To maximize performance by completely ignoring radiation and environmental protocols, we can push nuclear engineering to its absolute, unrestrained limits. Achieving an Earth-to-Moon-and-back Single-Stage-to-Orbit (SSTO) flight with a SpaceX-style retro-landing requires a massive thrust-to-weight ratio ($>1$) and an immense specific impulse (Isp​) to cover the roughly 22–25 km/s total $\Delta v$.

Without radiation constraints, the following three unshielded, high-thrust nuclear architectures become fully viable.

1. Open-Cycle Gas-Core Nuclear Thermal Rockets (GCNTR)

Traditional solid-core nuclear rockets (like NERVA) cap out at an Isp​ of 900 seconds because solid fuel rods will melt. An open-cycle gas-core system bypasses this by allowing the uranium fuel to exist as a supercritical, uncontained gaseous plasma at tens of thousands of degrees. Hydrogen propellant is pumped directly through this radioactive plasma torrent.

• Performance: It yields an Isp​ between 1,500 and 3,000 seconds coupled with millions of pounds of thrust.

• The Mission: Because we are ignoring radiation protocols, the engine can safely vent highly radioactive uranium fission products directly into Earth’s atmosphere during liftoff. This raw thermal power blasts the single-stage rocket directly into orbit, handles the lunar landing and takeoff, and provides the throttling capability needed to execute a precise, backward propulsive landing back on Earth.

2. Pulsed Nuclear Propulsion (Project Orion)

The ultimate brute-force SSTO architecture is pulsed nuclear propulsion. This design abandons traditional combustion chambers. Instead, the ship drops low-yield nuclear fission bombs out of the rear, detonating them against a massive, shock-absorber-mounted steel pusher plate.

• Performance: It delivers an Isp​ ranging from 3,000 to 10,000 seconds, combined with a thrust-to-weight ratio that can easily lift battleship-sized payloads.

• The Mission: An Orion ship shatters the tyranny of the rocket equation. By setting off a rapid sequence of nuclear explosions directly on the launchpad, it can lift off from a standing position on Earth. It transits to the Moon, lands vertically via micro-detonations, launches back to Earth, and uses a final sequence of precisely timed atomic blasts to cushion its descent, landing upright like a giant Falcon 9.

3. Nuclear Salt Water Rockets (NSWR)

The Nuclear Salt Water Rocket is essentially a continuous, controlled nuclear explosion. It stores a solution of water and highly enriched uranium salts in geometrically shielded, boron-lined tubes to prevent criticality. When pumped into a central reaction chamber, the solution reaches critical mass and goes prompt critical.

• Performance: The resulting continuous nuclear detonation provides an Isp​ of up to 10,000 seconds paired with massive, high-acceleration thrust.

• The Mission: Free from environmental restrictions, the NSWR acts as a literal flying torch. The steam and superheated fission products create a devastatingly radioactive but hyper-efficient exhaust plume. It possesses more than enough energy to single-handedly manage Earth departure, lunar touchdown, and the final retro-propulsive landing maneuver back at home base.