The Naked Scientists

The Naked Scientists Forum

Author Topic: thrust does not work in space  (Read 58233 times)

Online chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1867
  • Thanked: 143 times
    • View Profile
Re: thrust does not work in space
« Reply #175 on: 24/01/2015 16:53:30 »

A rocket works, for us laymen, by pushing off something. A terrestrial-launched rocket goes nowhere until thrust is given, pushing the rocket off the pad because the expelled air and heat push off the ground. Or the submarine. When launched from a plane, the rocket is dropped from under the wing, and shortly thereafter its thrust begins and the rocket goes forward, this time because it is pushing on air. Were you to take a Saturn-type setup and drop it from, oh, half a mile up, point upwards, somehow, and let it drop, then fire the thrusters, whether or not you make it up, and so escape that gravitational pull, would be a risky proposition. This is just 1/2 a mile, in Earth atmopshere.


False. A rocket pushes off of itself. By forcing propellent in one direction, the rocket goes the opposite direction. If the rocket were pushing off of something, wouldn't it matter what it was pushing off from? Wouldn't it be more effective to push off the hard, stable ground or launch platform than pushing off the air? Well, it doesn't make a difference what is behind the rocket, so I posit that it isn't actually pushing off.
 

Offline Ethos_

  • Neilep Level Member
  • ******
  • Posts: 1277
  • Thanked: 14 times
    • View Profile
Re: thrust does not work in space
« Reply #176 on: 24/01/2015 17:42:31 »
To the OP and Mr. "SteinUntStein"

In an effort to regain some civility in this thread and to likewise offer an explanation for why rockets work in space, please consider the following detailed information.

Place your attention to the inside of the rocket. A tube, if you will, with one end open to space and the other end sealed off. An explosion takes place located in the center of the tube. From this location at the center, there will be force directed in two basic directions, out the back at the open end and likewise, toward the front end which is sealed off. The force directed out the back end is not what propels the craft, it is the force applied toward the front of the rocket. Because the explosion is initiated in the confines of the rocket, the sealed end of the craft feels the push from the pressure wave and experiences movement away from the initial explosion. Thus the rocket moves thru the vacuum. The expanding gases out the back of the rocket have little to do with the motion of the craft.
« Last Edit: 30/01/2015 11:42:16 by CliffordK »
 

Offline SteinUntStein

  • Jr. Member
  • **
  • Posts: 11
    • View Profile
Re: thrust does not work in space
« Reply #177 on: 25/01/2015 13:38:24 »
Place your attention to the inside of the rocket. A tube, if you will, with one end open to space and the other end sealed off. An explosion takes place located in the center of the tube. From this location at the center, there will be force directed in two basic directions, out the back at the open end and likewise, toward the front end which is sealed off. The force directed out the back end is not what propels the craft, it is the force applied toward the front of the rocket. Because the explosion is initiated in the confines of the rocket, the sealed end of the craft feels the push from the pressure wave and experiences movement away from the initial explosion. Thus the rocket moves thru the vacuum. The expanding gases out the back of the rocket have little to do with the motion of the craft.

Even on Earth, if you plug up the exhaust of a rocket it will go nowhere.
Your analogy is explaining things in a half-empty/half-full glass sort of way. Yes  what you say is half true. The rocket does go up (or straight out from under the wing...well kind of straight up, I mean those manned trajectories themselves could use examination...) because of pushing against the rocket. BUT if there was no thrust there, outside against the resistance of air, there would be no movement.

Blow up a balloon. Keep the end closed. No forward movement. Let the hole open, you get forward motion. You get it because the air inside is being expelled outside. Use the air inside as analogy for the spend fuel. Without exhaust it goes nowhere. Seems to me in an alleged vacuum that air would be expelled, but just kind of dribble out the end.
« Last Edit: 30/01/2015 11:39:25 by CliffordK »
 

Offline SteinUntStein

  • Jr. Member
  • **
  • Posts: 11
    • View Profile
Re: thrust does not work in space
« Reply #178 on: 25/01/2015 13:46:25 »

False. A rocket pushes off of itself. By forcing propellent in one direction, the rocket goes the opposite direction. If the rocket were pushing off of something, wouldn't it matter what it was pushing off from? Wouldn't it be more effective to push off the hard, stable ground or launch platform than pushing off the air? Well, it doesn't make a difference what is behind the rocket, so I posit that it isn't actually pushing off.

Yes a rocket pushes off itself, but it needs a catalyst, as you say very next sentence "by forcing propellant in one direction."
No propellant force, no motion, necessary vs. sufficient conditions.
 

Online chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1867
  • Thanked: 143 times
    • View Profile
Re: thrust does not work in space
« Reply #179 on: 25/01/2015 15:38:55 »

Blow up a balloon. Keep the end closed. No forward movement. Let the hole open, you get forward motion. You get it because the air inside is being expelled outside. Use the air inside as analogy for the spend fuel. Without exhaust it goes nowhere.

*That ↑* is a perfect explanation!   ;D

Seems to me in an alleged vacuum that air would be expelled, but just kind of dribble out the end.

But this ↑ is mistaken. If anything, the air would be expelled faster into a lower pressure environment or vacuum, not slower.
 

Offline PmbPhy

  • Neilep Level Member
  • ******
  • Posts: 2760
  • Thanked: 38 times
    • View Profile
Re: thrust does not work in space
« Reply #180 on: 25/01/2015 20:35:37 »
Quote from: chiralSPO
But this ↑ is mistaken. If anything, the air would be expelled faster into a lower pressure environment or vacuum, not slower.
That's precisely the case. The rate of flow is a function of the difference in pressure.
« Last Edit: 26/01/2015 14:51:39 by Georgia »
 

Offline SteinUntStein

  • Jr. Member
  • **
  • Posts: 11
    • View Profile
Re: thrust does not work in space
« Reply #181 on: 27/01/2015 12:36:04 »
Quote from: chiralSPO
But this ↑ is mistaken. If anything, the air would be expelled faster into a lower pressure environment or vacuum, not slower.
That's precisely the case. The rate of flow is a function of the difference in pressure.
The exhaust would just kind of be absorbed by the vacuum (alleged) of space. Low pressure is also not the same as no pressure (vacuum).
« Last Edit: 30/01/2015 11:11:46 by CliffordK »
 

Online chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1867
  • Thanked: 143 times
    • View Profile
Re: thrust does not work in space
« Reply #182 on: 27/01/2015 15:31:44 »
Absolute vacuum is unachievable. But we can call very low pressures "partial vacuum," or even just shorthand it as "vacuum" because no one would be expected to confuse the "vacuum" being discussed with absolute vacuum.

The vacuum of space is also actually just extremely low pressure (down to about one particle per cubic meter!)

I'm not exactly sure what you mean by "absorbed by the vacuum."

______________________
Time try a molecular picture of pressure and thrust!

Pressure (P) is proportional to the number of gas molecules (n) per unit volume (V), as well as proportional to temperature (T, on the Kelvin scale). Hence the handy equation P = nRT/V, where R is a constant.

If we have a box of some gas at room temperature (273 K) then the gas molecules will be zipping around inside the box, bumping into the walls with a rate determined by n/V, and kinetic energy determined by T. The pressure is a result of this: a greater number of molecules in the box will hit the walls more often, and at higher T, they will hit the walls harder.

If we now imagine an immobile wall dividing the box into two compartments, we can calculate the pressure on either side of the wall, given the number of gas molecules and volume of each side (let's assume T is the same on both ends of the box, so the molecules are all moving with the same average kinetic energy). If the left side has twice the number of molecules per liter as the right side, there will be twice as many molecular impacts per second against the left side of that wall as on the right side. As defined, the wall can't move, but if we allowed it to move, it will go to the right, being pushed by all of the molecules on the left. As it moves, the density of gas on the left decreases (because the available volume increases and number of molecules remains the same), and the density of gas increases on the right (because the volume is decreasing). Eventually the densities (pressures) equalize, and the wall stops moving because the molecules pushing it from the right perfectly balance the molecules pushing on the left.

Let's try this thought experiment again, but with an immobile wall that has a hole in it that we can open by pressing a button. Initially there are twice as many molecules on the left side of the box as on the right side, and both sides have the same volume (and temperature). Let's say the hole is circular, with an area of 1 mm2. While the hole is closed, molecules from each side of the box are bumping into the cover and bouncing back the other way (just as above). In any given second, there are twice as many molecules hitting the left side of the hole-cover as are hitting the right side. Now, we push the button, and the cover slides over, opening the hole. Now, instead of bouncing back, the molecules pass right through the hole, with twice as many molecules per second passing from the left side to the right side as there are from the right side to the left side. There are molecules going both ways through the hole, each with the same average speed, but the overall rate of flow is from the left to the right. If the density on the left side of the box were 10 times that of the right side, then there would initially be ten times as many molecules going from left to right through the hole. As the number of molecules on the left fall, and the number of molecules on the right increase, the rate at which molecules cross from right to left approaches the rate they cross from left to right, eventually becoming equal. There are still molecules crossing the hole, but when there are the same number going each direction, the overall flow of gas has stopped.

Now lets imagine that there are two boxes, one inside the other, and the smaller (inner) box has a hole that can be opened with a button. The inner box has 1000 times as many gas molecules in it, and is 1000 times smaller than the outer box (so the ratio of molecular densities is 1000000:1). This means that there are 1000000 times as many molecules bumping into the inside of the box as there are hitting the outside of the box (it will explode if the box isn't strong enough, but let's say it doesn't). When the hole is opened, 1000000 times as many molecules per second will move through the hole from the smaller box into the larger box, as will move from the larger box to the smaller box (note: if the larger box had an extremely low density of gas molecules, there might not be any that pass into the smaller box through the hole). So overall, molecules are leaving the inner box though a hole that's on one side.

The force exerted on the inner walls of the inner box is unbalanced now, as there are fewer molecules hitting the wall with the hole as there are hitting the wall without the hole. So if the inner box is free to move, it will naturally move away from the side that has the hole (being pushed by the gas inside it). As long as the pressure inside the inner box is greater than the pressure of the outer box, there will be a flow imbalance and the inner box will move accordingly. That is thrust.
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4698
  • Thanked: 153 times
  • life is too short to drink instant coffee
    • View Profile
Re: thrust does not work in space
« Reply #183 on: 27/01/2015 20:43:45 »
Quote from: chiralSPO
But this ↑ is mistaken. If anything, the air would be expelled faster into a lower pressure environment or vacuum, not slower.
That's precisely the case. The rate of flow is a function of the difference in pressure.

No I'm afraid you are wrong again. The exhaust would just kind of be absorbed by the vacuum (alleged) of space. Low pressure is also not the same as no pressure (vacuum).

Presumably you are happy to admit that rockets work OK at low altitude. You might also admit that atmospheric pressure decreases with altitude - or is my altimeter operated by fairies? So at what altitude do they stop working?
 

Offline Ethos_

  • Neilep Level Member
  • ******
  • Posts: 1277
  • Thanked: 14 times
    • View Profile
Re: thrust does not work in space
« Reply #184 on: 27/01/2015 21:45:34 »


No I'm afraid you are wrong again. The exhaust would just kind of be absorbed by the vacuum (alleged) of space. Low pressure is also not the same as no pressure (vacuum).
Nowhere in the cosmos does a perfect vacuum exist. So, I'm afraid you are misinformed.
« Last Edit: 30/01/2015 11:13:06 by CliffordK »
 

Offline PmbPhy

  • Neilep Level Member
  • ******
  • Posts: 2760
  • Thanked: 38 times
    • View Profile
Re: thrust does not work in space
« Reply #185 on: 27/01/2015 22:32:57 »
Quote from: SteinUntStein
The exhaust would just kind of be absorbed by the vacuum (alleged) of space.
As I said above, the thrust on, say, a high pressure gas container, is a function of the pressure outside the container and the pressure inside the container. If the pressures are equal ten the container doesn't accelerate. If the outside pressure is near zero then the thrust is at a near maximum. In between those extremes the thrust is also in between and is an increasing function of the difference.
« Last Edit: 30/01/2015 11:40:23 by CliffordK »
 

Offline SteinUntStein

  • Jr. Member
  • **
  • Posts: 11
    • View Profile
Re: thrust does not work in space
« Reply #186 on: 28/01/2015 19:14:29 »
ChiralSPO: I appreciate the explanation attempt however there is one problem, it's that the box would be need to constructed of such material that nothing could go through it. Many perceived impermeable materials can be penetrated by the likes of subatomic particles, light, etc. Also, if that box is truly sealed, in the absence of any compelling factor, there should be no movement because any alleged vacuum would make that an impossibility. Maybe I'm missing how this applies to either thrust in space or the existence of any vacuum.
AlanCalverd: They stop working when there is no atmosphere at all. No atmosphere = no means of propulsion. Pretty basic actually. Maybe your altimeter is off, possible you know ;)
Ethos: I in fact was the first person on this board to assert the non-existence of the vaccum in relation to the thrust question.
PmbPhy: The converse (vacuum exists) is taken as a fundamental truth, without the qualification, about the constituency of space.

There is no vacuum, as Ethos has confirmed. If indeed thrusters work in space IT MUST BE BECAUSE the space you are in IS NOT a vacuum.
« Last Edit: 30/01/2015 11:22:09 by CliffordK »
 

Online chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1867
  • Thanked: 143 times
    • View Profile
Re: thrust does not work in space
« Reply #187 on: 28/01/2015 20:07:20 »
ChiralSPO: I appreciate the explanation attempt however there is one problem, it's that the box would be need to constructed of such material that nothing could go through it. Many perceived impermeable materials can be penetrated by the likes of subatomic particles, light, etc. Also, if that box is truly sealed, in the absence of any compelling factor, there should be no movement because any alleged vacuum would make that an impossibility. Maybe I'm missing how this applies to either thrust in space or the existence of any vacuum.

The box doesn't have to be impervious to everything, just the gas that it contains.

The box won't move when it is perfectly sealed, but will begin to move if a hole is opened on one side, allowing the gas to escape (assuming the environment has a lower pressure of gas).

I know it was a really long post, but I suggest giving it another read to see how it applies to thrust and pressure differences.

We understand that space is not a true vacuum, but that is not why thrusters work in space. If you know there is no vacuum in space, why claim that thrusters can't work in space, and be so hostile to the notion of spacetravel?

By the way, let's try to keep this civil, please... everyone...
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4698
  • Thanked: 153 times
  • life is too short to drink instant coffee
    • View Profile
Re: thrust does not work in space
« Reply #188 on: 28/01/2015 20:20:52 »
Nowhere in the cosmos does a perfect vacuum exist. So, I'm afraid you are misinformed, or more likely, you are just being contrary and or contentious.

So the moment you introduce a rocket, it's no longer "space" (particularly if it is belching out exhaust gases), ergo the question is meaningless.
 

Offline Ethos_

  • Neilep Level Member
  • ******
  • Posts: 1277
  • Thanked: 14 times
    • View Profile
Re: thrust does not work in space
« Reply #189 on: 28/01/2015 22:15:38 »
They stop working when there is no atmosphere at all. No atmosphere = no means of propulsion. Pretty basic actually.


Wrong.........................A rocket needs no air behind the exhaust to move. The motion of the rocket occurs because; While the force is applied in both directions, front and aft, the only end that feels the force is the sealed end.
« Last Edit: 28/01/2015 23:14:50 by Ethos_ »
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4698
  • Thanked: 153 times
  • life is too short to drink instant coffee
    • View Profile
Re: thrust does not work in space
« Reply #190 on: 28/01/2015 22:46:09 »
AlanCalverd: They stop working when there is no atmosphere at all. No atmosphere = no means of propulsion. Pretty basic actually. Maybe your altimeter is off, possible you know ;)

So in your experience (not opinion - this is a science forum), rockets become less efficient as the atmospheric pressure decreases. Have you told NASA and RFSA? Or the Chinese satellite companies, and everyone else who regularly flies rockets? Their experience seems quite different from yours.
 

Offline PmbPhy

  • Neilep Level Member
  • ******
  • Posts: 2760
  • Thanked: 38 times
    • View Profile
Re: thrust does not work in space
« Reply #191 on: 29/01/2015 02:08:33 »
Quote from: SteinUntStein
AlanCalverd: They stop working when there is no atmosphere at all. No atmosphere = no means of propulsion. Pretty basic actually. Maybe your altimeter is off, possible you know ;)
It's a very well established fact which can be derived as a theorem based on Newton's Third Law, and verified countless experimentally and in practice countless times, that what you keep claiming is 100% wrong? It's been demonstrated time and time again with every launch by NASA and other agencies of rockets into space that your claims are 100% BS. They simply do not stop working or even become less efficient when there is less atmosphere.
« Last Edit: 30/01/2015 11:19:17 by CliffordK »
 

Offline dlorde

  • Neilep Level Member
  • ******
  • Posts: 1441
  • Thanked: 9 times
  • ex human-biologist & software developer
    • View Profile
Re: thrust does not work in space
« Reply #192 on: 29/01/2015 12:07:05 »
It's quite an amusing and interesting exercise, to claim some fundamental law of physics doesn't apply in some fairly common situation (rockets in space are fairly common these days - probably about 10,000 launches since sputnik). It allows one to refresh one's understanding of how these laws apply. But to persist as if serious smells of trolling. Many people are unable to grasp the principles of aerodynamics, yet they don't deny that planes fly.

The idea that if you don't understand it, it can't happen, or that the world necessarily conforms to one's naive understanding of it, is understandable, but to deny such a vast amount of evidence is perverse or delusional.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: thrust does not work in space
« Reply #193 on: 30/01/2015 11:26:12 »
I've had a couple of complaints about this topic.
Please strive to keep the comments civil, and on topic.

I have been going through and removing some of the personal attacks, but I would expect board users to refrain from petty name calling and belittling others.

- Moderator -
 

Offline SteinUntStein

  • Jr. Member
  • **
  • Posts: 11
    • View Profile
Re: thrust does not work in space
« Reply #194 on: 30/01/2015 14:59:44 »
Well this should be my last post since I have been censored and singled out as an insulter when all I am doing is counter-punching.
For everyone who "complained", and you know who you are: Thanks for proving my point. ALL my posts have been on topic and ALL were civil unless I was insulted first.
Your science is a fairy tale, a modern myth believed rabidly, tooth and nail, with all the blind acceptance of religious zealotry.
None of you here have been in space. None of you have seen thrusters work in space. None of you, I presume, has made any contribution to real science.
You are just parrots, rehashing the "science" I first knew as bad a decade ago.
So, in humble gratitude for your lack of any help, for your backstabbing and clandestine complaints to moderators because you got your little feelings hurt, because you cannot fight your own battles and prefer to ban the problem rather than correct it, for being hypocritical (YOU HERO FAGS can insult, but noone else can...), I want to say, go **** yourselves, and thanks for the new mission ;)
 

Offline Ethos_

  • Neilep Level Member
  • ******
  • Posts: 1277
  • Thanked: 14 times
    • View Profile
Re: thrust does not work in space
« Reply #195 on: 30/01/2015 16:00:52 »
The understanding concerning propulsion has been misunderstood by the layman ever since rocketry first came on the scene. I remember my physics teacher asking our class in high school how we would explain the motion of a rocket. 100% of the class got it wrong! The common interpretation was that the exhaust coming out of the aft side of the rocket was pushing against the air behind it. Even after our physics teacher attempted to explain that the reason for the rockets acceleration was caused by the force applied to the forward end of the rocket, many of us were still questioning how that could be.

Not until I was much older and attending college did I finally understand the laws of motion. So when people don't understand how these phenomenon work, I can appreciate why they have their doubts. Nevertheless, when individuals resist the facts and refuse to be taught why things react the way they do according to the laws of physics, it becomes quite frustrating.

Given some time and proper examination of all the facts, those who misunderstand why a rocket works will eventually have that light bulb go off in their minds. These moments of growth in our understanding are the WOW moments in our lives and this is why I love science!
« Last Edit: 30/01/2015 17:38:46 by Ethos_ »
 

Offline PmbPhy

  • Neilep Level Member
  • ******
  • Posts: 2760
  • Thanked: 38 times
    • View Profile
Re: thrust does not work in space
« Reply #196 on: 30/01/2015 17:27:23 »
Well this should be my last post since I have been censored and singled out as an insulter when all I am doing is counter-punching.
Who're you kidding?? We all know that you were the one to respond with insults when we explained to you that you were wrong and ignorant of the correct physics. Like too many people you mistook the term "ignorant" to be an insult when in fact all it means is you lack knowledge in a particular area. We're all ignorant in some field but you used that and other things to launch ad hominem responses. Cliff is far to smart to be fooled by your claims. He's read your responses and knows what you did all along.
 

Offline PmbPhy

  • Neilep Level Member
  • ******
  • Posts: 2760
  • Thanked: 38 times
    • View Profile
Re: thrust does not work in space
« Reply #197 on: 31/01/2015 06:13:48 »
The easiest way to understanding this is to look at the simplest situation. In a vacuum, think of a coin with a firecracker next to it right above its flat side. When the firecracker detonates the fragments smash against the coin and bounce off of it. Newton's third law tells us that the coin will rebound. You can look at a rocket engine as a controlled continuous explosion. This has nothing to do with the presence of an atmosphere. In fact it doesn't work as well with an atmosphere present.
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4698
  • Thanked: 153 times
  • life is too short to drink instant coffee
    • View Profile
Re: thrust does not work in space
« Reply #198 on: 31/01/2015 10:41:36 »
There seems little point in discussing this further with SuS until he has answered my question: at what point, during its ascent through the atmopsphere, has he observed the efficiency of a rocket to decrease?

Science is begins with data, not conjecture or analogy, and this is a science forum.
 

Offline Ethos_

  • Neilep Level Member
  • ******
  • Posts: 1277
  • Thanked: 14 times
    • View Profile
Re: thrust does not work in space
« Reply #199 on: 31/01/2015 15:25:49 »
There seems little point in discussing this further with SuS until he has answered my question: at what point, during its ascent through the atmopsphere, has he observed the efficiency of a rocket to decrease?

Science is begins with data, not conjecture or analogy, and this is a science forum.
I agree Alan, but some members come to our forum expecting us to answer their questions in the affirmative while feeling no responsibility to answer ours. Such displays only prove their goals have nothing to do with sharing information but everything to do with exaggerating their egos.
« Last Edit: 31/01/2015 15:28:04 by Ethos_ »
 

The Naked Scientists Forum

Re: thrust does not work in space
« Reply #199 on: 31/01/2015 15:25:49 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums