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I Inertial integrity

  1. Sep 1, 2018 #1
    Maybe i should pose this query in a different area, but since it involves basic physical laws of motion...

    Space flight? I know this should be obvious enough but i was wondering how a shuttle is capable of maneuvering once in an area with which gravity and atmosphere play too little a part on its movements..
    Pressurized propellants such as oxygen have been sighted but there are some issues that need resolved before this is plausible. If the pressurized oxygen, (or anything), being released from a container is allowed to expand at the rate of its release or more, does it then act upon the vessel in such a way as to create momentum?

    And does the loss of mass from a vessel create a thrust in an opposite direction of the loss if there is no possitive point of inertia to equate to the vessel?

    These are two questions i seem to have trouble finding an answer that seems plausible.
  2. jcsd
  3. Sep 1, 2018 #2


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    1. As far as I am aware, the rate of expansion doesn't affect whether or not it provides a force, it only affects the magnitude of the force. I believe that releasing the same amount of oxygen quickly or over a long time provides the same impulse either way. Impulse is the application of a force over a length of time and equates to a change of momentum of an object. Since the rate of release doesn't affect the impulse, it also doesn't affect the change in momentum.

    2. I'm not sure what a 'positive point of inertia' is and a quick google search didn't turn up anything except useful. Perhaps I never got that far in my physics classes in school. Can you elaborate or give a reference? You didn't mean moment of inertia did you?

    In any case, the expulsion of mass in the form of exhaust is exactly how all rockets work. It doesn't matter if the exhaust is expelled at a high velocity because it was heated and forced through a constriction, or if it was simply pressurized and then released. Heck, you can even test these principles yourself at home if you're creative enough and have the right equipment. The first idea that comes to mind is to jump off of a diving board into a pool with something heavy and push off of it while in mid-air. You'll find that both you and the object go in opposite directions.

    Note that gravity is still very much affecting the shuttle. Its the entire reason the shuttle stays in orbit instead of flying off into interstellar space! What's not affecting it is anything to oppose its motion except for a few molecules or atoms of gas here or there, far too sparse for flight surfaces to function.
  4. Sep 1, 2018 #3
    The rate of release is not the point of my conjecture. The rate at which it is allowed to expand is my problem. Because it allows the propellant to be released without a transfer of energy. Or at least one efficient enough to accomplish such tasks as maneuvering a shuttle.

    By saying positive point of inertia is meant to say that an object has an inertial framework to start with in order to gain momentum by the loss of mass.
    The divingboard and heavy object scenario is still played out in an atmosphere of over 12 psi. This inertial point must be considered.

    It seems that the laws of motion used to shuttle through space can easily be verified in an atmosphere but once they reach a place where there is nothing to act upon, the same laws dont seem to be applicable.
    Like a rocket for example. It doesnt leave the ground due to the rate its propellant is released, it doesnt create any meaningfull thrust until that propellant is ignited. Thrust is then created from the force of that ignition acting upon a possitively pressured surrounding. So it is in fact pushing off of that surrounding. But if that same rocket were to do that in space (if combustion were possible there also) it would be like a swimmer in a pool trying to kick off of themselves instead of the water to swim foreward.

    You said it yourself that there is nothing to effect its motion. Nothing but a few particles. So since ignition of that propellant is out it leaves us with the pressured release, which loses its validity once its rate of release is complicated by the rate its expansion.

    Think of a balloon full of air being released into a room. The release of air propells the balloon forward.
    Now think of that balloon with vacuum cleaner hose behind it. Do you think that the balloon would be propelled forward if the vacuum suck the air out at a rate equal to or greater than it is released. Or at least that is the closest analogy that comes to mind right now.
  5. Sep 1, 2018 #4


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    Can you provide something to back this up? It doesn't look correct to me.

    There is something to act upon. The shuttle and the exhaust. The shuttle pushes on the exhaust molecules as they bounce off of its engine, and the molecules do the same to the shuttle. That's literally all there is to it unless you somehow think that molecules can bounce off of each other without exerting a force. And why would there need to be anything else? Air just gets in the way of the exhaust and the shuttle and makes things more difficult. Rocket engines actually perform better in a vacuum than they do in atmosphere because of this.

    No, this is entirely incorrect. The rocket and the exhaust push on each other. This is easily verified by placing a rocket engine or pressurized container of gas in a vacuum chamber.

    Of course combustion is possible. Rockets literally carry their oxidizer and fuel with them and mix them in a combustion chamber where they are then ignited and burned. Are you going to try to tell me that mixing oxygen and hydrogen (or any other fuel/oxidizer mix) together in space somehow makes them not combust? And no, it's not like a swimmer kicking off of themselves, it would be just like the example I already gave with you pushing off of a heavy object in midair. And keep in mind that the combustion chamber is not a vacuum. It's full of fuel and oxidizer.

    On the outside of the vehicle.

    No, because the air in front of it is pushing it backwards. That's how vacuums suck things up. The air in front of the object pushes it into the vacuum tube. There is no air in space to push the shuttle backwards.

    As I already said, this entire thing boils down to whether or not you believe that colliding molecules impart a force on each other. That's it. Everything else can be derived from that fact.
  6. Sep 1, 2018 #5


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    I have no idea what you're getting at. I've never heard of an 'inertial framework' before but I can tell you that all objects (free to move about) have a momentum change if they expel mass. Note that I used expel for a reason. Just letting go of a mass doesn't get you anything. You have to apply a force to it and accelerate it away from you. By Newton's laws the object is doing the same to you. Unless you don't believe in Newton's laws of course, but if so we have a serious problem.
  7. Sep 1, 2018 #6


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    Not every maneuver changes the kinetic energy of the Shuttle, and some of them decrease it. Kinetic energy in some random reference frame is rarely useful to consider in spaceflight. Studying momentum is easier.
    Releasing gas changes the energy of this gas (the direction of the change depends on the direction of the gas release) and it can free some thermal energy. This can also change the kinetic energy of the spacecraft.
    Calculate it yourself! Conservation of momentum makes it easy. Take some fuel mass, take some exit velocity (~3000 m/s for rocket fuel, ~500 m/s for cold gas thrusters), look up the mass of the spacecraft you want to consider, calculate the change in velocity of the spacecraft.

    This has nothing to do with an atmosphere. The spacecraft acts upon the gas it expels.
  8. Sep 1, 2018 #7


    Staff: Mentor

    From your response below it sounds like by “positive point of inertia” you mean “inertial reference frame”. Any inertial frame will work for explaining the motion.

    Umm, it is not just plausible, it has been built and used and confirmed.

    There is a difference of pressure on the gas. On one side of the gas there is vacuum (0 pressure) and on the other side there is the canister pressure. Due to this unbalanced pressure the gas accelerates. By Newton’s third law, there is an equal and opposite force on the shuttle. Assuming this force is unbalanced, the shuttle accelerates in the opposite direction. The amount of acceleration can be found either by Newton’s third law or by conservation of momentum.

    Last edited: Sep 1, 2018
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