Page 1 of 16

European Journal of Business &

Social Sciences

Available at https://ejbss.org/

ISSN: 2235-767X

Volume 07 Issue 04

April 2019

Available online:https://ejbss.org/ P a g e | 1364

STUDY OF HTPB-PARAFFIN BLENDS FOR AEROSPACE

APPLICATIONS

RAMAMOORTHY .J

ABSTRACT

Research on hydroxyl terminated polybutadiene (HTPB) for use in

hybrid rocket propulsion has been sporadic for many years. The HTPB fuel grain

suffers from low fuel regression rate. This low regression rate, results in low heat

and mass transfer from the fuel surface which affects the thrust of the hybrid

rocket. As a result, hybrid rocket has not yet found widespread use in commercial

and military applications. The challenge is to improve the low regression rate

without compromising the appealing features of the hybrid rocket which includes

safety, low cost environmental cleanliness and simplicity. The present research

discusses two methods to mitigate this low regression rate of the HTPB. One of

the methods used was adding activated charcoal (AC) as an additive and the

second method proposed was to blend the HTPB with paraffin. The purpose of

this work was to carry out the thermal analysis of HTPB with various percentage

of AC as additives and HTPB-paraffin blends.

I.INTRODUCTION

In the past years significant development has taken place in the field of

hybrid rocket technology. Hybrid rocket propulsion can be used for future launch

missions since it offers several advantages over others, such as improved safety

and reliability, low development cost, minimal environmental impact (green

fuel/propellant) and on-off operation capability. The development of

environmental friendly propellants with low signature characteristics and high

energetic properties has also been of great interests of researchers. Paraffin is

considered as a green propellant because the byproduct of combustion is water

and carbon dioxide. The hybrid rocket motor was chosen to power the stage II of

SpaceShipOne.

Page 2 of 16

European Journal of Business &

Social Sciences

Available at https://ejbss.org/

ISSN: 2235-767X

Volume 07 Issue 04

April 2019

Available online:https://ejbss.org/ P a g e | 1365

However, major disadvantages of hybrid propulsion systems include

low regression rate, combustion efficiency problem, reduce motor mass fraction,

combustion instability with pressureoscillation.

Pyrolysis can occur only if the temperature of the fuel is higher or equal

to the depolymerization temperature. Fig.1. shows a typical temperature

distribution zone of a hybrid fuel grain. In the gas phase reaction zone (Zone III)

the pyrolysis product temperature increases to the flame temperature of the

combustion zone. The condensed phase reaction zone (Zone II), in which the

phase change from solid to liquid and/or gas occurs in endothermic or exothermic

reaction. In this region the heating rate is of higher order (> 100 deg/s). In the

condensed phase zone (Zone I), the fuel will be in solid state, no chemical

reaction will occur as the temperature of the fuel increases from initial

temperature to decomposition temperature. The heating rate in this zone is in the

lower order (< 100 deg/s) compared to Zone II. This is the preparation zone for

the fuel when the thermal stability of the material decreases breaking the

molecules of the polymer. During combustion, under the effect of high heat flux

near the fuel surface, a sharp temperature profile (Figure 1.2) is observed along

the thickness of the grain. As a result the heating rate is not uniform along the

thickness of the grain. It is very high in the fuel grain on the combustion side

(Zone II) and a much lower along the outer walls (Zone I). In the present research,

the heating rate is varied from 3 to 43 K/min. The study is carried out to examine

the effect of adding AC and paraffin on the pyrolysis process in Zone II. The fuel

characteristics study in Zone II is important for hybrid rocket propulsion to

determine the solid fuel regression rate for performanceprediction.

Page 3 of 16

European Journal of Business &

Social Sciences

Available at https://ejbss.org/

ISSN: 2235-767X

Volume 07 Issue 04

April 2019

Available online:https://ejbss.org/ P a g e | 1366

Fig.1. Temperature distribution zone of a hybrid fuel grain

Hydroxyl-terminated polybutadiene (HTPB) is widely regarded as a

workhorse fuel for fuel grains in hybrid rockets. An overview image of classical

regression in a HTPB-based hybrid model is shown in Fig.2. The basic model

involves transferring heat to the solid fuel surface to the extent that pyrolysis

occurs at the fuel surface. The solid fuel regresses and releases gaseous fuel into

the fuel-rich zone area pictured. Incoming oxidizer enters the boundary layer of

the flow and mixes with the fuel. At the point where the mixture is sufficient, a

combustion flame forms. This combustion flame transfers heat to the solid fuel

surface through radiation and convection to enable further pyrolysis. Heat transfer

through convection is generally much larger than radiation unless metal additives

are mixed into the fuel as reported by George P Sutton (2001). But the main

disadvantage with HTPB is that it has high heat of ablation and this resultinal

owregressionrate of the fuel grain in hybrid rocket operation. The low regression