Mechanical Systems Design Flashcards
What is the primary emphasis in spacecraft structural design compared to aircraft design?
The primary emphasis is on minimum weight, vibration interactions, and material behaviour, along with multifunctionality, such as providing environmental protection and thermal control.
What are the primary structural forms used in spacecraft design?
Beams: Long straight components supporting axial or lateral loads.
Trusses: Collections of beams connected for enhanced strength.
Panels: Thin walls, often in honeycomb form, for mounting and enclosure.
Monocoque structures: Thin-walled shells, sometimes with stiffening ribs.
What are CubeSats, and how are they classified?
CubeSats are small, self-contained nanosatellites deployed via spring-loaded pods. They are classified by “U” units, where 1U = 10 cm³. Examples include 1U, 3U, 6U, and 12U configurations.
Define specific strength and specific stiffness in the context of spacecraft materials.
Specific strength and specific stiffness refer to the strength and stiffness of a material per unit mass, critical for minimizing spacecraft weight while maintaining performance.
What are three key requirements for spacecraft structural materials?
- High specific strength and stiffness.
- Resistance to environmental conditions like radiation and extreme temperatures.
- Suitability for manufacturing and integration processes.
What is the function of honeycomb panels in spacecraft structures?
Honeycomb panels are lightweight and rigid, used for mounting equipment. They have a core (usually aluminum) bonded between face sheets and resist distortion using inserts for load distribution.
Describe the difference between primary and secondary spacecraft structures.
Primary structures provide direct load paths between major elements and the launcher interface.
Secondary structures support equipment and connect loads to the primary structure.
What is Hooke’s Law, and how does it relate to spacecraft materials?
Hooke’s Law states that stress is proportional to strain up to the elastic limit:
𝜎 =𝐸𝜖
where 𝐸 is Young’s Modulus. It defines material elasticity and stiffness, key for structural design.
Explain the concept of the factor of safety (FS) in spacecraft design.
The FS accounts for uncertainties in material properties and fabrication. It is applied as:
Proof level: Limit load × yield FS (no significant deformation).
Ultimate level: Limit load × ultimate FS (no rupture or collapse).
What role does Finite Element Analysis (FEA) play in spacecraft design?
FEA predicts:
- Natural frequencies and structural response to vibrations.
- Load distributions and maximum deflections.
- Optimized material allocation to meet mission requirements.
Why are monocoque structures advantageous and disadvantageous in spacecraft design?
Advantages: High strength-to-weight ratio.
Disadvantages: Susceptible to elastic instability and buckling, mitigated by stiffening ribs or honeycomb panels.
What is Poisson’s Ratio, and why is it important in material selection?
Poisson’s Ratio (
𝜈
ν) measures the lateral contraction relative to axial extension under stress. It influences material deformation behavior and structural integrity
Describe the concept of bending stress in beams using an equation
Bending stress,
𝜎𝑏 depends on:
𝜎𝑏 = 𝑀 / 𝑍
where 𝑀 is the bending moment, and
𝑍 is the section modulus. Positioning material away from the neutral plane increases stiffness.
What are the advantages of frame structures in spacecraft?
Frame structures, such as trusses, are lightweight and highly stiff. They use compression struts and tensile ties to optimize load-bearing with minimal material
What three types of testing are required to verify spacecraft structural designs?
- Static load tests: Check against proof and ultimate loads.
- Dynamic tests: Simulate vibration and shock during launch.
- Acceptance tests: Performed on the flight model to confirm readiness.
How do vibration tests simulate launch conditions?
- Sinusoidal sweeps: Identify natural frequencies in X, Y, Z axes.
- Shaped random vibrations: Mimic launch vehicle’s vibration spectrum.
What are some challenges associated with thin-walled tubes in spacecraft design?
- Bowing instability: Predicted by Euler’s formula.
- Local buckling: Depends on thickness-to-radius ratio.
What is the significance of the Q-factor in vibrations?
The Q-factor measures the damping efficiency:
𝑄 = SQRT (𝑘𝑚 / 𝐶)
Low Q-factors indicate better damping, reducing resonant amplification.
Why is mass optimization critical in spacecraft design?
Launch costs are approximately $25,000 per kg, so minimizing mass while ensuring structural integrity significantly reduces mission expenses.
What is the purpose of modular spacecraft designs like SSTL’s series?
Modular designs allow for scalable configurations, faster integration, and reuse of proven structural elements across missions.
What is the relationship between Young’s Modulus (E) and shear modulus (G)?
They are related by:
𝐺 = 𝐸 / ( 2((1 + 𝜈)
where 𝜈 is Poisson’s Ratio.
How do spacecraft shapes relate to attitude control strategies?
- Three-axis stabilized: Box-shaped for balanced mass distribution.
- Spin-stabilized: Cylindrical to enable smooth rotation.
- No control: Spherical for uniform stability in all directions.
What is the difference in behaviour between ductile and brittle materials under stress?
- Ductile materials: Show plastic deformation before fracture, with significant elongation.
- Brittle materials: Fracture with minimal elongation and no plastic deformation.
