Hydrolox Rocket Revisited

I had previously proposed a hydrogen-powered rocket—a hydrolox rocket using HTP as a third propellant. Today I will propose a pure hydrogen and oxygen rocket without the HTP add-on. The objective of my design is to allow a pure hydrolox heavy-lift rocket without solid boosters. I solved the problems of current designs one by one to come up with this design revision.

In order to increase the thrust of the engine to eliminate boosters, we need to increase the combustion pressure while keeping the engine weight and vehicle dry mass low. The final design is an aggregate of my previous proposals adapted for these new requirements.

I solved the pressure problem of the engine by using an open-cycle pumping architecture and using LOX as the regenerative coolant. This setup removes the losses inherent to closed cycles and eliminates the pressure drop hydrogen suffers when used as the coolant. Pressurizing hydrogen is already difficult, so eliminating coolant pressure drops ahead of the combustion chamber solves the primary bottleneck. Using LOX as a regenerative coolant is highly feasible thanks to advancements in metallurgy; Inconel 718 suits this application well.

A key physical advantage of this choice is density matching: liquid hydrogen's density is much closer to superheated oxygen (at roughly 400 °C) than liquid oxygen is to gaseous hydrogen. This enables a more compact combustion chamber and boosts combustion performance.

I drive both the liquid hydrogen and LOX pumps using a single turbine, resolving the volumetric flow differential via reduction gears. This setup locks the ideal mixture ratio mechanically without requiring complex dual-loop valve controls.

As with my previous concepts, I opted for an aerospike nozzle. The nozzle features a truncated base rather than a full plug. The open-cycle turbopump exhaust discharges directly through this truncated section, fluidically extending the aerospike expansion ramp without a mass penalty while recovering open-cycle pumping losses. This setup allows a 200 bar combustion pressure to yield high thrust on a compact, lightweight engine.

For the airframe, I propose a quad-tank layout inspired by my naked ultimate rocket design, where hydrogen and LOX tanks are strapped in a quad formation. Four structural studs sit at the tangential interfaces where opposite propellants meet. This is where the engines mount; these studs transfer thrust directly up through the airframe, isolating the thin tank walls from primary axial thrust loads. Additionally, mounting engines at these nodes enables direct dual-propellant feeding, bypassing the heavy, complex common bulkheads of traditional rockets.

The propellant tanks utilize a metallic sandwich shell: a thin Inconel inner skin surrounded by an open-cell Inconel metal foam. Evacuating this foam layer provides thermal insulation for the cryogenic tanks. The entire quad-tank assembly is wrapped in a corrugated Haynes 214 outer shell, with the internal void spaces pulled to a hard vacuum. The corrugated Haynes 214 skin protects the assembly from aerothermal heating during ascent and reentry, accommodating thermal expansion without stressing its structural attachment points.

These vacuum voids also serve as protected utility channels for autogenous pressurization lines. The hard vacuum prevents the warm GH₂ and hot GOX lines from transferring heat to the cryogenic liquid tanks as they route upward to the top ullage spaces.

To manage vehicle flight control without heavy mechanical gimbals or flex joints, the open-cycle gas-generator setup enables rapid differential throttling across the four engine pods. Modulating the fast-acting gas generator control valves allows rapid thrust adjustments across the 30%-100% throttle range. Furthermore, injecting cold liquid hydrogen directly alongside hot supercritical GOX provides the closest possible density match for a hydrolox system (≈ 71 kg/m³ vs ≈ 105 kg/m³). This balanced momentum ratio promotes rapid micro-mixing and flash-vaporization inside a compact combustion chamber, minimizing fluid lag so valve adjustments across the four stud nodes deliver a control response that matches or exceeds the speed of heavy servo-gimbals while eliminating hundreds of kilograms of actuator mass.

Complementing differential throttling, dedicated hydrolox micro-thrusters are integrated along the outermost perimeter of the corrugated outer skin to handle high-frequency attitude adjustments. These micro-thrusters tap directly into the warm autogenous GH₂ and hot GOX lines running through the vacuum voids, drawing high-pressure gas without requiring separate propellant tanks or gas bottles. Placing these gaseous thrusters at the maximum radius of the vehicle provides extreme geometric leverage for precise pitch, yaw, and roll control during atmospheric flight and reentry maneuvers.

Finally, the quad-strapped, stud-supported architecture creates an exceptionally stiff airframe. This high structural rigidity allows the vehicle to initiate its gravity turn earlier and execute more aggressive pitch angles through Max-q than conventional thin-skinned, foam-insulated rockets.