Section 4.5: Phase 4A - Low Orbit Locations
Phase 4A is the first set of program locations beyond the Earth itself. It begins the process of using space to upgrade and expand civilization. In time sequence it begins after the start of Phase 2B Industrial locations, because industrial-scale transport is needed to reach low orbit. It comes before the remaining locations in space because travel to them must pass through low orbit, and this region is physically closer and easier to reach than the rest of space. Like other phases, it continues in parallel with them once started. The Low Orbit region is already used for a range of purposes, and many additional uses are possible. Like the program as a whole, this makes it a complex engineering situation. So we apply the Systems Engineering methods from Section 1.5 to develop this phase.
0.1 - Conceptual Stages 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
1.0 - Concept Exploration
1.1 - Region Definition
- 1.1.1 - Low Orbit Boundaries
The Low Earth Orbit region is defined as orbits averaging 160 to 2700 km above the mean radius of the Earth. It would be very difficult to build static structures extending up from the surface to this altitude range, and motionless objects would rapidly fall back to the surface. Therefore physical objects in this region need to be in orbital motion with a particular range of velocity and direction, such that they do not intersect the Earth or rise too high. This is unlike on the Earth, where locations can have fixed coordinates of latitude, longitude, and altitude. Instead, orbital 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 referenced to the plane of the Equator. An object's position constantly changes due to orbital motion, but can be projected from the given time using the set of elements. 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 orbits 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 between the atmosphere and space, as defined by various methods. The 80-160 km altitude range has space-like conditions, such as near-vacuum pressure levels. It can only be transiently occupied by unattached objects. More permanent occupation requires attachment to either the ground or objects in higher orbits. We therefore assign items in this 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 between 50 and 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 ) 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 the definition for inclusion in the region.
- 1.1.2 - Low Orbit Environment
Designs for the Low Earth Orbit region must be adapted to the local environment conditions. We start by looking at the primary environment parameters used for the prior Earth phases. Where the conditions are outside the ranges of previous designs, they must be modified accordingly. If there are unique conditions in Low Earth Orbit, then further design changes or new designs will be needed.
1. 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 size. For the environment temperature range, we assume a 50% reflective grey body that has a back side facing away. This 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 245-261.6 K, and get reflected light from the Earth on the sunlit side, raising the temperature somewhat. The net result is a temperature within the moderate to extreme ranges encountered on Earth, and not requiring immediate modification for the average temperature.
Space equipment is generally designed for low weight, and therefore low thermal mass. The vacuum of space also lacks convective and conductive contact with the surroundings, which can moderate temperatures. 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.
2. Water Supply - Natural water essentially doesn't exist in this region, requiring it be imported from elsewhere. Equipment and processes that use water on Earth may need modification or substitution.
3. Atmospheric Pressure - 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/m2, or 1.8 million times lower than static pressure at sea level.
4. Ground Pressure
5. Energy Supply
6. Gravity Level
7. Radiation Dose
8. Ping Time
9. Travel Time
10. Stay Time
11. Transport Energy
- Day Length - Orbits in this region range from X to X hours, with a day/night cycle if it passes into the Earth's shadow. Some orbits are "sun synchronous", with their path oriented to avoid darkness, but most are not.
- 1.1.3 - Low Orbit Resources
- 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.
- Energy: Solar, magnetic, kinetic & potential
- Raw Materials: Upper atmosphere, artificial debris. Particle & meteor flux is minimal.
Identify Phase Activities
We want to identify useful activities for the Low Orbit region, which meet our program goals