Orbital Speed/Altitude

[RWebb]

Inversion is good - I agree with masraum - you do an inversion down to the other guy's orbit.

I bet NASA has this explained on their site somewhere - you know high school science kids are asking this all the time...

[svandamme]

Originally Posted by vash

not enough info.

does your imaginary spaceship weigh the same as your friend's imaginary spaceship?

There is no weight in space, just mass.

It doesn't matter really, more mass, more energy in the burn to accomplish the same.
Since the lower craft is not changing, the relation to one another , mass wise, is irrelevant.

[svandamme]

It has a name :

Hohmann transfer orbit - Wikipedia, the free encyclopedia

The diagram shows a Hohmann transfer orbit to bring a spacecraft from a lower circular orbit into a higher one. It is one half of an elliptic orbit that touches both the lower circular orbit that one wishes to leave (labeled 1 on diagram) and the higher circular orbit that one wishes to reach (3 on diagram). The transfer (2 on diagram) is initiated by firing the spacecraft's engine in order to accelerate it so that it will follow the elliptical orbit; this adds energy to the spacecraft's orbit. When the spacecraft has reached its destination orbit, its orbital speed (and hence its orbital energy) must be increased again in order to change the elliptic orbit to the larger circular one.
Due to the reversibility of orbits, Hohmann transfer orbits also work to bring a spacecraft from a higher orbit into a lower one; in this case, the spacecraft's engine is fired in the opposite direction to its current path, decelerating the spacecraft and causing it to drop into the lower-energy elliptical transfer orbit. The engine is then fired again at the lower distance to decelerate the spacecraft into the lower circular orbit.
The Hohmann transfer orbit is theoretically based on two instantaneous velocity changes. Extra fuel is required to compensate for the fact that in reality the bursts take time; this is minimized by using high thrust engines to minimize the duration of the bursts. Low thrust engines can perform an approximation of a Hohmann transfer orbit, by creating a gradual enlargement of the initial circular orbit through carefully timed engine firings. This requires a change in velocity (delta-v) that is up to 141% greater than the two impulse transfer orbit (see also below), and takes longer to complete.[citation needed]

[Tim L]

Higher orbits require LOWER speeds, and my speed is now increased increased. Increased sufficient to maintain a lower-altitude orbit.

It's all about energy. When the you fire your rockets you're adding energy, at first its speed then as you go up against gravity you slow( the initial speed is converted to potential energy (You slow down). When you throw a ball up it slows from the fore of gravity.

[vash]

Originally Posted by svandamme

There is no weight in space, just mass.

It doesn't matter really, more mass, more energy in the burn to accomplish the same.
Since the lower craft is not changing, the relation to one another , mass wise, is irrelevant.

huh?

we are talking orbit not outer space right? maintaining orbit is the perfect balance of...nevermind, someone mentioned it.

unless the two imaginary ships have a compatable docking system..you cant meet for the beer anyways like i said, not enough info.

[masraum]

Originally Posted by svandamme

There is no weight in space, just mass.

It doesn't matter really, more mass, more energy in the burn to accomplish the same.
Since the lower craft is not changing, the relation to one another , mass wise, is irrelevant.

Actually, in orbit, whether it's LEO or GEO, there is weight, absolutely. You aren't far enough into space to actually be weightless. The astronauts feel weightless in orbit for the same reason you feel weightless in the vomit comet on it's downward portion of the cycle.

[masraum]

Originally Posted by Superman

Let's say I am in a stable orbit at a certain speed, altitude and period. If I fire my rockets in exactly the same direction as I am moving, I will go faster. Obviously. Now, I'm going too fast to maintain my previous stable orbit. What happens? Viscerally, I would expect to go into a higher altitude orbit, but that runs contrary to the graphs. Higher orbits require LOWER speeds, and my speed is now increased increased. Increased sufficient to maintain a lower-altitude orbit. In other words, if I am to believe the graphs, and if I fire my rockets to increase speed, since I am now traveling too fast for my previous orbit, my centrifugal force would take me to a higher orbit where I am still going too fast. And so......it would seem that ANY increase in speed beyond what is required for a stable orbit at that altitude would take me to a higher and higher and higher orbit, all of which would require my speed to be slower. In other words, any firing of my rockets to propel me faster in the direction I am heading.......would cause me to escape the Earth's gravity.

Doesn't make sense.

The problem with a lot of this discussion is that we are dealing with circles. I think some of this discussion forgets about the second dimension and thinks linearly.

But, I understand what you're saying. To orbit at high altitude you go slower, but if you slow down in orbit your orbit will decay.

[red-beard]

Originally Posted by Superman

Right. So....higher orbits required faster speeds. That is what makes sense. And faster periods for lower orbits. But the data matches the period thing, but not the speed thing. In reality, if I believe the graphs, higher orbits mean lower speeds. WTF?

Higher orbit = slower speed. When you accellerate, you go higher in orbit, but your speed relative to ground goes down.

The issue is that the further you get away from earth, the lower the gravitational attraction.



F is the attractive force between the objects

m1 is the mass of one object

m2 is the mass of the second object

r is the distance between center of mass of each object.



As the orbit increases, the attractive force decreases, requiring less velocity for orbit.

[red-beard]

Originally Posted by masraum

The problem with a lot of this discussion is that we are dealing with circles. I think some of this discussion forgets about the second dimension and thinks linearly.

But, I understand what you're saying. To orbit at high altitude you go slower, but if you slow down in orbit your orbit will decay.

Well, not exactly. You reduce your energy by firing an engine in the direction of your motion. You "think" you are slowing, but you are actually decreasing altitude, which if you still have enough energy to be in orbit, will actually increase your speed.

Eventually, you get low enough that the atmosphere starts to drag on you enough that your orbit will fully decay.

It is a lot easier to conceptualize this in terms of energy than "velocity". It is a combination of speed and gravitational potential.

It has been a while since I calculated orbits (1984 or so) when I was studying AreoSpace Engineering at Texas A&M.

[Superman]

Originally Posted by svandamme

It has a name :

Hohmann transfer orbit - Wikipedia, the free encyclopedia

Okay....here is something that makes sense to me. But unfortunately, it disagrees with the graphs I have seen. In this Wiki description, in order to go from a lower orbit to a higher one, one would increase speed. Vice versa for going from a higher orbit to a lower one. That makes sense to be, but runs contrary to the graphs which show that a higher orbit requires a LOWER speed. And vice versa.

My gut believes this Holmann thing. But it flies in the face of the data.

Maybe I should take this to PARF after all. The guys over there seem to know everything.

[Superman]

Oops. The Holmann description did not "quote," so here it is:

______________________________________________

The diagram shows a Hohmann transfer orbit to bring a spacecraft from a lower circular orbit into a higher one. It is one half of an elliptic orbit that touches both the lower circular orbit that one wishes to leave (labeled 1 on diagram) and the higher circular orbit that one wishes to reach (3 on diagram). The transfer (2 on diagram) is initiated by firing the spacecraft's engine in order to accelerate it so that it will follow the elliptical orbit; this adds energy to the spacecraft's orbit. When the spacecraft has reached its destination orbit, its orbital speed (and hence its orbital energy) must be increased again in order to change the elliptic orbit to the larger circular one.
Due to the reversibility of orbits, Hohmann transfer orbits also work to bring a spacecraft from a higher orbit into a lower one; in this case, the spacecraft's engine is fired in the opposite direction to its current path, decelerating the spacecraft and causing it to drop into the lower-energy elliptical transfer orbit. The engine is then fired again at the lower distance to decelerate the spacecraft into the lower circular orbit.
The Hohmann transfer orbit is theoretically based on two instantaneous velocity changes. Extra fuel is required to compensate for the fact that in reality the bursts take time; this is minimized by using high thrust engines to minimize the duration of the bursts. Low thrust engines can perform an approximation of a Hohmann transfer orbit, by creating a gradual enlargement of the initial circular orbit through carefully timed engine firings. This requires a change in velocity (delta-v) that is up to 141% greater than the two impulse transfer orbit (see also below), and takes longer to complete.[citation needed]

_______________________________________

[lane912]

gloobs of beer foam floating from misshandeled bottles-

[Superman]

If indeed orbital speed decreases as altitude increases (as the formulas and graphs suggest), then Holmann is apparently incorrect, since it describes increasing speed, twice, in order to change from a low orbit to a higher orbit.

[red-beard]

Originally Posted by Superman

If indeed orbital speed decreases as altitude increases (as the formulas and graphs suggest), then Holmann is apparently incorrect, since it describes increasing speed, twice, in order to change from a low orbit to a higher orbit.

The delta-V is at the lower part of the orbit. As the craft reached the upper part of the elliptical orbit, it slows down.

Think about throwing a ball up in the air. As it reaches the top of the arc, it slows down. That is what is going on. To circularize the orbit, you add energy at the top of the arc.

[Superman]

I'm not that smart, James. The Holmann thing makes sense to me. It makes sense to me that a lower-orbit vehicle, in order to achieve a higher orbit, would need to speed up. Similarly, it makes sense that a higher-orbit vehicle, in order to get closer the Earth, would slow down. Let gravity do the work. This makes sense.

What doesn't make sense is the notion, or fact I guess, that higher orbits are accomplished with lower speeds. With a tennis ball at the end of a string, I can vary its "orbit" around my hand. A larger orbit would surely take a longer "period" of orbit. This all makes sense. But it just feels, viscerally, like a ball in a five-foot orbit, while having an orbit "period" much longer than a 6" orbit, would have a greater velocity. Not a smaller one.

Indeed, I can imagine a vehicle in a high-altitude orbit, wanting to be in a lower altitude orbit.....I can imagine that vehicle slowing down just enough that gravity brings it closer the center and if that delta-V were small enough, I can imagine the vehicle nearly slipping effortlessly into that lower orbit. Perhaps one of the things I am missing here is that if this maneuver were performed, by the time the vehicle reaches the desired lower orbit, gravity would have accelerated it significantly beyond its original velocity, enough to almost match the velocity required for the lower orbit. And vice versa.

And if this is true, if this Holmann procedure is accurate, then:

Two bodies in orbit. One high one low. From the perspective of the higher object, the lower object would seem to be outpacing it. Certainly in terms of orbital period. And indeed, in actual velocity. But to catch up with this object, which is much speedier, the higher object would primarily need to slow down. Maybe do a little accelerating to slip into the lower orbit, but for the most part catching up with the lower, speedier thing, it would need to reduce its speed a tad bit. Gravity would do most of the necessary accelerating.

Oops......gotta run. Phone's ringing. It's probably those NASA guys, wanting my advice again......

[red-beard]

Originally Posted by Superman

I'm not that smart, James. The Holmann thing makes sense to me. It makes sense to me that a lower-orbit vehicle, in order to achieve a higher orbit, would need to speed up. Similarly, it makes sense that a higher-orbit vehicle, in order to get closer the Earth, would slow down. Let gravity do the work. This makes sense.

What doesn't make sense is the notion, or fact I guess, that higher orbits are accomplished with lower speeds. With a tennis ball at the end of a string, I can vary its "orbit" around my hand. A larger orbit would surely take a longer "period" of orbit. This all makes sense. But it just feels, viscerally, like a ball in a five-foot orbit, while having an orbit "period" much longer than a 6" orbit, would have a greater velocity. Not a smaller one.

Indeed, I can imagine a vehicle in a high-altitude orbit, wanting to be in a lower altitude orbit.....I can imagine that vehicle slowing down just enough that gravity brings it closer the center and if that delta-V were small enough, I can imagine the vehicle nearly slipping effortlessly into that lower orbit. Perhaps one of the things I am missing here is that if this maneuver were performed, by the time the vehicle reaches the desired lower orbit, gravity would have accelerated it significantly beyond its original velocity, enough to almost match the velocity required for the lower orbit. And vice versa.

And if this is true, if this Holmann procedure is accurate, then:

Two bodies in orbit. One high one low. From the perspective of the higher object, the lower object would seem to be outpacing it. Certainly in terms of orbital period. And indeed, in actual velocity. But to catch up with this object, which is much speedier, the higher object would primarily need to slow down. Maybe do a little accelerating to slip into the lower orbit, but for the most part catching up with the lower, speedier thing, it would need to reduce its speed a tad bit. Gravity would do most of the necessary accelerating.

Oops......gotta run. Phone's ringing. It's probably those NASA guys, wanting my advice again......

The problem with the ball on string analogy is that the attractive force does not reduce over distance like it does with gravity. To keep the string in tension, the force actually has to increase!

Go back to my explanation. Throw a ball in the air and see an arc. This is EXACTLY a ballistic arc. And orbit is a ballastic arc as well. You throw the ball up and add velocity, it goes up an slows as it goes up. Velocity energy (mass*v^2) is being turned into gravitational potential energy (mass*gravity*heigth). As it comes back down, the velocity increases.

The difference is that in an orbit, the ball tries to come back down, but it misses the earth.

There is a picture that Newton put out using cannot, describing ballistic arcs, where the last one is fired so strong, it misses the earth and starts to orbit.

In the diagram above, you accellerate the object in orbit. Initially, it goes up higher in orbit, and loses speed in the process. Again, you are trading velocity for gravitational potential. The speed at the top of the ellipse is not fast enough for that orbit, which is why it "comes back down".

As the eliptical orbit continues, it comes back down to the same place, and the speed increases.

To round out the orbit, you have to increase the speed at the top of the ellipse to the orbital speed of that point.

[Superman]

I get that part. The part I want to get my mind around is the slowing-down-to-speed-up part. vehicle in a high orbit. fires retros to catch a smaller orbit. But needs more speed in that orbit. I suspect that in this instance, the vehicle does indeed slow down, orbit decays a bit (now it's on a collision trajectory but......) speed is only slowed down a little, so that the vehicle continues to 'go around' the Earth and eventually it falls nearly into a lower orbit requiring a tad bit of thrust to place it into that orbit. I suspect that gravity's force is what allows this vehicle to accomplish this maneuver without expending much forward (accelerating) thrust. So again.....slowing down to speed up.

Frankly, I'd known or at least heard about this principle, and have used it as an analogy. Sometimes, in our lives, we have to slow down in order to speed up.

[red-beard]

Originally Posted by Superman

I get that part. The part I want to get my mind around is the slowing-down-to-speed-up part. vehicle in a high orbit. fires retros to catch a smaller orbit. But needs more speed in that orbit.

No no no....

You do slow down when you fire retros. And you are making your orbit an ellipse instead of a circle. And as you drop in altitude, you speed up.

Look at it this way: At the high point in the orbit, you are going too slow to be in a circular orbit at that point. At the low point, you are going too fast to be in a circular orbit there.

[vash]

swing a ball on a string. that tension of the string is the force is gravity. holding it in orbit. now the force of the ball trying to pull it away..centrifugal..or centripetal.. needs to equal that of gravity. all things being equal...you have orbit.

faster it swings the more force outwards so it needs to be closer to earth..or have more mass.

ow..my head hurts.

superman. google "the cartoon guide to physics". it is a comic book that explains physics. i keep it around my office. i love that book.

[Superman]

Originally Posted by red-beard

No no no....

You do slow down when you fire retros. And you are making your orbit an ellipse instead of a circle. And as you drop in altitude, you speed up.

Yes yes yes, this is the part I've been looking for.