Artificial Gravity on JumpShips

[VhenRa]

Only the Wagonwheel puts its gravdecks visibly outside the hull.

Not entirely true. Concordat, New Syrtis, Dreadnought and possibly some of those from the 2765 Field Reports have visible gravdecks.

Also have some of the largest grav-decks in setting.

[Weirdo]

Visible, but still fully enclosed. Those ships can use the space inside the ring for anything the designers want(cargo, quarters...okay, maybe not guns), while the Wagon Wheel has simply support spokes and empty vacuum.

[idea weenie]

Not sure that's 100%. I would imagine maintaining the sail's position to the system's primary would require some expenditure of thrust. While Jump Points aren't exactly close to a star's gravity well, objects will still move.

If the area of the sail is large enough vs the sail's mass being attracted by the star's gravity, it could be 'pushed' back at a similar rate as the star's gravity is pulling it towards the sun, effectively canceling each other out.

[cray]

If the area of the sail is large enough vs the sail's mass being attracted by the star's gravity, it could be 'pushed' back at a similar rate as the star's gravity is pulling it towards the sun, effectively canceling each other out.

No solar sail in BT produces enough thrust from light pressure to lift itself against stellar gravity, let alone the entire ship. The gap is about a factor of one hundred for the solar sail alone, if I recall correctly the last time I ran the calculations.

There is a completely different "light sail" for satellites that will hold a satellite at a jump point, but it isn't available for larger vessels and should not be confused with the "solar sail" aka "jump sail."

[Cryhavok101]

I found the references that made me think ships don't move with deployed sails:

Fluff from SO pg 124:

Sails are only deployed when a JumpShip is parked at a jump point because any real acceleration will rip them apart.

It goes on to talk about the station keeping drive being used to counter the stars gravity, because as Cray said, the sail provides no propulsion, however, the effect of the station keeping drive is have no movement.

In addition, in the Advanced Battleforce rules on pg 276 of SO it says:

Furling/Unfurling Jump Sails: Prior to any maneuvering, JumpShips must furl (close) their jump sails. Reeling in a jump sail takes 350 turns. Unfurling a jump sail takes 180 turns.

And again on pg 277 of SO it says:

Detaching Jump Sails: Prior to any maneuvering or executing hyperspace jumps, WarShips must furl (close) their jump sails. Times for this are the same as for JumpShips. Additionally, WarShips may detach their jump sails. This process takes 4 turns. Reattaching a jump sail takes 12 turns and requires the WarShip to successfully dock with the jump sail first

I couldn't find any reference under standard rules about needing to not be moving around with a deployed jump sail, but hopefully this illustrates why I might think that to be the case.

(disclaimer: All quotes were taken from my PDF, which drivethruRPG tells me is up to date, I did not bother checking for any errata on the matter)

[cray]

There hasn't been an errata on moving with deployed sails.

It goes on to talk about the station keeping drive being used to counter the stars gravity, because as Cray said

Well, do note who wrote the fluff sections in StratOps on spacecraft operations. ;) My statements are just summarizing (parroting) my writing in StratOps.

[VhenRa]

You can tell who wrote a section like that.

The science to word count rises.

[idea weenie]

No solar sail in BT produces enough thrust from light pressure to lift itself against stellar gravity, let alone the entire ship. The gap is about a factor of one hundred for the solar sail alone, if I recall correctly the last time I ran the calculations.

There is a completely different "light sail" for satellites that will hold a satellite at a jump point, but it isn't available for larger vessels and should not be confused with the "solar sail" aka "jump sail."

(I had been thinking just the solar sail would use solar pressure to hold steady while the ship used station-keeping thrust, but I am wrong.)

Didn't know these.  Very good stuff here, thank you.

[vidar]

Those sciency sections are my favorites.  I like when I have to look up the word >:D

[Kovax]

Am I correct in assuming that the light sail hangs "down" toward the star, and that the hole in the center is for the station-keeping drive to fire through without damaging the sail?  If so, that pretty much verifies that light pressure from the star does not move the sail, much less the entire ship, against gravity at that distance.  Stray particles from the drive stream would also tend to push against the sail, keeping it rigidly extended in the weak gravity.

[cray]

Am I correct in assuming that the light sail hangs "down" toward the star, and that the hole in the center is for the station-keeping drive to fire through without damaging the sail?  If so, that pretty much verifies that light pressure from the star does not move the sail, much less the entire ship, against gravity at that distance.

p124 StratOps: "At a standard jump point, jump sails dangle under the JumpShip facing the star, pulled down under by their own weight. Yes, weight. The zenith and nadir points aren’t in orbit, hence the station-keeping drives that keep JumpShips from plummeting into stars. ... Solar sails offer no propulsion to speak of—they’re not real sails, and light pressure is usually much weaker than stellar gravity at standard jump points. And before you ask, the solar wind is even weaker than light pressure."

[Kovax]

Thanks, thought so, but don't have Strat Ops.

[croaker]

I believe the impression that the sail provided sufficient thrust to cause appreciative acceleration comes from the very early texts where the sail size was approximately a factor of 100x the current sizes.

[The_Caveman]

I believe the impression that the sail provided sufficient thrust to cause appreciative acceleration comes from the very early texts where the sail size was approximately a factor of 100x the current sizes.

I don't remember having ever seen a reference to 100km+ jump sails.

Sword and the Dagger, maybe? BattleDroids makes no mention of JumpShips even being a thing (DropShips were hyperspace capable at first) and by the time DropShips and JumpShips came out, sail size was fixed at the current dimensions.

[cray]

I don't remember having ever seen a reference to 100km+ jump sails.

DropShips & JumpShips and StratOps cover it, sort of. Croaker was off by a factor of 2.

DropShips & JumpShips, p. 14, has a sidebar section called, "Jump Sail." "The second major improvement [in the 22nd Century] wrought major changes to both the operational characteristics of the JumpShip and its outward appearance . This new development was the energy collecting jump sail, whose development was made possible by new polymer and advanced metallurgic technologies . The huge parasol-shaped sail was some 50 kilometers in diameter and constructed of high-strength material coated with a special energy-absorbing surface." It rapidly shrank in size as the technology advanced.

StratOps p. 124: "The advantage to sails is that they give JumpShips almost limitless endurance and greatly accelerated human colonization when they were introduced in the 2200s. The first sails were giant arrays up to 50 kilometers in diameter, but they have shrunk considerably to an average of about 1 kilometer"

I believe the impression that the sail provided sufficient thrust to cause appreciative acceleration comes from the very early texts where the sail size was approximately a factor of 100x the current sizes.

Even ye olde DropShips & JumpShips noted the jump sail had a hole for the stationkeeping drive to fire through and avoid falling into the star. The ancient 50km sails wouldn't generate enough thrust from light to lift a JumpShip against stellar gravity, either.

I suspect the impression comes from the name "solar sail." Real life solar sails and the hypothetical ones of the 1960s to 1980s generate thrust rather than photovoltaic power.

[Goose]

Yes, they do use thrust with the sail deployed. The sail is most often used at standard jump points, which are stationary with respect to the star and thus subject to gravity. To avoid falling (slowly) into the star, JumpShips use their stationkeeping engines, which necessarily fires through the dangling sail. (No, light pressure is not stronger than gravity at the jump points.)

OK: How does hanging out at a pirate point complacent this? Would you need more or less thrust?  O:-)

[worktroll]

General rule - more, much more.

Pirate points are a) much closer in system than regular points, so more solar pull, and b) also in proximity to other masses, which - being closer than the star - will exert their own pulls.

Short form, pirate points are probably more challenging for safe sail deployment.

[cray]

OK: How does hanging out at a pirate point complacent this? Would you need more or less thrust?  O:-)

That depends on the pirate point. If it's anywhere on the proximity limit like the standard jump points, then you fire up your stationkeeping engine like normal. If it's an L1 point, then you can go into a halo or Lissajous orbit around the point and not need your engine after the injection burn.

[The_Caveman]

OK: How does hanging out at a pirate point complacent this? Would you need more or less thrust?  O:-)

Pirate points are usually gravitationally balanced, at least to the degree that a KF bubble can form inside them. The point itself is in orbit, being created by the interaction of large bodies in the system.
My assumption of what goes on there is, the JumpShip arrives stationary relative to the target jump point (which is usually fixed to the star, so we can say the ship arrives stationary relative to the star), which in this case is a moving target. The JumpShip shows up in the pirate point's inertial reference frame and is therefore automatically in orbit of the star.
Unfortunately, pirate points aren't permanent fixtures because the gravitational forces are always changing, so you're going to need some stationkeeping thrust if you want to be there for more than a few hours/days. Otherwise you either wind up in a weird quasi-orbit like some Earth-crossing asteroids, or your orbit decays into the closest planet/moon.

General rule - more, much more.

Pirate points are a) much closer in system than regular points, so more solar pull, and b) also in proximity to other masses, which - being closer than the star - will exert their own pulls.

Not quite. The entire idea behind pirate points is they're areas where the various pulls cancel one another out. Over the short term, at least, you shouldn't need any thrust to remain on station. The need for thrust comes in when you're wanting to stay for longer periods and your orbit starts getting perturbed by n-body interactions within the system.

But because pirate points, being gravitational nexes (nexuses? nexi?), are going to attract all kinds of naturally occurring objects, you'll probably be burning a fair bit of fuel avoiding collisions with small trojan asteroids and the like.

[Goose]

 ??? I get how non-standard ProxiLimit points are just like the standard ProxiLimit ones, but I thought halo orbits where for the Legrange(s) 4 and 5, 'cause L1 was perfectly stable?

[The_Caveman]

??? I get how non-standard ProxiLimit points are just like the standard ProxiLimit ones, but I thought halo orbits where for the Legrange(s) 4 and 5, 'cause L1 was perfectly stable?

I believe L1 is only stable in a two-body system. Once you start adding the effect of other planets, moons, etc, it becomes unstable over the long term.

[cray]

My assumption of what goes on there is, the JumpShip arrives stationary relative to the target jump point (which is usually fixed to the star, so we can say the ship arrives stationary relative to the star), which in this case is a moving target. The JumpShip shows up in the pirate point's inertial reference frame and is therefore automatically in orbit of the star.

Exactly that.

Unfortunately, pirate points aren't permanent fixtures because the gravitational forces are always changing, so you're going to need some stationkeeping thrust if you want to be there for more than a few hours/days. Otherwise you either wind up in a weird quasi-orbit like some Earth-crossing asteroids, or your orbit decays into the closest planet/moon.

The L1 JUMP point is not quite at the same place as the L1 Lagrange point because the KF drive is blind to centripetal forces. Without centripetal effects, it would be further from the main body than the real L1 point and thus would fall into a highly elliptical orbit around the smaller body. Because the L1 points are not moving as fast** around the larger body as the smaller body, the resulting elliptical orbit would be retrograde around the smaller.

**They have the same radial velocity, but a lower translational velocity.

But because pirate points, being gravitational nexes (nexuses? nexi?), are going to attract all kinds of naturally occurring objects, you'll probably be burning a fair bit of fuel avoiding collisions with small trojan asteroids and the like.

The only viable Lagrange jump points are the L1 points - see p. 134 StratOps. They are not stable over periods of weeks to months, and thus will tend to shed anything not actively stationkeeping. The astronomical satellites at the Earth-sun L1 point need 30-100m/s per year, which is 0.1 to .33 fuel points.

[edit] I see you noted the instability while I was typing.

??? I get how non-standard ProxiLimit points are just like the standard ProxiLimit ones, but I thought halo orbits where for the Legrange(s) 4 and 5, 'cause L1 was perfectly stable?

L4 and L5 are not viable jump points. They are constructs of both centripetal force and gravity. Only L1s work.

[Goose]

I know only the L1 is jumpable (that's why I didn't mention Ls 2 and 3;) But I've only heard of halo orbits in association with Ls 4 & 5 …

So: Tell me about the need for a halo orbit at L1. :-\

[edit][reads second paragraph] [face palm]

Shouldn't we come up with a better name then "sort'a-near-de-L1?"[/edit]

[cray]

I know only the L1 is jumpable (that's why I didn't mention Ls 2 and 3;) But I've only heard of halo orbits in association with Ls 4 & 5 …

So: Tell me about the need for a halo orbit at L1. :-\

L4 and L5 don't NEED a halo orbit per se. The L1 point is another matter:
Explanation: https://en.wikipedia.org/wiki/Lagrangian_point#Stability
Example: https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory#Orbit

[Goose]

Welp: That's the page, other then the part where I remember the two types of L-points bass ackwards. [run away]