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Question 1

L1 - State Kepler’s three laws of orbital motion

Answer 1

1. Kepler’s First Law:
   The orbits of the planets are ellipses with the Sun at one focus
2. Kepler’s Second Law:
   The line joining a planet to the Sun sweeps out equal areas in equal times
3. Kepler’s Third Law:
   The square of the orbital period (how long it takes the planet to go around the Sun) is directly proportional to the cube of the mean distance between the Sun and the planet.

Question 2

L1 - State Newton’s law of gravitation

Answer 2

Newton’s Law of Universal Gravitation:

Any two bodies attract each other with a force proportional to the product of their masses and inversely proportional to the square distance between them

Question 3

L1 - Describe the restricted two-body problem

Answer 3

Restricted Two-Body Problem:

If there are only two bodies which have constant mass and are both exactly spherical, then the small one will follow a trajectory which is a conic section - a circle, ellipse, parabola or hyperbola.

If we apply Newton’s laws where:

• There are only two bodies
• The bodies have constant mass
• The bodies are exactly spherical

We find a small body will fly around a big one on a trajectory which is one of:

• Circle; e = 0
• Ellipse; 0 < e < 1
• Parabola; e = 1
• Hyperbola; e > 1

Eccentricity (e) is a measure of how non-circular the orbit/trajectory is.

Key Facts:

• A satellite rotates around the attracting body in a plane.
• The plane passes through the centre of mass of the attracting body
• A satellite moves along a circle, ellipse, parabola or hyperbola
• The size of the orbit is fixed

Question 4

L1 - Describe the geocentric equatorial (ECI) co-ordinate system - PART 1

Answer 4

For Earth orbiting spacecraft we use the geocentric equatorial (ECI) system:

• Origin - centre of the Earth (geocentric).
• Fundamental plane - Earth’s equator (equatorial), soperpendicular plane is in the North Pole direction.
• Principal direction - vernal equinox/first point of Aries direction, ^.
• Third axis - found using right-hand rule

(For Sun-orbiting spacecraft we use the heliocentric ecliptic system)

Question 5

L1 - Describe the geocentric equatorial (ECI) coordinate system - PART 2
But what is the vernal equinox/FPA?

Answer 5

• The vernal equinox/first point of Aries (FPA) is the point in the Sky at which the Sun appears to be on the first day of spring (March 21) when the night and day have the same length.
  
  • Vernal: spring
  • Equinox: equal night
• This is also where the ecliptic (the plane defined by the Earth’s orbit) and the celestial equator (plane of the Sun) cross.
• The FPA moves about 1 degree every 70 years. It is no longer in Aries but in Pisces.

Question 6

L1 - Explain different general types of orbit

Answer 6

Circular Orbit:

the smaller body has a close orbit around the bigger body, the altitude at the apoapsis equals the one of the periapsis and the eccentricity equals zero.
E.g.: a body in a circular orbit around Earth at 222 km altitude must have a velocity of 7,77 km/s

Excentricity: e = 0

Elliptical Orbit:

the smaller body has a close orbit around the bigger body, the altitude of the apoapsis is greater than the one of the periapsis and the eccentricity is greater than zero and smaller than one.

Excentricity: 0 < e < 1

Parabola Orbit:

the smaller body has an open orbit with respect to the bigger body, it moves upon the minimum escape trajectory and the eccentricity is strictly equal to one.

Excentricity: e = 1

Hyperbola Orbit:

the smaller body has an open orbit with respect to the bigger body, it moves upon an escape trajectory and the eccentricity is greater than one.

Excentricity: e > 1

ISS - Low Earth Orbit

• h = 400 km
• V = 7670 m/s
• T = 92 min

GPS - Medium Earth Orbit

• h = 20,000 km
• V = 3890 m/s
• T = 12 hr

Geosynchronous Orbit

• h = 35800 km
• V = 3070 m/s
• T = 24 hr

Inclination can vary between 0o and 180o and is used to classify a variety of
different sorts of orbit:

• Equatorial orbits i = 0^°; i =180^°
• Polar orbit i = 90^°
• Direct/prograde orbit (moves in directions or Earth’s rotation) i
• Indirect/Retrograde orbit (moves opposite to the direction of Earth’s rotation)

Question 7

L1 - Draw and label an orbit diagram

Answer 7

Question 8

L1 - Use the Classical Orbital Elements (COEs) to define orbits

Answer 8

The orbit of a spacecraft is usually described using the Classical Orbital Elements:

(position)

• Size - Semimajor axis, a
• Shape - Eccentricity, e
• Position - True anomaly, ע

(velocity)

• Inclination, I
• Right Ascension of ascending node, omega
• Argument of perigee, ω

Question 9

L1R - Define Orbital Velocity

Answer 9

Orbital velocity is constant only in circular orbits. In elliptical orbits it is greatest at perigee and smallest at apogee

Question 10

L1R - Circular orbit period and velocity

Answer 10

For circular orbits, the period increases with altitude while the velocity decreases.

Question 11

L1R - Co-ordinate Systems

Answer 11

• Earth orbiting - geocentric equatorial (ECI) system.
• Sun-orbiting - heliocentric ecliptic system.

Question 12

L1R - Orbital Elements: What do we need to know to describe the motion of a satellite?

Answer 12

Six pieces of information: x,y,z (position) and x’,y’,z’ (velocity)

• Not very user-friendly though, as these all change all the time.
• Instead we use the six ‘orbital elements’ orginally created by Kepler.
• These are often known as Keplerian elements or Classical Orbital Elements (COEs).
• There are many other formulations of orbital elements, e.g. the equinoctial orbital elements.

Question 13

L2 - Explain what is 3-level model of Space systems Engineering

Answer 13

1. Program Level – The uppermost level of the hierarchy. The individual who understands the highest-level interrelationships (including missions, politics, national relations, etc.) can serve in several positions - Space Program Architect, Program Manager, Administrator, Minister of Space…
2. Mission Level – The individual who understands the detailed interrelationship among the many vehicle and operational components that make up a mission is the “Mission Designer”
3. Spacecraft Level - The individual who understands the detailed interrelationship among the many subsystems which make up a spacecraft is often designated the “Spacecraft Systems Engineer”

Question 14

L2 - Understand program definition

Answer 14

Program definition can be a very simple statement with task(s) to be performed with all necessary information to begin work:

• Budgetary envelope
• Time frame
• Task to be performed

Example: the Apollo Program

1. Time frame = before the end of the 60’s
2. Budgetary framework = “no other space mission will be so expensive to accomplish”
3. Task = Land a man on the moon and return him safely to Earth
   Ø Apollo and Gemini projects resulted
   Ø Produced the Saturn Launch Vehicles
   Ø Created the lunar orbiter and lander project
4. Task was achieved in July 1969 (budget was largely overrun)

Program Definition

1. It can be very complex with hundreds pages;
2. Technical specification and statement of works:

• With different mission statements and all details of mission definition
• Requirements on mission design;
• Large document (hundreds of pages);
• Internally consistent with Technical Specification and Design Specification documents and statement of works with different mission

Space Program Level:

The Program level may integrate multiple
missions along with:

• Internal Agency Strategic Plans
• National Policy Issues
• International Space Law
• International Partnership Agreements

The ideal architect should be a man of letters, a skilled draftsman, familiar with historical studies, a diligent student of philosophy, acquainted with music; not ignorant of medicine, learned in the responsibilities of jurisconsults, familiar with astronomy and astronomical calculations. - Vitruvius, 25 B.C.

Space Program Architecture:

1. Program Architecture: The topmost structure, arrangement, configuration of system elements and relationships required to satisfy both constraints and requirements
2. In the 3-level Model, we characterize this highest level of integration as the Space Program Level
3. Space program may contain several space missions.

Question 15

L2 - Understand Space mission characteristics

Answer 15

Space mission:

Space mission refers to human activity for the exploration, development and utilization of space and celestial resources outside the Earth, which is an engineering project that uses space technologies to achieve specific goals and covers a wide range of areas.

Space and Celestial Resources:
The vast expanse of space has unique resources not available on Earth, such as
high and remote positions, high vacuum, ultra low temperature, microgravity,
ultra cleanliness, electromagnetic spectrum, solar power and celestial bodies.

Mission Design Level

1. The Space Mission is an integration of
   many systems for:

• Spacecraft(s)
• Launch Vehicle(s)
• Orbital Mechanics
• Communications Infrastructure
• Operations

2. One Mission may contain several
   spacecrafts

Mission Example:

Chang’e 4

Mission definition:

The Chang’e 4 is a mission of Chinese Lunar Exploration program (2nd phase) for the soft-landing and robotic exploration on the far side of the moon.

Mission configuration is comprised of a lander, a rover and a relay satellite:

• Lander and rover are developed from mature Chang’e 3 and adaptively modified according to far-side terrain and relay communication conditions and the operation at moon night.
• A data relay satellite, launched 6 months ahead of the lander and the rover

Features of Space Missions:

• Major space missions basically come from the national or commercial space projects;
• Space missions require huge investments and are highly risky;
• Spacecrafts are normally not repairable and have to be adapted to harsh space environment;
• Space missions have high returns, especially from various application satellites such as communication satellites, navigation satellites, and earth observation satellites

Question 16

L2 - Describe space system breakdown

Answer 16

Space system: is a system integration which consists of spacecraft; space transportation system; launching site; ground operation and control system(TT&C* network); recovery and landing site; astronaut system(for manned missions) and application system to complete specific space missions.

*Telemetry, Tracking, and Command

Space System

1. For space mission, Space system is a system integration of following Systems:
   
   • Spacecraft system and/or Space Infrastructure; (ENG-L04)
   • Space transportation system; (ENG-L08)
   • Spaceports (Launching, recovery and landing); (ENG-L13)
   • Ground operation and control system (TT&C network);
   • Astronaut system (only for manned missions);
   • Space application system to complete specific space missions.
2. According to the general requirements and interface control documents,The systems work together to complete the mission objectives.
3. It is essential to optimize all the systems with restrictions.

Space System Design

System design brings together many design elements to create a successful, end-to-end (pre-launch to payload disposal) space mission. See the 5 areas below:

• Payload(s)
• Science instruments / Human crews
• Spacecraft
• Transportation System
• Launch vehicles and upper stages/Cruise stages/Entry vehicles and landers
• Orbit and Flight Mechanics Planetary and transfer orbit selection
• Mission Operation

Communications infrastructure / Mission control facilities Success can be measured by the accomplishment of all the mission-level requirements within the mission constraints.

The core of space system design is to provide the optimal system solution and product under specified constraints including timeline, funding etc.

• Meet the user’s requirements of functions and performance, schedule and cost constraint;
• Carry out system optimization to prevent the local high performance;
• Implement step by step in accordance with the design procedure of the spacecraft;
• The design effort accounts for less than 15% of the life cycle cost of the system, but it decides more than 85% of the life cycle cost of the system.

Space System Design Content

The main tasks of system design :

1. Analysis and breakdown of mission objectives and system requirements;
2. Selection and analysis of payload;
3. Selection and analysis of spacecraft orbit; definition of constellation and
4. Analysis and preliminary design of spacecraft platform ;
5. Selection and analysis of launch vehicle and launch site; analysis of TT&C and operation during launch and in-orbit operation; determination of interface between spacecraft and other system segments;
6. Analysis and estimation of cost and schedule.

Space engineering requirements and trade-off study are the basic methods for system design. (ENG L06 and L10)

Question 17

L2 - Describe how space projects are broken down into major phases

Answer 17

A space mission could be broken down into 7 phases:

• Phase 0: Mission analysis/needs identification
• Phase A: Feasibility study
• Phase B: Preliminary Design
• Phase C: Critical Design
• Phase D: Development, Manufacture, Integration and Test
• Phase E: Mission Operations and Data Analysis
• Phase F: End of File and disposal

Phase 0 and A:

1. Development of system functional and technical requirements;
2. Identification of system concepts;
3. Selection of an optimum system;
4. Demonstration of the feasibility of the project by design and analysis (assessments of technical and programmatic risk);
5. Initiation of pre-development activities: Definition of a technical solution to the level necessary to produce realistic date for later stages.

Phase B:

1. Definition of the system and subsystem designs in enough detail to allow Phase C/D to proceed smoothly.
2. Production of subsystem requirements, design specifications, development plans, programs and proposals.
3. Initiation of advanced C/D activities such as ordering of long lead-time items

Phase C/D

1. Completion of all designs and analyses.
2. Preparation of drawings and procedures
3. Completion of all development and qualification testing.
4. Manufacture of flight hardware;
5. Finish Assembly, Integration and Testing.

Phase E/F

1. Launch campaign
2. In-orbit operations
3. End of Life operations

• Disposal into graveyard orbit
• Disposal into atmosphere

This phase usually accounts for most of the cost of the mission – an important factor in optimization.

Question 18

L2 - List important tasks that a system engineer carries out in a project team

Answer 18

Basic Requirements:

The tasks of mission design normally are done under the leadership of the systems engineer with team efforts.

• Systems engineer must have a general knowledge of space system engineering;
• Systems engineer should be expert of the details of mission element;
• Systems engineer should have a rich expertise of interaction with users and good skill for team and engineering management.

Key Tasks

1. Key tasks for the system engineer in early phases include:

• Requirements identification/analysis
• Options identification and trade-offs
• Mission assessments
• Feasibility assessments
• Cost comparisons
• Concept selection

2. These tasks continue through later phases and include:

• System optimization
• System and subsystem specification and definition
• Performance analyses
• Budget allocation/management
• Interface specification
• Support to assembly, integration and test
• Support to mission operations

Question 19

L2 - Explain the term ‘system budget’ and list important parameters for which budgets are prepared.

Answer 19

About Budgets:

Budgets are used to ensure that all the elements are considered.

System Budgets

• A budget is a quantitative list of a system parameter.
• The first-order evaluation is always based on mass, then

Safety and reliability budgets:

• Reliability budget
• Crew Safety budget
• Specialized risk analyses
• Probability of mission success

Financial Budgets:

• Mass can be used to calculate first-order costs (Cost Estimating Relationships)
• Launch vehicle costs are known at most of the cases.

Understand System Budgets:

Major budgets at the spacecraft system engineering level:

• Mass and mass properties;
• Delta-V and associated propellant quantity;
• Power and energy;
• Pointing accuracy, alignment and stability rate;
• Data rate and storage capacity;
• Communication link budget;
• Reliability and Safety (mainly for Human mission);
• Cost

Question 20

L3 - Characterise common Earth applications orbits

Answer 20

• Low-Earth orbit - LEO:
  
  • 180 – 3000 km eg//ISS
• Medium Earth orbit - MEO:
  
  • between LEO and GEO eg//GPS
• Geostationary orbit - GEO:
  
  • period of ~24 hours (altitude 35790 km) and inclination of ~00 (in the plane of the equator). This is where the money is made! ITU regulates slots.
  • (example – operational telecom satellites)
• Geostationary transfer orbit - GTO:
  
  • an inclined elliptical orbit connecting LEO and GEO used to transfer communications satellites to their operational orbits in GEO.
• Sun-synchronous orbit - SSO:
  
  • a polar orbit with permanent stable orientation of the orbital plane in respect to the Sun (more on this later) (example – SPOT orbit is ~825 km, i = 98.70)
• Highly elliptical orbit - HEO):
  
  • an inclined orbit with an eccentricity e>0.2 (example – space observatory ISO: orbit 1000 ́ 70500 km).
  • Mainly used for astronomical craft.
• Molniya orbit:
  
  • high elliptical orbit (500 ́ 35790 km) with a period of ~12 hours and inclination of 630 (more on this later) (example – Molniya 1 satellite).
  • Used for places far north or south on the planet.Eg//Russia

Question 21

L3 - Interpret orbits based on spacecraft groundtracks

Answer 21

Ground Track

• The path of the satellite traces on the Earth’s surface as it orbits
• From the ground track, you can measure many interesting things:
  
  • Orbital period - by measuring the westward shift of the ground track.
  • Inclination of a satellite’s orbit - by looking at the highest latitude reached on the ground track (for direct orbits).
  • Approximate eccentricity of the orbit - nearly circular orbits appear to be symmetric, whereas eccentric orbits appear lopsided.
  • Location of perigee - by looking at the point where the ground track is spread out the most.

MORE:
The six COEs (classical orbital elements) allow us to visualise orbits in space, but it is important to be able visualise how orbits are seen from/on the Earth.

A ground track is the trace of the path that the satellite takes over the Earth’s surface.

All ground tracks are great circles, because the satellite orbits the centre of the Earth and so the orbit plane must also pass through this point.

• Great circle - any circle which ‘slices through’ the centre of the Earth, e.g. lines of longitude.

The main complexity when interpreting ground tracks arises from the fact that:

• The satellite orbits about the Earth
• The Earth rotates

Each time the satellite completes one orbit, the Earth has turned eastward underneath it, so the ground track shifts to the west.

See Image

We can determine useful information from this shift:

• We know that the Earth rotates at 15^o/hour.
• We can measure the node displacement, ΔN, the shift in the track from one orbit to the next.
• From this we can calculate the orbit period and then the semimajor axis.
• As orbit size increases, the semimajor axis get bigger so ΔN gets smaller.
• This is because the satellite takes longer to make one revolution as the Earth rotates underneath it.
• When the period of the orbit is twenty-four hours, the orbit appears to perform a figure of eight and ΔN is zero.
• Geostationary orbit just looks like a spot.

We can also determine the orbit inclination by looking at the ground track.

• Direct orbit (0o < i < 90o) - inclination is given by highest latitude.
• Retrograde orbit (90o < i < 180o) - inclination is given 180o less highest latitude.

Inclination is very important in deciding on coverage patterns for satellites.

SEE image(in Slide p19)

All these orbits have a period of 4 hours, but they have different inclinations.

This can be seen by looking at the maximum latitude of their respective tracks.

Elliptical orbits ones are not symmetrical - they look different above and below the equator.

SEE image(in Slide p21)

Both orbits have P = 11.3 hours, i = 50o and e = 0.5.

However, the yellow has its perigee over the northern hemisphere, and red over the southern hemisphere.

Question 22

L3 - Explain the major perturbations to Earth-orbiting satellites and their implications for the design of space missions

Answer 22

In the real life, a pure Keplerian orbit can never exists because of perturbations by forces other than central body gravity.

Peturbations:

Disturbing forces that act on a spacecraft to alter its orbit in some way

The primary forces perturbing the orbit are:

• Atmospheric drag - Drag acts in the direction opposite to velocity vector, used for aerobrakeing.
• Non-spherical mass distribution of the planet (Earth) - effects on the node and perigee positions have two very practical applications: 1) Sun-synchronous orbit, and 2) Molniya orbit
• Third body gravity (Sun, Moon, planets, etc.) - GEO orbits do not station in a point but in a bow.
• Pressure of solar radiation
• Solar wind
• Earth’s magnetic field
• Unexpected thrust from outgassing

Lead to:

• Need for drag make-up (LEO)
• Station-keeping (GEO)
• Sun-synchronous orbits (useful)

Question 23

L4 - List and describe 5 different types of spacecraft

Answer 23

A spacecraft is composed of Spacecraft Bus (or platform) and Payload.

1. Telecommunication satellite
2. Navigation satellite
3. Earth Observation satellite
4. Space Science Spacecraft
5. Human Space Spacecraft
6. Earth observation

• Telescopes, radiometers, radars, spectrometers, IR sensors
• Typical missions: Landsat, Envisat, Hysat, RadarSat, Spot.

2. Telecommunication

• Mainly transponder with different frequencies, also Laser terminals, etc.;
• Typical missions: Hotbirds, Intelsat, DFH satellites.

3. Navigation

• Tracking and ranging hardware;
• -ypical missions: GPS, Galileo, Glonass and Baidou, etc.

4. Human Spaceflight

• Spaceship and crew;
• Typical missions: Vostok, Apollo, Shenzhou

5. Science mission:

• Payload directly links with mission objective
• Typical missions: Hubble Telescope, Mar express, Voyager-1…

Question 24

L4 - List the main subsystems that make up a spacecraft

Answer 24

Main subsystems of Platform:

1. Structure and mechanism;
2. Power supply;
3. Attitude and orbit control (including spacecraft propulsion);
4. Communication (TT&C);
5. Data handling (Data management);
6. Thermal control.

Satellite platform is subsystems of Spacecraft to provide supporting functions necessary to make the payload work in all phases (launching, in-orbit operation and disposal).

Question 25

L4 - Describe the two most common spacecraft configurations

Answer 25

• Robotic spacecraft
• Human spacecraft

1. For a few types of mission some relatively ‘standard’ configurations have emerged, e.g.

Astronomical observatory-type satellites, inertially pointed

• Telescope feeding into instrument module
• Specific configurations are still mainly very different

LEO Remote sensing

• 3-axis stabilized; nadir pointed (often with a ‘single-sided’ solar array)

GEO telecommunication and meteorology

• Spin stabilized (spin axis normal to orbit plane)
• 3-axis stabilized (solar array along N-S axis)

2. For most others, configurations are mission-specific.

Question 26

L4 -Explain the main design “drivers” that influence the design of a spacecraft

Answer 26

Spacecraft Configuration Drivers

1. Payload is the fundamental design driver:

• Number of instruments or equipment;
• Instrument mass, size, power, field of view, and data rate;
• Required orbit (LEO, GEO, HEO, deep-space, or others);
• Trajectory and orbit control requirements;
• Pointing/attitude control requirements.

2. Spacecraft platform is secondary:
   * It exists to provide for payload and mission objectives in terms of mechanical support, thermal control, power, data handling, and other functions。

Question 27

L4 -Describe the principle requirement for, and types of, spacecraft structure

Answer 27

Requirements:

Structure Provides:

• shape
• strength & support
• protection mounting surface
• attachment points
• relative movement

1. Strength: the amount of load a structure can carry without fairing;
2. Stiffness: a measure of the load required to cause a given deflection;
3. Structural Stability: structure’s ability to maintain its configuration – ability to resist buckling
4. Structure size: jointly determined by the strength, stiffness, and stability requirement;
5. Structure life: number of loading cycles a structure can withstand before failure;
6. Damping: the dissipation of energy during vibration.

Types:

Primary Structure – Load bearing

• Spacecraft Frame or Skeleton
• Comprises most structural mass

Secondary Structure – no-Load bearing

• Brackets, panels, deployable appendages
• Not as heavy as primary structures in general

Question 28

L4 - Describe the basic types and functions of the spacecraft propulsion system

Answer 28

From L06

Propulsion Systems can be classified by the type of energy source.

Chemical Propulsion

The energy to produce thrust is stored in the propellant, which is released by chemical reactions and then the propellant is accelerated to a high velocity by expanding it on the form of a gas through a
nozzle.

Electrical Propulsion

The energy to produce thrust is supplied from outside by an extra power source (solar panels, batteries or nuclear sources).
Thrust is produced by:

• the expansion of hot gas heated by electrical current and then passed through a nozzle;
• accelerating charged particles in electric or magnetic fields to high expulsion velocities

Nuclear Propulsion

• *Functions of Space propulsion systems -** generate thrust:
  1. to get spacecrafts into space;
  2. to move them around in their orbit;
  3. to change their attitude (the direction they are pointing);
  4. to de-orbit them at the end of their life.

Question 29

L4 - Describe the spacecraft electrical power system functions

Answer 29

Electrical Power Systems Functions

1. Power is a critical subsystem: no power = no mission;
2. Generate the required power for spacecraft in the duration of the mission;
3. Provide for Energy Storage (e.g., rechargeable batteries);
4. Regulate, control and distribute the power to all subsystems onboard:

• Support power requirements for average and peak electrical loads
• Provide transformation DC to AC, if required
• Protect against power supply faults (redundancy)

Question 30

L4 - Describe the spacecraft thermal control system functions

Answer 30

Controlling the level of temperature of equipment, payloads on-board satellites is essential during all phases of a space mission to protect flight hardware and to guarantee the optimum performance and success of the mission.

Importance of Thermal Control

1. Keep spacecraft survival in the temperature extremes, e.g. from -200 oC to + 150 oC to in Lower Earth Orbit.
2. Matters are made worse by temperature gradients across the spacecraft between parts in sunlight and parts in shadow;
3. The spacecraft will also move into and out of eclipse, creating large changes in temperature across the entire spacecraft;
4. Thermal Control System maintains all the elements of the spacecraft within their acceptable temperature ranges for all the Mission Phases
5. Thermally Critical Components have a very narrow range of acceptable temperatures

✓ Components internal to the Spacecraft

✓ Propellants like hydrazine

Thermal Control Method

1. Passive thermal control devices
   - No moving parts, use surface coatings (paint, mirrors, reflectors, multi-layer insulation, etc.) to keep the desired surface temperature
2. Active thermal control devices
   - Use mechanical pumps, thermal louvers, heaters, thermostats, heat pipes, fans
3. Hybrid thermal control devices
   - Use reasonable composition of passive and active thermal control elements

Passive thermal control relies on:

1. Radiators: radiate heat into deep space;
2. Heat sinks: absorb heat due without temperature change i.e. high specific heat;
3. Heat pipes: phase-change absorption;
4. Thermal isolation: thermal decoupling of elements;
5. Insulation, e.g. MLI (Multi-Layer Insulation);
6. Surface coatings.

Active thermal control relies on:

1. Heaters: to keep shaded components warm (e.g. mechanisms)
2. Louvers: blades that open and close to change the heat ejection from a radiator
3. Fluid loops:

• Single-phase loop uses liquid to transfer heat;
• -apillary-pumped loop uses a two-phase system using a temperature gradient to drive the fluid.

Question 31

L4 - Identify the two main categories of attitude control

Answer 31

Attitude and Orbit Control System (AOCS)

Two main types of attitude control:

1. Spin stabilization is accomplished by setting the spacecraft spinning, using the gyroscopic action of the rotating spacecraft mass as the stabilizing mechanism.
2. Three-axis stabilization is a method of spacecraft attitude control in which the spacecraft is held fixed in the desired orientation without any rotation.

Question 32

L4 - Describe the basic functions of the communications, control, and data handling system

Answer 32

Communications - Telemetry, Tracking & Command (TT&C)

TT&C subsystem provide communication for ground-to-space link to handle external communications for satellite.

• Receive commands (uplink)and transmit data (downlink);
• Links may be direct spacecraft and ground station or through data relay satellites;
• The subsystem includes antennas, receivers, transmitters and amplifiers.

Spacecrafts usually have following antennas:

• Omnidirectional antenna for emergency and prior to deploying the main antenna;
• High gain antenna for main communications.

Command and Data Handling subsystem (C&DH)

= essentially the “brains” of spacecraft and controls all spacecraft functions by using Space Flight Computer and Flight Software

The main function of C&DH subsystem in all the mission Operations Phases:

1. manages all forms of data on the spacecraft;
2. carries out commands sent from Earth;
3. prepares data for transmission to Earth;
4. manages collection of solar power and charging of the batteries;
5. collects and processes information about all subsystems and payloads;
6. keeps and distributes the spacecraft time;
7. calculates the spacecraft’s position and attitude;
8. carries out commanded maneuvers; and,
9. autonomously monitors and responds to a wide range of onboard problems that might occur.

Question 33

L4 - Understand spacecraft configuration drivers

Answer 33

Spacecraft Configuration is an overall arrangement of payload and subsystems inside the spacecraft body as well as an external architecture of the spacecraft.

Key word: Compromise

Spacecraft Configuration Drivers

1. Payload is the fundamental design driver:
   - Number of instruments or equipment;
   - Instrument mass, size, power, field of view, and data rate;
   - Required orbit (LEO, GEO, HEO, deep-space, or others);
   - Trajectory and orbit control requirements;
   - Pointing/attitude control requirements.
2. Spacecraft platform is secondary:
   - It exists to provide for payload and mission objectives in terms of mechanical support, thermal control, power, data handling, and other functions。

Common Classes of Configuration

1. For a few types of mission some relatively ‘standard’ configurations have
   emerged, e.g.

• Astronomical observatory-type satellites, inertially pointed
  
  • Telescope feeding into instrument module
  • Specific configurations are still mainly very different
• LEO Remote sensing
  
  • 3-axis stabilized; nadir pointed (often with a ‘single-sided’ solar array)
• GEO telecommunication and meteorology
  
  • Spin stabilized (spin axis normal to orbit plane)
  • 3-axis stabilized (solar array along N-S axis)

2. For most others, configurations are mission-specific.

Payload and Instrument Requirements

1. Payload requirements may include location, pointing accuracy, temperature, magnetic field, radiation, field of view;
2. Payloads may also need a specific location on the Spacecraft to meet all the requirements;
3. Pointing accuracy requirements can drive configuration very heavily may require extreme rigidity temp. stability to minimize distortion.

These requirements will dictate structural
design, material choice and configuration.

Launch vehicle constraints:

• Mass and Dimensions
• Launching Environment (Vibration, Acoustics and shock)
• Safety (esp. Human flight)

1. The biggest constraint is that of launch mass capability;
2. Next constraint is the spacecraft dimension by fairing envelope.

Question 34

L5 - Explain what a space propulsion system is for

Answer 34

Space propulsion systems generate thrust:

1. to get spacecraft into space;
2. to move them around in their orbit;
3. to change their attitude (the direction they are pointing);
4. to de-orbit them at the end of their life.

The most common form is the rocket, there are also solar sails, space
tethers, others means of propulsion.)

Question 35

L5 - Explain how a rocket works

Answer 35

A rocket is a system that takes mass (propellant) and energy and converts them into force (thrust) to move a vehicle.

• Energy is used to accelerate (V -> V + ΔV) the mass away from the rocket
• Thrust is produced by conservation of the linear momentum.

1. A rocket ejects propellant at a rate called the mass flow rate
2. The (ideal) thrust of the rocket is given by:
3. This means that the thrust can be increased by:
   
   1. Increasing the mass flow rate
   2. Increasing the exhaust velocity
4. The total time that the rocket operates for is given by the burn time, tb.
5. The total mass of propellant used, Mprop, is: Mprop =M tb

The Rocket Equation was first derived by the Konstantin Tsiolkovsky (also called the Tsiolkovsky Equation)

The law of conservation of momentum (or the law of conservation of linear momentum) states that the momentum of an isolated system remains constant. Momentum is therefore said to be conserved over time;[1] that is, momentum is neither created nor destroyed, only transformed or transferred from one form to another.

Question 36

L5 - Describe different types of space propulsion system and their uses PART 1/4 - Overview

Answer 36

1. Chemical Propulsion

1. 1 Liquid Propulsion
2. 2 Solid Propulsion
3. 3 Hybrid Propulsion

2. Electrical Propulsion

2.1 EP Power Sources

• Nuclear Fission
• Solar

2. 2 Electrothermal Propulsion
3. 3 Electrostatic Propulsion
4. 4 Electromagnetic Propulsion

3. Nuclear Propulsion

3. 1 Continuous nuclear fission propulsion
4. 2 Pulsed nuclear fission propulsion

Question 37

L5 - Describe different types of space propulsion system and their uses PART 2/4 - Chemical

Answer 37

Chemical propulsion is propulsion in which the thrust is provided by the product of a chemical reaction, usually burning (or oxidizing) a fuel.

Chemical propulsion is the most common form of rocket is the chemical thermodynamic rocket:

• chemicals react to generate heat and by-products;
• combustion products are accelerated through a nozzle, generating thrust.

There are three main sorts of chemical rocket:

i. Liquid propulsion

For liquid propulsion, propellants are stored in liquid phase, and there are two basic types:

• Bipropellant – 2 propellants
• Monopropellant – 1 propellant

Bipropellant – fuel and oxidizer, react together to produce heat and exhaust products

• Higher performance
• Ve up to 5 km/s
• Complex systems with pressure fed or pump fed

ii. Solid propulsion

• For solid propulsion, the fuel and oxidiser are mixed with a binder and cured in the combustion chamber with the required grain geometry.
• Typically aluminium powder (fuel), ammonium perchlorate (oxidiser) and Hydroxyl-Terminated Polybutadiene (HTPB binder) + catalyst.
  
  • Solid rockets burn until all the propellant is gone, the way to control thrust over time is through grain geometry variation (surface area)；
  • Designers choose different burn rates, e.g. thrust/time curves by using different geometries;
  • Simple, storable, a bit lower performance (Ve up to 4 km/s).
• Solid rockets are used for:
• Core stages and boosters on launch vehicles;
• Upper stages for putting satellites into transfer orbits;
• Escape tower for human mission;
• Most of Missiles.

iii. Hybrid propulsion

• Hybrids combine aspects of liquid and solid systems by using a liquid oxidiser and a solid fuel
• Typically Nitrous Oxide and a rubber-based fuel (e.g. HTPB)
• A moulded fuel grain forms the combustion chamber and the oxidizer is injected into it
• Offer simplicity & safety and throttling & restart capabilities
• Scaled Composites’ SpaceShipOne was propelled by a hybrid motor as with Virgin Galactic’s SpaceShipTwo

Question 38

L5 - Describe different types of space propulsion system and their uses PART 3/4 - Electrical

Answer 38

Electric propulsion (EP) uses electricity as the energy sources to accelerate a propellant.

There are three main types of EP (Each of these has a number of sub-types):

1. Electrothermal (electrothermodynamic)

Electrothermal thrusters are the simplest form of electric thrusters.

• Resistojets produce thrust through heating a propellant by passing it over a resistively heated element;
• Arcjet produces an electrical discharge (arc) in a flow of propellant in a form of gas.

2. Electrostatic

An electrostatic propulsion uses electrical energy to accelerate charged ions, and it is also called an ion propulsion system.

There are three main types of electrostatic thruster:

• Gridded Ion Thruster (GIT)
• Hall-effect Thruster (HET)
• Field Emission Electric Propulsion (FEEP)

3. Electromagnetic (electrodynamic)

Electromagnetic (EM) thrusters make use of the Lorentz force that arises from the interaction between orthogonal currents and magnetic fields.

There are two main types of EM thruster and they fulfill their function in different ways:

• Magnetoplasmadynamic (MPD) thrusters use radial currents and azimuthal magnetic fields,
• Pulsed Inductive Thrusters (PITs) use azimuthal currents and radial magnetic fields.

Question 39

L5 - Describe different types of space propulsion system and their uses PART 4/4 - Nuclear

Answer 39

Nuclear propulsion uses a nuclear power source to provide energy to heat a propellant.

1. Continuous nuclear propulsion uses a nuclear energy source to heat the propellant which is then expanded through a nozzle (nuclear thermal/ thermodynamic propulsion).
2. Pulsed nuclear propulsion uses ‘pellets’ of material which are the source of both the mass and the energy for propulsion.

Question 40

L6 - Understand space mission requirements definition

Answer 40

Space Mission Requirements are the quantitative expression of an approach
to meet mission objectives, there are 3 type of basic requirements:

1. Function requirements that define what a system is supposed to do andhow well it must to do it;
2. Operational requirements that define how the system is to be used;
3. Constraints that define limitations imposed on the system such as operating environment and requirements imposed by other system elements to minimize interfaces.

For Functional Requirements:

• Performance: primary objective, payload size, orbit, pointing…
• Coverage: orbit, swath width, number of satellites, scheduling.
• Responsiveness: communications architecture, processing delays, operations.
• Secondary mission: the same as above.

For Operational Requirements:

• Duration: Experiment and operation, level of redundancy, altitude;
• Availability: number of satellites of the system;
• Survivability: orbit, hardening, electronics;
• Data distribution: communication architecture;
• Data Content, Form and Format: users’ needs, level and place of
• processing, payload.

For Constraints:

• Cost: Manned or robotic flight, number of satellites, size and complexity
• Schedule: technical readiness, program size
• Regulations: policy and law
• Political restriction: Sponsor, whether international program
• Environment: Orbit, lift time
• Development Constraints: Sponsoring organization

Question 41

L6 - Understand space Engineering requirements definition

Answer 41

Requirement engineering shall ensure the following:

1. Proper interpretation of the customer needs and constraints concerning technical requirements for a product that satisfies the customer needs, produced, consolidated and agreed with the customer.

NOTE: This can be done in interaction with the customer.

2. Generation, control and maintenance of a coherent and appropriate set of system and lower level specifications.
3. Full traceability of the requirements within the set of specifications stated in b. above, down to final verification close-out.

System engineering requirements are called technical requirement specifications, every specification must consist of function, performance and Interface:

1. Functional requirements are to identify what characteristics are required;
   - it is important in the early design process.
2. Performance requirements are to quantify the required functions;
   - it is important later in the design process.
3. Interface requirements show elements which have dependencies on one another.

Question 42

L6 - Describe the main steps in the design cycle for a space system

Answer 42

Design of a space system is not made at once. The iteration loop needs to be used as many
time as needed until a suitable configuration
is reached.

Main steps: (like the 0, A, B.. Phases)

1. Requirement definition (mission objective, orbit, payload, subsystems, etc.);
2. Analysis (How can this be achieved?);
3. Design (Design parameters of key components are derived);
4. Development (Equipment List, Package components, Calculate System Mass properties) (Develop Subsystems);
5. Integration and Testing (Is this solution fulfilling the outputs of the analysis phase?);
6. Solution Analysis or Deployment (Is this solution fulfilling the requirements).

Question 43

L6 - Explain the importance of requirements definition and expansion

Answer 43

Requirements are statements which expand mission aims and objectives, and state them at a quantitative rather than qualitative level of detail for implementation by the designer, manufacturer, operator, etc.

Requirement Importance:

1. Requirements are the means by which we document the thought processes as a problem is ‘decomposed’;
2. Requirements ensure we get exactly what we want;
3. Consistent language avoids different interpretations;
4. Effort spent formulating good requirements in the early project phases pays for itself many times over;
5. Changing requirements late in a project is very expensive - majority of resources are committed early on.

-As the program develops more detail is built up and ‘performance’ may be further divided; This is expansion

Question 44

L6 - List key parameters for which requirements must be established at both early stage and late stage in the system design cycle

Answer 44

Requirements include:

• Functional - a statement of what something has to do
• Performance - a characteristic of something
• Characteristics - an interrelation between subjects

Question 45

L6 - Understand the key points on engineering requirement

Answer 45

1. Requirements development is:
   
   1. at the heart of the systems engineering process;
   2. a critical part of implementing any space program.
2. Requirements should be verifiable and quantifiable;
3. A logical and traceable hierarchy should be developed;
4. In general the requirements can be divided into:
   
   1. “What should it do”
   2. “How well should it perform”
   3. “Relationship with other elements”
5. Bad or changed requirements are the main source of cost overruns in the aerospace industry.

Question 46

L6 - List characteristics of good requirements

Answer 46

Characteristics of good requirements

1. Use clear language:

• stated simply, positively and concisely
• Technical requirements should be expressed in a positive way, as a complete sentence and grammatically correct

2. Provide a consistent hierarchy:

• traceability
• system - subsystem - element - unit, etc.

3. Define characteristics that can be quantified & verified;

4. Provide effective systems engineering organization:

• careful management
• experienced personnel

Required verbal form:

Uses; Shall, Should, May, Can

Say NO to ‘nice’ words like:

Adequate, Quick, As far as possible, Necessary

Question 47

L7 - Explain what a launch window is

Answer 47

Launch windows:

The period of time during which a spacecraft can be launched directly into a particular orbit from a particular launch site with restrictions.

From a particular launch site, this means launching:

• At the right time;
• With the right velocity
• In the right direction.

the launch window exists only
when the orbit plane intersects
with launch site.

Question 48

L7 - Explain how many opportunities there are to launch from a particular launch site into a particular orbit directly

Answer 48

• When latitude equals the orbital inclination, Lo = i

​One launch opportunity per day.

Launch site intersects with the orbital
plane once a day

• When the launch site latitude is less than the orbit’s inclination

Lo < i,

Two opportunities per day (ascending node and descending node).

As Earth rotates, launch site intersects with the orbital plane twice a day;

Question 49

L7 - Outline the different phases of a launch

Answer 49

A launch vehicle goes through four distinct phases on its way from the launch pad into orbit:

1. Vertical climbing; The launch vehicle begins by flying straight up, gaining both vertical speed and altitude. Gravity acts directly against thrust causing ‘gravity drag’.
2. Pitch over; The pitch over maneuver consists of the rocket gimbaling its engine(s) slightly to direct some of its thrust to one side. Thrust is gimballed to one side so launch vehicle starts to gain horizontal velocity. Want to keep the angle of attack close to zero (in line with velocity direction)
3. Gravity turn; After the pitch-over, the LV flight path is no longer completely vertical, so gravity acts to turn the flight path back towards the ground.
4. Acceleration in vacuum. Finally, launch vehicle is out of Earth’s atmosphere; Launch vehicle continues accelerating to gain necessary orbit velocity

Question 50

L7 - Describe the different components of launch vehicle design velocity

Answer 50

ΔV_design = ΔV_burnout - ΔV_launchsite + ΔV_{gravity loss} + ΔV_drag

• ΔV_burnout: Velocity needed at burnout to be in the correct orbit;
• ΔV_launchsite: Velocity of launch site due to Earth’s rotation;
• ΔV_{gravity loss}: Extra velocity needed to overcome gravity;
• ΔV_drag: Extra velocity needed to overcome atmospheric drag and other losses.

Question 51

L8 - Discuss the launch vehicle selection process as a balance of performance, availability, risk and cost

Answer 51

• *Performance** = spacecraft mass delivered to given orbit, must include performance and its margin + size of the fairing for spacecreaft;
• *Availability** = launch vehicle itself, select the existing LV or develop a new launch development;
• *Risk** = user’s attitude to risk => Safety/reliability and political decisions??
• *Cost** = more mass into orbit for the lower cost or with sharing options

Four key factors in selecting launch system are:

1. Launch vehicle capability
2. Launch vehicle availability and reliability
3. Launch vehicle/Spacecraft compatibility
4. Cost and risk

Launch System Selection Process

1. Define your final target orbit and injection accuracy tolerance;
2. Estimate your launch mass and define a mass margin policy;
3. Evaluate whether you need an on-board propulsion system;
4. Identify key dimensional/volume (envelope) requirements;
5. Determine your launch cost budget, schedule needs, and risk tolerance;
6. Evaluate launch vehicle capability and reliability;
7. Contact launch service providers to discuss requirements and rough cost estimates with them;
8. Select candidate vehicles and assess the associated launch environments;
9. Proceed with detailed spacecraft design while preserving the capability to fly on different vehicle… if necessary;
10. Select launch vehicle (Practically with an option).

Question 52

L8 - Discuss the impact of the launch vehicle choice on spacecraft design

Answer 52

Once launch vehicle has been identified, following areas must be defined and communicated early in design process between launch service provider
and spacecraft designer through Interface Control Documents (ICD) , and reviewed by joint teams:

Structural and electrical interfaces

• The spacecraft is attached to the launch vehicle by a launch adapter – need to determine if spacecraft needs additional support;
• The adapter connects the payload and any required kick motors, spin tables, separation systems or electrical interfaces.
• Launch system providers must provide physical, electrical, RF and optical access to the payload while it is enclosed by the fairing.

Fairing description;

• Fairing provides a good condition before lift-off and protects spacecraft(s) from aerodynamic loads during flight.
• The fairing envelope determines the shape of the spacecraft allowed, as well as the separation plane between launch Vehicle and Spacecraft(s)

Flight environments

The environments consist of:

1. Natural environment at launch site;
2. The thermal and mechanical environment during satellite processing;
3. The mechanical and thermal environment (acceleration, vibration, shock & noise) experienced during the launch vehicle flight; Vibration tests are carried out to ensure spacecraft can withstand the loads it will encounter during flight.
4. The electromagnetic environment during ground processing and the flight. Also acoustic environments must be considered Eg// reflected sound energy from pad and structures
   - Protect spacecraft during ground handling, transportation, hoisting, launch and ascent.

Question 53

L8 - List launch vehicles which are commercially available (but not the systems themselves)

Answer 53

Commercial Available Launch Vehicle - China:

Maiden flights of LM-5 & LM-7 in 2016

• LM-5: New generation heavy-lift launcher
• LM-7: Cost-effective Launch workhorse

Commercial Available Launch Vehicle - Europe:

• Ariane V is a large launch vehicle designed for LEO/GTO/HEO missions
• Vega is a small launch vehicle to deliver small satellites into LEO.
• Ariane VI will be a large launch vehicle designed to replace Ariane V for LEO/GTO/HEO missions with competitive price.

Commercial Available Launch Vehicle - India:

-Polar Satellite Launch Vehicle (PSLV) is a four- stage rocket for launching payload to LEO using

a mixture of solid and liquid propulsion.

-Geosynchronous Satellite Launch Vehicle (GSLV) is intended to make India less dependent on

foreign rockets.

-GSLV III is designed for both geostationary orbit and human spaceflight:

Commercial Available Launch Vehicle - Japan:

-H-2A/B is substantially redesigned of the earlier H-II, it is liquid-fuelled LV manufactured by Mitsubishi for JAXA

Commercial Available Launch Vehicle - Russia:

• Soyuz is a medium launch vehicle, the most famous launch systems in the world
• Proton is a large launch vehicle with different versions, and it is still in use for commercial & government mission.
• Angara is intended to make Russia completely independent in space launch capability, it will be primarily launched from Plesetsk and Vostochny.

Commercial Available Launch Vehicle - USA:

• Antares is a designed to launch up to 5000 kg to LEO with liquid propulsion1st stage, and solid propulsion for 2nd stage:
• Falcon launch vehicles is developed by Space Exploration Technologies Corporation (SpaceX), a private company founded Elon Musk.
• Electron is a two-stage launch vehicle developed by Rocket Lab; a private company founded Peter Beck.
  1. Delta IV family built by Boeing and operated by ULA to meet US military needs.
  2. Atlas V is built by United Launch Alliance, based on earlier Atlas vehicle technology.

Future Launch Vehicle - USA:

-The Space Launch System (SLS) is the US replacement for the Space Shuttle for Lunar and/or Mars mission (Orion Capsule) or launch other heavy payloads into space.

Question 54

L9 - Explain the most energy-efficient means of transferring between two points - the Hohmann Transfer

Answer 54

It is an elliptical orbit used to change the orbit from LEO to GEO: one manoeuvre takes place at LEO to transfer the spacecraft into the apogee of the elliptical orbit.

Other manoeuvre takes place at the perigee of the elliptical orbit to place the spacecraft into higher orbit (GEO.)

Question 55

L9 - Explain plane changes

Answer 55

• To rotate the orbit plane/velocity vector by an angle θ, we must apply an out-of-plane ΔV.
• Simple plane change - only the direction of the velocity vector changes.
• Combined plane change - the magnitude and the direction of the velocity vector changes
• Plane changes should always be done at apogee

Question 56

L9 - Explain combined manoeuvres and the reasons for them

Answer 56

Hohmann Transfer + Plane Change.

This would require three manoeuvres, though.
We can do it in two and save propellant by combining the [PlaneChange] manoeuvre with one of the [Hohmann burns].

Question 57

L10 - Understand space engineering trade-offs definition, and explain the importance of trade-off studies in space systems engineering approach

Answer 57

Trade-off study is a structured comparison of alternative approaches (or options) to perform required system functions with constraints, which provides data, not decisions.

In early conceptual design phases, important decisions must be made between system-level options. Nearly 80 % of the program cost is committed (not spent) in the early phase before flight hardware manufacture begins.

Question 58

L- 10 Explain how to carry out trade-offs to reach a preferred solution

Answer 58

• Draw up a trade (or option) tree;
• Establish evaluation criteria (selection rules);
• Use quantitative analysis if possible/sensible.

Question 59

L10 - Briefly outline the main methods to conduct trade-offs;

Answer 59

• Broad approach without much quantitative detail;
• Weighted trade-offs;
• Parametric analysis and performance assessment

Question 60

L10 - Understand the trade-off analysis procedures

Answer 60

1. Definition of trade-off requirements;

2. Identification of Potential Solutions

• Initial screening should eliminate infeasible or unsuitable solutions;
• Feasible solutions should be defined in sufficient detail for the next phase.

3. Criteria Selection:

• Should be quantitative to avoid subjectivity;
• Different criteria should be independent of one another (to avoid multiple assessments of the same characteristic);

4. Criteria Weighting

• Some criteria will receive equal weighting (if equally important);

5. Scoring Functions

• Same range of scores must be applied;
• Remember that weighting has already been assigned.

6. Sensitivity Study

• If problems are identified, additional action may be required;
• Highlight potential risks associated with a possible incorrect decision and carry out risk management.

Question 61

L11 - List and discuss the requirements of entry design

Answer 61

For atmospheric entry, there are three main aspects to be considered:

1. Deceleration: max deceleration must be low enough to prevent damage/injury; but it must be high enough to slow spacecraft to re-enter, rather than skip off the atmosphere.
2. Heating: Intense heat is caused by the air molecules’ hit; this can damage/destroy the spacecraft; the entry trajectory and spacecraft must be designed to prevent this.
3. Accuracy of landing/impact: A spacecraft will usually be aiming for a particular location on the planet, with a given accuracy. The entry trajectory of the vehicle must be designed to fulfil this requirement

Question 62

L11 - Define an entry corridor and discuss its importance

Answer 62

All the possible trajectories define a permissible re-entry corridor.

Question 63

L11 - Explain the effects of changing trajectory entry velocity and flight-path angle

Answer 63

• Changing trajectory entry velocity impacts deceleration caused by Drag force.
• Changing flight-path angle influences entry velocity.

Question 64

L11 - Discuss the effect of changing vehicle ballistic coefficient.

Answer 64

Different Balistic Coefficient implies different decelerations and heating rates.

High coefficient –> low deceleration.

Question 65

L11 - Describe three types of thermal-protection systems and how they work

Answer 65

• Heat Sinks - Uses extra material to absorb the heat, to keep peak temperature lower. If a spacecraft faces a fixed amount of heat energy, we can reduce peak temperature by increasing the volume of its TPS material to absorb more heat. Simple but heavy – can reduce payload mass significantly.
• Ablation - Ablative materials melt/evaporate while absorbing energy; carbon-ceramic and similar surfaces can be used to coat the spacecraft; the main drawback is that spacecraft must be completely refurbished if it is to be re-used.
• Radiative Cooling - High emissivity materials can emit almost as much energy as they absorb. This means they reach thermal equilibrium sooner and at a relatively low temperature. This is called radiative cooling; typically a surface-coating with high emissivity and melting point is used on top of a very efficient insulator to protect spacecraft structure.

Question 66

L 11- Discuss the advantages offered by lifting entry

Answer 66

1. Including lift makes analysis much more complex, but it gives us more flexibility.
2. E.g., lift can be used to ‘stretch’ the size of the entry corridor, giving greater margin of error in V_entry and γ and improve accuracy.
3. With dedicated shape of spacecraft (lifting body), and we can change the angle of attack to improve lift and make the spacecraft behave more like an aircraft (Space Shuttle).

Question 67

L 11 - Explain aero-braking and discuss how interplanetary missions can take advantage of it

Answer 67

1. Aerobraking uses aerodynamic forces (drag and lift) to change a spacecrafts trajectory.
2. For example, atmosphere drag can perform the equivalent of the DV retro burn on planetary approach.
3. Using aerobraking, instead of propulsive deceleration is almost ten times more efficient in terms of mass

Question 68

L 12 - List the four main environments that a spacecraft experiences during its lifetime

Answer 68

• Earth environment
• Launch environment
• Atmospheric environment
• Space environment

Question 69

L12 - Describe the main attributes of each of the four environments

L12 - Explain the implications of these attributes for the design and operation of space systems

Answer 69

> Earth environment - Main problem = atmosphere

H2O + O2 can cause corrosion which in turn can cause problems,

Need to control humidity ~ 40-50%. For some scientific instruments, dry N2/He2 purge may be needed, but risk of static electricity build-up if humidity too low.

Atmospheric particles/dust can: – Interfere with delicate mechanisms, – Reflect light and fool star sensors, – Contaminate other planets.

Cleanrooms used for assembly and test (whole S/C or subsystems.), • Static charge build-up can also be a problem:– On human skin, plastics – Metal-oxide semiconductors (MOS) v. sensitive

Transportation can cause vibration and shock – Road: can be worse than launch

> Launch and re-entry environment à • Launch and re-entry are brief but very stressful.

Aspects include: ▪ Axial loads (acceleration) ▪ Lateral loads (steering, wind gusts) ▪ Mechanical vibration ▪ Acoustic energy (rocket engine noise at lift-off) ▪ Thermal loads (aerodynamic heating) ▪ Shock transients (fairing jettison, stage separation) • Mitigated by careful design and testing.

> Atmospheric environment - Three potential areas of impact: – Launch – In-orbit (if LEO) – Entry (depending on mission)

• As well as drag, we must consider the effects of the chemical composition of the atmosphere.
• This changes with altitude because the lighter components float upwards.
• Near - vacuum + thermal extremes + solar UV causes yellowing/browning of surfaces

>Space environment - In space, we need to consider the effects of:

• Electromagnetic radiation

Three wavebands of interest:

–Ultraviolet, can damage organic materials (e.g., paints, coatings), some glasses (e.g. solar cells) and electronics.

–Visible, is frequently converted into electricity using photovoltaic solar cells. Design needs to consider specular reflection, missions consider solar radiation force (5 N/km2) which can disturb S/C orientation and perturb S/C orbits.

–Infrared, radiation hitting the spacecraft heats the sunlight side to temperatures around 150 oC. The side of the spacecraft in shadow will be at around -200 oC, while the electronics typically need to operate at around 20 oC. This means that good thermal control systems are needed.

• Charged particles - There are three main sources of charged particles: The solar wind and flares, Galactic cosmic rays (GCRs), The Van Allen radiation belts. They can damage S/C by: Charging, Sputtering, Electrostatic discharge, Single-event phenomena (SEP), Total dose effects
• Micrometeoroids
• Orbital Debris
• Vacuum
• Gravity
• Drag
• Solar pressure