From a similar point of view, you could say fluid mechanics is basically just F=ma.
Even just orbital mechanics has a decent amount of complexity. Transfer between celestial bodies in general requires solving the three-body problem (or four for transfer between planets). And it's one thing to solve for just the trajectory of an object given some initial conditions, and another to figure out the correct initial conditions to give to get the trajectory you want, all the while staying within a fuel budget. Then you have to contend with not having instantaneous impulse in real life.
A simplification here is that you can get a decent approximation to the three/four-body problem with patched conics, which is where you assume that the gravity outside of a celestial body's 'sphere of influence' is zero, and within that SOI, all of the gravity comes from that body; in this way, you can treat it as a series of two body problems, where you 'patch' together the solutions (which are conic sections) for the orbital trajectories of the all the bodies involved. This is by no means a perfect approximation, though, and in practice I would expect that one would want to check a given solution found with patched conics with a more complete n-body simulation.
Even simpler than this, though, at least mathematically, is orbital rendezvous. Here, you only have to contend with the gravity of a single body. Yet it's very difficult to get the timing right, and the first couple attempts by the USSR and the US failed, and Buzz Aldrin even submitted a doctoral thesis based entirely around orbital rendezvous (two spacecraft meeting in Earth orbit):
> In its first human spaceflight program Vostok, the Soviet Union launched pairs of spacecraft from the same launch pad, one or two days apart (Vostok 3 and 4 in 1962, and Vostok 5 and 6 in 1963). In each case, the launch vehicles' guidance systems inserted the two craft into nearly identical orbits; however, this was not nearly precise enough to achieve rendezvous, as the Vostok lacked maneuvering thrusters to adjust its orbit to match that of its twin. The initial separation distances were in the range of 5 to 6.5 kilometers (3.1 to 4.0 mi), and slowly diverged to thousands of kilometers (over a thousand miles) over the course of the missions.[1][2]
> In 1963 Buzz Aldrin submitted his doctoral thesis titled, Line-Of-Sight Guidance Techniques For Manned Orbital Rendezvous.[3] As a NASA astronaut, Aldrin worked to "translate complex orbital mechanics into relatively simple flight plans for my colleagues."[4]
> First attempt failed
> The first attempt at rendezvous was made on June 3, 1965, when US astronaut Jim McDivitt tried to maneuver his Gemini 4 craft to meet its spent Titan II launch vehicle's upper stage. McDivitt was unable to get close enough to achieve station-keeping, due to depth-perception problems, and stage propellant venting which kept moving it around.[5] However, the Gemini 4 attempts at rendezvous were unsuccessful largely because NASA engineers had yet to learn the orbital mechanics involved in the process. Simply pointing the active vehicle's nose at the target and thrusting was unsuccessful. If the target is ahead in the orbit and the tracking vehicle increases speed, its altitude also increases, actually moving it away from the target. The higher altitude then increases orbital period due to Kepler's third law, putting the tracker not only above, but also behind the target. The proper technique requires changing the tracking vehicle's orbit to allow the rendezvous target to either catch up or be caught up with, and then at the correct moment changing to the same orbit as the target with no relative motion between the vehicles (for example, putting the tracker into a lower orbit, which has a shorter orbital period allowing it to catch up, then executing a Hohmann transfer back to the original orbital height).[6]
> As GPO engineer André Meyer later remarked, "There is a good explanation for what went wrong with rendezvous." The crew, like everyone else at MSC, "just didn't understand or reason out the orbital mechanics involved. As a result, we all got a whole lot smarter and really perfected rendezvous maneuvers, which Apollo now uses."