Section 3.1 - Design Factors
This section covers factors which influence a design across multiple types of subsystem elements. The most obvious influence is the set of requirements which define what the system is supposed to do and the criteria used to score how good the design is. Requirements and criteria were discussed in Section 1.5.
These involve the performance levels of different technologies, and how mature they are. A project or design should assume consistent performance values and assumptions for a given technology. This needs to be done both across different subsystems within a design, and across different alternative design options. For example, if a solar panel power/mass ratio of 100W/kg is used in one alternative, the same value needs to be used in other design alternatives.
The maturity or Readiness of a technology is a measure of how far it has progressed from initial idea to commonplace use. NASA developed a scale of Technology Readiness Levels (TRLs) to describe the status of a technology. The higher the TRL, the less uncertainty about the cost and performance of the given technology. Care should be taken to understand the actual state of a technology, and allow appropriate risk margins.
These involve whether a given material, component, subsystem, human skills, or facility will be available for a given project. A given design option might require components which are in limited supply. Even if they would perform better, if they are not available when the project needs them they will cause a delay which might be unacceptable. Reasons for limited supply include the number of suppliers with the skill and capacity to make the item, their current work backlog, upstream resources they need from their suppliers, import or export restrictions, possible production disruptions, intellectual property, and existing contractual agreements. Any item which is not obviously in abundant supply should be verified as to availability, to at least identifying a supplier who could meet the project needs. In some cases, the project itself can develop the capacity to provide a given item, but that imposes additional tasks to do so.
Physical Design Factors
- Materials Selection - This involves how to choose materials for different parts of the design. Usually materials selection involves multiple factors beyond the obvious ones like strength and melting point. Strength/density ratio is important when weight matters, which is most of the time for space systems. Materials cost. Qualification for a material mean sufficient testing to know how well a given material will work for a given purpose. New materials may not have sufficient testing.
- Design Margin
- Design Life
- Corrosion and Fatigue
Integration is the process of combining lower level components into a higher level element which performs a set of functions.
- Design Budgets
A system will have budgets for finite items such as component mass, maintenance time, communications, or command inputs. These budgets apply across multiple subsystems, and must be estimated, allocated, and tracked.
- Subsystem Interactions
This is consideration of the effects different systems impose on each other. These may include acceleration, vibration, thermal, electromagnetic, radiation, and others.
You cannot design humans (not yet, anyway), therefore you have to factor in human features into a design. This includes physical factors like acceleration tolerance, and mental limits such as the finite ability to learn and execute operational tasks.
Humans come in a range of sizes. Therefore seats and control interfaces have to accommodate different eye positions, eye focus, arm reach, hand size, and other characteristics. Devices such as space suits either need to fit the range of users, or crew selection needs to restrict the size range to fit the equipment. One source for data for space projects is the NASA Man-Systems Integration Standards, but this topic has gotten a lot of attention on Earth because most systems interact with humans, and thus have to be designed to interact with them.
Operating a complex system is not intuitive, therefore human operators must be trained for the task. It imposes a requirement that the amount of natural skills plus training fit within what a given crew is capable of. For long duration operation, re-training may be needed for infrequent tasks. Simulators are used to do training at lower cost, and to train for hazardous conditions, such as loss of an engine. So in addition to designing operating hardware, training media, instructors, and simulators need to be considered in the design.
Any design must be capable of withstanding the various environments in which it finds itself. These will include the production, storage, transport, and operating environments. We can divide environments in general into two groups, objects and space, since the former have distinct local conditions.
There are a wide variety of objects in the Solar System and beyond, of which the Earth is one. The environment conditions vary widely between them, and locally on individual objects, so we will not try to list individual details. Instead we will note the types of environment parameters which should be considered in a design, and what effects they may have. For specific locations, previous scientific or other data sources can be used for detailed information. If that is not available, new observations or visits may be needed to get the local data.
If a body has an atmosphere, a number of conditions should be considered. These include:
- Static Pressure - Internal elements of the system, humans for example, may require particular conditions. If the outside pressure is too high or too low, then the design will need to include a pressure shell to maintain the desired level.
- Dynamic Pressure - These are the forces generated by a moving atmosphere (wind), or from moving the system through the atmosphere. The structure must be designed to withstand these forces in addition to any static pressure. Lightweight or large structures may bend or shift bodily from dynamic pressure loads.
- Composition - An atmosphere may contain gases which react with system hardware. Examples include oxygen and water on Earth, and sulfuric acid on Venus. Some gases are combustible (react quickly) with exhausted or leaked materials from the system. For example, a leaking oxygen tank may combust in a methane atmosphere.
- Dust - An atmosphere can transport dust and larger particles. These can abrade surfaces, or get deposited on them and accumulate. Dust may interfere with mechanical devices and be a hazard to living things.
- Condensation - Under certain conditions, atmosphere components can condense to liquid or solid form and then precipitate as rain and snow due to their higher density. Very small condensed particles may remain aloft as fog and clouds. The effects on a design include condensation on surfaces, and accumulation of fallen rain and snow on equipment and the ground.
- Opacity - An atmosphere can reduce visibility, communication, and filter incoming and outgoing light and heat due to being partially or totally opaque in particular wavelengths. The opacity can be caused by the gases, dust, or condensation within the atmosphere, and can be variable.
The equilibrium temperature for a system element includes solar input, heat transport from any atmosphere present, and from the solid or liquid ground. Most objects rotate, so the Solar input will vary with time. If their orbit is significantly elliptical, the solar input will also vary with distance. Atmospheres can transport heat by radiation, convection, and conduction. To the extent an atmosphere is transparent or not present, heat can be lost to the very cold background temperature of the Universe. Systems operating close to, or on or below the surface of an object will get some heating from the object, in addition to solar input.
A functioning system also usually generates internal heat from operation of components, so the equilibrium temperature is what results when internal and outside thermal flows are in balance. If this temperature is higher or lower than desired for system operation, then components like radiators, heaters, and thermal insulation must be added to bring the internal temperature into the desired range
On Earth, gravity level is within a few percent of the standard value (9.80665 m/s2) for all locations. For other objects, except for the Gas Giants, it is generally lower and more variable. When the level is below biological or industrial process needs, then generating artificial gravity by rotation may be necessary. If the level is so low that traction or anchoring by weight does not work as it does on Earth, then special methods for movement and staying put may be required. When significant gravity levels are present, the design needs to account for structural loads caused by it, and bearing loads against the object, and ability of the object to support them.
Almost all locations receive some radiation from a combination of radioactive decay, solar wind and flares, trapped particle belts, and cosmic rays. In addition to natural sources, a system may contain artificial sources such as radioisotope generators, accelerators, and reactors of various types. Humans and other biologicals, sensitive electronics, and some instruments are affected by high radiation levels. Protection comes from distance in the case of point sources, and shielding of various kinds from other sources. The best type of shielding varies by radiation type, and other parts of the design may provide shielding by their mass and arrangement.
The Sun is the major light source in the Solar System. It provides a source for heating and power, photosynthesis and other chemical reactions, and natural lighting. When not filtered by an atmosphere, the high energy part of the spectrum (ultraviolet and above) is a hazard to humans and other biologicals, and may degrade other materials. Lack of sunlight either from nighttime shadowing on an object, or inside system elements, may require artificial lighting.
On objects without a significant atmosphere, designs in exposed locations should account for the natural flux of meteors. Long term exposure causes pitting, and in rare cases larger impacts can cause more severe damage.
Object surfaces can experience transient events such as earthquakes, vulcanism, heavy precipitation and flooding, high winds, dust storms, fire, and others. They can also experience seasonal and long term changes such as surface melting and terrain shifts. A system should account for the frequency and severity of such events either by design features or by financial insurance against low frequency events.
The space environment does not have new classes of parameters which do not exist for large objects, but the details of each parameter will be different:
Plasma and Atomic Species
Significant atmospheres are bound by gravity to massive objects, but they shade imperceptibly into the background medium that exists between distinct objects. The upper reaches of the Earth's atmosphere extend past the lowest stable orbits. Although thin, the conducting plasma and atomic species in this region can affect hardware. This group of particles are distinct from the radiation group by having lower energy, and thus unable to penetrate solid objects.
Due to the low density in the space environment it has low rates of heat conduction, and can therefore have very different internal temperature (defined by particle velocity) than that of objects embedded in it. For design purposes, system elements will mostly be affected by solar input, reflected light or shadowing by nearby objects, and heat loss to the cold cosmic background.
Since gravity operates by an inverse square force law, it never vanishes entirely, merely decreasing in strength with distance. For objects in orbit, gravity forces manifest in the shape of the orbit, which influence design in ways like varying communications distance, sun and shadow times, and travel times to reach a desired destination. Free orbit trajectories for the different parts of a system are nearly identical when the hardware elements are small relative to the distance of a massive object. Therefore the design needs to account for the lack of forces between the elements. Large structures will see differences in gravity, called Tides, and objects using propulsion or rotation will see artificial forces that act like gravity.
Space environments typically have higher levels of radiation than found on the Earth's surface. The sources include UV and particle radiation from the Sun and cosmic rays. Bodies with strong magnetic fields, such as the Earth and Jupiter, can trap particles and create Radiation Belts with particularly high levels. Radiation levels can vary significantly from short term events like Solar flares. See the radiation heading under the previous Object Environments section for additional design factors from radiation.
Light flux in the space environment has the same design influences noted previously under Object Environments. The differences have to do with it being unfiltered by any atmosphere, and a different or lack of day/night cycle, depending on location.
Meteor and Debris Flux
The space environment contains natural solid particles ranging from dust grains up to whatever size distinct tracked objects are (nominally 1 meter). In addition to the natural particles, human-made debris, defunct hardware, and still functioning hardware also exist. Designs need to account for random impact of small particles, and tracking and avoiding larger objects, or otherwise accounting for the risk of damage from such impacts.