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The advantage of going up by pulling oneself up on a space elevator over pushing oneself up with a rocket, is that one doesn't have to push oneself up with a ro

by drdeca 8y ago

The advantage of going up by pulling oneself up on a space elevator over pushing oneself up with a rocket, is that one doesn't have to push oneself up with a rocket. The tyranny of the rocket equation! The energy use of a rocket isn't just "how much energy is needed to have that kinetic energy and that potential energy", but also includes the energy needed to lift and accelerate the fuel used to provide that energy, as well as the fuel needed to provide that energy, as well as the ... and so on.

imh 8y ago
Isn't the rocket equation just a momentum equation? Does it escape that? If we brought something up, would we not pull the satellite it's attached to down, requiring more momentum from somewhere?
- drdeca 8y ago
  If by "Does it escape that?" You mean "would a space elevator not be subject to the rocket equation?", then yes, it would not be subject to the rocket equation. The rocket equation is about reaction mass that is carried by the thing expelling it. It does not apply to climbing a rope, nor does it apply to the flight of a helicopter. Regarding pulling the satellite down, Pulling down on the weight at the end is something we have to do anyway to keep it from flying out away from earth.
- mrep 8y ago
  No, the elevator has counterweights at both ends which each pull on the tether more than the object being moved due to gravity and centripetal force. The object on earth pulls more though which keeps it tethered. The elevator that is moving than exerts extra force to propel itself up but not more than what would drag the top counterweight down, thus, the rope still stays fully extended. Think of it like holding a rope tight between your arms and putting a robot that moves on it but ends up not exerting enough force to cause it to slack. Your arms (earth/space teather and the in space counterweight) and the object being moved would keep the same overall motion between all 3 objects (your body would just turn). The space elevator is similar except earth is way more massive than the object being moved so nothing really changes. Also, due to gravity, you have to expend extra energy to move from the bigger object (earth) to a smaller object (space elevator counterweight), aka, the minimum energy cost of moving things out of gravity wells.

by drdeca 8y ago

imh 8y ago

Isn't the rocket equation just a momentum equation? Does it escape that? If we brought something up, would we not pull the satellite it's attached to down, requiring more momentum from somewhere?

drdeca 8y ago
If by "Does it escape that?" You mean "would a space elevator not be subject to the rocket equation?", then yes, it would not be subject to the rocket equation. The rocket equation is about reaction mass that is carried by the thing expelling it. It does not apply to climbing a rope, nor does it apply to the flight of a helicopter. Regarding pulling the satellite down, Pulling down on the weight at the end is something we have to do anyway to keep it from flying out away from earth.
mrep 8y ago
No, the elevator has counterweights at both ends which each pull on the tether more than the object being moved due to gravity and centripetal force. The object on earth pulls more though which keeps it tethered. The elevator that is moving than exerts extra force to propel itself up but not more than what would drag the top counterweight down, thus, the rope still stays fully extended. Think of it like holding a rope tight between your arms and putting a robot that moves on it but ends up not exerting enough force to cause it to slack. Your arms (earth/space teather and the in space counterweight) and the object being moved would keep the same overall motion between all 3 objects (your body would just turn). The space elevator is similar except earth is way more massive than the object being moved so nothing really changes. Also, due to gravity, you have to expend extra energy to move from the bigger object (earth) to a smaller object (space elevator counterweight), aka, the minimum energy cost of moving things out of gravity wells.