Ringworld type object...

[fsci123]

I was wondering how fast would a ring 243km in radius would have to rotate to provide .69G... I was also wondering whether could this ring be built out of steel or some other normal material being CNTs a last resort... And how much fuel could be used to rotate the ring...

 

[Quick_Nick]

Centripital acceleration = tangential velocity squared / radius
So, you need a tangential velocity of about 1280m/s (?)(though maybe angular velocity would be more useful)
0.0053 radians/second I believe
Ac = w^2 * r

As for material, i'd say anything you can get into space.
Fuel: depends on ISP. Theoretically, attach enough solar sails and wait a while.

 

[Axertan]

If you're a bit lazy like me, you can also try this:
SpinCalc

 

[Manichean]

If I remember correctly, though, for the structure as discussed in the books you'd need materials that have tensile strengths in the order or magnitude of nuclear forces. It would probably be worth doing a quick calculation and finding out if this is true for your structure, as well.

 

[agentgonzo]

0.0053 radians/second I believe

That works out at one revolution every ~20 minutes

 

[fsci123]

If I remember correctly, though, for the structure as discussed in the books you'd need materials that have tensile strengths in the order or magnitude of nuclear forces. It would probably be worth doing a quick calculation and finding out if this is true for your structure, as well.

Well last time i checked space elevators on earth could be 10x the length and they dont require nuclear type forces to hold them together...
And we could have built a space elvator on mars or the moon using either kevlar or steel and thats still much longer than this object...

 

[agentgonzo]

If I remember correctly, though, for the structure as discussed in the books you'd need materials that have tensile strengths in the order or magnitude of nuclear forces. It would probably be worth doing a quick calculation and finding out if this is true for your structure, as well.

I question this as, by definition, the maximum stress that you'd be putting on any component would be 1G. I'm pretty sure that we have materials now that can hold their own weight.

 

[T.Neo]

It's about the combined stress of the ring trying to rip itself apart. That would require far stronger material than anything known in science.

As for fsci's ring, CNTs should do the trick... there was [ame="http://en.wikipedia.org/wiki/McKendree_cylinder"]a study[/ame] of a 460 km radius cylinder using CNTs, so it should be possible. I've even heard radii of up to 1000 km, but I'm not sure if this is a serious number of a slight handwave.

As for the propellant needed to spin it up, the short answer is: A lot of propellant. The long answer is that RisingFury posted a way to do this in another centrifuge-related thread, but I can't think of it offhand.

 

[jangofett287]

to get this thing spinning: put a fly-wheel in the centre of the station. Using an electric motor spin the wheel up so it has the same amount of rotational energy as your station will need. Then apply the breaks to your flywheel. the rotational energy has to go somewhere, so your station starts to turn. This idea may require some modifications, but it works in my head.

 

[Izack]

to get this thing spinning: put a fly-wheel in the centre of the station. Using an electric motor spin the wheel up so it has the same amount of rotational energy as your station will need. Then apply the breaks to your flywheel. the rotational energy has to go somewhere, so your station starts to turn. This idea may require some modifications, but it works in my head.

The rotational energy also has to come from somewhere, remember. Newton's Third works both ways...

 

[Linguofreak]

I question this as, by definition, the maximum stress that you'd be putting on any component would be 1G. I'm pretty sure that we have materials now that can hold their own weight.

Well, the question is over what length they can support their own weight. IIRC, a ring with mass-per-unit-length M and circumference C, rotating for centrifugal forces of magnitude F needs tensile strength equivalent to what a dangling string with mass-per-unit-length M and length C would need in a gravitational field providing acceleration F.

Where Manichean is confused is that, yes, indeed, a *full scale* Ringworld (with 1 AU radius, ~6 AU circumference) would require a tensile strength similar to that provided by interactions by the strong nuclear force, if it was rotating for 1G, but the OP specified a radius of 243 km here, which would require a lot less tensile strength.

My concern for the OP's design would be atmospheric escape.

 

[jangofett287]

The rotational energy also has to come from somewhere, remember. Newton's Third works both ways...

Like I said, an Electric motor, which you can power using solar panels. This would work perfectly if you could get frictionless axles/ bearings, but since you can't the station will start to spin before you apply the brakes (you could also use a clutch). If you wish for a part of the station to not spin, you will need something that rotates in the opposite direction to cancel the rotation of the part you don't want to spin. one way is to have 2 counter-rotating centrifuges, with a non-rotating part in the middle.

 

[fsci123]

Well this ring is just a ring it orbits in a lagrangian point and has a latice structure to support it... It has a hydrogen atmosphere and its been sitting in the same position for who know how long... It is not designed for humans and the g force is not earth like...

Just as a hint...

 

[Hlynkacg]

A hydrogen atmosphere would leak away/disperse without somthing to replenish it or a strong gravitational force to hold it in place.

 

[tblaxland]

Like I said, an Electric motor, which you can power using solar panels. This would work perfectly if you could get frictionless axles/ bearings, but since you can't the station will start to spin before you apply the brakes (you could also use a clutch).

So you connect the rotor of your electric motor to the flywheel. What do you connect to the stator to? If you say nothing, then you will end up with a rapidly rotating stator (contradiction in terms :shrug. The only other foreseeable answer is the station, but what that means is that you never apply the brakes on the flywheel (unlike your suggestion) otherwise both the station and the flywheel will come to a stop again (conservation of momentum and all...).

 

[fsci123]

So you connect the rotor of your electric motor to the flywheel. What do you connect to the stator to? If you say nothing, then you will end up with a rapidly rotating stator (contradiction in terms :shrug. The only other foreseeable answer is the station, but what that means is that you never apply the brakes on the flywheel (unlike your suggestion) otherwise both the station and the flywheel will come to a stop again (conservation of momentum and all...).

Well my understanding is that youl still use the same amount energy with or without the flywheel...

 

[jangofett287]

So you connect the rotor of your electric motor to the flywheel. What do you connect to the stator to? If you say nothing, then you will end up with a rapidly rotating stator (contradiction in terms :shrug. The only other foreseeable answer is the station, but what that means is that you never apply the brakes on the flywheel (unlike your suggestion) otherwise both the station and the flywheel will come to a stop again (conservation of momentum and all...).

Umm... scratch that idea then. :lol:
Wait, I've got it:
You need to have a stationary core. You clutch the fly-wheel to a motor in the stationary part, spin it up, declutch from the stationary part, then clutch to the part you want to move. If you aren't going to have 2 counter rotating wheels, you will need to bring some extra RCS fuel to cancel the rotational energy the centrifuge will impart to the core section.

Does all that make sense?

 

[T.Neo]

My concern for the OP's design would be atmospheric escape.

That should not be a problem if the atmosphere is contained...

Though I am not sure why anyone would give it a hydrogen atmosphere, as it is doubtful for a terrestrial planet to have a thick, hydrogen atmosphere...

 

[fsci123]

That should not be a problem if the atmosphere is contained...

Though I am not sure why anyone would give it a hydrogen atmosphere, as it is doubtful for a terrestrial planet to have a thick, hydrogen atmosphere...

Well it us not known whether it originaly had a hydrogen atmosphere the builders of this dosnt leave any computers robots or notes... Nor did the alien explorers that came before the US scientist...

 

[T.Neo]

US scientist? Aren't there other types of scientist?

There's no reason to put a hydrogen atmosphere in a space habitat, you want to put in an atmosphere that your species can survive in- an atmosphere that mimics that of your homeworld.

However, I strongly doubt that there would be anything living out there that had a hydrogen-covered homeworld.