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To be clear, is the reason this is not sufficient for flying forever is due to orbital mechanics making "point and shoot" not feasible if aiming in a straight line for the void? Or because the boost isn't sufficient to escape the planetary system's influence and thus still predominantly subject to its gravitation pull? Or both?
Yes
As long as the object doesn't reach escape velocity, it's in orbit and thus bound by the primary object. Now this orbit can by very weird and huge, but it's still a loop. Only above escape velocity does the loop "break" and leaving the object without a guaranteed return is possible.
So in theory if an object is in orbit and the boost is sufficient, it can just leave. However even then it is subject to the gravity fields and will make an arc instead of just a straight line. So "point and shoot" is never really an option. But often in movies a small spacecraft is seen making a small maneuver and somehow being seen as lost to space. That will for certain not be the case, a small boost just gives you a different orbit, but an orbit still.
For example the movie Life (2017) comes to mind. Spoiler alert. In the end they decide to use an escape pod to launch into "deep space" with the alien. The escape pod just points up and fires the rocket for a short while and now is lost to deep space forever. This is total nonsense. The reason escape pods can work with very little fuel is they often have just enough oompf (or delta-V if you want to be technical) to put the pod into a slightly lower orbit. This lower orbit means more drag from the atmosphere which slows it down further, lowering the orbit again etc. until the thing is slowed enough it can totally re-enter and land. It isn't like an escape pod pointed down goes to Earth and pointed up goes into outer space. The pod actually fires in the direction of the orbit, so horizontally, in order to slow it down.
Orbital mechanics get really weird really fast. For example slowing down can cause the orbit to become higher before it goes lower. And putting in energy sideways can alter the angle of the orbit just like those spinning flywheel desk toys. Playing around with orbits in Kerbal Space Program can give a better understanding and can even make the concept of delta-V very easy to understand. KSP players would be unable to watch the movie Gravity (2013) for example without screaming at the screen: "THIS IS NOT HOW ANY OF THIS WORKS".
It's hard to figure out, that's why we refer to hard things as "rocket science". It's not just the complexity of the rocket as a machine and engineering challenge. But also figuring out stuff like orbits, taking into account the different gravity fields of objects that are of note. Doing things like gravity assists or Hohmann transfer orbit, taking into account the influence the extremely thin atmosphere has. And remembering everything moves, so shoot for where the target is going to be, not where it is now.
Love the write up!
Another great example is how insanely hard it actually is to hurl something into the sun. Intuitively, people think gravity does the work. Like the escape pod, you should be able to just point at the sun and go.
But the moment you escape Earth's gravity well, you're still going almost (or more than) the speed of Earth. You're now orbitting the sun by yourself. The only way to reach the sun is to slow down and lose all of that orbital velocity. It's significantly easier to escape the sun than it is to actually touch it.
can confirm. which incentally led me to playing KSP with my nephew while everyone else watched the very boring movie.