Nuclear-Electric Space Tug with Fusion-Assisted Exhaust Heating

1. Concept Summary

The proposed system is a high-power nuclear-electric space tug designed for in-space transport only (no planetary ascent). A compact fission reactor supplies steady electrical power. That electricity drives:

High-power electric propulsion (baseline thrust and efficiency)
A fusion-assisted exhaust heating stage that increases exhaust energy without requiring net-positive fusion power

Fusion here is treated as a thrust multiplier, not a power source. Any fusion reactions simply add kinetic energy to the propellant already being expelled. Even modest gains (10–20%) are valuable, and the architecture allows incremental improvement over time.

Key philosophy: Avoid the “fusion must break even” trap. Fission provides guaranteed electrical power; fusion improves exhaust performance as an add-on.

2. Mission Definition

Payload: 200,000 kg from LEO to Earth-Moon L5
Transit time target: ~1 month
Return: Tug returns empty to LEO
Secondary case: 600,000 kg payload

A low-thrust spiral out of LEO followed by cislunar transfer is assumed. Total Δv (including inefficiencies): ~6 km/s.

3. Power Scale

Nuclear-Electric Power Level

Parameter	Value
Electrical power	20–30 MW_e
Thermal power	80–120 MW_th

This is large, but plausible for a space-only reactor using liquid-metal cooling and high-temperature radiators. Below ~10 MW, the 1-month transfer becomes difficult with a 200-ton payload.

4. Mass Breakdown (Order-of-Magnitude)

Subsystem	Estimated Mass
Fission reactor core	15–25 t
Radiators	30–40 t
Power conversion & PMAD	10–15 t
Electric + fusion-assisted thrusters	10–15 t
Structural & shielding	10–15 t
Total tug dry mass	~80–110 t

This mass is heavy, but acceptable given the very high reusability and multi-hundred-ton payload capability.

5. Fusion-Assisted Exhaust Physics

Baseline (No Fusion)

High-power electric propulsion (MPD / VASIMR-like)
Exhaust velocity: 50 km/s (Isp ≈ 5,000 s)

With Fusion Assistance

Fusion reactions deposit energy directly into the exhaust plasma
No requirement for net energy production

Case	Energy in Exhaust	Effective Exhaust Velocity
No fusion	1.0× (baseline)	50 km/s
Early fusion assist	1.2×	55 km/s
Aggressive near-term	1.5×	61 km/s
Long-term aspirational	4×	100 km/s

Even the 20% case is a major win: higher Isp, lower propellant mass, or shorter trip time.

6. Thrust Levels

For a 25 MW system:

Parameter	Value
Exhaust velocity	55 km/s
Total thrust	~900–1,100 N
Initial acceleration (200 t payload)	~0.003–0.004 m/s²

This looks tiny, but sustained for weeks it produces large Δv.

7. Transit Times

200,000 kg Payload

LEO → L5: ~30 days
L5 → LEO (empty): ~10–14 days

The return leg is much faster due to far lower mass.

600,000 kg Payload

Same tug, same power
Transit time: ~70–90 days

Alternatively, clustering two or three identical tugs would restore ~1-month transit times.

8. Why This Architecture Is Attractive

Does not require net-positive fusion
Performance improves incrementally over time
High reusability amortizes reactor mass
Ideal for sustained cislunar industrial traffic

This is exactly the kind of system where “Elon-scale funding” and aggressive iteration would pay off: build something that works, then make it better every few years.

9. Development Timeline (Aggressive, Well-Funded)

Phase	Time
High-power space fission demo (5–10 MW)	5–7 years
Electric tug without fusion assist	7–10 years
Fusion-assisted exhaust (non-breakeven)	10–15 years
Operational LEO↔L5 logistics	~15 years

With exceptional funding, political support, and tolerance for risk, the first operational version could plausibly fly in the mid-to-late 2030s.

10. Final Assessment

Your intuition is sound:

Fission for power is the right anchor technology
Fusion as a thrust enhancer avoids the hardest problem
Even modest fusion gains are transformational for space logistics

This kind of tug would be a backbone asset for a high-traffic Earth–Moon economy and a realistic stepping stone toward eventual net-positive fusion propulsion.