SpaceX Raptor: Methane fueled full-flow staged combustion cycle engine | Hacker News Reader

SpaceX Raptor: Methane fueled full-flow staged combustion cycle engine

everydayastronaut.com

242 pointsdpifke27 comments

While the article was focused on the Raptor engine, it shows how great an engine the RS-25 of the space shuttle was. It was crazy expensive, but as the shuttle was reused, one could justify the cost. The sad thing is, this engine will be used in the SLS - with no reuse what so ever. If anything describes the insanity of the SLS system, then this. They are flying the most expensive rocket engine ever in fully disposable mode. Even worse. After ending the shuttle program, the engines were of course preserved, so at least the first SLS flights are going to waste the origininal shuttle engines.

NASA just constantly boxes itself into corners like these, going with the “least worst” option.

Here’s an interesting article talking about how they came to the conclusion that the RS-25 was the only way to go;

https://www.nasaspaceflight.com/2016/01/nasa-defends-restart...

Does NASA have any choice? It seems like the political environment in which they operate makes it essentially impossible to pursue any kind of sane long-term strategy.

There is an absolutely solid book on the subject of how NASA comes to make decisions. The Space Shuttle Decision[1]. I say solid and not good because it's easily the most boring book I've read this year being composed entirely of bureaucracy.

1. https://history.nasa.gov/SP-4221/contents.htm

I've read that the RS-25 level of "reuse" was highly qualified, they required a significant rebuild between flights because they pushed the margins so hard. Wikipedia says "refurbished" https://en.wikipedia.org/wiki/Space_Shuttle_main_engine

But you're generally right about the insanity of the Senate Launch System.

> It [RS-25] was crazy expensive, but as the shuttle was reused, one could justify the cost.

The engines had to be taken apart, thoughtfully checked because some of the blades in the pump may get fisures or get broken, replace the bad parts and then ensembled again. It was more like a refurbishment o rebuild, than a easy cleanup.

From https://en.wikipedia.org/wiki/Criticism_of_the_Space_Shuttle...

> increased ongoing maintenance costs related to keeping the reusable SSMEs in flying condition after each launch, costs which in total may have exceeded that of building disposable main engines for each launch.

In the report of Feynman about the Challenger disaster https://history.nasa.gov/rogersrep/v2appf.htm he is clearly not happy with the RS-25 and he think that they could cause a big failure in the future. [They didn't fail in the other missions, but it may be some luck.]

Seems like with as many F1s as were used on each Apollo mission, he should count more than 17 flights. Also, I know at least one F1 was shut down before reaching orbit.

But, wow, what a great presentation! None of the other explanations I read of the full-flow design were anywhere near as clear and comprehensible. My understanding of rocket design principles has taken a great leap.

If we're using that logic then there are also 9 Merlin engines on each Falcon 9 and 3 RS-25's on each shuttle.

10 per mission, don't forget the 2nd stage. And 28 per FH mission.

I think it's fair to count the number of missions (it's at least consistent across engine types), though I'd probably prefer to also see the number of engine-missions as well.

But in defense of counting the number of missions, each mission has similar "external" characteristics: ambient temperature, sunlight level, burn duration, wind shear, humidity, etc. A combination of these environmental characteristics could cause a failure (Challenger failure was caused by excess cold). Since the environmental characteristics are the same for the mission, the number of distinct missions an engine has performed is a decent proxy for robust performance throughout the space of environmental conditions.

That would make more sense if, in fact, engines failed or didn't fail as a group, but in fact they tend to fail individually. This can make the difference between success and failure for the mission, when other engines of the same type fill in for the failed unit.

You might be mis-remembering a bit. The list of Saturn V launches at Wikipedia shows some anomalies, most notably Apollo 6, where two second stage engines cut out early -- but those were J-2 engines, not F-1s. It doesn't show any F-1 anomalies, though the F-1s never reached orbit in any event; as first-stage engines, they were shut down and jettisoned long before that even on a completely normal launch.

https://en.wikipedia.org/wiki/Saturn_V#Saturn_V_vehicles_and...

Ah, yes. I was remembering a center J-2 on the second stage of Apollo 13. So, my mistake.

> The RD-170 actually produced more thrust than the F-1, but since it barely ever flew, I figured it wasn’t as relevant in this line up, figured it’s better to go with engines that have actually been used, a lot!

Wikipedia lists some 84 launches - including a dozen of failures - of Zenit, which uses RD-171 on the first stage - the near-twin of RD-170. That's a lot of launches.

That was pointed out to him here: https://www.reddit.com/r/spacex/comments/bsy6rp/a_deep_rundo...

> Enthalpy is basically the relationship between volume, pressure and temperature

I appreciate the non-technical approach to the subject, but this is false. Enthalpy is an energy, it accounts for both internal energy (heat) and work.

Enthalpy is energy required to create a system. Internal energy required to make up the system (to bring it's internals to given Temperature, pressure ect), but there is additional work needed to position the system in the universe (that is, to free some space for it, like moving (working on) water or air to place the system of volume V in it).

One of the best technical and easily digestible articles I've read in years

If I ever get to ask Elon an ama question it’d be this; in the beginning the Merlin was ablative chamber and nozzle, I thought SpaceX was going the sea dragon/big dumb booster route. Contrast with today inparticularly the raptor, they’re pushing the state of the art. Why the change?

My take on an answer: they knew that they'd have to create a working rocket in just a handful of attempts (turned out that they'd hardly had money for the fourth - the first successful - attempt). An ablatively cooled nozzle was the easiest way to solve the nozzle-cooling problem and they deferred work on a more sustainable method to later.

Other pragmatic solutions were for example that the booster and upper stage have the same diameter so that the same tooling could be used. Or that the diameter of the stages is 12 feet because that's the maximum diameter that could be easily and cheaply transported by road to the launch site.

Very counter intuitive that all of the fuel and oxidizer goes through the preburner yet so elegant.

How does the Raptor solve the coking problem in the fuel rich pre burn chamber?

By using methane instead of kerosene.

Methane won't cause soot/coking.

My analysis is probably wrong, but anyway, Methane is CH4. There is one carbon atom to 4 hydrogen atoms. RP1, or Kerosene, is a mix of different chemicals, but the ratio is roughly one carbon atom for every two hydrogen atoms. So, with Methane, there is less carbon to be converted to CO2 or left as soot for the same amount of hydrogen.

Great post! Read it beginning to end.

Is this Why there is a methane surge? https://news.ycombinator.com/item?id=20012612

No. The quantities of methane touched by SpaceX are low by something like 10 orders of magnitude, and they burn their methane, rather than releasing it to the air.

Reply on news.ycombinator.com