AS-2 and 3 Student Notes (Chapter 9)
Space Systems Engineering
February 28, 2008
VOCABULARY (IN ALPHABETICAL ORDER)
Attitude and orbit control subsystem (AOCS) – The subsystem that adjusts a spacecraft’s orientation and orbital path. From sensor measurements, it determines current attitude and orbit, and then calculates any adjustments, if needed. Finally, it uses its small attitude rockets (also called resistojets) to make the adjustments.
Bus – All the supporting subsystems of a spacecraft that enable the payload to perform its mission.
Communication and data-handling subsystem (CDHS) – The subsystem that interacts with all of a spacecraft’s other subsystems, moving data efficiently to storage units or transmitters for downlink to ground stations. It consists of computers that collect, process, and store data, as well as radios to communicate with the outside world.
Concept of operations (CONOPS) – How the space mission elements (people, systems, etc.) will work together to meet the mission requirements. The ground-based and space-based elements must communicate precisely and cooperate to get the mission data to the users.
Constraints – Limits on what can be done during a space mission. The 3 most common constraints are:
- Budget constraints…how much money can be spent?
- Time constraints…how quickly must a mission be done?
- Performance constraints…how exactly or precisely must a mission be done?
Electrical power subsystem – The subsystem that includes any solar power collection panels (arrays) and rechargeable or non-rechargeable sources of electricity for a spacecraft.
Environmental control and life-support subsystem (ECLSS) – The subsystem that adjusts a spacecraft’s temperature, atmosphere, and other conditions to keep the payload healthy and happy. The payload may be human or it may be robotic but still have some environmental limitations.
Mission ground station – A control station on the ground for one spacecraft or for a network of satellites performing exactly the same mission or similar missions.
Need statement – Simple definition of what a space mission must accomplish.
Payload – The heart and soul of a spacecraft; the part that performs the actual mission.
Propulsion subsystem – The subsystem that boosts the spacecraft to the proper orbit to perform its mission. This subsystem often consists of a combination of solid- and liquid-fueled rockets to achieve an orbit, plus resistojets for minor course corrections while in orbit.
Structure and mechanisms subsystem – The subsystem that functions like the framework or chassis of a car or truck, to which all other subsystems are attached.
Systems engineering – A well-tested process developed by engineers to translate simply stated needs into complex systems. From the mission statement to the subsystem design, systems engineering keeps everything straight and in order.
Tradeoffs – Because each subsystem within a spacecraft’s bus (plus the payload itself) is represented by engineers, the design of any spacecraft involves some compromises, in an attempt to please all parties involved.
NOTES
Here are the 3 most important steps in the systems engineering process, in order:
- Define mission requirements first (for example, “get to the Moon before the Russians do”).
- Derive system requirements next (for example, “build a powerful enough rocket to transport a spacecraft large enough for 3 American astronauts to travel to the Moon and back to Earth safely”).
- Design subsystems last. The most common subsystems on a spacecraft are:
-- Attitude and orbit control subsystem (AOCS).
-- Communication and data-handling subsystem (CDHS).
-- Electrical power subsystem.
-- Environmental control and life-support system (ECLSS).
-- Propulsion subsystem.
-- Structural subsystem.
A spacecraft’s launch point and orbital path often provide hints regarding its mission. For example:
- Most imagery satellites are launched into a polar orbit from Vandenberg AFB in California.
- All crewed (manned) launches in the U.S. take place at the Kennedy Space Center in Florida.
Imagery satellites will generally have 2 opportunities per day to observe any target in the world.
- One will be during an ascending pass (while the satellite is headed toward the North Pole).
- The other will be during a descending pass (while the satellite is headed toward the South Pole, about 12 hours later).
