Taking off from moon

  1. Dear Friends!
    I am curious to know about take off from moon of a space craft.
    How Apollo landing module after detaching itself from orbitter module lands on moon and how later it takes off to attach to orbitter module in the absance
    of any atmosphere which could give continuous support to craft while landing and push to craft while taking off?
  2. jcsd
  3. You know that rocket motors do not need an atmosphere to work, right?
  4. How external reaction is generated to give it force to move?
  5. The fuel (and oxidizer) that the craft carries along acts both as an energy source and as reaction mass. The rocket motors expel the exhaust gasses out the back at very high speed. As the gasses are hurled one way the resulting force propels the craft in the opposite direction.
  6. HallsofIvy

    HallsofIvy 40,310
    Staff Emeritus
    Science Advisor

    A rocket engine does NOT "push" against anything. it throws gasses out the back end, having momentum "mv" where m is the (very small) mass of the gasses an v is (very large) speed of the gasses. That gives it an increase in speed V given by MV= mv with M the mass of the rocket. Do you know what "conservation of momentum" means. What grade are you in school?
  7. On earth rockets use the reaction force provided by reaction from air as well.On moon there can not be such reaction except in the beginning from floor.
    Rocket is continuously pulled towards floor due to gravitation.
  8. The rocket fuel is what pushes the rocket engine. The air is not needed. The fuel combusts, expands and pushes the rocket engine.
  9. A.T.

    A.T. 5,503
    Gold Member

  10. russ_watters

    Staff: Mentor

    Sorry, but this is false. Backwards even. Since the exhaust gases are opposed by the atmosphere, thrust is LOWER in the atmosphere, not higher.

    The same propulsion principle applies to air reaction devices as well (jets, helicopters, etc): they are moved by action-reaction, not by pushing on the air, just like rockets.
  11. How is the action-reaction effect of air reaction devices not "pushing the air"? That seems very much like pushing the air to me. Air gets pushed down, device gets pushed up such that momentum is conserved and the center of mass of the air/device system remains unchanged. That is what pushing is. At least, that is what I think of when I think of pushing and I think that is what most people think. I never studied aerospace engineering or flight in depth... But from my studies in physics I believe that this is how all motion is achieved - through pushing. Nothing can move without pushing something else.
    Last edited: Jul 23, 2014
  12. A.T.

    A.T. 5,503
    Gold Member

    action - reaction : helicopter pushes air down - air pushes helicopter up
  13. sophiecentaur

    sophiecentaur 13,407
    Science Advisor
    Gold Member

    Perhaps heavier than air flight could be regarded as the plane pushing against the Earth (effectively) - just with a rather bungee substance in between. After all, the weight of the plane (level flight, of course) is ultimately being supported by the Earth's surface.
  14. I guess the point is (as can be seen from the thrust equation) that if ambient pressure is "pushing back" against the rocket exhaust, then the exhaust leaves with a lower downward momentum and the rocket gains less upward momentum. The rocket isn't "using" the reaction with air so much as it is "hindered" by it.
  15. russ_watters

    Staff: Mentor

    That isn't what the OP is saying. The OP is saying that the air coming out of the nozzle pushes against the air in the atmosphere, generating lift.
  16. russ_watters

    Staff: Mentor

    The airplane does push down on the air of the atmosphere to push itself up and thus will not generate lift in a vacuum. UNLIKE a rocket, which only pushes down on its own exhaust.
  17. russ_watters

    Staff: Mentor

    Which air? The air that went through the rotor or the air that didn't? That's the point/difference.
  18. Right... My reply was to your claim that air reaction devices dont push the air. They do.
  19. russ_watters

    Staff: Mentor

    Perhaps the issue is less clear than I realized when talking about vehicles that don't carry their own reaction mass and instead use the air around (above) them. The point in all cases is that the reaction mass - whatever it is - is not pushing on the air below it to support the vehicle.

    For air propelled devices, accelerating the air through it, not that air pushing on the air below it, is what causes the lift.

    The proof would being in comparing a jet of air from any source in an atmosphere to a similar jet of air in a vacuum. In a vacuum, it accelerates faster and generates more thrust.
  20. russ_watters

    Staff: Mentor

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