Find out the Physics behind the Raptor Interplanetary Transport Engine of Elon Musk
Why did SpaceX select methane as the fuel source for its Raptor rocket engine?
Wednesday, December 8, 2021 | Chimniii Desk
Key Highlights
- To transport humans to Mars, SpaceX is developing the powerful Starship spacecraft, which will be powered by the Raptor engine – one of the first methane-fueled engines designed to be reused 1,000 times.
- These enormous machine, when combined, are capable of transporting up to 100 people into space, maybe to the Moon or Mars, with a tentative launch of humans around the Moon scheduled for 2023.
- Normally, some fuel is spent to initiate this process in a traditional open cycle engine, but Raptor will use every drop available, making it one of the most efficient rocket engines ever developed.
- As a result, Raptor operates at a far higher pressure than Merlin, almost three times higher, making it the highest pressure rocket engine in existence and contributing to its aforementioned stronger thrust than Merlin despite its similar size.
- Of course, Raptor's other significant advancement over its predecessor is its usage of methane, a nod to SpaceX's ultimate goal.
Advertisement
Raptor Interplanetary Transport Engine
Why did SpaceX select methane as the fuel source for its Raptor rocket engine? Robert Goddard's initial rockets were powered by liquid oxygen (O2) or liquid oxygen (LOX) plus gasoline. The first stage of the Saturn V moon rocket used LOX and refined kerosene. The second stage of the Saturn V employed LOX and hydrogen, which burn to form water in my favourite chemical reaction, 2H2 + O2 2H2O. Methane CH4, gasoline (a mixture of C7H16 and C11H24), and kerosene (a mixture of C12H26 and C15H32) are all linear hydrocarbons CnH2n+2 that burn to produce carbon dioxide and water (and residual carbon if the burning is incomplete).
Hydrolox is the most efficient method but requires enormous tanks (because to hydrogen's low density) that must be cooled to a few degrees above absolute zero. Kerolox is less efficient, but requires fewer tanks (because to the high density of kerosene), which can be kept at normal temperature and pressure. Methalox is a middle ground. However, unlike kerolox, methalox does not coke the engines, making reusing them more difficult. Additionally, SpaceX will densify the oxygen and methane to increase efficiency by cooling them to just above their freezing temperatures (rather than just below their boiling points).
The true advantage of methalox is that it can be synthesised on Mars from carbon dioxide and water using the CO2 + 2H2O CH4 + 2O2 reaction.
To transport humans to Mars, SpaceX is developing the powerful Starship spacecraft, which will be powered by the Raptor engine – one of the first methane-fueled engines designed to be reused 1,000 times.
Elon Musk, CEO of SpaceX, is hell-bent on colonising Mars. To do this, a massive new spacecraft will be required, fueled by a new type of rocket engine never seen before. Raptor is the future of SpaceX's endeavours.
Raptor is being developed to power SpaceX's next reusable vehicles, the Starship spaceship and the Super Heavy rocket. These enormous machine, when combined, are capable of transporting up to 100 people into space, maybe to the Moon or Mars, with a tentative launch of humans around the Moon scheduled for 2023. SpaceX tested the first flight-ready version of Raptor in February 2019 and flew one for the first time last week during a "hop" test of a Starship prototype named Starhopper.
For the better part of a decade, the engine has been under development, going through numerous revisions. At its heart, it is similar to other engines in that it generates thrust by burning chemical fuel. However, because to its usage of liquid oxygen and methane – which is largely unique in the rocket industry – and its novel design, it just might be SpaceX's ace in the hole for exploring the Solar System.
"No other rocket engine is capable of generating as much energy from liquid methane and liquid oxygen as the Raptor engine," says Tim Dodd, an industry expert and host of the popular Everyday Astronaut YouTube channel. "And the objective is to make it more reusable with little to no refurbishing. That will undoubtedly improve their business case if they can simply fly over and again."
Advertisement
Reusability is critical, as Musk has stated that each engine must be capable of flying up to 1,000 times to sustain Starship's ambitious operations. That is a significant issue; the most often reused engines in the history of space flight were the main engines on each Space Shuttle, which flew only a few hundred times. "It's fairly audacious," Dodd admits. "I'm not sure 1,000 flights is a realistic goal for the foreseeable future. If it lives up to its potential, perhaps 1,000 is a possibility someday."
SpaceX currently uses the Merlin engine on its operating Falcon 9 and Falcon Heavy rockets, but Raptor represents a substantial advancement. One is that it has double the thrust of its predecessor due to a substantially higher pressure, 380,000 pounds at sea level against 190,000 pounds, although having the same size.
The other is methane use. No methane-fueled rocket has ever reached orbit, with Starhopper's test hop earlier marking the first time a methane-fueled rocket engine took flight. In comparison to other fuels such as kerosene, methane prevents the formation of deposits in the engine, a process known as coking, while its superior performance results in reduced costs.
"The cost of propellant for liquid rockets is such a negligible percentage of the total launch expenses," space consultant Rand Simberg explains. "With reusable vehicles, we aim to reach a point where we don't care about the cost of propellant. Fuel accounts for around 35% of total operating costs in airlines. Historically, it is less than 1% with a rocket."
Raptor also employs what is known as a full-flow staged combustion engine, making it just the third in history, although Merlin employs the more common open cycle method. The two previous attempts, one in the Soviet Union in the 1960s and another in the United States in the early 2000s, never progressed past testing.
A full-flow stage combustion engine describes how a pump spins a turbine to power the engine, with a tiny amount of fuel being injected to initiate the process. Normally, some fuel is spent to initiate this process in a traditional open cycle engine, but Raptor will use every drop available, making it one of the most efficient rocket engines ever developed.
"Raptor burns the fuel at a high enough pressure to direct the fire from the preburner back into the combustion chamber, where it is totally consumed along with the other propellants," explains Massachusetts Institute of Technology space analyst Charlie Garcia (MIT). "And it accomplishes this in a way that only the Russians have previously accomplished... by routing all propellant in the engine through the preburners."
Advertisement
As a result, Raptor operates at a far higher pressure than Merlin, almost three times higher, making it the highest pressure rocket engine in existence and contributing to its aforementioned stronger thrust than Merlin despite its similar size. Musk referred to the "insane pressure" of 300 bars within the engine's main chamber in 2016, which necessitated the development of a new metal alloy.
Of course, Raptor's other significant advancement over its predecessor is its usage of methane, a nod to SpaceX's ultimate goal. SpaceX is not the only corporation pursuing methane as a fuel source; Blue Origin, directed by Amazon CEO Jeff Bezos, is also constructing its own methane engine, dubbed the Blue Engine 4. (BE-4). With various advantages, this is a switch that almost seems overdue.
Historically, the majority of rocket engines have depended on fuels such as kerosene in place of methane. However, the primary advantage of using methane is that it has a higher performance than other fuels, which allows for a smaller rocket. Additionally, its cheaper cost means that the entire cost of launching can be reduced.
This could be critical, as the amount of Raptor engines SpaceX intends to construct is enormous. Each Starship vehicle is equipped with six Raptor engines, as well as 35 Super Heavy engines - a total of 41 engines per launch. The Falcon Heavy, SpaceX's largest rocket to date, is powered by 28 Merlin engines (27 on the bottom and one on the upper stage). With the number of planned launches for Starship and Super Heavy, SpaceX's rate of production will need to dramatically rise.
Look at this Critical Methane Feed System for Super Heavy Spotted | SpaceX Boca Chica
Advertisement