Speed In Space

Originally posted by D_Hoffman
Now, suppose you have a craft in space, and you ignite your propellant for a little while to get you up to 100 KPH. If there's no drag to slow you down, wouldn't you then drift at 100 KPH for eternity?

Theoretically, yes. Practically, pretty much.

Originally posted by D_Hoffman
Suppose you fire the propellant again, increasing your speed to 500 KPH. Same rules apply? You'd drift at 500 KPH forever?

Yes, it's called inertia. Objects with mass don't like having their state or motion changed.. If they are still, they prefer to remain that way, and if they are moving at a certain speed, they also prefer to remain that way.

That's why we need force to change this state, or accelerate them. Or deccelerate them, which is basically the same thing, only in the opposite direction.

Originally posted by D_Hoffman
What I'm getting at here, is if the above is accurate, why couldn't you keep firing your propellant to continue going faster and faster with no limit to how fast you could go?

I know what you're trying to get at - the speed of light, or even beyond..

As we all "know", for some strange reason, the faster something is moving, the weirder things become.

This is where the results start to differ for the person inside the craft and for an outside observer.

Things like time dilation and mass increase come to mind.

Let's say you have an engine that can continuously supply a constant force and thus a constant acceleration.

As this craft is going faster and faster, it starts becoming more and more massive and at the same time, time inside it becomes slower. This is true even at slow speeds, but it's effect is negligable, even tho measurable (cessium atomic clocks on airplanes).

As it slowly starts approaching the speed of light, this means, that the engine which supplies the constant force, won't be supplying constant acceleration anymore, due to the increasing mass this force is trying to accelerate:

F=m*a -> a=F/m -> as m(ass) is becoming greater, the a(cceleration) from the F(orce) will be smaller and smaller.

Why this happens is something that is hard to understand and even harder to explain.

The real question is speed in relation to what. This is where relativity comes into play.

To the person inside the craft, the time becomes slower, so is this person even going to notice that his craft is accelerating less and less?

What about the outside observer? What is he going to observe?


The thing is, there is no ABSOLUTE frame of reference here, which you could compare these changes to.

All these changes are relative to the observer. Each will percieve them differently.


Let's say person A is traveling at near the speed of light. He flies past person B.

To person B it appears A is travelling by very very fast, so his mass must be immense and his time very slow.

To person A it appears B is travelling in the opposite direction very very fast, so his mass must be just immense and his time slow.


So who's mass is really increased and who's time is really going slow?


The whole point here, is that since there is no absolute frame of reference available, you have to CHOOSE one, to make your calculations from.

So both of these two people are right about what they percieve about the other, but only from their OWN perspective.


This is as much as i'm capable of explaining right now. I hope someone else can shed some more light on it.

[edit on 17/1/08 by deezee]

Originally posted by Soylent Green Is People
reply to post by D_Hoffman

 

That's one of the concepts behind the 'Ion Engine' or 'Ion Drive', such as the one that is now powering NASA's Dawn mission that will visit the asteroids Ceres and Vesta.

One of the characteristics of the ion drive is that it puts out very little thrust, but since they are electrically powered instead of chemically-propelled, this electrical power source allows the ion drive to fire continuously for a very long time, which in turn -- slowly but surely -- gets the spacecraft up to very high speeds. An ion engine can burn continually for months, or potentially for years.

Chemical rockets, on the other hand, require plenty of very heavy fuel to quickly get up to speed in a big burst of energy expelled in a relatively short duration engine burn. Once the fuel in a chemical rocket is used up -- usually very quickly -- the thrusters can no longer fire (although in practice, some chemical fuel is saved for mid-course corrections and orbital insertion burns).

An ion drive may take a long time to accelerate, but it potentially can reach fantastic speeds. Also, an ion drive allows a spacecraft to do more things, since it will have enough of an energy source to move on to a second destination after it has reached it's first destination. Chemical spacecraft don't usually have the luxury of moving from place-to-place using multple engine burns, since the amount of chemical fuel it can carry is so limited.

Very interesting stuff

What kind of speeds are we talking about with Ion Engines/Ion Drives ?. Seems like NASA are actually doing something worthwhile for once. I wonder this technology could spell the end for the conventional 'phallic' rockets

Originally posted by mug2k


Very interesting stuff

What kind of speeds are we talking about with Ion Engines/Ion Drives ?. Seems like NASA are actually doing something worthwhile for once. I wonder this technology could spell the end for the conventional 'phallic' rockets

In short, no, it won't replace chemical rockets. The "ISP" for ion drives is great, which means you get great "gas mileage," but the thrust is incredibly low, which means it only works once you get into orbit. The ion drive can then be left turned on at "full thrust" for literally months at a time to achieve the full speed. It's the equivalent of the tortoise beating the hare, but really, unless you're going far beyond mars the hare will still win because it takes the tortoise so long to accelerate (and you better have some way of generating electricity that far out - generally speaking, solar won't cut it anymore beyond mars). Ion propulsion is certainly useful if you want endurance rather than speed, such as a mission where you want to visit multiple targets. If you were to just burn the engine straight though you'd get about 10 times the delta-v (speed) that a chemical rocket with the same amount of fuel would give you, but it'll taken you months to reach that speed instead of minutes.

Originally posted by mug2k
Very interesting stuff

What kind of speeds are we talking about with Ion Engines/Ion Drives ?. Seems like NASA are actually doing something worthwhile for once. I wonder this technology could spell the end for the conventional 'phallic' rockets

Ion propulsion is for now most usefull for small probes that have to go very far, but eventually it could even power a manned mission to Mars

Unlike a rocket, which accelerates very fast but also uses up all of it's fuel very fast, ion propulsion accelerates rather slowly, but can do so for a very long time.

It still requires propellant, a gas to ionize and shoot out at high speeds - 30km/s. It uses Xenon, since it's a heavier gas, so it gives more thrust.

Due to the very high exhaust speeds, less of this "fuel" is spent, so it can continue accelerating much longer.

Basically, the faster you can throw "stuff" out of the engine, the less of this stuff you need to achieve a certain acceleration.

I've read somewhere, that given enough time it could reach as much as 10% of the speed of light.

Some information can be found here:

Ion Propulsion FAQ

Boeing Xenon Ion Propulsion Thruster

NASA: Ion Propulsion - Over 50 Years in the Making

[edit on 17/1/08 by deezee]