Wed. Dec 1st, 2021

The starship, SpaceX’s underdeveloped rocket, is turning up the power.

In a video shared via Twitter last week, the space company hosted a static test fire of a prototype of the Starship rocket. The ship, designed to send the first humans to Mars and establish a city, is currently being tested at SpaceX’s facilities in Texas. This is the first time that Starship has fired a vacuum variant of its Raptor engine, specifically optimized to propel the rocket through the depths of space.

The test fire marks another step forward in CEO Elon Musk’s overall mission to establish a city of 1 million people on Mars by 2050. To achieve this goal, Musk has calculated that SpaceX will have to send about one million tons of cargo to the planet.

The starship, when paired with the Super Heavy Booster, is designed to transport about 100 tons to Mars at a time – meaning SpaceX will host about 10,000 flights to Mars.

The vacuum variant of the Raptor engine is the key to achieving this. The Super Heavy booster, which lifts the starship away from Earth, is expected to have 33 Raptor engines optimized for use in the atmosphere at sea level. The starship, which detaches from the Super Heavy to complete the voyage, will have three sea level and three vacuum Raptor engines.

In the dim light of the Texas plant, SpaceX demonstrated this powerful rocket with a burning screen.

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SpaceX’s Vacuum Raptor: What You Need to Know

The vacuum variant of the Raptor engine is optimized to better propel the rocket as it moves through space.

An earlier size comparison from SpaceX shows that it’s not just a minor tweak: the vacuum variant is gigantic.

The first SpaceX Raptor vacuum engine, captured before being shipped from SpaceX’s California plant to the Texas rocket development site.SpaceX / Twitter

So why is it so much bigger? Due to the difference in pressure out in the room.

NASA explains that in the simplest terms, a rocket engine sends hot gases out through a nozzle to produce pressure. The design is based on Newton’s third law: for every action there is an equal and opposite reaction. Gas pushes out through the nozzle and the engine is in turn pushed forward.

The Mars Society of Canada, which advocates exploring the planet with rockets like Starship, explains that making the nozzle larger forces the gas out into a wider area and increases propulsion. The problem is when the gas expands so much that the pressure is lower than the surrounding atmosphere, so the atmosphere squeezes the gas and pushes it away from the nozzle walls.

Different nozzle sizes. While the top one is underexpanded, the bottom two are overexpanded. The middle one is the optimal size for the ambient pressure.Wikimedia Commons

An oversized nozzle leads to what is known as flow separation, which can cause damage to the engine.

Atmospheric pressure is of course not a problem in the room’s atmospheric vacuum. This suggests that SpaceX could continue to make the nozzle bigger and bigger, but there are practical limits to this.

So how was SpaceX able to launch this vacuum engine on the sea surface without encountering any problems? Musk claimed on Twitter, in response to a question from YouTuber Everyday astronaut, that the engine has a “very high chamber pressure”, which allows it to be fired at sea level without suffering from flow separation.

How much more powerful is this variant? A measure is specific impulse, or how much thrust comes from a particular fuel unit. Musk claimed in September 2019 that where the Raptor variant at sea level could reach a specific impulse of 350 seconds, the vacuum variant could reach 380 seconds.

When trying to establish people as a multiplanet species, each lift helps.

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