Module 1: Space Environment

Question 1

What are the space mission requirements?

Answer 1

The goal in engineering is to simplify things. The high-level requirements of space missions are:
1) Mass
2) Performance (eg accurate measurements)
3) Reliability (because it’s not possible to repair everything in space)

Question 2

What are the definitions of material, process, mechanical part and qualification?

Answer 2

1) Material : A raw, semi-finished or finished purchased item (gaseous, liquid or solid) of given characteristics.

2) Process: A set of inter-related steps which transform a material into a semi-finished product or a semi-finished product into a final product.

3) Mechanical part: One or more elements which perform a function, mechanical, optical, thermal or electromechanical and not classified.

4) Qualification: A set of data collected (typically test results) to demonstrate that a material, a process or a part performs as intended with respect to technical requirements, with sufficient margins and sufficient confidence.

Question 3

How are materials selected?

Answer 3

Materials shall be chosen giving preference to the following:
- Those materials succesfully used for an identical application in identical programmes similar with respect to environment constraints and lifetime to the proposed application
- Those materials for which satisfactory evaluation results are obtained on samples representative of the application with a sufficient margin as regards to the conditions of use.
- Those materials included in approved data sources.

In order to have material heritage, we should have same processes, same loads, same duration and same conditions. We should always re-do qualifications taken by other sources.

Question 4

What are the main and secondary drivers for materials selection?

Answer 4

Main drivers:
1) Properties requirements, both functional (eg mechanical, thermal, structural) and environmental (eg ATOX, corrosion, radiation resistance)
2) Lightweight, high stiffness and strength
3) Reliability, cost

Secondary drivers:
1) Manufacturing complexity
2) Product availability
3) Ease of integration
4) Safety

Question 5

What are some properties that should be considered for material selection and what are some operational requirements?

Answer 5

Material properties:
- Mechanical properties
- Fracture toughness
- Flammability and offgassing characteristics
- Corrosion and stress corrosion
- Thermal and mechanical fatigue properties
- Glass transition temperature
- Vacuum outgassing
- Fluids compatibility
- Microbial resistance, moisture resistance
- Fretting, galling
- Susceptibility to electrostatic discharge
- Contamination

Operational requirements:
- Operational temperature limits
- Loads
- Contamination
- Life expectancy
-Moisture or other fluid media exposure
- Vehicle-related induced and natural space environments

Question 6

What are some metallic materials used in space?

Answer 6

1) Light metals
2) Steels
3) Nickel and nickel based alloys
4) Refractory metals
5) Copper-based alloys
6) Precious metals
7) Welding, brazing and soldering alloys
8) Various plating alloys

Question 7

Describe the ground environment.

Answer 7

1) Manufacturing. It has the most impact on the material properties.
2) Tests (demanding for quality and production). They simulate launch, ascent, re-entry and flight. Also vibrations, accelerations, shocks, acoustic and thermal loads.
3) Transport loads. Can be demanding because of vibrations, accelerations, shocks. A Monte Carlo analysis can be done to determine the loads instead of a deterministic analysis.
4) Atmosphere humidity, temperature and biological. Contamination of products
5) Ageing and transport (eg corrision, polymer can become brittle, pressure changes).
6) Storage (maybe salt formation)
7) Launch

Question 8

Discuss about the corrosion issues.

Answer 8

There are corrosion issues due to manufacturing, ground storage etc, which doesn’t dissapear once the system is in space.

• Satellite systems remain for long times on Earth
• Many facilities are located close to the sea, Cl- is present and exhausts from launchers.
• The critical corrosion types are:
  a. General corrosion. It can generate particles and particulate contamination which leads to loss of mission.
  b. Galvanic corrosion. Can lower electrical contact and open circuit up to loss of mission.
  c. Stress corossion. Can provoke premature failure and loss of mission.
• General corrosion is treated by paints and coaitings.
• Galvanic corrosion treated by subsituting metals in contact
• Stress corrosion is a killer.

Question 9

Discuss launch site corrosion.

Answer 9

• Launch sites such as the European Spaceport suffer from severely corrosive environments owning to their close location to the atlantic ocean.
• Very high saltfalls are deposited on these coastal sites throughout the year as they are exposed both to norteast and southeast trade winds.
• Salt from the ocean comibed with the launch vehicles acidic rockets exhausts make corrosion (in all its forms) an important concern for spacecrach, launchers and ground segments infrastructures, both from a safety as well as economic point of view.

Question 10

What is included in the launch and space environment?

Answer 10

• Lightning strike, birds strike and vibrations/shock during the launch phase.
• Atomic Oxygen
• Inner and outer radiation belts with high energy electroncs and protons.
• Electrostatic charge/discharge, vacuum/zero gravity and violent temperature changes in the magnetosphere.
• Solar protons and ultraviolet rays from the sun

Question 11

Discuss the launch and ascent considerations

Answer 11

1) Vibrations during ignition
2) Accelerations
3) Shocks (eg separation)
4) Thermal flux
5) Lightning impact
6) Rain
7) Birds

All these have to be addressed at sub-system or system level.

Question 12

What are the issues on the launch vehicle from mechanical loads, thermal loads. atmosphere, nature and weather?

Answer 12

1) Mechanical loads
- Vibrations, accelerations, shocks
- Depressurization
- Noise

2) Thermal loads
- Fairing re-radiation
- Plume

3) Atmosphere
- Free molecular heating
- Friction air-rocket
- Friction air-satellite
- After fairing jettison

4) Nature
- Bird strike

5) Weather
- Lightning impact
- Rain
- Wind gusts
- Frost

Question 13

What are mechanical loads mainly due to?

Answer 13

1) Transportation
2) Rocket motor ignition overpressure
3) Lift-off loads
4) Engine/motor generated acoustic loads
5) Engine/motor generated structure-borne vibration loads
6) Engine/motor thrust transients
7) Pogo instability, solid motor pressure oscilations
8) Wind and turbulence, aerodynamics sources
9) Liquid sloshing in tanks
10) Stage and fairing separation loads
11) Pyrotechnic induced loads
12) Manoeuvring loads
13) Flight operations, onboard equipment operation.

A launch is made of different flight events from lift-off to spacecraft separation. Each event generates mechanical loads on the spacecraft: static, acceleration, acoustic noise, vibrations, shocks.

Question 14

What is included in the launch mechanical environment?

Answer 14

1) Steady state accelerations
2) Low frequency vibrations
3) Broad-band vibrations (random vibrations and acoustic loads)
4) Shocks.

Loads (vibrations) are transmitted to the payload (eg satellite) through its mechanical interface.

Acoustic loads also directly excite payload surfaces.

Question 15

Discuss about the steady-state accelerations.

Answer 15

The axial acceleration on the launcher and the payload comes from the engines thrust. When the mass decreases due to consumption of the propellant, the acceleration increases.

Lateral accelerations from wind gusts or changes in trajectory.

Question 16

Discuss about steady-state, low frequency transients and shocks.

Answer 16

In some time intervals, transient events associated to vibrations on top of the static acceleration are clearly identifies.

During the lift off and the early phases of the launch, an extremely high level of acoustic noise surrounds the payload.

For example, in transonic flight there are transients, after the first stage and second stage burnout and in the third stage cutoff.

Question 17

Discuss about acoustic noise.

Answer 17

The principle sources of noise are the engine functioning and aerodynamic turbulence. Acoustic noise (as pressure waves) impinging on light weight panel-like structures produce high responses.

Question 18

Discuss about broad band random vibrations.

Answer 18

Broad band random vibrations are produced by
- Structural response to broad-band acoustic loads
- Engines functioning
- Aerodynamic turbulent boundary layer

Question 19

Discuss about shocks.

Answer 19

They are mainly caused by the actuation of pyrotechnic devices used for release mechanicms for stage and satellite separation and for deployable mechanisms (eg solar arrays).

Question 20

Discuss about quasi-static laods and vibration levels.

Answer 20

During launch the spacecrach is submitted to a combination of static and dynamic accelerations (vibrations).

This is translated in the user manual in:
- Quasi-static loads computed from the loads and moments at the SC/launcher interface. Sometimes the dynamic and static contributions are detailed.
- Vibration levels in terms of frequency spectra computed from the transient time histories of acceleration at the SC/launcher interface.

Question 21

What is a Shock Response Spectrum?

Answer 21

• For each frequency, we consider a DOF system with a given Q (quality) factor.
• We apply to this system the time signal
• We retain the maximum acceleration.

Question 22

Describe the design diagram of a component.

Answer 22

1) Start with defining design features in a rough-high level design manner.
2) Generate FE models (substructure moels, coupled spacecraft model and coupled spacecraft/booster model).
3) Quantify design environments for all significant events (forcing function, steady state accelerations, sound pressure levels, power spectral densities). This is an input for 4).
4) Predict design parameters (loads, displacements, loading spectrum)
5) Analytically verify requirements per criteria (strength, life, dynamic envelope).
6) Reiterate

Question 23

What are the two types of tests according to the objectives to be reached?

Answer 23

1) Simulation tests for structure qualification or acceptance.
2) Identification tests (aka analysis-validation tests) for structure identification (the objective is to determine the dynamics characteristics of the tested structure in order to “update” the mathematical model).

Question 24

What are some testing techniques?

Answer 24

1) Generation of mechanical environment
- Small, large shakers
- Shock machines
- Noise generators + reverberant acoustic chamber

2) Measurements
- Force sensors, calibrated strain gauges
- Accelereometers, velocity or displacement sensors

Question 25

What are the classes of tests used to verify requirements?

Answer 25

1) Development test in order to demonstrate design concepts and acquire necessary information for the design.
2) Qualification test.
3) Acceptance test
4) Analysis validation tests

Question 26

What are the tests for verifying mechanical requirements?

Answer 26

1) Static loads test
- Verify strength
- Verify stiffness

2) Sinusoidal vibration test
- Verify strength for structures that would not be adequately tested in random vibration or acoustic testing
- Cyclic loads at varying frequencies are applied to excite the structure modes of vibration
- Sinusoidal vibration testing at low levels are performed to verify the natural frequencies.

3) Acoustic test
- Verify strength and structural life
- Verify adequacy of electrical connections

4) Pyrotechnic shock test
- Verify resistance to high frequency shock waves caused by separation explosives.

5) Random vibration test
- Verify strength and structural life by introducing random vibration through the mechanical interface.

Question 27

What are the main materials properties needed for the mechanical environment characterization/verification?

Answer 27

1) Static strength, E modulus, Yield. eg, AM materials are anisotropic, different behaviour in vertical and horizontal direciton.

2) Fatigue strength

3) Fracture toughness

4) Fatigue crack propagation

Question 28

What is included in the space environment?

Answer 28

1) Atomic oxygen
2) Space debris
3) Micrometeorites
4) Synergetic effects

1) Temperature
2) Vacuum
3) Radiation
4) Cosmic rays

Question 29

Discuss the temperature effects

Answer 29

• Thermal stresses occur on the outer surface of satellites due to periodic in and out of the sunshade during orbiting
• In function of the orbit, thermal stresses occur from 2000 to 50000 times and materials can reach temperatures from -180 to 180 degrees celcius.

Temperature -> Cycling -> Thermal fatigue -> Deboding, fracutres and loss of protective coating.

Temperature -> cycling -> high -> molecular degradiation -> increased outgassing and degradation of operational properties of materials

Temperature -> cycling -> low -> modification of electrical and mechanical properties (leads to fragilisation) ->increased condensation

Question 30

Discuss about thermal fatigue

Answer 30

Typical thermal conditions depend on type of orbit and of the satellite configuration. Temperature around earth is between -150C and +150C. For other missions, extreme T can be higher or lower and T range is increased.

The repeated temperature variation causes thermal stresses in materials. The difference in CTE could casuse cracking, lack of adherence and finally spalling of the surface treatment layers as well as of electronic assemplies. This could lead to degradation of other properties.

Question 31

What is a Tin Whisker?

Answer 31

Hair like crystal structure that may grow from mostly pure tin (or zinc) finished surfaces. Uknown growth mechanism.

It can result to short circuits (in citu and everywhere if its broken during vibrations) and contamination.

Question 32

Discuss the vacuum effects.

Answer 32

Vacuum -> outgassing/sublimation -> change in operational properties of materials -> numerous problems mainly dimensional stability and lubrication

Vacuum -> outgassing/sublimation -> gas cloud -> peturbation of measurements -? numerous problesm especially on scientific satellites

Vacuum -> outgassing/sublimation -> gas cloud -> corona -> arc -> electrical problems

Vacuum -> outgassing/sublimation -> gas cloud -> condensation -> modification of thermo-optical properties, radiation effects and electrical properties -> thermal problesm and electrical problems

The effects of vacuum in materials are mainly due to desorption of water from ceramic/oxide layers and of the polymeric materials and outgassing of gaselous light species released by organic materials or organic residues entrapped in inorganic layers.

The effect of water desorpiton could have an impact on the layer structure weakening it and/or modifying electrical and /or thermoptical propeties.

Outgassing could provoke contamination of highly sensible optical devices.

Question 33

Dicuss the contamination resulting from vacuum in polymers.

Answer 33

Under vacuum polymers can release molecules
- Outgassed molecules may redeposit/recondense
- Generally on cold surfaces
- Generally on optical surfaces
- Lower performances

Question 34

Dicuss the contamination resulting from vacuum in metals.

Answer 34

Also metallic materials outgass (zinc and cadmium) causing:
- Short circuits due to cloud of conductive material
- Surface changes
- Metallic clouds
- Contamination/Obscuration of optics and sensible surfaces
- Used for ground support equipment -> can go to vacuum
- Often used on launchers, may be a problem depending on stage.

Question 35

Discuss the cold welding-fretting issue.

Answer 35

Metals are generally covered by an oxide layers.
- If an oxide is removed under vacuum it does not form again.
- Two bare metals in cotnact may cold-weld if they have similar crystal structure (fretting and impact)
- Development of coatings to avoid cold welding.

Question 36

What are the radiation effects and where do they come from?

Answer 36

1) Solar energetic particles (flares, CMEs - coronal mass ejections) (protons, heavy ions, electrons)
2) Solar wind (electrons and protons)
3) Galactic cosmic rays (protons and heavy ions)
4) Van Allen belts

Radiation(UV,protons,electrons) -> modification at molecular level -> increased outgassing

Radiation(UV,protons,electrons) -> modification at molecular level ->modification of thermoptical properties -> thermal problems

Radiation(UV,protons,electrons) -> modification at molecular level ->modification of the electrical charge state -> peturbation of measurements which lead to numerous problems especially on sicientific satellites and breakdown which leads to electrical problems.

Radiation(UV,protons,electrons) -> modification at molecular level -> Increased sensitivity

Radiation(UV,protons,electrons) -> modification at molecular level -> modification of mechanical properties which lead to fractures.

Mostly a problem for polymers

Question 37

What are the radiation effects on mechanical properties?

Answer 37

1) Tensile strength, elasticity, elongation, impact resistance, fatigue, hardness, shear strength and dimensional stability
2) The effect on metals is the creation of lattices vacancies and interstitials atoms. creep, fatigue initiation, stress corrosion cracking.
3) It could have an impact on surface treatments due to effects on adherence (micro-cracks, flaking off)
4) Polymeric materials are considerably more sensitive and siginificant effects are to be expected (peel-off, colour changes, crazing)
5) The formation of new cheical bonds after irradiation could result in changes in apperance, in chemical and physical states and in mechanical properties.

Question 38

What are the effects of radiation on thermo-optical properties?

Answer 38

1) Thermal conductivity and stores energy, emissivity, absorbance and reflencance.
2) The thermo-optical properties of coatings of the exposed surfaces of a spacecraft are critical for temperature control of the system so the effects of the radiation on these surfaces are hence of conern in all aspects of the reliability of the spacecraft.
3) The principle effect of radiation in thermo/optical properties are changes in surfaces structure, color, loss of transmission, variations of emissivity/absorptibity and modifications of refractive index

Question 39

What are the effects of radiation on electrical properties?

Answer 39

1) Conductivity, dielectric properties
2) Impact on semiconductors and electronic devices
3) Circuit failure due to change of electrical properties.
4) Displacement damage effects grdually reduce the power output of solar panels
5) On surface treatments, isolation layers are conductive, radiation could modify the intrinsic electrical properties of the layer compromising the funcionality of the surfaces treatment.

Question 40

Discuss the space debris and micrometeoroids effects.

Answer 40

High velocity particle -> impact - > through hole, cratering and debris

High velocity particle -> impact - >through hole -? loss of material integrity -? exposure of underlayer and numerous problems particularly fragilisation/leak

High velocity particle -> impact - >through hole -? conductive track -> electrical problems

High velocity particle -> impact - >through hole -? change in thermo-optical properties

High velocity particle -> impact - >cratering -? conductive track -> electrical problems

High velocity particle -> impact - >cratering -? conductive track -> change in thermo-optical properties

High velocity particle -> impact - > debris -? contamination cloud

Question 41

How can you design against debris and micrometeoroids?

Answer 41

Multiple layers of walls, with the first one being harder (eg tungsten), which creates a debris clouds with lower energy particles.

Question 42

Dicuss the Atomic Oxygen effects.

Answer 42

Atomic oxygen -? Oxidation -> Oxide layer, Erosion and Elimination of contaminants

Atomic oxygen -> Oxidation -> Oxide layer -> Breakage -> Erosion -> Mass loss (-> generation of particles -> contamination cloud) and (-?degradation of mechanical properties -> ruprure/deformation)

Atomic oxygen -> Oxidation -> Oxide layer -> Breakage -> Erosion -> texture change -> change of thermo-optical properties

Atomic oxygen -> Oxidation -> Oxide layer -> Protection

Atomic oxygen -> Oxidation -> Elimination of contaminants -> property recovery

Main effects of atomic oxygen is the erosion of materials. The degradation of silver when exposed to atomic oxygen (a layer of oxides is rapidly formed and this layer is unadherent and particles flake-off with considerable risk of contamination and degradation).

Polymeric materials are sensitive to atomic oxygen and the thickness of the material my decrease.

Polymers containing silicones, fluorides are bleieved to be moderately inert for short exposeures to atomic oxygen, but are degraded in long-term exposure.

Question 43

What are the challenges with manned volumes?

Answer 43

Life supporting atmosphere -> combustion _> loss of material function -> catastrophic failures

Life supporting atmosphere -> combustion _> ageing/corossion -> toxic gases -> aggressions to life

Life supporting atmosphere -> combustion _> ageing/corossion -> degradation of material function -> numerous problems particularly insulation/gasketing

Life supporting atmosphere -> combustion _> biological growtth -> toxins -> diseases

Life supporting atmosphere -> combustion _> biological growtth -> smell -> discomfort

Although designed to support life in space, the presence of breathable air,water,organic materials and human activity can negatively affect the materials properties.

Damage of materials can occur in the form of:
- Corrosion
- Stress corrosion
- Biological growth
- Friction and wear

Question 44

Discuss the effects of biological growth/

Answer 44

Bacteria and fungi feed on skin epithelia, lipids and other products of human activity.
- These products are introduced into the habilatble atmposphere from human breath, sweat and adhere to internal surfaces.
- Biological growth generate products of metabolism such as organic acids which can corrode steel, glass and plastic.

Question 45

Discuss the re-entry effects.

Answer 45

Re-entry -? Ultra-high temperature and dynamic pressure

Re-entry –? dynamic pressure -> erosion/corrosion -? loss of material integrity -? fragilisation

Re-entry –? dynamic pressure -> erosion/corrosion -? surface texture changes -? modification of heat exchange -? thermal problems

Re-entry –? Ultra-high temperature -> loss of mechanical resistance -> rupture

Re-entry –? Ultra-high temperature ->plasma -> chemical reactions -> erosion/corrosion

Question 46

What are some contradicting requirements for re-entry?

Answer 46

1) Design for demise to avoid space debris
2) Re-entry

Question 47

What are some problems of propulsion systems?

Answer 47

1) Stress corrosion cracking in space propulsion systems
- Propulsion systems, in particular tanks and pipes are exposed to extremly reactive and aggresive fluids, such as fuels, oxidizers, cleaning agents which can promote SCC
- Significant residual stresses due to welding or machining processes could be still present in the tank evena fter stress release treatment due to the tank pressurization are present in service.

The propulsion systems are exposed to extremly high temperatures, erosion occurs, aggresive environment (HNO3) and highly loades structures.