Can any of you Rocket heads explain this?

[Diogenes]

Last week I saw a video about Russian rocket engine developments during the space race. According to the video the Russian engines are 25% more efficient than American engines.



Apparently they operate at greater combustion chamber pressures, and this is asserted to be the consequence of them being a "closed cycle" system. "Closed Cycle" in this context apparently means the turbo pump exhaust discharge is fed into the combustion chamber rather than just being vented.


Why would feeding exhaust from the turbo pump increase pressure?

Why couldn't pressure be increased from simply redesigning the turbo pump to produce more pressure?

[paperburn1]

Yes but wiki does it better.
Pay special attention to the advantages of open cycle engines and why Musk would use them instead of a Russian design.
https://en.wikipedia.org/wiki/Expander_cycle
Because of the necessary phase change, the expander cycle is thrust limited by the square-cube rule. As the size of a bell-shaped nozzle increases with increasing thrust, the nozzle surface area (from which heat can be extracted to expand the fuel) increases as the square of the radius. However, the volume of fuel that must be heated increases as the cube of the radius. Thus there exists a maximum engine size of approximately 300 kN of thrust beyond which there is no longer enough nozzle area to heat enough fuel to drive the turbines and hence the fuel pumps. Higher thrust levels can be achieved using a bypass expander cycle where a portion of the fuel bypasses the turbine and or thrust chamber cooling passages and goes directly to the main chamber injector.

In an open cycle, or "bleed" expander cycle, only some of the fuel is heated to drive the turbines, which is then vented to atmosphere to increase turbine efficiency. While this increases power output, the dumped fuel leads to a decrease in propellant efficiency (lower engine specific impulse). A closed cycle expander engine sends the turbine exhaust to the combustion chamber

[Diogenes]

I'm still confused. There are pieces of this that do not fit with what I remember.



In the original V2, they used a Kerosine powered engine to build the fuel pressure. It was effectively a separate engine that ran at the same time as the main rocket engine.


My understanding of the turbo pump is that it has it's own combustion chamber that burns fuel as if it were a jet engine.

Like this.



The article to which you linked me appears to indicate that the turbo pump runs entirely off of the expansion of fuel heated by the nozzle. Sort of like a steam turbine.


If that is the case, it's easy to see why a steam turbine design would be less powerful than a combustion powered design.


Were we using this sort of steam expander design instead of a combustion driven design?


That seems like ignoring low hanging fruit.

[hanelyp]

There are several turbo-pump arrangements used for liquid fueled rockets.

In the expander cycle (used by the RL-10), a propellant is pumped, passed through coolant passages, through the turbine, then into the combustion chamber. Reliable and efficient, but doesn't scale up well.

In the gas generator cycle a gas generator fueled by the same chemicals as the rocket (F1 or SpaceX Merlin) , or a separate monopropellant like hydrogen peroxide (V2, many Goddard rockets) drives the turbine and then exhausts overboard. Not as efficient, but reliable and scales up well.

The precombuster cycle (SSME) burns part of the propellant in a gas generator, drives the turbine, then into the combustion chamber. Capable of both high thrust and efficiency, but more temperamental than the above options.

[paperburn1]

It is another one of those rob peter to pay Paul things
Open cycle engines are easy to throttle and bullet proof as long as you have done your math right.
There is a host of things that you generally do not have to worry about.
For that security you lose some performance.
Closed cycle engines you get best performance but you have more worry.
Getting the un-burned/ burned gasses back into the combustion chamber running at pressure.
hot spots/ melt though on the turbo-pumps because of back pressure/ coolant flow issues or contamination.
So for more thrust you have more reliability problems.
These pump were (still are?) manufactured at tolerances you would not believe, the machinist working in tolerances that they had to take what type of lubrication was used in to account. I found that fascinating.
( Note most of my recollection are over 50 years ago from a man named Charlie Weinberg. The smartest man I have ever met. When NASA had a problem they went to see the man and when the man had a problem he went to see Charley. He is the reason I got off the farm and went into aviation A good man and my fathers friend.)

[Diogenes]

The precombuster cycle (SSME) burns part of the propellant in a gas generator, drives the turbine, then into the combustion chamber. Capable of both high thrust and efficiency, but more temperamental than the above options.

And this is the Russian system. I'm back to my original question. How does this produce greater pressures than venting the exhaust gases?


If anything, you would think it would cause back-pressure and lower the differential pressure applied to the turbine wheel.

[Diogenes]

It is another one of those rob peter to pay Paul things
Open cycle engines are easy to throttle and bullet proof as long as you have done your math right.
There is a host of things that you generally do not have to worry about.
For that security you lose some performance.
Closed cycle engines you get best performance but you have more worry.
Getting the un-burned/ burned gasses back into the combustion chamber running at pressure.
hot spots/ melt though on the turbo-pumps because of back pressure/ coolant flow issues or contamination.
So for more thrust you have more reliability problems.
These pump were (still are?) manufactured at tolerances you would not believe, the machinist working in tolerances that they had to take what type of lubrication was used in to account. I found that fascinating.
( Note most of my recollection are over 50 years ago from a man named Charlie Weinberg. The smartest man I have ever met. When NASA had a problem they went to see the man and when the man had a problem he went to see Charley. He is the reason I got off the farm and went into aviation A good man and my fathers friend.)

I get how not venting fuel will improve efficiency. What I don't get is how dumping exhaust gas from the turbine preburner into the combustion chamber boosts pressure.


If you are using a preburner to drive the turbine wheel, why not just vent the exhaust instead of feeding it back into the combustion chamber? The fuel thus used would seemingly be a trivial part of the overall fuel supply.

[hanelyp]

The back pressure of the main combustion chamber does work against turbine performance in a closed cycle. But with an open cycle you want to limit how much gas you're using for the turbine and dumping overboard. A closed cycle turbopump can use a larger portion of propellant (even all of it with two turbines, one fuel rich, one oxidizer rich) to drive the turbine without worrying about being wasteful.

[paperburn1]

If you are using a preburner to drive the turbine wheel, why not just vent the exhaust instead of feeding it back into the combustion chamber? The fuel thus used would seemingly be a trivial part of the overall fuel supply.
In the gas generator cycle a gas generator fueled by the same chemicals as the rocket and can be vented but this is basically an open cycle design. It scales well but you have to worry about keeping it cool.

[paperburn1]

Sorry , are you still interested in the question. lost my internet in the middle of my answer.
This link may explain it far better than I ever can
http://www.braeunig.us/space/propuls.htm#cooling

[Diogenes]

Sorry , are you still interested in the question. lost my internet in the middle of my answer.
This link may explain it far better than I ever can
http://www.braeunig.us/space/propuls.htm#cooling

Yes, i'm still interested in the answer, because much has been said that is true, but I don't see how it explains what I asked.

I'll look at your link. Thanks.

[hanelyp]

Here's a thought: finite energy and reaction mass. With an open cycle turbopump you're dumping a portion of your potential reaction mass with little energy. With a closed cycle turbopump you can accelerate all the reaction mass equally, to better effect.

[paperburn1]

point to ponder, when feeding your turbo pump exhaust back into the reaction chamber it has to be at a much higher pressure than what is in the chamber making the pump more acceptable to failure.In a staged combustion cycle, the turbine exhaust gases get fed back into the main injector and get “burned again.” This is possible since the combustion in the preburner is off from stoichiometric conditions, meaning that in addition to combustion products you also have lots of leftover propellant (either fuel or oxidizer depending on the scheme). The leftover propellants from the turbine exhaust then become part of the mix of propellants in the main combustion chamber.
https://blogs.nasa.gov/J2X/tag/closed-cycle-engine/
As for why we use what we do for rocket engines. I guess the simplest way to put it back in the 60s we were blowing up a lot of rockets because our material science was behind the Russian for high temperature material. So Nasa errored on the side of safety because the whole world was watching us and we did not want to fry some astronauts on live TV like the Russians did...maybe (note Russian launches were not live for the public.). By venting the gas overboard we did not risk fuel pump failure adding a factor of safety. Now with our better understanding of material science we are making closed cycle engines high performance engine not limited to lower thrust levels and if you’re willing to accept consequent greater complexity then staged combustion is what to use.

But for simplicity and reliability and re-usability open cycle engines win at a cost of less thrust /lower isp. most likely the reason Musk is using them.
This is why I can not teach, i though I had hit the high points but after rereading my posts I clearly missed the question.

[D Tibbets]

I'm not sure this has been mentioned, but an open cycle engine may be lighter for the same amount of thrust. This thrust to weight advantage would compensate for at least some of the greater efficiency in a closed cycle engine. The Falcon 9 engines are, I believe, the highest thrust to weight main rocket engines in a production rocket. That simplicity and cost are advantages is pertinent, but so is the weight of the rocket, especially if you are trying to fly it back to the launch site.

Dan Tibbets

[paperburn1]

so it looks like Musk is going after that low hanging fruit, Elon Musk stated that SpaceX were instead working towards a potential staged cycle engine. The forthcoming engine currently under development by SpaceX has been named "Raptor". Raptor will use liquid methane as a fuel, and was stated as having a sea-level thrust of 6,700 kilonewtons (1,500,000 lbf). Since the initial announcement of Raptor, Musk has updated the specification to approximately 230 tonnes-force (2,300 kN; 510,000 lbf)—about one-third the original published figure—based on the results of optimizing for thrust-to-weight ratio.