Movement of different-sized crafts in space


by Jiketsu
Tags: crafts, differentsized, movement, space
Jiketsu
Jiketsu is offline
#1
Jan5-11, 02:37 AM
P: 2
Hello and thanks for reading! You'll have to excuse my general ignorance of physics and the jargon, and also if I am not posting in the correct spot (I looked up astrophysics and it seems to be closest to my inquiry)

Backstory:

This is not for school or anything like that. I have for awhile now been working on a game, where the player is given a planet, gains productions, and eventually builds fleets and colonizes other systems. I have come to the crossroad where I'm designing different classes of ships, their sizes, capacities, etc.. and its really becoming technical. I suddenly thought about SPEED. And I had to stop, and think really hard. And I admit it hurt my head. So I need more, qualified direction in this matter (did I just make a joke?) so that I can better wrap my head around this.

Actual Question(s):

There are two metal crafts deep in space. One is the size of an economy car, like a Honda Civic. The other is the size of a Cruise Ship. Mounted on the back of each of these crafts, is a single identical 'thruster' that both produce the same force of propulsion. Let us pretend there are no nearby planets or objects that apply any forces of gravity (or other, if others exist, and they probably do) ...

Will both of these crafts move at the same speed? Why or why not?

Will one begin movement sooner than the other or anything?

Would adding a second thruster to either change its speed potential?

Does volume, mass, or weight (but no weight in space right?) have any affect (ex if I have a cargo ship filled with cargo would it move slower than a cargo ship that's empty) or would the shape of the crafts make any impact? (I'm assuming there's not really any resistance in space right?)

Also I am curious.. about light speed haha.. is there anything faster?

Oh and a bonus question if anyone has heard of this... I remember seeing sometime on TV something about a car, traveling at light speed... but then it turns on its headlights or something, and it poses the question if the light of the headlights would double its speed. I can't quite remember. Something about time travel. Anyone know what this is about?


I really appreciate your help with this stuff. I don't know if I'll ever finish the game or anything, but this would really be insightful nonetheless. I'm always up for learning something!

-Ji
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cjl
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#2
Jan5-11, 03:18 AM
P: 976
1) Given the same engine, the smaller craft will accelerate much faster. If the smaller craft is 1/100 the mass, it will accelerate 100 times as quickly.

2) They will both start moving at the instant the thruster is turned on.

3) Adding a second thruster to either will double its acceleration

4) Mass has an effect, but the shape of the craft really doesn't.

(Note that in the answers above, I talk about acceleration, not speed. This is because the speed is dependent both on the acceleration and the duration. If the larger ship is 100 times more massive than the small one, but it leaves the thruster running 100 times as long, it will end up at the same speed as the smaller one. It just took longer to get there.)
barasawa
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#3
Jan5-11, 04:47 AM
P: 2
If I could toss in a couple of cents on this.
Real world hard physics without exotic (currently non-existent sci-fi) technology tends be be rather slow on both planetary and especially interstellar games. Though I have no idea what timescale you'll be using.

A ship with twice as much thrust accelerates at twice the rate. A ship with twice the mass accelerates at half the rate. A big problem with reaction thrusters (what we use) is the limited 'burn time' since you only have so much reaction mass on board. Our liquid fuel and solid fuel rockets actually burn the reaction mass. An ion drive ionizes the reaction mass and is of a higher efficiency. To reach high speeds you'll need lots of reaction mass. If you choose a reactionless drive (a few physicist think it's possible but most don't) would avoid the problem of having to carry reaction mass.

If you can thrust for the entire trip, the generally accepted fasted method is to thrust towards destination till halfway point, flip ship and thrust for deceleration the rest of the way so you slow down and don't go flying past destination.

Top speed. Limited by Light Speed, aka C. As you get closer to C time will apparently slow down. You'll never break C because time dilation will slow you down to the point where you'll never get to do that final thrust. This assumes you have the fuel/thrust to reach those kinds of speeds, much like a 100mph speed limit on a 10' section of road.

Yes, cargo would increase your mass and slow your acceleration rate. An easy way to determine your efficiency ratios is to divide the total mass by newtons of thrust.

Theoretically there may be things faster than light, but we haven't actually found them, and their existence is doubted by many. Look up Tachyon for one example, it's minimum speed is C.

Does shape mean anything for spaceships... Conventional thought is no. Unconventional though is that it depends on your speed and the local particle density. The vacuum of space isn't actually empty, it just has very little in it compared to our atmosphere. Let's say in a measured area, a billion atoms smack into it normally. In space, that may only be one or two atoms, probably hydrogen. (These aren't even appx, they're just for illustration.) If you are traveling so fast you are encountering as many particles over that test area as an airplane would in an atmosphere, you may want to think about streamlining. (I saw some calculations a long time ago that a bussard ramjet could really pile up a huge mass if it wasn't processing it fast enough. You might want to look up Bussard Ramjet to see what I'm talking about.)

Turning on your headlights when traveling at a significant percentage of C. It's all relative, literally. Due to time dilation and the theory of relativity, you will still see, but the wavelengths will shift. Things in the direction of travel will shift into the blue, while things you are moving away from will shift into the red. Yes, wavelengths can shift into or out of visible light range due to this. Of course, stuff in the ship will look normal because it's traveling at the same relative speed the passengers are. There are also other visual effects that will happen as you speed up, but I'm not even going to try and describe those here. (Really weird stuff)
By the way, light in a total vacuum always travels at the same speed, it doesn't add the speed of the emitter to it, that just alters the wavelength of the light. I'm sure you can find some good articles and videos on this, just search for red shift, and blue shift.

Just to throw some strangeness at you, a recent article indicates that the fine structure constant (aka alpha), of which C is on part, actually varies in different parts of the universe. This is being investigate to eliminate equipment and analysis errors. If it's true, then light speed (and other things) may be different in various places. A lot of scientists are ignoring it out of a kneejerk reaction, and not because it's impossible. Don't worry, similar things have happened many times, for example not that long ago everyone knew that the atom was the smallest possible thing and was utterly indivisible. Now we play with atom smashing and sub atomic particles all the time.

I've rambled way too long, but better descriptions of all this stuff is out there, in written and video form, check it out. I'm sure you'll like it, you are writing a game based on it after all. :)

Skolon
Skolon is offline
#4
Jan5-11, 07:36 AM
P: 82

Movement of different-sized crafts in space


Quote Quote by barasawa View Post
...Top speed. Limited by Light Speed, aka C. As you get closer to C time will apparently slow down. You'll never break C because time dilation will slow you down to the point where you'll never get to do that final thrust. This assumes you have the fuel/thrust to reach those kinds of speeds, much like a 100mph speed limit on a 10' section of road. ....
Not really true. Two objects can depart one to each other with a speed greater then C. For instance, because of expansion of Universe right now exist a lot of cosmic objects that are departing from us with a speed greater than C.
This is a real effect not just theory so a future spaceship maybe it will can use it. From this is derived the warp speed concept used in Star Trek.

But inside classical GR relativity an object can't exceed speed of light.
barasawa
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#5
Jan5-11, 01:34 PM
P: 2
Quote Quote by Skolon View Post
Not really true. Two objects can depart one to each other with a speed greater then C. For instance, because of expansion of Universe right now exist a lot of cosmic objects that are departing from us with a speed greater than C.
This is a real effect not just theory so a future spaceship maybe it will can use it. From this is derived the warp speed concept used in Star Trek.

But inside classical GR relativity an object can't exceed speed of light.
Right, that may be possible, but the original poster seems to be wanting known offshoots of technology, not sci-fi types. An article I recently saw on the warp drive indicated that the energy requirements may be absolutely absurd. (As in greater than the total energy of all the matter in the ship.) I mentioned the reactionless drive because there are reports of one scientist exploiting an oddity in physics to actually make one which supposedly does have a measurable thrust. I haven't seen it independently verified, so it may be a hoax, although some scientist say it may in fact work. The site for that is http://emdrive.com if you want to check up on it. I'd really love for it to be real, but again, without independent verification or the majority of the physics community doing headslaps and declaring it obvious, you have to maintain a 10 kilo grain of salt.
As to the mention of tachyons, I did it because he did ask if anything goes faster than light, and that's completely the realm of unsubstantiated theory, which I did mention.

Of course if you want to go with unproven methods, there are many to choose from. I think it was just last year some magazines did articles on the various theorized methods. Although a Sci-Fi game has the advantage of defining a travel method without defining the physics involved, and when a modern person asks you to explain it, you just mutter something about how it takes advanced studies from some futuristic university to explain it, but that doesn't stop people from using it, just like a modern 8 year old can use a microwave but hasn't a clue how it works. :)

Please understand that I'm not trying to argue with you, I am just trying to clarify what I thought the original poster was after. I could be way way off target. :)
Skolon
Skolon is offline
#6
Jan5-11, 02:22 PM
P: 82
Quote Quote by barasawa View Post
Please understand that I'm not trying to argue with you, I am just trying to clarify what I thought the original poster was after. I could be way way off target. :)
Yes, I am understand your point of view and I am totally agree with it.
I mentioned the warp speed just because it is based on real phenomenon from Universe. Even if expansion Of Universe and warp drive are not based on the exact same phenomenon, are related because represent two types of spacetime manipulation.

I hope I can say that the accelerated expansion of Universe is proved (and the fact that existence of objects that are departing in Universe from other objects with a relative speed greater then C is proved too). On the other hand the warp drive is not just a sci-fi creation but is based on some theoretical studies (like Alcubierre drive).
Yes, I know about huge amount energy needed by warp drive and a lot of other problems with this theory but maybe solutions exist. Why I think so? Because if "nature" can make an object to depart from other object with a speed greater then C, then I'm sure we will can do it too! Somehow. Sometime.
Jiketsu
Jiketsu is offline
#7
Jan7-11, 02:32 AM
P: 2
Thank you ever so much for your responses. I really appreciate your acumen in some of the explanations. I am sorry for my delay in responding; I had intended to revisit within 24 hours of my posting.

I definitely got the answers I was looking for and its certainly given me a clearer perspective on how ship travel should be implemented for my game.

The additional information offered on the theoretical drives was actually quite useful as well. Despite where their arguments stand, in terms of game mechanics it will be likely necessary that several forms of upgradable drive types or travel methods will need to be devised. I will be looking over these suggestions for that, thank you very much.

I am absolutely pleased with the quality of your feedback. Much appreciated!

-Ji
Chronos
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#8
Jan7-11, 12:38 PM
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An easy way out of this relativity bind is to equip your spacecraft with wormhole generators. Even lightspeed is unbearably slow in the vastness of space. The traveler does not notice the passage of time, but, one trip to the nearest star and your new born infant would be half grown by the time you returned [which may not be all bad]. We, of course, have no idea how to generate a wormhole [or if it is even possible], but, it is at least a theoretical possibility. More conventional means of propulsion will do just fine once you get close to your destination.


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