The Engineering behind SpaceX's New Raptor Engine will blow your mind (Rvac)!

Work never stops at SpaceX's Starbase facility where engineers are working to develop the Starship launch system that will enable humanity to land the first crew on Mars within this decade. In the meantime, the company carried on its merry way testing out a variant of its super-powered Raptor engine named the Raptor Vacuum Engine. Interestingly enough, this somehow became an almost inconceivable feat of engineering, breaking the single biggest hurdle standing between Starship and orbit. Now, without any delays, let's get started in today's episode of Great SpaceX to learn the engineering behind SpaceX's new Raptor Vacuum Engine, and I swear it will blow your mind. Boom! SpaceX puts its new Starship engine through its paces in a video shared in September of 2020, which shows the Raptor engine completing a full duration test fire at the firm's rocket development facilities. The 15-second clip shared via Twitter shows a new vacuum variant of the Raptor engine during a test at the McGregor, Texas facility. This variant of the Raptor engine will help the Starship travel through the vacuum of space as opposed to the smaller, sea-level variants that have been demonstrated so far. The new variant is a beast. A previous photo shared by SpaceX showed the new version towering over the previous one, with a relatively tiny human for scale. It's an exciting step forward in Starship, which is possibly SpaceX's most ambitious project. Known as Raptor Vacuum or R-Vac, the engine is almost entirely based off of its sea-level optimized cousin, taking all of the turbo-machinery and combustion chambers that represent the bulk of a rocket engine. Things start to diverge below the throat of the combustion chamber, which is the narrow part of the central hourglass-like curve, where SpaceX has expanded Raptor's existing bell nozzle by a factor of five or more. SpaceX's reusable Starship spacecraft will use a mix of three sea-level Raptors and three Raptor Vacuum engines to give it the thrust it needs to reach orbit and ensure efficient operations both in atmosphere and vacuum. In simple terms, a rocket engine can benefit from a vacuum-optimized nozzle because the added surface area, more or less, gives the extremely high-pressure gases, exiting its combustion chamber, even more footholds to push against. Rocket nozzles are at their most efficient when the engine's exhaust gas finishes expanding to match ambient pressure at the exact moment it exits the bell. Logically, at sea-level on Earth, the ambient pressure is quite high, meaning that the rocket exhaust doesn't have to expand as much to equalize. In the vacuum of space, however, exhaust gases must expand far more to reach the same pressure as its surroundings. For rocket propulsion, that extra expansion can be exploited to make a much more efficient engine. Squeezing extra energy out of the same propellant and in a perfect vacuum, the most efficient nozzle would technically be infinite. But how powerful is SpaceX's new engine? One measurement would be a specific impulse. This measures how much movement you get from each unit of fuel used. NASA's Glenn Research Center explains how teams arrive at that figure, but the important thing to note is that it can be measured in seconds. A 2013 version of SpaceX's website explained that the Merlin engine used for its current Falcon 9 launches had a specific impulse of 282 seconds for the sea-level and a specific impulse of 311 seconds for the vacuum. In September of 2019, Musk wrote on Twitter that the sea-level Raptor engine is around 350 seconds and it can reach 380 seconds for the SpaceX took another step toward validating the rocket engine technology that will power its Starship rocket on October 21st of 2021. For the first time, engineers at the company ignited a vacuum version of a Raptor rocket engine that had been attached to the Starship upper stage. The test firing at sunset in South Texas lasted only a few seconds, but it appears to have been successful. Rocket engines have many parts, of course, but the largest and most prominent is the nozzle, which channels the flow of exhaust gas. This exhaust originates in the combustion chamber where oxidizer and propellant combust. This exhaust gas is then pushed through a narrow opening, called a throat, to accelerate it. Now traveling supersonic, the exhaust expands as it enters the nozzle, where the longer and wider the nozzle is, the faster the exhaust moves. Faster gas coming out of a rocket engine is good because it delivers more thrust. More thrust means your rocket can lift more mass. An expanded nozzle, therefore, means better performance. So why don't all rocket engines have giant nozzles? Because of a phenomenon known as flow separation, which happens when the flow of gas inside an engine separates from the nozzle walls. This can induce turbulence and vibrations. In a worst-case scenario, it could result in the engine blowing itself up. There is no absolute value for when this occurs, but the risk of flow separation increases when the pressure of exhaust exiting the nozzle falls below 50% of the ambient pressure. This isn't a problem in space, where the atmospheric pressure is essentially zero, but at sea level, the larger the nozzle, the greater the risk of flow separation. The most common way to address this issue is to design a rocket's first stage with engines optimized for performance at sea level and an upper stage with vacuum-optimized engines. The Falcon 9 rocket, for example, has a first stage with nine Merlin engines with smaller nozzles that do all the work in the lower atmosphere, and a Merlin vacuum engine with a much larger nozzle for outer space. NASA's Space Shuttle took a more hybrid approach. Its main engines, which flight profile from launch into orbit, sacrificed performance on both ends. The Shuttle ended up with a nozzle as large as possible at sea level, which it really pushed the limits on flow separation without going over the edge, but considerably smaller than would be optimal in a vacuum. SpaceX's Starship upper stage is designed to fly in both thick atmospheres and space. It aims to solve the nozzle size conundrum by flying with three sea-level Raptor engines and three vacuum Raptor engines. Thursday's test marked the first time one of the vacuum engines was attached to a Starship vehicle and test-fired. The most experienced U.S. upper stage engine, the RL-10 manufactured by Aerojet Rocketdyne, has a massive expansion ratio, in that its nozzle size is much larger than its throat. So, this engine can only be tested on the ground in a large vacuum chamber. SpaceX's test on Thursday took place outside, in South Texas, a few feet above sea level. So, how did SpaceX complete the test-firing of the vacuum-optimized engine without actually destroying it? In response to this question, SpaceX founder Elon Musk said on Twitter that the company solved the problem by building the Raptor engine to produce a very high chamber pressure. The engine is also not yet fully optimized for a vacuum, so there was enough margin to prevent flow separation from destabilizing it. This allowed SpaceX to complete its October test without anything blowing up. In the future, Starship will be upgraded to nine engines, with three sea-level gimbling and six vacuum fixed, with the increased propellant load. That means it delivers more thrust. This, of course, is not easy to get. Regardless, SpaceX's RVAC, with its current achievements, completely deserves its title as a masterpiece of engineering. And that's it for today's episode. 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