Hybrid Ultimate Rocket

I would like to propose another variant of the Ultimate Rocket architecture that I introduced previously. My original design utilized turbopumps in its first stage to achieve a 45 bar combustion chamber pressure. However, High-Test Peroxide (98% pure HTP) is a highly unstable molecule, meaning even a carefully engineered turbopump presents severe risks of localized heating and friction-induced explosions. If a reliable high-pressure turbopump cannot be safely developed, we need a viable alternative.

This new proposal is a hybrid design, combining the structural concept of my Yurt Rocket with the stage configurations of the recent Ultimate Rocket. Because the second and third stages of this architecture already bypass the use of turbopumps, they will remain as originally planned. The primary modification focuses entirely on the first stage fluid dynamics and structural layout.

Hydrostatic & Low-Pressure Propellant Architecture

To generate sufficient engine injection pressure for the first stage without mechanical pumps, I opted to utilize gravity and vehicle acceleration. The stage requires a high aspect ratio—a long, narrow "pencil" layout—to build up the necessary hydrostatic fluid head. Because HTP is highly dense, a propellant tank height of 45 meters combined with just 1 atm of inert nitrogen or helium gas pressuring the upper ullage area is enough to passive-feed the bottom manifolds at 6 to 7 bars.

It is important to note that this initial hydrostatic pressure setup is calculated for take-off conditions only. As the engines burn propellant, the fluid level inside the 45-meter tanks drops drastically, which would normally cause a severe drop in injection pressure. However, because the rocket continuously sheds massive amounts of propellant weight, its acceleration rate spikes sharply during the climb. The increasing inertia from this high acceleration compresses the remaining fluid column downward. This automated inertial throttle perfectly counteracts the depleted tank levels, maintaining the required 7 bar injection pressure at the engine manifolds all the way until first-stage shutdown.

LPG (Liquefied Petroleum Gas), on the other hand, has a lower density than HTP and requires a higher baseline ullage pressure headroom to reach the exact same 7 bar manifold injection target. Small-diameter, elongated tanks are lightweight and can easily withstand these low structural pressures without requiring thick walls. For the propellant tank shells, I propose the use of marine-grade Al-Mg (Aluminum-Magnesium) alloy. This material contains virtually zero copper, making it highly compatible with 98% HTP by preventing catalytic decomposition, while remaining lightweight and durable.

Concentric Tank Layout and Structural Studs

The first-stage tanks do not sit below the upper stages; instead, they completely surround them in an alternating circular perimeter. Because the tanks are nested in this outward-to-inward configuration, the overall height of the vehicle is drastically limited. At every vertical seam where two adjacent tank walls meet, we place high-strength structural studs running the full length of the joints. To trim dry weight while handling the massive axial forces, these external studs are manufactured from Al-Li (Aluminum-Lithium) alloys.

The modular 3D-printed aerospike engine blocks are mounted directly beneath these studs. Each engine is fed directly by the two adjacent propellant tanks. This layout represents a "Naked Rocket" architecture: the close proximity eliminates extensive piping, massive manifolds, and complex valves. Minimizing the internal fluid routing paths directly reduces the risk of premature, catastrophic HTP decomposition while keeping the structural mass low.

Sub-Orbital Ballistic Separation Profile

The flight profile relies on a high-altitude ballistic staging sequence rather than traditional continuous-thrust hot staging. The first stage will fire its low-pressure outer engine ring, providing a pure vertical ascent up to approximately 50 km before shutting down its engines. From there, the entire vehicle will coast upward passively through the thin atmosphere on its own kinetic momentum, reaching an apogee near 100 km where its vertical velocity drops close to zero.

Because the vehicle is momentarily stationary at apogee, mechanical separation forces are minimal. The upper core stages (Stages 2 and 3) will unlatch and disengage smoothly from the outer Stage 1 sleeve. Once unlatched, Stage 1 fires a small, short-duration separation burn to back away and clear the area, leaving the upper core behind. The second and third stages are stacked linearly one above the other inside the core. Once cleared, Stage 2 ignites its engines, performs a standard gravity turn, and begins horizontal orbital acceleration. When Stage 2 is depleted, it is expended, and the final Stage 3 separation proceeds as a classic linear stage separation in the vacuum of space.

Dual-Purpose Variable Geometry Fairing

As the separated Stage 1 booster begins its descent back to Earth, it does not rely on heavy fuel reserves for a massive retro-burn. Instead, the top aerodynamic fairing remains attached to the booster ring and transitions into a deployable mechanical braking shield, doubling as a solid parachute during free fall.

Controllable vent windows integrated directly into the fairing structure open and close to regulate the internal air mass flow, keeping peak deceleration forces under control. Furthermore, by modulating these vents asymmetrically, the system can actively steer and maneuver the falling stage, guiding it back toward the launch site for a low-velocity touchdown. Unlike traditional rocket fairings that are discarded as waste in space, this design utilizes the fairing for a double purpose throughout the entire flight lifecycle.

During liftoff, this same fairing provides a secure aerodynamic shield over the open payload bay of the third stage. Because the fairing covers the full length of the nested core, it creates an elongated, spacious payload room. This structural volume allows satellite operators to launch larger diameters and highly irregular payload geometries that would never fit inside standard commercial rocket fairings.