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Old 12-29-2017, 01:20 AM   #1
bcbarnes
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Question TransX Escape vs Eject

I think I've got these two terms figured out:

Escape: Used when escaping from a minor body into the orbit of a major body (i.e. like from earth to sun)

Eject: Used when ejecting from a major body orbit into the influence of a minor body (i.e. like from the sun to mars)

It just doesn't feel quite right (or maybe not complete?). Can anyone give me a more detailed and specific definition of these to terms as they are used by TransX. Also, are these TransX specific terms or do they apply to orbital mechanics in general?

Thanks,
Brian
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Old 01-01-2018, 09:09 PM   #2
MontBlanc2012
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Quote:
Originally Posted by bcbarnes View Post
 I think I've got these two terms figured out:

Escape: Used when escaping from a minor body into the orbit of a major body (i.e. like from earth to sun)

Eject: Used when ejecting from a major body orbit into the influence of a minor body (i.e. like from the sun to mars)

It just doesn't feel quite right (or maybe not complete?). Can anyone give me a more detailed and specific definition of these to terms as they are used by TransX. Also, are these TransX specific terms or do they apply to orbital mechanics in general?

Thanks,
Brian
Hi, Brian

I don't think this interpretation is quite right. TransX uses the term 'escape' to refer to trajectories that have sufficient energy (kinetic and potential) to leave the gravitational influence of a minor body such as the Earth. So, in TransX's stage 1, to go from Earth to Mars, say, one has to select the 'Eject' option since the goal of the planned trajectory is to leave the gravitation well of the Earth far behind.

In terms of Keplerian orbits, to 'escape' a minor body, one has to add enough energy such that the orbit shifts from being 'elliptical' (wrt the minor body) and becomes hyperbolic (wrt the minor body). Almost always, once the vessel has 'escaped' the gravity well of the minor body, it will have an elliptical orbit wrt to the Sun. It is possible to keep on adding energy to the vessel such that it also 'escapes' the gravity well of the Sun system as well. So, when you 'escape' you have to have a mental model of which gravity well you plan on leaving.

Rather confusingly, to set up a manoeuvre using in TransX that 'escapes' Earth and sets one up on a trajectory to head to Mars, say, one first needs to set up an 'Eject Plan'. The Eject Plan identifies four things: 'when' your vessel will escape the gravity well of the minor body; and the three prograde/outward/plane components of your vessel's velocity vector after you have left the gravity well. In other words, TransX asks "OK, so you want to leave Earth. You need to tell me when you want to leave; and the direction (and speed) you want your vessel to have once you leave."

Having set up an 'Eject Plan', standard TransX protocol is to translate this Plan into a manoeuvre that you can execute in Low Earth Orbit such that once that manoeuvre (colloquially the 'Escape Burn') has been completed (and once the vessel has escaped the Earth's gravity well) it has a final velocity vector that matches that of your Eject Plan.
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Old 01-02-2018, 12:48 AM   #3
bcbarnes
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Thanks MontBlanc2012. I'll have to read that a couple more times, but I think it will sink in to my head.

Brian
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Old 01-02-2018, 09:55 PM   #4
francisdrake
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If you just started with TransX you may want to have a look on these 2 pages of basic commands: http://orbiter.quorg.org/TransX_desc.pdf
When I started with TransX I was completely lost. I felt the need to write a short description for myself. Most of it is still valid.
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Old 01-03-2018, 02:41 PM   #5
bcbarnes
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Thanks, francisdrake. I've printed that out to keep by my computer.
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