# Space Transport and Engineering Methods/Phase4A

Low Earth Orbit (LEO) was the first region in space to be developed, starting in 1957 with the launch of the first orbital satellite. It still accounts for the largest segment of artificial mass in orbit, but it is not yet a complete economy. As of 2017 the activities there are mainly public programs, such as scientific and military, and support of activity on the ground, such as Earth observation and some communications. The limited range of activity is a function of the high cost of reaching orbit compared to most activity on the ground. Industrial development on Earth in Phase 2B should greatly reduce transport cost, and exploiting materials and energy already in space should further reduce total costs. The combination should enable larger scale development and a wider economic base in the region.
For the purpose of our program we consider Phase 4A to have started once industrial development on Earth (Phase 2B) enabled equipment delivery to orbit. We recognize the existing activities, but consider additions or changes that can be made in the future. New activities in this phase will generally precede other phases in space because Low Orbit is the physically nearest and lowest energy region to reach from Earth. However, most of the useful resources in space are beyond Low Orbit. So a large part of the development in this region will be to support the more distant regions. In turn, the Low Orbit region needs continuing support from Phase 2B industries for space, because industrial-scale production and transport systems are needed to reach low orbit. This phase, like others, will continue in parallel once started.
The Low Orbit region is already used for a number of purposes, and quite a few additional ones are possible in the future. This makes it a complex engineering challenge. Therefore we apply the Systems Engineering methods from Section 1.5. That begins with concept exploration, which we pursue in this section. We first describe the features of the region and do an industry survey to identify additional future activities. We then look at drivers like motivations, economics, and technology, which would cause projects to be initiated. This information is combined into a development approach and a set of activities and projects by time and function. Finally we link the projects to each other and other program phases. Projects for which we have developed additional details and calculations are then more fully described as the last part of the section. As an output from our concept exploration, we identify what R&D will be needed to prepare for this phase, and feed it back to the preceding Phase 0G - R&D for Low Orbit Development.

## Low Orbit Features
### Region Definition

Our program defines the Low Earth Orbit (LEO) region as orbits averaging 160 to 2700 km above the mean radius of the Earth. When it would not be confused with low orbits around other bodies, we will often shorten this "Low Orbit". It is entirely surrounded by the High Earth Orbit region, and in turn completely surrounds the Earth. It would be very difficult to build static structures extending up from the surface to more than 160 km, and motionless objects would rapidly fall if unsupported. Therefore physical objects which persist in this region need to be in motion with a particular range of velocity and direction, such that they do not intersect the Earth or rise too high. This is unlike locations on Earth, where objects can have fixed coordinates of latitude, longitude, and altitude.
Locations in Phases 4A to 4F are identified by a set of Orbital Elements. These parameters determine the size and shape of the orbit, how it is oriented in three dimensions, and a position along the orbit at a given time. For Low Earth orbits, the parameters are usually referenced to the center of the Earth and a fixed direction on the Celestial Sphere. An object's position constantly changes due to orbital motion, but can be projected from the given into the past and future. Orbit parameters can change over time, either from natural forces or using any of the transport methods listed in Part 2. So a built-up site, like a space station, may change its orbit over time.
The lower bound of the region is set by where atmospheric drag would rapidly cause orbital decay without compensating propulsion. This is somewhat higher than the 80-122 km Boundary Designations between the atmosphere and space determined by other methods. The 80-160 km altitude range has space-like conditions, such as near-vacuum pressure levels, but it can only be transiently occupied by unsupported objects. More permanent occupation requires attachment to either the ground or objects in higher orbits. We therefore assign items in this transition range to earlier phases if transient or attached to the ground, and to this phase if attached to objects above 160 km.
The region's upper bound is set halfway in energy terms between the lowest stable orbits and Earth escape, or 75% of the energy from the Earth's surface to escape. This is an arbitrary limit, but lower orbits have different enough conditions from higher ones to warrant a distinction. Orbits can be elliptical, where the altitude varies as you move along them, so we define the region by the average of perigee and apogee along the major Orbit Axis. The highest point of an elliptical orbit in the region is then 5240 km (0.82 $R_{\bigoplus }$) above the surface. Since orbits have many possible orientations and shapes, the region has a fuzzy boundary in physical space. Instead, objects meet or do not meet our definition for the region.
The Earth's gravity is the dominant force in the region, at least 74,200 times stronger than than the Moon's and 500 times stronger than the Sun. Total volume of the region using the average upper altitude bound is 1.8 times the volume of the Earth, and cross section is 85.7% of the Earth's land area. Not all of this can be used because of intersecting orbits and blocking sunlight from reaching the surface. Despite this, the usable space is substantial.

### Environment Parameters

Designs for long-lasting objects in Low Orbit must accommodate the local environment conditions. We consider the same environment parameters as for projects on Earth and other space regions. Where local conditions are outside the ranges of previous designs, they must be modified accordingly. We also note unique conditions for Low Orbit which must be considered in designs.

#### Primary Parameters

Temperature
A Black Body at the Earth's average distance from the Sun has an equilibrium temperature of 393.7 K (120.5 C) on the Sun-facing side. The movement of objects in Low Orbit are centered on the Earth, and therefore share the same theoretical equilibrium temperature. Actual hardware temperature will be a function of its time in the Earth's shadow, orientation, infrared contribution from the Earth as a function of altitude, albedo, emissivities, and thermal properties. As an example, 50% reflective grey body that has a back side facing away from the Sun would have a temperature of 331.0 K (57.9 C) if it were in sunlight all the time. However, low orbits are typically in the Earth's shadow 22-40% of the time, reducing the temperature to 245 to 262 K (-28 to -11 C) The result for various hardware designs is an average temperature within the moderate to extreme ranges encountered on Earth, but with short-term fluctuations from crossing the Earth's shadow.
Space equipment is currently designed for low mass, and therefore low thermal mass. The vacuum of space also lacks convective and conductive heat transfer with the surroundings. So the internal temperature can vary significantly as the equipment moves into and out of sunlight. Parts of a satellite may also be normally oriented to face towards or away from the Sun, and therefore be hotter or colder than average. So while average temperatures may be reasonable, specific equipment elements may need adapted designs to account for their operating conditions.
Atmosphere and Water Supply
The Earth's Atmosphere extends from the surface to about 10,000 km, and therefore fills the entire Low Orbit region. However, the density decreases with altitude, and at 160 km is one billion times lower than at sea level, placing it in the extreme range as far as people, and hard vacuum condition for many design purposes. Dynamic pressure at 160 km due to orbital velocity is less than 0.05 N/m$^2$. This is 1.8 million times lower than static pressure at sea level and about 180,000 times lower than passenger aircraft at cruising altitude. So it is negligible for most design purposes, although enough to cause orbital decay. Natural water essentially doesn't exist in this region, requiring import from elsewhere. Since transportation costs are currently high, equipment and processes that currently use water would likely need modification to reduce their use, or substitution by methods that don't require it.
Ground Strength
Since this is an orbital region, soil or rock strength for construction or transportation purposes is not relevant.
Gravity Level
The Earth's gravity in the region varies with altitude from 9.3 to 2.95 m/s$^2$, or 95 to 30% of surface gravity. The variation from 160 to 5240 km altitude goes as the inverse square of the distance from the Earth's center. However, most objects in the region will be in orbits with free fall conditions. Gravity still accelerates them downwards, but their horizontal motion is enough that the Earth curves away a compensating amount. Different parts of smaller objects see about the same acceleration, so they don't see net acceleration among themselves. The effect at small scales is as if there were no gravity acting, and thus no structural loads for design purposes.
The situation changes for larger objects. Their parts are different distances from the Earth's center, and the direction to the center varies. So the gravity forces are different strength and direction, resulting in net forces between the parts. The larger the object, the more noticeable these differences become, to where they can be the primary design load. Artificial gravity is desirable for human health, and for some production and transport methods. This can be produced by rotation, and creates additional design loads.
Radiation Level
The Earth has natural Radiation Belts containing high energy charged particles. They mostly come from the Solar Wind, and are trapped by the planet's Magnetic Field. The inner belt generally extends from 1000 to 6000 km altitude, but parts may reach as low as 200 km. It therefore fills most of the Low Orbit region. The magnetic field, and thus the belts, are generally toroidal (doughnut) shaped, so the radiation levels vary strongly by altitude and latitude. The magnetic field is tilted and off center with respect to the Earth's polar axis, and solar wind pressure causes variable distortions to it. So the belts can move, and radiation levels will vary by orbit position and time.
An unshielded person in the dense parts of the belts can get a lethal radiation dose in a matter of days or months, and the radiation can cause permanent damage and transient upsets to equipment. To date, the main ways to avoid human exposure have been to stay in lower orbits, where their strength is less, and, in the case of Lunar missions, to transit them quickly at higher latitudes. A number of satellites have orbits that require them to be in higher radiation regions of the belts. This requires Radiation Hardening in their designs. Future approaches to the radiation problem include local shielding by bulk mass, weakening the belts by intercepting the particles with mass or electrostatic devices, and reducing their particle sources with devices "upwind" between the Earth and Sun.
Communication Time
Most long distance communication in the region would be by electromagnetic waves (radio or laser) in a vacuum. So round-trip (ping) communication time with Earth and within the region is mainly a function of distance and the speed of light. Direct communication with Earth can be as little as 1 millisecond (ms), though that would be rare. From the highest orbit altitude in the region to the Earth's horizon is 9700 km, and therefore 65 ms ping time. There would be additional transmission time from the space-to-ground terminal to the end point of communication on the ground. Direct communication between points in the region can take up to twice that with the ground, for paths which graze the Earth from maximum altitude on both ends, so 130 ms. Direct signals can't pass through the Earth, so some space-to-ground and space-to-space communications will have to go through one or more relay points. Today this is commonly via satellites in synchronous orbit, because it provides a fixed target for ground stations and three satellites can provide coverage for most of the planet. A worst-case link might require two satellites in synchronous orbit at maximum distance, and therefore 1100 ms ping time (1.1 seconds). Low orbit point-to-point satellite networks are under development. They require many more satellites, because each one only sees a small part of the Earth at any time. Such networks would reduce the worst-case ping time to about 180 ms.
Travel Time
Orbit periods in the region range from about 90 to 144 minutes. The Earth rotates beneath given orbits, and objects are in constant motion at different speeds in different orbits. Therefore travel time from Earth to points in the region, or between points in the region, are usually governed by waiting times for proper alignment rather than the orbit periods themselves. The combination of the Earth's rotation and safe launch directions from a given site typically result in one launch window per day. Once reaching Earth orbit, careful matching with the destination may take another day. So total travel time will be 1-2 days, although planning for space travel today involves much longer lead times for training and securing a ride.
The planes of inclined orbits shift due to the Earth's equatorial bulge on the scale of several degrees per day. So minimum energy orbit-to-orbit transits, which require the orbit planes to align, can require on the order of 100 days waiting time. Point-to-point travel between orbits can be accomplished much faster, but at great expense in propellant if using chemical rockets. Electric or other propulsion can be much more efficient, but are themselves typically slow in providing the needed velocity changes. Future traffic within the region and points beyond will have an incentive to concentrate on equatorial orbits. These reduce waiting time for launch, because the orbit always passes over the same points on the ground every time. They also eliminate the plane shift effect from the equatorial bulge. There will still be a need for orbits with other inclinations, so not all traffic will be equatorial. They will still have to deal with travel delays.
Stay Time
Recent missions to the International Space Station, which is in the Low Orbit region, have averaged 6 months, with a maximum of 1 year. By comparison the US average stay time in a given county, which for this purpose is a single location, is 25 years, with the most rapidly growing ones averaging 7 years from growth plus mobility. Therefore orbital stay times are short compared to those on the ground. The short orbital stay times impose extra transportation requirements, but relieve the personal space and comfort needs because the crew understand the conditions to be temporary. Current stay times are limited by radiation exposure and the long-term effects of zero gravity, despite attempts to counteract them. If longer-term stays are desired, then designs would have to address the radiation and gravity problems, and provide increased personal space and comfort. The current ISS is designed for zero gravity research, so those kinds of design changes would imply a new orbital installation.
Transport Energy - The minimum theoretical transport energy from the Earth's surface to the lowest altitude in the region is 32 MJ/kg, accounting for kinetic and potential energy, less the contribution of the Earth's rotation. The highest orbits in the region require about 50% more, or 48 MJ/kg. However, current chemical rockets such as the Falcon 9 deliver about 4% of takeoff mass as payload, and most of the rest is propellant whose energy is consumed. Assuming 90% of the takeoff mass is RP-1/Oxygen with a chemical energy of 13 MJ/kg, the transport energy consumed is 285 MJ/kg of payload, or a system efficiency of 11%.

#### Unique Conditions

Day Length
Orbits in this region range from 87.5 to 143.5 minutes, with a day/night cycle if a satellite crosses into the Earth's shadow. Some orbits are "sun synchronous", with their path oriented to avoid darkness, but most are not.

### Available Resources
#### Energy Resources

Solar has been the primary energy source for satellites in the region since 1962. A notable exception was the Space Shuttle, which used fuel cells, and a small percentage of satellites which use Nuclear Power. Many satellites have batteries to cover time in the Earth's shadow and occasional eclipses.
Solar Energy - The Solar Constant at 1 AU is 1361 W/m$^2$ at solar minimum and about 1 W higher at solar maximum. The Earth's orbit varies from 0.987 to 1.017 AU from the Sun, which changes the local intensity from +2.65% to -3.3% of the reference value. Orbit radius has a negligible effect on solar distance and flux. Depending on orbit parameters, satellites can spend up to 40% of their time in shadow, generally decreasing with altitude, and varying cyclically due to orbital precession. Total available energy in the region is large (352,000 TW), but it cannot all be used, because it would block sunlight from reaching the Earth. A reasonable limit is 1% of this, which is still 175 times total 2017 energy use by civilization. Sunlight in natural form is useful for illumination and plant growth, and conversion to electricity using solar panels is reliable and up to 30% efficient. Concentrating reflectors can produce high temperatures, and somewhat higher electrical efficiency at the cost of complexity and mass.
Other Energy Sources - Batteries are the usual way to handle the 35 minutes or less of darkness per orbit, and fuel cells and nuclear power have been used in the past. Some future possibilities are beamed energy from the ground or other satellites, and tapping the satellite's orbital energy using electrical conductors or momentum exchange. In order for the orbit not to decay, the orbital energy must be replaced from other sources.

#### Material Resources

Earth's Atmosphere - The atmosphere above 160 km contains about 32.5 kg/km$^2$, or 17.5 million tons in total. If some of this gas is collected, more will replace it from below, so it is a large, although low density, resource. The scale height, over which pressure drops by a factor of e, at this elevation is 26.4 km, increasing to 36 km at 200 km altitude, so most of gas is concentrated in the 160-200 km altitude range. The composition in this range is mostly nitrogen (N$_2$), and monatomic oxygen (O) rather than diatomic oxygen (O$_2$)found at low altitude. Collection of air from this region can be accomplished from orbit with a reverse nozzle that compresses the incoming flow. However, the drag this creates must be balanced with thrust produced with electric propulsion, using a portion of the collected gas.
Orbital Debris - As of 2017, the region contains several thousand tons of non-functional satellites, empty upper stages, and smaller fragments generated by collision and other reasons. The debris mass continues to increase faster than orbital decay removes them. All the artificial debris was designed for use in space, and some of it likely still contains functional parts, even though the satellite as a whole no longer works. It is a potential source of usable materials and parts which are already in the region, although how useful is yet to be proven. At the least, efforts should be made to remove them because they are a hazard.
Other Sources - Other natural sources of materials are negligible, such as fluxes of meteors, dust, and particles, and the Earth has no natural satellites in the region. So other needed materials will have to be imported, either from Earth or more distant regions. Delivery from higher orbits to the region can mostly be accomplished by slow Aerobraking against the Earth's atmosphere. This uses the thinner atmosphere at higher altitudes than Atmospheric Entry, so it does not produce high accelerations or extreme heating. The remainder of the maneuvers are accomplished by regular propulsion methods, but overall it is much more efficient. Even though the Earth's surface is physically close, delivery from there requires much more energy.

## Industry Survey

* Existing satellites, projects & programs already using the region. They can supply inputs or accept outputs from the phase, or adapted for phase needs
* Proximity to Earth, so products (materials, parts, equipment) & energy (beamed, chemical, nuclear) can be delivered from existing civilization. Labor can be supplied directly or remotely.

## Development Projects
### General Approach
### Long-Term Projects
#### Low Orbit Production
#### Low Orbit Transport

Orbital Tugs
Crew Transport
Transport Infrastructure
(skyhooks, spaceports, storage)

#### Low Orbit Services
## [still to be merged]
### Concept Exploration Approach

Systems evolve through a sequence of life cycle stages, from initial idea to final disposal. For projects and locations in the Low Orbit region, we will address their early design stages, which are concept exploration and conceptual design, with the following set of tasks, which are further detailed below:
* 1. Concept Exploration
       1.1 Region Definition - Boundaries, environment, and resources
       1.2 Phase Candidates - Activities from program goals & objectives, reference architecture, industry lists, and plans for future space projects
       1.3 Phase Needs - Compare to current space programs to identify new projects & locations
       1.4 Phase Concept - Organize into a logical sequence & link to other phases
* 2. Develop Reference Architecture
* 3. Identify Requirements & Measures
* 4. Perform Functional Analyses
* 5. Allocate Requirements
* 6. Model Alternatives & Systems
       6.1 Collect External Technical Information
       6.2 Develop Alternative Options
          6.2.1 Identify Relevant Fields by Function
          6.2.2 Develop Candidate Technology & Methods List
          6.2.3 Assess Candidate Feasibility
          6.2.4 Size Relevant Options
          6.2.5 Quantify Option Parameters & Configurations
       6.3 Build System Models
* 7. Optimize & Trade-Off Alternatives
* 8. Synthesize & Document Conceptual Design
       8.1 Write Conceptual Books & Articles
       8.2 Write Design Technical Reports