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SpaceX’s first orbital starship engine was breathing fire

Less than three weeks after shipping to Texas, SpaceX said the first Starship Raptor Vacuum engine passed a “full-duration test fire” en route to test flights in orbit.

Known as the Raptor Vacuum or RVac, the engine is based almost entirely on its sea-level-optimized cousin and takes over all of the complex turbo machines and combustion chambers that make up the majority of a rocket engine. Below the neck of the combustion chamber (the narrow part of the central hourglass-like curve), where SpaceX expanded Raptor’s existing bell nozzle by a factor of five or more, things start to diverge.

SpaceX’s reusable spaceship spaceship will use a mix of three sea level raptors and three raptor vacuum motors to give it the thrust it needs to reach orbit and ensure efficient operation in both atmosphere and vacuum .

Raptor Vacuum (roughly) true to scale next to the Raptor Sea Level, a Space Shuttle Main Engine (SSME) and a Saturn V F-1. (Teslarati)

In simple terms, a rocket engine can benefit from a vacuum-optimized nozzle, as the additional surface (more or less) gives the extremely high pressure gases emerging from its combustion chamber even more support against which they can press. Rocket nozzles are most efficient when the exhaust gas from the engine expands to ambient pressure just as it exits the bell. At sea level on Earth, the ambient air pressure is logically quite high, which means that the rocket exhaust doesn’t have to expand as much to equalize.

In the vacuum of space, however, the exhaust gases have to expand much more to reach the same pressure as their surroundings. For rocket engines, this extra expansion can be used to make a more efficient motor that squeezes extra energy out of the same propellant, and in a perfect vacuum the most efficient nozzle would technically be infinite. Unfortunately, technical and physical infinities do not exactly understand each other, so that vacuum rocket engineers are forced to commit themselves to a nozzle size on a scale that humans can realistically manufacture.

In theory, Starship doesn’t need Raptor Vacuum engines are said to be a working orbital spacecraft, and CEO Elon Musk himself launched a seven-engine design at sea level two years ago. Since then, SpaceX’s CEO announced that Raptor has made such good progress that the company has reversed the removal of vacuum-optimized motors from Starship’s base design.

Look, ma, no vacuum motors! (SpaceX)

It is unclear what exactly SpaceX means when it says that Raptor Vacuum SN1 has completed a “test fire of full duration”. For Starship, a burn would likely be no less than five or six minutes during the entire orbital insertion – beginning immediately after the super heavy booster disconnects. Even for SpaceX, going from the first engine (Raptor Vacuum), which was manufactured in less than three weeks, to a successful several-minute static fire would be an almost unimaginable feat of engineering. The feat would mean that SpaceX is already very comfortable with raptor burns lasting several minutes – perhaps the biggest hurdle between spaceship and orbit.

More likely, “full-duration test fire” simply refers to the fact that the Pathfinder Raptor Vacuum engine managed to ignite, burn, and shutdown on schedule – in other words, to avoid an early shutdown. For an engine as large and complex as Raptor, even that downgraded interpretation would be an impressive feat.

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