Space Forum / Space Flight / December 2003

I have just improved design of the bolo and the sling.
Here is complete, revised text:

This system of Earth-to-orbit transportation is called
"lunavator bolo exchange." It is trivial when compared
to rocket launchers, and it may reduce the cost of space
access to a few dollars per kilogram! The system is based
on GPS, a reusable sounding rocket, terrestrial bolo,
lunar rotovator, cargo sacks, and a small Zylon sling.

The lunar rotovator is called lunavator. It hurls sacks
filled with regolith (Moon dust) toward the Earth. The
lunavator is mounted on a rotating arm which is attached to
a large, rotating, toroidal greenhouse. The arm rotates
independently of the greenhouse, so it can easily change
the angular velocity of the lunavator. The maximum length
of the lunavator is about 200 km. When a winch reels its
cargo in, the cargo moves faster to conserve its angular
momentum. This fact makes it possible to increase the
orbital energy of the lunavator and the greenhouse without
the need for any external thrust. It is as simple as
picking the cargo from the Moon, reeling it in, and tossing
it backward. The orbital velocity of the lunavator is only
1.6 km/s, much less than the Moon's escape velocity (2.4
km/s). When the cargo is released from the lunavator, its
velocity relative to the Moon is 3.2 km/s. It is gradually
slowed down by the lunar gravity to 0.8 km/s (3.2 km/s -
2.4 km/s = 0.8 km/s). Gravitational pull of the Earth
accelerates the cargo by 11.2 km/s, which is the Earth's
escape velocity. An ion thruster guides the cargo toward
the terrestrial bolo. When the cargo is captured by the
terrestrial bolo, its velocity relative to the bolo is 4.3
km/s. (The cargo gains 3.5 km/s, which is the difference
between the Earth's escape velocity, and the orbital
velocity of the bolo, which is 7.7 km/s). The bolo is
larger than the lunavator, but it has the same design, and
is mounted on a rotating arm, which is attached to a large,
rotating, toroidal greenhouse. The bolo captures the cargo
and reels it in. Human crew divides the cargo, which is
called lunar cargo into 4 identical cargoes, which are
called terrestrial cargoes. The terrestrial cargoes and a
sling are secured to the bolo. The sling is placed at the
outer tip of the bolo, while the terrestrial cargoes are
evenly spaced along the bolo. When the bolo is accelerated
to its maximum angular velocity, the sling and the
terrestrial cargoes are released almost simultaneously. The
bolo reverses velocity of the cargoes and thus gains
orbital energy. The sling is released first. Its velocity
relative to the Earth is only 2 km/s. Before the sling is
released, small rocket engines permanently attached to both
ends of the the sling make it spin about the outer tip of
the bolo. At the same time a sounding rocket lifts a
payload to the altitude of 100 km. (My favorite propellant
for the sounding rocket is the hydrogen peroxide
monopropellant because it is safe and clean.) The payload
and the sling have the same mass; their total mass is the
same as the mass of the lunar cargo. The sling captures the
payload, which at this moment is stationary relative to the
Earth. The sling is made of Zylon and is strong enough to
reverse relative velocity of the terrestrial cargoes and
the payload. If we treat the momentum exchange as perfectly
elastic collision, the principle of conservation of linear
momentum implies that when the payload and the sling unite,
their velocity relative to the Earth is 1 km/s. The small
rocket engines permanently attached to the sling control
its angular momentum and guide it toward the terrestrial
cargoes. When the outermost terrestrial cargo is released
from the bolo, its velocity relative to the Earth is 3
km/s. After the exchange of momentum with the payload, its
velocity is reversed and reduced to only 0.2 km/s. At the
same time the payload velocity relative to the Earth is
increased from 1 km/s to 1.8 km/s. After momentum exchanges
with the four terrestrial cargoes the payload has the same
velocity (3.4 km/s relative to the Earth) as the bolo tip,
and it is captured by the bolo. If the payload is going to
be used in the greenhouse orbiting the Earth, the bolo's
winch reels it in. If it is going to be used in the
greenhouse orbiting the Moon, The bolo reverses payload's
velocity and hurls it toward the lunavator, which captures
it. Modern GPS technology guarantees high precision of all
maneuvers.

The lunavator and the bolo do not have to be made of
unobtanium, buckytubes, or even Zylon. Carbon fibers and
S-glass fibers are strong enough, and they are immune to
the radiation and temperature extremes of the outer space.
Perhaps the most practical material for the lunavator and
the bolo is a rope made of S-glass fibers fused together
under high tension and high temperature.

=========================================================

PS. This is not just another newsgroup post, but history
in the making. I am going to post drawings and updates at:
http://www.islandone.org/LEOBiblio/SPBI132.HTM#lunavator_bolo_exchange

We also plan that most of the traffic to the GEO hotel is round
trip tourists.  If the LEO-tether and GEO-tether are picking up
10 tourists at the same time they are sending 10 tourists home,
then there is no need for thrust (or regolith).  

Only once per day do we do one-way traffic.  We plan to open the
hotel when only some of the rooms are finished and just let
people stay for a shorter time.  For example, if 1/7th of the rooms
are finished and the designed hotel capacity is for 1 week stays
with the tether transport capacity, then the average stay can only
be 1 day.  

And that 90 minutes should really be 100 (I often forget this).
The problem is that while the orbit is 90 minutes, the launch
site on Earth and the GEO-hotel you are tossing to rotate during
this time.  So for things to line up again it takes an extra
10 minutes, for a total of about 100.

 -- Vince

VC> The same device gets a different name depending on
VC> where it is?

You did not read the web page which explains the difference:
http://www.islandone.org/LEOBiblio/SPBI122.HTM

The name depends on whether the device picks up
its cargo from the surface of a planet or a moon. If it
can do that, it is called the rotovator. If it cannot,
it is called the bolo. I have seen the rotovator term in
several publications.

AN> The rotovator hurls the sacks filled with regolith
AN> (Moon dust) towards the Earth.

AN> In order to do this more than once, it needs to get
AN> momentum back from someplace.

No, it does not.

VC> The next problem is that your sack will need some
VC> guidance and thruster control to get exactly where
VC> it needs to go to. You can not fling something 1/4
VC> million miles and have it get within a few meters
VC> of the spot you were aiming for at exactly the right
VC> time.

You did not read the web page about GPS accuracy:
http://gipsy.jpl.nasa.gov/igdg/system/index.html#accuracy

VC> With the mass of the tether and payload on a 200 km
VC> lever arm you would need a *really* huge greenhouse
VC> to store up an equal amount of angular momentum.

VC> For a similar idea I like 2 hotels in GEO connected
VC> by a 20 km tether and rotating fast enough to get
VC> 1/6th G.  This is 131 m/sec tip speed which is so
VC> low that the tether can be like 0.8% of combined
VC> mass of the 2 hotels.  Of course my hotels would have
VC> greenhouses, but they are mostly hotels.

I agree. The first, small system of this kind can be any
large spinning object. Note that the system can "bootstrap"
itself, which means that a small, one ton system can lift
into space a much more massive system.

AN> When a winch reels its cargo in, the cargo moves
AN> faster to conserve its angular momentum. This fact
AN> makes it possible to increase the orbital energy
AN> of the rotovator and the greenhouse without the need
AN> for any external thrust.

VC> You conserve both your angular momentum (around own
VC> center of mass) and your tether systems momentum
VC> around the moon.  Since you picked up something that
VC> was not moving, your overall orbital speed is slower
VC> (more mass and less speed for same momentum). So the
VC> opposite side of the orbit from where you pick up
VC> gets lower. Depending on how high it was to start,
VC> you can only do this a few times before you hit the
VC> moon.  Why do you think you don't need thrust?

The lunavator picks up cargo from the Moon, reels it in
and hurls it backward before it gets to the other side
of the Moon. Its orbital energy is increased, but its
angular momentum about its own center of mass is
decreased. This is why the arm is needed to adjust the
angular momentum of the lunavator. When you run out of
the angular momentum, you have to reverse the angular
velocity of the lunavator before you release its cargo.
(If you reverse the angular velocity of the lunavator,
you cannot pick up anything from the Moon unless you
reverse it again.)

AN> When the cargo is captured by the terrestrial bolo,
AN> its velocity relative to the bolo is 4.3 km/s.

VC> For spectra-2000 you would need a tether like 600
VC> times as heavy as your payload to handle a 4.3 km/s
VC> tip speed.

You did not read the web page which explains it:
http://www.islandone.org/LEOBiblio/SPBI1SL.HTM

The characteristic velocity of Spectra 2000 is 2690 m/s,
so the minimum tether mass is about 40 times greater
than the cargo mass.

AN> It is easy to design a reusable sounding rocket which
AN> lifts the payload to the altitude of 100 km and accelerates
AN> it to the velocity of 2.5 km/s. (When the payload separates
AN> from the rocket, its total energy is equivalent to the
AN> kinetic energy of only 3 km/s.)

VC> I think it is easy to get a reusable sub-orbital rocket
VC> going much faster than 2.5 km/sec.   I think you are
VC> putting too much work on the tether and not enough on
VC> the rocket.

We could argue about that point for ever. I prefer to use
the tethers because they use almost no propellants. The
only consumables are Xenon gas used by the ion engines
and the cargo sacks.

VC> In order for your LEO tether to pickup something every
VC> 90 minutes, I think it has to be in an Equatorial orbit.
VC> But the moon only crosses the equatorial plane every 2
VC> weeks, so you can not toss to it all the time from an
VC> Equatorial orbit.

No. The bolo orbit must be in the same plane as the Moon
orbit. This means that the sounding rockets are launched
from a different latitude in summer than in winter.

VC> Have you tried things on tether simulator?
VC> http://spacetethers.com/spacetethers.html

Yes, but it is too simple to simulate the system we are
talking about (lunavator bolo exchange). The system
is a complex celestial ballet of 3 rotating tethers. There
is a great need for a simulator that explains the ballet,
and you are the best guy to make such a simulator!

PS. I made another sling improvement: terrestrial cargoes
are attached to both ends of the sling when the sling is
released from the bolo. I also made a drawing of the
lunavator. It is posted at:
http://www.islandone.org/LEOBiblio/SPBI1SL.HTM/SPBI1324.JPG

Andrew Nowicki wrote:
AN> I also made a drawing of the lunavator. It is posted at:
AN> http://www.islandone.org/LEOBiblio/SPBI1SL.HTM/SPBI1324.JPG

Wrong URL. The drawing is posted at:
http://www.islandone.org/LEOBiblio/SPBI1324.JPG
and http://www.medianet.pl/~andrew/SPBI1324.JPG

VC> I think it is easy to get a reusable sub-orbital
VC> rocket going much faster than 2.5 km/sec. I think
VC> you are putting too much work on the tether and
VC> not enough on the rocket.

This system of space transportation is much simpler
when the sounding rocket lifts the payload to the
altitude of 100 km and accelerates it to a horizontal
velocity of 3.4 km/s, which matches the tip velocity
of the bolo. When the bolo hurls the payload forward,
payload velocity relative to the Earth is 12 km/s,
which is more than the Earth escape velocity. An ion
thruster may be used to guide the payload to its final
destination and change its velocity.

There is no sling in the simplified system; only one
lunavator, one terrestrial bolo, and one sounding
rocket.

VC> I think it is easy to get a reusable sub-orbital
VC> rocket going much faster than 2.5 km/sec. I think
VC> you are putting too much work on the tether and
VC> not enough on the rocket.

This system of space transportation is much simpler
when the sounding rocket lifts the payload to the
altitude of 100 km and accelerates it to a horizontal
velocity of 3.4 km/s, which matches the tip velocity
of the bolo. When the bolo hurls the payload forward,
payload velocity relative to the Earth is 12 km/s,
which is more than the Earth escape velocity. A Hall
thruster may be used to guide the payload to its final
destination and change its velocity.

There is no sling in the simplified system; only one
lunavator, one terrestrial bolo, and one sounding
rocket.

AT> What orbital inclination would the Earth orbiting
AT> rotovator be in?

Lunar orbit. Actually, it is a bolo.

AT> In theory it could be orbiting the Earth in the same
AT> plane as the moon. This means that:
AT> 1. The rendezvous latitude would change throughout the
AT> year, shifting between the two tropics. This would
AT> necessitate and air launch of the rendez-vous cargo.

I prefer a ship as the launching platform. There will be at
least 8 launches of the sounding rocket a day. It would be
difficult to launch a full size rocket launcher from a
bobbing ship, but launching the small sounding rocket
seems feasible.

AT> 2. The orbit is not stable. Henry Spencer's view in
AT> an earlier post was that it would be difficult, though
AT> perhaps not impossible, to maintain this orbit for the
AT> rotovator.

I can't find the post; perhaps my news server is at fault.
The only two stable terrestrial orbits are polar and
equatorial orbits. Every other orbit undergoes precession
due to the pear shape (geoid) of the Earth. This is not a
big problem because the annual mass of Moon dust handled
by the bolo is much larger than the mass of the bolo. If
the bolo is tilted slightly so that it throws the sacks
off the lunar plane, the precession problem is solved.

AT> Note also that the rotovator is not good for reaching
AT> low Earth orbits. With something like this in place
AT> High Earth Orbit would rapidly become the place to be.

I had the same knee jerk reaction to this space
transportation system. It is ideal for moving cargo
between the Moon and the Earth. (Does the not-so-smart
president follow our conversation?) The only practical
way to bring the cargo from the Earth to the orbit of
the bolo is to have a small electric cart (manipulator?)
riding on the bolo and pulling the cargo to the center
of bolo's mass. The good news is that direct current
motors using Samarium Cobalt magnets and have high
power-to-weight ratio: about 7 watts per gram not
counting the gearbox. This means that the cart would be
small. Another good news is that the bolo with build
in aluminum wires is excellent round-the-clock source
of electric power due to the electrodynamic tether
effect (interaction with terrestrial magnetic field).
The used up electric power is replenished by the orbital
energy of the lunar sacks filled with Moon dust.

The full description and lunavator spacecraft picture are at:
http://www.islandone.org/LEOBiblio/SPBI132.HTM#lunavator_bolo_relay
mirror site:
http://www.medianet.pl/~andrew/SPBI132.HTM#lunavator_bolo_relay