by colonising Mars, one piece of the gargantuan interplanetary jigsaw had to fall into place to give SpaceX’s ambitions credibility: the Raptor rocket engine.
And thanks to the no doubt phenomenal engineering brains at SpaceX, it did just that. On September 27, Elon Musk tweeted a photo of SpaceX’s first successful test firing of the Raptor interplanetary transport engine at the company’s McGregor, Texas facility. But why does SpaceX need to design a new engine, and why is the Raptor so special?
Getting 100 colonists, their life support, their waste, and their luggage the 57.6 million kilometers from Earth to Mars is not as easy as lobbing up, for example, a small satellite into a low Earth orbit (LEO). The engines required to propel the giant booster and spaceship into space need to be powerful yet more efficient than any other rocket engine created before, whilst possessing the ability to endure multiple flights and re-entry cycles.
Remember, SpaceX’s proposed Interplanetary Transport System (ITS), booster and spaceship combined, is 122 meters tall, with a diameter of 12 meters (the spaceship section had a diameter of 17 meters). In context, that’s 11 meters taller than NASA’s Saturn V rocket, and a tad wider. This ITS is so big, in fact, it would just about fit into NASA’s Vehicle Assembly Building (VAB) at Kennedy Space Center, with only 17 meters to spare.
“If you’re going to stand on the shoulders of giants, pick the right giants.”
“Also, [Raptors] are very, very high pressure rocket engines. That’s a characteristic of Russian rocket engine technology too. The closed cycle [as opposed to an open cycle gas generator system the Merlin engines use] is also a Russian rocket engine development. They’re taking from a very good example, and building on it. If you’re going to stand on the shoulders of giants, pick the right giants. And they have.”
The meaning of life is…42?
The next step for SpaceX is attaching the Raptor engines to the ITS colony ship (the first of which will be named after the “Heart of Gold” spaceship in Douglas Adams’ The Hitchhiker’s Guide to the Galaxy). There’s planned to be 42 of the beasts for the booster section, optimized for sea level. The engine configuration on the booster will consist of 21 Raptors on the outer ring, 14 on the inner ring, and then seven clustered in the center. This center cluster will also be gimballed for manoeuvrability.
The spaceship section will use six Raptors, optimized for vacuum flight, along with three extra Raptors for manoeuvring on landing.
“The ITS uses 42 engines in the booster stage. Now there are some advantages to that, for example if some of the engines shut off. Supposing if one of the engines has to be shut down, then you can shut down its partner on the other side, so you don’t have asymmetric thrust,” said Bonsor. “Then you throttle up the remaining engines. Obviously there’s a limit to how far you can go with that, but there’s a lot of cross-plumbing and really good propellant management, and it can compensate for an ‘engine out’ [scenario]. For larger engines in a smaller number, ‘engine outs’ are more difficult to deal with. That happened a couple of times with the Saturn V. However there are possible dangers. This has been tried before on the Russian N1 Lunar booster back in the late 1960s [through] early 1970s.”