Effects of adding powdered metals with the solid propellants – A review

Rocket engines that use solid propellants are widely used in the commercial space flight industry for powering orbital-class launch vehicles and small research rockets. Thrust and the specific impulse developed by a rocket engine are essential properties of a rocket propulsion system, and it specifies the overall performance of a rocket. These properties heavily influenced by the burn-rate properties of the solid propellants. One of the many methods to improve the burn rate of the solid propellant is the addition of metallic powders into the fuel-oxidizer matrix. This technique has been observed to enhance the burn rate of the solid propellants; several studies have concluded the same. On the other hand, these additives result in the release of metal oxides into the atmosphere and lead to a higher amount of environmental pollution. This paper summarizes the effects of adding metals and their concentrations on the burning properties of the solid propellants.


1.Introduction
Burn rate of a solid propellant is an important parameter in determining the operational characteristics of a solid rocket motor. The burn rate of a propellant is highly influenced by the chamber pressure, the initial propellant temperature, oxidizer particle size, propellant composition, etc. Burn rate modifiers like metal powders, catalysts also have a significant influence on the burn rate. Several studies have been conducted on the effect of addition of such burn rate modifiers into solid propellants. Metals have been observed to enhance the burn rate of composite propellants by either reducing the decomposition temperature of the Ammonium perchlorate oxidizer or by lowering the activation energy (the initial energy required by propellant molecules to start the reaction).
Burn rates are generally measured in a Crawford Burner: A Crawford burner is essentially a pressure vessel in which a propellant strand is placed with two fuse wires inserted into it, a time measurement is taken for the propellant to burn from one fuse wire to another, the pressure in the vessel is utilized to simulate the chamber pressure of a live burning rocket motor. The paper focuses on the various components of the solid propellants like binder and aluminium for improving the ballistic performance of the powder. The effect of Al size and binder nature on the burning rates and residue oxide products of the propellants has been studied in this paper. Recordings from high speed video camera, burn rate measurements and residue analysis done by XPS, XRD and SEM are obtained for several propellants formulated, designed and manufactured at the laboratory. The propellant formulation is given in the table below:  The residue was collected into a pressurized Plexiglas vessel containing nitrogen and ice. It was later ignited using Ni-Cr wire. After combustion the residue is washed and treated with HCl. All the weights are normalised to the original weight of metallic Al present in the sample. The presence of metallic Al in the uncombusted fraction represented the incomplete burning process. XPS was used to measure the residue composition and the atomic concentration of chemical elements. The table below shows the atomic concentration data obtained at different pressures. Carbon is partially oxidised and comes from the pyrolysis process. Al is detected as Al oxide. The metallic Al is present within the core of agglomerates. Cl comes from the decomposition of AP and is also trapped within the agglomerates. XRD spectra data results are given in the table below: XRD crystalline fraction of combustion residues after treatment with HCL  The result is the low content of metals in the residue in the P_01 sample. It is inversely proportional to pressure. Thus, the smaller the Al powder, more is the burning. The SEM observations show that the size of combustion products depends on: size of Al powder, nature of binder and the burning pressure. It helped to establish a relationship between structure and chemical composition to ballistic properties. The burning rate was increased by two factors with the use of Nano-sized Al powder and PPG binder which easily degrades during the process of burning limiting the agglomeration of Al. Thus, resulting in efficient combustion.

2.1.2.
Enhanced Propellant Combustion with Nanoparticles The dependence of burn rate on the size of the aluminium powder particles was studied by varying the particle size from micrometre to nanometre scales. The study employed 15% Alex particles and aluminium nanoparticles produced by Technanogany, LLC. Both these additives were tested at varying particles size of the aluminium particle and then the following graph was plotted. It is evident that there is a sharp rise in the burn rate as the aluminium particle size is decreased to a nanometre scale. The burn rate seems to reach an inverse particle diameter squared dependence.  Figure.2 Variation of burn rate with aluminium particle size [2]

Combustion of HTPB-based solid fuels containing nano-sized energetic powder in a hybrid rocket motor.
The effect of addition of nano sized metal particles or Boron (B4C) to solid HTPB fuel in a hybrid rocket motor is discussed. Nano sized particles were embedded into the solid fuel during the casting of the furl grain. A long grain centre perforated propellant grain was cast. It had already been demonstrated that addition of Alex particles into solid HTPB fuel can increase the fuel burn rate by up to 70%. It showed that an improvement in performance can be achieved by addition of aluminium powders. In addition to this, the production of finer WARP-1 aluminium powder with a much specific area which was also used.  Figure.3 Variation of burn rate with oxidizer flow rate [3] It can be seen that at the same oxidizer mass flux, addition of energetic particles enhances the burn rate of the fuel. Addition of just 6.5% of nano sized WARP-1 showed a significant increase in the burn rate of the propellant.

Development of Strand Burner Test by Using Aluminized AP/HTPB
The study focuses on the investigating the relationship between the burn rates of aluminised AP at low pressure. The aim is studying the burn rates of aluminised AP/HTPB propellant under varying pressure.

Figure.4 Schematic of the Crawford burner used [4]
International Conference on Thermo-fluids and Energy The test is done under varying chamber pressure (1atm to 7atm in steps) using the strand burner, an apparatus for measuring the burn rates of solid propellants at increasing pressure. Then the plot is drawn for burn rate, r and the chamber pressure. The same is done using a strand burner as mentioned above. There were four propellant formulations used for this test which contained AP, 5-micron Al with 85% solid loading and HTPB as the binder with 8-10% curing agent. The different formulations of these components used for the test are given in the table below. After the preliminary tests were run the tests under atmospheric conditions were conducted (by supplying air sans nitrogen) for each formulation. The same was repeated after filling the burner with nitrogen gas.

Figure.5 Burn rate variation with O/F ratio at different pressure [4]
Also, the tests were run by varying the chamber pressure from 1atm, 3atm, 5atm and 7atm in the nitrogen gas condition and the graph for the obtained data was plotted which showed that the burn rate was higher when the chamber pressure was increased. Thus, in conclusion, the paper says that the best and accurate way to conduct a burn rate test for varying formulations of the propellants is in an inert gas medium (like nitrogen). And it was found that the burn rate is directly proportional to the chamber pressure.

2.3.Addition of other metals 2.3.1. Burn rate enhancement of AP-NC solid propellant using CuO-graphene foam microstructures
The study focuses on experimentally enhancing the burn rate of the AP-NC solid propellant using CuO-graphene microstructures which are highly conductive, interconnected and porous. Here nitrocellulose acts as the binder. The authors made use of 3-D graphene foam with CuO as the metal oxide instead of the conventional 2-D graphite support metal oxide-which showed no significant improvement. Here further exploration has been done on the catalytic property of 3-D graphene foam and its application to any kind of propellant irrespective of the chemical composition. The tests for burn rates of AP-NC with various CuO-graphene loadings, determining the effects of addition of CuO to GF and the activation energy of AP-NC were conducted using SEM, XPS, TG and DSC. The GF structures were grown on Ni templates using CVD techniques with a combination of three gases CH4, H2 and Ar at a temperature of 1050˚C. Then they were functionalised with CuO using simple hydrothermal approach. The bonding between them was then investigated using XPS. In this study there was a difference of almost 20.3eV in the binding energy which confirmed the presence of CuO phase only. This was further confirmed using the XRD. Three samples of composite solid propellants were prepared: without GF and CuO, without GF but with CuO and with GF. For all the propellant samples burned they were ignited using a resistive Ni-Cr wire and a 10V DC voltage. The burn rates were measured tracking the brightest zones of the reaction zone as observed by the IR cameras. For pure AP-NC an average burn rate of 1.3±0.2 cm/s was obtained. But with addition of GF (around 25% loading) the average burn rates were 4.5 cm/s. With use of GF-supported CuO, burn rates up to 9 cm/s were obtained, which were 7 times that of the pure AP-NC burn rate. The positive effect came from the use of CuO acting as the catalyst, but the negative effect came from the fact that CuO was not taking part in the combustion thus, reducing the overall calorimetric value of the propellant. This indicated that there was an optimum CuO loading (~3%) which maximised the burn conditions. To determine the activation energies of the AP-NC propellants on addition of CuO-GF, TG and DSC analysis were performed. The activation energies were calculated using Kissinger method using 4 different heating rates along with HTD (High temperature decomposition) peaks. The analysis showed that the activation energy of AP-NC was lowered by 22% by addition of CuO-GF structures. This work presented a new approach of using 3-D GF with CuO catalyst for enhancing the burn rates and other characteristics of the solid propellants (AP-NC). The authors obtained a burn rate enhancement of 7% with 3% CuO loading for and 10% GF loading. Also, the AP-NC propellant samples with only the CuO nanoparticles at the same loading had 1.8% enhancement only. The GF-CuO structures not only enhance the transport of propellant but also gives an effective contact between propellant and CuO i.e., increase in catalytic effect.

The effect of Nd2O3 on thermal and ballistic properties of AP based composite propellants
The study was on the effects of Nd2O3 on the thermal and ballistic properties of AP based composite propellants. The composite propellants were prepared by slurry cast techniques using sigma blade Nd2O3 gives a typical burn pattern. In the pressure range of 50-70 kg/cm2 the pressure index for Nd2O3 based formulation is lower than that of Fe2O3 based formulation. But in the pressure range of 70-125 kg/cm2 the scenario is reversed which is booth beneficial and unique when compared with Fe2O3. The impact and sensitivity tests showed that the Nd2O3 based formulations were less sensitive than compared to Fe2O3. Thus, the study reveals that Nd2O3 appears to offer high potential to the propellants for specific applications.

An effective method to embed catalyst on AP and its effect on the burn rates of aluminized composite solid propellants.
A study aimed at learning the effect of embedding an Iron Oxide catalyst on the surface of Ammonium Perchlorate oxidizer in a solid AP/HTPB propellant [4]. Micro and Nano sized catalyst was embedded into the propellant grain and then burn rate measurements were made on a Crawford burner test rig. The results of the Crawford burner test for the micron sized Iron Oxide are as follows: List of various composition prepared for optimum IO fraction in aluminized solid propellant