Jumping To L1 Points

[SCC]

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]

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]

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]

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]

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.

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.

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.

[SCC]

Wait, how does the Terra-Luna point offset the Sun gravity? That makes no sense!

[cray]

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.

[SCC]

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]

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]

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.

  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.

[Frabby]

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.)

[Daryk]

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]

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.

[Daryk]

Wasn't that supposed to be one of the inherent risks of using pirate points near the Lagrange points?

[cray]

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.

[Kovax]

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]

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. ;)

[Lord Cameron]

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

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.

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.

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.

How big would the L1 Earth/Moon or Earth/Sol zone of null gravity be?
A few 10s of km wide?
Is their a danger of jumping onto another ship that’s recently arrived at the point?

[AlphaMirage]

I think the Jump Drive won't work if that point is wrong but it will still discharge the core meaning you have to recharge it again.

[monbvol]

How big would the L1 Earth/Moon or Earth/Sol zone of null gravity be?
A few 10s of km wide?
Is their a danger of jumping onto another ship that’s recently arrived at the point?

I know people have worked it out but I can't remember the exact size but because orbital positions can impact size but I seem to remember even at it's smallest it could handle Kerensky's entire fleet jumping in at the same time with more than enough space between ships to not have issues with annhilation field effects.

[Hellraiser]

but I seem to remember even at it's smallest it could handle Kerensky's entire fleet jumping in at the same time with more than enough space between ships to not have issues with annhilation field effects.

Yes/No.

Remember, Kerensky's forces had a major foul up when 1 warship arrived in the space of another because the first one didn't move out of the area quick enough due to engine trouble.

So is it big enough to handle a couple JS a week traveling through, probably, sure.
But enough to handle 2K+ DS/JS porting in over a matter of hours, not likely.
At least not w/o the risk increasing substantially.

[monbvol]

Yes/No.

Remember, Kerensky's forces had a major foul up when 1 warship arrived in the space of another because the first one didn't move out of the area quick enough due to engine trouble.

So is it big enough to handle a couple JS a week traveling through, probably, sure.
But enough to handle 2K+ DS/JS porting in over a matter of hours, not likely.
At least not w/o the risk increasing substantially.

That was uninformed writing.  Because that was happening at the Zenith/Nadir point, which is far larger.

Like I said the math was done.  Based off what we know now that issue should never have been an issue in the first place.

[Daryk]

That's an understatement... the zenith/nadir points are more like "minimum safe distances"...

[cray]

How big would the L1 Earth/Moon or Earth/Sol zone of null gravity be?
A few 10s of km wide?

Last time I crunched the numbers, it was a few hundred or thousand kilometers across (between Earth and Luna). The Earth-Sol L1 jump point would be much larger.

I don't think Mars and Phobos would have a point of usable size.

Is their a danger of jumping onto another ship that’s recently arrived at the point?

If multiple military ships were making a coordinated jump, say, the Earth-moon L1 point, then it shouldn't be much of a problem. Errors in military ship jumps are about 500 meters. You can stack quite a few ships at 30-kilometer spacing in a jump point hundreds of kilometers across.

If two random ships were aiming at the same jump point and unaware of each other...well, standard practice (per StratOps) is to clear out of jump points - even standard points - right after arriving. JumpShips, space stations, and everything else move away so accidents don't happen.

So it's a pretty low chance of two ships landing atop each other at a pirate point.

I think the Jump Drive won't work if that point is wrong but it will still discharge the core meaning you have to recharge it again.

The jump drive - via the Brandt Recoil Effect - has an idea if the target jump point is safe. However, using a pirate point deep in a star system is inherently unsafe and there's a degree of forcing the jump. Most often, jump malfunctions result in a spent charge and failed jump. However, if the jump goes off during a rolled misjump then, yeah, the ship can be borked.

[Lagrange]

I did an L1 jump point size calculation here.  It's a bit intricate so if anyone sees a flaw, please speak up.

The solution for terra/sol is about 139 space hexes wide.  For Mars it's 67 space hexes in the short dimension.  For Ceres it's just 5 space hexes.

[Daryk]

What gravitational field value did you use for a threshold?

[Lagrange]

What gravitational field value did you use for a threshold?

1/100th the gravitational force of the sun at earth's orbit (= the gravitational pull of the sun at 10AU).

[Daryk]

As I recall, that value used to hold across different stellar classes, but hasn't for some time now.  You'll need to recalculate for each star system.

[Lagrange]

As I recall, that value used to hold across different stellar classes, but hasn't for some time now.  You'll need to recalculate for each star system.

Yeah, that seems plausibly true.  The post lays out the way the calculation is done, so it should be relatively straightforward for whoever/wherever you want to do it.

[cray]

As I recall, that value used to hold across different stellar classes, but hasn't for some time now.  You'll need to recalculate for each star system.

I understand that the proximity limit gravity levels vary by stellar type in the transit time and recharge tables, but I suspect that was a result of an approximation when originally cooking up the tables in DropShips & JumpShips. I used a single value (Sol's proximity limit gravity) when calculating proximity limits of Uranus, Neptune, and Mizar.