[X] My Assistant

[dvandorn]

But, Bob -- not even the CMPs were able to see the LMs firing their engines from more than about 100 miles distance. Of course, they didn't use night-vision scopes, either...

-the other Doug

[Bob Shaw]

But, Bob -- not even the CMPs were able to see the LMs firing their engines from more than about 100 miles distance.  Of course, they didn't use night-vision scopes, either...

-the other Doug

oDoug:

I wonder whether they were looking at the engine from above, the side, or below? And against what sort of background?

I suppose that there's actually some virtue in this enquiry, as manned vehicles could act as a yardstick against which meteorite impacts on the moon could be measured!

Bob Shaw

[dvandorn]

I've read several different accounts of CMPs watching LM engine operations, and of course we have the TV record of the three J mission LM lift-offs. From these, I've determined that you pretty much have to be looking straight down the throat of the engine bell to see much in the way of actual self-illuminated exhaust with the nitrazine-UDMH engines they used in Apollo. Otherwise, the exhaust expanded far too rapidly for there to be much illumination. Or even much in the way of visible exhaust at all.

The best view of such an engine exhaust is the TV record of the Apollo 17 lunar liftoff. The engine exhaust is very briefly visible just as the stages separate, due I'm sure to the reflection of exhaust from the descent stage forrming interference patterns in the plume. As the ascent stage lifts clear, you see absolutely no indication of exhaust or even illumation from the engine. At pitchover, however, the ascent engine brightens, and as the stage flies almost directly away from the TV camera, you see a dramatic brightening as we look straight down the engine bell. The brightness of the exhaust at that point overcomes the brightness of the sunlight reflected off the stage. It's fairly dramatic.

I imagine the exhaust was somewhat more noticeable when burns occurred in darkness -- but even so, Mike Collins wrote of being able to see Eagle's DOI burn (in darkness), but that he was looking through his telescope pretty much straight down the descent engine's throat, which we know is the optimal angle. So the question of dark-condition visibility is still not that well defined.

Of course, we *do* have the record of Shuttle attitude control and OMS engine burns, which produce quite bright and noticeable exhaust patterns, when viewed from the Shuttle windows. And those are also nitrazine-UDMH engines. Maybe it's just a matter of the lighting...

-the other Doug