So why don't all rocket engines have giant nozzles? For the first time, the company's engineers ignited a vacuum version of the Raptor rocket engine that was attached to the upper stage of the spacecraft.
Starship's integrated Raptor vacuum engine launch for the first time pic.twitter.com/uCNAt8Kwzo
- SpaceX (SpaceX) October 22, 2021< p> SpaceX is already a Starship that has tested itself with Raptor engines. On some test flights, the car climbed up to 10 km with a maximum power of three "sea level" Raptor engines. But testing a rocket with a Raptor version optimized to operate in a vacuum is another matter entirely.
The expanding nozzle
Rocket engines have many sections, but it is the largest and most prominent nozzle that directs the flow of exhaust gases. This exhaust originates from the combustion chamber, where the oxidizer and fuel are burned. This exhaust gas is then compressed through a narrow opening - called the throat - to increase its speed. Now that the ultrasound is moving, the exhaust expands as it enters the nozzle, and the longer and wider the nozzle, the faster the exhaust is ejected. More thrust means your rocket can lift more mass. Therefore, the enlarged nozzle means better performance.
Finish this small animation showing the expansion ratios of the Merlin 1DSpaceX engines. It's crazy how big the vacuum crater is compared to the sea level crater! Are these numbers close to elonmusk? pic.twitter.com/N14MEXmeAh
- Daily Astronaut (@Erdayastronaut@) September 18, 2019
So why do all rocket engines have the giant crater? Due to a phenomenon called "flow separation", it occurs when the gas flow inside the engine separates from the nozzle walls. This can cause turbulence and vibration. In the worst case, this could cause the engine to explode. When this occurs, there is no absolute value, but when the outlet pressure from the nozzle drops to less than 50% of the ambient pressure, the risk of current separation increases.Advertising
This is not a problem in space, where the atmospheric pressure is essentially zero. But at sea level, the larger the crater, the greater the risk of disconnection.
The most common way to solve this problem is to design the first stage of the rocket with outboard-optimized engines and the upper stage with vacuum-optimized engines. For example, the Falcon 9 rocket has a first stage with 9 Merlin engines with smaller nozzles that do all the work in low atmosphere and a Merlin vacuum engine with a much larger nozzle for outdoor space. Alternative Methods The NASA space shuttle used a more hybrid approach. Its main engines, which went from the time of launch to orbit along the flight profile, sacrificed their performance on both sides. The shuttle ended up with a nozzle as high as possible at sea level - it really did increase the flow separation limit without crossing the edge - but it was much smaller than would be desired in the vacuum.
SpaceX upper stage is designed to fly in dense atmospheres. Its purpose is to solve the mystery of crater size by flying three "sea level" and three "vacuum" Raptor engines. Thursday's test marked the first time a vacuum engine had been attached to and tested on the Starship. Higher before the motors reach full power, external air pressure pushes into the motor's nozzles, causing unstable current disconnection. Watch the flexibility of the nozzle. pic.twitter.com/vYXjUl7nTK
- Scott Manley (DJSnM) Jan 17, 2019
America's most experienced engine, the RL-10 produced by Aerojet Rocketdyne, has an enormous expansion ratio, meaning The size of the orifice is much larger than the throat. Therefore, this motor can only be tested on the ground in a large vacuum chamber. Thursday's SpaceX test took place outside, in southern Texas, just a few feet above sea level. In response to this question, SpaceX co-founder Ilan Musk tweeted that the company had solved the problem by building a Raptor engine to create an extremely high pressure chamber. The motor is not yet fully optimized for vacuum, so there is enough margin to prevent instability in the current class.
This allowed SpaceX to blow up its experiment on Thursday with nothing.
How SpaceX lights up a new Raptor engine in Starship without an explosion
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