This document expands on Vince Cate's ideas for a first-generation space tether system designed to deliver small payloads to the Moon's surface at low cost. The goal is to create a functional system that can generate revenue through payload deliveries while demonstrating the viability of tether technology. The design prioritizes minimal initial costs, reusability, and incremental testing. It uses a rotating tether (rotovator) to lower payloads gently to the surface and incorporates high-ISP ion thrusters for orbit maintenance and transfer from LEO to lunar orbit.
The system is launched to LEO and uses its own propulsion to spiral to a low lunar orbit over approximately 7.7 months. Once in place, it can deliver up to 100 x 10kg payloads to the Moon's surface over time, with potential for bidirectional operations (catching payloads from the surface) to achieve true momentum exchange without constant thrusting.
The core is a 50 km rotating tether made of high-strength material (e.g., Dyneema or Spectra) in a bolo configuration: a heavy ballast end (movable module with thrusters and solar panels) and a light tip end (assembly for payload release and catching). The tether rotates to match the orbital velocity (~1.6 km/s) at perigee, allowing the tip to have near-zero velocity relative to the Moon's surface for safe payload release.
Key features:
This design is a good first use of a tether because it provides real utility (cheaper lunar deliveries than chemical landers) at small scale, with low risk through incremental testing. Total mass is far less than equivalent chemical systems, enabling breakeven at competitive $/kg rates.
We use 15 Turion TIE-20 GEN2 ion thrusters (55 mN thrust, 4500 s ISP, 2000 W power, 23 kg mass, $150,000 each). This provides total thrust of 0.825 N and power draw of 30,000 W. The number is driven by the need to transfer from LEO to low lunar orbit in under 8 months (~7.7 months with this config).
Propellant: Xenon or similar, with ~413 kg total (377 kg for transfer + 36 kg for 100 payload operations).
Minimized to ~10 kg: release mechanism, sensors (cameras, lidar for positioning), communications, small net (10m diameter with 5 cm holes for catching), and minimal solar (for backup). The net sweeps at ~30 m/s relative to surface for gentle catches.
~50 kg base (structure, robotics for tether traversal, inspection, payload handling). It moves along the tether for pumping, phasing, and maintenance. Can reel the entire tether for tip access.
30,000 W from panels (~150 kg at 200 W/kg efficiency). Provides power for thrusters, robotics, and communications.
50 km, ~200 kg (10x tip + payload mass for strength margin). Tapered design to optimize mass under centrifugal loads.
100 x 10 kg. Early ones: backhoe parts, solar panels, catapult components. Later: customer payloads (e.g., experiments, rovers). Crowdfunding: $50k/kg or $400k/10kg.
Assembled backhoes (solar-powered, always in sun via slow traverse) dig regolith, fill bags, and operate catapult for uplink. Incremental testing: practice launches, track net position with lidar/reflectors, time releases before real catches.
Low lunar elliptical orbit, perigee ~100-1000m above surface. Moon rotates every ~28 days; backhoe moves ~0.5 km/h to stay under perigee and in sunlight (sun-synchronous traversal). Tether orbit precesses slowly (once/year) for full coverage.
Water from Moon could resupply thrusters long-term.
| Component | Mass (kg) | Cost ($) | Notes |
|---|---|---|---|
| Payloads (100 x 10kg) | 1000 | 0 (customer-provided) | Revenue source |
| Tether (50 km) | 200 | 1,000,000 | High-strength material, space-qualified |
| Tip End Assembly | 10 | 500,000 | Sensors, net, release mech |
| Movable Module (base) | 50 | 1,000,000 | Structure, robotics, comms |
| Thrusters (15 x 23kg) | 345 | 2,250,000 | Turion TIE-20 GEN2 |
| Solar Panels (30,000 W) | 150 | 3,000,000 | ~100 $/W space-grade |
| Batteries & Electronics | 50 | 500,000 | For eclipses, control |
| Other (structure, tanks) | 50 | 200,000 | Misc |
| Propellant (transfer + ops) | 413 | 2,065,000 | Xenon ~5,000 $/kg |
| Total Dry Mass (excl. prop) | 1855 | 8,450,000 | |
| Total Launch Mass | 2268 | - |
Costs are rough estimates; development not included (~$10-20M additional for engineering/testing).
Delta-v: 8 km/s (low-thrust budget). ISP: 4500 s (v_exh ≈44,100 m/s). Propellant: 377 kg. Time: ~233 days (~7.7 months) with 0.825 N thrust.
Impulse needed: 10 kg × 1600 m/s = 16,000 N·s. Time: 16,000 / 0.825 ≈ 5.4 hours (every ~1 day possible).
Direct mission costs: Hardware $8.45M + Prop $2.065M + Launch (Falcon) $2.268M = ~$12.78M. For 1000 kg delivered: $12,780/kg.
With Starship launch: ~$11M total, $11,000/kg.
Revenue from 20 customers at $400k/10kg: $8M, reducing effective cost. Reuses lower future missions to <$5,000/kg. Compares to chemical landers (~$1M/kg small payloads).
This seems an excellent MVP: low mass/risk, revenue potential, scalable. Challenges: precise catching, tether durability, lunar assembly. Incremental testing mitigates risks. Could attract universities/companies for early payloads; crowdfunding viable given novelty.
For more: Momentum Exchange Tethers, Turion Thrusters.
Updated: January 04, 2026