Simulation debugging for Apollo Lunar Landing

  • #1
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Summary:
What am I doing wrong?
My simulator is trying to mimick the actual conditions of the Apollo landing. I have entered in numbers for my runge kutta simulator for LEM mass, dry mass, fuel, thrust of the rockets, fuel burn rate per second etc etc. But I am not able to land, only very close to landing. I copied the conditions from this source:

https://blog.adacore.com/make-with-ada-the-eagle-has-landed

So the LEM starts off at 15 km altitude (50,000 ft) at 1.6km/s speed. But I only seem to barely have enough delta V from the onboard DPS fuel. What am I doing wrong?

2nd failed attempt

1st failed attempt assuming 100km orbit

Does anyone know the ground distance to Tranquility Base before the Eagle started de-orbiting from it's 15km altitude?


Important note: This simulator has time acceleration so don't get confused why it sometimes fast forwards while deorbiting.

David

[Mentor Note -- personal website URL deleted]
 
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Answers and Replies

  • #2
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I suppose it's possible my simulator is fine and it's just my piloting skills, but can anyone use their sources to tell me the correct descent procedure? I cannot find it on the internet. I want to know the ground distance to target landing area from initial de-orbit burn from 50,000ft.


Anyone know Buzz Aldrin?


UPDATE:

FINALY DID IT! Is this how it was done?


David
 
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  • #3
Drakkith
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As you've seen, landing a spacecraft is very, very hard, especially when you don't know the optimal parameters for burn times, directions, etc. A sub-optimal set of burns can look reasonably good when you're just eyeballing it, but can waste a significant amount of fuel and leave you with a disassembled spacecraft on the surface.

Even with my 1000-2000 hours of playtime on Kerbal Space Program I still hesitate to give you advice on how to make your burns more optimal.

But I only seem to barely have enough delta V from the onboard DPS fuel.
You're burn(s) aren't optimal. That's all. The LEM really did have just barely enough fuel to get down to the surface.
 
  • #4
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I don't have the hard numbers (and it is after midnight) but the principle used for the Apollo landings where not that complicated in principle, just do a launch in reverse.

Do you know how to do a a launch using a gravity turn and circularization? The landing is pretty much the same reversed except that you need a bit of margin around the landing and the mass of the space craft is highest in orbit and the lowest on the surface instead of the other way around which messes with the acceleration curve.

Without adding any margin it becomes.
1. De-circularize so that your perilune is round about (depends on the details) the surface. How you do this will determine where you land.
2. Start braking at the correct (last possible) time and follow the retrograde vector until you become stationary just above your landing site.
3. Touch down.

And yes, getting both 1 and 2 correct so you land efficiently where you intended is very tricky.

Lander, the NASA program that did the calculations.
https://ntrs.nasa.gov/citations/19890005786
 
  • #5
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Thanks for these replies to everyone, but what was the ground distance between deorbit burn and landing?

What were the mission parameters?


David
 
  • #6
Tom.G
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  • #7
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"At 6:55:42 p.m. Dec. 11, the LM was placed into an orbit with a perilune altitude of 6.2 nautical miles. Approximately 47 minutes later, the powered descent to the lunar surface began. Landing occurred at 7:54:57 p.m. Dec. 11,..."

Plenty of details in the rest of the article.

Above from:
https://www.nasa.gov/mission_pages/apollo/missions/apollo17.html

(above found with:
https://www.google.com/search?&q=apollo+landing+altitude+deorbit)

Cheers,
Tom
Doesn't have the data I want, altitude when powered descent started and distance to landing site at that point. Not enough data to even extrapolate the points of data I want.
 
  • #8
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"At 6:55:42 p.m. Dec. 11, the LM was placed into an orbit with a perilune altitude of 6.2 nautical miles. Approximately 47 minutes later, the powered descent to the lunar surface began. Landing occurred at 7:54:57 p.m. Dec. 11,..."

Plenty of details in the rest of the article.

Above from:
https://www.nasa.gov/mission_pages/apollo/missions/apollo17.html

(above found with:
https://www.google.com/search?&q=apollo+landing+altitude+deorbit)

Cheers,
Tom
Doesn't have the data I want, altitude when powered descent started and distance to landing site at that point. Not enough data to even extrapolate the points of data I want.
 
  • #10
Drakkith
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Doesn't have the data I want, altitude when powered descent started and distance to landing site at that point. Not enough data to even extrapolate the points of data I want.
websterling's link has this info on page 3. The ground distance from PDI to landing is approximately 260 nautical miles and the altitude at PDI is approximately 50,000 ft. Detailed information on the thrust settings, spacecraft attitude, etc starts on page 7.
 
  • #11
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websterling's link has this info on page 3. The ground distance from PDI to landing is approximately 260 nautical miles and the altitude at PDI is approximately 50,000 ft. Detailed information on the thrust settings, spacecraft attitude, etc starts on page 7.
Hey thanks for this! Was the Eagle (Apollo 11) at full MTOW? I tried landing the Eagle at 15200kg and at this weight with the rocket firing constantly I overshot 260 nautical miles (400km). Was the Eagle lighter than 15.2 tons?
 
  • #12
Drakkith
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Hey thanks for this! Was the Eagle (Apollo 11) at full MTOW? I tried landing the Eagle at 15200kg and at this weight with the rocket firing constantly I overshot 260 nautical miles (400km). Was the Eagle lighter than 15.2 tons?
Wikipedia says it was 15,102 kg launch mass, but I can't guarantee that's accurate. Your last video shows a mass of over 22,000 kg, so that might be a big reason why you were just barely able to land with 8100 kg of fuel.

How far did you overshoot the landing site?
 
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  • #13
websterling
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You can get all of the numbers you need from APOLLO BY THE NUMBERS

You need to get the various weights from the various tables and do a little arithmetic to get your total, but there should be more than enough information there.

Also remember that 11 did it's own DOI so some fuel had already been consumed at PDI.
 
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Likes Drakkith and berkeman
  • #16
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Does anyone know the distance between RCS and center of gravity of the LM?
 
  • #17
Drakkith
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Does anyone know the distance between RCS and center of gravity of the LM?
Not sure, and the CoG will move over time as fuel is expended. A rough guess would be half of the diameter of the LM, or about 2 meters since the LM is a little over 4 x 4 meters (13 ft 3 in x 14 ft 1 in).
 
  • #18
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Not sure, and the CoG will move over time as fuel is expended. A rough guess would be half of the diameter of the LM, or about 2 meters since the LM is a little over 4 x 4 meters (13 ft 3 in x 14 ft 1 in).
Even if I were to use that, any idea how I can get hold of the moment of inertia and angular momentum of the LM?
 
  • #19
Drakkith
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Even if I were to use that, any idea how I can get hold of the moment of inertia and angular momentum of the LM?
I'd probably just use a simplified model. Perhaps a cube or a small cube on top of a big cube.
 
  • #20
pbuk
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I assume you have seen http://www.ibiblio.org/apollo/. There may be something there (presumably the AGC had to have some idea of MoI etc in order to compute burn times, or was it all done through feedback?)
 
  • #21
pbuk
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Paging @anorlunda - and this may be better off moved to the Computing sub-forum.
 
  • #22
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I'd probably just use a simplified model. Perhaps a cube or a small cube on top of a big cube.
What's wrong with a Sphere?
 
  • #23
Drakkith
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What's wrong with a Sphere?
The same that's wrong with an octagonal cylinder. Nothing.
 

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