Scope

• Design and machine a nitrous oxide and IPA liquid rocket engine with fully custom concentric tanks, floating piston, and a UC valve.
• Develop a valve and injector architecture compatible with manual lathe and mill fabrication while preserving performance and control.
• Characterise injector flow, discharge coefficients, and spray geometry through water and cold gas testing before hot fire.
• Build instrumentation for live pressure, temperature, thrust and video telemetry to validate system behaviour and safety.

Key Characteristics

Predicted thrust curve

Simulation thrust curve to compare against empirical results and inform design.

Predicted thermodynamics

Pressure and temperature modelling to used to find pressure drops and initial conditions for engine.

Design Overview

Concentric tank and floating piston

The engine uses a concentric tank geometry with a 48 mm inner oxidiser tube and a 76.2 mm outer fuel tube. A floating piston separates the nitrous oxide from the fuel and maintains a pressure on the fuel from the nitrous.

Concentric tanks allowed for simplified valve design, makes for a floating piston less likely to cam and doesnt required an extra line to bring either the fuel or oxidiser from the top tank which is needed in a stacked tank design. The design also accommodates stock circularity variance through generous O-ring cross-sections, making the sealed assembly forgiving to the measured 0.2 mm ID variation in the tube stock.

Valve design

The valve is based on a UC style valve, extended to support both oxidiser and fuel actuation using just the oxidiser fill tube. This approach preserves the compatibility of a classic burn-through hybrid UC while adding a true pintle injector and an independent check valve to regain ignition timing control.

Using the dump function on the ground support equipment, the valve can now be actuated without relying on uncontrolled burn through. That removes the two biggest UC drawbacks, poor timing control and lack of true injector integration.

Valve geometry

Cross-section detail shows the oxidiser and fuel paths, pintle profile, and sealing interfaces.

Injector development

The injector is a pintle injector with an outer fuel annulus and an inner oxidiser pintle. Injector geometry was chosen to match target OF ratio and momentum ratios. The CdA values determined from the model and water testing of the 3D printed injectors were used to size the effective areas of the injector elements.

Initial sizing and testing produced a fuel injector area of approximately 2.03 mm² and an oxidiser area of 20.34 mm², with discharge coefficients of 0.8 for fuel and 0.4 for oxidiser. These values were iterated through modelling and flow testing to support the expected 0.35 kg/s total massflow.

Valve motion

Animation directly illustrates the valve actuation and the transition between open and closed states with flow paths.

Valve open state

The open position occurs when the fill line is dumped, pressure on the underside of the piston drops to atmospheric and the excess pressure on top of the piston allows it to be actuated open.

Valve closed state

In the closed state the unequal area of the valve piston exposed to the nitrous pressure ensures there is a net force holding the piston up..

Injector testing and characterisation

Testing began with 3D printed injector prototypes to verify flow rates and spray geometry. Separate inner and outer flow measurements were taken with different prints, and the data was used to derive empirical Cd values for fuel and oxidiser.

Water tests validated the injector performance, including spray angle and flow distribution. The test programme also aims to establish a correlation between 3D printed and machined injector behaviour, testing to take place soon, machined components have just been finished.

• Initial test showed outer flow higher than the model, so annulus sizing was corrected to match the predicted spray angle within ~2%.
• Adding a pseudo annulus for the outer flow holes helped direct the flow and increase mixing while preserving the target OF ratio.
• Pump fed water tests measured a mass flow of ~0.28 kg/s, matching simulations at the time.
• Final tuning produced empirical discharge coefficients near Cd_fuel = 0.8 and Cd_ox = 0.4, which was used as the basis for the final design. I am expecting some difference due to surface roughness, simplified geometry of the 3D print and the difference in fluid characteristics between water and especially the nitrous oxide.

Water validation

Water testing captured spray formation and helped refine the injector's discharge coefficients.

Manufacturing

All components were manufactured on manual lathes and mills from local tube stock and billet. Radial bolts are shoulder bolts, matched drilled with bushings and designed to NASA 5020b standards for bolted joints.

An aft thread on the engine assembly lets internal components be compressed to manage axial tolerance and reduces the need for extremely tight axial fits. This thread preload strategy and generous O-ring seals support manufacturability and ease of assembly.

Finished hardware

Valve, injector and hydro/cold flow tank components.

Finished valve

Completed main valve assembly.

Lathe work

Manual turning for the inner concentric tank.

Hydrostatic and cold gas testing

Before committing to hot fire, a dedicated hydro and cold gas campaign will be done to validate the system with minimal safety risks. This phase will prove structural strength, valve actuation and injector performance.

• Hydrostatic pressure checks to confirm the concentric tank, floating piston and O-ring seals held under working load.
• Cold gas runs to verify valve functionality and steady flow through both oxidiser and fuel paths.
• Pump fed water tests to match injector behaviour to simulations prior to hot fire.

Hydrostatic test

Hydrostatic test of the complete valve and injector assembly with sub-scale tanks.