Basics of spacecraft instrumentation design

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Basics of spacecraft/instrumentation design

Design steps in a nutshell (a very tiny nutshell that is)

1. Mission objectives

2. Design

3. Selection of a launcher

4. Launch environment, space environment (from orbit)

5. Design

6. Performance / cost tradeoffs

7. Prototyping

8. Testing

9. Design

10. Elegant prototyping

11. Testing

12. Design

13. Flight model

14. Testing

15. Launch

Launch environment

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Basics of spacecraft/instrumentation design

Mechanical environment during launch – launch phases and design specs

• Engine’s ignition and lift-off – acoustic and blast waves

reflected by the launch pad

• Maximum dynamic pressure (max Q) – longitudinal trust

and lateral dynamic excitations (e.g. wind)

• Supersonic speed – acoustic and lateral dynamic

excitations due to fairing and launcher body

• SRB pressure oscillations – propellant is consumed and

cavities are formed. Those make the booster resonate.

• SRB separation – sudden drop in thrust and shocks

caused by pyrotechnics.

• Fairing jettisoning – pyrotechnics => shocks

Shocks: • Stage separations • Fairing jettisoning

The design and testing environment during launch is specified in the launch vehicle’s User’s Manual

• Quasi-static environment

• Low frequency vibrations (up to 100 Hz)

• Acoustic environment (20 Hz – 2 000 Hz)

• Random vibrations (covered by the acoustic envelope)

• Shock

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Mechanical environment during launch – quasi-static loading approximation

Definition of approximation: load applied slowly => acceleration assumed constant => inertia is

neglected

Function of approximation: to cover the static and low frequency launch environment. Determines

resistance and rigidity requirements.

Testing: Loads are applied with dead weights or hydraulic jacks. Two types of test:

• yield test – verifies ability to withstand load without permanent deformation

• ultimate test – verifies ability to withstand higher loads without rupture or collapse

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Basics of spacecraft/instrumentation design

Mechanical environment during launch – vibration test equipment

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Basics of spacecraft/instrumentation design

Mechanical environment during launch – low frequency environment

• Complex environment, depending on the dynamic behavior of the launcher-satellite assembly.

• Low frequency transients are simulated as a swept-sine vibration @ frequencies up to 100 Hz.

• Disadvantages: simultaneous undertesting and overtesting may occur.

• Sine sweep as a diagnostic tool

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Basics of spacecraft/instrumentation design

Sine test

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Basics of spacecraft/instrumentation design

Mechanical environment during launch – random vibrations

• Some load environments are treated as random, since the forces involved have random character,

e.g. high frequency engine thrust oscillations, aerodynamics of fairing, sound pressure on the

payload surfaces.

• All frequencies are excited simultaneously.

• The amplitude of motion at each frequency is described by a power spectral density function PSD.

• From the PSD, the root mean square (rms) amplitude

of the response is calculated as:

where

is the area under the curve

between the frequencies F2 and F1

The Grms value is an indicator for the overall energy of a random vibration.

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Basics of spacecraft/instrumentation design

Random test

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Basics of spacecraft/instrumentation design

Mechanical environment during launch – shock

Shocks are very short transients with high acceleration amplitudes, high frequency content, and are

characterized by rapid initial rising times.

Sensitive equipment:

• Electronics: relays, quartz, transformers, hybrids, tantalum capacitors, heavy components,

optoelectronics;

• Structural effects: cracks and fractures in brittle materials (ceramics, crystals, epoxies or glass

envelopes), local plastic deformation, or accelerated fatigue of materials for repeated shocks;

• Effects on mechanisms: degradation of bearings, gears, worm wheels and endless screws;

• Valves

Steinberg shock analysis for electronics

Empirical calculation of the minimum acceleration a

component can sustain, 𝐺, and compare it to the maximum

acceleration of the specified shock spectrum.

𝐺 =6.78𝑒−4𝐵𝑓2

𝐴𝐶ℎ𝑟 𝐿

B – length of PCB edge || component

L – component length

h – PCB thickness

C – component sensitivity

r – position factor

A – shock amplification factor

f – normal mode

Space environment

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Basics of spacecraft/instrumentation design

Space environment

1. Offgassing

2. Outgassing

3. Micro vibrations due to moving parts

4. Ultra-high vacuum

5. Ionizing radiation

6. Non-ionizing radiation

7. Thermo-elastic behavior

8. Atomic oxygen

9. Meteorites and space debris

10. Charging and plasma

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Basics of spacecraft/instrumentation design

Space environment

1. Offgassing

Assembly in clean rooms => water absorbed in organic

materials and composites => water evaporates in vacuum =>

deformations

2. Outgassing

Leads to contamination governed by vibration, electrostatics,

radiation pressure etc.

Leads to alterations of material properties

3. Micro vibrations due to moving parts

Critical for optics

Affect spacecraft’s attitude

4. Ultra-high vacuum

Accelerates offgassing and outgassing

Rapid depressurization can cause damages

Gas is a lubricant => no gas can cause mechanism lock-up

(cold welding)

Huge negative effect on heat conduction

Optical Solar Reflector – outgassing contamination by Chemglaze Z306

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Basics of spacecraft/instrumentation design

Space environment

6. Non-ionizing radiation

Types

Solar radiation

Albedo

LW radiation

Effects

Thermal

Radiation pressure affects attitude

UV alters material properties of polymers

7. Thermo-elastic behavior

Severe temperatures (-150°C to 120°C)

Important for both mechanical structures and electronic assemblies and critical for

mechanism design.

Coupled materials with

different CTE

=>

Recurring

deformations

=>

• Material fatigue and

cracking

• Mechanism lock-up

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Basics of spacecraft/instrumentation design

Space environment

8. Atomic oxygen

Between 200 and 1000 km UV dissociation of O2.

Density depends on solar activity.

Atoms impact spacecraft surface @ high speed => accelleration of oxidation rate => corosion

& erosion => surface deterioration, loss of shape, outgassing, contamination with

condensate or particles

MLI – Multi-layer Insulation LDEF – Long Duration Exposure Facility

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Basics of spacecraft/instrumentation design

Space environment

9. Meteorites and space debris

Space debris lifetime vs. altitude

200 km ~10 yr

800 km ~100-200 yr

GEO (35 786 km)

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Basics of spacecraft/instrumentation design

Space environment

10. Charging and plasma or how solar flares can

destroy a satellite

Satellites charge.

They tend to take on negative charge.

Charge to above 20 kV.

Discharges can occur between parts of

the satellite, satellite and surrounding

space plasma, satellite and its

propellant plume.

Sudden discharges can:

• Explode semiconductors, capacitors

etc.

• Vaporize metal parts

• Cause structural damages

• Destroy thermal shielding