Ammonium-nitrate-(AN-) based composite propellants prepared with a hydroxyl-terminated polybutadiene (HTPB)/polytetrahydrofuran (PTHF) blend binder have unique thermal decomposition characteristics. In this study, the burning characteristics of AN/HTPB/PTHF propellants are investigated. The specific impulse and adiabatic flame temperature of an AN-based propellant theoretically increases with an increase in the proportion of PTHF in the HTPB/PTHF blend. With an AN/HTPB propellant, a solid residue is left on the burning surface of the propellant, and the shape of this residue is similar to that of the propellant. On the other hand, an AN/HTPB/PTHF propellant does not leave a solid residue. The burning rates of the AN/HTPB/PTHF propellant are not markedly different from those of the AN/HTPB propellant because some of the liquefied HTPB/PTHF binder cover the burning surface and impede decomposition and combustion. The burning rates of an AN/HTPB/PTHF propellant with a burning catalyst are higher than those of an AN/HTPB propellant supplemented with a catalyst. The beneficial effect of the blend binder on the burning characteristics is clarified upon the addition of a catalyst. The catalyst suppresses the negative influence of the liquefied binder that covers the burning surface. Thus, HTPB/PTHF blend binders are useful in improving the performance of AN-based propellants.
Solid propellants are contained and stored in the combustion chamber of a solid rocket motor and are sometimes hermetically sealed in the chamber for long-term storage. Upon ignition, the propellants react to form hot gases within the chamber, which in turn are accelerated and ejected at a high velocity through a supersonic nozzle, thereby imparting momentum to the rocket motor. Solid rocket motors offer the advantage of having few moving parts. Therefore, they are used as propulsion systems for launch vehicles, spacecrafts, missiles, and other applications.
There are various types of solid propellants, and a suitable propellant is selected to meet the requirements of each rocket motor application. A composite propellant is a solid propellant in the form of heterogeneous propellant grains composed of oxidizer crystals held together in a matrix of a synthetic or plastic binder. Ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) are widely used as an oxidizer and a binder, respectively, because AP/HTPB-based propellants have excellent burning and mechanical characteristics. One of the few serious drawbacks of AP-based propellants are the products of combustion, such as HCl, chlorine, and chlorine oxides, which cause atmospheric pollution.
Recently, ammonium-nitrate-(AN-) based composite propellants, that is, propellants prepared with AN as the oxidizer, have become popular, although there are some major problems associated with their use. These problems include a low burning rate, poor ignitability, and low energy output compared to AP-based propellants [
Applying an energetic binder is an effective approach toward AN-based propellants. However, the synthesis processes of energetic binders are complicated and costly; therefore, it is difficult to manufacture these binders industrially. To date, such energetic binders have not yet been used for practical applications because they are expensive.
Polytetrahydrofuran (PTHF) is used as an ingredient in making rubber products. This inexpensive polymer is mass-produced commercially in several molecular weights. Table
Chemical properties of PTHF and HTPB.
Binder ingredient | PTHF | HTPB |
---|---|---|
Molecular structure | HO–(–CH2–CH2–CH2–CH2–O–)n–H | HO–(–CH2–CH=CH–CH2–)n–OH |
Density (g cm−3) | 0.970–0.981 | 0.902 |
Heat of formulation (kJ mol−1) | −219.2 | −21.1 |
The curing behavior, mechanical properties, and thermal decomposition behavior of HTPB binders are improved by the addition of a small amount of PTHF. PTHF is an effective plasticizer in the preparation of high-performance composite propellants [
The thermal decomposition behaviors of AN-based composite propellants with a HTPB/PTHF-blend binder are provided in [
AN was ground in a vibration ball mill (for 5 min) for use as an oxidizer. The weight mean diameter of AN was 125
The values of
Binder formulations.
Binder | Mass fraction (%) | |||
(—) | HTPB | PTHF | IPDI | |
PTHF1 | 0 | 92.5 | 0.0 | 7.5 |
0.2 | 69.7 | 17.4 | 12.8 | |
0.4 | 49.5 | 32.9 | 17.6 | |
0.6 | 31.3 | 46.9 | 21.8 | |
0.8 | 14.9 | 59.5 | 25.6 | |
PTHF2 | 0 | 92.5 | 0.0 | 7.5 |
0.2 | 72.5 | 18.1 | 9.4 | |
0.4 | 53.3 | 35.6 | 11.1 | |
0.6 | 34.9 | 52.3 | 12.8 | |
0.8 | 17.1 | 68.5 | 14.4 | |
PTHF3 | 0 | 92.5 | 0.0 | 7.5 |
0.2 | 73.8 | 18.5 | 7.7 | |
0.4 | 55.3 | 36.8 | 7.9 | |
0.6 | 36.8 | 55.2 | 8.1 | |
0.8 | 18.4 | 73.4 | 8.2 |
Ammonium dichromate (ADC) was used as a burning catalyst and added to the propellant at 6% [
Each of the prepared propellant strands were 10 mm in diameter and 40 mm in length. The burning behavior was investigated in a chimney-type strand burner that was pressurized with nitrogen. Each strand was ignited by applying 10 V to an electrically heated wire threaded into the top of the strand. The temperature of the wire was above 1200 K. Each propellant strand was combusted within a pressure range of 0.5–7 MPa. The burning phenomenon of the propellant was recorded by a high-speed video recorder, and the burning rate was measured from the images obtained. The combustion phenomenon was recorded at a shutter speed of 30–125 frames s−1. All measurements were checked in triplicate at each pressure, and the average values were used in the data analysis.
The specific impulse (
Theoretical values of
Propellant | AN/HTPB/PTHF1 | AN/HTPB/PTHF2 | AN/HTPB/PTHF3 | |||
AN content (%) | 70 | 80 | 70 | 80 | 70 | 80 |
0 | 189 | 198 | 189 | 198 | 189 | 198 |
0.2 | 190 | 199 | 189 | 199 | 189 | 199 |
0.4 | 190 | 201 | 190 | 200 | 190 | 200 |
0.6 | 191 | 202 | 191 | 201 | 191 | 201 |
0.8 | 192 | 203 | 191 | 203 | 191 | 202 |
0 | 1186 | 1353 | 1186 | 1353 | 1186 | 1353 |
0.2 | 1194 | 1403 | 1191 | 1393 | 1190 | 1388 |
0.4 | 1200 | 1456 | 1197 | 1438 | 1195 | 1429 |
0.6 | 1208 | 1507 | 1203 | 1485 | 1200 | 1473 |
0.8 | 1217 | 1553 | 1210 | 1531 | 1207 | 1518 |
The mass fraction of IPDI in the binder increases with decreasing molecular weight of PTHF, as shown in Table
The value of
Table
Theoretical mole fraction of main combustion products of AN/HTPB/PTHF2 propellants.
70% AN | |||||||
Mole fraction (—) | |||||||
CH4 | CO | CO2 | H2 | H2O | N2 | Graphite | |
0 | 0.0843 | 0.0358 | 0.1270 | 0.2397 | 0.2086 | 0.1653 | 0.1393 |
0.2 | 0.0852 | 0.0364 | 0.1338 | 0.2411 | 0.2156 | 0.1681 | 0.1198 |
0.4 | 0.0862 | 0.0368 | 0.1406 | 0.2423 | 0.2227 | 0.1709 | 0.1005 |
0.6 | 0.0873 | 0.0371 | 0.1473 | 0.2433 | 0.2299 | 0.1737 | 0.0814 |
0.8 | 0.0885 | 0.0374 | 0.1541 | 0.2440 | 0.2371 | 0.1765 | 0.0624 |
80% AN | |||||||
Mole fraction (—) | |||||||
CH4 | CO | CO2 | H2 | H2O | N2 | Graphite | |
0 | 0.0662 | 0.0507 | 0.1719 | 0.2642 | 0.2371 | 0.2099 | 0 |
0.2 | 0.0581 | 0.0469 | 0.1700 | 0.2636 | 0.2521 | 0.2093 | 0 |
0.4 | 0.0506 | 0.0434 | 0.1680 | 0.2620 | 0.2672 | 0.2088 | 0 |
0.6 | 0.0437 | 0.0401 | 0.1659 | 0.2596 | 0.2822 | 0.2085 | 0 |
0.8 | 0.0373 | 0.0370 | 0.1637 | 0.2564 | 0.2974 | 0.2082 | 0 |
A subsequent experiment investigated the thermal decomposition behavior and burning characteristics of propellants at 80% AN, because the influence of the HTPB/PTHF binder on the theoretical performance of a propellant at 80% AN is greater than that of a propellant at 70% AN.
Figure
Burning rate characteristics of propellants.
AN/HTPH/PTHF1
AN/HTPH/PTHF2
AN/HTPH/PTHF3
The thermal decomposition behavior of the AN/HTPB/PTHF propellant (
The burning propellant at 7 MPa was quenched by a depressurization of approximately −1 MPa s−1. Figure
Photograph of propellant quenched at 7 MPa.
On the other hand, the propellant containing PTHF did not leave a solid residue at
When a solid residue is produced in the combustion chamber of a rocket motor, it interferes with the combustion of the propellant and the mass ratio of the rocket decreases. Furthermore, the presence of a solid residue results in erosion of the nozzle and an exhaust plume is produced. Thus, the generation of a solid residue is undesirable. The use of PTHF as a binder ingredient prevents the generation of a solid residue for the AN/HTPB propellant, although the burning rate of the propellant does not increase, indicating that PTHF improves the propellant performance.
Figure
SEM photographs of burning surface of propellant quenched at 7 MPa.
AN/HTPB propellant
AN/HTPB/PTHF2 propellant (
Figure
As described in Section
The combustion process of the AN/HTPB propellant is as follows [
For the AN/PTHF/glycerin propellant, the liquefied binder covering the burning surface interferes with the evolution of the AN decomposition gases and the heat flux feedback from the flame to AN, and therefore, the AN/PTHF/glycerin propellant does not ignite [
In a previous study [
The effect of a PTHF/glycerin binder on improving the burning characteristics of an AN/PTHF/glycerin propellant was clarified by adding ADC, which suppressed the negative influence of the liquefied binder covering the burning surface [
Figure
Burning rate characteristics of propellants with ADC.
AN/HTPH/PTHF1
AN/HTPH/PTHF2
AN/HTPH/PTHF3
The burning rates of an AN/HTPB propellant without ADC, as shown in Figure
The burning characteristics of an AN-based propellant with a HTPB/PTHF blend binder were investigated. The use of a HTPB/PTHF blend is theoretically effective in enhancing the specific impulse and adiabatic flame temperature of an AN-based propellant. The AN/HTPB propellant left a solid residue on the burning surface, and the shape of the residue was almost the same as that of the propellant. On the other hand, an AN/HTPB/PTHF propellant did not leave a solid residue, and the burning rates of the AN/HTPB/PTHF propellant were almost the same as those of the AN/HTPB propellant. This was because some of the liquefied HTPB/PTHF binder covered the burning surface of the AN/HTPB/PTHF propellant. The burning rates of AN/HTPB/PTHF propellants with ADC were higher than those of AN/HTPB propellants with ADC. The addition of ADC to the AN/HTPB/PTHF propellant was required to clarify the effect of the blend binder on the improvement of the burning characteristics. The effect of the HTPB/PTHF blend on the increase of the burning rate was independent of the mass ratio of PTHF in the blend.
Specific impulse, s
Adiabatic flame temperature, K
Mass ratio of PTHF in binder, —.
Ammonium nitrate
Ammonium perchlorate
Differential thermal analysis
Hydroxyl-terminated polybutadiene
Isophorone diisocyanate
Polytetrahydrofuran
Thermogravimetry.