Movement in space.

[Ford Prefect]

A while back there was a thread in OSF about the vows all sci-fi writers should take, and this one came up:

When writing about space battles I will actually research what movement in space is like instead of blithely assuming it's the same as aircraft combat.

So, here I am, and I wonder, how does movement in space actually happen?

[Sharpshooter]

Certainly not like in the air, that's for sure.

Because there's no gravitational force in space that puls upon a vehicle in a specific direction, an object will always keep moving in a specific direction when motion is made, whether or not said motion is continued. Basic example, if a ship flies forward, then does a ninety-degree climb, it will continue to move forward at the same speed it was moving before initiating the climb because there is no counter-force to stop said motion, whereas if you did this in an atmosphere, gravity and air resistance would cause your forward speed to drop. This thus requires that a ship either spins a hundred and eighty degrees and applies an equal amount of force that it deployed earlier to bring itself to a standstill before beginning its next maneuver, or simply deploys a series of vernier and reverse thrusters at the fore area of the vehicle so that they can do that without the need to muck about with turning.

Basically, it means that there's a lot of drifting, a lot of spinning and applyig counter-forces, and a whole lot of wallowing in space.

[Imperial Overlord]

There is gravity in space. But the pull is usually pretty feeble.

[Lord Zentei]

When you are not in planetary orbit, you are in solar orbit.

When you are not in solar orbit, you are in galactocentric orbit (or in orbit around some massive nearby object or complex combination of orbits).

Simple orbits - around a single body- can be circular, elliptical, parabolic or hyperbolic. The former two leave you bound to a system, the latter two allow you to escape the system (causing you to start orbiting the next higher order system or neighbouring system).

Complex orbits - around multiple bodies - can be extremely complex and unpredictable.

Powered spaceflight causes you to move from one orbit to another, either of a similar type of orbit (for instance to a higher elliptical orbit or from an elliptical orbit to a hyperbolic one) or of a different type. This can be very energy demanding.

[Molyneux]

Closest I can come to understanding zero-G flight (far from any large mass such as a planet or star) is the game "Asteroids"...

Of course, in the gravity well of a massive object, that would go right out the window, as you'd have to worry about the force of gravity affecting your flight path and imparting strain on your ship.

[outcast]

anothing thing to take into account is the lack of friction

[Admiral Valdemar]

Play Independence War or the sequel and then you'll see what a real space-sim is like. None of this X-Wing or FreeSpace crap, the real deal instead. When you accelerate to 20 km/s, it takes as much power and distance to decelerate back to a stop again, you do not instantly come back to zero km/s.

When you want to turn, you use thrusters all the time and unless your thrusters are amazingly powerful, as you yaw when flying forward, you will slide. This can, however, be used to your advantage. Many a time I have copied the Furies from B5 and flown one direction in a patcom sized vessel and pointed my hull and thus my fixed guns another direction. Bandit on your six? No problem. Accelerate then flip 180 and blow him to pieces while still moving.

There is infinitely more freedom flying via Newtonian mechanics than you see in soft sci-fi where it's just like flying in an atmosphere which is entirely incorrect.

And the only time you are really in a zero G part of space is when you're in a Lagrange point which is where the gravity ofa body like a planet and moon cancel eachother out (in I-War, these allow for jump points since gravity in the game affects the energy required to open a wormhole to jump).

[Dakarne]

Nexus, the PC game had ships with thrusters on every side... for turning and stopping, I found it to be more realistic than other sci-fi games I've played..

[fgalkin]

Play Independence War or the sequel and then you'll see what a real space-sim is like. None of this X-Wing or FreeSpace crap, the real deal instead. When you accelerate to 20 km/s, it takes as much power and distance to decelerate back to a stop again, you do not instantly come back to zero km/s.

When you want to turn, you use thrusters all the time and unless your thrusters are amazingly powerful, as you yaw when flying forward, you will slide. This can, however, be used to your advantage. Many a time I have copied the Furies from B5 and flown one direction in a patcom sized vessel and pointed my hull and thus my fixed guns another direction. Bandit on your six? No problem. Accelerate then flip 180 and blow him to pieces while still moving.

There is infinitely more freedom flying via Newtonian mechanics than you see in soft sci-fi where it's just like flying in an atmosphere which is entirely incorrect.

And the only time you are really in a zero G part of space is when you're in a Lagrange point which is where the gravity ofa body like a planet and moon cancel eachother out (in I-War, these allow for jump points since gravity in the game affects the energy required to open a wormhole to jump).

Well, b5IFH allows you to fly a fury with *gasp* actual physics. Many a times have I turned around to kill something behind me while continuing to accelerate forwards. It was...refreshing.

Have a very nice day.
-fgalkin

[Lord of the Abyss]

One of the more common laws ignored in Scifi space flight is "An object in motion tends to stay in motion". If you shut down the drive or blow something up, the object ( or it's fragments ) will keep moving.

[outcast]

didn't frontier: elite 2 have realistic physics? i seem to recall so. you needed front thrusters and such. hell, i remember one of the in-character reviews of ships that came with the game said something about one ship: "look how ridiculous this thing is, it's got BIGGER thrusters at the front than at the BACK"

[drachefly]

Zero-G? You're never in a zero G-field*. Certainly not at a Lagrange point. Perhaps you're thinking of free-fall. But that kicks in as soon as you lose atmospheric resistance and aren't landed on something (e.g. an airless moon)

Yes, vacuum combat is different. You can build up very high velocities, and if you do you have to spend some time slowing down and coming back, afterwards...

I think one characteristic of space combat that comes up from time to time may be extremely high relative velocities of the combatants, e.g. 1-20 kilometers per second (3600-72000 kph). The entire combat would occur in one pass. This would apply when one side rushes to engage the other (e.g. a chase situation). In an intercept situation in which the attacker thinks they will need multiple passes to achieve their mission, there may be much lower relative velocities.


* I am neglecting Gauge symmetry here. If you know enough to quibble on this point, you'll probably agree with the rest.

[Admiral Valdemar]

Zero-G? You're never in a zero G-field*. Certainly not at a Lagrange point. Perhaps you're thinking of free-fall. But that kicks in as soon as you lose atmospheric resistance and aren't landed on something (e.g. an airless moon)

I was defining it as basically being outside the effects of any net gravitational field exertion. You may still be within two gravity wells, but they nullify one another and so bring the "free fall" effect up. So long as the forces working on you from either direction in the immediate vicinity are cancelling one another out, it may as well be considered a zero gravity zone for this purpose (ignoring the extragalactic influence, in either case, given the way the wormholes need no localised gravity well, it has a purpose in this respect).

Yes, vacuum combat is different. You can build up very high velocities, and if you do you have to spend some time slowing down and coming back, afterwards...

A lot of designs in the I-War games have ships with large bow and aft thruster arrays. If you go full throttle forward, you can at least slow down as quickly should you need to. Course, the designs not like this can have you flip 180 and then burn your main thrusters in the opposite direction.

I think one characteristic of space combat that comes up from time to time may be extremely high relative velocities of the combatants, e.g. 1-20 kilometers per second (3600-72000 kph). The entire combat would occur in one pass. This would apply when one side rushes to engage the other (e.g. a chase situation). In an intercept situation in which the attacker thinks they will need multiple passes to achieve their mission, there may be much lower relative velocities.


* I am neglecting Gauge symmetry here. If you know enough to quibble on this point, you'll probably agree with the rest.

In those high velocity passes, I arm missiles. You can strafe with high-output Gatling weapons or beam weapons if you want, but you may miss. In this attack tactic, the added speed gives your missiles a nice little KE boost, less time on target and are generally going to hit 9 times out of 10. Real dogfighting I find works best around 5 km/s. The stuff you see in SW and ST etc. where they seemingly cross hundreds of klicks in seconds won't work when you actually want to hit something.

[drachefly]

Zero-G? You're never in a zero G-field*. Certainly not at a Lagrange point. Perhaps you're thinking of free-fall. But that kicks in as soon as you lose atmospheric resistance and aren't landed on something (e.g. an airless moon)

I was defining it as basically being outside the effects of any net gravitational field exertion. You may still be within two gravity wells, but they nullify one another and so bring the "free fall" effect up. So long as the forces working on you from either direction in the immediate vicinity are cancelling one another out, it may as well be considered a zero gravity zone for this purpose (ignoring the extragalactic influence, in either case, given the way the wormholes need no localised gravity well, it has a purpose in this respect).

That's a total brain bug. You are experiencing Free fall if the only forces acting on your ship are gravitational. End of story. You can be five inches from the surface of the moon and you are in free fall. You are roughly in free fall if you jump down the stairs. Nothing to do with gravitational cancellation.

Yes, vacuum combat is different. You can build up very high velocities, and if you do you have to spend some time slowing down and coming back, afterwards...

A lot of designs in the I-War games have ships with large bow and aft thruster arrays. If you go full throttle forward, you can at least slow down as quickly should you need to. Course, the designs not like this can have you flip 180 and then burn your main thrusters in the opposite direction.

... I don't see what you're getting at. If your ship can accelerate at 15G, and you accelerate at this maximum thrust for three hours, then engage the enemy at high velocity, then you'll need another three hours to decelerate, no matter where you put your thrusters.

I think one characteristic of space combat that comes up from time to time may be extremely high relative velocities of the combatants, e.g. 1-20 kilometers per second (3600-72000 kph). The entire combat would occur in one pass. This would apply when one side rushes to engage the other (e.g. a chase situation). In an intercept situation in which the attacker thinks they will need multiple passes to achieve their mission, there may be much lower relative velocities.

In those high velocity passes, I arm missiles. You can strafe with high-output Gatling weapons or beam weapons if you want, but you may miss. In this attack tactic, the added speed gives your missiles a nice little KE boost, less time on target and are generally going to hit 9 times out of 10.

I'm glad to see you agree with me completely. I don't see why you think I'd use lasers or gatling guns in these cases.

[Alan Bolte]

I never got the chance to play I-War. I did, however, greatly enjoy the Starshatter demo; I didn't buy the game because of some irritating AI issues. Missiles are definitely more effective when you're making a high-v pass, but in that game that wasn't really the norm. The problem is that your sensor range relative to your acceleration wasn't always sufficient to be able to build up enough velocity for such a fight. Anther problem is that you better hope they have poorer missiles or poor reaction times, because otherwise it's a head-to-head and they have the same velocity advantage you do. Of course, this is why they named the heavy cruiser class with powerful long-range missiles the Devastator.

As to equal thrusters fore and aft, that's much more appicable for low-v fights where manueverability is key.

[drachefly]

Short sensor ranges are somewhat unrealistic. Any ships with multi-G accelerations are going to have a bright signature that can be seen a long way off.

And once spotted, it's very easy to keep a tracking telescope on a target even if it's very very dim. After all, we can see small asteroids from Earth...

[tharkûn]

I was defining it as basically being outside the effects of any net gravitational field exertion. You may still be within two gravity wells, but they nullify one another and so bring the "free fall" effect up. So long as the forces working on you from either direction in the immediate vicinity are cancelling one another out, it may as well be considered a zero gravity zone for this purpose (ignoring the extragalactic influence, in either case, given the way the wormholes need no localised gravity well, it has a purpose in this respect).

No it shouldn't. Langrange points are critical points in the gravitational field where the centrifugal force balances out gravity. 3 L points are unstable and only attractive in the plane perpendicular to orbit while the other two are are gravitational anti-nodes. Lagrange points are where the net gravitational force provides a centripetal acceleration that keeps the body in a fixed position relative to the two massive bodies.

So, here I am, and I wonder, how does movement in space actually happen?

Newton's laws (unless you are well inside Mercury's orbit or have an absurd degree of precision/absurdly long baseline to play with). Most real spacecraft make heavy use of gravity to make fuel economizing trajectories. When you can't use gravitational slingshots and the like then you have to through some high velocity particles (Xenon ions, flaming hydrazine, small nuclear bombs, etc.) out the back.

Once you have your velocity, you keep it until you actively rid yourself of it (firing the engines against your momentum vector, gravitational braking, atmospheric dives, EM resistence, etc.).

This means when you are not actively accelerating you can face whatever damn direction you want. This means that you need to have enough fuel to bring your ship back to a stable orbit, or you will end combat with a oneway ticket to the great black nothingness of interstellar space. This also means that THOU SHALT NOT BANK in a turn.