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Space Systems Term 1 > Structural > Flashcards

Structural Flashcards

1
Q

What environments, prior to launch, must a spacecraft structure be designed to withstand?

A

Manufacture, testing and transportation

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2
Q

State and briefly describe two typical requirements for spacecraft structures

A
  • strength: amount of load a structure can carry without collapsing or deforming so much to risk the mission
  • structural life: number of loading cycles the structure can withstand before the material fatigues or creep/crack sets in from stress corrosion
  • structural response: magnitude and duration of vibration caused by external loads
  • natural frequency: frequency/cycles per sec that structure vibrates at when excited by transient load and then left undisturbed. Mass and stiffness dependent and particular attention on fundamental frequency (lowest nat freq)
  • stiffness: measure of load that causes structure to deflect
  • damping: dissipation of energy in vibration to limit the magnitude and duration of force response
  • mass properties: mass, centre of mass, moments and products of inertia
  • dynamic envelope: physical space S/C or substructure must be within whilst deflecting under load
  • positional stability: ability to maintain location or orientation within a specified range. Concerns of thermo-elastic distortions, material yielding and mechanical joints shifting
  • mechanical interface: how structure and components attach by location and flatness
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3
Q

Define what is meant by static and dynamic envelopes for launch vehicles

A

Dynamic envelope is the space the s/c must be within during launch, including tolerances and structural deflections

Static envelope is the space the satellite must be within, including tolerances, when it is integrated with the LV

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4
Q

What is a Frequency Response Function?

A

Ratio of the output amplitude w.r.t the input. Response can be displacement, velocity, acceleration

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5
Q

State how many fundamental frequencies there are for a satellite structure

A

Two - lateral and longitudinal

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6
Q

What is a complex mode and when do they occur?

A

Solving models with damping creates complex numbers. Means that maximum displacements do not occur at the same time in the structure at all points in the structure - phasing between the maximum displacement

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7
Q

What is the grouping effect of inserts in a honeycomb sandwich panel?

A

Less structure around the inserts, less uniformity to carry the loads so likelihood of small cracks. proportional to the relative distance/shape between the inserts

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8
Q

What is a satellites primary structure?

A

Structure that carries the main load path i.e. main structural element, external panels, main framework, interface adaptor etc.

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9
Q

Define what is meant by transient loads

A

Short duration, high intensity acceleration loads in shock environments

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10
Q

What does the g_rms value of an Acceleration Spectral Density (ASD) plot denote and how can it be calculated from the plot?

A

G_rms describes the intensity of the environment, can be used to compare power and power spectral density. Calculated from the plot as the square root of the total area under the plot

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11
Q

In launch manuals the Sound Pressure Level in the payload bay is defined in frequency bands. What frequency band sizes are commonly used?

A

1/3 octave bands - 32, 40, 50, 63, 80 Hz etc

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12
Q

Explain what a Shock Response Spectrum curve represents

A

Describes the peak accelerations that components need to withstand and comes from the max acceleration response of a SDoF models and defines damage potential of a time history . a consistent shock will have a repetitive curve

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13
Q

What are micro-vibrations and why can they b important in the design of a satellite structure?

A

Vibrations transmitted to sensitive instruments that can cause issues to sensors/optic payloads. If a payload on the satellite is sensitive, the design of the load path between the vibration source and payload will be affected. Need to include vibration suppression or isolation into the structure.

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14
Q

What types of packing architectures are used for closed body s/c?

A

Panel mounted and deck mounted

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15
Q

Briefly explain what Power Spectral Density is and how it can be obtained from a random vibration signal

A

defines the random vibration environment where power refers to the input signal and measures the acceleration. Signal processing done by Fourier transforms and from low pass filters to obtain the mean square value of each filtered frequency

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16
Q

Briefly describe the procedure known as Coupled Load Analysis and explain why it is carried out

A

Combining the validated FEM with the launch manual to predict the loads in launch. Used for final verification and is more accurate at this later stage than the launch vehicle user manual used in the design process. The CLA outputs allow for test specs to be derived to finalise the final design

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17
Q

What are the advantages of using a Marmon clamp band over other separation system designs?

A
  • efficient to join cylindrical structures
  • simple
  • loads transferred in relatively uniform manner due to the flanges being held together across whole perimeter
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18
Q

What are the most common modes of failure of honeycomb sandwich panels?

A
  • facing failure: in compression or tension face caused by insufficient panel thickness, facing thickness or facing strength
  • transverse shear failure: from insufficient core strength or panel thickness
  • local core crushing: from low core compression strength
  • *general buckling: from insufficient panel thickness or core shear rigidity
  • *shear crimping: can follow from buckling, from low core shear modulus, or low adhesive shear strength
  • *face wrinkling: buckles in or out depending on relative strengths of core in compression and adhesive in flat wise tension
  • *dimpling - for cellular cores, with v thin facing and large cells. Can propagate across adjacent cells
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19
Q

What are the basic steps of finite element analysis?

A
  1. Build solid model
  2. Mesh solid model
  3. Apply loads and constrain
  4. Solve
  5. Post processing
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20
Q

Briefly describe what is meant by the terms ‘cold bonded’ or ‘hot bonded’ inserts

A

Hot bonded is integrated in during the sandwich panel production and is blocks of material in cut outs of the core
Cold bonded is post panel production and potted with curing resin into the existing panel

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21
Q

In the context of the mechanical launch environment, briefly explain what the Quasi Static Loads are and how they are used

A

Steady state accelerations and low frequency dynamics. Used in preliminary design to constrain the loads and design a structure that can withstand application of these defined load factors

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22
Q

Briefly describe the hardware for which shock loading is typically a cause of concern

A
  • LV stage separation and combustion process
  • fairing separation (nose cone opening)
  • firing satellite release mechanism (release of clamp band)
  • release mechanisms for all deployable structures i.e. solar arrays
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23
Q

In the context of satellite structural design, define the parameters that are typically used to describe the acoustic loads

A

Sound Pressure Level (SPL) - strength of load
Directivity - direction of load

If SPL is equal along all directions then the field is reverberant

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24
Q

Describe the main issues that have to be taken into account in the mechanical design of a hinge and hard stops for example for deployable solar arrays

A

A hinge requires a hold down device to keep structure in until deployment.
In a hard stop the stiffness of the deployed array is good (array not floppy) but in deployment a high level of shock is exerted. To minimise this, damping can be included in the hinge spring but this will reduce the array stiffness so there is a trade off between stiff deployed arrays and high level shock

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25
Q

Describe the functioning of a typical pyro-cutter for space application discussing advantages and disadvantages w.r.t other devices

A

Cuts retaining bolt and is a one time device releasing in an explosive manner
Used in clamp band,
Simple but causes a hard stop without damping included. Can only be used once, rapid

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26
Q

Explain under what condition a quasi static test can be performed on a shaker

A

Only performed if the 1st fundamental mode is at least 3 times higher than the test frequency and for very low frequency vibrations where freq is less than resonance

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27
Q

Briefly describe the main advantages and drawbacks of the Aluminium Beryllium alloys compared with standard Aluminium alloys

A
  • AlBeMet allows for a higher Young’s modulus to steel with a lower density to standard Al by varying Be content
  • machined with techniques similar to high strength Al alloys therefore manufactured with increased mechanical efficiency without design effort
  • AlBeMet produces toxic materials and poses a serious health risk so needs to be machined in controlled environment (added expense)
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28
Q

Explain why bolt preload has to be applied

A

Predicts amount of torque and preload needed for each point of attachment. Keeps parts firmly together, avoids gapping and bolts under vibration coming loose

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29
Q

List the main causes of shock in satellite design and briefly describe the main characteristics of a typical shock load

A
  • LV stage separations and combustion process
  • fairing separation
  • firing satellite release mechanism
  • release mechanisms of deployable structures eg solar array

Short duration, high intensity loads

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30
Q

Explain why the SRS is used as a shock specification and what is the purpose of the Shock Response Spectrum Synthesis

A

SRS describes the shock in terms of the effect on a number of SDoFs identifying its intensity over the frequency range of interest - identifies damage potential

SRSS - process where a time history is created so that its SRS is exactly the same as the one required. Preferably done by summation of damped sinusoids

31
Q

Briefly describe the mechanical properties of the fibres typically used in composite materials for spacecraft applications

A

High specific strength and stiffness - mass savings
Dimensional stability - stable in different thermal environments
Signal transparency
Electrical insulation/conduction
Impact resistant
High damping characteristics
Fatigue resistance

32
Q

Describe the functioning of the separation known as Marmon clampband

A

Circular belt with a v-shaped grove which engages on 2 circular mating flanges holding them together.
The mechanism is activated by a pyrocutter, when the bolt is cut the belt opens by torsional springs which pulls out the shoes and frees the flanges. satellite is pushed away b separation springs

33
Q

Define the two different load paths that transmit loads to a satellite structure in the payload bay

A
  • structural or mechanical borne transmission via the launcher structure in which the s/c is excited by mechanical forces at its interface with the launcher
  • acoustic or air borne transmission via the ambient air in which the s/c is excited by the acoustic pressure field inside the fairing and acting on all exposed surfaces
34
Q

Define what is meant by deterministic, transient and random loads

A

Deterministic - loads we can predict as a function of time
Transient - relatively short excitation for pyrotechnic events (shock) and transient thrust loads
Random - deterministic analysis impossible. Behaviour must be considered in probabilistic manner. Excitations caused by various sources which are more or less independent

35
Q

What are handling loads and why are they important for satellite structural design?

A

Manufacturing, testing and transportation

Structure should be designed to withstand loads prior to launch. I.e. preventing damage for light structures, transportation when s/c is horizontal (diff load path), suitable mech ground support equipment

36
Q

In the analysis of static/dynamic systems what is meant by a ‘rigid’ body and how many degrees of freedom can be applied?

A

One that is assumed not to deform to simplify analysis by model the structure with a finite DoFs. Can be allowed up to 6

37
Q

In terms of the s/c structural design process, what standard tests need to be performed at what stage?

A

During design - development tests to verify use of new technologies
After manufacturing:
- prove compliance with appropriate requirements and specifications
- verify and update s/c FEM
- qualification tests - demonstrate existence of adequate margins of safety
- acceptance tests - ensure no manufacturing error introduced

38
Q

What typical ‘life phases’ will a s/c structure be designed to withstand?

A
  • manufacture & assembly
  • handling and transportation
  • testing
  • launch - lift off, transonic, stage & s/c separation from LV
  • appendage deployment
  • mission operations
  • reentry/demise
39
Q

What is the aim of a dynamic analysis for satellite structural design?

A

Calculate more accurate design parameters such as members loads, stresses and displacements

40
Q

How many modes of vibration does a real s/c have?

A

Infinite no of DoF each with unique mode of vibration therefore infinite modes of vibration

41
Q

Which of a s/c structures natural frequencies are of most concern?

A

Lowest frequencies with large amplitudes

42
Q

Why are sandwich panels so structurally effective?

A

Optimum strength to weight ratio and stiffness to weight
V good fatigue resistance
High damping
Relatively low cost
Thermal versatility

43
Q

What is a satellites secondary structure?

A

Other significant structural elements that aren’t primary I.e. booms, support trusses, platforms, solar panels, antenna dishes etc

44
Q

Define what is meant by modal effective mass and state why it is useful

A

The amount of mass being excited by a specific mode of a structure. Provides a method for judging the significance of a vibration mode

45
Q

Does damping affect the natural frequencies of a s/c structure?

A

No wn = sqrt(k/m)

46
Q

List 5 key material properties that drives the selection of s/c structural materials

A
  • strength (ultimate and yield)
  • stiffness (Youngs modulus)
  • specific mass
  • thermal characteristics - thermal conductivity, heat capacity and thermal expansion coefficient
  • toughness and fatigue resistance
  • resistance to s/c environment - outgassing, atomic oxygen corrosion
  • availability
  • cost - raw materials & manufacturing
47
Q

What two panel location properties reduce the pull out strength of an insert?

A

grouping effect & proximity of panel edge

48
Q

What is a satellites primary structure?

A

Major load path between s/c components and LV, including body structure & LV adapter

49
Q

What are the main LV used by the ESA?

A

Vega and Ariane

50
Q

What is meant by vibro-acoustic environment?

A

Very different load paths (structural/mechanical transmission & acoustic/airborne) can occur at the same time i.e. during lift off or transonic flight

51
Q

Define what is meant by dynamic unbalance

A

Effect on s/c separation. Less that 1 degree to achieve spin up mode

52
Q

What property dominates the response at low frequency?

A

Stiffness dominated

(W=wn damping dominated, high freq mass dominated)

53
Q

What are the two worst case launch periods for the acoustic environment and what are the sources for this acoustic noise?

A

Lift off and transonic flight
From engine functioning and aerodynamic separation and turbulence

54
Q

Does the primary structure include deployable booms and support trusses?

A

No

55
Q

In the s/c system engineering process what does abbreviation CDR stand for?

A

Critical Design Review

56
Q

For main European launchers, is there standardisation of the typical adapter interface geometries

A

No

57
Q

Would the introduction of a fully reusable LV have a significant impact of the structural design of LV payload?

A

Yes due to the re usability creating very different loading environment compared to standard expendable launchers

58
Q

Do Quasi Static Loads include some high frequency dynamics?

A

No, only low frequency

59
Q

Do SDoFs tell us anything useful about real systems?

A

Yes

60
Q

Can a spring in a SDoF model approximate a realistic system with multiple sections of different stiffnesses?

A

Yes

61
Q

Is it easy to position fundamental frequency of a large s/c above 2 kHz?

A

No

62
Q

Are simplified base acceleration time histories used to calculate the SRS?

A

No

63
Q

How many natural frequencies does a real structure have?

A

Infinite

64
Q

What natural frequencies are of concern to s/c structural engineer?

A

Typically those lower that 200 Hz

65
Q

When calculating normal modes of a structure, is damping included?

A

No

66
Q

Can the eigenvector matrix be scaled up by any value without affecting final analysis result?

A

Yes

67
Q

What does the output from Coupled Load Analysis replace?

A

The design acceleration info in the LV manual

68
Q

What is the most efficient packing for small satellites?

A

Panel mounted packaging

69
Q

Can 5XXX grade Al alloys be used in cube sat primary structures?

A

Yes

70
Q

Which fibre has the highest specific strength of the fibre options available for composite materials?

A

Carbon fibre

71
Q

Do joints that can be disassembled affect the s/c stiffness?

A

Yes, fewer disassembling joints increases the stiffness

72
Q

Is an accurate physical geometry needed when building a FEA model?

A

No

73
Q

Describe two types of general testing that need to be performed on mechanisms

A
  • Design - development tests to verify use of new tech
  • Post manufacture - to prove compliance with requirements and specifications, update FEM, qualification tests (margins of safety), acceptance tests (manufacturing error)

Space Systems Term 1 (4 decks)

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