How did the descent engine throttle thrust

[bknight]

I was watching a YT video by hunchbacked where he was questioning the descent engine.  I looked but could not find the mechanism of thrust throttling.
Anyone know what is was.  Jay?

[Allan F]

Both the fuel and oxidizer was led through valves which was controlled by the computer. Both flows were throttled by one single mechanism, which was powered by an actuator.

I found this pretty picture:

[JayUtah]

The actual injector is a pintle design, just like a garden hose spray attachment.

[bknight]

The actual injector is a pintle design, just like a garden hose spray attachment.

So the constant 1.8 ratio was only 1.8 at say 100% thrust?
Most of the time running fuel lean?

[Allan F]

I seem to remember it was running at optimum mixture - if it was leaned, the oxygen surplus would erode the nozzle ablative material too fast.

[bknight]

I seem to remember it was running at optimum mixture - if it was leaned, the oxygen surplus would erode the nozzle ablative material too fast.

You are right at 100% thrust, but the thrust was reduced during the final phases of the landing, which by my understanding the fuel was reduced not so much the oxidizer thus my comment fuel lean for the last few minutes.  And that may have eroded the nozzle somewhat during that reduced thrust time.

ETA: If I am incorrect then please explain how the injector restricts both fuel and oxidizer?  From the diagrams I have seen it looks like only the fuel is restricted.

[JayUtah]

I may have oversimplified it when I said the pintle injector was "just like" a garden hose.  In the pintle injector, both propellants can be metered, and this was the case for the LM DPS.  The pintle meters one propellant while a sort of sleeve meters the other propellant.  Collectively this is known as the pintle injector.

If you do your best to decipher Fig. 1 in this document,
https://web.archive.org/web/20170809041152/https://www.usbid.com/datasheets/usbid/2001/2001-q1/pintleenginepaperaiaafinal.pdf and to comprehend the rather lavishly jargonesque description on that page, you might be able to see how the mechanism works.

The "orifice adjusting sleeve" meters the magenta propellant by varying that tiny gap that's not really well depicted.  The range of motion for that part is thousands of an inch.  The pintle meters the green propellant.  The impingement point is where the "oxidizer orifice" label points.  A little later in the document there are photographs showing the two flows operating separately in an actual injector.

[Allan F]

I seem to remember it was running at optimum mixture - if it was leaned, the oxygen surplus would erode the nozzle ablative material too fast.

You are right at 100% thrust, but the thrust was reduced during the final phases of the landing, which by my understanding the fuel was reduced not so much the oxidizer thus my comment fuel lean for the last few minutes.  And that may have eroded the nozzle somewhat during that reduced thrust time.

ETA: If I am incorrect then please explain how the injector restricts both fuel and oxidizer?  From the diagrams I have seen it looks like only the fuel is restricted.

The powered descent was one single burn, initiated at 10% thrust to ensure ullage. It was one single continous burn all the way down to landing.

[JayUtah]

I seem to remember it was running at optimum mixture - if it was leaned, the oxygen surplus would erode the nozzle ablative material too fast.

The oxidizer surplus -- in this case, N₂O₄.  But yes, unreacted propellants of either species would probably not be good for the ablative liner.  They're both corrosive in dangerous ways.  We have the same problem in LOX-rich staged combustion engines.  The hot oxygen starts attacking any ferrous compounds in the ducting leading from the turbine exhaust to the powerhead.  This is why American staged-combustion engines generally opt to be fuel-rich designs instead of LOX-rich designs.

[bknight]

I may have oversimplified it when I said the pintle injector was "just like" a garden hose.  In the pintle injector, both propellants can be metered, and this was the case for the LM DPS.  The pintle meters one propellant while a sort of sleeve meters the other propellant.  Collectively this is known as the pintle injector.

If you do your best to decipher Fig. 1 in this document,
https://web.archive.org/web/20170809041152/https://www.usbid.com/datasheets/usbid/2001/2001-q1/pintleenginepaperaiaafinal.pdf and to comprehend the rather lavishly jargonesque description on that page, you might be able to see how the mechanism works.

The "orifice adjusting sleeve" meters the magenta propellant by varying that tiny gap that's not really well depicted.  The range of motion for that part is thousands of an inch.  The pintle meters the green propellant.  The impingement point is where the "oxidizer orifice" label points.  A little later in the document there are photographs showing the two flows operating separately in an actual injector.

I read the paper and the method to control the fuel was not really explained very well.  I don't doubt that the injector reduces the amount of fuel and oxidizer, I just don't understand how one does that.
Thanks Jay
Alan I thank you for your input, but I don't believe I ever indicated that the firing was done in stages, but in percent thrust during the descent.

[JayUtah]

I read the paper and the method to control the fuel was not really explained very well.

Indeed, that's why I said to do your best.  Most of that paper is TRW chest-thumping.  Whoever did the graphics evidently did them in a hurry.

[bknight]

I read the paper and the method to control the fuel was not really explained very well.

Indeed, that's why I said to do your best.  Most of that paper is TRW chest-thumping.  Whoever did the graphics evidently did them in a hurry.

LOL, ok could you describe the method?

[JayUtah]

LOL, ok could you describe the method?

I'd need to do an animation or something.  Maybe I can find a better description someone has already done.

[bknight]

LOL, ok could you describe the method?

I'd need to do an animation or something.  Maybe I can find a better description someone has already done.

Ok, I'm patient.

[Allan F]

Alan I thank you for your input, but I don't believe I ever indicated that the firing was done in stages, but in percent thrust during the descent.

No, of course not. Wasn't my intention to imply that.