Info on ascent module

[Willoughby]

Does anyone have info - or preferably a source of info - on the ins and outs of the ascent propulsion system, module - and any actual data on the launches from the moon?  Just going by the Wikipedia page and using the gross mass of the module and the thrust of the engine, I come up with a net force of about 8,480 Newtons - resulting in an acceleration of about 1.8 m/s².  This of course assumes the engine is running at max thrust (which I have no idea about).  Also, does anyone know if the ascent module actually got any boost from the separation charge.  I remember reading somewhere where one of the astronauts had an "oh sh&t" moment because the module actually sank for an instant before the ascent engine kicked on.  This would lead me to believe that the ascent module wasn't accelerated by any separation charge.  I'm just trying to reconcile the three launches we have footage for with the math involved.  Any help would be appreciated.  Thanks!

[Allan F]

The ascent engine didn't have a throtte, so it was either on or off. At lunar liftoff, it had a thrust-to-weight ratio ensuring about one lunar G of acceleration.

There was some gas flow issues between the ascent and descent stage at liftoff, which provided a small extra "kick" for the first few centimeters.

[Willoughby]

The ascent engine didn't have a throtte, so it was either on or off. At lunar liftoff, it had a thrust-to-weight ratio ensuring about one lunar G of acceleration.

There was some gas flow issues between the ascent and descent stage at liftoff, which provided a small extra "kick" for the first few centimeters.

Perfect.  Thanks.  So my estimation of 1.8 m/s² wouldn't be that far off then.  The only issue I was having was that it was a little low for what I visually see at the initial liftoff which is why I was wondering if the separation of the stages added anything right at the start.  What you say makes more sense and is consistent with the personal testimony of the astronaut who said the module actually dropped a little on separation (momentarily).  This is what I couldn't reconcile, and you've filled the gap.  I appreciate it.

For anyone interested, I came across this Apollo 17 ascent stage liftoff simulation second-by-second analysis since my original comment :

http://www.braeunig.us/apollo/LM-ascent.pdf

[Allan F]

Yes, that is written by Bob B.

He has lots of good information on his site.

[ka9q]

Separation involved cutting the cables and bolts between the stages with piston-driven blades powered by a small explosive charge. A relatively large explosively-driven cutter called a guillotine severed electrical cables, oxygen and water lines. This was timed to happen just a fraction of a second before ascent engine ignition.

There's a valve in the high pressure oxygen line from the descent stage that should close at separation.

In other words, there would be no impulse at all from the separation itself. I don't know what effect the "fire in the hole" aspect of ascent engine ignition would have, i.e., the fact that the nozzle is so close to the top of the descent stage. It would be brief, though.

The first thing the guidance system did after liftoff was to orient the stage to the local vertical and then yaw to the correct launch azimuth. I suppose this could have been a little disconcerting to the crew when the lander was at an angle. On some of those ascents you can see the stage move horizontally a little before it corrects.