Author Topic: Change satellite speed without changing its trajectory  (Read 2918 times)

Offline Jim

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Re: Change satellite speed without changing its trajectory
« Reply #20 on: 08/07/2017 04:43 PM »
I need solid references to include into a paper. I am ready to believe anything you tell me here, but my belief won't be enough. I am searching everywhere on the internet but I can not find nowhere a scientific description of a thrusted satellite that remains on its circular orbit.


You haven't given us enough data or purpose.  What are you trying to do with this "thrusted satellite that remains on its circular orbit."? 

Offline Stan-1967

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Re: Change satellite speed without changing its trajectory
« Reply #21 on: 08/07/2017 04:46 PM »
I need solid references to include into a paper. I am ready to believe anything you tell me here, but my belief won't be enough. I am searching everywhere on the internet but I can not find nowhere a scientific description of a thrusted satellite that remains on its circular orbit.
At the contrary all the books and paper are describing thrusts that change the orbit, wether the thrust is impulsional or low and continuous. Here is an other example :
Fundamentals of Astrodynamics, Karel F. Wakker, paragraph 19.3 and 19.4
https://www.researchgate.net/publication/272507882_Fundamentals_of_Astrodynamics

This is why I ask you here if you can drive me to formal scientific evidences and demonstrations of how to perform a satellite acceleration by staying on the same circular orbit. My aim is not to offence any one, but just to get some solid scientific evidences.

Here is a "proof" for you:

Your condition of a satellite accelerating ( increasing/decreasing velocity) over a planetary surface, while also maintaining a given altitude only work out in a system where the planet surface is an infinitely long, & massless flat plate. 

If you can find a condition like that in the universe, what you are proposing is possible.


edit:  The rest of the "universe" in which this happens must also contain zero mass, so as to not perturb your satellites trajectory.
« Last Edit: 08/07/2017 04:48 PM by Stan-1967 »

Offline hclatomic

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Re: Change satellite speed without changing its trajectory
« Reply #22 on: 08/07/2017 04:56 PM »
May be I should precise that I am not looking for help with a homework but that I have some skills about orbital mechanics. See here for instance : http://vixra.org/pdf/1504.0128v2.pdf, and this work led to the building of an impulsional space transfer simulator, here : http://www.hclatom.net/transferSimulator/

From the theorem of the keplerian kinematics (theorem, not theory), it is very simple to prove that thrusting on a circular orbit can not leave the satellite into its circular orbit, in no way at all. What I am interesting in is to have this demonstration done from the newtonian mechanics, instead of the kinematics, if it is possible.

Offline DarkenedOne

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Re: Change satellite speed without changing its trajectory
« Reply #23 on: 08/07/2017 04:57 PM »
You keep asking for literature references for things that are simply basic facts about orbital mechanics that people familiar with orbital mechanics clearly understand. If you continuously fire a thruster directly towards the Earth, this adds to the force of gravity, meaning you need a faster velocity to stay in the same circular orbit. Combined with an initial impulse to increase your velocity, you could travel faster while maintaining the same orbit at least until you run out of fuel.

This kind of idea is the only way to travel from one side of the Earth to the exact opposite in less than about 45 minutes.

Sorry to ask for references but the answers above are contradictory, and the link that I give above says that you can not stay on the circular orbit by thrusting in any direction. So who should I believe ?
As far as I know in science any statement has to be proven, with either a demonstration or a reference of the literature. Do you think that I could refere to your post in a scientific paper that only says "meberbs, at forum.nasaspaceflight.com, stated so" ? I would be very happy if I could but I figure out that it won't do it.
This is why I am looking for references. Sorry for that.


You're in a circular orbit right now, maintained by the earth thrusting (wrong term, I know) on your feet. No difference 200 miles up if you use an engine instead of the ground. With constant thrust, you can do anything you want.

I think people are adding a great deal of confusion here.  If you have a satellite that is not currently thrusting then its motion will be governed strictly by orbital mechanics.  The only forces on the spacecraft the force of gravity and the centrifugal force.  I have linked to a nice tutorial on orbital mechanics below.  Kepler's 3rd law of orbital dynamics states that "the squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits."  Since orbital period (the time it takes to complete one orbit) is directly related to the velocity, you can easily deduce that any two objects with the same trajectory have to share the same velocity.  We can come to this conclusion assuming that no other forces are acting on the satellite.
http://www.braeunig.us/space/orbmech.htm
https://en.wikipedia.org/wiki/Orbital_mechanics#Laws_of_astrodynamics

Of course Proponent decided to complicate things, and say that this can be achieved if the spacecraft is continuously thrusting.  It is true because you have added this third force created by thrusting that will counter the forces of gravity and the centrifugal force.  The problem with this idea is that satellite operators practically never do so.  Fuel on satellites is a precious resource that can't be replenished.  Great time and care is taken to plan every satellite maneuver in order to conserve as much fuel as possible.  Realistically no satellite can maneuver like this for very long to any significant degree before running out of fuel.

So to answer your question the technically yes, but practically no.  A satellite that is thrusting can manage to do this for a short period of time until it runs out of fuel.  A satellite that is not powered cannot because you cannot have two unpowered objects on the same trajectory, but with a different velocity.

« Last Edit: 08/07/2017 05:01 PM by DarkenedOne »

Offline DarkenedOne

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Re: Change satellite speed without changing its trajectory
« Reply #24 on: 08/07/2017 05:22 PM »
May be I should precise that I am not looking for help with a homework but that I have some skills about orbital mechanics. See here for instance : http://vixra.org/pdf/1504.0128v2.pdf, and this work led to the building of an impulsional space transfer simulator, here : http://www.hclatom.net/transferSimulator/

From the theorem of the keplerian kinematics (theorem, not theory), it is very simple to prove that thrusting on a circular orbit can not leave the satellite into its circular orbit, in no way at all. What I am interesting in is to have this demonstration done from the newtonian mechanics, instead of the kinematics, if it is possible.

It is all newtonian physics.  Keplers laws of orbital dynamics describe the motion of an object when no forces other than gravity and the centrifugal force are operating on the object. 

In a circular orbit the force of gravity and the centrifugal force are perfectly balanced.  You can increase the velocity by thrusting forward.  Of course this will increase the centrifugal force since the centrifugal force is directly related to gravity.  Normally this would cause orbit to expand as the centrifugal force grows stronger than gravity, but if you were to simultaneously thrust into the gravity well you could essentially reinforce the force of gravity.  With thrusters you can make up the difference between the two forces, and keep the spacecraft  on the same trajectory. 

Like I said before thrusting this way would be considered a stupid waste of fuel, but technically it can be done.

Offline Donosauro

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Re: Change satellite speed without changing its trajectory
« Reply #25 on: 08/07/2017 05:45 PM »
This kind of idea is the only way to travel from one side of the Earth to the exact opposite in less than about 45 minutes.

Well, a gravity train could theoretically make the trip in about 42.2 minutes: https://en.m.wikipedia.org/wiki/Gravity_train
http://www.uh.edu/engines/epi2703.htm
You probably would want to stock up on a good supply of unobtanium before starting in on this bit of extreme engineering!

Edit: added second reference.
« Last Edit: 08/07/2017 05:51 PM by Donosauro »

Offline hclatomic

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Re: Change satellite speed without changing its trajectory
« Reply #26 on: 08/07/2017 06:04 PM »

It is all newtonian physics.  Keplers laws of orbital dynamics describe the motion of an object when no forces other than gravity and the centrifugal force are operating on the object. 

In a circular orbit the force of gravity and the centrifugal force are perfectly balanced.  You can increase the velocity by thrusting forward.  Of course this will increase the centrifugal force since the centrifugal force is directly related to gravity.  Normally this would cause orbit to expand as the centrifugal force grows stronger than gravity, but if you were to simultaneously thrust into the gravity well you could essentially reinforce the force of gravity.  With thrusters you can make up the difference between the two forces, and keep the spacecraft  on the same trajectory. 

Like I said before thrusting this way would be considered a stupid waste of fuel, but technically it can be done.
If I understand what you say, the phasing maneuver (http://ccar.colorado.edu/asen5050/projects/projects_2012/bartkowicz/website/orbital_mechanics_4.html or https://en.wikipedia.org/wiki/Orbit_phasing), which is finally at the center of my question, is performed presently by changing the orbit 2 times (see the above links), but it could be done in an other way : by accelerating the satellite on its circular orbit. This last solution being too expensive in fuel, we choose the first way.

If so, I would like to have the reference of the scientific work that demonstrates this difference of cost in fuel. Such a work would certainly contain the orbital mechanics equations that I am looking for.
Thanks for help.


Online meberbs

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Re: Change satellite speed without changing its trajectory
« Reply #27 on: 08/07/2017 06:08 PM »
This kind of idea is the only way to travel from one side of the Earth to the exact opposite in less than about 45 minutes.

Well, a gravity train could theoretically make the trip in about 42.2 minutes: https://en.m.wikipedia.org/wiki/Gravity_train
http://www.uh.edu/engines/epi2703.htm
You probably would want to stock up on a good supply of unobtanium before starting in on this bit of extreme engineering!

Edit: added second reference.
I almost mentioned that concept, although I didn't know it was the "gravity train" name for it.

The 45 minutes was approximate, the lower the orbit altitude the shorter the period. Because the semi-major axis of the orbit is the sole determinant of the period, the 42.2 minutes would also be half the period of a satellite orbiting at the Earth's surface (in a globe encircling vacuum tube :)).

Offline Jim

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Re: Change satellite speed without changing its trajectory
« Reply #28 on: 08/07/2017 06:10 PM »

If so, I would like to have the reference of the scientific work that demonstrates this difference of cost in fuel.


You would have to do that yourself.  Because it is intuitive that constant burning is going to be more than two burns
« Last Edit: 08/07/2017 06:15 PM by Jim »

Online meberbs

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Re: Change satellite speed without changing its trajectory
« Reply #29 on: 08/07/2017 06:17 PM »
If so, I would like to have the reference of the scientific work that demonstrates this difference of cost in fuel. Such a work would certainly contain the orbital mechanics equations that I am looking for.
Thanks for help.
The difference in fuel is so huge that people with experience in this field usually would have no reason to calculate it, it is just obvious to them. As a result, there may not be any references, but it is any easy problem to solve. If someone else doesn't beat me to it, I could work an example later, but I don't have time now.

Offline hclatomic

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Re: Change satellite speed without changing its trajectory
« Reply #30 on: 08/07/2017 06:42 PM »
If so, I would like to have the reference of the scientific work that demonstrates this difference of cost in fuel. Such a work would certainly contain the orbital mechanics equations that I am looking for.
Thanks for help.
The difference in fuel is so huge that people with experience in this field usually would have no reason to calculate it, it is just obvious to them. As a result, there may not be any references, but it is any easy problem to solve. If someone else doesn't beat me to it, I could work an example later, but I don't have time now.

Oh Don't. If the Nasa, the ESA and the russians have chosen a solution for the phasing, against an other, there is certainly a scientific paper giving the arguments for why doing so. They are not taking any decisions without scientific calculations, evidences and argumentation.

Because all the space agencies have chosen the same method for phasing, I figure out that it is a classic of orbital mechanics that we can find it in any good academic book. But so far, I searched in many of them, and found nothing.

 



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Re: Change satellite speed without changing its trajectory
« Reply #31 on: 08/07/2017 06:56 PM »
You won't find writeups on this because  no one that understands orbital mechanics would try to do this[1]. Some things are pretty intuitively obvious and don't get derived. This is one of them. Study orbital mechanics and you'll see why.

Locked.

1 - Arguably whoever asked Jim about doing this didn't understand orbital mechanics either.
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