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The Raptor getting 380 seconds specific impulse is a pretty big deal. However, I'd say it's an even bigger deal that it's running at 300 bar (30 MPa real units) chamber pressure.

 

image054.jpg

 

Looking at the P-V diagram for an idea rocket engine, there's pressurization  (1-->2), constant pressure combustion (2-->3), and isentropic expansion to ambient pressure (3-->4), which takes place in the nozzle.

 

 

The two ways to crank up the area inside the curve (work of the cycle) are to drop your ambient pressure or increase your operating pressure (in reality, the nozzle expansion won't be completely isentropic, but its a close enough approximation). Ambient pressure is a function of, well, the ambient environment. The lower the pressure, the greater the work (and efficiency), this is why engines are more efficient in a vacuum (an underexpanded nozzle drives up your ambient pressure). Conversely, the chamber pressure can be controlled by the design.

 

The Raptor engine is designed to run at 30 MPa. The RD-180 (Russian witchcraft engine) runs at about 25. The F-1 engine on the Saturn V was at 7, and therefore was much less efficient.

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BFR is now Starship (well, Super Heavy + Starship).

 

There is some concern over the number of changes BFR seems to be going through.

 

Regardless of whatever changes happen to the design, I am going to assume they will be keeping the 9m diameter, since they have already started cutting metal. Would be stupid to switch to completely new tooling at this point.

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Popular Mechanics interview with Musk on the BFR

 

https://www.popularmechanics.com/space/rockets/a25953663/elon-musk-spacex-bfr-stainless-steel/

 



On the windward side, what I want to do is have the first-ever regenerative heat shield. A double-walled stainless shell—like a stainless-steel sandwich, essentially, with two layers. You just need, essentially, two layers that are joined with stringers. You flow either fuel or water in between the sandwich layer, and then you have micro-perforations on the outside—very tiny perforations—and you essentially bleed water, or you could bleed fuel, through the micro-perforations on the outside. You wouldn’t see them unless you got up close. But you use transpiration cooling to cool the windward side of the rocket. So the whole thing will still look fully chrome, like this cocktail shaker in front of us. But one side will be double-walled and that serves a double purpose, which is to stiffen the structure of the vehicle so it does not suffer from the fate of the Atlas. You have a heat shield that serves double duty as structure.

Yeah.

To the best of my knowledge this has never been proposed before.

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