What environments, prior to launch, must a spacecraft structure be designed to withstand?
Manufacture, testing and transportation
State and briefly describe two typical requirements for spacecraft structures
- 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
Define what is meant by static and dynamic envelopes for launch vehicles
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
What is a Frequency Response Function?
Ratio of the output amplitude w.r.t the input. Response can be displacement, velocity, acceleration
State how many fundamental frequencies there are for a satellite structure
Two - lateral and longitudinal
What is a complex mode and when do they occur?
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
What is the grouping effect of inserts in a honeycomb sandwich panel?
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
What is a satellites primary structure?
Structure that carries the main load path i.e. main structural element, external panels, main framework, interface adaptor etc.
Define what is meant by transient loads
Short duration, high intensity acceleration loads in shock environments
What does the g_rms value of an Acceleration Spectral Density (ASD) plot denote and how can it be calculated from the plot?
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
In launch manuals the Sound Pressure Level in the payload bay is defined in frequency bands. What frequency band sizes are commonly used?
1/3 octave bands - 32, 40, 50, 63, 80 Hz etc
Explain what a Shock Response Spectrum curve represents
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
What are micro-vibrations and why can they b important in the design of a satellite structure?
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.
What types of packing architectures are used for closed body s/c?
Panel mounted and deck mounted
Briefly explain what Power Spectral Density is and how it can be obtained from a random vibration signal
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
Briefly describe the procedure known as Coupled Load Analysis and explain why it is carried out
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
What are the advantages of using a Marmon clamp band over other separation system designs?
- efficient to join cylindrical structures
- simple
- loads transferred in relatively uniform manner due to the flanges being held together across whole perimeter
What are the most common modes of failure of honeycomb sandwich panels?
- 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
What are the basic steps of finite element analysis?
- Build solid model
- Mesh solid model
- Apply loads and constrain
- Solve
- Post processing
Briefly describe what is meant by the terms ‘cold bonded’ or ‘hot bonded’ inserts
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
In the context of the mechanical launch environment, briefly explain what the Quasi Static Loads are and how they are used
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
Briefly describe the hardware for which shock loading is typically a cause of concern
- 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
In the context of satellite structural design, define the parameters that are typically used to describe the acoustic loads
Sound Pressure Level (SPL) - strength of load
Directivity - direction of load
If SPL is equal along all directions then the field is reverberant
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 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
