Application prospect of the hottest solid-liquid r

The application prospect of solid-liquid rocket engine in space launch

Abstract This paper introduces the working process of solid-liquid rocket engine, studies its characteristics and performance, discusses three principles for solid-liquid rocket engine to meet the development of low-cost, high reliability and pollution-free future launch vehicles, and emphatically studies its application prospect in space launch

keywords solid-liquid rocket engine, hybrid propulsion, space application

1 introduction

at present, most rockets or spacecraft use solid propellant or liquid propellant rocket motors. Both solid rocket motors and liquid rocket motors have their own advantages, but also have certain disadvantages. The continuous improvement of these two engines may improve some performance, but some shortcomings cannot be completely eliminated by using liquid or solid propellants alone. Therefore, engines using different aggregated propellants (solid or liquid) have emerged, which are called solid-liquid rocket motors. It is a development direction of rocket propulsion system at present, and has become an important competitor of liquid and solid rocket motors, with broad development prospects [1 ~ 4]

2 working process of solid-liquid rocket motor

due to the existence of a variety of physical and chemical processes, the working process of solid-liquid rocket motor is quite complex, and the reaction flow caused by two different flows in the solid fuel channel must be considered. One is the flow of oxidant injected into the front of the charging channel through the injector, and the other is the flow of high-temperature decomposition products of solid fuel. The flow of oxidant, fuel gas and combustion products in the engine conforms to the viscous hydrodynamic equation. The key to the problem lies in the interaction between wall reaction and flow [6]

Figure 1 is a system diagram of solid-liquid rocket engine

Figure 1 Schematic diagram of solid-liquid rocket engine

when starting the engine, valve 1 opens, and the heated compressed gas enters the tank at a certain pressure through the pressure reducer. After opening the valve 2, the compressed gas squeezes the liquid propellant in the storage tank to flow into the head of the injector. Under the action of the injector, the liquid components are broken into droplets and jets, and then flow into the channel of the solid grain. Then the solid components are ignited, the gas formed on the surface of the grain is mixed with the liquid components in the channel, the mixture of fuel and oxidant is burned, and the combustion products are sprayed outward through the nozzle, This generates thrust

the mixed combustion of oxidizer and solid propellant has two different combustion areas in the engine: inlet recirculation area and main combustion area. Figure 2 shows the combustion process of solid-liquid rocket engine

Figure 2 combustion process of solid-liquid rocket motor

in order to improve combustion, liquid oxygen HTPB solid-liquid rocket motor has a precombustion chamber in front of solid fuel grain to accelerate the gasification of liquid oxygen. The oxidizer forms a recirculation zone in the precombustion chamber. The airflow velocity in this recirculation zone is small, and the turbulent mixing in this zone is very strong, so that part of the oxidant and gasification fuel can be fully mixed and burned, so the combustion in this zone is controlled by chemical kinetics. The retreat rate of the burning surface in the inlet recirculation zone is directly controlled by fuel ablation and heterogeneous combustion at the interface

3 characteristics of solid-liquid rocket engine

solid-liquid rocket engine has advantages that other rocket engines do not have:

a) good safety

1) inert solid fuel - solid fuels commonly used in solid-liquid rocket motors, such as HTPB and PE, are inert substances, so they are safe in the process of production, storage and operation

2) non toxic oxidants - oxidants such as liquid oxygen are not toxic

3) no explosion and deflagration - in solid rocket motors, due to external impact or cracks and debonding in the grain, the engine may explode or deflagration and cause catastrophic accidents. In solid-liquid rocket motors, fuel and oxidant are separated into two tanks, and combustion usually occurs on the surface of solid fuel. The necessary condition for the gasification of solid charge is that the heat flow strongly enters the solid charge from the combustion area, that is, strongly enters the solid charge from the gas flow along the charge channel. Because the heat flow entering the crack from this gas flow is not as strong as entering the main surface, the gasification rate of the crack is low, which reduces the impact on the amount of charge gas generated due to the occurrence of cracks, and there will be no explosion and deflagration. Among them, the well cover pressure testing machine is widely used

4) no pollution of combustion products - in solid-liquid rocket engines, htpb/liquid oxygen is mostly used in propellant combinations, and their combustion products will not pollute the environment

b) simple operation and design

1) the change of external temperature has little effect on the engine - the change of temperature has little effect on the mixed combustion speed, so it is not necessary to consider the change of working pressure due to the change of external temperature, as the solid generator prevents the hidden danger of gas and coal dust accumulation, so the design of combustion chamber does not need additional safety factor

2) the combustion chamber does not need a cooling system - because the solid fuel is heated and decomposed, it can cool the combustion chamber shell itself. In addition, the unburned solid fuel can also act as the insulation layer of the combustion chamber shell, so the solid-liquid rocket motor does not need another regeneration or liquid film cooling system

3) the charge design can be rough - the solid motor will explode or deflagration due to the accidental crack and debonding of the grain, so the motor must be inspected before launching. The solid-liquid rocket engine is not sensitive to the occasional cracks and debonding in the grain, so the charge design can be rough

c) flexible operation

1) easy to shut down and restart - due to the independent on/off control valve in the solid-liquid rocket motor, the whole mixed combustion process is easier to control than that of liquid and solid motors

2) strong thrust regulation ability - some literature shows that as long as the oxidant flow rate is simply changed, the thrust can be changed by 90%, and the performance loss is only 10%. When adjusting the thrust of liquid engine, two control valves must be adjusted accurately at the same time to change the oxidant and fuel flow. If we want to adjust the thrust of solid motor, we have to change the charge design

3) there are a variety of propellant combinations that can be used - various additives such as aluminum powder can be added to solid fuel to improve the specific impulse of the engine

d) high reliability

compared with the two-component liquid rocket engine, it has only one liquid component, which simplifies the system and improves the reliability; Compared with solid rocket motor, fuel grain is simple in structure, high in strength and inert. It is insensitive to occasional cracks and debonding in the grain, which greatly improves the reliability

the main disadvantages of solid-liquid rocket motor are as follows: let's learn about the breaking strength test scheme of milk silk tensile testing machine:

a) the retreat rate of the gasification surface of solid fuel grain is low, which requires lengthening the combustion chamber or using porous grain, which is related to many factors. At present, it is mainly determined by motor experiments

b) the oxidizer is injected into the combustion chamber from the head, and there should be enough space to mix the gas decomposed by the solid fuel with the oxidizer, which reduces the volume filling coefficient of the fuel grain

c) the combustion efficiency is low, generally about 95%, which is slightly lower than that of liquid and solid rocket motors

d) in the process of steady-state operation and thrust adjustment, the mixing ratio of propellant will slightly deviate from the optimal value, resulting in the loss of specific impulse

e) the technology of solid-liquid rocket engine is not mature, and a lot of research work must be done if it is to achieve practical application

4 performance of solid-liquid rocket engine

solid-liquid rocket engine has a variety of propellant combinations to choose from. Solid fuels are mainly polyurethane rubber, such as hydroxy terminated polybutadiene, and some also contain metal powders (such as aluminum, magnesium, etc.) and energetic plasticizers. The most commonly used oxidant is liquid oxygen. Other oxidants that have been used include nitrogen tetroxide, nitrogen monoxide, nitric acid and hydrogen peroxide

thermochemical calculations show that [8] the storable hybrid propellant has a higher specific impulse than the solid propellant with metal additives, and its performance is equivalent to that of the storable liquid propellant. The combined specific impulse of nitrogen tetroxide and polyethylene and hydrogen peroxide and polyethylene can reach 2705 M/s. The semi cryogenic hybrid propellant using liquid oxygen and solid polymers is superior to the storable liquid propellant, and its performance is equivalent to that of liquid oxygen/kerosene semi cryogenic liquid propellant

Figure 3 shows the theoretical vacuum specific impulse (combustion chamber pressure exergy PC exergy =2.068 MPa, expansion ratio) when HTPB is combined with some oxidants ε= 60∶1）［9］。

Figure 3 theoretical specific impulse when HTPB is combined with various oxidants

5 the cost of solid-liquid rocket motor is low

foreign research shows that [4] the cost of solid-liquid rocket motor is only a small part of liquid or solid rocket motor. Taking a large solid-liquid rocket engine with a thrust of 14000 kn, which is intended to be used in the starting stage of space rocket, as an example, the cost relationship of different types of rocket engines can be identified. The cost of rocket engine is determined by the final development, production and operation costs, as well as the cost of materials and propellants. American experts believe that among large rocket engines with a thrust of 14000 kn, solid-liquid rocket engines have the lowest cost. It was also pointed out that when a 14000 kn solid-liquid rocket engine was created, its cost would be $37million, or $5.3 per kilogram of mass, while the cost would be reduced to $3.6 per kilogram of mass in the future. The mass of large solid rocket motors with the same thrust is $6.6 per kilogram, and the cost can only be reduced to $4.85 per kilogram in the future. The cost of liquid rocket motor is much higher than that of solid rocket motor. The low cost of solid-liquid rocket engine is mainly due to the following reasons:

a) good safety - because the solid-liquid rocket engine will not explode there or deflagration, and the propellant is also non-toxic, there is no need to take too many preventive measures in the process of production and transportation

b) low production cost of hardware - cheap steel shells can be used for oxidant storage tanks and combustion chamber shells, and commercial standards can also be used for the production of some components and parts

c) low detection cost - it is not necessary to conduct close monitoring to prevent explosion like liquid and solid rocket motors in the production process, and it is not necessary to conduct nondestructive testing on solid charges before launch

d) low development risk - the design of solid-liquid rocket motor can refer to the mature technology of liquid and solid rocket motor, so that the investment risk is small and it is easy to develop the rocket