June 11, 2013

Spacecraft Systems Engineering

I've had little enough time to write or update for a while now for a number of reasons.  I'm currently teaching an aerospace structures intro class for a local college's aerospace technology program.  In addition to my summer job I'm using the break to work out new lesson plans, as I was less than satisfied with how class progressed the last two semester's.  I'm posting a version of my notes as I develop my lesson plans.  I hope this will help me work out what material I want to present, how best to present it, and idea how long each lecture will be.  I also hope that these notes will be of use to others, who may be interested in aerospace technology.

The first topic to cover in class is an introduction to systems engineering and an overview of spacecraft systems.  This first lecture will introduce concepts we'll refer back to throughout the class and describe the systems that we'll be looking at.
We start by defining the mission.  At the outset we have a mission objective statement, which defines the purpose of the mission and what the end deliverable will be to its users.  Users in this context refers to the people or groups who ultimately benefit from the mission, and generally from whom funding is obtained.  An example might be the users of the Iridium satellite communications system.  Users pay the company for the ability to communicate from anywhere, without much concern about how or why the system works.  The company builds and operates a satellite network to achieve this goal.
Mission analysis is a process to develop a mission plan - an outline of how to accomplish the mission - from the mission objectives.  We must first define the mission requirements and constraints.  Requirements are those objectives which must be achieved for the mission to be considered a success.  These are stated as positives, as what a component must be capable of doing.  Constraints are limitations on selection imposed by practical concerns and are stated as negatives, as what a component must not do.
Component and element are general terms for any item that are part of the spacecraft or mission architecture.  The terms used for specific levels of component:
  • part - hardware item that cannot be broken down. (e.g. resistor)
  • sub-assembly - functional subdivision of an assembly. (e.g. sun tracker)
  • assembly - group of items that performs a function. (solar array)
  • subsystem - collection of elements that perform a major system function. (power generation)
  • system  - group of smaller components that performs a mission objective. (electrical power system)
Once we define the mission requirements and constraints we can use these to begin selecting systems to accomplish our mission.  As we define the requirements and constraints for one system these will narrow our selections for another, resulting in what is called the requirements loop.
As identifying the system requirements and constraints are it becomes possible to select subsystems to accomplish those requirements within those constraints.  Each subsystem selected imposes a new set of requirements and constraints on another system.  This cycle is called the design loop.
Finally the subsystem selections refine the mission plan and impose new restrictions on the entire spacecraft.  This requires going back to the mission requirements and constraints to ensure that these are still met and that the decisions already made can fulfill the new requirements and constraints, called the validation loop.  The whole process is show graphically below:
The selection of subsystems is called a trade study.  Its steps, in order, are:
  1. identify alternatives - there will be multiple options for each component.  Each has its own benefits and drawbacks and these must be defined.
  2. selection criteria - determine what the component must do.  Establish what the criteria are that the alternatives will be judged by.
  3. establish weights - assign each of the selection criteria a weight in terms of its importance.
  4. utility functions - given to each attribute from 0 (for least desirable) to 1 (most desirable).
  5. compute benefits - multiply the weights by the utility functions and sum for each alternative.  That alternative with the highest benefit should be the best.
  6. assess sensitivities - check how sensitive the result is to small changes in the weights and utility functions before making a selection.
  7. select preferred alternative
There are 3 criteria which will always come up when selecting systems and subsystems for any mission: risk, cost, and performance.  Ideally each component will be reliable, inexpensive, and efficient.  In reality the best we can usually hope for is two of the three.  Increasing performance means either an increase in cost or risk, if not both.
As the design is refined the individual subsystem requirements are documented in an Interface Requirements Document (IRD), which defines precisely the requirements and constraints placed on each subsystem.  All the components selected must interact together correctly.  They must not only physically fit together, but must work together.  How all the components fit and work together is detailed in an Interface Control Document (ICD) which defines the physical, environmental, functional, operational, and procedural interfaces between components.

Now that we've considered how systems are selected and why we'll look at what the systems are that are to be selected.  Part of how we'll describe systems is in terms of budgets.  In addition to the overall budget for the mission - how much money we have to get a spacecraft in orbit and operate it - there are other budgets for most systems.  These budgets represent the limited capabilities of each system, so as the mission goes on this budget can be expended.
  • payload - accomplishes mission specific tasks and varies greatly between missions.
  • attitude and orbit control (momentum budget) - determines the spacecraft location and orientation and ensures these match mission requirements.
  • communication (link budget) - relays information to ground support and commands to systems.
  • data handling (memory budget) - interprets data collected by spacecraft sensors and instructions sent by ground support. 
  • electrical power (power budget) - provides power to spacecraft systems.
  • environmental control and life support - ensures components remain within operational temperature range.  For manned missions life support systems provide all necessary requirements for human life.
  • structures - supports components and provides spacecraft stability.
  • propulsion (delta-v budget) - provides thrust to enter mission orbit and alter spacecraft orbit as necessary.
All of these together add to the mass budget.  Since each kilogram lifted into orbit has a direct launch cost from fuel there is a maximum amount of mass that can be included in any spacecraft.  Each additional component subtracts from this mass budget, limiting how much can be added to other systems.

Many of these systems are a form of control system.  Control systems come in two forms: open loop and closed loop.  Open loop control systems operate continuously and approach a natural equilibrium.  In this case the body being acted on, called the plant, is put in a situation where it naturally will approach the desired state.  While this is desirable for simplicity, there is no way to modify the system if requirements change.
Closed loop, or feedback, controllers include a sensor which monitors the plant.  The actuator, which modifies the plant in some way, does not operate continuously, but rather based on the current state of the plant.  This allows the controller to keep the plant close to the desired state.  The obvious limitation is an increase in complexity, cost, and weight.
A simple example is the heating system in a house.  An open loop solution is a fireplace.  Heat from the fire heats the room, which in turn loses some heat to outside.  If the fire stays the same temperature eventually an equilibrium is reached.  But there's little control except to add more fuel to the fire or not.  A closed loop system is more like a thermostat.  It decides, based on the difference between room temperature and desired temperature, at what level the heater should be run.  As the room heats up is reduces the amount of heat it adds, until it is providing just enough to stay at the desired temperature.  Though more complicated than the fireplace, it allows for much finer control.

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