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A quick 10 minutes to explain what's really going on with rockets. It's not rocket science. Oh, wait.. Well, Enjoy!
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By Fran

13 thoughts on “The Illusion Of Rocket Thrust”
  1. Avataaar/Circle Created with python_avatars Glenn Stasse says:

    I think people confuse jet engines and rocket motors. Kinda looks the same, right? Oddly, the rocket motor sorta suggests you could stand in a box and lift the box off the ground. Not sure which of Newton’s laws that one is, though. 😉

  2. Avataaar/Circle Created with python_avatars Jocram Krispy says:

    Alas, while an attractive argument, it's not (just) pressure.

    The F1 engines ran at 70 bar (call it 1000 psi). The max diameter of the engine was 12.2 feet, or about 120 square inches. pressure would, therefore, give you 120,000 pounds of thrust. (nothing to sniff at)

    The actual thrust of the engine was more than ten times higher. The extra thrust comes from the heat of the combustion. How that works is some rocket science stuff, involving fluid dynamics and thermodynamics and the weird (1-M²) component of compressible flows…

    Basically the exhaust of a rocket is much cooler than the combustion chamber. (that pillar of flame the rocket is sitting on – that's actually unburnt hot fuel coming into contact with the atmosphere and burning with the oxygen in the air – as the rocket gains altitude ant the air gets thinner that flame disappears)

    by converting the pressure and temperature to velocity, the Staurn V engines managed to accelerate 90000 oz of propellant to 5700 mph every second. Every action has an equal and opposite reaction, so the rocket got accelerated a little bit the other way.
    .

  3. Avataaar/Circle Created with python_avatars A Webuser says:

    "Pressure" just makes it too complex to visualize… It's simply Newton's third law; throw (accelerate) any mass one direction and you get the same acceleration in the opposite direction (recoil in a gun), or in this case, it's the MASS of the hot gas being "thrown" out the nozzle that accelerates the rocket in the opposite direction. By 3+ minutes into the video the action/reaction principle had not yet been stated, that needs to go first!

    One might say, well, the "gas" doesn't weigh anything! Well, a Saturn 5 fist stage had 4.4 MILLION pounds of propellant to "throw" out the end of the rocket in about 2.5 minutes!

  4. Avataaar/Circle Created with python_avatars ampeyro says:

    What kind of crowd are you hanging out with that try to argue against rockets working?

    I suggest slowly walking away while not loosing the eye contact until you're near the door, then call for professional help.

  5. Avataaar/Circle Created with python_avatars PistonAvatarGuy says:

    I just think of Bender, flying through space, then throwing objects forward to slow down. All a rocket really does is throw combustion products in one direction, while the combustion process itself is used to accelerate those combustion products.

    I think that the main benefit of the skirt extension is to force the combustion products to accelerate rearward, because they'll have a tendency to want to pop out of a shorter nozzle sideways when there's no surrounding pressure to keep them from doing so.

  6. Avataaar/Circle Created with python_avatars StarkRG says:

    I wonder if the "there's nothing to push off of in a vaccum" people have never tried throwing anything heavy before. If you throw something in one direction, you get pushed in the other direction. That the thing you're throwing is explosion exhaust really doesn't matter, the effect is the same.

  7. Avataaar/Circle Created with python_avatars TheKitsuneCavalier says:

    So, rather than the rocket pushing the exhaust backwards, the pressure is pushing the rocket forwards. Of course, due to Newton's Third Law of Motion, the housing of the rocket engine is pushing back with a force equal and opposite to that of the thrust, and thus the rocket accelerates forward.

  8. Avataaar/Circle Created with python_avatars Iron Horse says:

    As the Flat-Earther in the room, I guess it's up to me to apply some common sense. Now your simplistic explanation here is not in dispute, it's basic rocketry which we all understand. What's in dispute, is whether it WILL actually work in a vacuum, not "technically it should".

    We've probably all seen the large tanker vessel collapse under the weight of the surrounding atmosphere, when all the air was removed, right? The part of your equation you've ignored, to this point, is the surrounding air pressure .

    Apparently, a rocket works as well in water, as it does in air. Despite being some 800 times denser, the thrust has more to push against, but the 800 times more density also resists it, thus having no net gain.

    Apply this to a vacuum, however, and while there's zero resistance, there's zero to push against, no external pressure. All that will happen is a violently fast extreme reaction of the burning fuels, with once again, zero net gain. Either the rocket won't move at all, or if it's even the slightest bit imbalanced in one place, as it must be with human payload somewhere in or in front of the nose, the thing will pinwheel out of control.

    Further, the thinner the air gets as it ascends, the less steerage or control it will have. This is precisely why they always, without exception, start leaning over to one side, and eventually fall into a parabolic arc to descend back and land in the ocean somewhere.

    In short, the powerful thrust of rocketry is most suitable for straight flight in a medium. In a vacuum, the laws of physics practically reverse, and it's near impossible to create any type of design where seals won't eventually give way to the lack of outside pressure. Literally the reverse of a submarine only able to go so far deep, before it starts falling apart at the seams.

    You're welcome.

  9. Avataaar/Circle Created with python_avatars Barry Davis says:

    The pressure inside the container would escape and NO thrust is achieved if inside a vacuum. And Gas at pressure cannot reside next to a vacuum without spontaneously moving towards the lower pressure of the vacuum. No thrust attained inside vacuum.

  10. Avataaar/Circle Created with python_avatars Hola! Zepherius Collins says:

    It's no different than a balloon releasing the air or a firework releasing its fuel. Should have more pressure outside an atmosphere as there is nothing in the way of blocking the rocket from moving forward. I do like how this explains why most rockets are launched at sea level.

  11. Avataaar/Circle Created with python_avatars Jim Morrison says:

    Another way to think of it:

    Imagine a motionless rocket in space.

    It contains propellant, but it is not yet ejecting the propellant.

    The momentum of the rocket/propellant system is zero, and this momentum (of the whole system) must be conserved.

    Now, the rocket starts to expel the propellant, and the propellant now gets a certain non-zero momentum.

    In order for the momentum of the rocket/propellant system to stay at zero, the rocket must also get an equal momentum, but in the opposite direction to that of the propellant.

    So the rocket moves, and the momentum (of the whole system) is conserved.

  12. Avataaar/Circle Created with python_avatars Steve Jones says:

    Famously, the New York Times printed and editorial article with a scathing criticism of Professor Robert Goddard, who'd written an article in Popular Science in 1920 explaining how rockets could launch space craft.

    This is what the New York Times editorial had to say in a rather patronising manner :-

    "That professor Goddard, with his 'chair' in Clark College and the countenancing of the Smithsonian Institution [from which Goddard held a grant to research rocket flight], does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react — to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools."

    In July 1969, in the light of the Apollo missions, the New York Times finally issued a public apology for the error :-

    "Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th Century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error."

    Unfortunately, Robert Goddard had been dead for 24 years, so he was not around to read it.

  13. Avataaar/Circle Created with python_avatars Rinoa's Auspicious Travails says:

    I've never realized people would think of it this way. Really I just think of the engine as thrust being the opposite side of the opening. Have a 6" round exhaust opening, and you get force against the other 6" back side of the rocket. The nozzle is just there to make it flow better.

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