Abstract
Densified liquid hydrogen/liquid oxygen is a promising propulsion fuel in the future. In order to systematically demonstrate the benefits and challenges of densified liquid hydrogen/liquid oxygen, a transient thermodynamical model considering the heat leakage, temperature rise, engine thrust, pressurization pressure of the tank, and wall thickness of tank is developed in the present paper, and the performance of densified liquid hydrogen/liquid oxygen as propulsion fuel is further evaluated in actual application. For liquid hydrogen/liquid oxygen tanks at different structural dimensions, the effects of many factors such as temperature rise during propellant ground parking, lift of engine thrust, mass reduction of the tank structure, and extension of spacecraft in-orbit time are analyzed to demonstrate the comprehensive performance of liquid hydrogen/liquid oxygen after densification. Meanwhile, the problem of subcooling combination matching of liquid hydrogen/liquid oxygen is proposed for the first time. Combining the fuel consumption and engine thrust lifting, the subcooling combination matching of liquid hydrogen/liquid oxygen at different mixing ratios and constant mixing ratios are discussed, respectively. The results show that the relative engine thrust enhances by 6.96% compared with the normal boiling point state in the condition of slush hydrogen with 50% solid content and enough liquid oxygen. The in-orbit time of spacecraft can extend about 2–6.5 days and 24–95 days for slush hydrogen with 50% solid content and liquid oxygen in the triple point state in different cryogenic tanks, respectively. Due to temperature rise during parking, the existing adiabatic storage scheme and filling scheme for densification LH2 need to be redesigned, and for densification LO2 are suitable. It is found that there is an optimal subcooling matching relation after densification of liquid hydrogen/liquid oxygen as propulsion fuel. In other words, the subcooling temperature of liquid hydrogen/liquid oxygen is not the lower the better, but the matching relationship between LH2 subcooling degree and LO2 subcooling degree needs to be considered at the same time. It is necessary that the LO2 was cooled to 69.2 K and 54.5 K, when the LH2 of 13.9 K and SH2 with 45% was adopted, respectively. This research provides theoretical support for the promotion and application of densification cryogenic propellants.
Highlights
Liquid hydrogen/liquid oxygen (LH2/LO2) is the most widely used as fuel for large launch vehicles because of its advantages of non-toxic, pollution-free, low cost, high specific impulse, and high thrust, whose specific impulse is 30–40% higher than of normal temperature propellants
Researchers are often keen to use densified cryogenic propellant as propulsion fuel, and its thermodynamic performance will be significantly improved compared with the saturated sate, such as increasing its own density, reducing gasification pressure, increasing cooling capacity per unit volume, etc
It is found that when the LH2 is subcooled to the slush state with 50% solid content, the relative engine thrust increases by 6.96% compared with the normal boiling point state
Summary
Liquid hydrogen/liquid oxygen (LH2/LO2) is the most widely used as fuel for large launch vehicles because of its advantages of non-toxic, pollution-free, low cost, high specific impulse, and high thrust, whose specific impulse is 30–40% higher than of normal temperature propellants. There are still some disadvantages, such as low density, easy evaporation, difficult storage, and so on At this time, researchers are often keen to use densified cryogenic propellant as propulsion fuel, and its thermodynamic performance will be significantly improved compared with the saturated sate, such as increasing its own density, reducing gasification pressure, increasing cooling capacity per unit volume, etc. Many researchers identified the advantages of subcooled cryogenic propellants in reducing take-off weight and extending in-orbit time, the advantages were mainly considered from the simple ideal situation, such as the benefits from the increase of density and cooling capacity.
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