Movement in Space

[AdmiralToguroAni]

I was just wondering: Why can't an imperial Star Destroyer move as fast as a TIE fighter in space? Since they're in a weightless environment without intertia or any other form of resistance, it would seem as if a Star Destroyer COULD move as fast as a TIE fighter I'm no physics major so I might be missing something, but if anyone has an idea about that, post it.

 

Incidentally, notice how in the movies the ISDs were regularly outrunning the Milennium Falcon? Have they just been slowed down in games for balance purposes?

[Eko]

Whilst there may be no resistive force there is always inertia from the force of the engines. The sheer force of the enigne will matter, therefore a ship with larger engines relative to its mass will be faster. Gravity I suppose also plays a small part in determining velocity, based upon trajectory you could boost velocity by aiming for a gravity well such as a star.

[AdmiralFishface]

First of all, one thing:

Since they're in a weightless environment without intertia or any other form of resistance...

Weightless, yes. Without inertia, NO.

Inertia exists everywhere. It's the force you experience when you try to change the velocity of any object, even in space! And to clarify, inertia does not change your speed nor put a limit on your top speed, it works against changes of movement, meaning it tries to keep you moving in a straight line at constant velocity.

 

Just felt I had to point it out. But you are correct that there should be no speed limit in sapce.

 

In the movies, there is indeed no reason to assume that large ships were limited to low speeds. The Star Destroyers hunting the Falcon are the best example.

 

But: Small ships have lower mass. For this reason, they have it easier to accelerate further with even less thrust than the large ship.

So while the Falcon could accelerate to a certain speed in a matter of seconds, the SD' would have to accelerate for a much longer time, or put incredible amounts of energy into the engines to keep up (assuming their engines were powerful enough for that, which I doubt).

They would also have a hard time changing their direction of flight as inertia would try to keep them moving on a straight line.

 

So in short, while capital ships could move at the same speeds as small ones, they would require more time to get to that speeds, which they don't always have in combat situations.

 

As for why they work the way they do in games, it's just a decision by the game-makers. People expect big things to be slower and more sluggish than small ones. The analogy for SW space combat has always been WW2 naval battles - Large, lumbering warships with small, nimble fighters buzzing around in between. So that is what combat tends to look like.

[Skynxnex]

There is in fact still MASS. One has to remember that the words 'weight' and 'mass' are not the same. Weight in fact means the effect of gravity on you. Mass is something different.

 

A TIE fighter has much less mass than a Star Destroyer and as a result requires less energy to get it going.

[Krytos]

Well, pretty much everything Fishy said - plus a little extra: official SW RPG (D6 ) stats have most warship speeds very close to the maximum speeds of smaller fighter crafts.

There are some ships, like gunships and corvettes, that are faster than Y and B Wings and most New Republic warships are as fast as a Y-Wing.

 

Here's a site I go to often to look at some official D6 stats - by the way, it hasn't chagned since . . . well about 5 years now, maybe more

http://members.tripod.com/dex_10/index.html

[BigChimpChili]

logically a limiting factor on the speed of something with the disign of a star destroyer would be stopping time. you would have to get over its enertia to get it going quickly but you would also have to do the same to get it to stop. as physics teachers love to say, objects in motion want to stay in motion. here on earth with an atmosphere and what not there is friction to slow you down, but not in space. since there are not huge engines puting out large amounts of thrust in front of the ship, like in the back, max ship speeds would have to be limited in order to be able to bring the ship to a controlled stop within a reasonable amount of time.

 

however its worth noting that this doesnt much matter for things like star destroyers because nothing in the star wars universe adheres to this particular rule of physics.

[AdmiralToguroAni]

sort of like the laser sounds and explosions in space?

[Eko]

O've thought about explosions and they are possible. As a ship explodes it loses atmospheric containment, this mean that oxygen and other gases will be available for combustion, therefore explosions are possible. As for Lasers and sounds, you'll have to ask someone else, i.e. someone who actually know rather than myself who just akes hypotheses based on limited physics and guesswork.

[MDk]

Well, I am a physics, but I am not good in english. But most things Admiral Fishface wrote was correct and is explaining everything, with one exception: There actually is a speed limit for ships: It is the speed of the particels emited by the engine. For example a nowadays rocket: The gases expelled from the engines have a fixed speed, so a rocket can never fly faster than this speed. A modern ion engine is emitting faster particels than a rocket, but the overall mass of the emitted particles is lower, so the acceleration of an ion engine is lower than the one of a rocket.

 

So it is possible, that a Star Destroyer is as quick as a TIE if the SD engines does emit particels as quick as a TIE engine. But normaly a fighter does repulse more mass per second with regard to his total mass than the SD does, so that is short range. So the fighter has a much better acceleration and agility.

 

For the sounds in space: You will never hear a explosion or engine in space, if you are not hit by the engines radiancy or the blast of the explosion, because sounds needs a fluid or gas to be transported. But soundless explosions an engines would be boring in a movie.

[AdmiralFishface]

Wait, MDk... No.

 

Consider a rocket (at rest) with mass M and expelled gases with total mass of m. The rocket pushes the gas out with a speed of v. Then, according to Newton's third law,

mv = MV , where V is the resulting speed of the rocket. Most of the times, gases will weigh a lot less than the rocket, but we can still assume the following scenario:

 

Consider m to be 10*M. V would then equal 10 v. The rocket would speed away from the gases at ten times their speed.

 

Also, if you fire more gases from your rocket, the total impulse will still have to be balanced out, so you will accelerate further. your own speed matters only if you consider relativistic effects (which would be useless for this discussion).

 

The speed of the gases does matter, but it is only a limit to acceleration, not top speed.

The only real top speed is the speed of light (assuming we don't find a way to break it sometime in the future).

 

The real technical limitation is that if you want to accelerate to desired speeds, you'll want to avoid running out of fuel. So the solution is, as you've said correctly, to accelerate smaller particles at higher speeds. Then, if you remember the equation MV=mv above, v will be very large, so m doesn't have to be.

This is what ion drives accomplish.

[DarthTex]

I believe the term you're looking for (and somebody has already mentioned it) is "Impulse", mass times velocity. Chemical rockets have a low "impulse" value (lots of gas to be burned), ion or (better yet) nuclear ion drives have higher "impulse" but technologically, we're not quite there yet (especially economically).

 

Consider "laser shots" in space. If light travels at ~186,000 miles per second and let's say it takes your laser canon 1/100 th of a second to get off a shoot, that "laser bolt" is 18,600 miles long! Talk about a long range weapon! The way "dog fights" are shown, or even capital ship engagements, there should NEVER be "gaps" shown between shots. By the time you see the shot, it's miles long. To the human eye, it would "instantly" appear and disappear as the trigger was pulled. For movie effects, they're shown like "tracer" gunfire so you can "see" where the shots are going.

[MDk]

You are part right, Admiral Fishface. What I wanted to say is, that there actually is a speed limit. Of course you are right, that a rocket won't reach the speed of the propulsion gases. I was assuming a rocket with unlimited fuel as an idealization. In that chase a rocket would theoretical reach the speed of the gases (not practical).

 

To come to your excample: If you are want to solve it about pulses, lets take mv=MV as you said. So, the maximum speed of the rocket would be V=v*(m/M). Because of m/M allways being smaller than 1, the rocket will never reach the speed of the gases, so the speed limit still exists. But pulses will only work, if the rocket will pump out all fuel instantaneous. Normally you must calculate the endspeed with an integral, because the the fuel is emitted gradually. I assume that the endspeed V will be somewhere between v*(m/M) and v.

[MDk]

I believe the term you're looking for (and somebody has already mentioned it) is "Impulse", mass times velocity. Chemical rockets have a low "impulse" value (lots of gas to be burned), ion or (better yet) nuclear ion drives have higher "impulse" but technologically, we're not quite there yet (especially economically).

 

But we do allready have ion engines. Have a look here, please:

http://www.bbc.co.uk/science/space/exploration/futurespaceflight/ionengines.shtml

[EvilleJedi]

not to mention that you need exponentially more energy to reach relatavistic speeds.

 

the speed limit is not only the fraction of the exhaust speed and your efficiency, but also the ability to store the fuel, the reactor power needed to excite and accelerate it and the fun stuff like time dilation that happens when you go too fast too much also the acceleration rate is directly linked to the hull integrity and the inertial dampers ability to maintain a comfortable operating 'gravity'.

 

anyway star destroyers can go at around 2000km/s at 'top speed' after an hour or so of acceleration (this is the 8 hour intra innersystem transit time claimed in many books) our current ion engines can only reach 40km/s after months of acceleration. a star destroyer is capable of much higher velocities simply because it has a larger emitter area and a phenomnally larger reactor (if you scale the ties engines emission rate/sq area by the reactor capacity of an SD and the emitter area you get some really big numbers)

 

there is also something to be said for the energy density of large star ships (in the supernova+ range/ second) and the mass implyed that must be moved. but since star wars has repulsor lifts and gravity negating technologies I'm pretty confident that they can 'reduce their effective mass' by enough to overcome this.

[DarthTex]

MDk wrote:

 

But we do allready have ion engines. Have a look here, please:

http://www.bbc.co.uk/science/space/exploration/futurespaceflight/ionengines.shtml

 

There you go, ONE spacecraft . How many "chemical" rockets/satellites have there been? Many more. Currently, the best effective method of speeding a deep space satellite along its way is with gravity assist. Ion engines show great promise, just like the first computer. But until you can mass produce them (to get the costs down) and get them in use, upgrades will be few and far between.

[MDk]

MDk wrote:

 

But we do allready have ion engines. Have a look here, please:

http://www.bbc.co.uk/science/space/exploration/futurespaceflight/ionengines.shtml

 

There you go, ONE spacecraft . How many "chemical" rockets/satellites have there been? Many more. Currently, the best effective method of speeding a deep space satellite along its way is with gravity assist. Ion engines show great promise, just like the first computer. But until you can mass produce them (to get the costs down) and get them in use, upgrades will be few and far between.

 

Yes, I know that we don't massproduce them, but at least, we do have the technology to produce them.

 

Allthough the ion drive has to major advantages,

1) Higher endspeed

2) Longer fuel reserve

and allthough the production is not much more expensiv like a conventional engine, it still has two major disadvantages:

1) Slower acceleration

2) It needs nuclear power core. While Deep Space One was launched there were quite many discussions and protests against the start, because if the carrier rocket would explode a large area of Earth would be conterminated.

 

I think that is the bigger problem than the costs.

[Stellar_Magic]

2) It needs nuclear power core. While Deep Space One was launched there were quite many discussions and protests against the start, because if the carrier rocket would explode a large area of Earth would be conterminated.

 

Ha, thats really funny considering what their planning to use to propell the mars express mission. An impulse drive aka radiation drive which emits radioactive particles for thrust!!!

 

Actually there are some other speed limits in space...

 

Relativity = You can not exceed or reach the speed of light!

Photon Pressure = Though there may not be air resistance, there is resistance to propulsion in space, however it only begins to come into play at extremely high velocities.

[Eko]

1) Slower acceleration

 

When you are planning a mission to deep space this isn;t a bad thing, as it will eventually accelrate past the maximum speed of the alternatives and will evetually outstrip them.

[AdmiralFishface]

Ah, I've thought about it some more and I can see what you mean now, MDk. Of course, if the mass of the fuel doesn't exceed the mass of the spacecraft, the craft will never accelerate past the relative speed between rocket and gas without aids such as gravity slingshots.

[MDk]

I feel relieved now, Admiral Fishface, that you understand what I meant to say. I was fearing, that my english isn't good enough to describe what I wanted to say. Well, now I am just imagining a SD using gravity slingshots... That will surely be funny.

 

Stellar Magic: You are right, both speed limits does exist, but a SD is making less than 1% of lightspeed (EvilleJedi was talking about 2000 km/s). With this speed both effects won't be relevant.

 

Eko: You are right, for deep space missions this won't be a too bigh disadvantage. If the probe is flying long enough the higher endspeed will be of more importance. But for example for a spaceship to the moon the ion engine would be nearly useless. As for myself I am waiting for the microwave engine to be realized, which would have a predicted top speed of up to 20% of lightspeed.

[Eko]

20%? Wow, to the edge of the Solar System in under two days, I suppose that's a long way off. Practicality must be taken into account with the ion engine of course, to the moon would be pointless. I have a question realting to the solar sail:

 

Can it travel towards the sun or only away?

 

Oh and if you have a link for the microwave one that'll be facsinating to read.

[MDk]

The question about the solar sails is easy to answer: Yes, it is even possible to travel towards to sun arcording to this articel:

http://www.planetary.org/solarsail/whatis.html

 

A link for the microwaving engine I do not have unfortunally. I read about it several years ago in a journal in an articel about future technologies. But the concept of the microwave engine was quite faszination. For the engine you would install strong microwave guns against the direction you want to fly to. The emited microwaves itself won't have any repulsion strength. In front of the microwave guns you would install something like solar sails. Where the microwaves hits on the sail, they would create a plasma field which radiates so much heat, that a propulsion will be achieved.

 

I think too, that this technology is quite futuristic. While it won't be a problem to create enough energy with a nuclear power core the costs of such a shipwould be this high, that a single country won't build something like this.

[Eko]

Cheers for the Solar Sail article, its very interesting.

 

A point for the microwave concept:

 

Now from limited physics knowledge I know that energy is neither created nor destroyed. Is the effect of heating caused by the energy changing form? For example from a battery (eletrical energy) to propulsion (heat energy). If so would the engine have to be completley energy efficient in order to maintain a constant velocity. Wouldn't outside factor cause the process to lose energy?

[Skynxnex]

I think we are taking the whole inertia and lasers have sounds in space thing a little too far.

 

 

It wouldnt be exciting as a movie for us to simply see a light show and not hear anything.

 

As for the inertia part its been stated in many novels and technical stuff that these vehicles have inertia compensators which, somehow have the ability to counter the effects of inertia which is why these ships act the way they do.

 

As for the faster than light thing, I think most recently in the Ep3 technical thing that SW ships have relativistic shielding/stasis shielding which 'slows down' the aging a pilot or passenger goes through when they accelerate faster than light that way when they are done with the journey they arent like 300 yrs old and everything in the galaxy is different.

 

At any rate its sci-fi and just interesting.

[MDk]

Now from limited physics knowledge I know that energy is neither created nor destroyed. Is the effect of heating caused by the energy changing form? For example from a battery (eletrical energy) to propulsion (heat energy). If so would the engine have to be completley energy efficient in order to maintain a constant velocity. Wouldn't outside factor cause the process to lose energy?

 

Yes, it is a form of energy transformation. Microwaves is a high energie radiation. The microwave engine would be a little bit compareable with a solar sail, where the microwave gun would replace the solar radiation. The problems of energy efficience and constant velocity would need some work to be solved. Especially energy efficence would be a problem because of a tripple energy transformation: nuclear fission into electrical energy, electrical energy into microwaves, microwaves into heat energy. Each one would cause a energy loss. But I can't remember very much details of the article, it is quite a while now, when I read this.