Speed In Space

Hi

I was thinking about the topic in question the other day but due to my non-existant knowledge of physics I'm not able to workout the answers for myself. Sorry if these are stupid questions

1. There is no gravity in space, so no matter how heavy your aircraft is their should be no slowing down once you've accelerated ?

2. As the aircraft would essentially weigh nothing, would it theoretically be possible to accelerate using a simple propelled house fan ?

3. A normal engine on a fighter jet only allows you to get to a certain speed due the weight of the jet. Would the same jet be alot faster if travelling through space ?

Thanks,
Marcus

Originally posted by mug2k
1. There is no gravity in space, so no matter how heavy your aircraft is their should be no slowing down once you've accelerated ?

Since the vacuum is empty space, there is no drag, so once you reach a certain speed and stop accelerating, the speed remains constant.

But not because of lack of gravity.

Also, if you were then to "roll down a window" and put out an object, and many years later opened that "window", the rock would also still be there if there were no changes in acceleration in between, and the speed was constant.

Besides, there is gravity in space, depending on where you are. There is only one point between the earth and the moon, where the sum of the gravity of both is zero.

Otherwise people usually confuse the counteraction of the gravity and the centrifugal forces present in a sattelite or space station orbiting earth, with lack of gravity.

In fact the two forces just cancel each other out, so you get weightlessness. It's the same thing that keeps the sattelites up there in the first place, not the lack of gravity.

Originally posted by mug2k
2. As the aircraft would essentially weigh nothing, would it theoretically be possible to accelerate using a simple propelled house fan ?

First of all, a fan doesn't work in vacuum. A fan only blows air.

Secondly, even tho there is no weight, there still is MASS. The more massive an object, the more "stuff" you have to throw out in the opposite direction (rocket exhaust) to propell this object.

The more massive an object is, the more force you need, to overcome inertia from the mass.

Without gravity this mass just cant push "down" against a scale, so there is no weight. Weight and mass are not the same.

Originally posted by mug2k
3. A normal engine on a fighter jet only allows you to get to a certain speed due the weight of the jet. Would the same jet be alot faster if travelling through space ?

In the atmosphere, the speed of a jet is limited by it's mass (not weight) and the drag from the atmosphere.

But the same engine wouldn't even work in space, since jet engines need air.

That's why rockets have to use propellants with mass, ejected at a very high velocity, to push themselves forward.

Still, the same rocket in space would accelerate faster than in atmosphere, due to less drag.

But not due to less weight. The mass stays the same.

Of course, if a rocket starts accelerating upwards from earth, it's acceleration is slowed down by the force of gravity. Further away from the planet, where the gravity is lower, it would accelerate faster with the same amount of force.

[edit on 15/1/08 by deezee]

ahh but weight is from the force of gravity. your confusing weight with mass...common mistake. thigns still have mass in the absence of a force..... take planets LOTS of mass....if what your saying is correct evrytiem we launch a missile it would push the planet b/c it has no "weight"

Originally posted by engenerQ
if what your saying is correct evrytiem we launch a missile it would push the planet b/c it has no "weight"

Now you're catching on. In fact, every time we launch a missile it actually does push the planet. But the difference is so small and there are so many other forces at work it can't be accurately measured. And every time a rocket goes into space, the planet loses that much mass and the total gravity of the planet decreases.

Originally posted by mug2k
Sorry if these are stupid questions

IMO there's no such thing as a "stupid question".

Originally posted by mug2k
1. There is no gravity in space, so no matter how heavy your aircraft is their should be no slowing down once you've accelerated ?

Not quite true. There is gravity in space. There is gravity everywhere. It's just very very weak in places away from large masses. The strength of gravity is inversely proportional with distance, so it diminishes quickly, but never altogether.

Space is not empty, there is microscopic dust, gas, and meteoroids which will gradually slow anything down over time (could take billions of years depending on speed, mass and a bunch of other variables). If you are talking about space within a solar system, then I would think there are major differences with interstellar space which would be much freer of obstructions (by a factor of around 10x would be my guess, but perhaps more).

On the other hand, once something is in motion in space, it will never come to rest fully... nothing is at rest in relation to other parts of the universe anyway. Left to their own devices, most objects set adrift in space will eventually end up being captured in the orbit of some other object, it's only a matter of time and perhaps luck.

Originally posted by mug2k
2. As the aircraft would essentially weigh nothing, would it theoretically be possible to accelerate using a simple propelled house fan ?

You are confusing weight and mass here. The craft would have mass, and therefore momentum, although in the complete (artificial) absence of gravity it would be safe to say it had no weight.

Originally posted by mug2k
3. A normal engine on a fighter jet only allows you to get to a certain speed due the weight of the jet. Would the same jet be allot faster if traveling through space ?

A normal jet fighter would never be able to work in space for a number of reasons, so such an experiment would not work. A jet needs a thick atmosphere to fly - the engine needs oxygen, and there would be no way to steer since aerodynamic surfaces need some form of resistance (ie thick atmosphere) to be effective. Spacecraft steer using "maneuvering thrusters".

If you did heavily modify a jet (may as well just design a spacecraft from scratch!), by adding Oxygen fuel tanks etc, the comparison would be technically invalid (comparing two very different beasts - apples and oranges), but putting those aspects aside, it could in theory go much faster since there is no air resistance in space, although the speed would probably end up being a fraction of what is possible now with purpose built craft equipped with new cutting edge propulsion technology like "ion drives".

Also, keep in mind that back here on earth, lightness in aircraft contributes towards speed, in that they have a better mass/thrust ratio - the lighter an aircraft is the faster it can go (as well as conserving fuel), which is why aluminum alloy is used in their construction. There is much more to it than this, but I'll leave it to someone who knows more about aircraft technology than me to explain.

Hope that answers most of your questions.

[edit on 15-1-2008 by C.H.U.D.]

Forgot to add, re the household fan: It would work in theory, but you might have to wait for a while to get your craft up to any kind of speed!

Originally posted by C.H.U.D.
Forgot to add, re the household fan: It would work in theory, but you might have to wait for a while to get your craft up to any kind of speed!

What would the fan push against? The few randomly drifting particles present even in the vacuum of space? In this case, the universe would probably end and this "houshold fan propulsed space craft" still wouldn't crawl even close to the speed of a snail.

I still don't understand, how the OP imagined this question. I could almost get a feeling it was a joke, if it wasnt for the other, more reasonable questions...

reply to post by deezee


Yes, I agree, it would be extremely inefficient, and usless from any practical point of view, but never the less, some propulsive force would be generated.

Originally posted by deezee

Originally posted by C.H.U.D.
Forgot to add, re the household fan: It would work in theory, but you might have to wait for a while to get your craft up to any kind of speed!

What would the fan push against? The few randomly drifting particles present even in the vacuum of space? In this case, the universe would probably end and this "houshold fan propulsed space craft" still wouldn't crawl even close to the speed of a snail.

I still don't understand, how the OP imagined this question. I could almost get a feeling it was a joke, if it wasnt for the other, more reasonable questions...

Yeah it was kind of a joke. I just thought to myself what is something that provides miniscule pushing power and came up with a fan. I knew it would n'tget going to the speed of light but was curious to know if it would be able to actually move an aircraft.

[edit on 16-1-2008 by mug2k]

[edit on 16-1-2008 by mug2k]

A related question. Up in the vacuum of space, there's no resistance, no drag, correct? So a body in motion will remain in motion, and let's also remove any gravitational forces that could act on this motion for my question.

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?

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

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?

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.

I think you have a good point. However I'm a bit unsure of how thrust works in space. I thought thrust was created by whatevr is coming out of the thruster pushing against the air or surface outside it. With no atmosphere for the thrust on a spacecraft to push against, how does it move? Probably a simple answer but i do sales not science

Originally posted by C.H.U.D.
Not quite true. There is gravity in space. There is gravity everywhere. It's just very very weak in places away from large masses. The strength of gravity is inversely proportional with distance, so it diminishes quickly, but never altogether.

I love this one.

If you could hypothetically visit somewhere that is totally devoid of gravity, then simply by being present you provide gravity... (unless you travelled there but left your mass somewhere else - but then your mass which is else where would exert gravity on you - which it wont, cos your not there)

That was clearer before the words left my mind

Originally posted by fiftyfifty
I think you have a good point. However I'm a bit unsure of how thrust works in space. I thought thrust was created by whatevr is coming out of the thruster pushing against the air or surface outside it. With no atmosphere for the thrust on a spacecraft to push against, how does it move? Probably a simple answer but i do sales not science

hmm good question.

i believe it has to do with "if there is a force there has to be a reacting force"

im not sure how to answer it really. hope that helps

reply to post by muzzleflash


Yeah, thrust works on the principal that every action has an equal and opposite reaction. Pretty straightforward.

Edit for typo

[edit on 17-1-2008 by C.H.U.D.]

reply to post by Now_Then


I see what you are saying, and it makes good sense. It's the same problem from quantum mechanics - you can't make a measurement without inadvertently affecting the result. A paradox if ever there was one!

Ok well that helps in a way. 'it just does' is enough of an answer in some cases or it might open up the flood gates and totally wash my brain away.

reply to post by fiftyfifty


The net forces inside the rocket's combustion chamber provide the forward thrust. All other forces cancel out, except the force pushing forward, since the engine nozzle is open in the opposite direction. Therefore, technically it is the forces pushing forward on the inside of the combustion chamber that is providing the forward motion not the exhaust at the back of the chamber through the nozzle.

Here's a graphic that explains what I'm saying:



EDIT: Link and spelling

[edit on 1/17/2008 by Soylent Green Is People]

Ahhh that makes sense.. thanks!

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?

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

I think it would work something like this:

Assuming you were drifting without trying to control your direction, over time, you would accelerate until you were at equilibrium with your environment. Although there is resistance from dust, gas etc, all objects are effectively in orbit around something (the sun in the case of our solar system), and therefore there is a trend to accelerate something until it is "going with the flow" as it were. A good model example would be a roulette wheel.

In the solar system, objects tend to have orbital speeds of between around 10-80 Km/S (at a rough guess), but in interstellar space (objects in a galactic orbit) objects would travel at much higher speeds, in excess of 100 Km/S and possibly above and beyond 300 Km/S (according to some new research).

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?

The limit at this point in time is light speed. As you approach it, your craft gains mass, and when you travel at light speed your mass would be infinite, which would require infinite energy. So only a fraction of the speed of light is possible at this time, at least till we find a way around the problems involved!