A simpler version of the Rocket Equation:
For a reasonable mission, Δv must be smaller than ve.
Now, this is not a strict rule: you can stretch it, but the mission profile won't be reasonable. By the same token, if you can swing it, you really want a margin on this. But as a quick rule-of-thumb, it's true enough and simple: Δv<ve.
Δv is a measure of distance in space, which adds up just like normal distances: it takes 9-10 to get from Earth's surface to orbit, then 5½ to get to the moon's surface, for a total of about 15. On the way back you can aerobrake, so it only comes to 2.3. All up, a moon mission is around 17½. There's a nice chart here, giving the Δv distances around the Earth, Moon and Mars.
ve depends on the rocket engine you use: the rockets we have top out around 3-4. The Space Shuttle engines make 4½, but they use hydrogen, which is somewhat inconvenient (it needs to be kept cold, and it's bulky - bigger tanks, bigger pipes, bigger pumps). Ion drives make 30 easily, but you can't use them to take off from a planet - only to change orbits.
Of course, if the ve we can get is in the region of 3-4 and even the shortest orbital space missions need a Δv of 9-10, it doesn't exactly satisfy the rule Δv<ve - which means that we would expect the missions not to be reasonable, and the moon missions even more so.
Thus we find them: our space rockets are huge, staged monsters, while the ships themselves are tiny, made from exotic metal alloys so as to be as light as possible, usually discarded after just a single use.
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