Jump points

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cmryan
02/24/04 10:35 PM
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A star has 2 jump points a zenith and a nadir jump point. Is this true of all stars? Are there stars that don't have jump points? Under what conditions do planets get jump points? Do jump points form in deep space far away from planets and stars? From my reading of BattleTech books I get the imprssion that no Great House or Clan is really interested in cataloging and using pirate points.
CrayModerator
02/25/04 03:47 AM
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For answers to these questions, you'll want to pick up a copy of Explorer Corps or the upcoming Aerotech 2, Revised.

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A star has 2 jump points a zenith and a nadir jump point.



Actually, stars have many, many more jump points than that.

The requirement for a piece of space to be a viable jump point is for the gravity to be below a certain level. The overwhelmingly dominating factor in a star system's gravity well is the local star(s). You have to be a minimum distance from the star to find a valid jump point, with certain exceptions (see Pirate Points, below). Because the gravity field of the star is spherical, so is the minimum distance shell around a star. Big planets can render a very small sphere of space unusuable for jumping.

The minimum distance that a jumpship can approach a star is defined by the Zenith and Nadir points. These mark out a diameter on the "minimum jump distance" sphere, approximately the marking the "north" and "south" poles of the sphere. Outside that sphere, a jumpship can arrive ANYWHERE in the star system. (Except maybe giving a million kilometers of leeway around big planets.)

Sol's zenith and nadir points happen to be 10AU (AU = astronomical unit, the distance from the Earth to the sun) above and below the sun. This is about the same distance as Saturn's orbit, though Saturn orbits around the Sun's equator while the zenith and nadir points are above and below the Sun's poles. It would be quite possible for jumpship to jump from a zenith point to Neptune, near Saturn, Uranus, Pluto, or the Oort Cloud...or out into interstellar space. After all, you can jump anywhere outside of that minimum distance from a star, and that includes the deepest reaches of space.

Zenith/Nadir points happen to represent particularly easy navigation targets, nothing more. Because most of a star system's planets tend to orbit in a single plane (a single disk around the local star's equator, a side effect of how star systems form), the zenith and nadir jump points are as far removed from the systems' planets' gravitational effects as possible. Computers can calculate jumps to zenith/nadir points in a few minutes, while human navigators can manually crank through the calculations in several hours. Jumping to other points in a star system (or deep space) take computers hours, while humans cannot make the calculations at all.

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Is this true of all stars?



As noted above, yes. All of space, except very near planets and kinda near stars, is a valid jump point.

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Under what conditions do planets get jump points?



An interesting question.

Condition 1: The planet is outside the minimum jump distance from a star. Then, assuming you don't arrive too close to the planet, you can jump anywhere around the planet.

Condition 2: Two Body Lagrange Points. These are points where gravity from two bodies (a planet and star, or a planet and moon) cancel each other. I'll let you websearch for "Lagrange Points" (I spelled it right) to get a diagram of "two body" Lagrange points. They're quite predictable and stable if you know the size of the two bodies involved. NASA has an itching to put space stations at the L4 and L5 points, and has put satellites (like SOHO) at them. Lagrange points are convenient because they let you jump inside the minimum distance from a star. For example, it would be quite possible to jump to the L4 point formed by the Earth and moon, only ~400,000km from Earth, instead of the zenith/nadir points some 1.5 billion kilometers from Earth.

Condition 3: Multi-Body Lagrange Points. For planets with multiple moons, Lagrange Points (places where gravity is canceled between the objects) are shifting and transitory things. These Transitory Jump Points are very difficult to target, because they might only exist for moments.

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Do jump points form in deep space far away from planets and stars?



As noted above, almost all of space is a valid jump point. It's only near large objects (like planets and stars) that you cannot jump.

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From my reading of BattleTech books I get the imprssion that no Great House or Clan is really interested in cataloging and using pirate points.



Correct. They're a pain to target.

Imagine jumping to a zenith or nadir point. With late 20th Century astronomical equipment, you can see the star, determine its motion relative to the star you're at currently, and determine the plane most of its planets orbit in from light-years away. You can estimate the star's mass and diameter. Thus, you can predict where the zenith/nadir jump points are without ever having visited the star system. If you're uncertain, aim a few more AU away from the star. After all, zenith and nadir points mark the usual MINIMUM distance that a jumpship can approach a star.

Now, consider the Earth and moon Lagrange points. You don't need to catalogue them, really. You just need to note how big the Earth and moon are, how big Sol is, and how the Earth and moon move around Sol. From there, you can just PREDICT the jump points - this is an 18th Century mathematical feat. The problem is, when you're in another star system, it's not easy to SEE something as small as an Earth-sized planet and its moon. (This feat cannot be accomplished now.) You need to know the motion of the Earth and moon really, really well, and be absolutely certain your ship's clock is very, very well synchronized with the Earth and moon's motion.

After all, the Sun is moving at several kilometers per second relative to most nearby stars. The Earth is circling the sun at 30 kilometers per second (100 AT2 hexes per turn). The moon is zipping around the Earth at a kilometer per second. This means the Lagrange points formed by the Earth and Sun are moving in a circle around the moving Sun at high speeds. The Lagrange points formed by the Earth and Moon are doing a funky spiral dance in a circle around the Sun. If your navigational clock is off by a few seconds, you'll MISS the jump points.

Sure, they're predictable. If you have a system mapped out, you can predict where the jump points are, no problem. The risk is in the timing.
Mike Miller, Materials Engineer

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.
cmryan
02/25/04 08:28 AM
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Thanks for the explanation. It could be done, but no one wants to make the effort except of course for the astronomers. Late 20th Century astronomy tecnology can do a lot more. Just visit "www.Space.com" There are plans to do additional upgrades to the Hubble Telescope. Even planet based telescopes with 100 meter mirrors are being designed right now look up "Overwhelmingly Large Telescope" or the 30meter California Extremely Large Telescope.
davion76
02/29/04 10:30 PM
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As for the Lagrangin points, don't forget that they rotate with the bodies involved (Earth/moon etc) so therefore if you want to jump back out, you either need to have the thrust to keep up with the moving points (as after a jump you have no initial velocity and will need to catch up with them and then slow back down to the proper speed to stay there), or enough time to travel outside of the Sphere of Influence of the sun.

Additionally, at pirate points you have to think about object that are orbiting the star in a system. The earth doesn't meet ALL that many extraterrestrial objects (meteors) as it is travelling the same speed as most of the objects in/near its orbit. A vessel that jumps into a pirate point has no velocity, and thus has a decent chance of meeting quite a few fast moving objects.

Interestingly I wonder how much fuel (for the dropships) a pirate point saves. I bet its not all that much. Any other aerospace people out there wanna take a crack at it?
Toontje
03/01/04 01:42 AM
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I take the 'no initial velocity' to be not absolute to the universe. Otherwise even jumping to nadir/zenith will leave you quite possibly stranded. So it would mean moving at the same speed as the no-gravity point. This still doesn't stop the point from possibly being temporary, or shifting/rotating, in a binary+-body system. In a 3-body system, predicting the return of a point to a spot could be a mayor plotline in a BT campaign or RPG. You've got x hr to finish the job, then we'll have to get out or we'll be blasted from the sky.

And for the DS it means quite savings on fuel, from say a week trust down to mere hr's. With a consumption of a mere 1.84 tons a day, that isn't worth the risk in fuel, only in time.

Now I'm left wondering where the magical 1.84 comes from for mill DSes.
Rather to blow up, then.
CrayModerator
03/01/04 03:39 AM
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Quote:

As for the Lagrangin points, don't forget that they rotate with the bodies involved (Earth/moon etc) so therefore if you want to jump back out, you either need to have the thrust to keep up with the moving points (as after a jump you have no initial velocity and will need to catch up with them and then slow back down to the proper speed to stay there), or enough time to travel outside of the Sphere of Influence of the sun.



You can also just establish an orbit matching the Lagrange points, or put your ship in a halo orbit around the point itself. Jumpships arrive stationary with respect to their destination point, so you don't have to worry about the jump point racing away from the ship.

For an excellent example of objects moving with a Lagrange point, look up Jupiter's Trojan asteroid clusters.
Mike Miller, Materials Engineer

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.
CrayModerator
03/01/04 03:41 AM
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Quote:

I take the 'no initial velocity' to be not absolute to the universe. Otherwise even jumping to nadir/zenith will leave you quite possibly stranded.



That's correct. Star systems are moving quite a bit with respect to each other, so jumping ships always arrive stationary with respect to the target jump point. This will be clarified in AT2-Revised.

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Now I'm left wondering where the magical 1.84 comes from for mill DSes.



It's consistent with warships. Civilian dropships are the oddities and apparently represent more primitive engines.
Mike Miller, Materials Engineer

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.
Daryk
04/19/04 05:56 PM
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The 1.84 tons per burn-day really is magical, but so are all the values for the civilian dropships, too. Even continuing the most optimistic fusion processes up through oxygen with 100% efficiency (including the energy normally lost to neutrinos), the best I can get down to is needing about 2.1% of the mass of the ship per burn-day (at 1G). I have to admit I haven't taken physics in about 10 years, so it's possible I've overlooked something in the relativistic energy/momentum calculations or the fusion physics.
CrayModerator
04/20/04 03:25 AM
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Quote:

The 1.84 tons per burn-day really is magical, but so are all the values for the civilian dropships, too. Even continuing the most optimistic fusion processes up through oxygen with 100% efficiency (including the energy normally lost to neutrinos), the best I can get down to is needing about 2.1% of the mass of the ship per burn-day (at 1G).



I'm not surprised. Even 100-ton fighters have a specific impulse of 100000 sec^-1. Small craft in strategic fuel use mode must be nuts, and civilian dropships further overboard.
Mike Miller, Materials Engineer

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.
Gnome76
04/21/04 12:45 AM
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What is "specific impulse" and what is it used for?
CrayModerator
04/21/04 03:49 AM
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Quote:

What is "specific impulse" and what is it used for?



Specific impulse is a measure of the efficiency of a rocket engine, the equivalent to "mileage" in a car.

Specific impulse is specifically (heh) "pounds of thrust per second per pound of fuel used." This means the units for measuring specific impulse are 1/seconds, or sec^-1. (You can substitute other measurements of force for pounds, like the metric newtons or the bastard measurement, 'kilograms-force.' The number won't change since the alternate units are canceled out.)

Anyway, what specific impulse says is, "If I have a rocket engine that uses 1 pound of fuel for every second the engine produces X pounds of thrust, then the engine has a specific impulse of X sec^-1."

The space shuttle's solid rocket boosters have a specific impulse of about 269, meaning if they produce 269 pounds of thrust for 1 second, they'll use one pound of fuel. As it happens, each SRB develops about 2.5 million pounds of force, so they gulp close to 10000lbs of fuel per second. Each.

The space shuttle's main engines are more efficient, having a specific impulse of about 455 (in a vacuum; worse in an atmosphere because of the back pressure). Since they each develop about 500000lbs of thrust, they each use about 1100lbs of fuel per second.

A good nuclear rocket engine (Timberwind, for example) has a specific impulse close to 1000. A hot-running turbojet with afterburner on has a specific impulse of about 1500, while a sedate airliner turbofan might exceed 10000.

Now, compare specific impulses like "455," "269," and "1000" to the specific impulse of a 100-ton aerospace fighter: 100,000. (Wait...is it 125,000?)

Let me check that:
A 100-ton fighter expends 2 thrust points in one turn (Accelerates 1 G for 1 minute).
That requires 220000lbs thrust for 60 seconds.
It consumes 2 points of fuel (25 kilograms, 55lbs) per minute, or 0.918lbs per second

Gee...that works out to a specific impulse of 240000. Where did I get 100K?

Anyway, if you want a longer discussion on specific impulses, look up my thread on ICE Aerospace Fighters
Mike Miller, Materials Engineer

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.
UncaRat
04/23/04 11:37 PM
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The problem with using Trojan points for jump entry deep in the planets system is just that they are dust magnets. Those orbits are VERY stable. So junk collects there.

Ships jumping in are blind! Radar must send out a signal and the echo must be interpeted before protective action can be taken.

being run over by a 30 ton or more rock traveling at a few KPS relative to you or having you jump footprint set off a mine will definitely ruin your whole day.

Pirate points were used by the Dragoons and the Kell Hounds to reach Luhtien durring the clan attack on that world. Only well surveyed worlds with recent surveys would be safe however, the rocks move.

UncaRat's opinion
CrayModerator
04/24/04 06:12 AM
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Quote:

The problem with using Trojan points for jump entry deep in the planets system is just that they are dust magnets. Those orbits are VERY stable. So junk collects there.



No, not necessarily. Only Jupiter's (Jupiter-Sol's) L4 and L5 points have show a strong accumulation of stuff. Neptune-Sol L4 and L5 points should also have a lot of debris according to the latest modeling work. Further, the distribution of dust and whatnot is very spread out over a broad arc (about 120 degrees around Jupiter's orbit for each the L4 and L5 points), the debris doesn't cluster right at what astronomers might term the "trojan point."

And the Lagrange points of other places like the Earth-moon Lagrange points do not seem to have an accumulation to speak of because, no, they aren't all that stable in the long run (millions of years) - not with King Jupiter and sister Venus wobbling stray debris around. Jupiter is why Saturn and Uranus do not have significant Spartan/Trojan asteroid accumulations.

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Ships jumping in are blind!



Hardly. Space is crystal clear at Earth's Lagrange points. Astronomical satellites like SOHO are even parked at them.

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being run over by a 30 ton or more rock traveling at a few KPS relative to you or having you jump footprint set off a mine will definitely ruin your whole day.



Jump points aren't that small, and most of them do not have rocks like the odd Jupiter-Sol L4 and L5 points. As of yet, no one has even *found* any such rocks in the Earth-Sun or Earth-moon Lagrange point, and astronomers are looking. If they were dense enough to blind radar, they'd be visible to the naked eye, let alone Earth-based astronomical gear.
Mike Miller, Materials Engineer

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.
UncaRat
04/26/04 12:15 AM
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Jumping Blind has nothing to do with dust. Jumping blind is because the ship has no current info on local conditions at the arrival point befor arrival. Makes sence if any scan gear could reach 20 to 30 light years and return data, battle tech would have to be totaly rewritten.
The ship therefor must collect all local data after it arrives. If you are aiming for an L point near a planet then the easiest (and cheapest) way to prevent safe usage is to mine it. How hard that is depends on how large a volume is involved. I'll issue a judgment call on that one. Until radar reflects off an object the ship is blind to its presence, then the reflection has to be analized for its likely threat to the ships safety. Granted the short range of battletech weaponry however the threat may be more of an appearance than a reality.
Stable orbits by the way only need to be stable since the fall of the Star League as the resources to locate rocks into dangerously exposed L points would be available, in 3005, they would not.
Cannon writings indicate that these are amoung the conerns jumpship captains with their nearly irreplaceable and fragile ships would need to concern themselves with. I reference the Black Thorn's arrival at their posting in the service of the Kurita's and how they bypassed the clan blockade.
As for SOHO it was placed at an L point in part to reduce fuel consumption due to the stable orbit, yielding a longer lived device. As for lear of dust and junk, certainly compaied to Low Earth Orbits I would have to agree
RESPECTFULY
an Unca Rat oppinion
CrayModerator
04/26/04 03:37 AM
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Quote:

Jumping Blind has nothing to do with dust. Jumping blind is because the ship has no current info on local conditions at the arrival point befor arrival.



Since, as you said, captains probably won't risk their fragile and rare jumpships on LaGrange points, so:
1) When using LaGrange points for commercial traffic in heavily developed sections of space, you should have the benefit of constant correction by HPGs or incoming jumpships from the destination. (Otherwise the merchants arranging that trade route need to be spaced by the captains of their jumpships).
2) When using LaGrange points for sneaky military attacks, use a Scout ship to jump to the edge of a system (beyond detection range of its arrival pulse, like 40-100 AU), map the target system with emphasis on planetary mass and motion, and return with the data. And if the system is not totally unknown, older system maps will speed the plotting of planetary locations.

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Makes sence if any scan gear could reach 20 to 30 light years and return data, battle tech would have to be totaly rewritten.



Not really. Allowing astronomical instruments to resolve target planets and their moons as dots is all that's needed. That's not far beyond our current astronomical reach. Slapping some future 20m multi-mirror array telescopes on a jumpship for navigational purposes doesn't seem over the top.

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If you are aiming for an L point near a planet then the easiest (and cheapest) way to prevent safe usage is to mine it.



Well, actually, mining a jump point runs into a problem: there's no rules for space mines in AT2 (or AT2R). You'd have to create house rules for them.
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Stable orbits by the way only need to be stable since the fall of the Star League as the resources to locate rocks into dangerously exposed L points would be available, in 3005, they would not.



Excellent point on the length of stability, though I disagree with the point about 3005 (or 3050) technology. The engines available for even a Union-class dropship are fine for moving large asteroids, you just need a few hundred tons of fuel and patience. A 2km nickel-iron asteroid (8 tons/cubic meter) could be nudged by 1m/s after the Union-class engines were run at the equivalent of 2 thrust (for the Union) for 11 days, using about 20 tons of fuel. That's enough delta-V to save a planet with several month's warning.
Quote:

As for SOHO it was placed at an L point in part to reduce fuel consumption due to the stable orbit, yielding a longer lived device. As for lear of dust and junk, certainly compaied to Low Earth Orbits I would have to agree



Actually, the point I was working toward on SOHO was only that it was just free of dust and junk at the Earth-Sun L1 point. When it comes to stability, SOHO's halo orbit is unstable on the order of months. L1, L2, and L3 points just do not have the stability of L4 and L5 points.
Mike Miller, Materials Engineer

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.
UncaRat
04/30/04 10:49 PM
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Cray we seem to be talking in different dialects, that being the case I'll try again.

1. Jumping blind has nothing to do with planets as the major topic of risk. both the methods you outline would work for that, it is the smaller pieces that at 40 to 100 AU are not easily trackable. I specified 30 to 300 tons in an earlier post, object of less that 12 meters in thier longest dimention (if made of iron, if furnace slag then they would be somewhat larger). Such objects would be hard to passively detect and any sort of orbital mineral processing at less than very high tec would produce it by the kilo ton in short order.

2. To the best of my knowledge their is no cannon report of L points being used for anything but military trafic, their may be such an entry but it is not known to me, unless it was used by smuglers, I'm not sure on that point. But the arrival of the Dragoons and Kell Hounds was treated as a very non standard event.

3. The likelyhood of the use of L point transits for commercial use would go up as time passes in the cannon. If I were the planetary government I would assume any arrival by any ship I didn't own and have schedualed at an L point was hostile.

Just Unca Rat's oppinion.
Toontje
05/01/04 01:56 AM
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---
Well, actually, mining a jump point runs into a problem: there's no rules for space mines in AT2 (or AT2R). You'd have to create house rules for them.
---

There are already. TO-Capital missiles, when out of LOS they still make attacks on autopilot, and even without fuel they do have a chance to hit. Fueled up and keep it updated on IFF and you've got a space mine, attacking like either a TO missile, or a single ordinary Capital missile attack at a 8 to hit.

Now that is a danger area when things fry while jumping, as the IFF system can be fried as well, thus resulting in friendly fire.
Rather to blow up, then.
CrayModerator
05/01/04 07:23 PM
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Quote:

Cray we seem to be talking in different dialects, that being the case I'll try again.



Focusing on different parts of each other's conversation as important.

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1. Jumping blind has nothing to do with planets as the major topic of risk.



Planets are everything with regards to pirate points. Jump calculations for even standard points need to be recalculated, IIRC, every 2 weeks as a result of planetary motions, sometimes more often.

Quote:

both the methods you outline would work for that, it is the smaller pieces that at 40 to 100 AU are not easily trackable.



The smaller pieces aren't relevant to jump calculations. Objects of 30 to 300 tons are easily spotted and dodged after arrival, and they would also be cleared out by the zone of annihilation an arriving jumpship. As intentional debris placed in jump points, that happens as often as Lagrange points are used for commercial traffic, or less. (BT seems very neglectful of jump point defenses). And as a random threat to arriving jumpships, they aren't. You're more likely to hit by lightning three times than to land on a significant (30-300-ton) chunk of debris in the outer system. Space is big and empty.
Quote:

2. To the best of my knowledge their is no cannon report of L points being used for anything but military trafic,



That's correct. A vastly overlooked commercial opportunity, but correct.
Mike Miller, Materials Engineer

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.


Edited by Cray (05/01/04 07:32 PM)
CrayModerator
05/01/04 07:26 PM
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Quote:

There are already. TO-Capital missiles, when out of LOS they still make attacks on autopilot, and even without fuel they do have a chance to hit. Fueled up and keep it updated on IFF and you've got a space mine, attacking like either a TO missile, or a single ordinary Capital missile attack at a 8 to hit.



That's kinda pushing the teleoperated missile rules, but I suppose. I admit to using TO missiles as the basis for my home "mine" rules.
Mike Miller, Materials Engineer

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.
UncaRat
05/06/04 10:27 PM
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Another point I should bring up is that the Ryan Cartel proves, as does the survival of all jump and drop ships that the grav turbulence that pulverizes things does not extend throughout the entire volume of the jump field. Just how big is this 'quiet zone'?

I know of no answer in cannon, it could be mentioned in an area I haven't seen. If it's recent I haven't seen it. The local rules we played with state that only the edges/surface of the field were turbulent. anything inside that field when you leave, leaves with you, anything in the boundary layer is pulped, anything outside is uneffected. We drew these conclusions from the story about the Grey Death leagion fighting to hold out against a seperatist force, (I don't remember the title).

The local rule also stated anything close enough to the ship to be inside the boundary layer at the time of the ships arrival, or outside the boundary layer are both uneffected by the Grav tubulence. Under our rules it is still in there to hit you.

At a normal jump point this doesn't seem a factor. As you say space is big.
At Tojan points the area involved is smaller but probably still to large to effectively mine, though the TO cap ship missle is a use I hadn't considered.
At an L point however the area will by nature of the beast be according to cannon much smaller, and a simple small station on the edge. a few tens of thousands of drifting hunks of slag with a few dozen of those missles drifting at its edge. not someplace I'd take my only ticket home, and definitely not before jump batteries were reintroduced to use.

I suppose it does depend on the time period. Durring the active raiding stages of the states wars, (say 2800 to 3030) deffending what you have would I think be a nessessary priority requireing such efforts wherever the technology would suppoert them.

Just a parinoid old cat,
Unca Rat
CrayModerator
05/07/04 03:28 AM
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Quote:

Another point I should bring up is that the Ryan Cartel proves, as does the survival of all jump and drop ships that the grav turbulence that pulverizes things does not extend throughout the entire volume of the jump field. Just how big is this 'quiet zone'?



The Ryan Cartel ships do not necessarily prove anything. The icebergs might well be pulverized. Water doesn't care. You just need some capture nets ready at the destination, and since you're going to be deploying sun shields around the iceberg anyway (to avoid it turning into a comet...).

Dropships and jumpships survive because they're attached to the drive, by KF booms in the case of dropships, and the drive knows to adjust the field around the jumpship and dropship. Living Legends gets pretty detailed about jump drive component descriptions.

No KF boom, no direct connection, and then you use the damage/injury rules for being too close to a jumping ship. See AT2, pg37. No matter what...interesting notions...the novels suggest, the rules take precedence and say nearby objects take damage. Novels often stretch or try to rewrite the rules; they work in a different world than the board game. Frex, when I was contributing to the CBT:Companion, I had to bend over backwards to reconcile "Stackpoles" with the inability of fusion reactors to explode.

Quote:

At Tojan points the area involved is smaller but probably still to large to effectively mine, though the TO cap ship missle is a use I hadn't considered.
At an L point however the area will by nature of the beast be according to cannon much smaller,



Er...trojan point = L point, specifically the L4 and L5 points.
Mike Miller, Materials Engineer

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.
Daryk
05/08/04 08:30 PM
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I did some research into the Lagrange point issue, and I don't think the L2, L3, L4 and L5 points are viable jump points. If my calculations are correct, for a jump to work the gravitational field must be less than 3.45x10^-6 N/kg (drawn from reverse engineering the jump point distances from the original Dropships and Jumpships; I note the tables have changed slightly in AT2). What makes the L4 and L5 points "stable" is the interaction of the gravitational fields from the star and planet with the centripetal (centrifugal) and Coriolis forces at those points. As the gravitational field wouldn't be affected by centripetal forces, I'm certain that the a pure field calculation would not yield a result within the limit at any L point with the possible exception of L1. The L1 is probably near the zero-field point, but I strongly doubt it's exactly co-located. The difficulty of calculating the gravitational fields in a dynamic multi-body system to better than one part in a million would certainly explain why it takes 31st century computers hours.
CrayModerator
05/08/04 09:15 PM
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Quote:

I did some research into the Lagrange point issue, and I don't think the L2, L3, L4 and L5 points are viable jump points.



The Explorer Corps Sourcebook said they are viable jump points, so they are valid, unless you want to use your calculations as the basis for a house rule banning the use of L2-L5 as jump points. I can have the exact quote and page number from the ECSB to you either Sunday evening or Monday.
Mike Miller, Materials Engineer

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.


Edited by Cray (05/08/04 09:18 PM)
CrayModerator
05/09/04 12:07 PM
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pg40 Explorer Corps, sidebar, "Lagrange Points."

Last paragraph: "Daring captains with powerful computers and accurate system models may use these transient stable points to jump into or out of a system, shortening their journey times and achieving tactical surprise."

No Lagrange point is identified as being more or less suited for jumping than any other.

Also, pg41, under "Non-Standard Jump Points." Righthand column, second paragraph.

"Non-standard jump points, commonly called pirate points, include any points of gravitational equilibrium within a star system other than the system's standard zenith and nadir points. (The most commonly used type of non-standard points are Lagrange Points.)"

Lagrange points as jump points should receive further treatment in AT2-Revised.
Mike Miller, Materials Engineer

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.


Edited by Cray (05/09/04 12:08 PM)
Daryk
05/09/04 08:29 PM
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I think the "gravitational equilibrium" citation explains the problem. In their explanation of Lagrange points, they neglected the centripetal (centrifugal) and Coriolis forces that actually make L4 and L5 "stable" (and L1-L3 "pseudo-stable"). If the writers assumed all Lagrange points have no gravitational field, then that would lead them to saying Lagrange points are valid jump points. Since they don't actually give an explicit limit for the gravitational field in the rules section, they can get away with that. Effectively, they leave it up to individual GM's to determine what makes a valid jump point (i.e., what each GM takes "gravitational equilibrium" to mean; Explorer Corps page 88 has the actual optional rule). The inconsistency in maintaining that situations like those at Lagrange points are sufficient to enable a jump is that a ship could simply accelerate appropriately and create those conditions at any point in the system (as opposed to the impossibility of reducing the gravitational field). Of course, for players and GMs without the inclination and supercomputers to simulate the gravitational field interactions in a hypothetical solar system, it's simpler to just use the page 88 rule as written and move on (multiply a 3d6 roll by 300,000 or 3,000,000 km and that's how far you are from a valid transient jump point at a given time). It's all about willing suspension of disbelief in the end. Internal consistency helps, but is not required for an enjoyable game.
CrayModerator
05/10/04 03:32 AM
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Quote:

I think the "gravitational equilibrium" citation explains the problem. In their explanation of Lagrange points, they neglected the centripetal (centrifugal) and Coriolis forces that actually make L4 and L5 "stable" (and L1-L3 "pseudo-stable").



"Neglected" might be too strong of a word. The description of Lagrange points on pg40 takes the time to discuss L1 to L3 separately of L4 and L5, identifying the different stability characteristics of L1-L3 vs L4/L5. The description does not touch on centripetal forces, but it's not unusual for BT writers to leave detailed scientific descriptions from print, even though they understand them. The draft reviewing boards are currently littered with scientific explanations that will never get into a book, because there's no need to get that detailed (and there's word count limits, and there's a possibility that putting in such an explanation will serve to trip up the writers later).
Quote:

Internal consistency helps, but is not required for an enjoyable game.



Which is why I like using all Lagrange points as jump points: nice, predictable, & consistent jump points.
Mike Miller, Materials Engineer

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.
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