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  Apollo 11: Glide slope angle and trajectory

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Author Topic:   Apollo 11: Glide slope angle and trajectory
Lou Chinal
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Posts: 1332
From: Staten Island, NY
Registered: Jun 2007

posted 06-24-2015 01:47 PM     Click Here to See the Profile for Lou Chinal   Click Here to Email Lou Chinal     Edit/Delete Message   Reply w/Quote
In studying the Apollo 11 landing profile, it seems to be about 15 degrees of a glide slope angle. Steep, but not too steep.

I know a lot of things were involved but it was also very familiar. Traditional would be another word for the approach. Any trajectory types out there?

David C
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Posts: 1039
From: Lausanne
Registered: Apr 2012

posted 06-25-2015 10:10 AM     Click Here to See the Profile for David C     Edit/Delete Message   Reply w/Quote
I'm no expert Lou but in selecting any flying machine's approach angle there are a number of factors involved. Some of these are: terrain clearance, energy management, crew visibility, escape options, structure clearance and structural loads. I'm guessing that you derived 15 degrees as the average from High Gate to touchdown, which seems reasonable.

To start with, I don't think that the term "glide slope" has any real meaning in this context since it really implies some kind of equilibrium state between aerodynamic forces, thrust and gravity — and there are no aerodynamic forces in this case. Secondly, whilst helicopters, and V/STOL airplanes may descend at a 90 degree angle (or even more depending on how you measure the angle), they don't normally start this from over a mile above the landing site and 300 knots. Finally, whilst the X-15, lifting bodies, shuttle etc flew very steep "Drinkwater" approaches, the vast majority of conventional airplanes fly an approach angle of between 2.5 to 4.5 degrees (with 3 degrees being by far the most common).

So yeah, 15 degrees is within the range of terrestrial experience (anything from just over zero to just under 180 degrees is - which covers everything). But I don't see how you get from there to calling the LM's final descent either "very familiar" or "traditional". Also remember that Armstrong had to extend significantly down range, so the plan was originally for an even steeper average approach angle.

Put another way, only one type of manned vehicle has ever flown an approach to touchdown in a vacuum and 1/6g - and it only did it 6 times. Nothing about that could be familiar or traditional.

All times are CT (US)

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