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ShinAce
2015-Sep-30, 02:53 AM
I'm sure this has been asked before, so if anyone could link me to that, I would appreciate it.

Where's the exhaust flame of the lunar ascent module? I'm under the impression that this engine cannot be throttled, so it clearly must be operating at its specified thrust of (1500 lbs?).

The lunar module ascent engine used hypergolic propellant, much like a Titan missile/rocket. While the Titan launches show huge flames in broad daylight, the lunar module appears to rely on anti-gravity instead of rocket propulsion. I just don't get it.

Please forgive me for beating a dead horse, but all I can find is talk about the blast crater. I couldn't care less about a blast crater. I want to know about the exhaust flame.

Reality Check
2015-Sep-30, 03:07 AM
I suspect that the "huge flames in broad daylight" in Titan launches are burning air rather than propellant burning. Rocket motors have a combustion chamber where the fuel and oxidizer burn. What we see coming out is the hot reaction mass and in atmosphere that will produce flames. In vacuum, the hot reaction mass will emit light according to its temperature. That should not be visible in an ordinary camera such as those that recorded ascents.

ETA: The lunar module ascent engine also uses a different hypergolic propellant from the Titan 2 engine (http://www.clavius.org/techengine.html)

In fact, once in operation the Aerozine 50 exhaust plume is essentially colorless and transparent.
That link also debunks the "We should see thick smoke from the hypergolic engine of the lunar module ascent stage" myth.

01101001
2015-Sep-30, 03:08 AM
First source for Apollo information: clavius.org
See http://www.clavius.org/techengine.html


[Question:]Th photo below (left) and descriptions of the space shuttle's RCS operation shows we should see a visible flame from the lunar module ascent stage. Both these engines burn hypergolic fuels.

Informative (and conspiracy-busting) answer is there.

ShinAce
2015-Sep-30, 03:33 AM
After looking at Falcon 9 footage, it would appear to be an atmospheric effect. Thanks RealityCheck!

We could reverse the question and say: show me footage of an exhaust flame for any rocket in the vacuum of space.

Reality Check
2015-Sep-30, 03:55 AM
http://www.clavius.org/techengine.html has an image from footage of an exhaust flame for a maneuvering rocket in the vacuum of space - with the note that was taken with a long exposure!

schlaugh
2015-Sep-30, 12:19 PM
We could reverse the question and say: show me footage of an exhaust flame for any rocket in the vacuum of space.

Starting around the 1:45 mark of this classic Apollo 4 footage, you'll see the S-IV-B ignite and the exhaust appears once the separation residue goes away. By then the S-IV-B was about 100 miles above the surface, or essentially vacuum. The exhaust is very light in color and not very large.

https://www.youtube.com/watch?v=q1vy4xXZynI

bknight
2015-Sep-30, 12:33 PM
Starting around the 1:45 mark of this classic Apollo 4 footage, you'll see the S-IV-B ignite and the exhaust appears once the separation residue goes away. By then the S-IV-B was about 100 miles above the surface, or essentially vacuum. The exhaust is very light in color and not very large.

https://www.youtube.com/watch?v=q1vy4xXZynI
View the A17 LM launch at the end of the video the camera is essentially looking into the throat of the ascent engine and the flame look similar to the SIV-B that schlaugh posted.
In addition look at these Gemini-Titan launches and you observe a exhaust, but not nearly the bright flames you might "expect to see"
https://www.google.com/search?q=image+of+gemini+titan+12+launch&biw=1366&bih=599&tbm=isch&imgil=Olrep-yKNEs9PM%253A%253Br5xKjPkML-My_M%253Bhttps%25253A%25252F%25252Fen.wikipedia.or g%25252Fwiki%25252FTitan_II_GLV&source=iu&pf=m&fir=Olrep-yKNEs9PM%253A%252Cr5xKjPkML-My_M%252C_&usg=__ZJE_LhlGyg2glMI5NG_vFbGHHSs%3D&ved=0CD4QyjdqFQoTCIS-mpDhnsgCFUfRgAodhvMJNA&ei=eNULVoSiAceigwSG56egAw#imgrc=Olrep-yKNEs9PM%3A&usg=__ZJE_LhlGyg2glMI5NG_vFbGHHSs%3D

GT-12 is the best of the bunch I think

https://www.youtube.com/watch?v=Obd_jTO66-0 A17 LM launch

JohnD
2015-Sep-30, 01:14 PM
RealityCheck, "burning air"??

Very high temps producing oxides of nitrogen?
Ditto ionising air molecules to a plasma, with that losing heat by emitting light?

This video may be of interest, Nitric oxide and Aniline being mixed in the fume cupboard. I think it may be, but from where I am, I can't hear the audio!
https://www.youtube.com/watch?v=bin_W1xVPfY
and this one that specificly deals with your Q
https://www.youtube.com/watch?v=JLCrZGgKD-k

John

schlaugh
2015-Sep-30, 01:14 PM
After thinking about it, it's worth noting that the camera angle is aimed straight up the throat of the S-IVB motor. So maybe it's possible that we're only seeing the exhaust before it exits the nozzle, and once it does then the flame essentially vanishes. Hard to tell from the angle in the film.

ShinAce
2015-Sep-30, 02:15 PM
With the Falcon what you see is the blackbody radiation of the nozzle glowing white hot.

For example, see the 3:00 mark of:
https://www.youtube.com/watch?v=as-s1OYAasg

I can't help but wonder what an ion thruster would look like in space when viewed in the umbra(shadow) of the Earth.

Elukka
2015-Sep-30, 04:04 PM
To add to the confusion, some rockets don't have much of a plume in the atmosphere either. :D

http://i.imgur.com/yf7iOPf.jpg

This is the British Black Arrow. I'm guessing for whichever reason the plume emits its light mostly in invisible wavelengths.

JohnD
2015-Sep-30, 08:27 PM
I'm guessing for whichever reason the plume emits its light mostly in invisible wavelengths.
See Post 8 and consider the emission spectrum of nitrogen plasma.
And give three cheers! The Bad AStronomer returns! In this article, he is delaling with the aurora, but shows a nitrogen spectrogram and sure enough, it doesn't emit in that part of the visual spectrum to which our eyes are most sensitive, the green/yellow part around 5-600 Angstroms.
http://blogs.discovermagazine.com/badastronomy/2012/06/18/aurora-in-the-pink/#.VgxDPMtVhBc

There's a full (?) list on Nitrogen partly and fully ionised emission lines here: http://physics.nist.gov/PhysRefData/Handbook/Tables/nitrogentable2.htm

John

Reality Check
2015-Sep-30, 08:38 PM
RealityCheck, "burning air"??
Yes -hot stuff + air = "burning air"
There is no "Very high temps producing oxides of nitrogen" in my post.
There is no "ionising air molecules to a plasma, with that losing heat by emitting light" in my post.
Nice videos - the first one shows a hypergolic reaction that is obviously visible. The second demonstration also has visible flames. But the lunar module ascent segments had no such flames. The main difference is no air on the Moon. Remove air from "hot stuff + air" and all you have is hot stuff which need not emit in the visible spectrum.

As the second video states nitrogen tetroxide (N2O4) reacts to give N2 which is hot. A nitrogen plasma of deionized N2+ (https://en.wikipedia.org/wiki/Ionized-air_glow) has emission lines in ultraviolet, visible, and infrared bands. There is a gap where human eyes are most sensitive but no astronauts were left behind on the Moon to witness the lunar module ascents. What the cameras left behind were most sensitive to is an open question.

ShinAce
2015-Sep-30, 11:05 PM
My understanding is that the sensor was a tube that used focusing coils to scan. A constantly spinning filter wheel was used to get one color per frame.

https://en.wikipedia.org/wiki/Field-sequential_color_system

It all boils down to the filters, really.

Reality Check
2015-Oct-01, 12:29 AM
It looks like you are right: Apollo TV camera - RCA J-Series Ground-Commanded Television Assembly (https://en.wikipedia.org/wiki/Apollo_TV_camera#RCA_J-Series_Ground-Commanded_Television_Assembly_.28GCTA.29)