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Rockets in space

  1. Mar 25, 2008 #1
    Once rockets leave the earth's atmosphere they would have a high velocity
    so how come rockets use their engines when they're in space if there's no air resistance? The only reason I can see why they would need to use them is if they wanted to change direction.
  2. jcsd
  3. Mar 25, 2008 #2
    The quick answer: It is not pushing against the air. It is pushing against its own fuel! This is due to the conservation of momentum.

    Imagine you are on a frictionless ice rink with a big rock. (this is space... no air resistance = frictionless) Now, if you throw the rock to the west, you will slide to the east! The bigger the rock, and the harder you throw it to the west, the more you'll slide in the opposite direction. You can imagine the same effect in space.

    You can try this with an office chair and something heavy, like a medicine ball, your little brother, or the giant book of pokemon cards everyone has from when they were popular. Now pick your feet off the ground, and put yourself in an open area, and throw the ball. You move too! You eventually slow down because of friction between the wheels and the ground, but thats the premise for how rockets fly in space.

    Alexandre Colavin
    Last edited: Mar 25, 2008
  4. Mar 25, 2008 #3
    If you're referring to the Space Shuttle, the Space Station, or other low orbiting rockets, they're at the fringes of the outer atmoshpere and experience some drag. If not for occasional boosts from the rocket engines, their orbits would eventually decay and they would return to earth (or burn up).
  5. Mar 25, 2008 #4


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    Rockets have to reach escape velocity or they will fall back to Earth. That's about 25 times the speed of sound. They must achieve this velocity before they run out of fuel (which happens pretty quickly). That is the primary reason why they thrust, whether or not they are in atmo or in space.

    BTW, it is easier to achieve that speed when in space since there's no air resistance. If they tried to achieve that speed while still in the atmo, they'd melt! So, in one sense, it would be more efficient to use their engines in space than in the atmo!

    Question for clarity: what rockets have you been watching in particular? There are rockets that go sub-orbital, rockets that go into low orbit, rockets that go into high orbit, rockets that go to the Moon, and rockets that leave the Earth Moon system altogether. They all have different requirements, and the answer to your question depends on which ones you refer to.
    Last edited: Mar 25, 2008
  6. Mar 25, 2008 #5


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    It really depends on what they want to do. Once the Space Shuttle or a satellite reaches space, it isn't in a proper orbit (basically, the orbit still intersects the atmosphere), so it needs to use its engines to reshape the orbit. For a probe going to the outer solar system, engines are used again for course correction and speed - "high velocity" is a relative term and the distances to be traveled are great, so more velocity is always desirable for deep space missions.
  7. Mar 26, 2008 #6
    Do the probes turn off their engines once the desired velocity is reached?
  8. Mar 26, 2008 #7


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    Staff: Mentor

    Most, yes. But we do have a few that keep their engines on in order to accelerate during the entire trip. Those use ion propulsion, which provides very low thrust for a very long time.
  9. Mar 26, 2008 #8

    D H

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    For the most of the time, yes. Vehicles still have to fire their engines for a variety of reasons. Some of them are
    • Maintain attitude/orbit. Some remnants of the Earth's atmosphere exist even at low Earth orbit altitude. The atmosphere extends about 10,000 km above the Earth's surface. Flying through that air slows a vehicle down. The ISS loses about 100 meters of altitude per day, for example. Some vehicles (the ISS, for example) expend fuel to maintain their altitude. They thrust along their velocity vector (thrusters pointing against the velocity vector) to increase the velocity and 45 minutes or so later fire the thrusters again to re-circularize the orbit. Typically they do this whenever the altitude reaches some minimal acceptable value.
      There is no atmosphere and hence no atmospheric drag at geosynchronous altitude. Other perturbative forces (moon, sun, Earth's non-spherical gravity, ...) act to move a vehicle out of geostationary orbit. Vehicles in geosynchronous orbit carry fuel to maintain their orbit.
    • Maintain attitude. The atmosphere and gravity not only generate external forces but also external torques. Vehicles use a variety of techniques to compensate for these torques. Some use fuel to do this.
    • Die gracefully. Vehicles in space are supposed to behave "nicely". This includes end-of-life operations. Vehicles in low Earth orbit typically carry enough fuel to make a planned reentry into the atmosphere, typically ending in the southern Pacific. Most LEO vehicles end there life this way. Sometimes things go wrong, USA 193 (the dead satellite the military recently "shot down"). Vehicles in high orbits are supposed to change to a graveyard orbit. Only about a third of the vehicles in geosynchronous orbits do this. This rest just die at GEO. We have a lot of space junk at GEO altitude that is no long geostationary.
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