Posts: 26
From: Garrison Iowa 52229
Registered: Jun 2009

posted 09-07-2009 09:37 AM               

---

I read something about a term called throttle castellations of the desent engine during powered descent traced to oscillations to a previously unrecognized phenomenon that came to be called "IMU BOB".

Was the cause of this being that the IMU was located above and 4 feet in front of the vehicle center of mass of the vehicle?

Posts:
From:
Registered:

Posts: 861
From: Danville, Ohio
Registered: Dec 2002

posted 09-17-2009 03:46 PM               

---

"Aaah yes, the IMU BOB..." [twists end of handlebar mustache slyly]

Thank you Scott for having a great answer. I had never heard of this before. But now that it's explained, of course there would be such a problem as stated.

------------------
John Fongheiser
President
Historic Space Systems, http://www.space1.com

Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006

posted 09-17-2009 09:42 PM               

---

If I recall, it required them to widen the "deadzone" limits between which the RCS thrusters would not be firing, or else they'd be firing almost all the time.

Is this related to the rocking motion that the Ascent Stage made during ascent? You'll notice in the films that the LM bobs forward and then kicks back every few seconds as it lifts off. I know that they used the thrust from the RCS engines to supplement the ascent engine. I didn't know if IMU bob also played into this motion.

Posts: 861
From: Danville, Ohio
Registered: Dec 2002

posted 09-18-2009 06:28 AM               

---

I suspect the slight rocking during ascent engine firing was because of a slight center of gravity offset in the thrust vector. For descent the descent engine was initially fired at 10% thrust for some 20 seconds to fine tune the c.g. alignment in preparation for full thrust (by gimballing the engine). But for the ascent engine there would be no opportunity for this fine tuning until after the moment of liftoff. Plus the LM had to align itself with the desired trajectory from a non-level launch pad (the descent stage).

------------------
John Fongheiser
President
Historic Space Systems, http://www.space1.com

Posts:
From:
Registered:

Posts: 1332
From: Staten Island, NY
Registered: Jun 2007

posted 09-19-2009 06:39 PM               

---

Keep in mind the ascent engine had an ablative nozzle also. During the ascent the nozzle would char, changing the center of mass. There were combustion instability problems as well. The nitrogen tetroxide used as an oxidizer and the fuel of hydrazine and unsymmetrical dimethylhdrazine were of different volumes (that's why the tank on the CMDR side bulges out more). As both propellants burned off the center of gravity constantly shifted.

There was also a crossfeed connection that could transfer fuel from the RCS to the ascent engine. A further reason for the changing center of mass/gravity.

Thanks to Scott for providing an answer that was intuitively obvious to the casual observer (I shaved off my mustache a long time ago).

Posts: 148
From: MD
Registered: Feb 2003