Started by randomvim, September 11, 2016, 03:14:56 AM
Quote from: Cavebear on August 01, 2019, 08:54:28 AMIt's time.When a body enters a planetary gravity, it speeds up. So it is in the approach longer than the escape. Therefore, more escape velocity. As simple as that may seem, I never heard it explained. And it bothered the hell out of me. So one night, I just sat down an thought about it. I didn't get it while awake. I went to bed. And in the night (brain sorting out questions), it hit me like a wet mackerel to the cheek...I woke up the next morning and understood what was happening in a flyby maneuver. My one great conception. (hey, no children jokes, OK)?
Quote from: Baruch on August 01, 2019, 03:23:49 PMIt has to do with gain and loss of kinetic energy. More KE, more speed.
Quote from: Cavebear on August 01, 2019, 03:47:11 PMI think spherical geometry. I don't know why. I can just visualize it easily.
Quote from: josephpalazzo on August 01, 2019, 10:07:12 PMSorry to rain on your parade, but that's not quite right. The reason the slingshot maneuver is effective it's that the body picks up energy from the rotation of the earth. Why it's also called gravity assisted - the rotation of the earth is effectively assisting the pull of gravity, both acting together to transfer energy to the body. Thus gaining more speed than when it entered into this maneuver. Hope this is clear... :-)
Quote from: Cavebear on August 05, 2019, 12:01:09 PMWasn't describing Earth orbits. I was describing non-orbital approaches to and from other planets. Though it IS accurate for Earth fly-bys from non-orbital approaches...
Quote from: josephpalazzo on August 06, 2019, 05:00:04 PMMaybe my mistake, but your question was: "Have you ever wondered WHY spaceship slingshot pass-bys increase speed?" The slingshot usually occurs around whether it's the earth or any other object like the moon or the sun. I don't know what non-orbital means. In the solar system, you are going to orbit something... unless you have enough energy to escape Helios and go where no man has gone before...
Quote from: Cavebear on August 08, 2019, 12:29:00 PMI have to rethink the explanation. So a ship goes around a Jupiter flyby to gain speed. Why does it gain speed? After all, the gravity pull should equal it towards and from Jupiter. What is gained should be equally lost.Now picture the fly-by in 2 equal parts of distance. Half coming to the nearest approach and half exiting. On approach, the spaceship is travelling X miles per hour. So it is in the approach for a certain length of time and Jupiter's gravity acts on it then. The spaceship gains velocity.On the exit half, the spaceship is still in Jupiter's gravity but not for as long because it is moving faster. And gravity is about time. So when the spaceship is exiting the half of the flyby, it is affected by Jupiter's gravity for less time. So it exits with a speed gain.And technically, Jupiter looses that same amount of speed. It's infitesimal but real. But the spaceship was approaching the closest approach (gravity well) longer then it took to exit. Wikipedia says "Important parameters are the time and distance of closest approach".I did see something about benefits from the direction of planetary rotation, but I'm pretty sure flybys don't depend on that.What am I getting wrong?
Quote from: josephpalazzo on August 08, 2019, 12:58:25 PMIt's well known as gravity assisted - The object flying by gets rotational energy from the planet. It's that simple. You can only reason this by looking at energy, since this is a conserved quantity. So the object cannot get more speed, that is more kinetic energy, it must get it from somewhere else. In this case it's not gravity, which would balance out (KE before = KE after), and so there would be no slingshot effect.
Quote from: Cavebear on August 08, 2019, 01:38:01 PMSo you are saying a flyby has to be in the direction of planetary rotation?
Quote from: josephpalazzo on August 08, 2019, 01:58:45 PMOf course.
Quote from: Cavebear on August 08, 2019, 02:13:20 PMDamn! I never realized the flybys (to increase velocity) had to go with the rotational direction of the gravity well. No one ever tells me these things (well, yeah you did, thank you). And here I thought I finally understood some minor thing about gravity and time...I think I should just buy 100 cases of wine, sit in my chair, and play Civ2 until someone comes by to dispose of my body and rescue the cats...