A Test System For Combustion Properties Of Solid Fuel And Ignition/Combustion Characteristics Of HTPB Based Fuels

Hybrid propulsion rocket combines distinguishing features of solid propulsion rocket and liquid rocket. Fuel and oxidizer are stored in different state in hybrid propulsion engine. Hybrid propulsion systems have many advantages over conventional solid- and liquid-propulsion systems, such as low-cost, safety, throtteability, high reliability and a wide range of appealing applications, making them very attractive for both military and commercial applications. However, the development of hybrid propulsion has been restricted by low regression rate of solid fuel. A large amount of works have been carried out abroad to improve the regression rate of solid fuel, however, few works were conducted in China. It is essential to establish a proper combustion apparatus for solid fuel in order to focus work on combustion characteristics of solid fuel and find some new methods to improve the regression rate of solid fuel.In this paper, a combustion system including a laser ignition system, an oxygen injection system, an optical test system, a pressure control system and a combustion chamber was build up in lab. At present, experiment can be carried out for different groups of oxidizer/fuel at pressure of 0.1～2.5 MPa (pressure can be up to 20 MPa once K9 glass was changed). Pressure in combustion chamber can be maintained in the range of ±6% of aimed pressure in order to ensure experiment will be carried out in quasi steady condition. Relationship between regression rate, mass burning rate, oxidizer/fuel mass ratio (O/F) and oxidizer mass flux were deduced. A program was written for data handling from the video recorded by high speed camera.Binder, plasticizer and curing agent selected for each fuel is HTPB R-20, dioctyladipate (DOA) and isophorone di-isocynate (IPDI), respectively. A reference composite fuel consisting of 78.86% of HTPB (mass fraction),13.04% of DOA,7.67% of IPDI and 0.43% of TIN was selected. Curing level was set at 1.04 in order to get good mechanical property. Fuels manufactured with the method of ultrasound and mixing at the same time in vacuum were demonstrated to have good properties:1) porosity of fuel is less than 1%; 2) micron sized powders were demonstrated completely in fuel without any agglomeration.Ignition characteristics of HTPB based fuel under CO2 laser irradiation were carried out at 0.1 MPa and 1.0 MPa. Results show that, when laser power density is in the range of 0 W/cm2～200 W/cm2, ignition delay of HTPB based fuel was decreased with the increasing of laser power density. For all of the fuels, both at 0.1 MPa and 1.0 MPa, the relationship between ignition delay (ti) and laser power density (q) could be described as ti= Aq-n, while n for all of the conditions is very close to 1.0, however, A for different conditions is quite different. Besides HTPB loaded with am_Al, ignition delay of all of the other fuels was shorted to nearly half of original when ambient pressure was increased from 0.1 MPa to 1.0 MPa. Ignition delay of HTPB loaded with am_Al was shorted 19.4% at most when pressure was increased from 0.1 MPa to 1.0 MPa over the shole range of laser power density investigated here. During ignition progress, distance between flame and solid fuel was decreased with the increasing of pressure, this could be one reason for pressure dependence of ignition delay. The concentration of gasified duel irradiated by laser could be another reason for pressure dependence of ignition delay.Gaseous oxygen was selected as oxidizer in order to investigated combustion characteristics of solid fuel at 1.0 MPa and 1.9 MPa, with initial oxygen flow of 380 kg/(m2·s). Results show that, relationship between regression rate and oxidizer mass flux usingrf(t)= aGoxn(t), and fit index n is 0.724 ± 0.003, which is consistent with literature values. All o f the added micron sized powers did have a positive effect on increasing both regression rate and mass burning rate. Both of high flame temperature and high density of am_Al contribute to much higher regression rate of HTPB loaded with am_Al compared with other fuels. When Gox=300 kg/(m2·s), mass burning rate of HTPB loaded with am_Al was increased to 83.2% compared with that of HTPB fuel without any additives. For aging problem, increase effect of MgB to regression rate of HTPB based fuel is lower than that of Mg.NASA-CEA program was use to calculate ideal chemical properties of fuels. Results show that, when 1.0<O/F<6.5, vacuum specific impulse of HTPB, HTPB loaded with Mg, HTPB loaded with MgB and HTPB loaded with am_Al comes to biggest value at the O/F of 2.3,1.9,2.3 and 1.9 respectively. Theoretical calculations show that vacuum impulse fuel ratio of fuel was decreased less than 1% with the help of three metal particles were added into HTPB fuel, however volume specific impulse was increased more than 5% with the help of addiation of Mg and am_Al. Ambient temperature of HTPB loaded with am_Al is higher than all of other fuels, while its condensed combustiopn product(CCP) content in combustion product at nozzle is 6.773 g/hg.