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  #51  
Old 11-06-2019, 03:56 PM
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I actually took an undergrad Astrophysics course in college, taught by one of the Professors who was putting together instruments and experiments for the Hubble Space Telescope (whose operational center is conveniently right on campus). The course included orbital mechanics and the like, and I remember my mind being completely blown over and over again by the seeming contradictions of it. To go up, you must go down. To slow down, you must speed up.

But every once in a while, I would be able to conceptualize why it had to be that way; those were Golden Moments when my brain briefly achieved a slightly higher level of functioning. Sadly they never lasted for more than 8 seconds before I lost the insight. But I remember I had it once, briefly. A tiny tiny bit of it, anyway.

QtM, no rocket scientist for sure.

ETA; Damn, that was a fun course though.

Last edited by Qadgop the Mercotan; 11-06-2019 at 03:57 PM.
  #52  
Old 11-06-2019, 04:01 PM
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Ah, I see. And you could actually crash into the Earth if the "low side" were low enough?
Yes absolutely. Your orbit describes an ellipse with the centre of mass at one focus of the ellipse. If that ellipse intersects the surface of the earth then (assuming you don't burn up in the atmosphere) you engage in what people like to call lithobraking. When you thrust "up" in this way you don't change the semi-major axis of the ellipse, but you can certainly make the minor axis a lot smaller.
  #53  
Old 11-06-2019, 04:08 PM
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These two answers seem to be different. Am I reading them wrong?
No. I'm trying to keep it as simple as possible. The simplest way to view orbits when learning, I think, is to picture them as rigid circles. If you're sitting on a ship that has rocket thrusters pointing forward, backwards, up, down, left and right, the only ones that will change the size and shape of your orbit are the ones that face forward or backwards. These increase or decrease your forward speed. If you are at the 3 oclock position and you fire your rear thruster, then your altitude at 9 oclock increases. You've made an oval. If you fire your forward thruster (decreasing your velocity), then you decrease the altitude at the 9. In both cases, your altitude at the 3 doesn't change. If you fire your left thruster, pushing yourself to the right, you will move your orbit without changing the shape. Picture a bike wheel suspended at the hub by a string. What happens when you press down on one side? The other side goes up. That's similar to what happens if you fire your left and right thrusters. It moves your current position at the 3, and moves your orbit at the 9 oclock and equal distance in the other direction. Just like the bike tire. Similarly, if you pull one side of the tire "away" from the center, the other side moves "toward" the center. If you fire your thrusters toward the Earth, then you move your 3 o'clock position closer to the Earth, and move your 9 o'clock position further away. It's very simplified, of course, but I think it helps people visualize what's happening.
  #54  
Old 11-06-2019, 04:08 PM
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Originally Posted by Gorsnak View Post
Yes absolutely. Your orbit describes an ellipse with the centre of mass at one focus of the ellipse. If that ellipse intersects the surface of the earth then (assuming you don't burn up in the atmosphere) you engage in what people like to call lithobraking. When you thrust "up" in this way you don't change the semi-major axis of the ellipse, but you can certainly make the minor axis a lot smaller.
Ok, thanks. And what happens if you thrust "down"?
  #55  
Old 11-06-2019, 04:11 PM
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Ah, I see. And you could actually crash into the Earth if the "low side" were low enough?
Crazy, right!? Trying to push yourself "away" from the Earth can actually make you crash into it.
  #56  
Old 11-06-2019, 04:11 PM
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Ok, thanks. And what happens if you thrust "down"?
The same thing, except instead of going up first and then down, you go down first and then up.
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Old 11-06-2019, 04:14 PM
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Ok, thanks. And what happens if you thrust "down"?
Down lowers the altitude at the current position of your orbit, while raising it on the opposite side. So trying to go "toward" the Earth will make you go further away. Of course, as you come back around in your orbit, you'll be closer again, but then you will get further away as you approach the other side of the orbit. You could possibly thrust enough to lower your current position enough to crash into the Earth (or at least enter the atmosphere which would slow you down enough to crash). But that takes much, much, much more energy than simple slowing down a little bit and lowering your altitude that way.
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Old 11-06-2019, 04:16 PM
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Dinsdale, keeping on the subject of geosynchronous satellites, I have a question for you. If you have a satellite at the end of your rocket and you launch it straight up to an altitude of 36km above the Earth, what happens to the satellite? Does it stay up there in orbit? After all, you've reached the required altitude, right? What happens to it?
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Originally Posted by Bear_Nenno View Post
Excuse me. I mean 36,000Km.
Assuming you launch from the equator, it'll stay in geosynchonous orbit. This is one case where you don't have to give it sideways thrust to stay in orbit. The satellite has sideways velocity inherited from being on Earth, which is rotating once per day. That's exactly how much speed it needs to stay in orbit at geosynchronous altitude.

Now if you launch from somewhere else, it'll have less velocity inherited from Earth, so the satellite won't stay in a geosynchronous orbit. Depending on the launch site latitude, it could either go into a different orbit or fall back to Earth.
  #59  
Old 11-06-2019, 04:20 PM
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...The course included orbital mechanics and the like, and I remember my mind being completely blown over and over again by the seeming contradictions of it. To go up, you must go down. To slow down, you must speed up.
....
One of my favorite moments from putting 3 kids thru college was one time I saw my son's upcoming semester courses. Just about every class was some kind of advanced math. Social science major that I was, I asked him, "How much math do you have to take?"
His deadpan response: "All of it, dad - ALL of it!"

I still smile remembering that!
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  #60  
Old 11-06-2019, 04:22 PM
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Sorry for the confusion, I meant the thruster is thrusting "up" as in the flames are 90 degrees away from the earth.

Sounds like it's more or less the same answer either way. Thanks! Was having trouble wrapping my head around it even though I understand the slow down to speed up counter-intuitiveness of orbital mechanics.
  #61  
Old 11-06-2019, 04:26 PM
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It's possible, given sci-fi level rocketry technology.

You can have it burn fuel throughout its flight. Or launch with a truly atrocious velocity. Or use that nifty warp technology they seem to have.
Except, as I recall that scene, it's not even possible with sci-fi level technology. The missile reaches the sun in about 10 seconds and the effect is instantaneous.
  #62  
Old 11-06-2019, 04:31 PM
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Crazy, right!? Trying to push yourself "away" from the Earth can actually make you crash into it.
"East takes you Out, Out takes you West, West takes you In, In takes you East. Port and Starboard bring you back."

It doesn't make intuitive sense to me either. It's still yet another sci-fi property I'd kill to see on the big screen.
  #63  
Old 11-06-2019, 04:40 PM
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You could just link to a Scott Manley KSP tutorial video on Youtube.
That's what I was going to say... someone really should make a video titled "Everything I ever needed to know about orbital mechanics, I learned playing KSP".

But... to answer the OP in a super-simplified fashion, there are three parts to it.

1. When something's in orbit around something else, they're being pulled toward it by gravity, but they're ALSO going fast enough to, in essence, fall PAST the item continually. That's why getting into space is more a matter of going fast enough, and not as much about going high enough. Another corollary is that since you're being pulled at a constant rate, adding speed means you orbit wider, and removing speed means you orbit closer. So what happens when a space capsule or whatever decides to re-enter, is that they turn around and burn their engines exactly in the opposite direction that they're traveling, slowing themselves down, and shrinking their orbit until it intersects with the Earth.

2. Everything in space is (for the most part) in orbit around something. We're in orbit around the Sun. The moon's in orbit around the Earth.

3. What Munroe is getting at is that the best way to get an object out of Earth orbit, and into a solar orbit that won't have enough speed to continue falling "past" the sun might well be to fire a rocket away from the Sun, because you could get just far enough away so that you're no longer in Earth orbit, but are instead in Solar orbit, but without much velocity, and you'll likely impact the Sun at some point in that orbit.
  #64  
Old 11-06-2019, 04:49 PM
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Originally Posted by dtilque View Post
Assuming you launch from the equator, it'll stay in geosynchonous orbit. This is one case where you don't have to give it sideways thrust to stay in orbit. The satellite has sideways velocity inherited from being on Earth, which is rotating once per day. That's exactly how much speed it needs to stay in orbit at geosynchronous altitude.
The circumference of the Earth is much, much less than the circumference of a geosynchronous orbit (GSO). One must travel a hell of a lot faster to circumnavigate that orbit in one day than to circumnavigate the Earth in the same period.

Quote:
Now if you launch from somewhere else, it'll have less velocity inherited from Earth, so the satellite won't stay in a geosynchronous orbit. Depending on the launch site latitude, it could either go into a different orbit or fall back to Earth.
Even if launched at the equator, it inherits roughly 1600kph of tangential velocity from the Earth. But the velocity required for GSO is close to 11,000kph. Launching straight up will result in the satellite falling straight back down. Well, not straight down, but generally straight, and regardless, it crashes back into the Earth.

Last edited by Bear_Nenno; 11-06-2019 at 04:49 PM.
  #65  
Old 11-06-2019, 10:01 PM
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The course included orbital mechanics and the like, and I remember my mind being completely blown over and over again by the seeming contradictions of it. To go up, you must go down. To slow down, you must speed up.
I think it is the way you are recalling it that makes it sound confusing. Like, what does "to slow down, you must speed up" mean? It could mean something like that you will move more slowly if you accelerate into a higher orbit. But what they always teach people at least sounds straightforward and not contradictory, e.g., an object sweeps out equal areas in equal times, energy is conserved, angular momentum is conserved, square of the orbit time is proportional to the cube of the semi-major axis.

Last edited by DPRK; 11-06-2019 at 10:04 PM.
  #66  
Old 11-06-2019, 10:31 PM
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Quoth Dinsdale:

Don't even TRY to tell me about the pixies inside my computer
Of course not. It's not pixies; it's leprechauns. Japanese leprechauns. And when the repairman comes, all he's really doing is slipping the little guy some sushi and soda bread. Or, for the really expensive problems, Guinness and saki.

At least, that's the way my mom explained it to me, and she wouldn't lie.
  #67  
Old 11-07-2019, 02:18 PM
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Cool thread folks. Lots of basic questions here about "if I do this in orbit, what happens?" I suggest to the curious: Get a copy of the game "Kerbal Space Program" (look it up online). It's a great place to learn orbital mechanics and play around with rockets and spacecraft.

Circling back to OP's mention of Randall Munroe, here is his comic where he claims that Kerbal Space Program really helped his understanding of orbital mechanics:

https://xkcd.com/1356/
  #68  
Old 11-07-2019, 02:20 PM
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I think it is the way you are recalling it that makes it sound confusing. Like, what does "to slow down, you must speed up" mean? It could mean something like that you will move more slowly if you accelerate into a higher orbit. But what they always teach people at least sounds straightforward and not contradictory, e.g., an object sweeps out equal areas in equal times, energy is conserved, angular momentum is conserved, square of the orbit time is proportional to the cube of the semi-major axis.
This was mind-blowing to NASA and the early astronaut corps too. They had to wrestle with these strange ideas, so different from flying an aircraft, in their first efforts at rendezvous and docking in orbit. Here's a pretty nice video documentary of the whole idea:

https://www.youtube.com/watch?v=RFmS5DL-SSM

Last edited by Limmin; 11-07-2019 at 02:22 PM.
  #69  
Old 11-07-2019, 03:24 PM
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Originally Posted by Qadgop the Mercotan View Post
I actually took an undergrad Astrophysics course in college, taught by one of the Professors who was putting together instruments and experiments for the Hubble Space Telescope (whose operational center is conveniently right on campus). The course included orbital mechanics and the like, and I remember my mind being completely blown over and over again by the seeming contradictions of it. To go up, you must go down. To slow down, you must speed up.

But every once in a while, I would be able to conceptualize why it had to be that way; those were Golden Moments when my brain briefly achieved a slightly higher level of functioning. Sadly they never lasted for more than 8 seconds before I lost the insight. But I remember I had it once, briefly. A tiny tiny bit of it, anyway.

QtM, no rocket scientist for sure.

ETA; Damn, that was a fun course though.
I was a history major. I didn't take those pre-med science courses. I had to learn my astrophysics on the street.
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Old 11-07-2019, 04:23 PM
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I had to learn my astrophysics on the street.
On the mean streets of Hell's Observatory?
  #71  
Old 11-09-2019, 08:35 AM
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I need to make a correction to my posts.

If orbiting clockwise at the 3 o'clock position, aiming "straight up" and firing your rockets will raise your orbit at the 6 o'clock and lower it at the 12 o'clock. My previous posts stated that it was the 3 and the 9 respectively. I didn't think that through carefully before posting, and I apologize.
  #72  
Old 11-09-2019, 12:59 PM
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"Set the controls for the orbit of Neptune, then set the controls for a -13.1 km/h retrograde burn, then shut the controls off and wait"
  #73  
Old 11-09-2019, 01:09 PM
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*13.1 km/s that is
  #74  
Old 11-11-2019, 11:25 AM
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Thanks for the interesting discussion!

Like Little Nemo, I'm a history major and then worked in software development. Reading this thread made me want to understand this better, so I went out and bought Kerbel Space Program to try.

Those poor Kerbels. (Think Galaxy Quest. "Those poor people.")

I returned it to save as many Kerbels as I could.
  #75  
Old Yesterday, 08:33 PM
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I remember a scene in a book I once read - my gut says Space by James Mitchener, but my memory does fail me sometimes. I would have read it in my teens, which was some many years ago.

The astronauts in training (hotshot pilots all) are taken to a racetrack where there are two rigged jeeps. The jeeps are set to go round in a circular path at a constant speed, but if the accelerator is pressed, it turns momentarily inward, and if the brake is pressed, it turns momentarily outward.

The jeeps are set 180 degrees apart, and the astronauts told to use the brake and accelerator pedals to bring the two jeeps together, one behind the other. As pilots, they expect this to be an easy task - of course, it is almost impossible.

This is deliberate, and is presented as a way to teach the astronauts that manually flying a spacecraft is completely counterintuitive for these pilots.
Once in space, they have to rely on ground control to make the burn calculations for orbital control.
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