In space, what do rockets push against?

[stompk]

Any thrust formula will have to take atmospheric pressure into consideration.

In space, there is no atmospheric pressure.

What is the space shuttle's rockets pushing against.

Some have said, it's the mass of the gas escaping from the nozzle.

How much mass does gas have? Enough to overcome the mass
of the shuttle?

The "scientists" like to give the explanation of releasing a balloon in a room.

The reason the balloon moves around the room, is because the escaping charged gas is pushing against the pressure of the atmosphere.

I just don't see how moving the shuttle without anything to push against is possible.

[harris999]

well there would also be no friction to stop the shuttle either, so it probably could just push itself from the gas being released?

Like on earth gravity pulls it back down, in space nothing pulls it back down, so it would need alot less to push against right?

I dont know the real answer, im just guessing!

its a good question tho, maybe someone more scientific can provide an answer :P

[supertzar]

I have wondered the same thing. A quick google indicates harris999's idea is correct.

[padirishjun]

That's a thought provoking question stompk.......it would bre interesting to see an experiment conducted, in a near vacuous environment, with your analogy of a baloon releasing its gaseous contents..........hmmm,wonder what the results would be............

[angelmoon]

that's one of those questions that when you hear it makes you think why the feck have we never thought of that before ????

just like the question my sons friend asked while an advert for donations for water in an African village was on the tv one afternoon, it was explaining how the children had to walk for miles to the nearest well to collect the water each day and he said "why don't they build there homes nearer to the well?"

out the mouths of babes an all that

[bluechip]

Picture a container with 1000 psi inside of it and completly sealed. There is 1000 pounds of 'push' in all directions, causing the container to sit motionless, all forces cancelling each other. Now remove one end of the container and the 1000 psi acting on the opposite side will push the container in the opposite direction as the open end. A rocket is designed to hold a pressure, while an end is open. So, to answer your question, a rocket 'pushes' against itself.

[tabea_blumenschein]

Rockets are a classic example of Newton's second and third laws in action.

Imagine you're on roller skates, and standing still.

You can push on your chest with your hands all you like, but all of that pushing will never move you backwards. An entity cannot exert a force on itself and change its own state of motion.

But if you have a bowling ball, you can push the bowling ball forward, away from you. By Newton's third law, the bowling ball exerts a force of equal magnitude on you, and it is this reaction force that propels you backwards.

This all happens because you and the bowling ball are two separate entities. And two separate entities are always capable of exerting forces on one another, and hence changing each other's state of motion.

When you look at the rocket in space, you have to remember that the rocket and the gas being expelled out the back are two separate entities, just like you and the bowling ball. The rocket and the gas are propelled in opposite directions because they're exerting forces on one another, and those forces change their state of motion. There is no need for air or some other medium "to push against".

As far as the "burn rate", or the mass of fuel burned per second, that will vary widely depending on the thrust force needed. You might see only a few kilograms of fuel burned per second by the Lunar Landers, or tons of fuel burned per second by the Shuttle or the Saturn V rockets.

There's a differential equation called the "rocket equation" you might learn if you study college level physics. This equation does not take aerodynamic drag into account, nor does it account for a rocket moving through a varying gravitational field. Aerodynamic drag or varying gravitational fields aren't too tough to work out in isolation (provided you know your calculus), but combining them, or factoring them into problems concerning rocket propulsion is PhD level physics.

[rodin]

two separate entities are always capable of exerting forces on one another, and hence changing each other's state of motion.

Precisely. Assume the rocket is in deep space. It will be travelling in a linear path - straight line - with a constant velocity. The rocket becomes its own 'frame of reference' just as the spinning Earth is your 'frame of reference'

Earth appears to you to stand still. So would anyone on the rocket appear to be motionless (discounting relativistic effects which are in any case a hoax IMO).

Thrust is produced by chemical reaction, propelling expanding combustion products at great speed in one direction. The rocket reacts by moving in the other direction since overall momentum must be conserved. Lets say the deep space rocket weights 10,000 Kg and 1 Kg of fuel+oxidiser is consumed per second. Lets say the fuel exits at 10,000 KPH and the burn is 1 seconds. The rocket will experience an accelerating force of 1 Km per second squared. After one second burn the spaceship will hardly have changed its velocity at all.

(to see how acceleration works check speed of gravity on Wikipedia)

You can see how impractical rocket fuel is for reaching high speeds. In order to achieve interstellar travel a new kind of space ship must be found - one that does not rely on thrust, but rather rides the ether - whatever that is.

[stompk]

Picture a container with 1000 psi inside of it and completly sealed. There is 1000 pounds of 'push' in all directions, causing the container to sit motionless, all forces cancelling each other. Now remove one end of the container and the 1000 psi acting on the opposite side will push the container in the opposite direction as the open end. A rocket is designed to hold a pressure, while an end is open. So, to answer your question, a rocket 'pushes' against itself.

The reason there is push in the container, is because the walls are containing it.

Sorry, you explanation leaves a lot unanswered.

If an astronaut was outside of the Shuttle in space, and pushed against the shuttle, who would move?

[stompk]

That's a thought provoking question stompk.......it would bre interesting to see an experiment conducted, in a near vacuous environment, with your analogy of a baloon releasing its gaseous contents..........hmmm,wonder what the results would be............

Now that would answer some questions. Thanks for your insight.

[bluechip]

The reason there is push in the container, is because the walls are containing it.

Sorry, you explanation leaves a lot unanswered.

If an astronaut was outside of the Shuttle in space, and pushed against the shuttle, who would move?

No.

I was trying to put it in the easiest words to visualise it. Even on a launch pad the rocket isn't pushing against the ground. Highly expanding gases in the rocket chamber due to ignition of fuel, creates a pressure. A pressure that is in all directions except at the nozzle. Thusly, it moves forward.

[stompk]

No.

I was trying to put it in the easiest words to visualise it. Even on a launch pad the rocket isn't pushing against the ground. Highly expanding gases in the rocket chamber due to ignition of fuel, creates a pressure. A pressure that is in all directions except at the nozzle. Thusly, it moves forward.

If you blew up a balloon, and released it in space, would the escaping gas move the balloon?

[bluechip]

If you blew up a balloon, and released it in space, would the escaping gas move the balloon?

Yes, the internal pressure on the opposite end of the opening hole in the balloon would be greater than on the open end. Same as here on earth.

[curtaincat]

Any thrust formula will have to take atmospheric pressure into consideration.

In space, there is no atmospheric pressure.

What is the space shuttle's rockets pushing against.

Some have said, it's the mass of the gas escaping from the nozzle.

How much mass does gas have? Enough to overcome the mass
of the shuttle?

The "scientists" like to give the explanation of releasing a balloon in a room.

The reason the balloon moves around the room, is because the escaping charged gas is pushing against the pressure of the atmosphere.

I just don't see how moving the shuttle without anything to push against is possible.

what on earth makes you think they bother with rockets in space ? :rolleyes:

[curtaincat]

well there would also be no friction to stop the shuttle either, so it probably could just push itself from the gas being released?

Like on earth gravity pulls it back down, in space nothing pulls it back down, so it would need alot less to push against right?

I dont know the real answer, im just guessing!

its a good question tho, maybe someone more scientific can provide an answer :P

of course you got it right first time, but if the OP really wants to get into this subject... has he been to http://www.physorg.com/ and they also have a forum. very very interesting stuff, i have lurked there for years, but i dont quite understand most of what they say. still like it. :)

unless of course, the OP wants to talk about it in these forums, if that is the case, forgive me please.

[curtaincat]

that's one of those questions that when you hear it makes you think why the feck have we never thought of that before ????

just like the question my sons friend asked while an advert for donations for water in an African village was on the tv one afternoon, it was explaining how the children had to walk for miles to the nearest well to collect the water each day and he said "why don't they build there homes nearer to the well?"

out the mouths of babes an all that

Maybe they have a belief in "gods" that tell them where to live?
Maybe there are too many mosquitoes around the well?
Maybe the local "chief" controls where the villagers live?
Maybe the land close to the well is stinky and horrible?
I wouldn't have a clue, myself

i know, too many maybe's. but i love the enquiring mind of the child that asked that question... Wonderful ! at least he thought of something.!;):)

[rodin]

The reason there is push in the container, is because the walls are containing it.

Sorry, you explanation leaves a lot unanswered.

If an astronaut was outside of the Shuttle in space, and pushed against the shuttle, who would move?

both, with velocity inversely proportional to mass

[stompk]

Let's look at this from the Harrier viewpoint.

The Harrier produces thrush on a horizontal plane.

When the nozzles are turned down, the plane
hovers, but doesn't move forward even though
it's still producing horizontal thrust.

If the theory that the mass of the gas is pushing
against the chamber withholding the expanding gas
molecules, the jet would move forward while in hover
mode.

But it doesn't. Thrust is only produced by pushing
against atmospheric pressure.

Therefore, the current explanation for space propulsion
is completely flawed, and made up.

;)

[bluechip]

Let's look at this from the Harrier viewpoint.

The Harrier produces thrush on a horizontal plane.

When the nozzles are turned down, the plane
hovers, but doesn't move forward even though
it's still producing horizontal thrust.

If the theory that the mass of the gas is pushing
against the chamber withholding the expanding gas
molecules, the jet would move forward while in hover
mode.

But it doesn't. Thrust is only produced by pushing
against atmospheric pressure.

Therefore, the current explanation for space propulsion
is completely flawed, and made up.
;)

I think I see the big picture as to where this is going. Have a nice day.

[tabea_blumenschein]

Let's look at this from the Harrier viewpoint.

The Harrier produces thrush on a horizontal plane.

When the nozzles are turned down, the plane
hovers, but doesn't move forward even though
it's still producing horizontal thrust.

If the plane is hovering motionless, then there are no horizontal forces acting on the plane; i.e. no horizontal thrust.

Also, if the plane is hovering motionless, the engines are pointed downward, and the force of their upward thrust is exactly equal to the force of gravity pulling downward on the plane.

We describe cases like this -- no motion and the vector sum of all forces acting on the object equalling zero -- as an object in static equalibrium.

All a consequence of Newton's laws of motion, and it's stuff everyone who studies physics cuts their teeth on.

Stompk, do you know something Newton didn't?

[stompk]

Stompk, do you know something Newton didn't?

Yes. I know that there is 0 atmospheric pressure in space. Did Newton know that??


In space, the surrounding atmospheric pressure is zero. In principle, the expansion ratio would have to be infinite to reduce the exit pressure to zero.

http://aticourses.com/rocket_tutorial.htm

In principle? So I am right.

Here is the formula for thrust.


The thrust of the rocket is given by the theoretical equation :

F = lm(dot) ve + ( pe - pa ) Ae


Theoretical equation?? WTF. Why is it, in theory?

In the equation, p stands for pressure. Atmospheric pressure has to be existent to produce thrust, according to the theory.
:rolleyes:

[majorlee]

space is not a complete vacuum? or so i thought? maybe wrong just a lot less dense than the 'space' we live on earth

[stockstalker]

All this assumptions about rockets propelling themselves thru space b/c we assumed man has done it before. Look up moon landing hoax for further proof that yes, you need atmospheric pressure for propulsion. Another reason why man never made it to the moon, or should I say, went half way to the moon, and staged the rest.

[tabea_blumenschein]

The thrust of the rocket is given by the theoretical equation :

F = lm(dot) ve + ( pe - pa ) Ae

In the equation, p stands for pressure. Atmospheric pressure has to be existent to produce thrust, according to the theory.



Check your boundary conditions, stompk.

If the rocket is in outer space, there is no atmospheric pressure, so the variable pa = 0. If you substitute this into your equation, you get:

F = lm(dot) ve + (pe - 0) Ae

F = lm(dot) ve + (pe) Ae

Does it look to you like the force F acting to accelerate the rocket drops to zero when the atmospheric pressure drops to zero? If it does, please explain your reasoning! :D

(I had a hard time reading that equation on the page you linked to; I'm assuming you transcribed/copied it correctly)

~

Since you want to discuss differential equations, let's work out a simple version of the rocket equation. We can do this using conservation of momentum (http://en.wikipedia.org/wiki/Conservation_of_momentum#Conservation_of_linear_mo mentum) considerations. We'll assume the rocket is in deep space, which means that there is no atmospheric pressure, and any gravitational forces acting on the rocket are vanishingly small and may be neglected.

Let m = the initial weight of the rocket plus fuel at time t. Let v be the initial velocity of the rocket at time t. The rocket's initial momentum is the product of the two, mv.

At a time of t + dt, some of the burning fuel will have been ejected out of the back of the rocket. Call the mass of this fuel dm. I'm taking dm to implicitly be a negative quantity, because it makes the integration come out right with less work. If the exhaust velocity of the burning fuel is V (capitalized to distinguish it from the velocity of the rocket itself), then its velocity in our frame of reference (http://en.wikipedia.org/wiki/Frame_of_reference) is (v - V).

The conservation of momentum law states that the momentum of the rocket at time t equals the combined momentum of the rocket and ejected fuel at time t + dt. So we get:

mv = (m + dm)(v + dv) + (-dm)(v - V)

Expanding:

mv = mv + m dv + v dm + dm dv - v dm + V dm

the two v dm terms cancel, as do the two mv terms. The term dm dv is the product of two miniscule quantities and may be neglected. That leaves us with:

m dv = -V dm

dv = -V/m dm

Dividing both sides by dt, and recalling that acceleration (a) is change in velocity over change in time (dv/dt):

a = dv/dt = -V/m dm/dt

This is the basic differential equation for a rocket accelerating in gravity-free space. dv/dt is the acceleration of the rocket. dm/dt is the rate at which fuel is consumed. We can see from looking at the equation that burning fuel at a fast rate (large dm/dt) results in a larger acceleration, as does ejecting it at a high speed (large V). But a more massive rocket + fuel load (large m) means a more modest acceleration.

What we don't see, and didn't even have to take into account, are any effects of atmospheric pressure. The acceleration of a rocket is entirely due to conservation of momentum; any supposed atmospheric effects have nothing to do with it.

Not that I really need to, but I'll go ahead and finish the derivation.

dv = -V/m dm

(integral sign) dv = -V (integral sign) 1/m dm

With the limits of integration [u,v] on the left and [M,m] on the right.

If you integrate correctly, you should get something like this (the negative sign in front of the V goes away when you flip the fraction inside the ln function):

v - u = V * ln(M/m)

Or:

v = u + V * ln(M/m)


M is the mass of the rocket plus the mass of the fuel, and m is just the mass of the rocket. V is the exhaust speed of the ejected burning fuel, u is the initial velocity of the rocket before any of the fuel is burned, and v is the final velocity after all the fuel is exhausted.

~

With a bit more work, you can derive the most general form of the basic rocket equation. It looks like this:

v = u + V * ln[(M + m)/(M + m -ut)] - gt

v is the velocity of the rocket after some time interval t
u is the initial velocity of the rocket
V is the exhaust velocity
M is the mass of the rocket
m is the total mass of the fuel
u is the "burn rate" of the fuel, in mass per unit time
t is the length of time the rocket has been firing
g is acceleration due to gravity

After all the fuel has been exhausted, t = m/u. Therefore, the rocket's final velocity after exhausting its fuel is:

v(final) = u + V * ln[(M + m) / M] - mg/u

Again, nothing in these equations about pushing against an atmosphere.

~

Stompk, are you really sure you want to continue trying to defend your position?

[tabea_blumenschein]

All this assumptions about rockets propelling themselves thru space b/c we assumed man has done it before. Look up moon landing hoax for further proof that yes, you need atmospheric pressure for propulsion. Another reason why man never made it to the moon, or should I say, went half way to the moon, and staged the rest.

The Earth's atmosphere doesn't extend halfway to the Moon. You've debunked yourself.

EDIT: If you seriously believe those "Moon landing hoax" stories, you've been thoroughly bamboozled.

[stompk]

Stompk, are you really sure you want to continue trying to defend your position?

Sure.

You conveniently ignored this statement


In space, the surrounding atmospheric pressure is zero. In principle, the expansion ratio would have to be infinite to reduce the exit pressure to zero.


Now, unless they have proved Newton wrong, the scientist explaining this
has debunked their own formulas.

The way the formula is set up,

F = lm(dot) ve + ( pe - pa ) Ae

pa = atmospheric pressure,

the LESS the atmospheric pressure, the greater the thrust???

Come on, do they think we are stupid.


They went to the moon? How did they pan the camera up to film the last liftoff from the moon? Apollo 17.

And the first person that suggests remote control, I'm going to laugh.

[stockstalker]

The Earth's atmosphere doesn't extend halfway to the Moon. You've debunked yourself.

EDIT: If you seriously believe those "Moon landing hoax" stories, you've been thoroughly bamboozled.

There is less air "half-way" to the moon.

Secondly, I don't know who's more bamboozled, the guy who didn't see that video of a NASA astronaut in Earth orbit placing a cutout frame on the window of the space shuttle (they also painted the panels housing that window pitch black-------- aka to simulate space-------- the same thing you would do in a studio) to make it seem like they were further away from the Earth (ie. on their way to the moon) than they were actually, or me.

Btw, this is the video I'm talking about:

http://video.google.com/googleplayer.swf?docid=1376152848542315216

[freethinker]

The reason there is push in the container, is because the walls are containing it.

Sorry, you explanation leaves a lot unanswered.

If an astronaut was outside of the Shuttle in space, and pushed against the shuttle, who would move?

the mass and inertia of the shuttle is much greater than the astronaut so the shuttle would push back on the astronaut and the astronaut would move backwards

the shuttle would also probably move a very very small amount

[hepcatdsm]

In space, what do rockets push against?...or where the FORCE comes from to push against the rocket??

very simple...Force = Mass X Acceleration (Newton's 2nd law)

It's not the velocity that give force, it's the change in velocity (acceleration)

The MASS is the gas molecule exiting the nozzle

The ACCELERATION is given by the nozzle geometry (converging/diverging for supersonic speeds)

that's it

[tabea_blumenschein]

Sure.

You conveniently ignored this statement

In space, the surrounding atmospheric pressure is zero. In principle, the expansion ratio would have to be infinite to reduce the exit pressure to zero.

Now, unless they have proved Newton wrong, the scientist explaining this
has debunked their own formulas.

The way the formula is set up,

F = lm(dot) ve + ( pe - pa ) Ae

pa = atmospheric pressure,

the LESS the atmospheric pressure, the greater the thrust???

Come on, do they think we are stupid.

Why didn't you quote the entire paragraph?

In space, the surrounding atmospheric pressure is zero. In principle, the expansion ratio would have to be infinite to reduce the exit pressure to zero. Thus optimum expansion is impossible, but it can be approximated by a very large nozzle diameter, such as can be seen on the main engines of the space shuttle with e = 77.5. There is ultimately a tradeoff between increasing the size of the nozzle exit for improved performance and reducing the mass of the rocket engine.

I'll try to explain this to you in simple English. For maximum thrust, you want the exit pressure and the ambient pressure to be equal. In space, the ambient pressure is zero because there is no atmosphere. Obviously, you can't reduce the exit pressure to zero to match, so you're necessarily losing a bit of efficiency. But as noted in the quoted paragraph, you can compensate with a large nozzle diameter.

Granted, you don't delve into the gory details of rocket propulsion in standard physics texts like the ones I have. But it's painfully obvious that what is being described is the effect that ambient pressure has on the efficiency of the rocket, not on whether it will perform in space or not. That rockets work in space where there is no atmosphere is proven by the conservation of momentum law: if mass is ejected out the back of the rocket, the rocket is damn well going to accelerate forward.

Argue that. If you can.

They went to the moon? How did they pan the camera up to film the last liftoff from the moon? Apollo 17.

And the first person that suggests remote control, I'm going to laugh.

Remote control. Enjoy your laugh.

[elton]

In space, what do rockets push against?...or where the FORCE comes from to push against the rocket??

very simple...Force = Mass X Acceleration (Newton's 2nd law)

It's not the velocity that give force, it's the change in velocity (acceleration)

The MASS is the gas molecule exiting the nozzle

The ACCELERATION is given by the nozzle geometry (converging/diverging for supersonic speeds)

that's it

At last someone talks sense. No wonder normal people laugh at the morons on this forum.

[stompk]

At last someone talks sense. No wonder normal people laugh at the morons on this forum.

Why is is that you have 301 posts, and haven't started a thread. And your posts are all anti-conspiracy.

Get out of here, you government funded phony.

I promise you wouldn't call me a moron to my face, punk.

[stompk]

In space, what do rockets push against?...or where the FORCE comes from to push against the rocket??

very simple...Force = Mass X Acceleration (Newton's 2nd law)

It's not the velocity that give force, it's the change in velocity (acceleration)

The MASS is the gas molecule exiting the nozzle

The ACCELERATION is given by the nozzle geometry (converging/diverging for supersonic speeds)

that's it


Could you please show me how many molecules at what speed it would take to counter the inertia of the mass of the shuttle?

[hepcatdsm]

Could you please show me how many molecules at what speed it would take to counter the inertia of the mass of the shuttle?

Well the 'how many' is not a number really...you could try to find the number of molecules, but its pointless...Normally, you use the burning rate of fuel(or mass flow rate), the exit velocity and pressure of the gas and the exit area of the nozzle. Combine all that together and you get:

Force = (mass flow rate) x (exit velocity) + (pressure difference between nozzle exit and ambient pressure which is 0 in space) x (exit area of the nozzle)

By combining, F= (Mass of shuttle) x (Acceleration of shuttle) with the previous equation, you could find out what MASS FLOW RATE is required to get an acceleration (getting an object to move from rest is an acceleration).

If you really want to know 'how much molecules' is required...you need know the thermodynamic properties of the gas at the exit and with the Avogadro number, the perfect gas law and the mass flow rate, you will be able to find the number of molecules exiting at each second at the nozzle.

But when talking about 'burning of fuels' there is a chemical reaction that changes the number of the different molecules during the burning. At the nozzle exit, it is possible that some fuel is still burning, meaning that the number of a particular molecule will always be changing until the chemical reaction as stopped. This is why I said at the beginning that finding out the number of molecules is pointless...
TOTAL MASS IS ALWAYS CONSERVED(in a closed system)...molecule types aren't...

[elton]

Why is is that you have 301 posts, and haven't started a thread. And your posts are all anti-conspiracy.

Get out of here, you government funded phony.

I promise you wouldn't call me a moron to my face, punk.

And I promise you that you won't get a GCSE in physics if you can't understand basic Newtonian motion! Teenager.