Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The Tech Behind SpaceX’s New Engine

hackaday.com

Post reply on HN
Print view View original

31–40 of 137 posts

Re: The Tech Behind SpaceX’s New Engine

#31

Post by Symmetry »

kumarvvr wrote: post #6

This is an excellent article. Just curious, how much more efficient is the full-flow engine? And, what exactly is the deal with the seals the article is talking about? Anyone have more info?

A staged combustion engine is intrinsically a lot more efficient than an engine where you dump some of your fuel to power the pumps, like in a tapoff or gas-generator engine.

Compared to a oxygen-rich full flow you can reach a higher pressure while having better safety margins which will allow you to re-use the engine. The higher your chamber pressure the closer the rocket's thrust at sea level is to its maximum thrust in vacuum. Also higher pressure tends to correlate with higher thrust to weight ratios.

Also, having both inputs already be gasses mean they mix better. This will have a tiny benefit in them burning more completely. But more importantly they'll burn better across a wider range of thrusts, which could be important for throttling down the engine for landing.

Re: The Tech Behind SpaceX’s New Engine

#32

Post by jandrese »

mynameisvlad wrote: post #22
kiba wrote: post #15

Earlier quoted context omitted.

There wasn't quotes around 'impossible' in the article.

Literally in the headline. And they used other words next to the unquoted impossible in the article itself like " deemed by scientists to be next to impossible". Context matters.

The US scientists didn't think there was a metallurgy that could survive the oxygen rich environment, but the Russians did the work to find it.

The part I'm confused about is how it talks about all of these 60s rockets that close the cycle but aren't as efficient because they mess up the mixture in the combustion chamber, but shouldn't the injectors be tuned for the excess fuel/oxidizer from the get go? It doesn't seem like you should be exhausting unburnt fuel/oxidizer if you have it tuned correctly.

Re: The Tech Behind SpaceX’s New Engine

#34

Post by twic »

wolf550e wrote: post #29
tehsauce wrote: post #23

Rocket lab's Rutherford engine uses a closed cycle with battery powered fuel pumps. They were actually to fly an engine like this in their "electron" rocket, as early as 2017. https://en.m.wikipedia.org/wiki/Rutherford_(rocket_engine)

This is only viable for small engines, like expander cycle is only viable for small engines. The specific energy of electric batteries cannot compare to that of kerolox or methalox. Closed cycle is not a new idea: https://en.wikipedia.org/wiki/Staged_combustion_cycle#Histor...

I still have a soft spot for expander cycles - they seem so elegant compared to staged combustion cycles, and apparently i am not the only person who thinks that:

https://blogs.nasa.gov/J2X/2014/03/24/inside-the-leo-doghous...

Re: The Tech Behind SpaceX’s New Engine

#35

Post by Eridrus »

buboard wrote: post #21
kumarvvr wrote: post #6

This is an excellent article. Just curious, how much more efficient is the full-flow engine? And, what exactly is the deal with the seals the article is talking about? Anyone have more info?

Wikipedia has some stats comparing their current engines (probably speculative for the raptor) Raptor: Thrust 1900 KN / 330 specific impulse Merlin 1D: 480KN / 275 s.i. i can't find the weight of the Raptor. Merlin is 630Kg

So, for the folks like me who don't know anything about rockets, it seems like "specific impulse" is the measure of how much "impulse" is generated per unit of fuel.

So 330/275 = 20% better.

Re: The Tech Behind SpaceX’s New Engine

#36

Post by Tuna-Fish »

JumpCrisscross wrote: post #20

“Benefits of the full-flow staged combustion cycle include turbines that run cooler and at lower pressure, due to increased mass flow, leading to a longer engine life and higher reliability.” TL; DR Full flow lowers turbine temperatures at the expense of parts complexity. Given turbopumps are the devil’s ass part of rocketry, this has been a sought-after technology. The pay-off isn’t so much efficiency as much as lon…

> TL; DR Full flow lowers turbine temperatures at the expense of parts complexity.

Ehhh. I'd argue it also lowers part complexity, because it eliminates interpropellant seals.

> The pay-off isn’t so much efficiency

It improves achievable chamber pressure (assuming similar maximum pressures and temperatures at the turbines), which improves both thrust and Isp.

> (This also explains why full flow hasn’t been a priority for anyone until SpaceX.)

The reason it hasn't been a priority is more that no new development has been a priority since the 70's.

Re: The Tech Behind SpaceX’s New Engine

#37

Post by mabbo »

> While the Space Shuttle has long since retired, a variation of the engine itself will go on to power the Space Launch System. It will be the most powerful rocket NASA has ever built and is slated to begin missions in 2020.

2020? I honestly doubt we'll see the SLS launch before 2024 at it's current rate.

Re: The Tech Behind SpaceX’s New Engine

#38

Post by hoorayimhelping »

twic wrote: post #8

Either approach, whether it recaptures the oxidizer or fuel rich preburner exhaust, is clearly an improvement over dumping everything overboard. But neither is an ideal solution as there’s still potentially combustible products being wasted. Are there? Here's a diagram of the RD-180, which uses an oxygen-rich preburner: https://en.wikipedia.org/wiki/RD-180#/media/File:Rd180schema... Where is anything escaping other t…

The diagram doesn't show that the fuel used in the preburner is combusted less efficiently than the fuel used in the main combustion chamber. It comes out through the nozzle, but isn't fully combusted (it comes out as liquid, while the main fuels are gasses) and so is wasted energy.

From what I understand, the fuel and oxidizer that has been through the preburners to spin the turbines is basically fungible with fuel and oxidizer that is used in the combustion chamber.

Re: The Tech Behind SpaceX’s New Engine

#39

Post by Diederich »

twic wrote: post #34
wolf550e wrote: post #29

Earlier quoted context omitted.

This is only viable for small engines, like expander cycle is only viable for small engines. The specific energy of electric batteries cannot compare to that of kerolox or methalox. Closed cycle is not a new idea: https://en.wikipedia.org/wiki/Staged_combustion_cycle#Histor...

I still have a soft spot for expander cycles - they seem so elegant compared to staged combustion cycles, and apparently i am not the only person who thinks that: https://blogs.nasa.gov/J2X/2014/03/24/inside-the-leo-doghous...

Excellent read, thank you.

Re: The Tech Behind SpaceX’s New Engine

#40

Post by mabbo »

kumarvvr wrote: post #6

This is an excellent article. Just curious, how much more efficient is the full-flow engine? And, what exactly is the deal with the seals the article is talking about? Anyone have more info?

Scott Manley has a fantastic video explaining all of this: https://www.youtube.com/watch?v=4QXZ2RzN_Oo

In terms of efficiency, it's worth mentioning that even tiny gains have big impacts because of the rocket equation. Having a 1% boost the entire time the rocket is running can have an enormous impact in the overall velocity, or reduce the overall amount of fuel needed for the same targets.

Post reply on HN