Communications and navigation on spacecraft rotating to create 'artificial gravity'

[meyst]

On a spacecraft Rotating/tumbling to create centrifugal force, 'artificial gravity', how would the motion of the ship complicate communications? navigation? How would one mitigate any such issue?

Many Science fiction ships and stations are depicted with 'rings' that rotate around the craft to create these forces for a crew while the ship proper does not seem to rotate in any meaningful way and thus eliminating this issue at the expense of perfect bearings, motors, thrusters, artistic license. Seeing as rotating/tumbling an entire ship seems (to me) to be relatively simple to engineer in a frictionless environment by comparison (thrusters, fuel and structure that can handle the stress).

This brought up my question, how would one communicate/navigate such a craft given that satellite dishes seem to respond poorly to being pointed in the wrong direction. What would you use to communicate and or navigate with such a craft while it cruises between planets?

[Accelerando]

could put the satellite dish on a ring itself, two dish rings counterrotating to conserve momentum, or have the dish on a joint on the end of the ship's rotating tube and track the target in the apparently spinning sky. alternatively have a communications rig trailing you and use omnidirectional antennae to keep touch with it and in turn message other worlds

[Nuke]

navigation is troublesome because all the sensors being mounted in a rotational frame of reference. but its fairly straightforward to go from the rotating ship frame to the orbital frame for example. thats just some pretty basic linear algebra.

any high gain antennas (or anything with a dish) would need to be mounted on rotating platforms about the z axis (probibly on your bow), so they can stay pointed at the thing they are communicating with, in both directions. to save power on the turret (especially the z axis), you might have a low gain wakeup signal on other antenneas to tell the high gain antennea to lock on. omnidirectional antenna arrays mounted all around the rotating hull could provide the wakeup signal, or any low speed communications if high gain is not needed.

thrusters are probibly the hardest problem to solve. you not only have to control thruster firings to compensate for the rotational frame of reference, but you must also compensate for gyroscopic forces as well. the control algorithms would be tricky but its something that can be done. but we already do spin stabilization in space probes, so this is likely all worked out already.

[Kerbin Dallas Multipass]

IIRC the Juno spacecraft is constantly rotating at something like 3 revs/min. Seems they have worked out the issues you mentioned.

Edited April 4, 2014 by Kerbin Dallas Multipass
3 rpm, not 1.4

[Ralathon]

IIRC the Juno spacecraft is constantly rotating at something like 1.4 revs/min. Seems they have worked out the issues you mentioned.

Yea, you just point the rotational axis towards the earth. That way the earth is at a constant spot in the rotating sky, just like polaris is for earth. Just point your dish along your axis of rotation and you'll have perfect communications 24/7

[einsteiner]

You could also have a ring of antennas, and use them as a phased array with beam-forming techniques to always point the virtual antenna towards the target. This would be a lot more complicated than just pointing the dish correctly, but it would be really cool.

[K^2]

Why would you put your sensors on the rotating ring? Ring should only contain the habitat and work stations. Maybe life support, if it's more convenient. Propulsion, power generation, comms, and sensors should all be on the hub. There just isn't any reason to do it another way.

There is also no reason for larger ship to have an exposed bridge. Whether for protection from possible attack, or just in case of collision with debris or asteroids, bridge should be well protected, located as deep inside the rotating ring as possible. All of the information, be it visual, radar, or other sensor data, can be relayed to the bridge from the hub. So from perspective of the bridge, there is going to be no rotation, and absolutely non problems related to rotation.

[Nuke]

i was under the assumption that we were discussing ships where the whole thing rotates along the z axis for gravity, no separate rotating and non rotating parts. you know like this:

[xcorps]

The Stanford Torus was a space station design proposed in the mid '70s. It envisioned a station capable of housing 10,000 residents. It had only 1 RPM to create gravity. Not a challenge for communications.

[magnemoe]

First it would probably be simpler to put communication and other stuff who needed to be pointed in various directions on a rotating frame than rotating the habitat section independently. the communication module is an tiny fraction of the habitat and only need power and data and this is a common problem, radars is one thing with the same problem. only change here is that its not an simple axis but an 3-5 meter wheel who is rotating but this is also not uncommon.

[Nuke]

im thinking a z aligned turret on the bow, full 360 degree rotation about z, with maybe 120 degree tilt, giving you near omnidirectional tracking with a small blind spot in back. then just stick all your high gain antennae on it. low gain hardware can just be bolted to the hull. this thing need not be very massive, and only a couple small cmgs to compensate for their motion. it might not always be locked on target when not in use. low gain traffic is probibly sufficient for most communications. if you need to transmit a lot of data, you can bring the dish to bear on the target. to initiate high speed communicate with the ship, send a wake up signal to the low gain antennae, allow the dish enough time to begin tracking, then begin transmission.

[Nibb31]

Why would you put your sensors on the rotating ring? Ring should only contain the habitat and work stations. Maybe life support, if it's more convenient. Propulsion, power generation, comms, and sensors should all be on the hub. There just isn't any reason to do it another way.

It's probably more convienient to have the entire ship spin and only the antennas and light fixed stuff on a rotating joint rather than having the entire ship fixed with a heavy rotating ring. It would avoid a lot of complexity in that rotating joint.

[Seret]

Any directional antennas would need to be on a rotating mount anyway. Really can't see how it would be a problem.

[Nibb31]

You could always despin the ship for manoeuvers, but that would cost propellant.

Or you could use Canfield joint thrusters like in this video:

[Seret]

I don't think you'd need to bother. I don't really see any major problems caused by having the ship spin. In space everything is always moving anyway.

[magnemoe]

You could always despin the ship for manoeuvers, but that would cost propellant.

Or you could use Canfield joint thrusters like in this video:

Downside of that is more mechanical stuff who can break down, yes you could have multiple for backup.

Not sure how much maneuvering you do during transit anyway. from KSP its typical the main burn, an adjustment burn, the main plane change burn around midpoint, then an correction burn.

You will have days to rotate the ship to point in burn direction before doing any of this, and yes you can use the 4 axis trusters then they point the right direction

[Seret]

Those thrusters might save on weight, but they'd be more complicated and therefore unlikely to be more reliable. You always want to reduce the number of moving parts, not increase them. There's no problem in using standard quad blocks to maneuver while spinning anyway, accounting for the spin would be pretty trivial. RCS systems are still perfectly useful while spinning, or they wouldn't be fit for purpose.

Generally speaking, since you're going to be rotating around your centre of mass and your engines will be aligned along the same then there's no problem with main engine burns either.

[Kermunist]

A phased-array antenna running round the 'equator' of your rotating vessel would be able to act like a dish pointed in any direction. Phased array systems already exist, the technology is fairly mature and they are replacing steerable dishes in some cases because they can be more reliable. Some examples off the top of my head: RAF Fylindales radar, SatCom systems on aircraft.

As for propulsion, putting the main engines to thrust along the direction of rotation is probably wise. When you attempt to change the heading of the craft prior to doing a burn, the gyroscopic effect might cause a bit of a headache, but nothing you can't work around with simple mechanics. You will need to apply torque (using reaction wheels or thrusters) at a different angle from what you would with a non rotating ship. Check out "Torque induced gyroscopic precession" for the details and the maths.

[Idobox]

Rings are a terrible idea for anything smaller than a city, it's much easier to use a tether to link your habitat to some counterweight (let's say fuel + power supply).

In this context, you only need a small platform at the barycenter to put you sensors and antennas, and a gyroscope to make sure the platform is not spinning.

[Nuke]

Rings are a terrible idea for anything smaller than a city, it's much easier to use a tether to link your habitat to some counterweight (let's say fuel + power supply).

In this context, you only need a small platform at the barycenter to put you sensors and antennas, and a gyroscope to make sure the platform is not spinning.

this might be useful for a one shot mars run, something you can stick on a rocket and go. but having to reel yourself in and spin down to do a burn could get costly pretty fast. for a large reusable platform with a never spin down policy is still quite possible.

also i dont think the ring needs to be that big. SpinCalc seems to indicate a minimum radius of about 224 meters would be acceptable. you can bring that down a lot if you dont need 1g. for example you can have a 179m radius @ 0.8g, or 112m @ 0.5g. so using the worst case radius of 224 meters, you end up with a circumference of 1407.4 meters. you dont need to make the ring very deep, 10 meters should do, and give you rooms the size of my apartment. 14070 square meters is hardly a city. but its still a fairly large ship (aircraft carrier large). so it might be better just to accept less gravity or have the crew put up with some adjustment period, so the ship can be made a sane size.

[KASASpace]

You could put the Satellite dishes on the back of the vessel, where they experience the least centripetal force.

Now, for navigation you would have three gyroscopes, all pointing ninety degrees to each other.

They are mounted in a way where they always face the same direction.

[Seret]

Well yes, that is how inertial navigation works! Pretty much every spacecraft and aircraft has done it that way for decades.

I think the OP was more curious about things like sighting stars, but I don't think that would a problem.

[Nuke]

ive seen all kind of spin stabilization schemes for cameras. you might use arrays of cameras where all the image data is algorithmically stitched together and transformed to a non rotating reference frame to provide adequate imagery for navigation.

[Seret]

ive seen all kind of spin stabilization schemes for cameras. you might use arrays of cameras where all the image data is algorithmically stitched together and transformed to a non rotating reference frame to provide adequate imagery for navigation.

Indeed. You'd probably need to do some calibration after you'd spun up, but after that the spin would be locked in at a constant rate and direction, so compensating for it wouldn't be hard.

[Idobox]

this might be useful for a one shot mars run, something you can stick on a rocket and go. but having to reel yourself in and spin down to do a burn could get costly pretty fast. for a large reusable platform with a never spin down policy is still quite possible.

Reeling in and out costs almost nothing, and if you burns are short, you can store your angular momentum in flywheels.

Also, nothing stops you from performing burns while spinning, as long as your center of thrust and center of mass are aligned (might require some precision pumping though)