I have a sci-fi setting that takes place very far in the future. I like to think it tends toward the harder side of sci-fi, so I'm trying to minimize the number of Clarketechs (fantastical technologies) in it, but a big one that they do have are these propulsion systems that basically convert electrical power directly into thrust. I would hesitate to call them reactionless drives since they do eject mass, just that the mass is electrically created on the spot rather than carried onboard the craft (never mind the implications of that; I have it all worked out). I'm not sure if that makes sense mathematically, since mass is a form of energy, but that's the basic idea (I'm sure there's some way to tie it into the whole E=MC^2 thing, but that's above my paygrade). Thus, the ships in this setting don't carry propellant onboard so mush as they just carry fuel for their fusion reactors, which in turn power their propulsion systems. Feel free to comment on that, but it's not the main goal of this question.
Space Launch Systems
In this setting, I've decided that planetary gravity wells remain prohibitive, so that ships are generally built in space and stay in space. Travel between orbit and planetary surfaces is mainly facilitated by infrastructure such as orbital rings, space elevators, and other spectacular megastructures. In fact, megastructures are a large part of the setting, all the way up to things like shell worlds and Dyson swarms, so that most people dwell in such artificial worlds rather than on planetary surfaces (because there's only so much surface area on a planet).
But ground-to-orbit infrastructure, and its associated mass-drivers and centrifugal throwing effects, are not always available or practical, especially in uncolonized star systems or during military operations. I want to fill this need in a way that feels "grounded" and realistic, that gets the point across about how hard planetary gravity wells are on space travel, the sheer violence of fighting gravity. For this reason, I've been considering having there be specialized rocketship-like vessels that land and lift off vertically, like modern launch vehicles, but much more powerful, more durable, and prettier. It's not something you see much of in sci-fi anymore. Mostly, it seems like people go for the more space-plane oriented approach, and so I think it would be a neat touch if I did it this way. A lander would basically look like a tower or pyramid on stilts or crab-like legs, an edifice in the middle of a barren landscape.
Image credit: Angus Mckie
But is it The Most Efficient Way?
I'm not a rocket scientist, so I guess what I'm really asking is what is the reason, ultimately, for a vertical design/tower shape? For modern launch vehicles, I take it that it's 1) to minimize drag with the atmosphere, and 2) to have multiple stages that can be detached and left behind as they are emptied of fuel. In my setting, these launch vehicles have much more powerful engines, presumably don't need to carry reaction mass or have stages, and are reusable as complete vessels with all systems remaining attached, not like our modern cutting-edge reusable rockets, which return to the ground immediately after use. I'm wondering, then, if there is some other reason why one would design landing/ascent craft to go straight up and down and then horizontally to enter orbit, rather than just taking off horizontally and spiraling out until orbital velocity is achieved outside the atmosphere, like in the idea of a space plane or similar craft (e.g. the landers from the film Interstellar, which needed to be lifted off from Earth with rockets for some reason but could take off from other planets just fine).
Basically, I'm looking for technological and physical reasons why vertical takeoff-style landers, of various sizes, I might add, would be more feasible or not for a civilization that has these energy-to-thrust engines. What is it about horizontal types of takeoff that make entering orbit so difficult? Is it just that they necessarily pass though more of the atmosphere and so experience more drag. And if they have engines capable of doing this, do they really rely so much on ground-to-orbit infrastructure. Sorry I can't provide specifics about thrust output or anything like that, but, as I said, I'm no rocket scientist, and this technology is, to my knowledge, unexplored territory. Anyway, I welcome any level of expertise in the responses.