I'm helping a friend with a hard sci-fi story and we ran into an issue. In this setting the technology for absurdly powerful and efficient fusion drives which seem not to have heat managment issues is ubiquitous across the solar system.
Reading a lot of The Expanse and ProjectRHO has told me that running an absurdly powerful engine is akin to detonating a modestly sized nuclear bomb in a pillar directly under your ship, continuously. Which would probably play hell with an atmosphere, should you try to use said engine in one.
I can count half a dozen reasons why, but mostly from shockwaves as the exhaust slams into the surrounding air, turbulence from the speed and temperature difference, and reflected heat energy screwing with the magic thermal managment system which seems to simultaniously not exist and keep the engine from melting.
I'll pretend that radiation isn't a concern here because this is a direct 'lensed' fusion drive burning some nondescript aneutronic fuel, so I'll leave that here.
Earth's navy force is built and maintained mostly in orbit, but rarely ships are built or brought down planet-side for repairs or secretive operations. So as to not go into detail let's just pull an any-ship.tm to use as an example. Let's pick the Leonidas-class from The Expanse. Noting the donnager's (only ship with a cannonical mass) quarter-million tonnes and 400-odd meter length, lets say our any-ship is 200-odd meters long and 90 thousand tonnes. Probably the biggest ship that could be brought down.
Bringing the ship down the gravity well should be relatively strait-forward. Turn on fusion engines and slow down from LEO-speed to hanging over the ocean with no relative velocity, drop strait through the atmosphere, and then using chemical booster skiffs attached (somehow) to the ship, slow it right down before splashdown off the coast of wherever the spaceport is, and use a tugboat to tow the ship home.
But how do you do this the other way around? A (practical) chemical booster, or hundreds of them, couldn't get this thing anywhere near the Karman line, where you can just kick on your fusion drive and burn the rest of the way to orbital speed.
Is there a way around this? Something better than chemical rockets, but not as powerful as LFDs?
Would the ship need to get into space to safely fire its LFD or could the boosters get it as far as the tropopause, where the drive might be able run for a precious few seconds without melting itself. (Probably out in the middle of the sea, as the shockwaves and noise would still be insane. Stand next to Starship IFT-2 at point-blank and then multiply by a few orders of magnitude.)
Could the booster skiffs use some modified fusion thermal rockets or airbreathing-something? Less powerful than an LFD but powerful enough to get to space? Maybe some kind of fusion drive designed to run in-atmosphere, at least partially? Perhaps a combination of the above?
If all else fails I guess we can ditch the entire idea, because as far as my understanding of physics goes this is burning up some of this book's precious supply of suspension-of-disbelief-ium.