Author Topic: Jumping To L1 Points  (Read 918 times)

SCC

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Jumping To L1 Points
« on: 09 February 2018, 04:54:10 »
OK recently someone linked my to a GIF showing the 2002-2003 close approach to Earth of J002E3 (Please see attached) which showed just how close the Earth-Sun L1 point is to Earth and it made me think about how it solves the problem of raiders being able to hit a planet before defenders can mobilize.

Now at this point someone's going to bring up the point that you can't see planets and determine their location from a distant system. While that makes sense for an unexplored star, if you don't where the target of you raid is going to be, well you've bigger problems.

Alsadius

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Re: Jumping To L1 Points
« Reply #1 on: 09 February 2018, 07:13:24 »
Lagrange points are not what you want for a pirate point. Lagrange points are areas where a body at rest will orbit the primary(star) with the same orbital period as the secondary(planet), due to the net gravity from the two and the altered orbital radius. Conversely, a K-F drive needs a low absolute gravitational pull - that's why you need to go so far above the ecliptic, to get away from everyone's gravity. The point of null gravity between Earth and the Sun is actually much closer than L1, because L1 is a point where the net gravity is fairly strongly towards the primary. Thing is, it's small and presumably pretty hard to hit(which is why ships using pirate points have a way of disappearing if they miss their target).

snewsom2997

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Re: Jumping To L1 Points
« Reply #2 on: 09 February 2018, 12:16:31 »
You would want the point between a moon of the target planet and the target planet as a pirate point.

While agree you don't blind jump into an unknown system. You would send a scout, track the objects in the solar system and work from there. Not sure how long it would take to track everything larger than a basketball in a unexplored system. It takes forever in our own system. You wouldn't need to survey the whole system just the orbitals of the inhabited planets, but even then you have things like comets moving in and out of the orbitals, or large planets perturbing the orbits of asteroids, etc.

SCC

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Re: Jumping To L1 Points
« Reply #3 on: 09 February 2018, 15:19:33 »
Lagrange points are not what you want for a pirate point. Lagrange points are areas where a body at rest will orbit the primary(star) with the same orbital period as the secondary(planet), due to the net gravity from the two and the altered orbital radius. Conversely, a K-F drive needs a low absolute gravitational pull - that's why you need to go so far above the ecliptic, to get away from everyone's gravity. The point of null gravity between Earth and the Sun is actually much closer than L1, because L1 is a point where the net gravity is fairly strongly towards the primary. Thing is, it's small and presumably pretty hard to hit(which is why ships using pirate points have a way of disappearing if they miss their target).
Well yeah, but there's a Jump Point associated with the L1 Points, probably a bit closer to the star.

You would want the point between a moon of the target planet and the target planet as a pirate point.
This will never work as what you really need to do is offset the mass/gravity of the star and planet/moon points don't do that

cray

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Re: Jumping To L1 Points
« Reply #4 on: 09 February 2018, 15:23:00 »
Lagrange points are not what you want for a pirate point.

Pirate points do, however, form near L1 points. See StratOps' discussion of Lagrange pirate points.

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Lagrange points are areas where a body at rest will orbit the primary(star) with the same orbital period as the secondary(planet), due to the net gravity from the two and the altered orbital radius. Conversely, a K-F drive needs a low absolute gravitational pull - that's why you need to go so far above the ecliptic, to get away from everyone's gravity. The point of null gravity between Earth and the Sun is actually much closer than L1, because L1 is a point where the net gravity is fairly strongly towards the primary.

Yep, all correct. But if I was using a pirate point, those near-L1 jump points are the ones you'd want to use because they're nice and predictable in their motion.

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Thing is, it's small and presumably pretty hard to hit(which is why ships using pirate points have a way of disappearing if they miss their target).

True, there are modifiers to the target number when targeting L1-ish jump points, but even civilian ships regularly hit their targets to within +/- a few kilometers.

This will never work as what you really need to do is offset the mass/gravity of the star and planet/moon points don't do that

Yes, they do. The Terra-Luna jump point does wiggle around quite a bit depending on where Sol is in comparison to the planet and moon, but you can find a net-zero gravity point between the three bodies.
Mike Miller, Materials Engineer

**"A man walks down the street in that hat, people know he's not afraid of anything." --Wash, Firefly.
**"Well, the first class name [for pocket WarShips]: 'Ship with delusions of grandeur that is going to evaporate 3.1 seconds after coming into NPPC range' tended to cause morale problems...." --Korzon77
**"Peace is that brief glorious moment in history when everybody stands around reloading." --Thomas Jefferson, or not

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.

SCC

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Re: Jumping To L1 Points
« Reply #5 on: 09 February 2018, 19:08:45 »
Wait, how does the Terra-Luna point offset the Sun gravity? That makes no sense!

cray

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Re: Jumping To L1 Points
« Reply #6 on: 09 February 2018, 20:04:58 »
Wait, how does the Terra-Luna point offset the Sun gravity? That makes no sense!

Look at your current situation: is the Sun's gravity currently yanking you off the surface of the Earth? Nope, because Earth's gravity is locally stronger than the Sun's. There's a point between the Earth and Sun where the Earth's gravity becomes stronger than the Sun's.

Continuing in that line of thought, you can also figure out that there's a point where Luna's gravity overcomes the Sun's because, obviously, there's not a steady rain of boulders, moon dust, and Apollo landers from the moon toward the sun.

A three-body situation is more complicated, but you're just adding three vectors together to find out where their sum drops below the level found at Sol's proximity limit, and you already know the Earth's and Moon's gravity can nullify the Sun's in some places.
Mike Miller, Materials Engineer

**"A man walks down the street in that hat, people know he's not afraid of anything." --Wash, Firefly.
**"Well, the first class name [for pocket WarShips]: 'Ship with delusions of grandeur that is going to evaporate 3.1 seconds after coming into NPPC range' tended to cause morale problems...." --Korzon77
**"Peace is that brief glorious moment in history when everybody stands around reloading." --Thomas Jefferson, or not

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.

SCC

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Re: Jumping To L1 Points
« Reply #7 on: 09 February 2018, 22:15:14 »
Ah, I see, but that would mean that anywhere between the Terra-Sol L1 point and Terra's Proximity Limit should be safe to jump into, and I'm pretty sure that that would be a pretty big area.

Daryk

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Re: Jumping To L1 Points
« Reply #8 on: 10 February 2018, 05:19:10 »
The Terra-Sol L1-ish point is the point you're describing, Cray.  From a distance, the Terra-Luna system can be modeled by a point mass.  The Terra-Luna L1-ish point is itself in orbit around Sol, and only accounts for the forces of Terra and Luna.  It's been a while since I had to do a three-body problem (like over two decades), but I don't think I'm wrong.

cray

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Re: Jumping To L1 Points
« Reply #9 on: 10 February 2018, 10:44:43 »
The Terra-Sol L1-ish point is the point you're describing, Cray.  From a distance, the Terra-Luna system can be modeled by a point mass.  The Terra-Luna L1-ish point is itself in orbit around Sol, and only accounts for the forces of Terra and Luna.

The L1 jump point between the Earth and Moon necessarily has to account for the sun's gravity if a JumpShip is going to arrive there. The Sun's gravity doesn't drop below the limit of the proximity limit for 10AU, so for a JumpShip to arrive at the Earth-Moon L1-ish point the other two bodies need to negate that gravity.

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  It's been a while since I had to do a three-body problem (like over two decades), but I don't think I'm wrong.

Ditto, but the math isn't bad for simple situations like setting the Earth, moon, and sun into a line. And the sun's gravity is fairly constant across cis-lunar space, given its radius.
Mike Miller, Materials Engineer

**"A man walks down the street in that hat, people know he's not afraid of anything." --Wash, Firefly.
**"Well, the first class name [for pocket WarShips]: 'Ship with delusions of grandeur that is going to evaporate 3.1 seconds after coming into NPPC range' tended to cause morale problems...." --Korzon77
**"Peace is that brief glorious moment in history when everybody stands around reloading." --Thomas Jefferson, or not

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.

Frabby

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Re: Jumping To L1 Points
« Reply #10 on: 10 February 2018, 14:14:54 »
One of the later Gray Death Legion books goes into the pirate point issue in some detail. The JumpShip in question (the Caliban if I remember right) had just jumped out of the system they wanted to go, and thus had a very up-to-date ephemeris on the system - i.e. they knew with great certainity where all relevant bodies in the system were. Having this ephemeris allowed them to plot a pirate point jump back into said system relatively quick and safely.

(I recall this because I liked how Keith correctly used the word "ephemeris" to describe the current condition of a star system, as in the current position of all bodies therein. I had previously only encountered the word in the context of astrology - fortune telling, card-laying, and the like.)
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Daryk

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Re: Jumping To L1 Points
« Reply #11 on: 12 February 2018, 15:22:18 »
One other aspect I just remembered... Don't JumpShips arrive stationary relative to the primary?  Using a point between a planet and moon (assuming that even works) would seem to have them zip past an arriving JumpShip at orbital velocity...

Frabby

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Re: Jumping To L1 Points
« Reply #12 on: 12 February 2018, 16:27:36 »
One other aspect I just remembered... Don't JumpShips arrive stationary relative to the primary?  Using a point between a planet and moon (assuming that even works) would seem to have them zip past an arriving JumpShip at orbital velocity...
That's not so much spelled out as it is inferred from the setup of the novel Far Country.
Another comment I read was that the JumpShip might arrive stationary relative to the jump point, if it is moving. Otherwise, using LaGrange points as pirate points would be pretty much impossible as you'd find yourself without a viable jumppoint to jump back out within hours or even minutes.
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Daryk

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Re: Jumping To L1 Points
« Reply #13 on: 12 February 2018, 19:18:44 »
Wasn't that supposed to be one of the inherent risks of using pirate points near the Lagrange points?

cray

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Re: Jumping To L1 Points
« Reply #14 on: 13 February 2018, 18:16:16 »
One other aspect I just remembered... Don't JumpShips arrive stationary relative to the primary?

Nope, relative to the jump point. See p. 131 StratOps: "A jump alters more than position, it alters velocity. A JumpShip arrives stationary with respect to its destination jump point."

The KF drive senses the target jump point through the Brandt Recoil effect early in the hyperspace field formation process and is able to fine tune the jump to match the destination's velocity. p. 131 goes into some further details about how that happens, and how the JumpShip always must match the destination's velocity.
Mike Miller, Materials Engineer

**"A man walks down the street in that hat, people know he's not afraid of anything." --Wash, Firefly.
**"Well, the first class name [for pocket WarShips]: 'Ship with delusions of grandeur that is going to evaporate 3.1 seconds after coming into NPPC range' tended to cause morale problems...." --Korzon77
**"Peace is that brief glorious moment in history when everybody stands around reloading." --Thomas Jefferson, or not

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.

Kovax

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Re: Jumping To L1 Points
« Reply #15 on: 14 February 2018, 10:35:17 »
That also avoids the obvious next question: "Stationary compared to WHAT?".  That could mean the jump point, the primary star, the center of the galaxy, or practically any other arbitrary point.  Tying it to the jump exit point's velocity makes it clear and simple, and even makes some sense, if there's any sense to be made about FTL travel.

cray

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Re: Jumping To L1 Points
« Reply #16 on: 14 February 2018, 21:14:24 »
That also avoids the obvious next question: "Stationary compared to WHAT?".  That could mean the jump point, the primary star, the center of the galaxy, or practically any other arbitrary point.  Tying it to the jump exit point's velocity makes it clear and simple, and even makes some sense, if there's any sense to be made about FTL travel.

A lot of Strategic Operations' discussion about KF travel was written in response to questions raised on this forum, questions just like that. ;)
Mike Miller, Materials Engineer

**"A man walks down the street in that hat, people know he's not afraid of anything." --Wash, Firefly.
**"Well, the first class name [for pocket WarShips]: 'Ship with delusions of grandeur that is going to evaporate 3.1 seconds after coming into NPPC range' tended to cause morale problems...." --Korzon77
**"Peace is that brief glorious moment in history when everybody stands around reloading." --Thomas Jefferson, or not

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.