A flywheel is only a temporary store for momentum/energy,
it is not a source nor a sink.

A flywheel has a maximum allowable spin before it disintegrates due to
tensile stresses.

Hence, all fyhweels need to be desaturated, or the momentum must be
dumped somewhere.
In space, the only way to dump momentum is to find some a method of
applying force,
i.e. some external thrust. In conventional spacecraft today moemtum
wheels are
desaturated by firing thrusters to take out the energy while electrical
"brakes" are
applied to the wheels.

In most cases, since you have to carry propellant anyway to desaturate
the wheels, the
wheels actually do not buy you very much. In practice, the primary
purpose for momentum
wheels on spacecraft is to provide an extremely stable platform for
accurately pointing either
cameras or radio antennae.

To despin an asteroid, a wheel would have little value, we might as well
use thrusters and cut out
the middle man. For such a wheel to be effective for depsinning an
asteroid it would have to be
very large, and would weigh more than the rest of the spacecraft put
together.

=====

Nobody has addressed the amount of propellant which would eb needed for
retrieving an asteroid.
It is very large.

Nobody has mentioned the studies done by Dr. Brian O'Leary in the 1970's
for retrieving asteroids using mass drivers. The mass of the asteroid
itself is used as reaction mass for the mass driver.
IIRC about half of the mass of a typical NEO would need to be ground up
and used as reaction mass to get the remained back to Earth. This
would save a lot by eliminating the need to ship
propellant all the way from Earth.

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>
> A flywheel is only a temporary store for momentum/energy,
> it is not a source nor a sink.
>
> A flywheel has a maximum allowable spin before it disintegrates due
to
> tensile stresses.

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.

> Hence, all fyhweels need to be desaturated, or the momentum must be
> dumped somewhere.
> In space, the only way to dump momentum is to find some a method of
> applying force,
> i.e. some external thrust. In conventional spacecraft today
moemtum
> wheels are
> desaturated by firing thrusters to take out the energy while
electrical
> "brakes" are
> applied to the wheels.

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.

> To despin an asteroid, a wheel would have little value, we might as
well
> use thrusters and cut out
> the middle man. For such a wheel to be effective for depsinning
an
> asteroid it would have to be
> very large, and would weigh more than the rest of the spacecraft put
> together.

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.
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.
You then refill the bucket and do it again. It does have the added
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.
The big question, would a flywheel (or other similar device) weigh
more than an engine and fuel to do the same job?

king_rodent (putting the eek in geek)

>
> 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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> > 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.

The total system will continue to have the same energy, yes... but
permanently jettison the flywheel and you can ignore that part of the
system -- the part with ALL the rotational energy -- from then on.
Right?

>
> > > 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.
>
> The total system will continue to have the same energy, yes... but
> permanently jettison the flywheel and you can ignore that part of the
> system -- the part with ALL the rotational energy -- from then on.
> Right?
>

Ouch !

Throw away the entire spacecraft ? !

Very expensive and inefficient.

If we were to use thrusters instead of wheels the lifetime of the
spacecraft
would be a lot longer, it could be refueled many times. And it could
even be sent
to other asteroids.

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>

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

You should be able to answer that question yourself using freely
available resources on the web.

To save you a bit of time, here is a list of companies who make
spacecraft reaction wheels and momentum wheels:

ADR http://www.adr.fr

Teldix http://www.teldix.com

Allied signal http://www.alliedsignal.com

Honeywell http://www.honeywell.com/space

Ithaco http://www.newspace.com/ithaco

===

As for an equivalent rocket engine, it depends on the type of engine you
have in mind,
in particular you need to consider specific impulse (exhaust velocity).

Good luck.

CR.

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> Nobody has addressed the amount of propellant which would eb needed for
> retrieving an asteroid.
> It is very large.

Not that large. Consider the rocket equation:

deltaV = exhaustVelocity * ln(initialMass/finalMass)

For a probe weighing 500kgs, attached to a rock weighing 1 tonne
the final mass is 1.5 tonnes. Let's assume we use a Hall thruster
with an ISP of 1600 seconds (exhaustVelocity is 9.81 * 1600 =
~16,000km/s).

This means that for a deltaV of 3 km/s [sounds ok, maybe even high
if we slingshot around the moon AND aerobrake at the earth] so the
initial mass before you head for home is:

1.81 tonnes i.e. ~310 kgs of fuel.

That's not all that much. Sure its going to be slow, you'd have to
wait many months, and you'd have to add more fuel for the way out,
but on the way out the probe is going empty so guesstimate for
doubling the fuel load and it probably comes out about right, i.e.
1.4 tonne initial vehicle drags back 1 tonne of rock plus itself,
i.e. you end up with more mass than you started. You could screw on
some new thrusters, refuel it, and send it back out again.

You can play with the numbers some more, bigger rock, more fuel,
more power, more thrusters, but that gives you the basic idea. You
might be able to start in LEO, or atleast MEO- if you do, don't
forget to add the delta-v to get to escape- it's ~4 km/s from LEO,
plus twice whatever the delta-v to get to the asteroid is.

Anyway, that's the basic idea; go away and play ;-)

Ok, here's the data for a Hall thruster:

Propellent: xenon [expensive...]
Thrust: 0.083 N [not Indy racing]
ISP: 1600 secs [c.f. 450s for the space shuttle that does
~9km/s deltaV]
Power: 1350 watts [hmm. quite a big solar panel...]
Mass flow: 5.3mg/s [very small]
Total Impulse: 1000,000 Ns [not bad...]
Mass: 3.5 kgs [I'll have 20!]
size: 15x22x12.5cm

So, from the total impulse and the thrust I calculate the rated
life of the thruster is 139 days.

For 1350 watts you can accelerate 1 tonne by 1km/s in that time.

Doubling up the power and adding many more thrusters would multiply
up the time you can thrust for and double the thrust levels.

Hall thruster are supposed to work with other, cheaper,
propellents, like oxygen, but I don't have performance figures for
that. The cost of xenon is huge, IRC, it comes from liquifying air
so there's no theoretical limit to supply. Hall thrusters can be
run on oxygen with 10% of something else [probably xenon or argon],
that might be another approach for the propellent.

I think this may compare quite favourably with going to the moon.
In LEO you can probably stuff the rock in the back of a Space
Shuttle or something or send it to the ISS for analysis.

Comments welcome.

Ian, this is an excellent contribution to the discussion.

Sorry for my tardy reply, the day job is quite demanding currently.

I will reply at more length later, when I have found out the cost per
kilo of Xenon, and can compare it to the cost per kilo to LEO and the
cost per kilo of platinum group metlas.

Cheers,

CFR

>
> > Nobody has addressed the amount of propellant which would eb needed for
> > retrieving an asteroid.
> > It is very large.
>
> Not that large. Consider the rocket equation:
>
[snip[

FYI, see http://www.chemicool.com/elements/ for a listing of prices for many elements.

Charles Radley
10/04/01 10:37 AM

Ian, this is an excellent contribution to the discussion.

Sorry for my tardy reply, the day job is quite demanding currently.

I will reply at more length later, when I have found out the cost per
kilo of Xenon, and can compare it to the cost per kilo to LEO and the
cost per kilo of platinum group metlas.

Cheers,

CFR

>
> > Nobody has addressed the amount of propellant which would eb needed for
> > retrieving an asteroid.
> > It is very large.
>
> Not that large. Consider the rocket equation:
>
[snip[

>
> For a probe weighing 500kgs, attached to a rock weighing 1 tonne
> the final mass is 1.5 tonnes. Let's assume we use a Hall thruster
> with an ISP of 1600 seconds (exhaustVelocity is 9.81 * 1600 =
> ~16,000km/s).
>
> This means that for a deltaV of 3 km/s [sounds ok, maybe even high
> if we slingshot around the moon AND aerobrake at the earth] so the
> initial mass before you head for home is:
>
> 1.81 tonnes i.e. ~310 kgs of fuel.
>
> That's not all that much. Sure its going to be slow, you'd have to
> wait many months, and you'd have to add more fuel for the way out,
> but on the way out the probe is going empty so guesstimate for
> doubling the fuel load and it probably comes out about right, i.e.
> 1.4 tonne initial vehicle drags back 1 tonne of rock plus itself,

So that means for an outlay 310 kg of Xenon propellant we get 1 tonne of PGM.

According to http://www.chemicool.com/elements/

Xenon costs $100,000/kg

Compared to:

Platinum, $47000/kg - $47M/tonne
Osmium, $77000/kg - $77M/tonne
Iridium, $42000/kg - $42M/tonne
Rhodium, $130000/kg - $130M/tonne

I am surprised to see here that the cost of the Xenon is a large part of the overall spacecraft cost, this is unusual. 310 kg of Xenon costs $31 million ! This is probably at least 50% of the overall spacecraft cost - wow !

Assume $60M overall mission cost...the mission could still make a profit for Osmium and Rhodium, but not for Platinum and Iridium.

> Doubling up the power and adding many more thrusters would multiply
> up the time you can thrust for and double the thrust levels.
>

Since the cost of propellant is so high, doubling up on thrusters/power could be attractive...
the percentage cost increase would be small and the mission performance would be dramatically improved.

> Hall thruster are supposed to work with other, cheaper,
> propellents, like oxygen, but I don't have performance figures for
> that. The cost of xenon is huge, IRC, it comes from liquifying air

Hmm, we definitely need those performance figures.

> so there's no theoretical limit to supply. Hall thrusters can be
> run on oxygen with 10% of something else [probably xenon or argon],
> that might be another approach for the propellent.
>

Yes, that would reduce the cost of propellant by a factor of ten.
So if the performance is halved we get an improvement of specific performance
for propellant cost increase of a factor of five.

But since I estimate propellant (Xenon) cost is about 50% mission cost,
we would wnat our performance to fall be less than 50%. Otherwise, we
might as well stick to the Xenon.

> I think this may compare quite favourably with going to the moon.
> In LEO you can probably stuff the rock in the back of a Space
> Shuttle or something or send it to the ISS for analysis.
>

ISS does not have any mineral analysis capabilities.

The analysis laboratories on Earth are much better equipped than anything we could put into
ISS in the forseeable future.
Best to land the samples on Earth an analysie there.

That is what they did with the Apollo moon rocks.

Hall thrusters can run on LOX or other propellents if they have to.
The electrical efficiency is reduced, but you can probably save
money by adding more solar panels. I think that a solar panel
gives 1 kw per 40 kg, but I don't have a figure for the cost of
solar panel per kg off hand; but I doubt its as high as $100,000
per kg.

LOX isn't very easy to store in space; some refrigeration is
probably required, but with that much power around, I doubt it
would be a show stopper.

Ian Woollard wrote,

"LOX isn't very easy to store in space; some refrigeration is probably
required,"

Is that true? What is the background temperature in the shade, and what
is the temperature at which LOX boils?

Ron

Ian Woollard ian.woollard@...
10/21/01 06:27 PM

Hall thrusters can run on LOX or other propellents if they have to.
The electrical efficiency is reduced, but you can probably save
money by adding more solar panels. I think that a solar panel
gives 1 kw per 40 kg, but I don't have a figure for the cost of
solar panel per kg off hand; but I doubt its as high as $100,000
per kg.

LOX isn't very easy to store in space; some refrigeration is
probably required, but with that much power around, I doubt it
would be a show stopper.

> In a message dated Mon, 1 Oct 2001 7:12:17 PM Eastern Daylight Time,
Ian Woollard ian.woollard@... writes:
>
>>For a probe weighing 500kgs, attached to a rock weighing 1 tonne
>>the final mass is 1.5 tonnes. Let's assume we use a Hall thruster
>>with an ISP of 1600 seconds (exhaustVelocity is 9.81 * 1600 =
>>~16,000km/s).
>>
>>This means that for a deltaV of 3 km/s [sounds ok, maybe even high
>>if we slingshot around the moon AND aerobrake at the earth] so the
>>initial mass before you head for home is:
>>
>>1.81 tonnes i.e. ~310 kgs of fuel.
>>
>>That's not all that much. Sure its going to be slow, you'd have to
>>wait many months, and you'd have to add more fuel for the way out,
>>but on the way out the probe is going empty so guesstimate for
>>doubling the fuel load and it probably comes out about right, i.e.
>>1.4 tonne initial vehicle drags back 1 tonne of rock plus itself,
>>
> So that means for an outlay 310 kg of Xenon propellant we get 1 tonne
of PGM.
>
> According to http://www.chemicool.com/elements/
>
> Xenon costs $100,000/kg
>
> Compared to:
>
> Platinum, $47000/kg - $47M/tonne
> Osmium, $77000/kg - $77M/tonne
> Iridium, $42000/kg - $42M/tonne
> Rhodium, $130000/kg - $130M/tonne
>
> I am surprised to see here that the cost of the Xenon is a large part of
the overall spacecraft cost, this is unusual. 310 kg of Xenon costs $31
million ! This is probably at least 50% of the overall spacecraft cost
- wow !
>
> Assume $60M overall mission cost...the mission could still make a profit
for Osmium and Rhodium, but not for Platinum and Iridium.
>
>>Doubling up the power and adding many more thrusters would multiply
>>up the time you can thrust for and double the thrust levels.
>>
> Since the cost of propellant is so high, doubling up on thrusters/power
could be attractive...
> the percentage cost increase would be small and the mission performance
would be dramatically improved.
>
>>Hall thruster are supposed to work with other, cheaper,
>>propellents, like oxygen, but I don't have performance figures for
>>that. The cost of xenon is huge, IRC, it comes from liquifying air
>>
> Hmm, we definitely need those performance figures.
>
>>so there's no theoretical limit to supply. Hall thrusters can be
>>run on oxygen with 10% of something else [probably xenon or argon],
>>that might be another approach for the propellent.
>>
> Yes, that would reduce the cost of propellant by a factor of ten.
> So if the performance is halved we get an improvement of specific
performance
> for propellant cost increase of a factor of five.
>
> But since I estimate propellant (Xenon) cost is about 50% mission cost,
> we would wnat our performance to fall be less than 50%. Otherwise, we
> might as well stick to the Xenon.
>
>>I think this may compare quite favourably with going to the moon.
>>In LEO you can probably stuff the rock in the back of a Space
>>Shuttle or something or send it to the ISS for analysis.
>>
> ISS does not have any mineral analysis capabilities.
>
> The analysis laboratories on Earth are much better equipped than
anything we could put into

> LOX isn't very easy to store in space; some refrigeration is
> probably required, but with that much power around, I doubt it
> would be a show stopper.
>

It should be even easier to store Oxygen.

It can be stored as either water (H2O) or hydrogen peroxide (H2O2) without refirgeration.

Water can be electrolyzed as needed, this would need somewhat larger solar panels.

Peroxide can be catalytically decomposed so would not need extra solar panels. It also has a higher Ox mass fraction, so has some advantages over water. Of course H2O2 decomposition leaves water as a by-product, this could be electrolyzed. If we do this, we get twice as much oxygen per watt by storing it as H2O2 rather than as H2O.

Another benefit, H2O2 when decomposed actually creates energy, so it could possibly be used as a power source,
driving a turbine for power, then collecting the exhaust to use as propellant.

All that is bit complex, but probably less complex and lighter than rerigeration.

Cheers,

Charles R.

Well, the H2 is really a waste product in this system, and could be vented into space as exhaust. There is
not really enough of it justify building in a
system to reclaim it, at least on the way there.

During the cruise TO the asteroid, relatively little propellant will be used, so the penalty for venting the hydrogen will be quite minimal.

But during the phase of actually towing the asteroid, the economics might be different.

Some NEO asteroids contain quite a lot of water, so rather than bring water or oxygen from Earth, we
could obtain it locally and electrolyze that to get oxygen propellant.

The higher SI of the Hall thruster means we would
consume much less of the asteroid as propellant than
with a mass driver.

The hydrogen produced could perhaps be stored as metal
hydride. I wonder if there might be oxides present on the NEO which could be reduced using the hydrogen.

CFR