Really big ships taking off and landing

[Batman]

Assuming the ship in question HAS a way to get orbital on antigravity or something, how HIGH an orbit would it need to be before they can fire up their main engines without undue worry about wear and tear on the planet, and UNTIL they do, what's the fastest acceleration they can afford without the displaced (and presumably superheated/ionized for the high end figures) seriously fucking up the local airspace/weather patterns/countryside?

[Junghalli]

The one place where I can think of rocket exhaust having serious weaponization potential is terrorism. Unassuming freighter pulls up to giant space station for routine docking and then at the last minute turns its rocket on the station and fires it up.

Even that has some relatively easy countermeasures but I can sort of see it as something you could pull off without the intended target being implausibly stupid. Maybe.

In any remotely coherent universe there would be rules of conduct to avoid things like huge warships lighting up their main engines in atmosphere/where they'd do lots of damage, just like IRL planes are usually forbidden from doing very low passes over cities at supersonic speeds and such.

Although if they're taking off from a planetary surface that opens up the question of what they're using to get to safe heights.

As I remember one of the proposals for Orion was to use booster rockets similar to what the space shuttle uses to get it up to some height before starting with the propulsion bombs.

Eh, it generally really makes a lot more sense to leave the giant spaceships in orbit and use shuttles/mass drivers/space elevators/whatever for ground to orbit transportation.

[Batman]

I DID assume going straight up but the going 90° left once you leave the worthwhile part of the atmosphere works, I hadn't thought of that.
Conceded on the speed vs acceleration part, the atmosphere couldn't care less how quickly you got that fast.

[Surlethe]

Mind you, I think the Kziniti Lesson is somewhat overhyped, at least when it comes to space combat. Sure, being in the exhaust stream of any serious interstellar drive is probably going to be like being hit with a nuke ... if you're close to it. But I doubt it would be very well focused so I imagine it would probably spread out pretty quickly, making if a fairly short range and therefore probably shitty weapon. As I remember, in the archetypal example the Angel's Pencil was using a photon drive, which is actually more or less a giant laser of ridiculous power, which is probably why it worked so well there.

Why would the drive be not-well focused? It would be as well-focused as technology permits: the more focused it is, the more bang you get for your buck. Think of it this way: the momentum you get is the vector sum of the momenta of the superheated propellant shooting out the ass-end of your spaceship. The off-axis vector components of your drive cancel (if you have a competently engineered ship, because otherwise you will start to tumble), so the more tightly you focus the cone of your exhaust, the better your momentum transfer, and the more efficient your drive.

Working out the dependence of efficiency on the spread angle, under reasonable assumptions, is left as an exercise in vector calculus.

I always wondered what a rapid exit, or entry would do within an atmosphere. Assuming your super advanced technology can deal with heat from engines, radiation, etc, what about displacing air? Or plunking into an ocean, wouldn't that cause a Tsunami? VACATING an ocean would cause at least a whirpool wouldn't it? if we're talking about multi kilometer ships?

Well, here's how you figure it out. Model the ship as a flat surface with equivalent cross-section (e.g., an ISD taking off while remaining oriented "upright" relative to the planet's surface would be kite-shaped). Figure out the force of air resistance (this is dependent on your speed). As you move and air piles up in front of the tremendous ship, figure out whether you pass the speed of sound or the air superheats enough to ionize. Also figure out the pressure as the ship excavates air from behind it (this will determine the sort of wind you get).

Preferably, you should do this after you've taken a comprehensive class in hydrodynamics and heat transfer.

[Junghalli]

Why would the drive be not-well focused? It would be as well-focused as technology permits: the more focused it is, the more bang you get for your buck.

Well, perhaps we can figure it this way, how well-focused is the exhaust from a real life rocket?

A exhaust velocity nuclear rocket in this context is similar to a plasma weapon (quite likely would be one, depending on the engine type). Of such weapons Atomic Rocket says:

Silly as they are, plasma weapons are a popular SF concept that just won't go away. They are encountered in such diverse places as the original Star Trek TV series, the Traveler role playing game, and the Babylon 5 TV series. They play the role of a futuristic flame-thrower.

Their main draw-back is that they won't work.

Plasma is the so-called "fourth state of matter", and is basically hot air. That is, it is a gas heated to temperatures comparable to the interior of a star or the center of a thermonuclear explosion so that all the atoms are ionized. Unfortunately, according to the virial theorem, the plasma wants to equalize its internal pressure with the external, i.e., it wants to expand into a diffuse cloud of nothing.

John W. Lewellen says that a "plasma beam" could be thought of as an exceedingly dense, slow-moving particle beam. Personally that seems a little strained, but what do I know?

For a definitive analysis of the worthlessness of plasma weapons, I refer you to Stardestroyer.net. Executive Summary: they won't work for the same reason that a gun shooting steam won't work.

A terawatt torch should be deadly enough at close range, given that even omnidirectional nuclear bombs can be, but how likely is it that it would be focused well enough to still be a reasonably dense stream hundreds or thousands of kilometers from the nozzle?

I don't know but I note that the starship on a string Valkyrie concept has a pretty energetic engine and puts the rest of the spacecraft behind it, hanging on a 10 km tether, with sheer distance and a small shield counted on to protect it.

[Surlethe]

Hmm. I'm honestly not sure how to mathematically treat this, and I don't have time to look it up. If you figure it out, I'll read it, though.