A SpaceX Falcon 9 rocket lifts off. Image: NASA/Alan Ault
The future of spaceflight will be powered by ion engines and warp drives, right? Not just yet. There is still some uncharted territory in the world of liquid-fueled rockets, which have been powering spacecraft since the 50s, and SpaceX is testing the waters with its new Raptor engine. It could be the engine that, if Elon Musk gets his wish, propels the first colonists to Mars.
Right now the Raptor looks like it will be developed to deliver 1 million pounds of thrust at launch, which is certainly a step up from SpaceX’s existing engines—SpaceX's Merlin 1D delivers about 147,000 pounds at launch. The Raptor will even beat out the Space Shuttle’s main engine, which delivered 375,000 pounds of thrust at launch. In other words, the Raptor is expected to be a huge engine, dwarfed only by the F-1 engine powering the gigantic Saturn V rocket.
Can SpaceX build a rocket engine to rival the giants? The firm has had good luck developing rocket engines in the past. Its Merlin 1D engine, which uses the traditional mixture of kerosene and liquid oxygen, has the highest thrust-to-weight ratio of any engine currently in use. But it still uses the same basic technology of all liquid rocket engines that came before it.
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Liquid-fueled rockets work by mixing a fuel and an oxidizer in a combustion chamber before the mixture is ignited and blasted out of a nozzle. For a powerful reaction to occur, large quantities of the liquid fuel and the oxidizer must be fed into a rocket's combustion chamber quickly and under high pressure. This flow of liquids is driven by a turbopump, a system that is powered by a small amount of fuel “pre-burning.” A similar process happens to get the oxidizer into the combustion chamber.
SpaceX's Merlin 1D rocket engine being tested. Image: SpaceX
Liquid-fueled engines typically send a small amount of fuel and oxidizer through the preburners; the bulk of the liquids are sent directly into the combustion chamber. But what if this weren’t the case?
The US Air Force and NASA have both considered what's called a “full-flow cycle” design, which sends all fuel through turbopumps. It has one important advantage: more fuel and oxidizer passing through the preburners will drive the turbo pumps harder, increasing the pressure inside the combustion chamber and in turn the rocket engine’s performance.
A full-flow design has never been used in the United States, but it’s the kind of design SpaceX is currently pursuing for its Raptor engine.
SpaceX first revealed its new engine design at the AIAA Joint Propulsion conference in July of 2010. They were introduced as the powerhouses for the company’s future Falcon X and Falcon XX rockets. The first new engine was the Merlin 2, an engine of similar design but more efficient than the Merlin 1-D. The second engine presented was the Raptor, a staged combustion engine using liquid oxygen and hydrogen to power heavy rockets.
SpaceX says its future Falcon Heavy rocket will be the most powerful rocket in existence. Its first stage will be powered by 27 Merlin rocket engines, which are about an eighth the size of the Raptor engine planned. Image: SpaceX
The initial choice of liquid oxygen and hydrogen for Raptor was an interesting one. Cryogenic liquid hydrogen is hard to handle, must remain insulated, and has a lower density than kerosene. That means a liquid hydrogen tank for, say, the Falcon rocket would have to be larger than the current kerosene tank. Switching to liquid hydrogen would also mean a cost increase for a company that has long focused on lowering the price of getting each pound of cargo to orbit.
Elon Musk continued to hint a change in SpaceX’s rocket engines in the years after that 2010 conference. Notably, he’s mentioned a new liquid methane and oxygen rocket that would have enough power to launch his envisioned Mars Colonial Transporter, a fully reusable system that could theoretically transport 100 colonists at a time to the red planet.
Swapping kerosene for methane yields about the same performance. Kerosene scores points for being slightly lighter, but methane scores points for being the more efficient fuel. It’s also possible to source methane on Mars, and its a cleaner fuel that leaves little to no residue in a rocket engine’s plumbing.
In late 2013, SpaceX revealed that its Raptor was the powerful stage combustion engine fueled by liquid methane and liquid oxygen. In a configuration similar to the Falcon heavy, with nine of these full flow engines powering three rocket stages, SpaceX could have an incredibly powerful way to get to Mars on its hands.
Raptor isn’t quite ready to start sending massive payloads off the Earth, but it's getting there. SpaceX will be testing the Raptor engine in cooperation with NASA at the agency’s Stennis Space Center in Mississippi. As for Mars, Musk says SpaceX hopes its Raptor rocket will be ready for interplanetary flight within a decade.