# In space, big ships moves slower than small ships...@_@

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Girsanov    136
If we are going by real life physics...should big space ships (battlecrusiers etc) move sluggish and slow compared to small fighters that are often portrayed as fast and maneuverable? Since space doesn't have friction and air resistance, won't the bigger engine on big ships compensate for its mass? But I guess for balance reasons we need to give big ships a weak point...

RDragon1    1205
inertia.

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Girsanov    136
Quote:
 Original post by RDragon1inertia.

The bigger engine should compensate for bigger inertia. (mass to propulsion ratio remains constant)

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Trapper Zoid    1370
Keep in mind you'd have to scale the power of the engines up by the cube of the size differences. If your capital ship is a hundred times larger than a fighter, then it'd need engines a million times more powerful.

Forgot to add: I also don't know what sort of properties a futuristic spaceship would have, but most of the ones I've seen in sci-fi appear to have thrust properties proportional to their outtake, or the area of "glowy bits" at the back of the ship. If your engine thrust varies on output area but the thrust you need depends on volume, then you'd expect larger ships to be slower by a factor of their size: if you scale a ship up a hundred times, you'd expect it to be a hundred times slower.

[Edited by - Trapper Zoid on March 2, 2008 1:25:22 AM]

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Xyphyx    142
I suppose the argument would then be this:
why not outfit the smaller ship with the larger engines allowing it faster speeds in a zero friction environment? And are we assuming a zero directional vector during propulsion? Because if the ship happens to be part of a gravitational pull (nearby planets / stars / or an orbit of some sort) wouldn't it then require more energy to move said battlecruiser compared to the smaller fighters? But then we're moving from a friction argument to gravitational force argument. Still - it would have an effect on propulsion speeds vs mass.

- Xyphyx

(Just a thought - I'm no physicist)

-Nevermind - I like Trapper Zoids answer better...

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Kaze    948
The best justification I can think of is that a fighter craft that will be docked in a carrier when not in battle is light since it doesn't need to haul around supplies and crew accommodations and can have a engine thats lighter but less efficient since it can refuel after a mission.

so its logistical independence vs speed and maneuverability

It could also be possible that capital ships have more armor since their more valuable or to compensate for being a bigger target.

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intrest86    742
Quote:
 Original post by Trapper ZoidKeep in mind you'd have to scale the power of the engines up by the cube of the size differences. If your capital ship is a hundred times larger than a fighter, then it'd need engines a million times more powerful.

Sorry, this isn't true. F=ma, so in order to accelerate a craft that is 100x larger you just need 100x the force. On top of that, you could probably make the larger engine more efficient because of the economy of scale.

If anything, Kaze's suggestion is the best. Large ships implies that the ship will tkae large trips that require increased fuel/supplies. In addition, they are larger targets and have to compensate with either stronger defense or offense, which invariably adds mass to the ship.

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ibebrett    205
[quote]Original post by intrest86
Quote:
 Original post by Trapper ZoidKeep in mind you'd have to scale the power of the engines up by the cube of the size differences. If your capital ship is a hundred times larger than a fighter, then it'd need engines a million times more powerful.

Sorry, this isn't true. F=ma, so in order to accelerate a craft that is 100x larger you just need 100x the force. On top of that, you could probably make the larger engine more efficient because of the economy of scale.

No hes right, its just he's saying 100x as big is 100x times the size as in 100*longer. Then its volume would be 100^3. Volume grows cubically with length.

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Trapper Zoid    1370
[quote]Original post by ibebrett
Quote:
Original post by intrest86
Quote:
 Original post by Trapper ZoidKeep in mind you'd have to scale the power of the engines up by the cube of the size differences. If your capital ship is a hundred times larger than a fighter, then it'd need engines a million times more powerful.

Sorry, this isn't true. F=ma, so in order to accelerate a craft that is 100x larger you just need 100x the force. On top of that, you could probably make the larger engine more efficient because of the economy of scale.

No hes right, its just he's saying 100x as big is 100x times the size as in 100*longer. Then its volume would be 100^3. Volume grows cubically with length.

Yes, that's what I meant. I should have written "longer" instead of "larger". Sorry about the confusion.

My point is that the engine power (probably*) grows proportional to the surface area of the ship, while the mass is proportional to the volume.

* This is sci-fi after all - who know how the engines work?

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Kaze    948
My new theory is that they are meant to fill different tactical roles. Large ships that have better offense and defense at the expense of speed for attacking at long range and holding lines as a semi mobile fortress and small fast ships that fight close up weaving through the enemy's lines make more sense than the other way around.

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Omid Ghavami    1007
Quote:
 Original post by Trapper ZoidMy point is that the engine power (probably*) grows proportional to the surface area of the ship, while the mass is proportional to the volume.

This is also one of the factors which limit the size of living cells; your input/output area is restricted to the surface, so increasing the size of the cell will eventually lead to a volume too large for the available input/output area to compensate.

Quote:
 This is sci-fi after all - who know how the engines work?

Indeed. If the OP feels that this is important then he should first decide how his engines should work (atleast psuedo-scientifically) and then think or ask about whether such a system would lead to larger ships moving slower or not.

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Girsanov    136
I think the "engine required to move a ship 10 times bigger at the same speed is 1000 times bigger than that of the smaller ship" is the loose explanation that I am looking for. Thanks!

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Edtharan    607
Quote:
 My point is that the engine power (probably*) grows proportional to the surface area of the ship, while the mass is proportional to the volume.

Yes, the Thrust of an engine is proportional to its surface area (X * Y) and the Mass of the Vehicle is proportional to the Volume (X * Y * Z).

Now for a larger engine to provide the same acceleration, you need to either move the propultion mass (fuel) faster or move more of it. Assuming you can get the same speed, this means that more propellent must be ejected. With a larger ship this means (proportionally) larger fuel tanks, leaving less room for all the other things that a capital ship needs (eg lots of weapons, armor, crew, etc).

So you could move less propellent mass, but this would mean that the capital ship would have to move slower.

Another thing to think about is the ability to move away.

If you had a capital ship and a fighter that could achieve the same acceleration which could move out of the way of an incomming missile easier?

Well, if your fighter is only 10m radius (simplified as a sphere) in size, then it only has to move 10m out of the way. So if the acceleration is at 10m/s (so a bit faster than 1g) then in approximately 1 second it has avoided the missile.

Now lets considder the Capital ship. Lets give it a 1km radius (again treating it as a sphere). As it has the same acceleration as the fighter it also acclerates at 10m/s. After 1 second it has moved 10m from where it started from, but it still has 990m to go to avoid the missile.

BOOM!!! :p

So in terms of avoiding taking hits, even with the same acceleration, the fighter is able to avoid missile fire where as the Capital ship can't. Even with the same acceleration, the Fighter could be seen as being more "manoeuverable".

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nihilisticod    204
Even though you are focusing on maneuverability, I would like to point out that speed caps in outerspace are unrealistic. There is nothing stopping a large ship from accellerating continuously. For example, the battle cruisers in homeworld 2 were understandably sluggish, but why did they stop accellerating after 1 inch per second?

Yeah, it would be wise for the pilot(s) to adjust their speed while moving so that they can stop before barelling through enemy lines, but when covering a large distance with a known destination, I would imagine a massive battlecruiser would have the computational facilities to optimize the travel (or atleast some guy who knows calculus). Of course, that introduces the tactical danger of being ambushed and not being able to stop your large ships immediately. I could see that as an interesting game mechanic.

On top of that, if these spacecraft have propulsion systems that point out the back, how do they slow down at the same rate that they sped up without turning around? Realistic 'stopping' is something that games don't seem to address.

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johdex    247
Quote:
 Even though you are focusing on maneuverability, I would like to point out that speed caps in outerspace are unrealistic. There is nothing stopping a large ship from accellerating continuously.

Is it so unrealistic? Space is not really empty, and eventually all the dust, rocks, sub-atomic particles may offer enough resistance to prevent further acceleration.

It may also be desirable to limit the maximum speed for safety reasons. A ship probably need to scan space ahead to avoid collisions. It should then not fly faster than its sensors allow.

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Rybis    109
Quote:
 Original post by GirsanovIf we are going by real life physics...should big space ships (battlecrusiers etc) move sluggish and slow compared to small fighters that are often portrayed as fast and maneuverable?Since space doesn't have friction and air resistance, won't the bigger engine on big ships compensate for its mass?But I guess for balance reasons we need to give big ships a weak point...

Well it is obviously cheaper to have a large engine on a small ship, a massive ship would have to have a giant (and expensive) engine to propel it at the same speed.

So if the big ship owners dont mind spending a billion or so on an engine, then it can go the same speed.

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MSW    151
In terms of real world realisam, space warfare will be fought with ultra long distance battles. Its some 238,000 miles from the Earth to the moon. NASA can reflect a lazer off a small mirror placed there by the Apollo space program. We can detect car sized objects between us and the sun 233,000,000 miles away. Just to reach 1/100th of lightspeed a tiny one manned craft will need to use about the same volume of rcket fuel as all the gasoline mankind has ever produced in the entire world to date. That will get you about 95 times faster than any spacecraft we have yet built. And it would still take you the better part of a day just to intercept some enemy fighter orbiting the planet Mercury.

Meanwhile they can see you comeing, and can slice your pretty little fighter into hundreds of bits useing beam weapons. You can't even evade them because you won't know they fired until its already too late. Your wingman won't even see the beam because in the vaccume of space its invisable. He will just see your ship rip itself to pieces. Fire a missle? too slow and way too easy to shoot down. Besides we could almost detect and track a baseball sized object orbiting Venus right now. In 50 years we might be able to count the stitches along the seams. Stealth technology will be pointless in the vaccume of space, and cloaking technology is pure fantasy. Infrared and gamma radiation signatures allow us to detect all sorts of spacebound junk undetected by sight and clouded by radar.

Space is realy, realy, realy, realy BIG and mostly empty. Makes our modern long range battles look like termites fighting ants in comparison.

But of course that doesn't make for exciteing video games, let alone movies and TV shows...but those can't exactly claim true realisam either.

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Talroth    3247
There are two ways to design a combat system for space in Sci-Fi.

1. Design the system, and come up with pusdoscience to explain it.

2. Design your pusdoscience, and then decide how you will have wars built around it.

Big factors:

1. Propulsion. How are your ships moving? Chemical rockets? They're slow to change acceleration, not going anywhere fast, and not carrying much.
Ion Drives? Still, slow to accelerate, but better than chemical drives. You're still going to take months to cross a solar system at best.

Advanced Sci-Fi tech? Your ships can project a new form of energy field around themselves, and by carefully changing and timing the fluctuations in that field your ship can swim like a fish in water, only the water is now the curvature of space-time. Using a tech like this easily gives you a reason for why very massive ships can move slow, while smaller ones go faster, even if it is a vacuum in space with nothing that should slow them down. It is simply a limit of the technology. If you move outside of the field fluctuations then normal space inertia wants to take back over (Since you're not actually applying a 'force' to the ship, your ship shouldn't have moved in the first place, therefore it is going to stop). Larger, more massive ships need larger, far harder to control fields, therefore they get out of sync more often and slow down. Pushing them faster through their field emissions greatly increases the risk of part of the ship falling out of their space bubble (and what happens if 10% of your 100,000,000 tonne spaceship suddenly has a velocity of 0km/h while the rest has a velocity of 10,000km/h?)

This could actually make for an interesting tactical portion of your game. Do you risk losing a whole ship to override your field regulators to try and get somewhere faster than your normally should?

2. Sensors. Stealth is an option, there are mathematical simulations on a real cloaking device, one that actually bends light around a position. Space is also full of background radiations. How do you tell the difference between some massive rock flying in from the Oort cloud and a large battleship that happens to look like a massive rock?

Radar is one part of the spectrum, and it can be deflected and masked. Why can't other parts be masked as well? Decoys. Is that a low emission battleship flying toward your base? or is it a probe outfitted with emitters to mimic a battleship? Do you send a force to intercept it? Or maybe your sensors are just glitching. Nothing is perfect when it comes to seeing things that aren't infront of your face.

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Quote:
 in order to accelerate a craft that is 100x larger you just need 100x the force
You have to take into account that adding 100x times as much thrust will require you to add considerably more structural reinforcements too, or your engine will blast your ship to bits.
Also, turning a large ship at the same angular velocity as a small one will have much higher forces acting on objects/people far from the center of rotation. So, even if the ship could do a fast manueuver without being torn to pieces, this is not necessarily true for the people within.

On the other hand, it is not unreasonable to assume that sci-fi spaceships do something which defies our normal understanding of physics.

Most things you see in any kind of Sci-Fi movie or series are grossly beyond reality, and yet it doesn't matter at all. People will happily accept anything that looks vaguely believeable. Therefore I wouldn't put too much effort into making everything 100% correct - make it fun instead.

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Quote:
 Original post by MSW...

I agree that space warfare will probably be fought at great distances. However, you assume that there will be little or no technological advances between today and a future age of interplanetary/interstellar war.

Who can guess what advances might be able to hide an object from today's sensors? I imagine that ancient mariners would think our modern submarines to be pure fantasy, but they have proven to be extremely stealthy. Why should we think space warfare will be any different?

Optical, IR, or radiation-based detection of the enemy may not be as easy as you suggest. As you said, "space is realy, realy, realy, realy BIG", so it would take a long time to scan everywhere with today's technology. Besides, assuming propulsion technology advances to the point where ships travel at a significant fraction of the speed of light, an incoming vessel might already be attacking you before your sensors pick up a signal.

Speaking of attacking, you assume that beam weapons will be very powerful and that missiles will be too slow. I could see directed energy weapons being very powerful, or completely useless due to dissipation at range. As for missiles, I'm betting they would be plenty fast to catch any ship. Just look at today's missiles: they are basically just an engine with a warhead, and are many times faster than all but the fastest of aircraft.

My point is that you should keep an open mind concerning future technology. Who knows what crazy, fantastical wonders the future world holds? I'm sure we will discover incredible ways to travel the galaxy, meet interesting and stimulating people, and kill them.

- Mike

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Quote:
 Original post by samothMost things you see in any kind of Sci-Fi movie or series are grossly beyond reality, and yet it doesn't matter at all. People will happily accept anything that looks vaguely believeable. Therefore I wouldn't put too much effort into making everything 100% correct - make it fun instead.

I totally agree! Make your world's rules believable if you can, but don't let it get in the way of making an interesting game.

As for the OP's original question, I think the concept of big, slow ships and small, fast ships would be based more on tradition and doctrine. Modern naval fleets have lots of smaller ships escorting a large carrier, so maybe that would carry over into space-based fleets.

Building smaller escorts allows you to have more of them, so a successful enemy hit would have a smaller effect on the total fleet. You could spread out your smaller, cheaper ships and increase both your sensor area and the distance enemy ships need to penetrate through them before attacking the bigger vessels.

Also, a single ship can only be in one place at a time. If you build two ships, each half as large, you now have twice the flexibility in strategic force deployment and tactical combat. On the other hand, you will have more overhead. A single ship wouldn't need two galleys, two command centers, two propulsion systems, etc.

- Mike

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MSW    151
Quote:
 Original post by doctorsixstring...

Oh, I agree make the game fun and interesting. Even toss in newtonian physics. But don't confuse it for reality.

Sure technology will advance. Engines will become more efficent and faster. But don't assume sensor technology wont advance with it. Sure you might be capable of masking a starship as a small spacebound rock. But spacebound rocks float about for millions and even billions of years. It doesn't take much to catalog them, even predicting thier flight paths and thus knowing where they will be anywhere from a hour from now to hundreds of years from now. The universe is huge and at our time scale its also very predictable too.

Besides, if we had FTL engines and could communicate just as fast. We will have advanced remote controlled AI drones fighting our wars by then...they don't need synthetic gravity to keep thier hearts healthy, they don't need breathable air, nor food or even a space potty. And, this is the most interesting part, they can take thier factory with them; replenishing thier ranks from all the elements they pick up along the way.

But then again the time travel abilities that come with FTL makes war obsolite.

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Edtharan    607
Quote:
 Its some 238,000 miles from the Earth to the moon. NASA can reflect a lazer off a small mirror placed there by the Apollo space program.

It also take around 1 second for that laser to actually reach the Moon. So if your ship can move its width or height (or more) in that time, then the laser will miss the ship entierly (actually, you should only need about to move half the ship's size - but I am working on the full size of the ship for a definite miss).

In terms of space battles, 1 light second would be relitivly close. How close you could get would depend on how fast the ships could move compared to their size.

So you would likely have Escort ships with a small front profile and relitivly powerful lateral engines. This will enable them to get in "close" and launch attacks.

How close? Well if you minimise the profile, then you might be looking at around 4 to 5 metres as the profil size (think fighter plane shape).

This means that at 1 light second (300,000km) from the target, you would only need an acceleration (perpendicular to the direction of the enemy) of 5 metres per second, or around 1/2g. A half a light second 150,000km) this rises to 10m/s acceleration (aproximately 1g).

This level of acceleration would be able to be handled by most humans, but it certainly would be a bit of a strain.

If you used a remotly operated ship (AI countroled), then it might be able to handle accleerations of much higher (maybe around 8g). At this level of acceleration they could get to around 37,500km or 1/8th of a light second.

An AI ship might also be able to have a much lower profile which would also get them in closer too.

Which brings me to missiles.

A Missile might ahve a profile of around 1/2m and have an latteral acceleration of around 16g. SO how close could this missile come?

At 8g that is an acceleration of around 80m/s. Giving 1.25m at 1/64th light second, or 4,687.5km.

This is before it even runs the risk of being hit by a beam weapon.

Yes, a capital ship with a lot of point defences could therefore establish a perimiter at around 4,500km from where missiles could be shot down. But too many missiles could easily overwhealm this defence and it would probably only take 1 missile to seriously damage such a large ship.

So while the principles (capital ships) might engage at a long range, you could get smaller escorts moving in closer and missiles would be nearly at your doorstep (well, airlock :D).

Now if your missiles were designed right (directed fragmentation blast) then at 4,500km you could do some serious damage that would not be able to be stoped easily by point defences.

Instead of having your missiles do damage in an explosion, if you instead have the warhead direct thousands of solid "shrapnel" at the target, then each of these small chuncks could end up traveling at a massive velocity. Each piece of shrapnel would not necesarily "blow the ship up", but it would put many small hull breaches throughout the ship. Think of these missiles as mobile buckshot shotguns. They would loose atmosphere fast (although they might operate in space suits and evacuate all the air from the ship while in combat to avoid this threat).

The main problem with all this is that it would be extremely fuel hungy. Fortunately, you would only need the really high accelerations as you get close to the enemy. For example if the Ship (5m profile) were 1 light minute away then it would only need around 1/60g acceleration (or 0.16m/s) to avoid being hit and a Capitalship 1km in size with 1g of acceleration could safely operate at around 100 light seconds (30,000,000km) from the enemy.

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For sub-light speed travel (I'm presuming you'll have FTL, but they'll need a totally different method that you'll need to basically make the science up for), one likely propulsion method for ships would be ion drives, which propel ions out the back, or something else based on this principle. A kilogram of ions could potentially accelerate a one ton ship to 300 km/s.

Now, with thrust based propulsion, it is speed of exhaust times by the density times the cross sectional area that determines the force (acceleration/unit mass). Therefore we get:

accel = density of exhaust * X-section * speed / mass

Presume the density is the same. Mass increases with the cubic, and cross section with the square. What happens to speed? Well, normally if you scale something up, speed of exhaust would increase...but not with ion drives. An ion drive will be firing it out very close to the speed of light, and hence the increased engine size will have little effect, and so acceleration will be roughly inversely proportional to the dimensions.

There is a certain amount of fiddle factor concerning how fast fighters throw out exhaust (maybe they do it somewhat below light speed due to energy concerns - since, even under Newtonian physics, energy is most efficiently used by using lower speeds, but then that needs more material for the exhaust). In addition, heavier ions for the exhaust will provide more thrust for power output, particularly as you approach the speed of light with the exhaust speed (since their speeds become almost identicle, where before the lighter one would have been faster).

So look up ion drives, since they basically provide exactly what you want, presuming that they manage to get them capable of fast speeds.

An alternative is to say that they don't get ion drives capable of high enough speeds. The fuel costs of capital ships using rocket based movement would, presumably, be impossibly high, but could be managed for smaller fighters, leading to a differential there (ion drives are much more efficient, in terms of fuel consumption compared with mass).

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Quote:
 It also take around 1 second for that laser to actually reach the Moon. So if your ship can move its width or height (or more) in that time
But how do you evade in the first place? If we're being realistic, you have no way of knowing that a laser beam will hit you, unless it is already happening. A laser is only visible if it passes through a scattering medium, but the scattered light won't reach your eye before the laser does.
Remotely controlled vessels are likewise doomed, even if you stand next to the cannon when it fires. Since any kind of remote control will need a reliable and secure transmission protocol, you would need a several-times-speed-of-light radio signal, no matter how good your ship computer's AI is, and how quickly it can manueuver.

Quote:
 have the warhead direct thousands of solid "shrapnel" at the target
Assuming a starship that does "real" interstellar travels, such a thing is no issue. A real starship would have no problems deflecting such things.
Space is by no means empty. Particles up to 0.1 mm in diameter are not rare. Even small particles which have no kinetic energy by themselves are a threat if only you move through them fast enough. Larger particles are rare, but do occur, and some move at high velocities and have a kinetic energy that is easily a thousand times higher than your missle shrapnel. Luckily, such encounters are really rare, but whenever the NASA says "we suddenly lost contact", you can guess what happened.
Now, imagine you have a starship which regularly travels through space at significantly higher speed than our present stone-age rockets (which do take months/years for a trip to just the next closest planet). Maybe it even travels to other solar systems regularly. We know only very little about what is outside the heliosphere, but it is likely that you encounter a lot more and higher velocity objects in absense of the solar wind and collisions with other matter.
So, realistically, a "real" starship would certainly have a kind of deflection shield which can cope with any realistic amount of impact your ballistic missles can produce, or else every normal voyage would be a suicide trip.
Typical explosive velocities are 5,000 (AN) to 9,100 (HMX) km/h. So let's assume your shrapnel achieves a grand total of 10,000 km/h. When we're at 100,000 km/h, we're talking.

Did I say "don't make it too realistic, make it fun"? :-)