Why do spaceships have a top speed?

[sao123]

I'd like to bring in the interpreted explainations Brian Greene on relativity.

According to his explanation... all matter is moving through a 4 vector spacetime.
3 vectors are the x,y,z of space, and the fourth one is t for time. All matter moves through spacetime at C, at all times, and the angle of the vector determines the path.

In other words, the sum of your 4 vectors, x(t) + y(t) + z(t) + t = C. Henceforth, if you should accellerate to close to the speed of light upon some path lets speculate along the standard X vector. When you reach C, your t component is now 0. Therefore at the instant your velocity hits C, time for you stops. Your instantanious velocity may appear to be C, but since no time passes, and since x,y,z are all functions of t, you actually go nowhere. This establishes the asymptotic nature of C.

 

[TTM77]

here is my 2 cents.

Simply put: The maximum acceleration is how much you can produce and how much your object can stand the strain.

1) The strain increases when resistances are introduced.
2) The ideal maximum strain your object can stand is by the maximum surface tention that it can hold it shape.
3) If the acceleration is more then your object can withstand then your object is totalled.

 

[theslickvik]

takes me back to physics....bad memories

 

[Calin]

Originally posted by: MetalStorm
If you have a rocket in space and assume a complete vacuum and no gravitational effects then when the rocket uses its engines the acceleration will be very close to linear... However it's not the speed of light where the acceleration starts to tail off in conventional rockets, its the speed of the exhaust gasses moving away from the rocket. Let me give you an example:

Assuming gravity is not taken in to account and there is no friction:
If you have a rocket at rest which starts its engines you will get the maximum acceleration possible. As the rocket increases in speed, the acceleration will reduce not due to how close it gets to the speed of light but due to the DIFFERENCE in speed of it relative to the exhaust gasses being produced. Eventually when the rocket gets to the same speed as the exhaust gasses going the opposite direction, the acceleration will be 0. In fact you will find when you get to that speed the exhaust will be left as a trail of STATIONARY gas.

Hence the speed of rockets is down to how fast the exhaust gasses can be made to exit the diffuser.

The rocket engine works in this way: it burns the fuel (using the fuel oxydizer) in a burn chamber. The hot gasses are exhausted thru the nozzles.
Now, the rocket engine does not know the vehicle speed (and it doesn't care). All he needs to know is to work, and for it work is throwing out exhaust gasses with a certain speed relative to the engine's casing.

The reason rocket engines have higher power at launch is:
while in space, the speed the craft gets depends only on the mass and speed of the fuel thrown thru rocket nozzles.
While on the launch pad, there is a column of hot gasses, having a certain pressure inside. The craft can ride on that column much like a hovercraft ride on its air pillow. Or like a chopper hovering over land or sea - the pressure under the main propeller is higher that in the same conditions, but high in air.

In the end, the maximum speed of a space craft is limited by the amount of mass he can carry for propulsion (action/reaction) and the energy (impulse) he can transfer to it before ejecting.

For your information, one of the most efficient "rocket" engine that exists uses some high mass ions (of some metal) that are accelerated in an electric field. The engine uses very little mass, which gets a very high energy. The space craft using this engine was able to get an acceleration of some fractions of g (like 0.01 or less compared to gravitational acceleration) for a period of time of weeks or months. The speed it gained can not compare to anything a liquid hydrogen/oxygen or anything else is able to get (in the same size constraints).
The low size? Launch was made with the ole liquid fuel rockets, which (while much less efficient), have a tremendous propelling force (rockets and shuttles accelerate with 4-5g during the initial phases of the ascent).

 

[Calin]

Originally posted by: RossGr
According to my copy of Halliday and Resnick the velocity of a rocket is given by

V = - v ln(m/M)

Where:
V = final velocity
v = velocity of the exhaust gas relative to the rocket
M= Mass of Rocket without fuel
m = mass of fuled Rocket.

This shows that the maximum velocity is only limited by the ratio of fuel to payload. The exhaust velocity is not a limit because it is measured wrt to the rocket, it will always be large as far as the rocket is concerned.

Now, in order to improve the figures, the current vehicles use several stages. This way, each of the propulsion method used will push a smaller and smaller vehicle into orbit (the M is reduced).
In the case of the russian space shuttle (Buran) (theoretical case only), the first stage are some lateral boosters, attached to the main engine. They used solid propellant (I think). The american shuttle uses solid rocket boosters (SRB).
The second stage of the russian shuttle is the main engine (the american have only the fuel tank there). The main engine (called Energia) is able to launch other vehicles, not only the space shuttle.
The third stage - the maneouver rockets on the shuttle itself. They are used to kickstart the shuttle from the trajectory it has at the dettaching of the main fuel tank into the desired orbit.

The end effect is that the manoeuver rockets create a higher delta V using less fuel because the main bulk (mass) of the propulsion is now on the ground (or in the water).

 

[Calin]

Originally posted by: Vee
(and all that about "drag" and "space is not absolute vacuum" is just fluff and need not be considered)

There is drag, and space is not absolute vacuum - however, for all current practical concerns (spacecraft speed in the range of kilometers a second), we can safely ignore that.
I don't know if the drag was taken into account for the probes that crossed the tail of comets - and the tail of the comet has the most concentration of matter of mostly everything you will find in cosmos (special areas notwistanding).

 

[Calin]

Originally posted by: inspire

Originally posted by: MetalStorm
If you have a rocket in space and assume a complete vacuum and no gravitational effects then when the rocket uses its engines the acceleration will be very close to linear... However it's not the speed of light where the acceleration starts to tail off in conventional rockets, its the speed of the exhaust gasses moving away from the rocket. Let me give you an example:

Assuming gravity is not taken in to account and there is no friction:
If you have a rocket at rest which starts its engines you will get the maximum acceleration possible. As the rocket increases in speed, the acceleration will reduce not due to how close it gets to the speed of light but due to the DIFFERENCE in speed of it relative to the exhaust gasses being produced. Eventually when the rocket gets to the same speed as the exhaust gasses going the opposite direction, the acceleration will be 0. In fact you will find when you get to that speed the exhaust will be left as a trail of STATIONARY gas.

Hence the speed of rockets is down to how fast the exhaust gasses can be made to exit the diffuser.

I liked this answer. Thanks.

After you accelerate enough, the exhaust will be a trail of stationary gas. Afterwards, the exhaust will move in the same direction as the rocket (but will still be able to give energy to the rocket, as its speed when entering the engine is much higher but forward).

 

[Calin]

Originally posted by: Kyanzes
I'm not considering to add to this thread anything useful as I'm short of the required education/dedication, but certainly would like to ask something I don't really "dig". What is exactly space? I mean everyone here seem to agree that space contains small particles of matter, radiation etc. But what's there where no matter or radiation could be found? Is there such space? If so, what is it? Some gravitational field? If so, is there any place where no gravity is present? And if there are such slices of space where no gravitation is present, what is it that allows light for example to radiate (read somwehere that photons have no mass)? Also, what makes space three dimensional? Would like to understand the nature of space on a VERY basic level if it's possible.

Well, space is... space. Everything.
Now, around big rocks (like Earth, Moon, Mars, Venus, ...) the gravity keeps some matter in gaseous form. While Earth keeps water, oxygene, hydrogene, methane in gaseous form, Pluto keeps its methane in liquid or frozen form (I forgot which). Also, while Earth keeps most of its water in liquid and gaseous form, Venus might keep all of its as steam.
Interplanetary space... what fills it?
-radiation (of all sorts - light, gamma ray, higher energy gamma ray, ...)
-gravity. There is no known way to stop gravity, and the only known way to decrease it is to go far far away. The huge mass of the galaxies allows them to affect each other. Stars (suns) affect everything around them, and their effect is strong enough to keep lots of things captive - rocks like planets, small planets (planetoids), asteroids, even rocks small as sand or smaller - and other things not rock - ice blocks big and small, ..., .... Comets are blocks of matter that under solar heat will start to "shed" their mass in gaseous form - the gasses will form the tail, and light reflecting on it make it visible (it's more complicated, as comets will have two tails, ...)

What matter can you find in space? Well, rocks big and small, pieces of ice, other frozen substances, atoms and molecules, ions and even incomplete molecules (like CH3 which needs one H atom to be methane). The sheer "density" of space makes such substances possible, as they don't find a hydrogen to combine to (this, and the high energy of particles that could be able to break molecular bonds).
Gravitational fields come from any mass there is
Photons have mass

Also, there are places where there is a higher density of space dust. If the density is enough, the clouds will collapse, and a star might be formed.

 

[msparish]

Originally posted by: Calin
Photons have mass

No they don't. When one speaks of having mass, it refers to the invariant mass, which photons do not have.

 

[MyK Von DyK]

I don't mean to be rude but it would seem that some of guys/ladies here would benefit greatly from actually attending primary school physics classes. Don't know if in US primary schools they actually teach basic physics, but from where I come from this question (the original one - the limits of accelerating) is what you'd expect from the 7th grade drop-out.

v(r-p)*m(r-p) = v(r)*m(r) - v(p)*m(p)

where:

v=velocity
r=rocket
p=used propulsion fuel
m=mass

read as:

Velocity of rocket minus used propulsion fuel times mass of rocket minus mass of used propulsion fuel equals velocity of rocket times mass of rocket minus velocity of used propulsion fuel times mass of used propulsion fuel.

Obviously you have to account for vectors here so in your case you get v(r-p)*m(r-p) = v(r)*m(r) - -1*v(p)*m(p) => v(r-p)*m(r-p) = v(r)*m(r) + v(p)*m(p)

Any questions? Like where did acceleration go? What acceleration? You throw your rocket's command console through the window and it's gone. Read: one time event. If you happen to find a spare reserve and decide to throw it off-board also, then the next push would be ever so slightly greater then previous as your total mass would be less than before you threw away your first command console. Thus "acceleration" less the "sqrts".

And while we're at it, relatively speaking I might be fat, but in absolute terms I'm massive

 

[msparish]

Originally posted by: MyK Von DyK
I don't mean to be rude but it would seem that some of guys/ladies here would benefit greatly from actually attending primary school physics classes. Don't know if in US primary schools they actually teach basic physics, but from where I come from this question (the original one - the limits of accelerating) is what you'd expect from the 7th grade drop-out.

v(r-p)*m(r-p) = v(r)*m(r) - v(p)*m(p)

where:

v=velocity
r=rocket
p=used propulsion fuel
m=mass

read as:

Velocity of rocket minus used propulsion fuel times mass of rocket minus mass of used propulsion fuel equals velocity of rocket times mass of rocket minus velocity of used propulsion fuel times mass of used propulsion fuel.

Obviously you have to account for vectors here so in your case you get v(r-p)*m(r-p) = v(r)*m(r) - -1*v(p)*m(p) => v(r-p)*m(r-p) = v(r)*m(r) + v(p)*m(p)

Any questions? Like where did acceleration go? What acceleration? You throw your rocket's command console through the window and it's gone. Read: one time event. If you happen to find a spare reserve and decide to throw it off-board also, then the next push would be ever so slightly greater then previous as your total mass would be less than before you threw away your first command console. Thus "acceleration" less the "sqrts".

And while we're at it, relatively speaking I might be fat, but in absolute terms I'm massive

For throwing such insults around, it's too bad that your answer is terribly wrong. The quantites are continually changing. You can't solve the problem without a differential equation, which is well past the scope of a 7th grader.

 

[MyK Von DyK]

Read the original question and you'll get to the part

I'm pretty sure there's a sqrt function in here, somewhere. I know the rocket's velocity can't just keep increasing - this isn't a constant acceleration problem. I imagine the rocket getting closer and closer to its max speed but never reaching it. Like at some point you hit really deminishing returns, and you can't throw stuff out the back of the rocket fast enough to propell the ship faster.

Now, obviously, this guy has plenty of stuff to learn before he gets to the differential equations, doesn't he? And don't take that as an insult to injury, it's only one of life's little facts - not everyone is a rocket scientist and it doesn't even take one to do some basic calculations to understand why rockets will only go this fast and will not accelerate continuously when your fuel supply is limited.

Besides, my point with that equation was that his understanding (Like at some point you hit really deminishing returns, and you can't throw stuff out the back of the rocket fast enough to propell the ship faster.) was basically wrong. And that equation shows exactly that. And my last "relative/absolute" joke shows where his understanding is flawed.

 

[TuxDave]

Originally posted by: MyK Von DyK

(Like at some point you hit really deminishing returns, and you can't throw stuff out the back of the rocket fast enough to propell the ship faster.)

Well to be completely accurate, if you're sitting on a rocket moving at whatever speed and you take anything and chuck it backwards at any speed (slow or fast) the rocket will still go faster. It's more the fact that adding more fuel doesn't give you more speed for free because you also end up increasing the intial mass of the rocket.

 

[MyK Von DyK]

Read before you quote! That's not what I was saying, it's a sentence from the original question. Besides, you're repeating the point I was making. A bit of an overkill if you ask me, so let's not confuse matters further. :shocked: THX, MyK

 

[TuxDave]

Originally posted by: MyK Von DyK
Read before you quote! That's not what I was saying, it's a sentence from the original question. Besides, you're repeating the point I was making. A bit of an overkill if you ask me, so let's not confuse matters further. :shocked: THX, MyK

My bad, I'm used to seeing the use of the quote brackets to distinguish what you're saying from what someone else has said.