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sweetplantveal t1_j7s2u0r wrote

The Raptor engines are unusual in that they can throttle down to 40% but as far as I know they still rely on turning some on and some off during different flight stages. If not takeoff and max q, for sure during landing. That coordination has to be super complex...

For context, the engine has been under development for a decade and testing for five years. It's a smaller engine than usual on heavy lift vehicles. It's height is 10 feet. The F1 engine from the Saturn V is almost twice as tall, could fit a Raptor sideways in its nozzle, and develops over 3x the thrust. But where the Saturn V has 5 F1s on the bottom, BFR has 33 Raptors. About twice the thrust in total.

This test is a big deal for Space X. A TON of their future plans hinge on the Raptor.

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upyoars OP t1_j7s62kc wrote

It’s also a full-flow staged combustion engine, the most complex but most efficient type of combustion engine possible. Meant for maximizing and optimizing efficiency to a ridiculous degree, squeezing every drop of juice possible. If it’s successful it would also be the first full flow stage combustion engine to ever be successfully operated and used in history.

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lapseofreason t1_j7trp7c wrote

Can you clarify what the efficiencies mean in practical terms ? Does it mean less fuel burn per unit thrust and less cost etc ? Why did they choose that technology ?

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CeeJayDK t1_j7tum7k wrote

See https://www.youtube.com/watch?v=LbH1ZDImaI8 or https://www.youtube.com/watch?v=Owji-ukVt9M

But if I understand it right, the temperatures when you drive a propellant turbine can get so hot they melt or otherwise break the turbine, so to get around this they normally burn the propellant in the preburner in a less optimal way with either too much oxygen or too much fuel.

The full flow engine however does both at the same time using two preburners and two turbines and the combined fuel and oxygen mix that reaches the main combustion chamber is optimal for the best combustion and it's super hot and in a gaseous form which helps it burn even more efficiently.

The downside is that it's probably twice the complexity and it's startup cycle is also very complex and must be managed precisely.

But it gets the most amount of thrust out of the available fuel of any current engine design today with the least amount of heat stress on the turbines - which you could use for added safety .. OR use to push the turbines even more to crazy pressures and more power. SpaceX does the latter.

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lapseofreason t1_j7u7una wrote

Thank you so much for taking the time to give such a good explanation.

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Bensemus t1_j7wdlho wrote

> If not takeoff and max q, for sure during landing.

They aren't turning any engines off during launch. Max Q doesn't' require a greater than 40% reduce in thrush.

Engines are turned off during separation. Then some are turned back on for the boost back burn and maybe a reentry burn. Then finally some are used for the landing burn.

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