It has been reported in literature that the use of protrusion in the combustion chamber of a hybrid rocket motor enhances the regression rate. A detailed study was carried out by using two types of protrusion (a) rubber (which burns along with fuel but at different rate, hence its shape changes with burn time), and (b) graphite (does not burns along with fuel and hence its shape does not changes with burn time). The results showed that the improvement in regression rate with protrusions was only up to a certain fraction of the overall burn time. From the results obtained, it was observed that an X/L of 0.5 was the best location for both types of protrusion. The maximum improvement in regression rate was around 45%. The combustion efficiency was observed to improve from 58% without protrusion to 69% with the use of protrusion, when placed at an X/L of 0.8. It was observed that the use of bluff body and protrusion improves combustion efficiency, when used individually. Hence, combination of the two was used and the combustion efficiency was determined. The combustion efficiency was around 90%, when bluff body was used at the head end and the protrusion at an X/L of 0.8 from head end. © 2014 Elsevier Masson SAS.

Periyapatna A. Ramakrishna

Department of Aerospace Engineering

Bluff body

HYBRID ROCKETS

PROTRUSION

REGRESSION RATE

Paraffin waxes

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Aerospace Science and Technology

Polytetrafluoroethylene is known to enhance the reactivity of aluminum. The enhancement in the reactivity is produced by mechanically activating the aluminum using polytetrafluoroethylene. In this study, mechanically activated aluminum powder is used along with paraffin wax as the fuel grain for a hybrid propellant. Here, gaseous oxygen is used as oxidizer. To achieve an optimum composition for the best specific impulse, the aluminum fraction in the composition is restricted to 50% of the base fuel composition along with 50% wax/ethylene vinyl acetate (combination of wax and ethylene vinyl acetate, in a ratio of 4∶1). The polytetrafluoroethylene in mechanically activated aluminum powder is added at the expense of wax/ethylene vinyl acetate in the fuel composition. When compared with the composition having pyral aluminum powder (which has flake like particles of very high specific surface area), mechanically activated powder increases the fuel regression rate by ∼40% at an oxidizer mass flux of 35 g∕cm2-s. It is also observed that the combustion efficiency of the hybrid motor is 59% with the use of mechanically activated aluminum powder in comparison to 39% obtained with micron-sized aluminum powder. Copyright © 2018 by Indian Institute of Technology, Madras, India.

Journal of Propulsion and Power

Utilization of mechanically activated aluminum in hybrid rockets

A fuel for the hybrid rocket system was developed. The attempts made to improve the mechanical properties of wax by adding EVA (ethylene vinyl acetate) are illustrated. It was observed that the mechanical properties obtained by adding EVA to wax were dependent on the process of curing the fuel specimen. A proprietary method of curing the fuel specimen was established. The percentage elongation obtained with 20 % EVA and 80 % of wax was around 17 %, which was much higher than the values obtained with pure wax (4 %). It was observed from the study that the higher the percentage of EVA content in wax was, the better the mechanical properties were. Regression rate studies with EVA and wax combination were carried out. It was observed that the regression rate decreased upon addition of EVA in wax. This reduction was compensated by using a bluff body at the head end. The regression rate obtained with 20 % EVA and 80 % wax even with the use of bluff body is lower than that obtained with pure wax, but is around 3.5 times the regression rate obtained with polymeric fuels. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Propellants, Explosives, Pyrotechnics

Studies on EVA-Based Wax Fuel for Launch Vehicle Applications

In this paper, an attempt is made to increase the regression rate of the hybrid fuel (paraffin wax) using a hemispherical bluff body placed near the head end of the motor. The oxidizer used for the present study is gaseous oxygen. The results show that the use of a bluff body increases the regression rate by around two times at the higher Gox (15g/Mcm2s) and by around three to four times at the lower Gox (3 g/Mcm2s). This increase in regression rate diminishes as theL/MDof the motor is increased from 9 to 27, whilst still being higher than the case without bluff body. The bluff body was seen to increase the regression rate even at a port diameter, which was four times the initial port diameter of 15 mm. The O/MF was nearly constant (0.75) for the case with the bluff body, whilst for the case without the bluff body, it increased from 1.5 to 2 during an operation time of 4.5 s. In addition, the port diameter after combustion was nearly uniform from the head end to the nozzle end as compared to the case when no bluff body was used, thus minimizing the sliver losses. © 2014 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Enhancement of hybrid fuel regression rate using a bluff body

The measurement of the regression rate of a hybrid rocket fuel is the subject matter of this paper. This is an involved problem because both the regression rate and the oxidizer mass flux Gox vary with the port diameter, which itself varies during combustion. This paper addresses the issue of how Gox should be varied to obtain the regression rate correctly. The most commonly used weight loss method has been employed here. Three methods used here to vary the Gox are artificially changing the port diameter, changing the mass flow rate of the oxidizer, and interrupted burn test. From the experiments conducted, it was evident that the interrupted burn test was the best method. These experiments were also carried out for a larger motor and the results compared quite well with established literature. The role of the recirculation zone size near the head end and the overall heat transferred back to the fuel surface in influencing the regression rate have been explained. The role of the recirculation zone diminishes as the length of the rocket motor is increased.

Issues related to the measurement of regression rate of fast-burning hybrid fuels

In this work, single-port, cylindrical grain laboratory-scale hybrid rocket motors are numerically simulated to study the effect of diaphragms. The effect of single and multiple diaphragms is investigated by varying diaphragm height, its axial location (for a single diaphragm), and spacing (for multiple diaphragms) at selected inlet GOX. Asingle diaphragm increases the local regression rate and its influence is prominent only in the region immediate downstream. Therefore, a tangible increment in average regression rate with a single diaphragm can only be realized for motors with small L/D(&lt; 10). The combustion efficiency in the presence of a single diaphragm may increase or decrease relative to the case with no diaphragm depending on the height and the position of the diaphragm. A diaphragm positioned near the nozzle end of the motor is most effective in increasing combustion efficiency. For long motors (L/D &gt; 10), use of multiple diaphragms are required to increase the regression rate and combustion efficiency. This requires diaphragms to be strategically spaced. Simulations show that regression rate and combustion efficiency improvements are maximum when diaphragms are spaced 8-10 times their height. The results also show that, at optimal spacing, a few tall diaphragms are more effective in improving regression rate compared with many short diaphragms. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Journal	Data powered by TypesetAerospace Science and Technology
Publisher	Data powered by TypesetElsevier Masson SAS
ISSN	12709638
Open Access	No