Staged Combustion Q&A

#80 by hkultala on 27 Mar, 2013 09:14
Quote from: Seer on 19 Jan, 2011 20:12
Does anyone know why the rd-180 only has a thrust/weight ratio of 80? The nk-33 has T/W of 136, with half the chamber pressure, in comparison.

One reason is that NK-33 does not have swivel mechanism in it's nozzle, RD-180 has 2-way swivel

N-1 was steered by throttling the engines,
Sojuz-2-1v will be steered by RD-0110R vernier thruster.

AJ-26 weights much more than NK-33 because the swivel mechanism is added, but it seems the weight of AJ-26 is reported nowhere, all links to AJ-26 just point to NK-33 even though the engines have considerable differences.

But AJ26 still propably has slightly better T/W ratio than RD-180.
#81 by baldusi on 27 Mar, 2013 12:16
Quote from: Hyperion5 on 26 Mar, 2013 22:58
I'm curious, but does anyone know how many different forms of staged combustion exist or have been thought up? I can think of at least three off the top of my head.

Twin-preburners (one oxygen rich, one fuel-rich)
Examples: RD-170 family
Single pre-burner & turbopump (runs oxygen-rich or fuel-rich)
NK-33, RD-162, RD-0120
Full-flow staged combustion
Examples??

Does the SSME count as a unique staged combustion cycle due to its resonance chambers?
You're confusing a few things.
1) About staged combustion the best way to think about it is the Russian nomenclature. They speak about how fuel and oxidizer arrive at the chamber's injectors. Thus, a gas generator is liquid/liquid, a staged combustion is gas/liquid and a full flow is gas gas. The only gas/gas engine that I know about, was the failed RD-270 engine. And there was the Integrated Powerhead Demonstrator. But I don't think they even had a thrust chamber for it.
2) RD-170/1/1M family has two identical oxidizer rich gas generators. That's why making the RD-180 required so little changes. When they developed it they didn't felt confident on making a single gas generator of that side. But both gas generators are oxidizer rich and only the oxidizer arrives as a gas to the chamber. The fuel is injected in liquid form.
3) The H2 and CH4 engines might extract some extra energy by doing something like an expander, but it's usually to avoid cavitation or such. In the RS-25D case the H2 LP turbine I think it does use expander power, but I'm not sure. I know that the only use of the LP turbines is to avoid cavitation.
4) You have to consider the expander cycle, for example. You have both closed cycle (like the RL-10/Vinci/RD-0146), that inject gas fuel and liquid oxidizer. And you also have open cycle expander (like the LE-5A/B and LE-X), where the expanded fuel is dumped and the fuel that does arrives to the injectors is in liquid form.
5) strangequark did teased about a new cycle for the RL10 replacement program.
6) You also have to consider the pressure fed and piston pump fed engines, you might consider them different cycles.
#82 by Hyperion5 on 27 Mar, 2013 16:13
Quote from: hkultala on 27 Mar, 2013 09:14
Quote from: Seer on 19 Jan, 2011 20:12
Does anyone know why the rd-180 only has a thrust/weight ratio of 80? The nk-33 has T/W of 136, with half the chamber pressure, in comparison.

One reason is that NK-33 does not have swivel mechanism in it's nozzle, RD-180 has 2-way swivel

N-1 was steered by throttling the engines,
Sojuz-2-1v will be steered by RD-0110R vernier thruster.

AJ-26 weights much more than NK-33 because the swivel mechanism is added, but it seems the weight of AJ-26 is reported nowhere, all links to AJ-26 just point to NK-33 even though the engines have considerable differences.

But AJ26 still propably has slightly better T/W ratio than RD-180.

I believe the t/w difference has a lot to do with the RD-170 family's combustion cycle. If you compared an single-chamber RD-191 with a single-chamber NK-33, the NK-33 is a nice, compact engine with a single turbopump and an oxygen-rich preburner. An RD-191 is a much larger engine with only modestly more thrust. If I'm reading this RD-180 schematic correctly (http://www.nap.edu/books/0309102472/xhtml/images/p20010c31g259001.jpg), it appears there are not one but two turbopumps feeding into a single pre-burner, which ignites some of what passes through the turbine that feeds the combustion chambers, or combustion chamber in the case of the RD-191. Looking at the NK-33 schematic, it presents a much simpler form of staged combustion: http://www.bing.com/images/search?q=nk-33+engine+schematic&qs=n&form=QBIR&pq=nk-33+engine+schematic&sc=0-23&sp=-1&sk=#view=detail&id=6B49A6F2969218BFD58C2EB34E09FED0C9561D0B&selectedIndex=1. Given an RD-191 has a respectable t/w ratio of 89 with two-plane swiveling up to 8 degrees, I'd guess it's the combustion cycle needed to achieve those incredible pressures that requires so much mass. Although the RD-193, essentially a lightened RD-191 lugging around 300 fewer kilograms, has a t/w ratio of 103.

I would guess that the AJ-26 probably still has a better t/w ratio than an RD-193, though the difference will be a lot less than between an RD-180 and an NK-33. Anyone know why an RD-191 has such a superior t/w ratio to its RD-180 cousin?
#83 by jsgirald on 01 Feb, 2015 18:48
After reading a few things, on this site and elsewhere, about this type of engine I've got a few questions:

Is it possible to change the fuel/oxidizer ratio in the preburners? I mean while the engine is running. Could this be used to change thrust (isp) and pressure in the combustion chamber?

In order to simplify plumbing, can a single set of pre-burners/turbopumps be used to feed several combustion chambers? Sort of using a single assembly for a small cluster of engines. Less mass per chamber/nozzle would improve T/W, right?

Actually, I'm thinking that 2-4 engines might be clustered as a single propulsion unit, combining also actuators for gimballing (i.e. the whole thing would gimball/throttle as a unit).

Thnx
#84 by R7 on 01 Feb, 2015 19:36
Quote from: jsgirald on 01 Feb, 2015 18:48
After reading a few things, on this site and elsewhere, about this type of engine I've got a few questions:

Is it possible to change the fuel/oxidizer ratio in the preburners? I mean while the engine is running. Could this be used to change thrust (isp) and pressure in the combustion chamber?

In order to simplify plumbing, can a single set of pre-burners/turbopumps be used to feed several combustion chambers? Sort of using a single assembly for a small cluster of engines. Less mass per chamber/nozzle would improve T/W, right?

Actually, I'm thinking that 2-4 engines might be clustered as a single propulsion unit, combining also actuators for gimballing (i.e. the whole thing would gimball/throttle as a unit).

Thnx

Yes on both accounts and you are describing what RD-170/171/180 engines are doing. There's a throttle for fuel going to the preburner and another throttle for rest of the fuel going to the nozzles. Oxidizer is free to flow to the preburner as fast the turbopump can pump it.
#85 by baldusi on 02 Feb, 2015 15:41
BTW, the RD-0124 has a single TP and preburner that ffeds the 4 main combustion chambers AND the four vernier engines. But the single nozzle version (the RD-0125), appears to be lighter (but longer).
The RD-170/1 family, uses two preburners to run a single turbopump. They had instability problems at the preburner level (and the MCC, too). That's why they use two preburners and four MCC. But generally speaking, changing the O/F ratio is usually done to handle propellant management.
#86 by Remes on 02 Feb, 2015 21:23
Quote from: Hyperion5 on 27 Mar, 2013 16:13
I believe the t/w difference has a lot to do with the RD-170 family's combustion cycle. If you compared an single-chamber RD-191 with a single-chamber NK-33, the NK-33 is a nice, compact engine with a single turbopump and an oxygen-rich preburner. An RD-191 is a much larger engine with only modestly more thrust. If I'm reading this RD-180 schematic correctly (http://www.nap.edu/books/0309102472/xhtml/images/p20010c31g259001.jpg),
I think the combustion cycle is very much the same.

The schematic for the NK-33 shows less details.

The NK-33 has the 2 boost pumps integrated into the same housing as the TP. That might be the main reason, why a RD-180 looks more complicated.

That is one NK-33 boost pump, the center shaft is the main shaft for fuel/oxidizer, and this boostpump is driven by the flow of the TP:
http://www.lpre.de/sntk/NK-33/img/tna_o.gif