## Powered Orbits and Gravity

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### Powered Orbits and Gravity

Hi,

I'm trying to figure out whether an artificial satellite in a powered orbit would have any gravity on board or not. E.g. a satellite that's orbiting too low for its speed, but is kept from falling to earth by a downwards thrust. Would the occupants be in free fall or not? I'm thinking not, and the gravity would be the equivalent of acceleration given by the thrust.
How can I think my way out of the problem when the problem is the way I think?

tomandlu

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### Re: Powered Orbits and Gravity

If a force (other than gravity) is being applied to keep it there then it is not in free fall and people inside will experience weight the magnitude of which will depend on the strength of the force applied. So yes, you're right.
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### Re: Powered Orbits and Gravity

I'm wondering if it's possible to stay in orbit under such conditions. Wouldn't you would drift sideways out of the orbital path (as if the planet's gravity was never there)?
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Solt

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### Re: Powered Orbits and Gravity

Yeah, you would have a force on everything inside the craft, gravity is applied to everything in your craft directly, whereas your engine just applies it to the shell of the ship, and everything from that gets applied indirectly to everything else through the structure (i.e. the floor pushing against your feet). A nice trivial case is one where your orbital speed is 0.

Solt wrote:I'm wondering if it's possible to stay in orbit under such conditions. Wouldn't you would drift sideways out of the orbital path (as if the planet's gravity was never there)?

You wouldn't have to cancel it out entirely to have different orbits than would otherwise be possible. Say for example you just apply 0.5g of thrust down, close to the Earth, you would be able to take a circular orbit that's much slower than what you would normally be able to do at that distance (as if the planet's gravity were less than normal). So you could use that to do something like have a geosynchronous satellite in a circular orbit much close than would normally be possible.

Or, you could do the opposite, give yourself a huge sideways velocity, and thrust straight down toward the planet, and you could have a circular orbit that's much faster than you could get otherwise at that distance(since without the extra thrust, you would go flying out into an ellipse, or out of the orbit entirely, etc.).

You don't even have to apply the thrust straight down, which could allow you to do some more unusual things, like have a geosynchronous orbit that's always directly over a point that's not on the equator. See also: http://en.wikipedia.org/wiki/Statite
Soralin

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### Re: Powered Orbits and Gravity

Soralin wrote:You don't even have to apply the thrust straight down, which could allow you to do some more unusual things, like have a geosynchronous orbit that's always directly over a point that's not on the equator.

Ooh - a shiny.

And, once again, tnx all...
How can I think my way out of the problem when the problem is the way I think?

tomandlu

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### Re: Powered Orbits and Gravity

tomandlu wrote:E.g. a satellite that's orbiting too low for its speed,

Too high for its speed?
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### Re: Powered Orbits and Gravity

No. Orbital speed goes down with distance, so orbiting too low for its speed would mean it has a speed that might be a perfectly acceptable orbital velocity higher up, but is too slow at the altitude the thing is currently at. (Probably a more intuitive way to say this would be that it's orbiting too slow for its height.)
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### Re: Powered Orbits and Gravity

Orbiting too slow for its height... So, basically, an aeroplane.
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bentheimmigrant
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### Re: Powered Orbits and Gravity

or orbiting too fast for its height, and using a downward force to avoid moving to a higher orbit.
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### Re: Powered Orbits and Gravity

bentheimmigrant wrote:Orbiting too slow for its height... So, basically, an aeroplane.
Or sitting here in my chair.
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### Re: Powered Orbits and Gravity

gmalivuk wrote:
bentheimmigrant wrote:Orbiting too slow for its height... So, basically, an aeroplane.
Or sitting here in my chair.
That's a great way of looking at it... I feel very productive now.
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bentheimmigrant
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### Re: Powered Orbits and Gravity

tomandlu wrote: kept from falling to earth by a downwards thrust.

Aaaaaw, I'm an idiot, and you all make sense - I read it too fast and "downwards thrust" became the image of a rocket pointing downwards in my head, so I was imagining a satellite going too slow. Stoopid.
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### Re: Powered Orbits and Gravity

Soralin wrote:Or, you could do the opposite, give yourself a huge sideways velocity, and thrust straight down toward the planet, and you could have a circular orbit that's much faster than you could get otherwise at that distance(since without the extra thrust, you would go flying out into an ellipse, or out of the orbit entirely, etc.).

You don't even have to apply the thrust straight down, which could allow you to do some more unusual things, like have a geosynchronous orbit that's always directly over a point that's not on the equator. See also: http://en.wikipedia.org/wiki/Statite

you could also describe that as a hovercraft depending on altitude.
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### Re: Powered Orbits and Gravity

Aelfyre wrote:you could also describe that as a hovercraft depending on altitude.

gmalivuk wrote:Or sitting here in my chair.
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