It really is rocket science

[ka9q]

Right, I know that keeping a mixture either extremely lean or extremely rich will keep preburner temperatures low by limiting the amount of heat produced and diluting that heat with a large amount of unreacted oxidizer or fuel, respectively.  What I don't understand is how a lean mixture could have any kind of advantage over a rich mixture, given that hot oxidizers are even more corrosive than cold oxidizers. Hot hydrocarbons are generally far more benign on metals with the possible exception of embrittlement by free hydrogen.

Maybe the Russians couldn't bring themselves to do something the same way as us western capitalist imperialist stooges.

[Bob B.]

Preburners are run lean to keep the exhaust gas temperatures low until they reach the thrust chamber, to prevent excess thermal stress on the preburner and turbine components.  Those are notoriously difficult to cool.

Or they are run rich, as is the case with the SSME.  In the SSME, most of the hydrogen is run through the preburner (about 70% of the total flow as I recall) with only a small amount of oxygen.  In either case, lean or rich, the preburners operate far from the stoichiometric ratio to keep the temperature down.  Same thing for a gas generator in an open cycle engine, though I think they almost always run rich.

[JayUtah]

Or they are run rich, as is the case with the SSME.

Wow, ignore anything I write before about 9:00 am Utah time.  Yes, all American engines I know run their auxiliary or prestage cycles fuel-rich, oxidizer-lean (which is how my pre-caffeinated mind was thinking) in order to keep combustion temperatures low enough for radiative cooling.  One big consequence of that in RP-1 fueled engines is coking of the turbine.  It's less a problem in hydrogen-fueled engines.

The RP-1 prestage exhaust products are very sooty.  It is those sooty carbon species that ignite and/or incandesce in the F-1 flow beginning a few feet below the exit plane.  Keep in mind that the Rocketdyne F-1 used the gas-generator exhaust (fuel-rich) as film cooling for the nozzle extension.

Metallurgy hasn't historically been Russian engineering's strong point.  I'll have to ask some colleagues who are more familiar with Russian engines why they can run fuel-lean.

I think the SSME ran its preburner absurdly fuel-rich in order to capitalize on some obscure adiabatic expansion properties, but that may be apocryphal.

[Bob B.]

What I don't understand is how a lean mixture could have any kind of advantage over a rich mixture, given that hot oxidizers are even more corrosive than cold oxidizers. Hot hydrocarbons are generally far more benign on metals with the possible exception of embrittlement by free hydrogen.

Maybe the Russians couldn't bring themselves to do something the same way as us western capitalist imperialist stooges.

I can't respond with certainty, but one possible explanation is that they're using the fuel as the coolant.  I know UDMH is adequate for regenerative cooling but I've never heard of using N₂O₄ for that purpose.  If the fuel is the only adequate source of coolant, then they obviously have to route that through the thrust chamber cooling channels.  That may leave them with oxidizer as the only option to power the turbines.  Though I don't suppose it's a real show stopper if the fuel was used as both the coolant and the propellant for the turbines, running it in series from the cooling jacket to the preburner.

Another possibility it that there isn't a large enough mass of fuel to power the turbines.  Gas generator cycle engines require only a few percent of the fuel to be diverted to the gas generator, but staged combustion engines require a large propellant flow through the preburner and turbines.  The large mass flow is required for a couple reasons: (1) higher pump horsepower due to the greater discharge pressure, and (2) less pressure drop across the turbines compared to gas generators.  The turbine pressure ratio of a gas generator engine is large because the exhaust is simply dumped overboard.  In staged combustion the exhaust from the turbines must remain at a very high pressure because it is then forced into the combustion chamber, thus the turbine pressure ratio is low.  In a N₂O₄/UDMH engine there is more than twice as much oxidizer as there is fuel.  Perhaps the turbines require a greater flow rate than the fuel alone can provide.

I honestly don't know the answer.  I just thinking out loud and suggesting possibilities.

[Bob B.]

I'll have to ask some colleagues who are more familiar with Russian engines why they can run fuel-lean.

If you can do that I'd be very interested in hearing their explanations.  Thanks.

[Bob B.]

I think the SSME ran its preburner absurdly fuel-rich in order to capitalize on some obscure adiabatic expansion properties, but that may be apocryphal.

Aside from the temperature issue, I can't confirm if there were any other reasons, but you're correct about the mixture being very fuel-rich.  I just found an article stating that 76% of the hydrogen flow and 11% of the oxygen flow was injected into the two preburners.  The fuel preburner had a mixer ratio of 0.86 and the oxidizer preburner had a mixture ratio of 0.60.  This compares to a mixture ratio of 6.03 at the main injector.

Source:  http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf

[cjameshuff]

With LOX/RP1, you have greater volumes of LOX to move than RP1. Running the preburner fuel-rich would further increase the burden on the LOX pumps relative to the RP1 pumps, running them lean might be part of why the Russian engines are so lightweight for their thrust. With LOX/LH2, the situation is reversed.

[Bob B.]

I took a look at the book History of Liquid Propellant Rocket Engines by George P. Sutton to see if it said anything about fuel-rich versus oxidizer-rich.  The book addressed the issue but didn't provide the answers I was hoping it would:

In the United States and several other countries, the GGs and PBs are usually operated at a fuel-rich mixture ratio.  There are only a few exceptions, such as Goddard's GGs, which were oxidizer rich.  In the Soviet Union and more recently Russia, the vast majority of GGs and PBs are usually operated at an oxidizer-rich mixture ratio.  The exceptions are LOX/LH₂ LPREs and a few others, where the gases are fuel rich.  There are some reasons for the choice of fuel-rich or oxidizer-rich gases, when driving turbines, but the rationales are usually not very strong, and a new engine could be operated with either one of these gas mixtures.

[ka9q]

With LOX/LH2, the situation is reversed

LH₂ is indeed extremely bulky (density 0.07 that of water) but it also has extremely low viscosity that makes it easy to pump.

[profmunkin]

Question on 1960's Soviet and USA soft landing of probes on the moon.
Could someone explain to me in plain english how this was accomplished?
The time needed to transmit and receive signals to and from a probe landing on the moon is significant.
To program a device to land via a set program could not take into account any discrepancies for significant alterations in parameters, such as altitude, terrain so on.

[raven]

I can't say for some of them, but the first successful soft lander, Luna 9, had a contact probe extending down from the main bus carrying the instrument unit and rocket. Upon contact, the main bus ejected the airbag clad instrument unit, which unfolded like a flower, revealing the camera and antenna. A rather elegant system I must say.

[Echnaton]

Question on 1960's Soviet and USA soft landing of probes on the moon.
Could someone explain to me in plain english how this was accomplished?
The time needed to transmit and receive signals to and from a probe landing on the moon is significant.
To program a device to land via a set program could not take into account any discrepancies for significant alterations in parameters, such as altitude, terrain so on.

Could someone explain to me in plain english how this was accomplished?

 A seemingly reasonable search for knowledge.

The time needed to transmit and receive signals to and from a probe landing on the moon is significant.

But this statement indicates you have already made some judgment in the matter.   Significance is relative to the problem and how much time is needed.  The round trip time was about about 3 seconds, a short time relative to a Mars landing for instance.  The Russians started landing on Mars in the early 70's.

To program a device to land via a set program could not take into account any discrepancies for significant alterations in parameters, such as altitude, terrain so on.
Why not?  This is a claim of fact that you cannot support.  Why do you just assume his to be true? 

[Echnaton]

...the main bus ejected the airbag clad instrument unit, which unfolded like a flower, revealing the camera and antenna. A rather elegant system I must say.

And a system still in use today for Mars landers.

[JayUtah]

Could someone explain to me in plain english how this was accomplished?

Well the Rangers did not soft-land.  They plummeted into the surface.

The Surveyors soft-landed.  The majority of the trip to the Moon was in a translunar coast.  At a certain point determined by the ground, based on telemetry, tracking, and a predetermined plan, the spacecraft was turned around and a powerful retrorocket was fired.  This rocket slowed the approach from cislunar speeds to an approach speed, timed precisely to leave the spacecraft at a certain altitude above the surface, descending at a certain rate.

Then the terminal descent program kicked in, which merely maintained a constant descent rate until contact.

The time needed to transmit and receive signals to and from a probe landing on the moon is significant.

Correct, which is why no real-time control was attempted from Earth during phases were close timing was critical.  The retrofire ignition was time-critical, but it was a singular event.  Once initiated, no further ground control was needed.  You just send the ignition command 1.3 seconds before you need ignition to occur.  After that, the spacecraft guides itself (i.e., maintains the proper attitude).

To program a device to land via a set program could not take into account any discrepancies for significant alterations in parameters, such as altitude, terrain so on.

Correct.  The model for retrofire ignition was based on an idealized shape of the Moon, which indeed would not allow for local variations in terrain such as hills or hummocks.  Hence the terminal descent program is meant to start relatively high (to account for any local maxima) and descent at a constant rate of descent until it contacts the surface.  This rate of descent is fast enough to be sustainable for several seconds, yet slow enough to be within the limits of the landing gear to absorb.

Imagine groping in your dark bedroom for the lightswitch.  If you get up from your bed and start walking, you know approximately how far it is to the wall.  You may walk briskly for a few steps to cover the distance, then slow down and walk slowly with your arm extended until you touch the wall.  You walk slowly enough that it won't hurt when you run into it.

Now you may ask how they would avoid boulders, craters, and other obstacles that may possibly upset the spacecraft.  The answer is that they didn't.  That was part of the risk of the mission.  They accepted the possibility that one leg may land on a large boulder and that the whole thing would tip over.  This is why manned landings had a generally higher expectation of success; the pilot could see and avoid obstacles because he is actually there seeing them and piloting the vehicle.

In short, Surveyor's descent was open-loop guidance, Apollo's descent was closed-loop guidance.

[ka9q]

Can anyone briefly summarize the relative advantages and disadvantages of the various turbine-driven rocket engine fuel cycles, i.e., gas generators vs staged combustion vs expander cycle? I've read the Wikipedia articles for each but I don't fully understand why they each have the advantages they do.

In particular, why is a gas generator cycle engine inherently less efficient? The turbine exhaust isn't actually thrown away, it is still ejected downward where it can add a little thrust. In single-engine stages it can even be gimbaled to provide roll control, which I would think is a very useful feature.

I know that the fuel-rich mixture used in most gas generators does represent unburned and therefore wasted fuel, but this can come in handy for nozzle protection as in the F-1, possibly allowing the bulk of the exhaust to be at a hotter temperature than the engine might otherwise tolerate.