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Engineering Halo: The Physics Of Building A Ringworld
A Thesis submitted for Albion College Honors
Kent Bornemeier
April 1, 2010
Albion College
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Table Of Contents:
I. Introduction and History of Ringworlds and Halo
II. Mechanics
a. Mass
b. Orbit
c. Rotation
d. Materials
III. Support
a. Atmosphere Retention
b. Power Generation
c. Station-Keeping (RCS)
d. Visiting Halo
IV. Conclusion
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Introduction and History of Ringworlds and Halo:
In 2001, Microsoft published a computer game called Halo: Combat
Evolved. The premise: a war between humans and aliens in the far future taking place on
the megastructure known as Halo, a ring the size of a planet orbiting a gas giant.1 Halo, and other planetary or cosmic scale structures like Niven rings2, are usually given the title
“megastructures,” referring to constructs greater than a million meters (a megameter) in
size. At ten million meters in diameter, Halo easily qualifies.
The idea of a ringworld is not new; some of the earliest space station concepts by
pioneers at the start of the 20th century, like Wernher von Braun, proposed toroidal space
stations that would rotate to generate centripetal force, which the astronauts aboard would
feel as simulated gravity.3 The famous movie 2001: A Space Odyssey contained a space
station built in exactly that fashion: two rotating rings mounted on an axle that simulated
gravity for the occupants.
In 1970, Larry Niven wrote his famous novel Ringworld. The structure it
describes is a ring nearly a million miles wide with a radius comparable to that of the
Earth’s orbit, called an Astronomical Unit. This ringworld was a cosmic-sized
megastructure that was built in orbit around a star. Constructs like this provide their
inhabitants with earth-like conditions of gravitation and atmosphere, as well as other
terran features like mountains, oceans, and continents, but on a massive scale; Niven’s
ringworld had a habitable surface area nearly three million Earths in size 2.
Historically, architects have been proposing megastructures of sorts for centuries,
with varied purposes. As the single largest artificial construct on Earth, the Great Wall of
China is considered to be the earliest example of a megastructure (other structures like Bornemeier 4
the Pyramids, being less than a megameter in size, do not technically count as
megastructures). In the 1960’s, a fresh wave of utopist megastructure designs surfaced,
including British architect Ron Herron’s ‘Walking City’4. These early designs have never
been built, remaining science fiction until such time as there is sufficient capital and
technology to build them. The ringworlds in the videogame Halo are in fact planetary-
scale structures similar to the orbitals used in the Culture novels by Iain M. Banks. 5
Two major reasons to build a ringworld are given by the Halo games and by Larry
Niven. In Niven’s novels, the Ringworld is created with the intention of increasing the available land area for the builder civilization to use. Without superluminal technology, the Ring Engineers found their civilization growing beyond the capacity of the planets of their solar system, and so they built the Ringworld by using all the matter in their star system.2 In Halo, the planetary-scale ringworlds are called Fortress Worlds. They are
used as massive laboratories and ultimately as weapons of mass destruction (via a
radiation pulse generator that can create a lethal dose of radiation even at a range of
25,000 light years) for combating a parasitic alien life-form known as the Flood.1 Other possible uses range from something as simple as power generation (which will be discussed later) to massive construction and launching facilities for spacecraft.
Although still well beyond the technical capabilities of modern civilization to construct, the Halo megastructures, through the games they’ve given their name to, inspire awe and incredulity at the very notion of such a large structure. The scientific knowledge to design one exists, and can be examined. This paper will explore the feasibility of a Halo-like megastructure using known physics and technology, assuming that the industrial capacity necessary to support such an endeavor existed. Bornemeier 5
Mechanics:
The greatest challenge of building a ringworld lies in its physical properties:
mass, volume, orbit, and composition to name a few. Structures the size of planets or
larger would require massive expenditures of time and energy to fabricate, place, and stabilize in their orbits. In the case of a Halo, they would also need to be constructed in space, and then set rotating once finished – a process that would take even more energy.
First we will explore the mass requirements of Halo, both how much material would be needed to construct it, as well as the energy requirements of lifting such material into orbit. Next will come a discussion of the orbital mechanics of Halo, then issues pertaining to its rotation and artificial gravity, and lastly, an analysis of the materials necessary for Halo’s construction.
Mass:
Constructing a ringworld is a sizable undertaking. Engineering, even on a
planetary scale as in the Halo series, requires mass and industrial capacity beyond
anything humanity has at its disposal. A simple physics analysis bears this out.
A Halo megastructure, as seen in the game, is an annulus with an outer radius of
5,000 km, a width of 320 km, and a thickness of 22.3 km (see Figure 1), giving a total
volume of 224 million cubic kilometers6. This entire space is devoted to all aspects of
the station’s infrastructure: landmass (as seen on the inner surface of the ring), power
generation, station keeping, the weapons system and laboratories that feature so Bornemeier 6
prominently in the games, and of course the physical superstructure that holds it all
together.
Fig. 1: Scale image of Halo showing relative radius, thickness, and width. 1 pixel = 22.3 km
Halo is a massive space station designed to be useful and habitable beyond the
ring’s inner surface. Several of the multiplayer maps lie deep within the structure, while many maps in both the multiplayer and campaign modes portray “bottomless” pits extending outward radially through the ring’s structure. These massive open areas are only part of a network of tunnels, corridors, and rooms that fill the structure, reducing its
overall density.1 Bornemeier 7
“The Covenant did a thorough seismic scan. My analysis shows that Halo is
honeycombed with deep tunnels, which circle the whole ring.” –A.I. Cortana in Halo:
Combat Evolved
As seen in the games, the Halo megastructures, though riddled with open space, contain many monolithic structures with thick walls, floors, and solid spaces. On the inner surface of the ring are also mountains and bodies of water. A conservative estimate
of the physical space devoted to open volumes of air (habitable volume) would be half of
the ring’s total size, though it could be larger or smaller depending on the needs of the
builders.
Assuming that only half of the annulus’ volume is made of solid structural
material (say, structural steel), and the rest is sea-level pressure air, the density of the ring
would be approximately 3900 kg/m3, nearly four times the density of water7. At 3900 kg/m3 and 223 million cubic kilometers volume, the Halo would weigh in at 8.73*1020 kg, just a little over 1% of the mass of Earth’s moon8. Given the assumptions made
about the ring’s composition, of the total mass, 99.98% would be steel.
Another possible construction material that would be much lighter is carbon.
Carbon fiber is already used in composite materials for a variety of applications, and
carbon nanotubes are predicted to be used for many different applications ranging from
electronics to high-stress tensile structures like space elevators or cables. If carbon
nanotubes (which will be discussed later) were used to build Halo instead of steel, the
mass would be much less, 1.57*1020 kg, five and a half times less than a steel Halo. Bornemeier 8
To build Halo in or near Earth orbit, all the material needed would have to be lifted from the surface. For Halo, normal low-Earth orbit wouldn’t be high enough to begin construction. With a radius nearly as large as the Earth’s, a halo ringworld would need to be built in medium Earth orbit, at least 5,000 km above the surface (medium orbit is the range from 2,000 km to just inside geosynchronous orbit)9 to guard against
accidents by making sure that, regardless of orientation, the entire ring is always far from
a drag-inducing atmosphere or a planetary surface. For a unit mass, the energy required
to lift it from the surface of the Earth into a stable orbit is proportional to the radius of
that orbit. Calculated out, to place a unit mass into a circular orbit around the Earth at a
distance of 3 Earth radii (2 Earth radii above the surface of the planet) takes nearly
52*106 joules of energy per kilogram. Multiplied times the mass of Halo, this yields a
total energy of 4.53*1028 joules for a steel ring, and 8.14*1027 J for the carbon ring,
figures roughly equivalent to 450 million and 81 million years worth of the U.S.’s total
energy consumption in 200610, or just under two minutes worth solar energy production.11
Mining the asteroids is a possibility for gathering the raw materials to build Halo,
but moving those materials to planetary orbit is even more costly than lifting them from
Earth’s surface. To move mass from the asteroid belt to a stable orbit around the sun at
the same distance as the Earth (moving mass from the asteroid belt to Earth and stopping
it there) requires 2.22*108 J/kg be lost by the asteroid. This energy would have to be
supplied by the mining and/or transport vehicle to move the mass into Earth orbit, but is
over 4 times the energy required to simply move the mass up from Earth’s surface. The
moon could also be mined for the material rather than Earth. Even at the same orbital Bornemeier 9 distance from the surface as the proposed Halo in Earth orbit (2 Earth radii), the energy required to lift a unit mass into orbit is only 2.65*106 J/kg, almost 20 times less.
Orbit:
As seen in the game, Halo is built somewhere between a large gas giant planet, called Threshold, and its moon Basis (see Figure 2).1 In Halo 2, a Halo is constructed merely in orbit of a gas giant planet, without the explicit presence of a moon.12 This brings up the important issue of orbits and the Halo structures.
Solar-scale ringworlds are known to be highly unstable structures due to their size and the presence of a star within the inner radius of the ring.13 A Halo, on the other hand, is much smaller (roughly the size of a rocky planet) and so lends itself to being placed into orbit like a natural satellite. In the case of the Halo series, the structures are found in orbit of a parent gas giant planet, Threshold for the first Halo and Substance for Halo
2.1,12
An orbit like that used in Halo 2 is relatively simple: the Halo is lifted/built in a stable orbit of a planet and left there without interference. The first Halo game poses a bit more of a problem: not only is there a moon present in the system, but in one scene from the game, a holographic display clearly shows Halo orbiting Threshold with precisely the same period as Basis, but at a lower orbital altitude. As seen in Halo’s control room, a holographic projection places it roughly halfway between Threshold and
Basis. This orbital location is highly unstable, particularly considering that Basis, a natural satellite, is in an inertial orbit where its centripetal acceleration precisely matches the gravitation attraction it feels from Threshold. If Halo truly were at half the orbital Bornemeier 10
distance of Basis, it would feel four times the gravitational force, but would only require half the centripetal force to stay in a stable orbit at that same orbital period, and so would
spiral down into Threshold and crash.
Fig. 2: Hologram of Halo orbiting Threshold, with Basis, in Halo’s Control room. Screen shot from
Halo: Combat Evolved, Microsoft.
Fortunately, it is apparent that the holographic display is not to scale: Threshold
is 107,302 km in radius, and Basis is 11,924 km,14 yet the hologram displays Basis as
being about a fifth of the size of Threshold. Also, Halo is portrayed as being thicker and
wider in the hologram than it actually is. This allows for the possibility that the Halo is
in the more logical L1 Lagrange point between Threshold and Basis (the masses of the
two bodies are not known, so finding the specific point isn’t possible, but for comparison
the Jupiter-Ganymede L1 point is 96.3% of the moon’s orbital radius).15 Bornemeier 11
The Lagrange points are gravitationally stable locations within a three-body
system. Two primary large masses orbiting a common center and a third satellite mass form the system. There are five locations relative to the two large masses where the third can reside in a stable configuration where it remains motionless relative to the axis between the two larger objects’ centers of mass: the L1 point between the two masses where their gravitational attractions on the satellite are equal, the L2 point beyond the second mass where the pull of the two masses in line balance its now higher centripetal acceleration, the L3 point opposite the second large mass around the first, and the L4 and
L5 points which are at the same orbital distance from the primary mass as the second, but are 60 degrees leading and trailing the second mass respectively.16 Halo, lying between
Threshold and Basis, would most likely lie at the L1 point due to its stability relative to
other orbits between the two bodies. At that location, only a little station-keeping would
be required to keep it on the balance point between Threshold’s and Basis’ gravity wells.
Rotation:
One of the primary issues with constructing Halo is its open inner surface environment. There is an atmosphere, water features, vegetation, and most importantly,
surface acceleration.1 Modern science has no way of generating a false gravitational
potential, but does allow for an artificial acceleration mimicking gravity. Centripetal
acceleration of a ring, and its partner, centrifugal force (the force felt by an object whose
reference frame is rotating with the ring), provide the answer. To generate one Earth
gravity, Halo has to rotate at a specific speed based on its radius: 1 revolution about every
hour and fifteen minutes. Bornemeier 12
This rapid speed leads to several problems for the structure. First is rotating the ring up to that speed from rest. Generating one gravity by rotation, Halo would be spinning with a tangential velocity of just over 7 km/s, only a little slower than the
International Space Station in its orbit of the Earth.17 The total rotational energy of such a system is 2.13*1025 joules, only 1/2,000th the energy required to lift it into orbit, but still 200,000 times the annual energy consumption of the United States. Fortunately, this expenditure of energy is a one-time input: rotating in space means that there is effectively no drag being applied to Halo to slow its rotation.
Unfortunately this rapid speed poses another problem for humans: an hour and fifteen minute long day-night cycle. Slowing down the ring’s rotation would decrease the strength of the artificial gravity, so another method of controlling the day-night cycle would be needed. Because no other orbiting structures are seen in Halo, we will assume that its day-night cycle is caused by something intrinsic to the ring.
Since both Halos orbit gas giant planets, they will inevitably orbit behind them and be in the shadow of the planets for some significant period of their orbit. Though it takes a long time for a satellite in high orbit to make one complete revolution of a planet
(the first Halo is orbiting at the same rate as a natural moon), this periodic eclipsing could produce a longer day/night cycle (treating a “day” as any period where the ring is not eclipsed by its parent body).
A small but important detail about Halo’s rotation is its orientation. Due to conservation of angular momentum, Halo behaves like a giant gyroscope when rotating, preserving its orientation no matter where it is in its orbit. A simple application of perpendicular forces to the ring can cause its axis to precess at a specific rate. Bornemeier 13
Controlling the orientation of the axis of rotation is extremely important for a structure as
large as Halo. Earlier, we calculated the energy required to lift Halo into an orbit two
Earth radii above the surface of the Earth. At that altitude, if Halo were oriented with its
axis of rotation parallel to its orbital path, one side of the ring wound be 10,000 km closer
to the Earth than the other. The centers of mass of Halo and the Earth would be 15,000
km apart, meaning that the far side of the 5,000 km radius Halo would be twice as far away from the Earth as the near side (gravitationally speaking), experiencing one fourth the gravitational attraction. Such a large differential would tend to put undue strain on
the structure and deform the ring. By precessing the axis of rotation with a period equal
to that of Halo’s orbit, the ring could be made to stay “balanced” and keep only one side
facing the planet. Even at larger distances such as those involved with Threshold and its
moon Basis, the size of Halo is still significant and would, without precession, cause
gravitation deformation of the ring. Precessing Halo would also require a new input of
rotational energy, though far less than would be required to rotate it for artificial gravity.
The nanotube Halo would take only 6.58*1021 J of energy to rotate around an axis
parallel to the plane of the ring at one rotation every 28 days (the approximate orbital
period of the moon), one millionth of the energy required to lift it into orbit.
Materials:
A primary challenge in constructing something as massive as Halo is determining
what materials to make the structure out of. Stresses are enormous for an object as
physically massive as Halo, particularly the stress of maintaining its shape as it rotates. Bornemeier 14
Another concern is what materials are readily available in quantities large enough to
produce a Halo.
Rotationally, Halo is under immense strain. The inertial tendency of the ring to
fly apart must be countered by some other force holding it in a circle, which is provided
by the tension within the ring’s structure. Dividing the ring up into infinitesimal slices
parallel to the width of the ring allows for a reduction of the problem to a classical
problem: a single point mass being supported by two forces on either side which pull on
the mass at an angle. Unfortunately, as the sections get smaller, the angle approaches
zero and the resulting tension approaches infinity.
The way around this is to construct the reference frame we’re using to be rotating with
Halo.
Bisecting Halo, we are left with two equal semicircles that are “suspended” from a common axis by one another (see Figure 3). Computing the centrifugal pull
perpendicular to the axis on each half of the ring gives a value for the tension of the ring.
Where λ is the mass per unit length of the ring, a is the centripetal acceleration of the ring
(in this case one Earth gravity), and r is the radius of the ring. If Halo were constructed Bornemeier 15
from structural steel and air, with a mass of 8.73*1020 kg, the force of tension in the ring
is 1.36*1021 N.
Fig. 3:
This presents a major problem to the ring’s builders. That force spread out over
the ring’s entire cross sectional area gives a tensile pressure of 1.91*1011 Pa, or 191 GPa.
Structural steel only has a maximum tensile strength of only 0.4 GPa,18 meaning that a
Halo made from structural steel would instantly fly apart.
Obviously for Halo to remain stable, it needs to be made of something other than steel. If the ring were made out of carbon nanotubes, a material known for its high strength and for its light weight,19 it would have a mass (still assuming the ring were 50% air at STP by volume) of only 1.57*1020 kg, five and a half times less massive than a steel
Halo. The resulting tensile pressure in a nanotube ring would be 34.3 GPa; good news for our Halo-builders. Carbon nanotubes have a theoretical maximum tensile strength of Bornemeier 16
300 GPA and have been proven to go to 63 GPa, just under twice what would be needed
to construct Halo.20
This change to carbon nanotubes presents a major boon for our Halo builders.
First, we have fixed the problem of the ring flying apart: nanotubes can handle the stresses involved. Not only that, but the large margin of additional strength nanotubes can handle allows for some heavier, non-nanotube materials to be used in the ring including water, steel, and stone (all materials we see in the game) without pushing the ring past its breaking point. The mass of the Halo has also been significantly reduced from out first estimate, requiring less mass and therefore less energy to move into orbit and set rotating, as well as less mining of material. An added bonus comes purely from the elemental change: carbon is nearly three times more abundant in the solar system than iron,21 requiring less work to locate abundant sources to mine.
Support:
Once a stable Halo megastructure frame is complete, another challenge appears.
Previously, we have discussed the purely physical requirements of simply building on a planetary scale and creating a stable structure to work with. Now we will discuss matters pertaining to actually using Halo. Of particular note will be the presence of an open, habitable environment on the inner surface of the ring. Other issues discussed will be power generation, reaction control systems, and landing/launching vehicles on Halo.
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Atmosphere Retention:
The major feature of Halo, besides its size, is the inner surface of the ring. On it
is a habitable biosphere with a breathable atmosphere, liquid water, vegetation,
geological features, and weather.1 Unlike most modern space station designs, Halo’s
environment is completely open to the vacuum of space, allowing ships to land or lift off
from the “ground.” Because of this open nature, something must work to keep the
atmosphere from escaping into space. Conveniently, Halo is already rotating to provide
artificial gravity. This acceleration alone is enough to compress the air into a thick
atmosphere just above the habitable surface, but there is still a problem.
As the air “falls” towards Halo from its own inertia, it will run into the surface
and be deflected outward towards the edges of the ring. If the habitable surface is precisely at the inner radius of Halo, then the atmosphere will spill over the sides and be quickly lost to space. Constructing a wall of sufficient height around the edge of the
Halo would hold in the air. In the Materials section, we established that if Halo were
made of carbon nanotubes, there would be a large margin for increasing the weight,
including adding large walls to the edge to prevent the atmosphere from spilling out.
The challenge then becomes determining at what altitude to build the walls.
Studies of the Earth’s atmosphere indicate that nearly all of the planet’s air is contained
within the first 100 km of the atmosphere,22 so walls 100 km high should be enough to
hold in the air. Above 100 km, the atmosphere of the earth starts to become ionized from
solar radiation, particularly ultraviolet radiation.23 This ionized remainder could be
partially contained by producing a strong magnetic field around Halo with a current
going in the same direction as the ring’s spin that would deflect positively charged ions Bornemeier 18
(atoms and molecules) headed towards the center of the Halo back towards the ring’s
inner surface.
Another possibility would be atmospheric replenishment. Both Halos seen in the games orbit gas giant planets, and Threshold has a gas-mining facility suspended in a
habitable band of the atmosphere, i.e. high oxygen content, within human pressure and temperature tolerance, and devoid of poisonous gasses in dangerous quantities, as evidenced by a lack of breathing apparatuses on characters in game who breath Earth’s
atmosphere comfortably. Though not explicitly stated in the game, transfers of mined
gasses, possibly ones as mundane as molecular oxygen and nitrogen, could be sent up to
Halo periodically and released to regenerate the atmosphere. Indeed, canisters identical
to those being processed in the gas mine can also be found on Halo.12
Power Generation:
A structure as large as Halo would have massive power requirements, both for any generic active internal systems, and for necessary systems like reaction control.
Several different methods could be used for generating power including internal generation and various methods of harnessing external energy sources, in particular solar energy.
Internal generation involves building power plants that use some kind of fuel to produce energy. Current technology would make nuclear power the leading choice for this, as it does not require oxygen for chemical reactions, and it requires a relatively small mass of fuel compared to chemical power to generate its energy. Nuclear fuel, however, is very heavy and not particularly abundant. Even if it were used, there would still be Bornemeier 19
problems with having enough fuel to last for 100,000 years, the minimum length of time
the Halos are stated in game to have been in existence. A single, average, modern
nuclear reactor operating for 100,000 years would consume 1.53*1010 kilograms of
uranium fuel, 386 years worth of current uranium production capacity (economically
available uranium supplies are not predicted to last more than 100 years) 24. Halo would
likely require far more than a single nuclear reactor, necessitating many times this mass
of fuel to run for so long. For such a lasting structure, a renewable source of power
would be more useful.
Solar power is a possible solution for all or at least some of Halo’s power requirements. At the given dimensions of Halo, and adding in 100 km high walls to hold in the atmosphere, each halo would have a surface area of 3.8 million square kilometers parallel to its axis of rotation, and 3.2 million square kilometers of cross sectional area perpendicular to the axis of rotation. If the axis of rotation of Halo is offset from the line between it and the sun by just over 40°, the perpendicular surface area exposed to solar radiation is at a maximum: 4.96 million square kilometers. The minimum exposed perpendicular surface area is the area obscured by the ring when viewed perpendicular to its axis of rotation, or 3.2 million square kilometers.
The sun gives off a huge amount of energy: 3.85*1026 W. At the orbital distance
of the Earth, that energy is attenuated to 1,368 W/m2, all of which would reach Halo because it is not shielded or obscured by the atmosphere.11 Modern solar cells have a conversion efficiency of 42.8%,25 meaning that the maximum energy that could be
harnessed by Halo with current solar technology is 2.9*1015 W, over 6,500 times the energy consumption of the United States. Even at the minimum exposed generation area, Bornemeier 20
64.5% (the ratio of the minimum to maximum area) of the maximum generating capacity
would still be available.
Even with the solar generation capacity of Halo, it would still be eclipsed by the
planet it orbits for some amount of time, and in the case of the first Halo, also by Basis
when the moon makes its closest approach to the sun. Such times would require storage
of power for later use. The simplest answer is to use a parallel plate capacitor built into
Halo’s structure. Carbon nanotubes are excellent conductors of electricity and behave
like wires, and our Halo has a primary structure made entirely of nanotubes. If the outer circumference of the ring were covered in two parallel sheets of nanotubes separated by a thin dielectric sheet like a polymer, it would create a gigantic parallel plate capacitor. At such small separations, the difference in circumference of the capacitor rings is miniscule. The capacitance of a cylindrical capacitor is given by:
where ετ is the permittivity of the dielectric, εo is the electric constant, l is the length of
the cylinder, ro is the outer radius of the capacitor, and ri is the inner radius of the
capacitor. If the dielectric has the same permittivity as a vacuum, the outer radius is
5,000 km, and the separation of the two “plates” is 0.1 mm, the capacitance becomes
8.90*105 F. In practice, the permittivity of the dielectric is higher than the permittivity of
a vacuum, further increasing the capacitance. If BT epoxy glass were used as the
dielectric, the constant would be 4.1,26 raising the capacitance to 3.65*106 F. The power stored in a capacitor is equal to half the capacitance times the square of the voltage applied across it, and the voltage safely applied across it depends on the dielectric used and its electric breakdown point. For BT epoxy, the breakdown occurs at 53,150 V/mm, Bornemeier 21
or 5,315 volts for a 0.1 mm thick layer. With this as the maximum allowable voltage, the
total storable energy of the capacitor is 5.15*1013 J.
This value represents just 1/36th the energy captured in a single second by Halo at
its minimum non-shaded capacity. Creating a layer of capacitors (as described) 1,000
layers thick would only take up space on the order of a meter, but would be able to store a
thousand times more energy, nearly 30 seconds worth of minimum generating capacity.
This would easily be more than enough energy to last Halo through shaded periods of its
orbit, which would last only a few hours at most, given that it is unlikely the structure
would ever require more than a token fraction of its energy at one time. If more storage capacity were required, a number of meter-thick layers could be added to the structure
(which is, after all, 22.3 km thick) as needed until the desired duration was reached. 24 hours of minimum generation capacity would, for example, require a capacitor layer that was 3.14 km thick
Station-Keeping (RCS):
Once in a stable orbit, Halo needs to be maintained in that orbit without crashing
into its parent planet, or in the case of the first Halo, becoming imbalanced from its L1
Langrange point. Even at orbital altitudes, there is still some atmosphere, as well as the
solar wind, that provide a tiny amount of drag on objects in orbit, gradually slowing them
down and decreasing their altitude until they finally burn up. Halo, with its immense
size, and therefore drag, would need to be protected from this slow orbital degradation by
some sort of active system that could compensate for and correct the changes in its orbital Bornemeier 22
path caused by drag. The common terminology used to describe systems that maintain a
spacecraft’s attitude or position is reaction control system, or RCS.
An object as massive as Halo requires lots of energy to move. Most RCS systems
on current space vehicles are chemical, using a reaction mass to change the orientation or
momentum of the vehicle or station. For Halo, this is highly impractical. First, Halo has
a long planned lifespan: in the games it is stated that the Halos are at least 100,000 years
old. Second, Halo has a huge mass to move, meaning lots of fuel would be necessary
even for a small change in motion. Over such a long lifespan, such fuel would inevitably
run out.
A popular and molecularly simple fuel used in some RCS systems, including
27 those on the space shuttle, is hydrazine, N2H4. When passed over a catalyst, the
naturally unstable molecule breaks down into nitrogen, hydrogen, and ammonia gasses
and releases tremendous thermal energy that goes into the reaction products. As mentioned earlier, there is the possibility that gas mined from the gas giants the Halos orbit might be shuttled to the structures occasionally. If the mining platforms harvested ammonia and synthesized it into hydrazine or a related chemical, it is also possible that it could be ferried to Halo for use as fuel along with replenishments of the air supply.
Rather than use chemical propellants, Halo could also use RCS systems that don’t require fuel, but simply a power source, to operate. There are two main RCS systems that can be used that do not involve an expenditure of mass: electrodynamic systems, and gyroscopic systems. Gyroscopic systems use the principle of conservation of angular momentum to change the orientation of an object. A gyroscope (or several) with a significant mass relative to the object using it, is set rotating by motors inside a housing Bornemeier 23
mounted in the object. Attempting to rotate the gyroscope causes it to exert a
perpendicular force on the housing, and by extension the object, causing a change in its orientation.28
For translational motion, the electrodynamic tether provides a potential alternative
to chemical fuel. In a tether system, an open circuit wire is stretched through a magnetic
field and passed through it (easily done by installing wires in Halo’s structure that are
insulated on all but the ends, and the wire moves as Halo rotates) with an electrical load
on it like a resistor or a battery. At each end of the wire, electrons flow either in or out
from the surrounding plasma of space or a planetary ionosphere due to an induced current
from the wire moving through the magnetic field. In this way, some of the translational
momentum of the object gets transferred into electrical energy, slowing the object’s
speed. Conversely, the circuit can be driven by a power source in the other direction,
using a current to increase the momentum of the tethered system.29
Gas giant planets are known for their strong and large magnetic fields; Jupiter is
the archetypal example. A network of wires throughout Halo could individually be used
like tethers to change the rotational speed of the structure (let one side slow down and the
other speed up: the net momentum change is zero, but the rotation of the ring is
increased), or as a group to increase the total translational speed of the Halo and thereby
alter its orbital altitude.
Visiting Halo:
Space vehicle interaction with Halo poses a problem for the pilots. At 7 km/s
rotational speed, a vehicle matching Halo’s orbital velocity will still be moving as fast, Bornemeier 24
relative to the ring, as the ISS moves in orbit, relative to the Earth’s surface. This presents three primary approaches to getting onto Halo: from outside the ring, from inside
the ring, and from next to the ring. Outside the ring and on the side of the ring can be
grouped together because of their technical similarities; neither has to deal with an
atmosphere (both occur outside Halo’s atmospheric retention walls, and so take place in a
vacuum), and if something goes wrong, the vehicle attempting to land will be inertially
thrown away from the ring, rather than into it as a vehicle approaching the inner surface
would be.
A natural impulse would be to try and land on the inner surface of Halo. It has
simulated gravity, an atmosphere, a surface just like a planet would. There are several
concerns involved with landing on the inside of a ringworld that landing on the outside
doesn’t pose. Any ship inside the inner surface of Halo wouldn’t feel any of the ring’s
natural gravity from mass, small though it would be, so flying inside and slowly drifting
down towards the inner edge would seem like a good plan. The problem is that the edge
would be moving at a relative 7 km/s, and to match that speed would send a ship crashing
into the inner surface in just over two minutes. Simply drifting down towards the ring
would produce progressively more and more aerodynamic drag as it entered the rotating
atmosphere, speeding up the vehicle, but also heating its surface just as re-entry of a
normal spacecraft would. The speeds involved would be, at every instant of the flight but
the last, lower than if the vessel immediately matched the spin speed, and the relative
speed compared to the atmosphere would allow for an airfoil to generate lift and slow the
“descent” once the vessel’s speed was fairly close to the ring’s. After the vehicle had
gained most of the necessary speed, it could fly in for a landing on the inner surface of Bornemeier 25
Halo, picking up the last bit of needed velocity as it landed and slowed on the runway.
Because of this heating and final gliding approach, a spacecraft similar to the Space
Shuttle would work well for inner-ring landings.
On the other hand, docking with the megastructure from outside of Halo’s atmosphere provides many advantages to a would-be pilot. First is the design and construction of a potential vehicle; without an atmosphere to enter, any ship trying to dock with Halo doesn’t need to have a heat-shield, or even be aerodynamic, potentially reducing mass and/or increasing carrying capacity. If something goes wrong on the vessel, its momentum will carry it away from the ring as it has no force acting on it to curve its trajectory with the Halo. Landing on Halo in this way would be easy in principle: simply approach the ring on a trajectory tangent to its surface and match the speed of rotation. If a docking bay is provided, the vessel will seem to drift slowly into it, where clamps or doors can shut to seal the ship inside and apply a centripetal force to it, “landing” the vessel.
Conclusion:
The Halo megastructure is a marvel not of engineering, but of industrial capacity and energy generation. Though modern technology has created the exotic materials necessary to build Halo, like carbon nanotubes, modern synthesis techniques are not yet practical on an industrial scale large enough for our merely planetary civilization, much less for a civilization with the material requirements of building a ringworld. Beyond that, the energies involved have been discussed, and they are unimaginably high: in some cases, even comparable to the output of the sun. If modern energy production were Bornemeier 26 several orders of magnitude greater than it is now, about 106 times greater if we wanted to build a Halo in a few hundred years, it would be physically possible to build the Halo megastructure for whatever purpose it might be required to serve for a future society, be it extra living space, research facilities, or even just for the challenge. Bornemeier 27
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