Why rockets don’t work in a vacuum

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This is the first post in a comprehensive thread that really summed it up for me. If it’s wrong then it sure fooled me.

Apollo, and more space hoaxes • Re: Does Rocketry Work in the Vacuum?
by Boethius on Jan 28, 2015 6:58 AM
The reason space travel is not possible is because the systems we claim to use to propel a rocket through space operate on gas pressure and there is no gas pressure in space.

Gas pressure requires molecules to be in contact with each other, bouncing off each other, causing millions of collisions per second, etc… If you release gas into the vacuum of space, the first molecule that pops out will shoot off into the distance at a constant speed, so will the one behind that, never catching up with the first one. The third, fourth, etc… all fly off into the distance trying to fill the vacuum by finding their empty corner. So no matter how much gas you produce none of it will ever change the pressure under a space ship. None it if will ever push a spaceship. To push a spaceship there must be some locally high pressure under it, which is impossible since the pressure in space is 0 everywhere.

Back the the Nozzle and the Massflow equation F=MA on earth
Think about a fire hose shooting water. A force comes directly back against the column of water shooting out. Why? Because the first drop of water has to pas through air, which is dense, causing many collisions, slowing down the drop of water. The second drop, directly behind the first, will not be slowed down by the air so it will collide with the first drop, the third drop hits the second drop and so on, the fast water coming through the hose pushing through the slower water outside causes Newton’s 3rd Law to push back on the column of water. This is why you need people holding the hose to add an unbalanced force otherwise the hose would not be able to push water through that column anymore, the water column would be diverted and the hose would flop around. It is obvious that one drop of water does not push back on the hose, you need a fast moving column.

The nozzle and the Massflow equation in space
Since the molecules leaving the combustion chamber and entering the vacuum never slow down, never collide with any outside objects, nor with each other, their force is always moving forward, away from the ship. There is no way for that force to be returned to the ship. There is no way for the force of the moving molecules to be extracted and used for propulsion. Their force is carried off into the far corners of space. This is also known as Joule Expansion. Remember that as soon as the nozzle is opened, the combustion chamber becomes part of the vacuum of space as is subject to its laws. A closed chamber is under pressure but not an open one.

NASA is lying at the molecular level
But that’s OK because most people don’t usually look there. The awesome, spectacular and heroic nature of space exploration is enough to cloud the most logical minds. Most respectable engineering schools won’t touch space flight and those who do have tiny departments. If it was really a multi-billion dollar government funded operation, every school in America would have their hands out for government grants like the do with Engineering, Computer Science and Biology. But why train thousands of the best minds of a generation in a field that doesn’t exist?


This thread has seen many forum members come and go. It’s been highly divisive, which must be why so many non fakeologists have avoided even discussing it. It’s like trying to reconcile the WTC twin tower imagery with any known method of building destruction. What we are told and what we know are simply irreconcilable, so regular people, from academia to the uneducated, simply don’t even try to explain what contradicts.

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67 thoughts on “Why rockets don’t work in a vacuum

  1. Hybrid Vre

    If rocket thrust works in space, the same reaction idea would work for every action. Why carry fuel when all you would need to do is bang the side of your craft with a large bat, the action would cause a reaction? Like hitting myself in the head with my right hand to move left.

  2. Victor Talha

    I’ve been scouring the net trying find a good, sound, and plausible answer to this query of mine. It’s neber sat with mw well. Not even in grade school. How in the heck does anything combust or propel in a vaccum??

    Thnx for being truthful!

    1. William Vietinghoff

      This is a reply to your question: “How in the heck does anything combust or propel in a vacuum??”
      The J-2 engines that powered the Saturn V vehicle that took astronauts to the moon were designed to ignite and operate at high altitudes at extremely low atmospheric pressures, essentially a vacuum. The J-2 engines used liquid hydrogen as fuel and liquid oxygen as the oxidizer. At Ignition Start, these propellants were injected into the combustion chamber which was a vacuum. The propellants were ignited by what were essentially “spark plugs” at the side of the chamber. Once the propellants started burning there was pressure in the combustion chamber (no vacuum) and the pressure in the combustion chamber exerted a thrust force on the circular flat injector of the engine. That combustion process generated the thrust that pushed the Saturn vehicle on to the moon.

      1. ab Post author

        William: No one has been to the moon, let alone left earth’s immediate vicinity. You can stop your debunked space myths now about Saturn and rockets.

        1. William Vietinghoff

          The mention of the Saturn V space vehicle in connection with the description of the combustion process in a rocket engine was not intended as any additional testimony as to whether we really went to the moon; that is a separate issue and not the purpose of my reply. The mention of the vehicle was only intended to put in context the engine being talked about. Therefore let’s put any statements about the moon aside and return to the original question being answered: ““How in the heck does anything combust or propel in a vacuum??”
          I interpreted that question as being a restatement of two questions I have heard before: (1) is the ignition of two propellants affected when they are injected into a vacuum? And (2) will combustion be possible since there is no oxygen in a vacuum?
          Number (1) is a valid question and in fact, the designers of rocket engines asked themselves that question when given the assignment to design upper stage engines.
          Rocket engines have been started and run in a vacuum. An example that comes to mind is the fourth stage of the Peacekeeper Missile; formerly know as the “MX Missile”. That propulsion system used storable propellants: monomethylhydrazine was the fuel and nitrogen tetroxide was the oxidizer. Because Stage IV was designed to ignite and operate only at a very high altitude the verification testing had to be done in a simulated high altitude. Several models of Stage IV were sent to Arnold Engineering in Tullahoma, Tennessee, where they were installed in a vacuum chamber and fired. The rocket engine in the stage always ignited, exerted thrust, and performed as designed. The answer to (1) is: rocket engines can ignite and operate in a vacuum.
          Number (2) question is sometimes asked because people think that rockets are air-breathing like aircraft with internal combustion engines or jet engines. Therefore how can you burn the fuel when there is no air? Vehicles propelled by rockets always carry the necessary oxidizer along with the fuel. The answer to number (2) is: rocket engines don’t need an atmosphere to maintain combustion.

            1. William Vietinghoff

              I am not clear how to answer the question “Can you direct me to the example that comes to mind.” The example was the vacuum testing of Peacekeeper Stage IV at Arnold Engineering, that demonstrated ignition and combustion performance at high altitude. There is no reporting or reference to the testing that can be accessed outside of the company records at Arnold Engineering and Rocketdyne.

              1. Vespadouglas

                So your example that comes to mind is reliant on the word of NASA. . There is no proof then of your mind(less) example . Why use examples of Nasa fakery as evidence in debunking allegations of nasa fakery .

                1. William Vietinghoff

                  The Peacekeeper Missile Program, from which the example of testing Stage IV in the vacuum chamber at Arnold Engineering was used, was an Air Force Program. NASA was not involved in that activity in any way. Having successfully demonstrated ignition and operation at altitude, the Stage IV was accepted by the Air Force and incorporated into the missile. Stage IV was the final stage and was the propulsion system for what was was termed the Post-boost vehicle. Stage IV was connected to the Deployment Module that carried the warheads. The Peacekeeper Missile was launched many times from Vandenberg Air Force Base in California and established a remarkable record of accuracy in dropping inert warheads at targets on Kwajalein Island in the Pacific. People who were not involved may choose not to believe that this testing was ever done and that the results were ever obtained if they have preconceptions of how rockets work, but this information was presented to at least make people who follow this forum aware of this activity. Hundreds of people were involved in the planning, conduct, witnessing and data analysis of these tests. Anyone attempting to deny this data exists would have a long uphill battle.

          1. Paris

            What about Joule expansion? An expanding gas will do no work in a vacuum it cannot fill. No thermodynamic change means no inertial or kinetic change right?

            1. William Vietinghoff

              A gas expanding in a vacuum is doing no work, but the gas in the rocket engine combustion chamber, is not in a vacuum; it is at a pressure of many hundred of pounds per square inch. It is pushing against the face of the injector and moving the rocket forward. There is very little opposing force as the gas is flowing out into a vacuum. Work is Force times Distance (W=F x D). Moving a rocket propelled vehicle from zero velocity on the launch pad up to hundreds if not thousands of miles per hour requires WORK. If the expanding combustion gas in the combustion chamber is not providing that work, then what is? ?

              1. arthur king

                The gas would get sucked out of the rocket chamber before doing any work. I think you know this.
                Missiles and Large Rockets Are Propaganda.
                Newton was wrong.
                Large rockets are a hoax.

                1. Justin Franks

                  Exhaust gases exit a rocket at hypersonic velocity due to the choked flow at the throat of the nozzle. The gases are able to propel the rocket and exit the nozzle long before the vacuum of space would ever be able to pull them out.

                2. William Vietinghoff

                  Replying to these comments requires some understanding and agreement on definitions. In what way or which aspect is referred to in the phrase “large rockets are a hoax”? What constitutes “large”? Are the rockets attached to aircraft or used by the military considered “small”? If yes, then these are not considered a hoax. What part is the hoax? I would assume that the comment refers to the large rockets that are used for vehicles such as the Atlas missile, or the Delta vehicles, or the space shuttle orbiter. If yes, is there any disbelief of their even being built? If no, is there any disagreement that they were fired on test stands to measure the thrust and flow rates? If no, is there any disagreement that they were used to actually launch vehicles, without debating whether they went very far? Again, at what stage is the hoax being pulled off? If there is any doubt that large rocket engines even exist all, what is the point of this discussion?
                  With regard to whether the gas pouring out of a rocket nozzle is doing any work, let us first define “work”. When you push on a piano and do not move it, regardless of how hard you struggle and work up a sweat, if the piano doesn’t move, you are not doing any “work” (in the physics definition). However, if you push the piano with a 50 pound force and move it 2 feet you have expended 100 foot-pounds of work (Work = Force x Distance). The combustion gases in a rocket engine are not “sucked” out. They flow out in response to the chamber pressure. Again, the chamber pressure, in some engines 500 pounds per square inch, pushes the injector face (with a surface area of say, 450 square inches) with a force of 225,000 pounds. The chamber pressure is maintained at 500 psi by the incoming flow of the propellants. The combustion of the fuel and oxidizer generates a tremendous amount of energy. When the engine is fired during launch, and the vehicle is lifted off, the combustion process is generating 225,000 foot-pounds of work for every foot the vehicle moves.

          1. William Vietinghoff

            This is my reply to Rygy:

            The phrase “essentially a vacuum” as applied to the operation of the J-2 rocket engines was used in anticipation that some reader of this forum might argue that those engines never had to fire in a truly “hard” vacuum. To repeat again, these comments were written to answer the original question posted here: “How in the heck does anything combust or propel in a vacuum??”

            One definition of “vacuum” is a space where the gas pressure is substantially lower than atmospheric. To others, the word “vacuum” refers only to a space without a single gas molecule. But in using that definition, be aware that even thousands of miles from Earth in deep space, there are still gas molecules flying around, perhaps one to ten molecules per cubic centimeter.

            The J-2 engines in the second stage of the Saturn V vehicle ignited at about 40 miles up. There is very, very little atmosphere there, but still more than 100 miles up. But the engine designer has to consider whether the absence of full atmospheric pressure (14.7 psi) will affect the ignition and combustion processes. The fact that the surrounding space is not a “hard” vacuum” is irrelevant. The concern is that the absence of normal air density may alter the spray pattern of the propellants. Therefore, instead of writing “essentially a vacuum”, the words “for design purposes is a vacuum” could have been used. The use of both fuel and oxidizer in the J-2 engine eliminates the need for atmosphere, and the ignition works fine. That is how rocket engines combust.

        1. William Vietinghoff

          The phrase “essentially a vacuum” as applied to the operation of the J-2 rocket engines was used in anticipation that some reader of this forum might argue that those engines never had to fire in a truly “hard” vacuum. To repeat again, these comments were written to answer the original question posted here: “How in the heck does anything combust or propel in a vacuum??”

          One definition of “vacuum” is a space where the gas pressure is substantially lower than atmospheric. To others, the word “vacuum” refers only to a space without a single gas molecule. But in using that definition, be aware that even thousands of miles from Earth in deep space, there are still gas molecules flying around, perhaps one to ten molecules per cubic centimeter.

          The J-2 engines in the second stage of the Saturn V vehicle ignited at about 40 miles up. There is very, very little atmosphere there, but still more than 100 miles up. But the engine designer has to consider whether the absence of full atmospheric pressure (14.7 psi) will affect the ignition and combustion processes. The fact that the surrounding space is not a “hard” vacuum” is irrelevant. The concern is that the absence of normal air density may alter the spray pattern of the propellants. Therefore, instead of writing “essentially a vacuum”, the words “for design purposes is a vacuum” could have been used. The use of both fuel and oxidizer in the J-2 engine eliminates the need for atmosphere, and the ignition works fine. That is how rocket engines combust.

          1. Vespadouglas

            WV .How much are you getting paid to be here ? It just does not make sense that someone who has found and digested these threads here and at CF ( threads which are full of irrefutable evidence of space fakery ) could still spout the shite you spout

            1. Justin Franks

              It really must be convenient to be able to dismiss outright any information which contradicts your beliefs, by labeling the provider of such information as an agent “in” on the conspiracy being paid to spread lies. It seems to be a common tactic — when you are unable to counter an argument, call the person a piece of crap who is willing to deceive the public for a fat paycheck.

            2. William Vietinghoff

              Thank you for making that comment. This gives me the opportunity reply and to zero in on what issues are outside the experience or understanding of the readers of this forum. My comment was not intended to include any evidence that space travel is not fakery and did not. That is a separate subject. My words were directed solely at the point that combustion in a vacuum is possible, that the fuel and oxidizer materials that are carried in vehicle tanks are sufficient for combustion; no air is required as with a jet aircraft. The topic of being in a vacuum was emphasized as rocket engine designers must take into account the environment and attitude that the rocket engine will be subjected to. Forget space. For example the sled in which Colonel Stapp conducted extreme acceleration tests on himself at Edwards Air Force Base in California and at Holloman Air Force Base in New Mexico was propelled by a rocket engine. There was no space travel involved with that engine. This engine was only used on a sled that ran along the ground. There was the special consideration that the engine had to operate horizontally, not vertically as with engines designed for vehicles. Is there any doubt that Stapp’s rocket engine required careful planning of the propellant injection and mixing process, was built, tested and eventually used successfully? There are many other engines that do not go into space, such as the one in the Lance, a 75 mile ground to ground missile. If anyone found an error, unverifiable, or unbelievable statement in my explanation of combustion in a vacuum, please point it out, but be specific. Merely suggesting that the explanation is part of space fakery leaves readers with uncertainty as to what is being talked about.

              1. Vespadouglas

                A rocket will not thrust in a vacuum . The thrust will dissipate into the vacuum immediately thus nullifying any ” work ” it may be doing . There is not one shred of evidence to dispute this . Repeating engine model numbers and stating heights , at which (alleged) incidences occur is not proof of anything .

                1. ab Post author

                  Agreed. There is no where on earth to simulate a vacuum either. A vacuum chamber has walls to push against. Also, there is no proof space is a type of vacuum since we’ve probably never been. This discussion lends itself to my ball earth skepticism blog, flatearth.fakeologist.com

                  1. Justin Franks

                    Rockets do not work by “pushing against” anything. Why is this so hard for some people to understand? Rockets work on one of the simplest and well-understood principles of Newtonian physics. Put a balloon in a large vacuum chamber much farther from the walls than the small amount of air stored inside the balloon could even have any effect on, and when the balloon is popped, it will fly around exactly as it would at sea level. Rockets work the same way — the mere act of expelling part of its mass is what generates thrust, not how that expelled mass interacts with other mass separate from the rocket. Also, there is no delay before the balloon starts to move. If its air were somehow pushing against the walls of the chamber, it would take a short but easily measurable time for the pressure to reach the wall, but this is not what is observed in any vacuum chamber test.

                    Stand on a skateboard with a heavy object and throw that object. You will be propelled in the opposite direction. And you think that the object you threw is pushing against the air to make you move?

                    1. William Vietinghoff

                      Fakeologist won’t allow me to repeat to you a reply I sent to Vespadouglas relative to rocket engines firing in a vacuum. This is to suggest you watch for that. In all seriousness, I am interested in the understanding that many people, including the writers to this forum, have relative to how engines operate. I guess I could have boiled down my reply to Vesadouglas to one of the sentences I included. I will repeat it here, and I would anyone and everyone to chime in.
                      If a rocket engine can’t develop thrust in a vacuum, then what is the lowest outside pressure in which it will develop thrust?

                    2. Justin Franks

                      You don’t need to reply to me also, as I get an email notification when you reply to someone else in this thread.

                      Rockets can work in a vacuum, this has been demonstrated many times in the massive (football field size) vacuum chamber in the Space Power Facility in Ohio. I think this whole “rockets can’t produce any thrust in a vacuum since it has nothing to push against” thing started when one person who both believed that all space travel is hoaxed and does not understand basic physics made this claim. Other space hoax believers saw this, and their confirmation bias caused them to accept the claim without actually taking the time to research and verify its validity (since it appears to support their beliefs, they just automatically accept it as true).

                    3. William Vietinghoff

                      I tried to log in to fill out informaton on myself and am having great difficulty. For example it says my user name must be only letter and numbers. Well the one I used is only lower case letters, but the error message keeps popping up.

                2. William Vietinghoff

                  Thank you for replying. I sincerely mean that. I often wonder if comments I make are ever read by anybody. I appreciate the time you took to think through your reply. As I read the inputs to this forum I am not always sure which statements of fact are agreed upon and by whom. The obvious position taken by many is that a rocket engine cannot develop thrust in a vacuum. Without arguing for or against that statement, I think it is fair to point out that it relates to other conditions. I have questions about those.
                  For starters, is there any disagreement that rocket engines, under certain conditions, can produce thrust?
                  Is there any disagreement that rocket engines on missiles and other vehicles have developed thrust in tests at Cape Canaveral sufficient to at least lift the craft off the launch pad?
                  Thrust level is determined by the rate at which the mass of burned propellants, as hot gases, flow out of the nozzle and the velocity of those gases. Agreed?
                  If a rocket has to push against the atmosphere, then a denser atmosphere should allow the engine to develop a higher thrust? Agreed? On days with higher barometric pressure, the same engine should produce more thrust.
                  Therefore, if we were able to keep increasing the outside atmospheric pressure (from 14.7 at sea level to say 25 psi, to 50 psi, to 100 psi, etc.), we should keep getting higher thrust. Agreed?
                  But wait, if the pressure inside a rocket combustion chamber is 500 psi, and the outside ambient pressure is raised to 500 psi, the same amount, the outside air will be extremely dense and easier to push against. A lot more thrust, right? But because the outside pressure is equal to the chamber pressure, no outward flow of gases can occur. Therefore there will be no thrust, which depends on the movement and velocity of the gases.
                  As a matter of fact, the thrust of engines being static fired on test stands has been demonstrated to increase at higher altitudes where atmospheric pressure is less. An engine fired at sea level at Cape Canaveral will have a higher thrust if test at Denver, Colorado.
                  That would suggest that as the outside pressure gets closer to a vacuum, the thrust will keep getting higher and higher.
                  Those who believe that thrust cannot be developed in a vacuum will argue that is not possible. In that case I would ask, will a rocket engine operate at any pressure lower than sea level pressure? If, so, how low? At what pressure does the engine stop developing thrust?

                  1. arthur king

                    How about a reproducible experiment anyone can conduct?
                    The one I watched on YOUTUBE makes use of a tube- so the whole apparatus acts like a gun barrel. This is not the same thing as a rocket in a vacuum.

                    Put an incredibly powerful vacuum cleaner tube at the end of an far less powered air powered rocket and see what happens.

                    The vacuum of space that Nasa claims isn’t a true Vacuum- –see links below for more– is supposed to be ‘infinite’ at least compared to a little rocket.

                    Vacuum and rockets aside. Orbital Mechanics are flawed.

                    Newton and Einstein both have that exact problem. Us normal folk cannot do any experiment to prove their ideas about gravity correct. I know. I know. Drop the mike and that proves gravity, Sure but it does not prove that the celestial bodies are physical objects flying around the imagined Cosmos.

                    What I mean specifically is the leap of faith it takes to buy into conceptual nonsense supported by bad math.

                    Einstein &; Newton Both Made the Same Mistake:
                    They claim an accelerated velocity can be balanced by a (fixed) constant velocity. This is obviously, conceptually and logically flawed reasoning despite any mathematical model fudgery.

                    Sir Isaac Newton imagines a cannonball fired from a tall mountain. He equates this physical body with celestial bodies like the Moon in order to ‘prove’ that an apple falling here on Earth is equivalent to the Moon’s assumed orbit around the Earth. Sir Isaac mathematically balances gravity, an accelerated phenomena (considered as velocity), with his concept of (an assumed) inertia which is defined as having a set, fixed and constant, velocity.

                    For example:
                    A is an apple that is magically growing without a tree. The Apple will never stop expanding and growing and taking on mass and weight. It gets heavier and heavier and over time will reach an unimaginable weight.

                    B is the table it rests on. The table can only hold 100 lbs before breaking.

                    At some point the Apple will gain too much weight and the table breaks.

                    read here for more:




                    1. arthur king

                      There;s a long history of film fakery:

                      “The lesson here, surely, is not that the camera can, and often does, lie, but that it has lied ever since it was invented. “Reconstruction” of battle scenes was born with battlefield photography. Matthew Brady did it during the Civil War. And, even earlier, in 1858, during the aftermath of the Indian Mutiny, or rebellion, or war of independence, the pioneer photographer Felice Beato created dramatized reconstructions, and notoriously scattered the skeletal remains of Indians in the foreground of his photograph of the Sikander Bagh in order to enhance the image.

                      Most interesting of all, perhaps, is the question is how readily those who viewed such pictures accepted them. For the most part, historians have been very ready to assume that the audiences for “faked” photographs and reconstructed movies were notably naive and accepting. A classic instance, still debated, is the reception of the Lumiere Brothers’ pioneering film short Arrival of the Train at the Station, which showed a railway engine pulling into a French terminus, shot by a camera placed on the platform directly in front of it. In the popular retelling of this story, early cinema audiences were so panicked by the fast-approaching train that—unable to distinguish between image and reality—they imagined it would at any second burst through the screen and crash into the cinema. Recent research has, however, more or less comprehensively debunked this story (it has even been suggested that the reception accorded to the original 1896 short has been conflated with panic caused by viewing, in the 1930s, of early 3D movie images)—though, given the lack of sources, it remains highly doubtful precisely what the real reception of the Brothers’ movie was.

                      Certainly, what impresses the viewer of the first war films today is how ludicrously unreal, and how contrived, they are. According to Bottomore, even the audiences of 1897 gave Georges Méliès’s 1897 fakes a mixed reception:

                      A few people might have believed that some of the films were genuine, especially if, as sometimes happened, the showmen proclaimed that they were so. Other viewers had doubts on the matter…. Perhaps the best comment on the ambiguous nature of Méliès’ films came from a contemporary journalist who, while describing the films as “wonderfully realistic,” also stated that they were artistically made subjects.

                      Yet while the brutal truth is surely that Méliès’s shorts were just about as realistic than Amet’s 1:70 ship models, in a sense that hardly matters. These early film-makers were developing techniques that their better-equipped successors would go on to use to shoot real footage of real wars—and stoking demand for shocking combat footage that has fueled many a journalistic triumph. Modern news reporting owes a debt to the pioneers of a century ago—and for as long as it does, the shade of Pancho Villa will ride again.”



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  4. boethius

    Here’s a simple explanation as to why rockets won’t work in space:

    What pushes a rocket upwards through the air? Expanding gasses in the combustion chamber shooting out the back.

    But if they’re shooting out the back towards the ground how can they be pushing the rocket upwards?

    Oh, some of the gas is still in the chamber and that gas is pushing up against the rocket (what the people at Physics Today claim)

    OK, but what happens in space, which is a vacuum? The gas in the combustion chamber exits at a a speed of 1 km/s (google The expansion of a Gas-Cloud into a vacuum). So if you open the nozzle all the gas goes shooting out into space within a fraction of a second.

    The gas cannot push the ship with the nozzle closed because gas trapped in the combustion chamber does no work but if you open the nozzle all the gas exits immediately before it can push against the ship. Therefore you cannot use gas in the vacuum to power a rocket ship.

  5. boethius

    This force pushing a rocket cannot be pushing on the inside of the rocket any more than you can push with your feet upwards against the inside of a cardboard box you are within to stop it from falling from a height. It sounds absurd but that is what NASA claims happens in a rocket.

    So how do rockets fly?

    An object sitting on the ground can only move upwards if it is pushed from underneath or lifted from the side/top. Since we know rockets are not lifted , they must be pushed. Therefore the gasses underneath the rocket must be pushing it up and off the launchpad.

    An object moving straight up into the air will eventually be pulled back down by gravity unless it is continuously pushed from underneath or pulled from the top/side by a force greater than gravity.

    Since I have shown the rocket cannot push against itself and is not pulled from above, the area under the rocket must be higher pressure than the area below. While I have several theories as to what causes the pressure I have not followed through on them and I do not need to. I only need to show that the rocket is rising due to higher pressure underneath it (as opposed to pushing itself, being lifted from the top/sides) because in the vacuum of space there can be no higher pressure underneath the rocket. In the vacuum of space the pressure will be equal and 0 on all sides of the rocket, hence it would not move under its own power and immediately fall back to earth.

  6. khammadkhammad

    Dear Lux,

    You wrote:
    it is the pressure of the gasses against THE GROUND, not the air that causes this (ground effect).

    When it comes to rockets, or airplane wings and ground effect, there is no difference between pushing against air and pushing against the ground. Think of molecules and how they have different densities in different objects. A pillow versus a brick. More dense means more resistance.

    When a rocket takes off from earth, it first pushes against the ground, which is pretty dense. The ground assists the rocket in taking off because the ground molecules are so close together, they behave like a thick dense mat, which is able to push back the rocket exhaust, giving the rocket extra lift.

    At higher altitudes, the thick dense mat that the rocket was pushing against is gone, and all that’s left are all these tiny particles, too many to count, too small to see, floating around. The rocket no longer has extra lift, just regular old lift which is pushing against these particles.

    We know the air molecules are there and floating about because they cause a lot of friction and other problems associated with overheating on objects flying through these particles. Seems to me that if air molecules can cause friction by rubbing up against objects then objects can rub up against air molecules. In other words, objects can push up against or down against air molecules.

    The fact that the rocket lifted at all when pushing against ground molecules means that “pushing against” is a good analogy in how rockets fly through the atmosphere. Ground versus air, just a matter of density.

    1. lux

      Rockets don’t move by “pushing against the ground” or “pushing against the air” behind them. The exhaust of a rocket isn’t a physical part of the rocket so it can’t impart a reactive “push” to the rocket.

      When you throw a ball against a wall you do not feel the ball hit the wall because the ball no longer has a physical connection to your body and thus can’t impart a force on it. It’s the same with rocket exhaust — it’s not part of the rocket.

      “Ground effect” is caused by a pressure buildup between the object and the ground and CAN push against a rocket or wing but this is not “the rocket pushing against the ground.” It’s simply a momentary pressure buildup beneath the rocket which quickly vanishes as the rocket rises.

      With the exception of magnetic effects and a few other special cases, “pushing” requires contact. One object can only push against another object when it is in physical contact with it. The exhaust of a rocket is not part of the rocket so it is not in physical contact with it so it cannot “return” any push it may impart on the air behind the rocket.

      An airplane propeller DOES push against the air and in so doing it DOES impart a reactive force to the plane because the prop is a solid object CONNECTED to the plane.

      Rocket exhaust isn’t connected to the rocket so it can’t function as a pushing medium to the rocket as a propeller does.

      Rockets move by creating an imbalance of forces within the rocket motor causing more internal pressure in the forward direction and very little internal pressure rearward due to the opening of the rocket nozzle. There is also a secondary forward thrust caused by Newton’s 3rd law as regards the rearward ejection of mass.

      If you blow up a balloon and hold onto its nozzle it will contain a higher pressure than the surrounding air. That internal pressure is equal in all directions thus creating an equilibrium of forces so the internal pressure creates no internal thrust in any particular direction. When you release the nozzle there is a sudden drop in pressure near the nozzle caused by the escaping gas. This localized drop in pressure creates an imbalance of forces and so the balloon moves in the direction of the higher pressure on the other side of the balloon until the pressures within equalize again.

      That is how rocket thrust works. The continued expansion of gasses caused by burning high energy fuel builds up pressure but the pressure is always lower at the rear of the rocket motor due to the open nozzle. The higher pressure in the forward part of the motor maintains an imbalance of forces so the rocket continues to move as long as fuel is burned.

      In addition to the above force there is also some thrust caused by rearward ejection of mass (the exhaust) in accordance with Newton’s 3rd Law.

      1. khammadkhammad


        I fear we are now arguing semantics instead of physics.

        To save time, I will tell you how I interpret Newtons 3 Laws of Motion. If you disagree then there is no longer a reason to continue this thread as we differ on basic laws of physics which won’t be resolved here. If you agree with me, then there is much to discuss.

        Let’s start with Newtons 3 Laws of Motion.

        Fist Law: For an object to remain as it is, either moving or not, the sum of the forces on it are zero.
        Sigma F = 0

        Second Law: For a body to accelerate, there must be a force on it.
        F = ma

        Third Law: For every force in one direction, there is an equal force in the opposite direction.
        F1 = – F2 or F1 + F2 = 0

        Notice how all of Newtons Laws of Motion contain the term ‘force’. Newton used the term ‘force’ to explain how objects are pushed and pulled in our universe.

        This is how I see Newtons 3rd Law applied to rockets flying through our atmosphere:

        If a rocket is moving through the air at 17,000 mph in a southwesterly direction, then there must be a force in the northeasterly direction also going 17,000 mph, which is the force produced by the jet engine exhaust coming out of the back of the rocket.

        The way you are explaining it, is that molecules hitting inside a chamber are moving the rocket forward, AND the rocket is moving forward. You did mention the perhaps the exhaust might move it forward also somewhat, but Newtons 3rd Law says the forward motion MUST be equal to the thrust only out the back because of the ‘opposite’ direction part of the law.

        You can’t have two positive forces. F1 + F2 would then be greater than zero, and that defies Newtons 3rd Law. My point is the exhaust out the back is not the minor part, it is the major part of the force. Newtons 3rd law says it has to be.

        I think these three laws of motion also must be followed in space. Space is no different than earth. It is just a matter of molecules and their ever decreasing amount as one moves farther and farther from earth.

        Dear Lux,

        Do you think the three laws of motion must be followed in space, just like on earth?

        1. lux

          Force and speed are 2 very different things. A bullet fired from a gun moves much faster than the gun in recoil yet equal forces are applied to each.

          My explanation of an imbalance of forces within a rocket motor is simply an alternate way of looking at rocket propulsion. Rockets move both by this imbalance and the action/reaction of the expulsion of exhaust gasses (ie, Newton’s 3rd law).

          If you stand on a small boat you can move the boat forward by throwing a mass astern. The boat does not move because you are pushing against air. It move because you are pushing against the mass and so the mass pushes back against you and you are physically connected to the boat. The air has nothing to do with it so removing the air from the equation (as in a vacuum) will not affect this action/reaction.

          “Do you think the three laws of motion must be followed in space, just like on earth?”

          Of course. Why wouldn’t they apply in space? They are laws of force and motion, not laws of air.

      2. William Vietinghoff

        The paragraph mentioning the imbalance of forces within the rocket motor is an excellent way of explaining the development of thrust to those readers and writers who have difficulty envisioning how the expulsion of gas can propel a rocket.

        A closed drum containing a highly pressurized gas is subject to tremendous internal forces in all directions. But the drum won’t move because the forces cancel out. As pointed out, that is not the case with a rocket engine. A typical rocket engine has a flat circular plate injector with channels on the back through which the fuel and oxidizer flow in. The fuel and oxidizer are injected into the combustion chamber through tiny drilled holes. The burning of the propellants produces an extremely high pressure in the combustion chamber. In the RS-27 engine, a very reliable engine used for the Delta launch vehicle, the chamber pressure is 700 pounds per square inch. For the purposes of illustration, assume the injector plate is 24 inches in diameter. The surface area of that plate would be about 450 square inches. The 700 psi combustion gas would exert an upward force of about 315,000 pounds. Opposite the injector, a few feet down, is the nozzle. Because the nozzle is open and because the sides are at a slight angle, the effective surface area that the gasses press upon may be about a third of the injector, say 164 square inches. The 700 psi would produce an opposing force, downward force of about 115,000 pounds which cancels out part of the upward force. The resultant upward force would be about 200,000 pounds. This, by the way, happens to be the thrust of the RS-27 engine.

  7. anthonycaralloanthonycarallo

    I think the use of the words “space” and “vacuum” are enough to dismiss that thread as a hoax- what, specifically, do those words mean in this context? The moon moves through “space”, therefore anything else set into motion could move through “space”. It doesn’t matter if it is has a rocket attached to it-nonoperational or otherwise. The rocket part of the equation is a straw man.

  8. khammadkhammad


    Thank you for your explanation.

    I think everyone can agree that if one throws a ball while standing on a skateboard, the skate board will move backwards slightly. One can also make a skateboard move while standing on it by just waving ones arms in a certain way while no ball is being thrown . Wouldn’t this mean the ball is unnecessary in the analogy?

    I am trying to understand how the principle of rearward expulsion relates a skateboard moving slightly on a sidewalk to a rocket barreling through the vacuum of space?

    1. lux

      Yes, there are a number of ways of propelling a skateboard. One could also hitch a horse to it or point it down a slope or attach a propeller or throw it through the air or kick at the ground and many other ways.

      My point is not that there is only one way to move a skateboard. I mentioned one particular way as an example of a particular type of propulsion that relates to rocketry.

      There are countless explanations on the net as to how rockets move (in or out of a vacuum). If you are curious I suggest that you read some of them and see if they seem true to you or not.

      1. simonshacksimonshack

        Lux wrote;

        “There are countless explanations on the net as to how rockets move (in or out of a vacuum). If you are curious I suggest that you read some of them and see if they seem true to you or not.”

        Yes, Lux – there are indeed countless such explanations on the net. I guess that the sheer number of these “explanations” means that they are all serious and truthful ? (*rolleyes*)

        Now, are you ready for my “VACUUM THRUST CHALLENGE” ? Give it a go!

        @Anthony Carallo:
        Dude, you are by all means free to criticize and disagree with our Cluesforum thread (“Do Rockets work in the Vacuum of Space?”) – but to call our thread a “hoax” is just stupid. We do not produce hoaxes at Cluesforum – we expose them.

        1. khammadkhammad


          I absolutely love your experiment. It’s exciting to think we could put an end to the debate.

          Imagine where our thoughts could take us if could all move forward in the same direction!

        2. lux

          No, Simon, the number of explanations does not necessarily mean that they are serious and truthful. I stated that there are numerous explanations merely because there ARE numerous explanations which makes them easy to find. I also suggested that one make up one’s mind as to whether one thinks they are true or not.

          I am anxious to see the results of your Vacuum Thrust Challenge experiment. Please post them ASAP.

          Perhaps these folks could assist in carrying out your experiment:

  9. anthonycaralloanthonycarallo

    The reason a rocket would continue to move after leaving earth’s supposed atmosphere is because of perpetual motion. Without as much gravity, it would continue at the same, or similar, speed. Theoretically, perpetual motion is why the moon continues on it’s trajectory, although reigned in by gravitational pull. If that is a true theory, then obviously “space” is not a vacuum. Leaving “the moon” would is another story. The moon landing is not believable for many other reasons.

    1. khammadkhammad

      I believe to be an object in perpetual motion the object has to be doing work. Planets are rotating around the sun but they are not doing work, so planets are not in perpetual motion.

      Another requirement for perpetual motion is that there is no fuel source.

      Another requirement for perpetual motion is that the speed of the object not decrease over time. There are calculations that show the earths spin is slowing down (even if ever so slightly) over time. Has anyone looked into if those calculations are valid?

      1. anthonycaralloanthonycarallo

        Perpetual motion is the prevailing theory behind the movement of the moon. So argue with an astronomer. PM is the explanation for why things like meteors continue to move- they were ejected from something and there is no force of gravity to stop their movement. If you threw a ball into the air, it would continue to go “up” if it were not acted upon by gravity. The same with a “rocket”, regardless of “fuel”. Technically, once free from gravity, a rocket would continue on it’s trajectory. (of course that doesn’t explain getting off the moon).

        1. khammadkhammad

          The prevailing theory of why planets rotate is not perpetual motion.

          It is this:
          Earth spins because of the way it was formed. Our Solar System formed about 4.6 billion years ago when a huge cloud of gas and dust started to collapse under its own gravity. As the cloud collapsed, it started to spin.

          The prevailing theory also says this:
          Due to a transfer of Earth’s rotational momentum to the Moon’s orbital momentum as tidal friction slows the Earth’s rotation.

          Again, this is not perpetual motion.

          1. anthonycaralloanthonycarallo

            I wasn’t talking about rotation, I was talking about movement through “space” or trajectory. The result of force acting upon an object or mass that will set the object in motion until something else (ie gravity) stops it. This is an observable phenomenon so I think it is a decent theory, unlike the “formation of the earth 4.6 million years ago from a huge cloud of gas”…

  10. lux

    Sorry to disagree but rocket propulsion does not work by “pushing against the air” behind the rocket so all of this “gas theory” is just a bunch of hot air. 🙂

    Rocket propulsion works by two factors: rearward expulsion of mass and by forward pressure exerted by gasses withing the rocket’s motor.

    If you stand on an easily movable platform such as a skateboard, wheeled dolly, etc and throw objects in a particular direction the platform will tend to move in the opposite direction. The heavier the objects and the harder you throw them the further you will move. If you throw lightweight objects which are the same size and dimensions as the heavier objects you will not move as far thus easily disproving that the motion is caused by “pushing against the air.” The same amount of air would be pushed by two objects of the same size which were thrown with the same effort but throwing the heavier objects results in more movement. Why? Because as you push against the objects they in turn push back against you and you are connected to the platform so the “pushing back force” is transferred to the platform and it moves. It has virtually nothing to do with “pushing against air.”

    The above principal works regardless of atmosphere or gravity. If you exert a force on an object it will exert an equal force in the opposite direction regardless of where you are.

    But, the above is only part of the rocket propulsion picture. In the rocket’s motor you have pressures caused by the expanding gasses resulting from burning fuel. The pressure in the forward direction are not countered because the rear of the rocket motor is open allowing gases to escape so the rocket moves forward.

    Place a firecracker under an empty inverted can and light it. When it explodes the can flies upward because the forces from the expanding gas of the explosion are not countered in the upward direction so that is the direction it moves. And, it doesn’t move merely because the gasses “push against the ground” under the can. It would work as well if the can were suspended by a string and away from the ground. And, if you could light it, it would also work in a vacuum.

    Another hole in the “push against the air” theory is that the exhaust gasses of a rocket are simply not part of the rocket any more than a cow pie is part of the cow. Since these gasses are not a physical part of the rocket and have no mechanical connection with it there is no way for these gasses to transfer motive energy to the rocket. And, so it doesn’t matter what they push against. If you throw a ball against a wall does your hand feel the ball’s impact on the wall? No, it doesn’t because as soon as the ball leaves your hand it no longer has any mechanical connection to your body so there is no way for the ball to transmit any forces to your body.

    A car moves because the spinning wheels push against the road and the road “pushes back” thus propelling the car forward. The wheels are connected to the car so this “push back force” is transmitted to the car and it moves in the opposite direction. If the wheels were not mechanically connected to the car then only the wheels would move forward, not the car.

    BTW, none of this means that NASA is “telling the truth” about anything or that I agree with NASA about anything. Rocket propulsion physics has existed since long before NASA was hatched and their fairytale space exploits and rocket ships are pure propaganda fantasy.

    1. khammadkhammad


      Real life analogies work great to explain scientific phenomena. In fact, I prefer them because we may not all agree on the definition of terms that Nasa and other space scientists use.

      You used this analogy:

      “Place a firecracker under an empty inverted can and light it. When it explodes the can flies upward because the forces from the expanding gas of the explosion are not countered in the upward direction so that is the direction it moves. And, it doesn’t move merely because the gasses “push against the ground” under the can. It would work as well if the can were suspended by a string and away from the ground.”

      Your assumption that a can suspended upside down by a string would also fly up in the air.

      I have tried this experiment and the can DOES NOT fly up in the air. The can moves slightly upward, but does not “fly up”.

      We taped a Black Cat firecracker to the inside of a green bean can with no lid, set it on the ground upside down with the wick sticking outside the can. With the same set up, we place another green bean can on the barbecue grill.

      For the can on the ground, when the firecracker blew, the can soared into the air about 20 feet. For the can on the grill, when the firecracker blew, the can only jumped up about 4 inches.

      Ya, we blow a lot of stuff up when its firecracker season using all manner of objects and environments. I have had a pretty active childhood and have experimented quite a bit with scientific principles.

      When it is said that a can on the ground behaves the same way as a can suspended in air when firecrackers are exploding inside them I have to disagree based on my own experience.

      Clearly, the ground is aiding the can somehow in gaining all that extra height. It could also be said that the grill and its lack of ground is prohibiting the can from flying up.

      Without using the idea of “pushing against the ground”, how would you explain the difference in heights of the two green bean cans?

      1. lux

        In aeronautics there is a term called “ground effect.” It mostly refers to the sudden increase in left of a plane during landing as it nears the ground. It is caused by air pressure increases between the wing and the ground. The same thing occurs with the can & firecracker on the ground. The air pressure builds between the two and assists in the “take-off” of the can. But, it is the pressure of the gasses against THE GROUND, not the air that causes this.

        The suspended can in your experiment was still sent upward, just not as far. Your experiment does not prove WHAT causes the can to move, only that it moved more in one circumstance than another.

    2. simonshacksimonshack

      Lux (actually ) wrote:

      “Another hole in the “push against the air” theory is that the exhaust gasses of a rocket are simply not part of the rocket any more than a cow pie is part of the cow. Since these gasses are not a physical part of the rocket and have no mechanical connection with it there is no way for these gasses to transfer motive energy to the rocket.”

      Dear Lux
      I’m afraid that what you wrote has to be about the silliest thing I have ever read in a long time. So do these two fire extinguishers (see below video) NOT push on air? Of course they do – and they clearly transfer motive energy to the man in the office chair :

      I really have to wonder how the heck an apparently intelligent person such as yourself can write such cow shit.

      1. lux

        “…what you wrote has to be about the silliest thing I have ever read …”

        Dear Simon, I do not respond to discourteous posts or ad hom criticisms. Ask your questions in a courteous manner and I will answer.

      2. Justin Franks

        “So do these two fire extinguishers (see below video) NOT push on air? Of course they do”. Actually, they don’t push on air. This is basic physics — I truly cannot figure out why people can’t seem to grasp this.

      3. Chris C

        It’s the weight and speed of the extinguisher’s ejected contents that is causing a momentum force to be created, which in turn is creating the opposite momentum force on the chair. This method of propulsion has NOTHING to do with the contents of the extinguisher ‘pushing on air’. You do realize that the contents of an extinguisher actually has mass?

    3. Vespadouglas


      This whole ramble of yours is neatly summed up by your sneaky insertion of these 13 words
      ” and if you could light it , it would work in a vacuum too ”
      Am I getting this right ?
      You are using the argument that a possible ( yet unproven) outcome of a process , in a vacuum , is proof of your theories on the outcome of the same process in a vacuum .
      Absolute balderdash .

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