¶ Introduction
This document details the components and margins for the hold-down assembly of the Firehorn rocket.
¶ Definitions and Abbreviations
- HD : Hold Down
¶ Applicable and Reference Documents
¶ Interfaces
The hold down mechanism interfaces with the hold down beam assembly, managed by GS and the thrust plate part, managed by ST.
¶ Overview
The assembly consists of a lifting ring attached to the GS beam, a turnbuckle to apply preload, and a cable assembly comprising the cable itself, thimbles, and cable connectors. An elastic element is integrated to ensure a clean disconnection at launch vehicle liftoff and the cable assembly sits within an insulated sheath to shield the steel elements from the engine exhaust. The lower part of the breakaway swivel is connected to the cable assembly, while the upper part—remaining attached to the launch vehicle—is fixed to a lifting ring bolted to the thrust plate. A 3D-printed chassis maintains the swivel within a defined volume during ascent and improves the system’s aerodynamic characteristics. The breakaway swivel can be replaced by a shackle to transform the hold down system in an anchoring point for VSFT activities.
¶ Rigging Analysis
¶ Hypothesis
To select the load rating of our rigging parts we will make the following hypothesis :
- Load is axial regarding the rigging parts main axis.
- 750 [kg] of thrust is produced by the engine.
- No load is dissipated by friction beetween the rocket and the rail.
- No additional Factor Of Safety is set on the load value, as rigging equipments include a FoS of 4 to 5.
- 10 [°] of launch angle is set beetween the vertical axis and the rocket axis. From a nominal elevation range of 0 to 10 [°].
¶ Load Rating
Let's compute the load when the rocket fires on a launch rail angled at 10 degrees :
F_{\text{axial}} = 750 /\cos(10^\circ) = 761.9 ≃ 800 \text
We should be looking for rigging parts with axial load ratings around 800 [kg].
¶ Parts Descriptions
¶ Order Sheet
- 2025_M_LOG_MEILI_04 Meili Order Sheet
¶ Breakaway Swivel
¶ Main Specifications
| Specification | Value | Unit |
|---|---|---|
| Dimensions | 146 x 32 | [mm] |
| Mass | 0.5 | [kg] |
| Cost | 550 | [CHF] |
¶ Lifting Rings
Écrou à anneau RUD VRM-M8, capacité 300 kg
¶ Main Specifications
| Specification | Value | Unit |
|---|---|---|
| Dimensions | M8 | |
| Mass | 100 | [g] |
| Design Load | 1000 Axial, 300 Random | [kg] |
| Factor of Safety | 4 to 5 | |
| Margin of Safety | 0.2 | |
| Cost | 36 | [CHF] |
| Fastening | 1x M8xXX Hex bolt | |
| Units | 2 |
¶ Interfaces
- Structure Thrust Plate
- Ground Segment Beam
¶ Analysis and Simulations
cf. "Rigging Analysis"
¶ Shackles
¶ Description
Manille lyre LA-1/4, capacité 1000 kg
¶ Main Specifications
| Specification | Value | Unit |
|---|---|---|
| Dimensions | D11 | [mm] |
| Mass | 100 | [g] |
| Design Load | 1000 | [kg] |
| Factor of Safety | 5 | |
| Margin of Safety | 0.2 | |
| Cost | 3.2 | [CHF] |
| Units | 1 | [m] |
¶ Analysis and Simulations
cf. "Rigging Analysis"
¶ Cable
Câble en acier Ø 8 mm, abgebunden (pces)
¶ Main Specifications
| Specification | Value | Unit |
|---|---|---|
| Dimensions | 8 | [mm] |
| Mass | 0.235 | [g/m] |
| Design Load | 41 | [kN] |
| Factor of Safety | 5.22 | |
| Margin of Safety | 4.22 | |
| Cost | 4 | [CHF] |
| Units | 5 | [m] |
¶ Analysis and Simulations
cf. "Rigging Analysis"
¶ Thimbles
¶ Description
Cosse 1608 pour câble en acier Ø 7.5- 8.0 mm
¶ Main Specifications
| Specification | Value | Unit |
|---|---|---|
| Dimensions | 8 | [mm] |
| Mass | 100 | [g] |
| Cost | 0.9 | [CHF] |
| Units | 2 | [m] |
¶ Cable Clamps
¶ Description
¶ Main Specifications
| Specification | Value | Unit |
|---|---|---|
| Dimensions | 8 | [mm] |
| Mass | 100 | [g] |
| Cost | 1.1 | [CHF] |
| Units | 8 | [m] |
¶ Turnbuckle
Tendeur LA-46/5, capacité 1000 kg
¶ Main Specifications
| Specification | Value | Unit |
|---|---|---|
| Mass | 0.8 | [g] |
| Design Load | 1000 | [kg] |
| Factor of Safety | 4 to 5 | |
| Margin of Safety | 0.2 | |
| Cost | 39 | [CHF] |
| Units | 1 |
¶ Chassis
The chassis contains the breakaway swivel and improves aerodynamic efficiency. It's bolted to the boattail using 4x M3 screws. The hole is a cone with a 1 degree angle to allow some positionning slack while mounting the lower beam on the launch rail. Active closure solutions were discarded due to their complexity.
¶ Main Specifications
| Specification | Value | Unit |
|---|---|---|
| Dimensions | 193 x 74 | [mm] |
| Mass | 60 | [g] |
| Material | PET-CF |
¶ Elastic and Insulated Sheath
Will be made from scraps ! They are bonus parts.