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Comment by dgritsko | Hacker News Reader

> the most advanced rocket engine ever made

Would love to learn more about this, can you expand? What makes Raptor so unique?

I am not a rocket scientist, but one of the main reasons it is so unique is that it will be the first full-flow staged combustion rocket engines to successfully be used on rockets.

Rocket engines have powerful turbo pumps to move large amounts of fuel quickly to the combustion chamber to create thrust. Usually these pumps are powered by burning some of the propellant in a turbine and exhausting the resulting gas on the side of the engine. A full-flow engine mixes this turbo pump exhaust gas back into the main combustion chamber. This makes the engine more fuel efficient but it is a difficult engineering problem. All previous types of these engines blew up too often to be used on spacecraft. Check out the Wikipedia page for more info [1].

[1]https://en.wikipedia.org/wiki/SpaceX_Raptor

To add on, one of the biggest challenges with a full flow staged combustion engine is that the exhaust gas from the oxidizer rich turbopump is almost pure gaseous oxygen at very very high temperature. This combination likes to corrode and eat away any thing, metal included.

To solve this, SpaceX had to develop special metal alloys that could withstand this incredibly corrosive environment for long durations.

Tim Dodd explains it very well: https://everydayastronaut.com/raptor-engine/

It's basically a sweet-spot between being very powerful, very efficient, cheap and easy to operate and highly reusable. To get all these, they implemented a design that never got past the basic demo stage before, which included independently working out the secret sauce of the best Russian-made engine.

>> which included independently working out the secret sauce of the best Russian-made engine.

Another hurdle was that there was no material available for some of the parts. SpaceX - so we'll get some materials science people to develop a new alloy that does what we need. Not an easy sub-task but one that needed a solution, so they found one.

There are two main metrics of rockets and rocket engines. Specific Impulse (ISP), and the Thrust to Weight Ratio (TWR).

ISP is essentially a measure of the fuel efficiency. And since there are no gas stations in space, the further you can go per kg of fuel, the further you can go, period.[1]

TWR is the measure of useful power.

TWR is the key measure for the first stage because a ton of power is required to escape Earth's gravity and atmosphere. For subsequent stages ISP is the main metric.

The Space Shuttle Main engines have an excellent ISP but because hydrogen needs massive tanks, it has a crappy TWR. Which is why the shuttle had a couple of strap on solids with a high TWR.

But instead of specialized engines, SpaceX designs engines that are good at everything. The ISP isn't as high as an advanced hydrogen engine and the TWR isn't as good as a solid, but both numbers are quite respectable and they bring other advantages such as the ability to relight and to be reused without refurbishment, they don't need use ablative cooling.

1: Although SpaceX is relying on in flight refueling for it's Moon and Mars plans. Nobody's done it before, but that doesn't stop SpaceX.

Full flow staged combustion cycle, never been flown* before, explained here:

https://everydayastronaut.com/raptor-engine/

* The Russians built one in the 60s but it never got past static fire tests

In addition to being full-flow, Raptor runs on methane (CH4, the simplest hydrocarbon), which already exists many places in the solar system (including Mars), and is easier to manufacture than kerosene or liquid hydrogen.

SpaceX developed an incredibly advanced simulator to make it happen: https://www.youtube.com/watch?v=vYA0f6R5KAI

Slight clarification: while there have been traces of methane detected on Mars, there's probably not enough to be able to easily use it as fuel. Methane is still beneficial for refueling on Mars, though, because you can make it in place out of water (which is plentiful on Mars as water ice) and CO2 (also plentiful, in the atmosphere) using the Sabatier process. So it remains the case that methane as fuel lets you avoid needing to bring enough fuel for a round trip.

Exactly. Elon has explicitly mentioned manufacturing metholox (the raptor engine fuel) using a really beefy ISRU factory. It would be feasible, but is still a lot of work required.

Mars could be the key to opening up the entire solar system. It's close enough to the sun for solar power to be practical, it's proven to have the mass amounts of subsurface ice and atmospheric CO2 needed for Sabatier-process methane/oxygen production, and it has a very shallow gravity well that would make it easy to lift huge propellant loads into orbit.

I don't think it's a stretch to imagine that Mars-based tankers supplying propellants and breathing oxygen would be commonplace through the solar system by the end of the century.

The raptor is a "full flow staged combustion engine" https://en.wikipedia.org/wiki/Staged_combustion_cycle#Full-f...

AFAIK it's the only flying, non-research example of such an engine.

The raptor uses a full flow staged combustion cycle. This makes a very efficient engine but increase the complexity. Everyday astronaut and Scott Manly on youtube do an excellent job explaining this in more detail

This might be the way to go - https://www.youtube.com/watch?v=Sdwy9fzQzl4

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