Artificial Gravity on JumpShips

[Achtung Minen!]

I always wondered this... JumpShips usually have grav decks to simulate gravity in space: essentially a ring that spins in the middle of the JumpShip. But why doesn't the entire ship just spin? Presuming the diameter of the ship is sufficient to produce the effect of gravity for the grav deck, it seems that it would be effortless to do the same for the entire ship, using small maneuvering thrusters on the outer hull to begin the rotation.

Granted, this would raise difficulties when the JumpShip either wanted to maneuver or dock with dropships, but given that JumpShips usually appear hundreds of thousands of kilometers away from planets and dropships have to travel for days from the jump points to the planet, it seems like there is a lot of downtime where the JumpShip is just waiting in deep space for the dropships to return.

Also, isn't the radius of a JumpShip (and thus also its grav deck) a little small to reproduce the effect of gravity in the first place (according to real-world physics)?

[Tai Dai Cultist]

It'd require dropships to unhitch if the JumpShip wanted to simulate gravity. Granted, it doesn't matter if all the DSs are in transit, but it is a bigger deal while recharging between jumps.  And besides, in commercial applications a JumpShip is more like a city bus than an airliner... normally passengers get onboard in a piecemeal manner rather than everybody on all at once and everyone off all at once.

As for the small size of JS gravdecks: they'd have to spin at nausea-inducing speed to achieve 1g... so they spin slower and generate lower gravity.  1/2 or even 1/3 standard gravity is enough to allow for more comfort and health than microgravity.

[Knightmare]

IIRC Grav Deck orientation to the ship's decks/spine are perpendicular to each other.

BattleTech JumpShips are built like skyscrapers, so the nose of the ship is actually "up." This way, when the ship's maneuvering, station-keeping or transit drive is fired the thrust creates artificial gravity. During active thrust, the Grav Deck is locked and non-rotating.

However, when then JumpShip or WarShip is in micro-gravity or not under thrust the Grav Deck can spin up, creating gravity via centrifugal force. Meaning, as the ring spins the outer portion of the deck becomes the floor, and the center of the deck—which is likely the ship's spine—becomes the deck's roof.

The point being, sending a ship into thrust while spinning a grav deck places the gravitational pull from the thrust horizontal across the grav deck.

Now, think of the grav deck as the whole ship, but with the drive still oriented along the ship's spine...messy. And JumpShips still need a station-keeping, maneuvering drive. They may not produce a ton of thrust at once, but they can still move a great distance if need be.

[ActionButler]

This is less of an Inner Sphere question than it is an Aerospace question, so we are moving it to the Aerospace sub-forum.
Carry on, all!

[Cryhavok101]

I have always wondered how valuable velcro was in battletech, with systems like that that regularly alter the orientation of gravity in an area, interspersed with areas of periodic micro/zero gravity.

[Knightmare]

96.7% sure I've read novels where the use of velco shoes, or something similar was used to maintain posture in low-micro gravity.

Although, I could be thinking of another setting...::shrugs::

I wonder if silent velcro is a real thing in the BTU?

[cray]

IIRC Grav Deck orientation to the ship's decks/spine are perpendicular to each other.

Gravdecks should circle the spine/KF core. It's the easiest orientation to handle stationkeeping thrust. If you use more than a milli-G of thrust, you can just shape the grav deck like a cone to absorb the combined translation/spin acceleration vectors into a single acceleration vector perpendicular to the tilted deck. It's how the gravdecks on low-G worlds like Luna are arranged.

BattleTech JumpShips are built like skyscrapers, so the nose of the ship is actually "up."

To answer the OP's question, this is a key point. Even JumpShips may be expected to accelerate at 0.1G on the rare times they cross a solar system for some reason. Arranging the decks for spin gravity makes these relatively slow transits into weeks of misery.

Another key point is that JumpShip crews are small compared to the volume of the ship. A 100-meter diameter gravdeck is a 314m length of space that could easily hold individual rooms for hundreds of people plus recreational, dining, and hygiene facilities. JumpShips have much smaller crews than that, so you end up with a lot of unused volume in even small gravdecks, never mind the entirety of the ship.

Further, docking collars aren't strong enough to DropShips under acceleration. You'd have to de-spin those from the rest of the ship.

[worktroll]

Wouldn't there also be issues with spinning a DropShip on its axis fast enough to generate useful pseudo-G, given their smaller diameters?

Cray signal lit! What's the amount of spin we humans can cope with before nausea sets in? And the differential spin rates between head & feet that produce similar issues?

[cray]

Cray signal lit! What's the amount of spin we humans can cope with before nausea sets in?

Per pg140 StratOps, you want to stay below 3rpm. NASA studies have found that 1 to 3rpm (maybe 4) isn't too bad on the inner ear and most people can adapt (key word: adapt). Above 3rpm and any movement of the head in that environment can be disorienting, though people who keep their heads stationary shouldn't have an issue - you can experience it yourself on high-speed spinning rides like "Gravitron" or Epcot's "Mission: Space." Below 1rpm and the spin isn't very noticeable to the inner ear.

If you're aiming for 1G, you get 1G at 1rpm at about 1800m diameter. 2rpm allows a 450m diameter grav deck, while 3rpm can be 200m. Most DropShip-scale spinning grav decks need to accept lower acceleration or risk nauseating their occupants.

And the differential spin rates between head & feet that produce similar issues?

I didn't think there'd be a big issue, assuming the peak G-force is only about 1G at the feet. Do you have a link or example of the effects of this?

[worktroll]

I'll look - I think it only tends to come into effect when height is a major fraction of the spin radius.

(Imagine 1.8m person standing in a 1.8m radius hamster wheel ...)

[cray]

I'll look - I think it only tends to come into effect when height is a major fraction of the spin radius.

(Imagine 1.8m person standing in a 1.8m radius hamster wheel ...)

That might be less troublesome than standing on your head on a 1G planet. On the planet, you have uniform gravity puddling blood in your head. Under spin across a 1.8m radius, you have centripetal force in the legs and mid-body fighting that tendency.

But spin gravity's weird. A good paper to the contrary would be interesting reading.

[Cryhavok101]

96.7% sure I've read novels where the use of velco shoes, or something similar was used to maintain posture in low-micro gravity.

Although, I could be thinking of another setting...::shrugs::

I wonder if silent velcro is a real thing in the BTU?

2001 space odessey used velcro for zero-g IIRC, and I think NASA uses it. No idea if it appears in any battletech novels.

[cray]

2001 space odessey used velcro for zero-g IIRC, and I think NASA uses it. No idea if it appears in any battletech novels.

NASA / ISS uses velcro to secure small items. Skylab used a system of locking cleats on an open gridwork floor; the ISS uses numerous restraint and mobility aids, but experience has found velcro shoes and magnetic boots aren't worth it. Some BattleCorps stories have mentioned magnetic shoes, but p. 256 StratOps addresses those and magnetic boots:

For spacers who can’t find a convenient freefall handhold or
foothold or other anchor, there are Velcro and magnetic shoes.
These are options of last resort. While cheap vid directors like to
use “magnetic shoes” as an excuse not to record their tripe in actual
freefall conditions (like Danger Dave), such shoes actually offer
little advantage. They’re basically good for holding you in place.
Trying to simulate walking in 1 G with a magnetic field around
your foot verges on folly because when you walk in gravity, your
moving foot does not have the weight of your body over it—
you put your weight on the stationary foot. Having to lift a foot
against magnetic or some other retaining force that approaches
full bodyweight is unnatural. The reality is that the experience of
simulating full bodyweight through such shoes or boots is like
fl oating underwater with heavily weighted shoes. You get to float
around like a waving seaweed anchored to the ocean floor.

[Cryhavok101]

NASA / ISS uses velcro to secure small items. Skylab used a system of locking cleats on an open gridwork floor; the ISS uses numerous restraint and mobility aids, but experience has found velcro shoes and magnetic boots aren't worth it. Some BattleCorps stories have mentioned magnetic shoes, but p. 256 StratOps addresses those and magnetic boots:

Oh, thanks for the reference. Yeah, I was talking about securing items, not for pretending to walk normally in zero-g lol. Having played with velcro walls/suits before, I know it would suck a lot. I do wonder how well it would work to secure larger cargo in zero-g though.

[cray]

Oh, thanks for the reference. Yeah, I was talking about securing items, not for pretending to walk normally in zero-g lol. Having played with velcro walls/suits before, I know it would suck a lot. I do wonder how well it would work to secure larger cargo in zero-g though.

The ISS uses bungies and nylon straps.

[Cryhavok101]

It's probably easier to undo those when you want to move the cargo lol. Anyway, enough of me derailing the thread with my idyll musings.

[Weirdo]

The ISS also uses smaller images. Yikes!

[Kovax]

It might make sense for individual "rooms" along a grav deck to be able to rotate slightly (perhaps 10-15 degrees at most) to accommodate thrust from the main engines.  At rest, the rooms would all remain "level" with the structure of the deck, but under thrust, they'd tilt slightly.  That would maintain level floors, things wouldn't roll off tables, and the wall wouldn't become the floor on short notice.  The rotating couplings between rooms and the access tubes leading to the rim would be the only areas where you'd notice the change in the direction of "pull".

[SethsMatches]

Would spinning the JS also mess with the jumpsail? Would the sails inertia cause it to, rotationally, fall behind a spinning JS and start twisting?

[Cryhavok101]

Would spinning the JS also mess with the jumpsail? Would the sails inertia cause it to, rotationally, fall behind a spinning JS and start twisting?

I was under the impression that jumpships don't expend thrust at all while the jump sail is deployed, I don't see why spinning instead of expending thrust would be any different that way.

[Knightmare]

I was under the impression that jumpships don't expend thrust at all while the jump sail is deployed, I don't see why spinning instead of expending thrust would be any different that way.

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.

[Weirdo]

They only expend station-keeping thrust, which is just enough to keep them stationary against the star's gravity at that distance.

[Fireangel]

A 100-meter diameter gravdeck is a 314m length of space that could easily hold individual rooms for hundreds of people plus recreational, dining, and hygiene facilities. JumpShips have much smaller crews than that, so you end up with a lot of unused volume in even small gravdecks, never mind the entirety of the ship.

And this does not even address the width of the gravdeck... or concentric decks on the same ring.

As a 'rule of thumb', about how wide is a plain-vanilla gravdeck?

[cray]

I was under the impression that jumpships don't expend thrust at all while the jump sail is deployed, I don't see why spinning instead of expending thrust would be any different that way.

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.)

And this does not even address the width of the gravdeck... or concentric decks on the same ring.

As a 'rule of thumb', about how wide is a plain-vanilla gravdeck?

No idea, but they don't have a lot of equipment or crew to support usually so they don't need to be wide. StratOps p. 140 gives an example of 10m for a 100m diameter gravdeck. p. 136 shows the Olympus with a wide-profile gravdeck that might be 200-ish meters for a 1200m diameter.

[marauder648]

With warships grav-decks it seems that they are more like tubes, they'll be X meters wide and then take up internal space rather than being a ring on the ships hull.  How this would work on ships with multiple grav decks is beyond me.

[cray]

With warships grav-decks it seems that they are more like tubes, they'll be X meters wide and then take up internal space rather than being a ring on the ships hull.  How this would work on ships with multiple grav decks is beyond me.

No, they're rings but without spokes. Like a subway train running in a circle. See StratOps p. 140.

[Kovax]

Obviously, there need to be at least one or two "spokes" (or access tubes) to connect the grav deck to the ship, unless the ship's own outside diameter is large enough to extend out to the inner rim of the grav deck.  That can be done by making a pancaked jumpship design, turning it into a disc (with the grav deck on the outer rim) skewered by a stick (the jump core), rather than a spoked wheel on a stick.

From a practicality perspective, having a grav deck with a central corridor and rooms on either side would seem to be the obvious solution.  That gives us a 1.5-3 meter practical minimum width (1-2 meters for the passageway, and another half-meter plus of walls, armor protection, and equipment) where there are no rooms, and something like 10+ meters width wherever there are rooms.

[Weirdo]

...unless the ship's own outside diameter is large enough to extend out to the inner rim of the grav deck.

That is already the case with the vast majority of spacecraft, or at least those that actually use grav decks. Aside from the Wagon Wheel, all ships are big enough(or the grav deck small enough) that the entire grav deck mechanism is completely within the ship's main hull.

[Wrangler]

Would spinning grav decks on real life cause a real life ship need fuel to keep. The ship on course?
I remember reading some where its a problem.

[cray]

Obviously, there need to be at least one or two "spokes" (or access tubes) to connect the grav deck to the ship,

Gravdecks are fully contained in ships, so there are other options than spokes like tracks paralleling the gravdeck with small transfer cars. Per p. 140 StratOps:

A space station can be built to almost any geometry with no concern for
a K-F drive core, ammunition feeds, power lines, data cables or
anything else that must run through the center of a JumpShip or
WarShip. ... Space station gravdecks therefore can
be built with simple systems for transferring people to and from
them, such as long elevator spokes to a slow-spinning hub.
JumpShips and WarShips generally cannot spare the volume or
passage room for such elevator spokes and must use “subway”
systems of transfer cars paralleling the gravdeck’s track.

This complete internal containment is why JumpShips tend to have tiny ( <100m diameter) gravdecks. Only the Wagonwheel puts its gravdecks visibly outside the hull.

Would spinning grav decks on real life cause a real life ship need fuel to keep. The ship on course?
I remember reading some where its a problem.

The gravdeck isn't used at more than stationkeeping thrust so gyroscopic precession wouldn't be a problem. However, if you find a ship that uses high acceleration and gravdecks, there are a number of options. For example, you could include a massive flywheel that spins in the opposite direction of the gravdeck to counter the precession. Alternately, you could include two gravdecks of equal size and spin in opposite directions.