The mystery of Apollo 10

[For a general refresher on the Apolllo 10 mission, the wikipedia article is good]

James Concannon writes...

         In their 2009 book Dark Mission, Richard Hoagland and Mike Bara wrote (p. 280)  "While the spacecraft [the Apollo 10 LM, Snoopy] was theoretically fully capable of landing on the moon, inexplicably, it was not given the capability to do so."

        In expat's critique of the book, he wrote: « FACT: Snoopy was emphatically NOT capable of landing (well perhaps, technically, it was, but not of successfully taking off again.) Grumman engineers had not yet implemented SWIP (the Super Weight Improvement Program) and the spacecraft was too heavy. More accurately, it would have been too heavy if it had been fully-fueled for a landing and takeoff. »

        It's not a widely-known fact that George Mueller, Director of NASA's Office of Manned Spaceflight, was actually in favor of a landing by Snoopy. He felt that, even if the astronauts couldn't exit the LM and walk on the lunar surface, just the fact of a brief touchdown would sufficiently fulfill JFK's challenge to land a man on the Moon.

        In Foothold in the Heavens (2010), Ben Evans wrote this, on the basis of an interview with Tom Stafford:

"Stafford told Mueller in no uncertain terms that if Apollo 10 was rescheduled to make a landing, "The flight crew won't be on it. There was just so much to do." The main problem was that 'his' lunar module, designated LM-4 and shortly to be re-named Snoopy, was overweight; it was only by a few kilograms, but still too much to satisfy the safety margins for a successful lunar liftoff. Grumman engineers had long known that LM-4 was earmarked for an Earth-orbital or lunar-orbital test flight, rather than a lunar landing, and had not subjected it to their Super Weight Improvement Program (SWIP)."

STATS: The mass of Snoopy, unfuelled, was 9,484 lb cf. 9,287 lb for Apollo 11 's Eagle. An overweight of 197 lb or 89 kg.

        On the basis of an online discussion with a space fan (in French, as it happens) I fell to wondering how come, if 197 lb overweight was a show-stopper for Apollo 10, it became possible to add a Lunar Rover weighing 463 lb for the later J Missions (Apollos 15-17). I'm a member of a Facebook group that regularly discusses space history and has several experts (James Oberg is one) who take an interest. So I posted to that group yesterday and got immediate response:

James Concanon: Snoopy was too heavy to have landed on the Moon and taken off again, right? The Descent Stage was 220lb heavier that that of Apollo 11. So how come for the J Missions it was OK to add a 463lb LRV?

Ronald Purviance:  Many things. Saturn V mods to increase payload, changes in way the LM was released for descent to the surface, and changes to the LM itself.

Alan McEwen: ...

Rolf Karlastad: Alan: If you're not too heavy to take off, more fuel = more delta V.
Even if you had too much fuel for takeoff, due to a lack of thrust, one could simply burn fuel until the thrust to weight ratio was greater than one, and you would lift off.
Sort of the opposite of a plane where you rely on lift.

Alan McEwen: I deleted my previous response because I realized it was wrong. Snoopy was short fueled in the ascent stage. That is why it could not have completed and an ascent to orbit. Thrust was not an issue. As I said it could lift off. There was just not enough energy potential in the amount of fuel to carry out a full ascent. The weight issue came into play with The descent. As you may remember , Buzz and Neil landed with almost empty tanks. If I'm not mistaken they also had 2 PLSSs and basic science cargo. Snoopy did not have those things if I remember correctly. Had Snoopy been fully fueled, plus carrying the cargo, it would not have enough descent propulsion fuel to make a landing.

John Breaux: Bigger descent engine is the main thing. Also in later missions the LM would be taken closer to the surface before undocking meaning it needed less fuel for the descent. I'm sure some of the bigger brains here can give you exact figures.

Shane Barry Penington: Absolutely Correct...BINGO you get the $64,000.00 prize! They also figured a more direct descent trajectory from a lower orbit...

David Paul:  Yes, in later missions they used the SM engine for the initial descent orbit insertion, letting that engine do some of the delta-v needed. Also, by the J missions, they had changed the fuel mixture ratio on the Saturn (I think it was the S-II) so they could get more payload up.

Greg Bigelow: At approx. 2:20 Tom Stafford himself says it was too heavy to land. https://youtu.be/DkS2BMeA2Io

Rolf Karlstad: he over simplified. Perhaps it didn't have enough delta V to land. Unless for some reason the landing legs were weak. See? Weight doesn't really factor in, only delta V. It didn't have enough delta V to land because it didn't have enough fuel, perhaps because it was too massive, and the tyranny of the delta V equation Delta V = natural log (mass fueled/mass empty) *isp *g
It's really the only factor at play here.

That's the History, now for the Mythology
        So, OK, I got my answer. But perhaps predictably it stirred up a well-known controversy about that mission. Gene Cernan must take part of the blame for this, because he himself suggested an alternative reason why Apollo 10 was not cleared to land.

 "A lot of people thought about the kind of people we were: 'Don't give those guys an opportunity to land, 'cause they might!' So the ascent module, the part we lifted off the lunar surface with, was short-fueled. The fuel tanks weren't full. So had we literally tried to land on the Moon, we couldn't have gotten off." --Cernan, quoted in Rocket Men by Craig Nelson

...and of course, some of those Facebook experts couldn't resist reviving the controversy:

Steve Pietrowski: NASA intentionally short-fueled the LEM so there was no way they could have landed and gotten back. Management assumed that the X crew would have tried for the surface if they had a viable chance. For the J-missions, they had better PDI, as well as a redesigned engine bell for the descent stage. I don't think the engine was uprated powerwise, just made more efficient.

Sarah Bowyer:  I don't think NASA short-fuelled the LM because they didn't trust the crew not to try and land on the surface! Yes, they were competitive, but not to the extent of ignoring orders and trying to land before the procedures had been worked out. Remember, when A10 flew, they had a good chance of being on a later lunar mission.

Steve Pietrowski:  I'm pretty sure I read it in "Chariots for Apollo." I don't recall management explicitly forbidding a landing either. They sort of just didn't bring it up; then mentioned, "by the way, you won't have the fuel." I know I've seen this story in the wild elsewhere. If anything it adds a layer to the legend.?????

James Concannon: That was not the reason for the half-empty tanks. The reason was they needed the spacecraft's mass at the moment of rendezvous to be exactly what it would have been if it had taken off from the surface.

Tom Faber: Will this myth that Snoopy's ascent stage was short fueled to keep the crew from trying to land ever die out? No, that is NOT the reason. It had a reduced propellant load so that the rendezvous maneuvers with the CSM were flown with the ascent stage at close to the same mass as one had after an actual ascent from the surface. If it had a full load its handling characteristics would have been very different.

Besides, these men were professionals. They would not have disobeyed orders. And what would have happened if they did? They would NOT have been heroes. They would have forever been branded as the crew who would follow orders. It would have cast a pall over the whole program. Congress may have pulled the plug on the rest of the program right there.

Ronald Purviance: Every time I hear this BS rumor, it makes me mad. It totally dishonors the crew.

Greg Kennedy: I’ve posted to similar threads several times. Suggest you check NASA history publication “Apollo by the Numbers”. It has statistical data for all the missions including vehicle weights, propellant loads, etc.

Just the facts
        I took Greg Kennedy's recommendation, and looked up the document. Here's a link to the ToC. In the tabulations labeled Spacecraft Key Facts, I found two very interesting facts.

FIRST, Although the Descent Stage of Snoopy is confirmed as overweight, the Ascent Stage was actually lighter than that of Eagle: 4,781 lb cf. 4,804 lb. So if that's true, why would there be any concern about Snoopy's capability to take off?

SECOND, The rated thrust of the Apollo 10 Ascent Stage engine is given as 1,650 lbf cf. 3,218 lbf for Apollo 11 and all subsequent missions. Even the Apollo 9 figure cited is 2,524.

        Nobody in the expert group ventured any explanation. So there is a mystery—two mysteries—about Apollo 10 after all.