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**Classification:** Wearable Elastic Propulsion
**Status:** DRAFT v0.1
**Date:** 2026-01-22
---
## 1. CONCEPT OVERVIEW
```
FOREARM SLINGSHOT - "ARACHNE LAUNCHER"
═══════════════════════════════════════
ELBOW VERTEX (Anchor Point)
┌───────────────────────────────────────────────────────────────────┐
│ │
│ ╔═══╗ │
│ ║ ◉ ║ ← Elbow mount (elastic anchor + tensioner) │
│ ╚═╤═╝ │
│ │ │
│ │ ══════════════════════════════════════ │
│ │ ║ ELASTIC BAND (high-tension latex) ║ │
│ │ ══════════════════════════════════════ │
│ │ │ │
│ │ F O R E A R M R U N W A Y │
│ │ │ │
│ ▼ ▼ │
│ ┌─────────────────────────────────────────┐ │
│ │▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓│ ← Forearm sleeve │
│ │▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓│ (guides elastic) │
│ └─────────────────────────────────────────┘ │
│ │ │
│ ▼ │
│ ╔═══════════════════╗ │
│ ║ WRIST COIL UNIT ║ ← Spring-loaded spool │
│ ║ ┌───┐ ┌───┐ ║ (stores elastic tension) │
│ ║ │ ◎ │──│ ◎ │ ║ (uncoils at MACH release) │
│ ║ └───┘ └───┘ ║ │
│ ╚════════╤══════════╝ │
│ │ │
│ ▼ │
│ ┌───────────────────────────────────────────────┐ │
│ │ BACK OF HAND │ │
│ │ ╔═══════════════════╗ │ │
│ │ ║ SPRINGBOARD ║ ← Launch ramp │ │
│ │ ║ ▲▲▲▲▲▲▲▲▲▲▲▲▲ ║ (curved for │ │
│ │ ║ / \ ║ trajectory) │ │
│ │ ╚═══════════════════╝ │ │
│ │ │ │ │
│ └─────────────────┼─────────────────────────────┘ │
│ │ │
│ ┌─────────────────┼─────────────────────────────────┐ │
│ │ T H U M B │ T R I G G E R │ │
│ │ ▼ │ │
│ │ ┌─────────────┐ │ │
│ │ │ ◉ LATCH ◉ │ ← Thumb holds elastic │ │
│ │ │ ╲ ╱ │ at tension vertex │ │
│ │ │ ╲ ╱ │ │ │
│ │ │ ╲ ╱ │ RELEASE = thumb lifts │ │
│ │ │ V │ → elastic snaps back │ │
│ │ └─────────────┘ │ │
│ └───────────────────────────────────────────────────┘ │
│ │
└───────────────────────────────────────────────────────────────────┘
```
---
## 2. LAUNCH SEQUENCE
### Phase 1: ARMED
```
[ELBOW]
║
║←─────────────────────┐
║ elastic stretched │
║ (potential energy) │
║ │
[WRIST] │
║ coiled tight │
║ │
[HAND] │
║ │
[THUMB]═══════════════════┘
↑
LATCHED (vertex)
```
### Phase 2: RELEASE
```
[ELBOW]
║
║ ←──── elastic SNAPS
║ (kinetic energy)
║
[WRIST]
║ UNCOILS → MACH WAVE
║
[HAND]
║
[THUMB] ← released
```
### Phase 3: TUNNEL ACCELERATION
```
Elastic wave propagates through wrist coil at supersonic speed:
════►════►════►
MACH WAVE
```
### Phase 4: SPRINGBOARD LAUNCH
```
Projectile hits curved ramp on back of hand, redirects trajectory:
╱ ↗ PROJECTILE
╱
═════╱ springboard
↖ incoming wave
```
---
## 3. COMPONENT SPECIFICATIONS
### 3.1 Elbow Anchor Assembly
| Component | Material | Dimensions | Notes |
|-----------|----------|------------|-------|
| Mount plate | Aluminum 6061-T6 | 60mm × 40mm × 3mm | Anodized |
| Elastic hook | Stainless 316 | 8mm diameter | Polished loop |
| Strap | Neoprene + Velcro | Adjustable | Silicone grip lining |
| Tensioner | Spring steel | 2mm wire | Preload adjustment |
**Load Rating:** 150N static, 300N dynamic
### 3.2 Forearm Runway Sleeve
| Component | Material | Dimensions | Notes |
|-----------|----------|------------|-------|
| Outer shell | Ripstop nylon | 250mm length | Low friction |
| Inner channel | PTFE tube | 12mm ID | Guides elastic |
| Reinforcement | Carbon fiber strips | 3mm × 200mm | Prevents twist |
### 3.3 Wrist Coil Unit
```
┌──────────────────────┐
│ SPRING HOUSING │
│ ┌────────────────┐ │
│ │ ╭──────────╮ │ │
│ │ │ ◎◎◎◎◎◎◎◎ │ │ │ ← Coiled elastic
│ │ │ ◎◎◎◎◎◎◎◎ │ │ │ (compressed spiral)
│ │ ╰──────────╯ │ │
│ └───────┬────────┘ │
│ │ │
│ ════╧════ │ ← Release channel
│ (MACH TUNNEL) │
└──────────────────────┘
```
| Component | Material | Dimensions | Notes |
|-----------|----------|------------|-------|
| Housing | Glass-filled nylon | 50mm × 30mm × 20mm | Injection molded |
| Spool core | Delrin | 25mm diameter | Low friction |
| Spring | Music wire | 0.8mm × 15 coils | 2N/mm rate |
| Channel | Polished steel tube | 8mm ID | Chrome lined |
### 3.4 Springboard (Back of Hand)
| Component | Material | Dimensions | Notes |
|-----------|----------|------------|-------|
| Ramp surface | Flex-polymer (TPU) | 80mm × 50mm | 15° curve |
| Base plate | Carbon fiber | 2mm thickness | Bonded to glove |
| Shock absorber | Silicone gel pad | 5mm thickness | Dampens impact |
### 3.5 Thumb Trigger
| Component | Material | Dimensions | Notes |
|-----------|----------|------------|-------|
| Latch body | Titanium | 20mm × 15mm | Lightweight |
| Release mechanism | Ball detent | 3mm ball | 10N release force |
| Thumb pad | Textured rubber | 15mm diameter | Ergonomic |
---
## 4. ELASTIC SPECIFICATIONS
### Primary Band
- **Material:** Natural latex surgical tubing
- **Dimensions:** 6mm OD × 3mm ID × 400mm length
- **Elongation:** 600% max
- **Tensile strength:** 25 MPa
- **Fatigue life:** ~5000 cycles at 300% stretch
### Reinforcement (at vertices)
- **Material:** Kevlar sleeve
- **Coverage:** 30mm at each anchor point
- **Purpose:** Prevent abrasion failure
---
## 5. PERFORMANCE ESTIMATES
| Parameter | Value | Notes |
|-----------|-------|-------|
| Draw length | 300mm | Elbow to thumb |
| Draw force | 50-80N | User adjustable |
| Stored energy | 7-12 J | At full draw |
| Projectile mass | 5-20g | Foam darts, etc. |
| Launch velocity | 25-40 m/s | 90-145 km/h |
| Effective range | 15-30m | Flat trajectory |
---
## 6. ERGONOMICS & FIT
### Sizing Chart
| Size | Forearm Length | Wrist Circumference |
|------|----------------|---------------------|
| S | 220-250mm | 140-160mm |
| M | 250-280mm | 160-180mm |
| L | 280-310mm | 180-200mm |
| XL | 310-340mm | 200-220mm |
### Fit Concerns & Solutions
| Concern | Risk | Solution |
|---------|------|----------|
| Elbow anchor slip | Misfire | Velcro + silicone grip |
| Wrist coil bulk | Discomfort | Low-profile spiral housing |
| Thumb fatigue | Strain injury | Mechanical latch (click-release) |
| Back-of-hand impact | Bruising | Flex-polymer springboard |
| Elastic wear at vertices | Failure | Kevlar reinforcement |
### Fit Rating
```
HAND-FIT RATING: ████████░░ 8/10
(Prototype viable, needs iteration on wrist bulk)
```
---
## 7. SAFETY WARNINGS
⚠️ **EYE PROTECTION REQUIRED** - Always wear safety glasses
⚠️ **ELASTIC INSPECTION** - Check for cracks/wear before each use
⚠️ **NEVER DRY FIRE** - Always have projectile loaded
⚠️ **TEMPERATURE LIMITS** - Latex degrades below 0°C and above 40°C
⚠️ **FINGER CLEARANCE** - Keep fingers clear of release channel
---
## 8. REVISION HISTORY
| Version | Date | Changes |
|---------|------|---------|
| 0.1 | 2026-01-22 | Initial draft |
---
## 9. RELATED DESIGNS
- ARACHNE-002: Dual-arm synchronized launcher (PLANNED)
- ARACHNE-003: Magnetic assist variant (PLANNED)
- ARACHNE-004: Compressed gas hybrid (PLANNED)
---
## 10. CROSS-REFERENCE: MARIONETTE SPOOL SYSTEM
The ARACHNE forearm launcher shares architectural DNA with the **MARIONETTE SPOOL**
sail-drone propulsion system (`src/physics/marionette_spool.py`).
### Shared Principles
| Principle | ARACHNE | MARIONETTE |
|-----------|---------|------------|
| Tether Energy Storage | Elastic band on wrist coil | Cables wound on spool drums |
| Release Mechanism | Thumb latch (vertex) | Centrifugal unreeling |
| Runway/Guide | Forearm sleeve | Cable channels per sail |
| Human Control Point | Thumb trigger | Throw + whistle command |
| Terminal Guidance | Springboard redirect | Champion brain + HOLD |
### MARIONETTE SPOOL Architecture
```
COLLAPSED (pre-throw) DEPLOYED (marionette mode)
┌───────┐ SAILS
│███████│ ⛵ ⛵
│███████│◄── All cables \ /
│███████│ wound tight \ /
│SPOOL │ ┌───┴───┐
└───────┘ │ SPOOL │◄── Champion brain
│ │CONTROL│ runs HERE
▼ └───┬───┘
Human │
hand individual
cable control
```
### Integration Concept: ARACHNE-MARIONETTE Hybrid
The ARACHNE launcher could serve as the **throw mechanism** for a MARIONETTE spool:
1. **Pre-throw**: SPOOL is wound, cables coiled
2. **ARACHNE Launch**: Forearm runway provides velocity + spin to spool
3. **Unraveling**: Centrifugal force deploys sails
4. **Marionette Mode**: Champion brain takes over cable control
5. **Recall**: Sails collapse, cables reel back to spool
```
FOREARM ──────────────────────────────────► DEPLOYED CONSTELLATION
[ELBOW] ⛵
║ ⛵ ╲ ╱ ⛵
║══elastic══[WRIST]══SPOOL══════throw══════► ╲│╱
║ [SPOOL]
[THUMB] │
↑ MARIONETTE
release CONTROL
```
### Source References
- `src/physics/marionette_spool.py` - Spool mechanism + Champion brain integration
- `src/visualization/dervish_viz.py` - Visualization of sail constellation
- `src/physics/hubless_dervish.py` - Sail physics + wind tacking
|