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Author Topic:   Apollo 11: Fuel remaining at lunar landing
Robert Pearlman
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posted 09-06-2013 09:54 AM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
Space artist Paul Fjeld contributes the cover article (and art) for the May/June 2013 issue of AIAA Horizons newsletter, "The Biggest Myth about the First Moon Landing."
We all know the Apollo 11 story. Many of us were "there." If you're older than 44 years, you were watching TV that summer of '69, even if you were a baby, though babies didn't understand what all the fuss was about. Neil Armstrong was guiding himself and his crewmate, Buzz Aldrin, in that odd creation of the early space program, Eagle, the Lunar Module (LM), nearer and nearer to the Moon's surface. It was very exciting to listen.

Aldrin was calling out numbers relating to where they were and how fast they were going as they worked to make that final first touch by humanity on a celestial object. The other voice was that of Houston (NASA Mission Control), CAPCOM (capsule communicator) Charlie Duke, mostly saying that things were going okay, but, at the end, doing a countdown.

"Sixty seconds." Then, "Thirty seconds!" Finally we heard, "Contact light!"

Ever since then the story headline has been, "The dramatic first Moon landing of Apollo 11 succeeded with only twenty seconds of fuel remaining!"

No! The biggest myth about the first Moon landing is those twenty seconds. Armstrong and Aldrin could have stopped their approach a few feet above the lunar surface and stayed there for more than a minute before letting Eagle drop safely to the surface...

moorouge
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posted 09-06-2013 10:41 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
Really!!! I'd like some evidence to support this contention. My understanding is that there was a margin of error in the fuel quantity light gauges, but a minutes worth of error?

Robert Pearlman
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posted 09-06-2013 10:48 AM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
Fjeld provides the details in the full article. To excerpt the basic details:
...the mission planners saved five seconds of descent engine burn time at full throttle to loft them to a safer altitude. During those five seconds, they would have time for staging, pressurizing of the ascent tanks, and ignition of the ascent engine.

Five seconds of thrust at full throttle corresponds to twenty seconds of thrust at 25% of full throttle.

...that adds another twenty seconds to the mythical twenty seconds, getting us to forty seconds of flight time left at landing. The remaining part of that "more than a minute" is a tale of slosh in the propellant tanks.

...after the flight, engineers concluded the low-propellant-level light was turned on between 30 to 45 seconds early!

So we now know that Armstrong had at least 20 + 20 + 30 = 70 seconds of flight time remaining (even though the LM crew and Houston could only count on 40 of them) before he would have crashed into the Moon in an Eagle LM starved of vital fuel.

garymilgrom
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posted 09-06-2013 12:28 PM     Click Here to See the Profile for garymilgrom   Click Here to Email garymilgrom     Edit/Delete Message   Reply w/Quote
Most know the countdown was to the abort point, not to crashing. Giving this more thought it makes sense they had additional fuel to position themselves for the abort. But that doesn't mean (to me) that they could have used said fuel to extend the landing phase.

I'm confused by this part of the article:

This is why the bingo call was actually a “Land in twenty seconds or abort now!” decision point. For example, if Armstrong had been flying when Duke’s countdown reached zero and Armstrong was still 60 feet above the lunar surface, but coming down smoothly with a three-feet-per-second velocity to a safe spot, his decision would definitely have been to continue with the landing.
If the LM could continue a safe descent after the abort now point, as this seems to say, then the bingo call was not land in 20 seconds or abort now. The author seems to be arguing both sides of that statement (abort now, continue landing) as true.

moorouge
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posted 09-07-2013 01:57 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
This confused me also. Surely, if mission rules stated that abort at this point, then the crew would abort regardless of how much fuel there theoretically remained. Otherwise we have a discussion on when is a mission rule not a mission rule.

On a related point - if an abort had taken place would the ascent stage engine be fired whilst the descent engine was still firing? Or would it have to be shut down before the abort took place?

David C
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posted 09-07-2013 05:09 AM     Click Here to See the Profile for David C     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by moorouge:
This confused me also. Surely, if mission rules stated that abort at this point, then the crew would abort regardless of how much fuel there theoretically remained.
It's not that simple. An abort, like a bale out or ejection is a course of action that is not available in all situations and in itself rather risky. Remember, that in an analogous manner to a single engine helicopter or an ejection seat the LM was subject to a low level "avoid curve" defined by height, sink rate and mass, within which assured abort was not possible.

I'm not commenting directly on Fjeld's article, and I'm afraid that my copies of Apollo Mission Techniques for Mission G are stored away.

"AMT Mission H-2 And Subsequent: Lunar Descent" states:

3.3 ABORT OR ABORT/STAGE DECISION

An abort or abort/stage decision is made when a malfunction has been detected that will affect crew safety or the capability of the LM to abort from powered descent... The decision to abort is based on the occurrence of one of the following:

  • (a) Propulsion... Impending DPS propellant depletion
  • (c) Trajectory constraints: Violation of specified abort limits on MCC-H displays
4.3.7 Low Level Propellant Monitoring

...the ground will assume the prime responsibility for warning the crew of impending DPS fuel depletion. Using the time at which the ground observes the DPS propellant-level low (DES QTY) "light on" signal as a zero reference point, MCC-H will give voice cues to the crew indicating 30 and 60 seconds elapsed time, and fuel depletion. The timing Of these cues will be biased (to account for transmission delay, fuel usage prediction errors, and sensor errors) so that the crew will hear "fuel depletion" when there will actually be 20 seconds of hover thrust (6 seconds of full thrust) remaining for an immediate landing or DPS abort.

"AMT Mission H2 And Subsequent: Abort From Lunar Powered Descent And Subsequent Rendezvous" states:
3.1 GROUND RULES

...the DPS should be used in abort situations whenever possible, even if there is only a few seconds burn available, to aid in establishing a positive radial rate and to improve propellant margins.

The "Final Flight Mission Rules: Apollo 11" document dated 16MAY69 states:
1-23 The Command Pilot may initiate such inflight action as he deems essential for crew safety.

2-25 LM Powered Descent: If a systems failure occurs and a choice is available:

  • Early in powered descent... (up to PDI +5 minutes)
  • During the remainder of the powered descent, it is preferable to land and and launch from the lunar surface than to abort. Only those systems failures and trends that indicate impending loss of the capability to land, ascend and achieve a safe orbit from the lunar surface, or impending loss of life support capability will be cause for abort during this period.
5-90 Powered descent will be terminated for: (H) Violation of the time biased (20 sec) DPS abort boundary.

5-91 There are no trajectory or guidance constraints which are cause for abort after crew take over of powered descent.

25-12 The total continuous burn time of the descent engine shall not exceed 910 seconds of continuous operation independent of thrust level (based upon a lunar mission duty cycle).

25-18 If possible the DPS will not be burned to propellent depletion, where ever possible the abort stage sequence will be initiated at low level plus 20 seconds during an abort from powered descent, however, at crew option abort stage sequence may be initiated at low level if a safe abort stage capability exists.

25-37 Low level confirms insufficient propellant to land - (B) Powered descent: Abort stage 20 seconds after low level

So what does all that mean? At low level there was no absolute hard "you must abort now" for propellant quantity. Propellant was only one part of the equation, and it all came down to the judgement of the command pilot. Frankly, any other "rule" would have been pointless. No stick - no vote. Arrogant perhaps, but ultimately true. Mission Rules in an area like this are more like guidance to assist the command pilot in exercising sound judgement.

BBlatcher
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posted 09-07-2013 10:12 PM     Click Here to See the Profile for BBlatcher     Edit/Delete Message   Reply w/Quote
In his autobiography, Armstrong makes clear that he was going to be the final decider on whether to land or not.

Lou Chinal
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posted 09-09-2013 03:34 PM     Click Here to See the Profile for Lou Chinal   Click Here to Email Lou Chinal     Edit/Delete Message   Reply w/Quote
After all was said and done, it came down to one guy's hand/eye co-ordination.

crash
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posted 08-01-2019 05:10 AM     Click Here to See the Profile for crash   Click Here to Email crash     Edit/Delete Message   Reply w/Quote
I have recently read somewhere that apparently, it is "generally accepted" that Eagle had at least 40 seconds of fuel remaining at touchdown. Well, that's news to me as all I've ever heard from the people I have met who were there at the time is that that there was 17 seconds left.

The background to the extra fuel is that the fuel calls during descent are to the decision point at which point they either land immediately or throttle up the DPS to initiate a climb before aborting and firing the APS. For either of these scenarios there was a fuel allowance built in. This is the first I've ever heard of this.

Can anyone either enlighten me or confirm that what I have read is nonsense?

Editor's note: Threads merged.

Altidude
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posted 08-01-2019 05:35 AM     Click Here to See the Profile for Altidude   Click Here to Email Altidude     Edit/Delete Message   Reply w/Quote
There is a great podcast called 13 Minutes to the Moon that goes into this estimation in detail. I think it will clear this up. The answer is a little complicated.

Headshot
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posted 08-01-2019 08:09 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
What would Duke have called out to Armstrong and Aldrin had they gotten to zero? Would he actually have radioed the word "bingo" to them?

crash
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posted 08-01-2019 11:54 AM     Click Here to See the Profile for crash   Click Here to Email crash     Edit/Delete Message   Reply w/Quote
Thanks, Altidude for the tip on 13 Minutes. I am currently working my way through the program which runs for hours and hours. I should be getting close to the fuel discussion point. I thoroughly recommend it to everyone if you haven't listened to it yet.

Fra Mauro
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posted 08-01-2019 12:06 PM     Click Here to See the Profile for Fra Mauro   Click Here to Email Fra Mauro     Edit/Delete Message   Reply w/Quote
Would there have been an indicator in Eagle for "bingo?"

Space Cadet Carl
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posted 08-01-2019 01:29 PM     Click Here to See the Profile for Space Cadet Carl   Click Here to Email Space Cadet Carl     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Headshot:
What would Duke have called out to Armstrong and Aldrin had they gotten to zero?
I'd like to know this too! What was the actual rehearsed audible call that Kranz and Duke practiced and they would have used at that actual moment if it occurred?

David C
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posted 08-01-2019 02:34 PM     Click Here to See the Profile for David C     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Headshot:
Would he actually have radioed the word "bingo" to them?
No, the call was "fuel depletion," as per my very long post above. There was no additional specific indication in the LM cockpit.

"Bingo" is an aviation term whose precise meaning varies by operator.

Kite
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posted 08-01-2019 04:00 PM     Click Here to See the Profile for Kite     Edit/Delete Message   Reply w/Quote
I seem to remember either reading or seeing a clip about Armstrong saying, with a grin, that it was the same as in your car, "When the fuel guage reads empty you know you can still get a few more miles!"

oly
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posted 08-01-2019 11:22 PM     Click Here to See the Profile for oly   Click Here to Email oly     Edit/Delete Message   Reply w/Quote
There are a few key points to consider when looking into this subject.

The first is the lunar module rate of descent, which when compared to an aircraft, was an extremely high rate. The time from 50000 feet to lunar surface was fast. This rate of descent is a velocity towards the lunar surface. This means that the people tasked with calculating the real-time fuel remaining had very little time to get the job done.

Engineers calculated that a certain amount of fuel was required to slow the rate of descent down to a speed slow enough that, once staged, the ascent engine could lift the ascent module away from the moon. The burn time at full power for the descent engine to slow, hover, or even lift the lunar module, depends on how much the lunar module weighs at that point in time, which was changing as fuel was consumed, and on how fast it was traveling towards the lunar surface.

The ascent engine has a fixed amount of thrust, designed to lift the LM from the lunar surface, at a predetermined weight, to a velocity suitable for rendezvous with the CM. Using kinematics, a (acceleration)= Delta-v (velocity) over Delta-t (time), and velocity towards the lunar surface at staging would negate the velocity away from the lunar surface.

The fuel remaining within the descent stage fuel tanks was an unknown value once the descent engine was ignited (assuming the quantity was the same as when launched) because the rate of burn varies due to throttle position. The voice and telemetry signals to and from the moon had a time delay, meaning the people on the Earth, tasked with monitoring the fuel remaining, had to reference the throttle position, duration of engine burn at that position, and make an estimate of remaining fuel and bingo time, (with built-in safety margin) and relay that information back to the LM, without being too far behind the game.

One characteristic to consider is fuel slosh within the tanks. Running the fuel supply dry was avoided because the combustion instability as fuel depletes could cause a catastrophic failure of the motor. This meant that a fixed reserve of fuel was needed to avoid an exploding rocket motor.

As the lunar module descended under the P63 program, the fuel burn could be estimated because the throttle position was predetermined. As the LM rolled over, the fuel tank slosh issue comes into the equation. At pitch-over, the P64 program also utilizes a predetermined throttle position, however, if landing radar altitude and rate of descent data were found to differ significantly from the programmed values, throttle position may become another variable.

Apollo 11 descent profile was found to be terminating at a location long from the originally planned location. The reasons for this have been discussed elsewhere. As Armstrong identified that the LM was tracking for a large crater and boulder field, the AGC P66 program allowed Armstrong to make throttle adjustments to arrest the rate of descent and track for a more favorable landing site. This resulted in staff on the ground needing to monitor the throttle position, calculate the instantaneous fuel burn, the duration of fuel consumption at that rate, subtract the used fuel quantity from the estimated fuel remaining, and calculate the bingo time/fuel quantity.

All the time this is going on, the abort requirements were changing. Abort sequencing has a requirement to provide a positive g loading on the LM ascent stage to provide the ullage function for the fuel system. This function was designed into the abort staging sequence and should be considered while thinking this problem over. LM Ascent motor ignition must be done under positive g load. This is one reason engineers considered that low-level staging was a higher risk than staging from the lunar surface.

What does all this mean?

At the time of the Apollo 11 landing, the person with the stopwatch measuring the fuel remaining, recorded 16 seconds of fuel remaining until bingo fuel level.

The Apollo 11 crew had received the 30 seconds radio cue, and armed with this information, continued on to achieve a successful landing.

How much fuel was actually remaining in the tanks at engine shut down means very little, because the quantity in the tanks at the beginning may not have been correct. Venting, leakage, or temperature change could all have resulted in a lower than loaded fuel quantity to start with.

There was enough to achieve the mission, and as the descent velocity slowed, the ascent velocity achievable by the Ascent Module was increasing. There is a “dead man's curve” drawn into the landing profile that considers the above information, used to evaluate phases where the chance of successful abort is reduced.

Any calculations done after the fact, having more time to read the data, make the calculations, understand the issues, and be aware of the outcome, may provide an accurate figure if the original fuel quantity is correct, "but lets not make matters worse by just guessing."

GACspaceguy
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posted 08-02-2019 12:22 AM     Click Here to See the Profile for GACspaceguy   Click Here to Email GACspaceguy     Edit/Delete Message   Reply w/Quote
Well written, thanks.

crash
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posted 08-02-2019 01:16 AM     Click Here to See the Profile for crash   Click Here to Email crash     Edit/Delete Message   Reply w/Quote
Great explanation, thanks.

Let's not get into the fixed thrust element of the APS during an abort when it would have presumably weighed a lot more than it would have done if the mission preceded as planned, which it did of course. Incredible work by the people on the ground who had to take all the many variables into account.

Space Cadet Carl
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posted 08-02-2019 12:45 PM     Click Here to See the Profile for Space Cadet Carl   Click Here to Email Space Cadet Carl     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by David C:
No, the call was "fuel depletion," as per my very long post above.
Thanks. So, the actual audible call would have been: "Fuel depleted." We're glad we never had to hear it.

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