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.
SpaceX, initially, has to get a planned 550 tonnes worth of payload into orbit, with 450 tonnes of payload heading beyond to Mars. The Saturn V, back in the Apollo era, had to take just 135 tonnes into LEO. Musk's proposed Falcon Heavy, a super heavy lift space launch vehicle with a first launch planned for early next year, is designed to carry 54.4 tonnes. Even on its lighter trips, when acting as an ITS tanker for refuelling other ITS spaceships, the booster will need to carry 380 tonnes of propellant into LEO. The Raptor engines are also needed for the ITS's second stage, or rather, the spaceship section, to carry the cargo and colonists to Mars. Yes, SpaceX clearly needs a new engine design.
So when it comes down to it, the Raptor engine claims to deliver 690,000 pounds (3,050 kN) of thrust at sea level, increasing to 738,000 pounds (3,285 kN) of thrust in the vacuum of space (no atmosphere makes for more efficient engines). In a tweet on September 26, Musk confirmed that the test firing had reached 670,000 pounds of thrust, far more powerful than SpaceX's own Merlin 1D engine (150,000 pounds of thrust) and the Space Shuttle's RS-25 engine (400,000 pounds of thrust).
To achieve this insane level of power, while still delivering an engine that can be reused over a long period life, SpaceX needed a design overhaul.
Why couldn't Musk just strap on Saturn V's F-1 engines onto ITS and be done with it? Well, while delivering an impressive 1,522,000 pounds of thrust at sea level per engine, older engine technologies such as the F-1 just weren't designed for longlife and re-entry. ITS engines also have to be efficient out beyond earth orbit, too.
While SpaceX initially planned to use a liquid oxygen and hydrogen (hydralox) mix for the Raptor engine, in 2012 Musk revealed that it will instead be powered by methane and liquid oxygen (methalox).
The switch to methane produces a more efficient engine, basically. It's also cleaner, and friendlier to a reusable design. Methane needs smaller tanks than hydrogen, cutting down on size and weight on the entire ITS. It's also Mars we're talking about, and guess what element is found by the fucktonne on Mars? Carbon dioxide. This CO2 can be used in a process called the Sabatier reaction that uses hydrogen to produce water, oxygen, and methane! Humans love oxygen and water, and they'll certainly need more methane to get the ITS back to Earth.
The Raptor is also one of the first rocket engines in the U.S. to use a full-flow staged combustion design. It works like this: instead of a smaller amount of liquid fuel and oxidizer being used to light a 'pre-burner' that powers a turbopump, which in turn is needed to pump the liquid fuel and oxidizer into the main combustion chamber to get the rocket going like most other rocket engines, a full-flow design shoves all the fuel through the turbopumps. This in turn works them harder with increased pressure, and you get more thrust. It's also handy for keeping the turbopumps cooler, as more liquid is running through them. Plus, it eliminates some complication in adding parts and having to seal off the liquid fuel and oxidizer, because they can now exist in their own sections until they're fed straight into the turbopump.
John Bonsor, British rocket engineer and both secretary of the United Kingdom Rocketry Association and the Scottish Aeronautics and Rocketry Association, told Motherboard, "Space X is wringing the last possible efficiencies out of liquid propellant rocket engines, and in some style. I'm particularly impressed by the oxidizer and fuel densification idea. That's really clever, and potentially solves several problems in one go."
Bonsor did note, however, how the technology the Raptor is based on borrows heavily from designs the Russian space program first tried to pioneer. "In actual fact, SpaceX is innovating, but a lot of their technology is actually based on Russian rocket technology," Bonsor told Motherboard.
"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."
SpaceX's overall ITS scheme is already having questions thrown over it regarding funding, with Musk himself joking that his company may need to steal underpants or launch a Kickstarter fund to get to Mars.
Nevertheless, Raptor itself is clearly well on the way to full production, partly thanks to the US Air Force. Yup, SpaceX isn't the only organisation interested in this technology. From 2009 to 2015 Raptor development was funded solely by SpaceX, but in January 2016 SpaceX pocketed $33.6 million from the US Defense Department to develop a prototype version of an upper-stage variant of the Raptor designed to be used on the upper stage of a Falcon 9 and a Falcon Heavy.
The Pentagon is specifically after engines for what it calls "Evolved Expendable Launch Vehicles" for satellite launches. SpaceX did have to put $67 million if its own money into the contract, but the award from the government shows SpaceX has increasingly been winning the attention of the military for its technology. And if the ITS succeeds, it will no doubt win the attention of the entire world.
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