Going over the Apollo fuel numbers and I have many questions

Question

I was reading through Apollo by the numbers and I noticed the LM descent to the moons surface consumed 17,414kg of fuel out of a total of 18,184kg leaving only 770kg of fuel.

But then in the very next table it says LM ascent consumed 4,836kg of fuel... when there's only 770kg available. This is with all the extra weight of rocks btw, that they somehow took off with nearly 1/5th the fuel it took to land, which was also somehow more fuel than was on the ship.

Is the data being presented badly? Is it just plain wrong?

Would love some explanation, new to this level of detail. Hoping there's a better explanation for the crazy numbers than "they had a fuel tank for descent and a separate one for ascent and for some inexplicable reason the fuel in the ascent tank isn't counted as being on board during descent".

I'm also pretty sure they didn't preemptively drop fuel tanks on the moon so.... Help?

I'd like to add that I would love more resources on detailed research. All the numbers, for absolutely everything. Museum of science and industry doesn't get that detailed.

KSP, which takes place at miniaturized scale, makes it really clear just how insanely difficult landing on, then returning from the moon is. According to the numbers just the fuel in the LM weighed 53,000lbs. I'm intimately familiar with delta-V, so big???Roughly 6380 gallons, which would require a tank 4ft high by 90ft long to store. I've touched the LM... it's maybe 15ft diameter.

I have many, many more questions, numbers aren't making sense. Send help.

Answer 1

Data isn't wrong. The LEM was a 2 stage vehicle, with separate engines and tanks for landing and ascent. The bottom part was the landing stage, the top part was the ascent stage. The fuel figures were separate for each stage, which is sensible as neither stage could use the other's fuel. During descent the ascent stage's fuel was simply payload.

Answer 2

According to the numbers just the fuel in the LM weighed 53,000lbs. I'm intimately familiar with delta-V, so big???Roughly 6380 gallons, which would require a tank 4ft high by 90ft long to store. I've touched the LM... it's maybe 15ft diameter.

Where are you seeing 53,000 lbs?

Apollo by the Numbers has the total LM weight, not just fuel, at about 33,000 lbs ("Launch Vehicle/Spacecraft Key Facts" in the summary tables PDF).

The tables "LM Descent Stage Propellant Status" and "LM Ascent Stage Propellant Status" show, for example, Apollo 11 having 18,184 lbs of propellant (fuel + oxidizer) loaded in the descent stage and 5,238 lbs in the ascent stage.

As GdD notes, the LM is a two-stage vehicle; the stages and tanks are arranged like this:

The fuel and oxidizer are stored separately; for the descent stage we have fuel: 6,975 lbs, oxidizer: 11,209 lbs.

The fuel is not very dense, 793 kg/m^3 -- sorry about the mixed units, but Wolfram Alpha can handle it -- yielding 1054 gallons. The oxidizer is denser, 1440 kg/m^3, yielding 932 gallons.

Four tanks, each 4 feet in diameter and about 6 feet in length, can store that volume, and the descent stage is in fact mostly fuel tanks.

Answer 3

The table on page 295 that you are looking at in Apollo by the Numbers is in fact just showing propellant consumed by the descent module. Since you are looking only at the Apollo 11 numbers then I will show just the pertinent numbers from that mission from the tables:

The 17,414 lbs of propellant consumed that you are referring to was in fact just propellent in the descent module propellant tanks, this number does not include propellant that was in the ascent module tanks.

The table on page 307 shows weights at various points in the mission, here I show just the two pertinent LM weights:

These two weights include both the LM ascent and descent stages as well as all propellant, and I would assume the weight of the astronauts and gear also. Subtracting these two numbers indicates that 17,530 lbs of propellant was consumed during the descent. However the previous table showed that only 17,414 lbs was consumed by the LM descent stage, which is a difference of 116 pounds. Some of this would have been RCS consumption, since all of the RCS thrusters were located on the ascent stage. So even RCS propellant consumed during descent would be included in the ascent stage propellant usage. However the table on page 296 for the ascent stage only shows a total of 69 lbs of RCS propellent used:

These values are not broken down by descent vs. ascent, so presumably this was the total RCS propellant used during both parts of the mission. But this still doesn’t account for the entire 116 pounds. Perhaps the consumption table for the LM descent stage used the later revised lower remaining propellant estimate (15 seconds fuel remaining adjusted to 45 seconds), but the mission weights table for the descent stage used the original lower estimated landing weight, when it should have been revised higher.

As you indicated the ascent stage used 4,836 pounds of propellant:

And yes that is 1/4th the amount of propellant consumed by the descent stage. But that’s because during landing the descent/ascent stage combination weighed between 33,683 and 16,153 lbs. Whereas during ascent the ascent stage weighed only between 10,776 and 5,738 lbs. This includes the 47 lbs of soil samples that they brought back with them. Although prior to liftoff they tossed out their PLSS (primary life support system) backpacks, boot overshoes, and a bag filled with miscellaneous trash, which at least in the early missions more than compensated for the weight of rocks that they brought back. Later missions brought back increasingly larger amounts of lunar samples, the most was Apollo 17 which brought back 250 pounds of samples. Even though this was more than the weight of the equipment that they left behind, it was still within the ascent stage fuel margins.