Lunar Momentum Exchange Tether: First Mission Design
A Minimum Viable Product approach to establishing routine, low-cost cargo delivery to the lunar surface
Executive Summary
This document outlines a revolutionary first-generation lunar tether system designed to deliver payloads to the Moon's surface at dramatically lower cost than traditional rocket systems. The key innovation is using a 30 km rotating tether in lunar orbit that can gently release payloads to the surface while using high-ISP electric thrusters and gravity gradient pumping to maintain its orbit.
Key Advantages:
- 10-100 small payloads delivered per mission
- Reusable infrastructure for future missions
- Path to full momentum exchange with surface catapult system
- Revenue potential from early commercial/research customers
- Drastically lower $/kg delivery cost compared to chemical rockets
System Architecture
Core Components
1. The Tether
- Length: 30 km
- Material: Spectra or Zylon fiber
- Safety Factor: 3:1
- Configuration: Tapered design, thicker at center
2. Tip End Assembly (Minimized Mass)
- Payload release mechanism (electromagnetic or mechanical latch)
- Capture net system (10m diameter, 5cm mesh with ratcheting hooks)
- Corner cube retroreflectors for tracking
- Minimal solar panels (~100W for sensors/communications)
- Positioning sensors and radio communications
- Net deployment/retraction mechanism
3. Mobile Ballast Module (All Major Systems)
- 8× Turion TIE-20 GEN2 electric thrusters
- Large solar array (20 kW)
- Xenon propellant tanks
- Battery system for eclipse periods
- Tether winch/crawler mechanism
- Main computer and communications
- Payload storage and transfer robot
Thruster Configuration Analysis
Selected: 8× Turion TIE-20 GEN2
| Parameter |
Per Thruster |
Total (8 Units) |
| Thrust |
79 mN |
632 mN |
| ISP |
4500 s |
4500 s |
| Power |
2000 W |
16,000 W |
| Cost |
$150,000 |
$1,200,000 |
LEO to Lunar Orbit Transfer Time
Assuming a delta-v requirement of ~4,000 m/s for LEO to low lunar orbit transfer:
- System mass: ~2,500 kg (initial, including propellant)
- Continuous thrust acceleration: 0.632 N / 2500 kg = 0.000253 m/s²
- Effective thrusting time: ~60% (accounting for solar panel orientation, eclipses)
- Transfer time estimate: 5-6 months
This meets the requirement of under 8 months and allows reasonable mission iteration rates.
Momentum Recovery Per Payload
For each 10 kg payload released at 1,600 m/s relative velocity change:
- Momentum change needed: 16,000 kg·m/s
- Thruster force available: 0.632 N
- Time to recover momentum: 16,000 / 0.632 = 25,316 seconds ≈ 7 hours
- Propellant consumed per payload: ~0.36 kg xenon
With 3-4 days between payload releases (for gravity gradient pumping and positioning), the 7-hour thruster burn is easily accommodated.
Mass Budget
| Component |
Mass (kg) |
Notes |
| Tether (30 km) |
300 |
~10 g/m average, tapered design |
| Tip End Assembly |
30 |
Minimized: net, release, sensors, small solar |
| Mobile Ballast Module |
|
|
| 8× Thrusters |
80 |
~10 kg each estimated |
| Solar Arrays (20 kW) |
200 |
Modern arrays ~100 W/kg |
| Batteries |
50 |
Eclipse power, system redundancy |
| Structure & Mechanisms |
150 |
Winch, crawler, payload handler, robot |
| Avionics & Communications |
40 |
Computer, radios, sensors |
| Xenon Propellant |
400 |
LEO→Lunar + 100 payload operations |
| Xenon Tanks & Feed |
50 |
Pressure vessels, valves, plumbing |
| 100× Payloads @ 10 kg each |
1,000 |
Customer payloads |
| Contingency (10%) |
230 |
Design margin |
| TOTAL SYSTEM MASS |
2,530 kg |
|
Cost Analysis
Component Costs
| Item |
Cost |
| 8× Turion TIE-20 GEN2 Thrusters |
$1,200,000 |
| Solar Arrays (20 kW) |
$400,000 |
| Tether (30 km custom) |
$300,000 |
| Xenon Propellant (400 kg) |
$400,000 |
| Structure & Mechanisms |
$800,000 |
| Avionics, Sensors, Communications |
$500,000 |
| Batteries & Power Systems |
$200,000 |
| Integration & Testing |
$1,000,000 |
| Subtotal Hardware |
$4,800,000 |
| Launch Costs (variable, see below) |
Launch Cost Scenarios
| Launch Vehicle |
Cost per kg |
Total Launch Cost |
Total Mission Cost |
| Falcon 9 (future pricing) |
$1,000/kg |
$2,530,000 |
$7,330,000 |
| Starship (target pricing) |
$200/kg |
$506,000 |
$5,306,000 |
Cost per Kilogram Delivered
For 100 payloads × 10 kg = 1,000 kg delivered to lunar surface:
| Launch Scenario |
Total Cost |
Cost per kg Delivered |
vs. Chemical Rocket (~$50k/kg) |
| Falcon 9 @ $1,000/kg |
$7,330,000 |
$7,330/kg |
85% savings |
| Starship @ $200/kg |
$5,306,000 |
$5,306/kg |
89% savings |
Revenue Potential from Customer Payloads:
- 20 customers @ $400,000 for 10 kg = $8,000,000
- This exceeds total mission cost in the Starship scenario
- Makes first mission potentially profitable while proving the technology
- Subsequent missions only need propellant and new payloads (~$1-2M)
Operational Sequence
Phase 1: Launch and LEO Checkout (Month 0)
- Launch integrated system to LEO
- Deploy solar arrays
- System checkout: thrusters, tether deployment, communications
- Begin electric propulsion burn to lunar transfer orbit
Phase 2: Lunar Transfer (Months 1-6)
- Continuous low-thrust spiral trajectory
- Multiple Earth flybys to raise apogee
- Final burn into elliptical lunar orbit
- Target orbit: 100 km × 2,000 km (perilune × apolune)
Phase 3: Initial Payload Deliveries (Months 7-18)
- Transfer first payload to tip end assembly
- Use gravity gradient pumping over 2-3 days to spin up tether
- Phase rotation so tip is at perilune when pointing moonward
- Release payload at 1,000m altitude initially, gradually lowering to 100m
- Recover momentum with 7-hour thruster burn
- Repeat every 3-4 days for 100 payloads
Phase 4: Surface Systems Assembly (Months 12-24)
Early payloads include components for surface infrastructure:
- Mini robot backhoes: 5-6 units assembled from multiple 10kg deliveries
- Solar charging stations: Keep backhoes powered continuously
- Regolith catapult: Assembled from ~15 payload deliveries
- LIDAR tracking stations: Measure net position precisely
- Catapult targeting system: Calculate intercept trajectories
Phase 5: Testing and Optimization (Months 18-30)
- Backhoes fill regolith bags to standard 10 kg mass
- Practice catapult launches, measure trajectories
- Lower net closer to surface on each orbit
- Refine timing algorithms
- Test net position measurement accuracy
- Practice approach without actual catch attempts
Phase 6: First Catch Attempts (Month 30+)
- Coordinate catapult launch timing with net passage
- 10m diameter net moving at ~30 m/s provides reasonable capture window
- Payload hooks engage in 5cm mesh
- Successfully caught payload recovered via tether crawler mechanism
- True momentum exchange achieved - minimal propellant needed
Gravity Gradient Pumping Mechanics
The tether uses gravity gradient pumping to convert orbital energy into rotational energy without expending propellant. This elegant technique was extensively analyzed by Tethers Unlimited, Robert Forward, and Robert Hoyt.
How It Works
- Shorten tether when tip rotates away from Moon: Mobile module reels in tether, reducing moment arm when rotational velocity opposes orbital motion
- Lengthen tether when tip rotates toward Moon: Let tether extend during the portion of rotation when tip velocity aligns with orbital motion
- Net effect: Orbital energy gradually converts to rotational kinetic energy
- Time scale: Can increase tip velocity by several hundred m/s over 1-3 days
Advantages for This Design
- No thrusters needed at tip end - keeps mass minimal
- Tether mass scales with payload, not with thruster/solar systems
- Propellant only needed for momentum recovery after release
- Can fine-tune tip velocity for different release scenarios
Backhoe Mobility Requirements
The tether orbit is fixed relative to the stars, while the Moon rotates beneath it with a period of ~27.3 days. To keep the backhoe/catapult system always in sunlight and near the tether's perilune point:
Lunar Surface Tracking Speed
- Lunar circumference at equator: ~10,920 km
- Rotation period: 27.3 days
- Required tracking speed: 10,920 km / 27.3 days = ~16.7 km/day = 0.7 km/hr
This is a very gentle pace - the backhoe only needs to move about 700 meters per hour, which is easily achievable with solar-powered wheels or tracks. The system can even move intermittently during peak sunlight hours.
Orbit Precession
The tether's orbital plane must precess ~360° per year to maintain optimal geometry. This is achieved through:
- Slight adjustments to orbital inclination using thrusters
- Natural perturbations from Earth's gravity and lunar mascons
- Small periodic burns distributed throughout the mission
Release Dynamics and Impact Survival
Release Altitudes and Impact Velocities
| Release Altitude |
Free Fall Velocity Gained |
Impact Velocity |
Impact Energy (10 kg) |
| 1,000 m |
57.5 m/s |
~58 m/s |
16,900 J |
| 500 m |
40.6 m/s |
~41 m/s |
8,450 J |
| 250 m |
28.7 m/s |
~29 m/s |
4,225 J |
| 100 m |
18.1 m/s |
~18 m/s |
1,690 J |
Note: Lunar surface gravity is 1.62 m/s², so impact velocity ≈ √(2 × 1.62 × height)
Payload Protection Strategy
- Crushable foam core: Aluminum honeycomb or expanded polypropylene
- Airbag outer layer: Inflated kevlar spheres
- Regolith cushioning: Soft lunar dust absorbs additional energy
- Robust electronics: Potted in epoxy, shock-rated to 500+ G
- Gradual approach: Start at 1,000m, work down to 100m as confidence grows
Impact G-forces at 100m release: If 10cm crush distance absorbs the impact, deceleration = (18 m/s)² / (2 × 0.1m) = 1,620 m/s² ≈ 165 G. This is survivable with proper packaging.
L5 Satellite Deployment
Some payloads will be small satellites designed for deployment to the Earth-Moon L5 Lagrange point. This serves as both a technology demonstration and a test of the future Moon-L5 cargo tether concept.
L5 Transfer Requirements
- Delta-v from low lunar orbit to L5: ~700-800 m/s
- Tether tip velocity for L5 injection: Adjust via gravity gradient pumping to provide ~750 m/s
- Satellite requirements: Small cold-gas or ion thrusters for final corrections (~50-100 m/s delta-v budget)
- Communication: Must maintain contact during multi-day transfer
Operational Approach
- Complete all lunar surface deliveries first
- Configure tether for L5 injection velocity
- Release satellites at optimal phase angle
- Satellites perform mid-course corrections
- Arrive at L5 region, station-keep with onboard propulsion
Catching from L5 Return: This is significantly more challenging than catching from lunar surface due to higher velocities and timing precision required. Only attempt after surface catch operations are fully proven.
Capture Net Design
Specifications
- Diameter: 10 meters (78.5 m² capture area)
- Mesh size: 5 cm hexagonal pattern
- Material: Kevlar or Dyneema braided cord
- Deployment: Centrifugal force plus spring-loaded ribs
Payload Hook Mechanism
- Payload has 8-12 spring-loaded hooks on flexible stalks
- Hooks are narrow enough to pass through 5cm mesh openings
- Once through, hooks rotate and lock (ratcheting mechanism)
- Multiple hooks provide redundancy
- Entire payload mass can hang from 2-3 hooks safely
Capture Window Calculation
- Net sweep velocity relative to surface: 30 m/s
- Net diameter: 10 m
- Time window for payload in net path: 10m / 30 m/s = 0.33 seconds
- Catapult timing precision needed: ±0.1 seconds
- Launch window per orbit: 1 opportunity every ~4-6 hours (orbital period dependent)
The relatively gentle 30 m/s net velocity reduces stress on caught payloads while still providing reasonable capture probability.
Future Scalability
Once this initial system is proven, it can be scaled up in multiple ways:
Near-Term Improvements (Missions 2-5)
- Larger payloads: Scale to 50-100 kg per delivery
- More propellant: Send up additional xenon tanks to extend mission life
- Additional nets: Multiple capture systems for higher throughput
- Longer tether: 50+ km for more flexible operations
Long-Term Vision
- Water propellant: Extract and purify lunar ice, electrolyze to H₂/O₂ for thrusters
- Continuous operations: Balanced up/down traffic requires minimal added momentum
- Moon-L5 tether: Transfer cargo between lunar surface and L5 colonies
- Multi-tether network: LEO↔GEO, GEO↔Lunar, Lunar↔L5 infrastructure
- Ton-scale payloads: Eventually deliver habitat modules, mining equipment, etc.
Key Insight: First mission proves the technology and creates reusable infrastructure. Subsequent missions only need to send up propellant and payloads, driving costs down dramatically. The tether, thrusters, and solar arrays last for years.
Funding and Revenue Model
Crowdfunding Approach
- Target: $8-10 million total
- 1 kg payload slot: $50,000
- 10 kg payload slot: $400,000
- 100 kg payload (future): $3,000,000
Customer Segments
- Universities: Lunar science experiments, regolith analysis, seismic monitoring
- Space agencies: Technology demonstrations, international cooperation
- Private companies: Communications relays, resource prospecting, manufacturing tests