Apollo 15 LM ascent module take-off data

[onebigmonkey]

I was tooling around with shadow length stuff for Apollo 15 and it struck me that in the ascent module take-off recorded by the 16mm camera we have the shadow of the LM passing a known landmark, in words the ALSEP.

Depending on whether I use Google Earth, or Photoshop's measuring tool using the LRO 50m scale to calibrate, I get a distance of around 100-105m from the descent module.

Now, assuming Stellarium is correct, the time at lift off gives a sun angle of just over 41 degrees. Using a bit of Pythagoras this equates to an ascent module altitude of around 115-120 metres.

My problem is this: this kind of altitude (according to a diagram on Bob's excellent page http://www.braeunig.us/apollo/LM-ascent.htm ) is what you ought to have after around 11.5 seconds, not the 10 seconds it takes from the first movement of the ascent module to its shadow it reaching the 100m downrange mark on all the various versions of the film I've seen (including my Spacecraft films one).

So: is the 16mm footage accurately timed on the various sites on which I've seen it?

And: Are the values given on Bob's page for 17 applicable to 15?

[bknight]

Looking the data from Bob's site, I get an altitude of about 250-260 m f at the 10 sec. mark from liftoff.  Am I missing something to arrive at the 115-120 m?

Edit: This was feet not meters, so it should read:
I get an altitude of about 76-79 f at the 10 sec. mark from liftoff

[onebigmonkey]

Actually I redid my workings out and things look more sensible!

This website:

http://www.wsanford.com/~wsanford/shadow_length.html

allows you to calculate a shadow length based on sun angle and a height.

Well, we know the sun angle, and while we don't know the height we do know the shadow length.

If we go with Photoshop's measuring tool we get a shadow length of 105 (ie the distance between the descent module in the ground and the top of the ascent module's shadow. So if you just keep punching the numbers until you get a value for the height above ground of the top of the ascent module that matches the shadow. In this case it works out as 91 metres, or 298.5 feet.

I loaded up one of the videos into Premier to get a more accurate time stamp and arrived at a figure of 10.7 seconds from lift off to the the point on the ground where the shadow arrives that I am using as a reference.

If I look on the Bob's diagram, this value of 10.7 seconds gives an altitude of around 300-310 feet, which I think is an acceptable margin of error for my back of the envelope calculations.

On top of that, the sun's angle relative to lunar north is 115 degrees, and the angle of the shadow on the ground is pretty much 295.

In other words the position of the shadow on the ground is exactly where it should be given the sun's angle

[bknight]

You have way better eyesight/reflexes than I!  I could only get the nearest second

[ChrLz]

Haven't actually looked at this properly, but my immediate thought was ...  aren't you making a rather rash assumption about 100% level ground?  You need to go through everything (eg also the ascent angle..) and ensure you haven't made that sort of (unjustified?) assumption/s - maybe even produce some error ranges..

[ka9q]

There may also be uncertainty in exactly how fast thrust builds up in the ascent engine, so I think you're well within the margin of error here.

[onebigmonkey]

All good points, especially the level ground assumption, which it most certainly wasn't!

I want to be as accurate as possible, but I'm no rocket scientist!

[Allan F]

In another thread, someone - I think it was JayUtah - wrote that the ascent engine reached sustained thrust in less than a second, so there should be constant thrust all the way up.

[bknight]

In another thread, someone - I think it was JayUtah - wrote that the ascent engine reached sustained thrust in less than a second, so there should be constant thrust all the way up.

Yes, simple effective design.