Flywheels / NEO Retrieval

>
> I know that there is an upper limit of spin. The idea being that
> the flywheel used would be able to handle the ammount of momentum
> added to it or a number of flywheels would be flown. I thought this
> would be assumed.
>

Assumptions must be explicitly stated.
I do not know where the gaps in your knowledge are,
so I cannot assume I know what all your assumptions are.

Actually, in real spacecraft, the limit is not the tensile strength
of the wheel, before that there is a limit on the life of bearings which
decreases rapidly with speed. Real spacecraft have to operate for
several years, so the wheels are run many times slower than the tensile
limit in order to reduce wear on the bearings.

Magnetic bearings are making this easier.

>
> You must have overlooked the section of my post where I describe
> the flywheel module would detach from the probe. Literally dumping
> the momentum by detaching from it.
>

No, that does not dump momentum.

The sapcecraft and asteroid prior to separation will each have
precisly the same rotation and energy following separation.

Assuming they were attached via a common axis of rotation.

After the spacecraft has separated the momentum wheel can only
be desaturated by applying an external force.

Rockets thrusters are the standard way of doing this.

>
> A follow up idea, you could use a tethered weight instead of a
> flywheel. You would be limited by the tensile strenght of the tether
> but this may be of less weight than a flywheel.

Yes.

"Yo weights" deployed on tethers are a common way to despin upper stages
today.

There are some interesting extensions of this idea which I have been
musing over lately.

> A third idea, the tethered weight could be send empty, basically
> just a bucket on a string. The bucket is filled with regolith or
> other material from the asteroid. Then spin the tether. Once you
> reach the tensile limit of the tether, the bottom of the bucket
> opens, allowing the momentum transferred to the rocks to fly away.

Yes, if you time the release at the correct part of the spin cycle it
will
impart a net delta-vee to the asteroid, so will have the dual benefit od
both despining it and moving it.

> You then refill the bucket and do it again. It does have the added

Refilling the bucket could be accomplished by having a tube alongside
the
tether which is filled with crushed material and continuously feeds
the bucket.

> complexity of needing to scoop up material off of the surface, but
> you would have to do the same for a mass driver. I would venture to
> guess that this device would be much simpler and weigh much less than
> a mass driver.

It wold certainly have a lower capital startup cost.
On the other hand, the mass driver probably has a higher specific
impulse
than the tether/bucket system, so would use up less of the asteroid
material.

It is a trade off.

> The big question, would a flywheel (or other similar device) weigh
> more than an engine and fuel to do the same job?
>

Yes, by definition.

I already explained that the flywheel is completely redundant if
you have thrusters and propellant available.

With or without the flywheel, the propellant used is exactly the same.

So the flywheel is just dead weight.

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