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Old 12-11-2017, 02:23 PM   #21
cvannrederode
 
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Default Re: [Spaceships] Breaking orbit with a low-thrust drive

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Originally Posted by Fred Brackin View Post
Some forms of increasing your orbital eccentricity would. If you started from an effectively circular orbit as your orbit became more and more highly elliptical it'd get "longer" on one axis but "narrower" on the other.

That would give you two places on your orbit to burn. If you're being very careful to later to an "egg-shaped" orbit with a narrow end far from Earth you've only got one place to burn.

The plans that I've seen for leaving orbit with ion drives or other very low thrust systems are actually spirals based on continuous thrust.

The practical trade is high thrust/low endurance v. low thrust/long endurance. Reaction mass isn't ever likely to be expensive enough that low thrust/low endurance ends up being the desrable choice.
A prograde burn at periapsis will cause your apoapsis to go up. Retrograde will make it go down. As long as your burn has no radial (towards/away the planet) component, your periapsis won't change. Specifically, the point your orbit where you're making the burn won't change. Albeit this is a continuous process, but it's a basic tenet of orbital maneuvering.

A spiral would work, since you would continuously raise the orbital point on the other side. Real life ion probes do this all the time in solar orbits, thrusting for months on end. Most probes leaving earth still have access to high thrust cyrogenic stages from their launch vehicles, and will make the escape burn as part of the launch from surface. Hence we haven't seen a "staged" departure from Earth IRL. I believe that staged captures have been done, but I may have been wrong.

I've seen and done both in KSP. You can also try Orbiter.
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Old 12-11-2017, 02:32 PM   #22
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Default Re: [Spaceships] Breaking orbit with a low-thrust drive

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Originally Posted by cvannrederode View Post
You need to make sure that your apoapsis doesn't leave the Hill sphere of what your orbiting.
Actually, that's exactly what you want to do -- preferably via the L1/L2 Lagrange points. You can save a fair amount of delta-V over the brute force, 2-body solution. Robert Farquhar pioneered the technique with the ISEE-3 satellite in the '80's. The standard references are a series of papers by Koon, Lo, Marsden, and Ross (sometimes as "KoLoMaRo"), but this one is pretty self-contained.
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Old 12-11-2017, 02:42 PM   #23
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Default Re: [Spaceships] Breaking orbit with a low-thrust drive

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Originally Posted by thrash View Post
Actually, that's exactly what you want to do -- preferably via the L1/L2 Lagrange points. You can save a fair amount of delta-V over the brute force, 2-body solution. Robert Farquhar pioneered the technique with the ISEE-3 satellite in the '80's. The standard references are a series of papers by Koon, Lo, Marsden, and Ross (sometimes as "KoLoMaRo"), but this one is pretty self-contained.
Fair enough. I'll amend to say "don't leave the hill sphere willy-nilly" Doing it in a carefully planned manner is okay.
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Old 12-11-2017, 03:33 PM   #24
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Default Re: [Spaceships] Breaking orbit with a low-thrust drive

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Originally Posted by cvannrederode View Post
A spiral would work, since you would continuously raise the orbital point on the other side. Real life ion probes do this all the time in solar orbits, thrusting for months on end. Most probes leaving earth still have access to high thrust cyrogenic stages from their launch vehicles, and will make the escape burn as part of the launch from surface. Hence we haven't seen a "staged" departure from Earth IRL
The SMART-1 probe demonstrated using an ion engine to travel from GTO to lunar orbit back in 2003.

Last edited by johndallman; 12-11-2017 at 03:48 PM. Reason: Correct orbit
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Old 12-11-2017, 03:53 PM   #25
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Default Re: [Spaceships] Breaking orbit with a low-thrust drive

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Originally Posted by Ulzgoroth View Post
EDIT: I shouldn't have said "nothing will be cheaper" - I haven't checked whether it's possible for a retrograde burn to lower your periapsis prior to doing an escape burn could be efficient. Seems unlikely, but my intuition isn't reliable.
It can be if you want additional velocity beyond what is needed to escape. For example, orbital velocity at Earth orbit is about 30 km/s. If you do a direct escape burn, your final interstellar velocity will be sqrt( v^2 - vesc^2 ), or sqrt( v^2 - 1800 ). Thus, a 60 km/s burn (total velocity 90 km/s) gets you to an interstellar velocity of sqrt( 90^2 - 42.4^2) = 79 km/s.

If instead you go into an elliptical orbit with a perihelion of 0.01 AU (requires about 26 km/s at Earth), your velocity at perihelion will be around 422 km/s (and escape velocity will be 424 km/s). Spend your remaining 34 km/s (new velocity is 456 km/s) and final velocity is sqrt( 456^2 - 424^2 ) = 167 km/s. However, if all you care about is escaping, you can't beat the 12.4 km/s required to just escape straight.
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Old 12-13-2017, 10:28 AM   #26
Michael Thayne
 
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Default Re: [Spaceships] Breaking orbit with a low-thrust drive

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Originally Posted by Humabout View Post
The delta-v needed should roughly be the difference between your current average orbital velocity and escape velocity. You aren't using the Oberth effect to get extra oomph from your burns, so there's nothing special going on here.
How do you figure—or even approximate—average orbital velocity in such a trajectory though?
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