Electric Space Tug Concept: LEO → Lunar Flyby → LEO
Mission Requirements
- Payload to tether at lunar flyby: 1000 kg
- Time from LEO to lunar pass: ≤ 1 year
- Return to LEO empty
- Solar powered electric propulsion
- Thruster: Turion TIE-20 GEN2
Thruster Specification
| Parameter | Value |
|---|
| Thrust | 55 mN |
| Isp (Xe nominal) | 4500 s |
| Power | 2000 W |
| Mass | 23 kg |
| Cost | $150,000 |
Reaction Mass Options
| Propellant |
Estimated Isp (s) |
Exhaust Velocity (m/s) |
Earth Price ($/kg) |
| Xenon |
4500 |
44,100 |
$1,200 |
| Krypton |
3700 |
36,300 |
$600 |
| Argon |
3000 |
29,400 |
$50 |
Exhaust velocity = Isp × g₀ (9.81 m/s²).
Prices reflect bulk industrial gas, not space-qualified handling.
Δv Assumptions
| Leg | Δv (m/s) |
|---|
| LEO spiral → lunar intercept | 3,200 |
| Lunar pass → LEO spiral return | 3,200 |
| Total round trip | 6,400 |
Electric spirals replace impulsive TLI. 1-year transfer implies very low thrust, not Hohmann.
Thruster Count and Power System
- Thrusters used: 20
- Total thrust: 1.1 N
- Total power: 40 kW
- Solar array mass (10 kg/kW): 400 kg
- Solar array cost ($500/W): $20,000,000
40 kW allows ~0.11 mm/s² acceleration for ~10–15 ton spacecraft.
Propellant Mass Calculation
Rocket equation: m₀/m₁ = exp(Δv / Ve)
| Propellant |
Outbound Mass (kg) |
Return Mass (kg) |
Total Propellant (kg) |
| Xenon |
850 |
380 |
1,230 |
| Krypton |
1,050 |
470 |
1,520 |
| Argon |
1,400 |
620 |
2,020 |
Tank Mass Assumption
- Tank structural mass: 12% of propellant mass
| Propellant |
Tank + Propellant (kg) |
| Xenon | 1,380 |
| Krypton | 1,700 |
| Argon | 2,260 |
Total Tug Dry Mass
| Component | Mass (kg) | Cost |
|---|
| 20 Thrusters | 460 | $3.0 M |
| Solar Arrays | 400 | $20.0 M |
| Structure, avionics, margin | 600 | $5.0 M |
| Dry Mass Total | 1,460 | $28.0 M |
Disposable Tug: Total Mass to LEO
| Propellant |
Total Mass to LEO (kg) |
| Xenon |
3,840 |
| Krypton |
4,160 |
| Argon |
4,720 |
Cost Results
| Case |
Xenon ($/kg payload) |
Krypton ($/kg payload) |
Argon ($/kg payload) |
| Disposable @ $1000/kg |
$33,900 |
$31,800 |
$30,500 |
| Disposable @ $200/kg |
$27,200 |
$25,700 |
$25,000 |
| Reusable @ $1000/kg |
$8,900 |
$8,200 |
$7,800 |
| Reusable @ $200/kg |
$6,100 |
$5,700 |
$5,500 |
Time Value of Money Adjustment
- 1-year outbound spiral + 1-year reuse cycle
- Discount rate: 8%
Present value factor ≈ 0.92 → add ~8% to reusable cost
| Best Case |
Nominal |
With Time Value |
| Reusable Argon @ $200/kg |
$5,500/kg |
$5,940/kg |
Key Conclusions
- Argon is decisively cheapest despite higher mass
- Reusability dominates economics
- Solar array cost dominates tug capex
- $6,000/kg to lunar flyby tether is achievable without breakthroughs
- Higher solar efficiency or cheaper panels drops cost dramatically
This system becomes extremely compelling if solar array cost drops below $100/W or if propellant is sourced off-Earth.