[General Battuta]

Well, jump drives are, by definition, not governed by the laws of physics as we know them. So, maybe there's an arbitrary limit on jumpout speed? Since ships cannot, for some reason, exceed an arbitrary speed limit relative to whatever reference frame they're all using, jumpdrives probably have the exact same limitation. It seems to be some sort of "snap to gravity well" kind of thing. That would also probably explain limited ship speeds. Since they all jump in into a common reference frame, their relative velocities would not be very high. The thing with phlebotinium is that since it isn't realistic in first place, all limits on what it can do and what it can't are set by the author.

you have to accept two things with subspace:
you must enter and exit at a very low speed, or TERRIBLE things happen
if you don't stick with that, the whole game falls apart

the way i have generally rationalized away the low speed of everything so far is that if you are flying too fast, the ship computers  and small side thrusters can't handle all the destabilizing effects of small particles hitting the ship, and then everything goes haywire
that's of course total BS, but hey, at least it's SOME reason

Jump drive requires a lot of energy, presumably part of this energy may be used by the jump drive to match velocity with the other location. Typical motion of stars is on the order of tens of kilometers per second. If my calculations are right, a 100 ton ship making a typical interstellar jump with 20km/s change of velocity would thus require at least 20 TJ of energy. Fusion reactor with 1GW output can recharge this jump energy in 5.5 hours. On the other hand relativistic projectiles would need orders of magnitude more energy, making them not feasible.

All this withers in the face of this

The problem is that if you accept relativity you can jump out at any speed you want, because there's no privileged referenced frame (and besides, if you can match velocities on interplanetary jumps you can reach kill vehicle velocities no problem).

The velocity differential involved in jumping from, say, an inner to an outer planet is spectacular, and easily enough to turn your ship into an impactor with kinetic kill capability (or, alternately, unstoppable 'peace, mother****ers' exit velocity). You can't just 'snap to the gravity well' because you must arrive at your location with a specific vector - if you have the capability to match orbits with a given ship, and another given ship in an orbit with exactly opposite inclination, you have the capability to arrive in a head-to-head collision with anything in orbit of the local body. Put in simpler terms: if you can place yourself into an arbitrary orbit of a target body (and apparently you can), you can place yourself into an arbitrary collision orbit.

There are probably ways to finagle some fake physics to make it all work: the relative velocities of your entry and exit frames are (as suggested above) constrained by the power output of your ship...but this still leaves you with the nightmare conclusion that the optimal weapons are fighter-sized drones with subspace drives that spot targets and then jump to maximum possible relative velocity.

This reminds me a bit of the 'sound in space is all rendered by your cockpit!' argument in that it's a fundamentally doomed attempt to work backwards from a narrative and design conceit to create something 'realistic'. No cockpit is going to render a series of nearby blasts as static-washed clippy booms, it's going to deliver a series of carefully calibrated informational sounds that convey useful information about the blast to a trained pilot. The subspace drive has the capability to move ships instantly between very disparate reference frames even at intergalactic distances (though who knows how the hell nodes work - the node connection geodesic may make the velocity differential irrelevant), so there is no way to avoid them being really, really useful for creating kinetic kill vehicles even if you restrict them to a narrow band of delta-V and try to create privileged reference frames. Dragon's right, of course, that the author has pretty wide license in establishing limitations, so I guess you could create some kind of rule that says...huh, no, I actually can't think of a way to do it that's physically consistent.

The best solutions are, of course

I think it's obvious every single point in space is pegged to a local velocity system we should henceforth call "Plot Velocity" and it masterfully manages every single ship nearby to synchronize their relative speeds to match all other local reference frames down to an error margin of 200 m/s, which coincidentally is on the same order of magnitude of speed of the anti-Newtonian dampened engines prevalent on all ships due to something called "Game Design Constraints". Any scientist who has scratched his head thinking about this problem has always been strangely attacked by some form of psychological malady that we believe is originated by some advanced form of MYOB* hacking attack from unkonwn alien origin. Further analysis is deeply unadvised precisely for the latter referenced reason.

*Mind Your Own Business.

~subspace is magic~.

[fightermedic]

I mean low speed relative to whatever. Yeah, it doesn't make sense itself, but it makes something else make more sense.

To be honest it can make perfect sense considering ~subspace is magic~.

exactly
subspace is the one thing that doesn't need to make sense in any way

[Lepanto]

Well, thanks guys, I guess the community's opinion is obvious.

Personally, I just sometimes loosely imagine that the battles are taking place at pseudo-realistic large distances (without really thinking about the physics involved; I'm no physics expert anyway.)

~subspace is magic~.

I used to wonder what subspace could be . . .

[Dragon]

You can't just 'snap to the gravity well' because you must arrive at your location with a specific vector - if you have the capability to match orbits with a given ship, and another given ship in an orbit with exactly opposite inclination, you have the capability to arrive in a head-to-head collision with anything in orbit of the local body. Put in simpler terms: if you can place yourself into an arbitrary orbit of a target body (and apparently you can), you can place yourself into an arbitrary collision orbit.

Note, at no place in FS does the ship warp into an arbitrary orbit - quite the contrary. They all arrive in the same, likely prograde orbit with only very slight variation in inclination, an it usually seems to be an equatorial one, too, or at least not far off it. You could probably fiddle with gravity distribution and define a certain privileged set of orbits in which it's easy to open a jump node. That doesn't exactly fix everything, but since jumpdrives don't seem to very accurate as well, it's possible that warping something in close, but in a highly different orbit was never precise enough to be a viable weapon system (notice noone thinks of jump-ramming, even without relativistic velocities, ships to move quite fast at warpout).
That would handle intrasystem travel, for interstellar one, I've always assumed this:

the node connection geodesic may make the velocity differential irrelevant

No matter what, the ship comes out of the node at a set velocity vector (see: blocades, not only you can predict how fast the ship goes, but in which direction!), and node itself seems to have a very well defined velocity as well. It's an inherent law of how jumpnodes work, apparently.

[General Battuta]

You can't just 'snap to the gravity well' because you must arrive at your location with a specific vector - if you have the capability to match orbits with a given ship, and another given ship in an orbit with exactly opposite inclination, you have the capability to arrive in a head-to-head collision with anything in orbit of the local body. Put in simpler terms: if you can place yourself into an arbitrary orbit of a target body (and apparently you can), you can place yourself into an arbitrary collision orbit.

Note, at no place in FS does the ship warp into an arbitrary orbit - quite the contrary. They all arrive in the same, likely prograde orbit with only very slight variation in inclination, an it usually seems to be an equatorial one, too, or at least not far off it.

We really don't have any canonical evidence for this. All we see in canon is ships arriving at a local rest relative to other objects in orbits. Some of these orbits are quite unusual: the transports in Surrender, Belisarius are moving away from the planet on fusion drives, so it's either highly elliptical or a brachistochrone escape trajectory. That's not an equatorial orbit, and in fact it's implied to be an escape orbit, so it seems that a subspace drive can put you in any orbit you please. In other cases ships arrive at installations that might be located at Lagrange points, or at relative rest to bodies that have very unusual orbital configurations or don't seem to obey orbital mechanics at all.

A great case of the latter is in High Noon: the Sath and Colossus are at relative rest to the Vega and Gamma Drac jump nodes, which are at relative rest to each other. Yet by the time of Their Finest Hour, the Gamma Drac jump node has moved far away from the Vega node - confirmed just two missions later in Apocalypse, when the Vega node is nowhere near the Gamma Drac node! Either these two nodes had a very close periapsis on elliptical orbits and then moved apart, or they skip around erratically as in BP canon. In the former case, we can conceive of a subspace drive dropping these ships at relative rest to the nodes, but if the latter, the drives must have picked a pretty weird velocity vector to arrive on.

So: two orbits, one of them definitely not a low-inclination equatorial, the second possibly a low-inclination equatorial but also possibly not an orbit at all. The idea that jump drives can only reach a narrow range of orbital configurations also violates parsimony: why aren't installations maneuvered into inclined or retrograde orbits, and thus rendered totally unassailable by subspace? What about all those deep-space engagements nowhere near an orbiting body - why didn't the NTF jump its ships to safe points in Gamma Drac and then use fusion drives to burn into 'illegal' target orbits, rendering them totally inassailable? It just doesn't work with the canon.

The reason nobody thinks about using jumps as weapon systems in FreeSpace is because that would make the story and gameplay bad, so the writing wisely just ignores it. It's impossible for some magical reason. The canonical precision of jumps is probably adequate for jump ramming to be a great tactic - the Psamtik's botched jump to Knossos II is only off by a few kilometers and we routinely see ships hitting much smaller targets, like the Colossus' faceoff with the Repulse.

[Dragon]

It might also have to do with the fact FS physics hate capship-capship collisions. I could see this being utilized in a mission, if the collision physics weren't so bad.
As for other problems, TBH, canon isn't really consistent about jump physics. Even "magic" explanation would be problematic for some of those points, because even magic usually has some rules. I mean, what's the point of talking orbital mechanics when things don't obey them even without jumping? Surrender, Belisarius is especially problematic, especially since burning radially doesn't really help you escape unless you're running a very powerful brachistochrone (on order of 1G or more), and if they were doing that, they'd vanish off mission in a few seconds due to huge acceleration required.

For the Vega-Gamma Drac node problem, I prefer just to take BP canon explanation. Any other only introduces bigger problems.

Also, there's little consistency on just how much FS sublight drives can do. If gameplay is anything to go by, then there's a good reason for nothing being in retrograde/high inclination orbits. Namely, they won't be able to get into them with those pathetic engines. Inclination changes are bloody expensive, and retrograde orbits are an extreme example of this. TWR also matters, changing inclination with an RL orbital engine takes ages. On the other hand, FS ships do seem to have unlimited dV, and there are, IIRC, quite a few cases that imply their TWR is very good, too.

[General Battuta]

Yeah, I agree. Basically it comes down to the fact that they have excellent sublight engines that could give them fantastic delta-V because they can seemingly burn forever, except that they have magic velocity caps.

[Dragon]

They're exactly the same type of "Sci-Fi space engine" Star Wars use. The closest thing to them I know of would be a magnetic confinement fusion drive, which has Isp of a few million seconds and can achieve quite high thrusts if the confinement magnets are good enough (a long shot, but nothing impossible). Though we also know that FS ships have separate reactors and engines, which implies some kind of electric (but not ion, too high thrust) engine, or at least an engine which can't produce electricity. There's also the velocity cap, which might be possible to explain in combat (B5:TGOS has flight physics in which you'll quickly realize this, keeping relative velocity down is required for any sort of combat), but less so during transit, where you're certainly better off with some sort brachistochrone if you've got a fusion drive. Oh, and that's not even mentioning the afterburner, which is among the strangest pieces of FS tech, physically speaking. It allows a ship to make well over 10Gs for a short amount of time, then drains energy to prevent velocity from dropping just about as fast. The best assumption would be that sublight engines are also based on some kind of subspace technology, which means they're just as good phlebotinium as the jump drives...

[AdmiralRalwood]

(B5:TGOS has flight physics in which you'll quickly realize this, keeping relative velocity down is required for any sort of combat)

And before that there was B5:IFH:D&O, in which attempting to accelerate to high speed just results in you flying straight past the battle and having to spend quite some time decelerating and then re-accelerating in the right direction (the bigger question is why your weapons have such a pathetic range, but hey, every sci-fi universe has its ~magic~ somewhere, right?).

[Dragon]

Well, with plasma weapons, you'd expect the range not to be too good. Which is a major reason not to use them IRL. I really liked TGOS and it's predecessor, B4:The Minbari Project, they had a nice, pretty realistic (if difficult to master) flight model.