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The point of Apollo 10 was to test the LM in Lunar Orbit, but without landing.

Why bother?

If the LM had any sort of bugs (say the upper stage wouldn't launch, or they couldn't re-dock), then the astronauts would be just as dead as on Apollo 11.

If anything, the in-flight abort added a further risk - if they're on the ground and the takeoff stage doesn't light, they can try to fix something (not much of a chance, but something). In Apollo 10, they'd be dead.

So what failure modes would be survivable in Apollo 10 that wouldn't be in 11+?

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  • $\begingroup$ You can also ask why would NASA want to have a manned test of the abort mode on the Shuttle? (They cancelled it because it was too dangerous, but had it been only somewhat dangerous, they would have done it). $\endgroup$ – 39a Feb 24 '17 at 20:25
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    $\begingroup$ It's not that bad. It was in orbit at all times, so if it couldn't return to CM, the CM could encounter it instead. If docking was impossible, astronauts might transfer through EVA. Although the spin LM entered was definitely dangerous - while CM could match the velocity it definitely wouldn't be able to match the spin. $\endgroup$ – SF. Feb 24 '17 at 20:29
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pericynthion is correct that the pacing item for the moon landing was LM weight.

In the case of ascent stage failure, CSM had fuel to maneuver to meet the LM. As pericynthion notes, this is an unlikely failure mode.

In the case of failure to dock, the LM crew would EVA to the CSM -- not a maneuver you'd want to undertake if you had a better option, but not all that risky.

As it happens, making Apollo 10 the landing was considered:

According to multiple sources, some consideration was given to assigning the first lunar landing to Apollo 10, which [Tom] Stafford commanded. It was ultimately decided that Apollo 10 could land no more than one month sooner than Apollo 11 could (because Apollo 11's lighter LM was required, and because the Apollo 10 crew would have needed some additional training), and that the additional tracking experience from a second lunar mission would be useful when planning the first landing. Apollo 10 therefore flew a lunar orbital mission as originally planned. Stafford later flew again as CDR of the ASTP mission, his fourth mission.

I can't find the reference, but I believe Stafford himself felt it would be better to make A10 a dress rehearsal flight rather than the first landing.

It's important to remember that Apollo 10 was only the second translunar flight ever, the second manned flight of the LM, and the third manned flight of the Saturn V. Every Apollo mission had had some anomalies, and it was by no means clear that the complete system was reliable. Another opportunity to work out the bugs was welcome.

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One aspect is that the Apollo 10 LM was not lightweight enough to land and return with safe reserves, even if it had been launched with full propellant, and a sufficiently lightweight one wouldn't be ready until later in the year. So with the end-of-decade deadline looming, Apollo 10 could be used to practice and gather information (e.g. about how the lunar mass concentrations affected a low altitude orbit) that could be applied to Apollo 11, at an earlier date than any landing attempt would have been possible.

As SF. mentions, you have the survivability of an ascent engine ignition failure backwards - on Apollo 10, the CSM could have rendezvoused with the LM. On surface missions the crew would likely have been stuck. Ascent engine ignition failure isn't a very credible failure mode though compared to other critical systems, because of its hypergolic nature, redundant plumbing etc.

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    $\begingroup$ Did they use redundant plumbing or redundant valves? $\endgroup$ – Uwe Jun 27 '17 at 16:42

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