Kylan Zhao - Phoenix-1 Internal Structure

Phoenix-1 Internal Structure

Background

After Marginal Stability was cancelled, I joined the Phoenix-1 structures team and started by validating an existing internal structure design for updated load cases with the goal of having minimal redesign.

Design

The design I was given had not considered parachute loadings; but once a worst-case prediction was found (2000 lbf), the structure was found to fail in tension at the bolt locations. My remedy for this was to add a thin Aluminum 7075-T6 brace spanning one horizontal bolt above the plate to another horizontal bolt below the plate in order to take some of the tension load. Aluminum 7075-T6 was chosen due to its higher yield strength, but most importantly its higher Young's Modulus. This was implemented with minimal changes to existing components: both the L-brackets and square tubes remained unchanged, and the plates had additional holes for the brace to pass through. However, with more detailed analysis and an updated worst-case parachute load (1400 lbf effective), it was determined that this component was no longer necessary, and we reverted to the original design.

Analysis

Conclusions

A Look Back

Requirements

Withstand loads
Minimal redesign
<10% mass increase
<10% cost increase

Load Cases

Parachute load (tension, static): <<600 lbf (old) -> 2000 lbf (old) -> 700 lbf (current, static) -> 1400 lbf (current, dynamic)
Maximum thrust + MaxQ (compression, static): 600 lbf
Engine vibration (undetermined)
Aerodynamic vibration (undetermined)

Design Choices

Added stiffening braces though plates, connecting top and bottom L-bracket to aid in tension
- Later removed due to improved analysis

Analysis

Parachute loading response factor
- Assumptions
  - 700 lbf is static load (conservative)
  - Applied instantly (conservative)
- Result: response factor = 2, actual load = 1400 lbf (tension)
Bolt tension
- Done by separate Analysis team
- Very high FoS
Bolt shear
- Done by separate Analysis team
- Very high FoS
Square tube tension
- Model: Single square tube with all bolt holes
- Material: Aluminum 6061-T6
- Connections: N/A
- Contacts: N/A
- Fixtures: Bottom face, all nodes 6 DoF constraint (reasonable approximation)
- Load: Top face, 350 lbf up (reasonable approximation)
- Min FoS: >1.5
Internal structure section buckling
- Model: Single section of internal structure with maximum expected square tube lengths
- Material: All components Aluminum 6061-T6
- Connections: Pin connections, free rotation, translation restricted
- Contacts: All no penetration
- Fixtures: Bottom face of bottom plate, all nodes 6 DoF constraint (reasonable approximation)
- Load: Top face of top plate, 1400 lbf up (reasonable approximation)
- Min FoS: >1.5
L-bracket tension
- Model: Quarter section of plate including bolt head with corner pointing inwards, 1 square tubes, 2 L-brackets, all components except L-brackets treated as rigid bodies
  - Bolt head inclusion and orientation was to consider the worst-case stress concentration due to the bolt, near the stress concentration of the inner corner of the L-bracket
- Material: All components Aluminum 6061-T6
- Connections: Pin connections, free rotation, translation restricted
- Contacts: All no penetration
- Fixtures: Bottom face, all nodes 6 DoF constraint (reasonable approximation)
- Load: Top face, 350 lbf up (reasonable approximation)
- Min FoS: >1.5