The Lunar Elevator System

--- In spacesettlers@y..., rmenich@m... wrote:
> Ian Woollard wrote,
>
> "A fiber with a tensile of 73 Gpa exists; the required strength
> is 72.5 Gpa. However, a) we need a safety factor b) we need to
> be able to splice the fibers together, and in practice that
> loses 15-30% of the strength."
>
> What is the required strength for a space elevator from the Lunar
surface
> to lunar orbit?
> Do currently-existing materials suffice to build such a lunar
elevator?
> Would a lunar elevator be a simpler technological solution to the
problem
> of moving large amounts of material off the moon than the O'Neill-
heritage
> mass driver/mass catcher system?

I've seen quite a bit of research on lunar elevators, often referred
to as lunavators. Every researcher seems to hold the opinion that
such a device is quite feasable with current material technologies,
in fact, steel, titantium and aluminum will work just fine.

What they're referring to is more commonly known as a rotating
elevator. A true lunar elevator would have to extend to greater than
300,000 km from the lunar surface to have the center of mass in a
lunar synchronus orbit. Such a long fixed length is not technically
feasable because the distortions that would be caused by the earth's
gravity would be immense.

Now, the lunar "rotovator" would be in either a lunar polar orbit,
which would destabilize in a few months (but there is a way to
stabilize the orbit by adjusting the center of mass of the rotovator
in its travels through its orbit) or it would be in a lunar
equatorial orbit, which is much more stable, but will only touch
down in 1 - 7 equatorial locations.

To further address your questions, I don't believe that a rotovator
would replace a mass driver/catcher system because the the system is
limited to how much material it can lift off of the surface of the
moon. The limitations are related to the mass ratio of the payload
to the rotovator, the experts state that the rotatvator must be at
least 16.7 times more massive than the payload.

To continue our analysis, let's compare the benefits of a polar
orbit to that of an equatorial orbit. A polar orbit will take the
rotovator over the entire lunar surface, but not consistently to the
same "launch pad" until the rotovator has made one lunar rotation,
whereas the equatorial orbit will take it over the same launch pads,
time and time again.

So, if you have a base at the lunar poles or if you want to explore
the entire surface of the moon, then the polar orbit makes the most
sense. If you have a mining base and want frequent launches, then an
equatorial orbit makes the most sense. In fact you could have up to
seven bases.

As you increase the number of touchdown points you'll decrease the
time you have to make your cargo attachments/detachments. After
looking at a number of factors, I've come to favor 5 touchdowns per
orbit. This works out to one orbit every 132 minutes. So, in 24
hours we could have 10.9 cargo uplifts. Let's say we were very
agressive in constructing the rotovator and it was capable of
lifting 100 tons per uplift. This works out to 2,181,818 lbs. To
lift the same material to orbit via a mass driver would require
firing only 25.25 lbs every second for the length of a day. That's
only 14.5% of 1 cubic foot of lunar regolith per shot.

Now the drawback with the rotovator is that to keep it properly
balanced it requires an equal amount of mass coming down to the
lunar surface to match the material we send up from the surface,
otherwise rocket fuel must be used to boost the rotovator back to
proper orbit. An ion thruster wouldn't work because it would have to
restore the orbit within the 132 minute period of one rotation in
order to maximize our use of the rotovator to its fullest potential.

So, I don't believe that the rotovator is a replacement for the mass
driver/catcher system, BUT

I think it will be crucial to lunar development.

The role I see for it is as a rocket replacement.

Here's how I think the lunar development process will work. First a
rotating skyhook in low earth orbit is built with the ability to
launch 5,500 lbs, and a mass/payload ratio of 62:1. The skyhook is
placed in orbit at 484 km. This will give it a rotation period of 94
minutes, 17 seconds. This is 5/7 of the lunar rotovator's 132 minute
orbit.

Concurrently, a Mid Earth Orbit Rotating Transfer Station is built
in LEO and when completed is transported to an orbit at 18,732 km,
giving it a period of 660 minutes per orbit. Thus it completes one
orbit per 5 lunar orbits. This station requires a mass/payload ratio
of 275:1.

Lastly, the rotovator is also constructed in LEO and then
transported to lunar equatorial orbit.

Now with the LEO skyhook, we would have two launch windows every
year because that is when the lunar plane would be in line with the
plane of the epliptic (?). But with the MEO Transfer station we can
now change the plane to increase our launch windows.

If we are very agressive with our launch capacity, we could launch a
payload to the moon once every 660 minutes (once per 7 orbits of LEO
skyhook will line up with the MEO transfer station, which will line
up with the lunar rotovator every 5 orbits.)

Now what would we launch to the moon so frequently? I propose we
launch the bootstrapping equipment and supplies that would be
required for the first tele-operated base building operation. But
first there would have had to have been a few rocket landings
delivering some basic equipment to gather lunar regolith, weigh it
and to sinter it into blocks. This is necessary because we will be
sending this mass to our 3 skyhooks for ballast. First off will be
to send the mass back to the lunar rotovator to restore its orbit
and then to increase its payload capacity, then more mass is sent to
the MEO Transfer station, and lastly to the LEO skyhook.

I've done extensive modeling on this system, using very agressive
launch schedules. To summarize my results it would take 25 LEO
launches to increase our 3 tether "Space Transportation System" :)
payload from 5,500 lbs to 5,766 lbs. This will take 11 days of
launches to accomplish. Now we can launch 5,766 lbs of payload from
LEO. Now it'll take 15 LEO launches to bump up to 6,045 lbs, then
more launches to get to 6,338 lbs, then 6,644 lbs and so on.

After 100 days of launching material from LEO to the lunar base, our
payload launch capacity is 10,658 lbs. After 200 days, the payloads
can now be 20,652 lbs. After 300 days, payloads can be 41,956 lbs.
After 400 days, payloads can be 81,301 lbs. After 500 days, the
payloads can be 165,167 lbs. It takes 533 days to break the 100 ton
mark.

Of course we can designate the payload capacity as much less than
100 tons, but I wanted to see how long it would take to bootstrap
the tether system up to that capacity.

In building up the system by launching needed materials to the lunar
surface and sending back lunar mass to build up our payload
capacity, we've built a space transportation system which degrades
its orbit less and less after every launch of the same mass or
allows greater mass payloads to be launched. We've also sent
millions of pounds of base material to the moon.

Now that the first part of the lunar rotovator's task has been
completed, namely bringing to the surface the materials needed to
build a base, its mission is modified to lifting the structural
materials from the lunar surface to L1, that will be used to build a
SPS for the operation of the mass driver at the lunar base.

The SPS will require many millions of pounds of material to be
completed and now we have a lift capability of 100 tons every 132
minutes.

Of course, now we'll have to correct the orbit of the lunar
rotovator with propellant because the payloads from LEO will
probably have diminished and will not match what is being launched
from the lunar surface to L1.

You may be interested to know that the mass of the LEO skyhook, with
a payload capacity of 100 tons, would be 6,217 tons, the MEO
Transfer station would mass at 27,520 tons, and the lunar rotovator
would mass 2,069 tons. The majority of that mass is lunar regolith
ballast that was sent from the lunar surface to match material sent
to the moon from LEO. Keep in mind that this isn't a direct 1:1
ratio because the Lunar Rotovator requires some additional ballast
to correct its orbit when it sends mass down to the MEO and LEO
tethers. This results in the rotovator reaching the 100 ton capacity
at day 116. In fact by day 225, its payload capacity is at 300 tons,
so by sending mass to the lunar rotovator to restore its orbit for
the additional mass it's sent to the MEO and LEO tethers (mass
beyond that required to restore orbits for mass sent to the moon)
the rotovator more quickly reaches payload capacity than its two
sister tethers.

The last missions for the tether system would be to continue as a
supply launch system and to return lunar processed material to LEO.

Just think that this whole system could be created with very few
rocket flights to the moon and can be kept functioning by matching
mass going to the moon with mass coming from it.

Remarkable!