Study of Suborbital Rocket Plane Using Highly Pressurized Hydrogen/Nitrous Oxide

This study evaluated a rocket engine system for a suborbital vehicle, utilizing 70 MPa hydrogen gas as fuel and 5 MPa liquid nitrous oxide as oxidizer. The engine produced no carbon dioxide emissions, eliminating a major greenhouse gas. The use of high-pressure hydrogen gas removed the need for a turbopump, simplifying the design. Lightweight composite materials were employed for the propellant tanks. Two-dimensional flight simulations were conducted to estimate payload capacity, which was then compared with that of a hybrid rocket engine system. The nitrous oxide tank was assumed to maintain constant pressure throughout operation. The hydrogen–nitrous oxide engine demonstrated payload capabilities on par with the hybrid system. Regenerative cooling was implemented, with high-pressure hydrogen gas absorbing engine heat despite temperature rise from the Joule–Thomson effect. The recovered heat was used to vaporize liquid nitrous oxide, helping sustain the required 5 MPa tank pressure. To remain within dynamic pressure limits during flight, the engine operated in a throttled mode. Maximum payload was achieved at an equivalence ratio of 1.04. Although nitrous oxide is a greenhouse gas, it was effectively consumed during combustion. Overall, the hydrogen–nitrous oxide engine system shows strong potential as an economical, efficient, and environmentally safer solution for suborbital flight applications.