Infrared signature of NEPE, HTPB rocket plume under varying flight conditions and motor size

Highlights:

• Simulating IR signatures and flow structure of plume at the rear of solid rocket motor.

• Using HTPB (weak afterburning) and NEPE (strong afterburning) propellants.

• Confirming the effect of altitude, flight Mach number, and motor size.

Abstract

In this study, radiation signatures of rocket plumes based on HTPB and NEPE propellants are simulated under varying altitudes, flight Mach numbers, and motor size. The flow field of plume is calculated by computational fluid dynamics, and the infrared signature is predicted using a Ludwig model. To verify the numerical method, experiments were conducted using small rocket motors and radiometric FTIR. The acceptable agreement between numerical calculation and the experiment was obtained in 2–5 μm wavelength region. The total radiance decreases with increasing altitude whether afterburning exists or not. A decreasing rate in the radiance of the plume with strong afterburning is larger than that of the plume with weak afterburning. The change in total intensity according to altitude depends on the afterburning and is directly affected by the radiating area. As the flight Mach number increases, total radiance, total intensity, and radiating area decrease. The motor size makes radiation intensity increase exponentially; exponent value is from 2.6 to 2.7. As the motor size increases, the effect of altitude increases, while the effect of flight Mach number decreases.

Introduction

Infrared signatures of exhaust plumes are determined by the thermodynamic property of media such as temperature, species and these quantities. The thermodynamic state of the plume is determined by the flow structure at the rear of the nozzle. Therefore, the infrared signature of gas in an exhaust plume appears differently depending on the propellant, propellant mixture ratio, shape of nozzle, and type of propulsion engine.

Most commonly, carbon dioxide and water vapor at the rear of the nozzle generate strong peaks at 4.3 μm and 2–3 μm wavelengths, respectively. This is the case particularly during afterburning, (secondary combustion between products of incomplete combustion and ambient air), where infrared signature is dramatically rising as temperature and amount of carbon dioxide increase at downstream [1], [2], [3], [4]. Niu [5] performed a study of the flow structure and infrared signatures relative to afterburning, chamber pressure, and ratio of propellant. This study showed that afterburning increased the length and intensity of the radiance area. Further, with increased chamber pressure and HTPB (Hydroxyl Terminated Polybutadiene) binder ratio, the spectral and total infrared intensity also increased.

In addition, even with the same fuel and propulsion system, the infrared signatures vary depending on the altitude, flight Mach number and engine size of the rocket. As the flight Mach number and ambient air condition are changed depending on the altitude and acceleration during flight, the flow structure of the rocket plume is changed over time. This is the main reason for variations in the radiation signature.

The study of radiation signature since the 1960 s has considered the variation due to altitude; it has been found that the increased altitude and reduced ambient pressure results in a dramatic expansion of the plume [4], [6], [7], [8], [9]. To consider afterburning according to altitude, Vitkin [6] developed and applied the code that corrects the chemical mechanism depending on altitude. Alexeenko [7] interpreted the radiation signature for AtlasⅡ at altitude ranges from 15 km to 40 km and compared these with experimental results. Through satellite measurements, Simons [4] explained the change in total intensity of the plume with variations of altitude—the main reason being the changing volume of the plume and the extinction of afterburning. Calhoon [8] also performed a study of the extinction mechanism of afterburning to estimate correctly the change of radiation intensity due to altitude. He stated that the extinction of afterburning is caused by Damkohler effect. Niu [9] analyzed spectral infrared signatures at altitudes between 5 and 70 km using infrared radiation analysis code, validated with AtlasⅡ data, while considering the effects of atmospheric absorption.

The study of the velocity effect of the rocket vehicle on the radiation signature while comparing with effect of altitude is limited. Stowe [10] has drawn the contours of the change of infrared radiation signature and flow structure relative to the velocity of the rocket, which indicates a tendency of the plume to get thinner with increasing velocity.

Further, the size of the rocket engine has a great effect on the infrared signatures. Most studies concentrate on small size rocket motors [11], [12], [13], [14], [15]; this is because these studies require validation with experiment in developing prediction method and real size experimental costs are very high [7], [16], [17]. Non-reacting supersonic jets have similarity in size [18], [19]; however, for reacting supersonic jets, there is no similarity due to the absence of scale law for chemical reaction. Therefore, in developing a rocket motor, the radiation signature of a real size plume and its size effect must be studied. Ludwig [3] conducted a study to confirm the size effect. However, the studies did not seem to consider changes in thermodynamic properties with varying size motors. In another study, Zhang and Li [20] confirmed the size effect of Trident D5 by reducing the size of engine gradually to 1/10 scale. The spectral and total intensity were compared for each size. Their study found that the total intensity is proportional to the exponential function of length scale and has a range of exponent value from 1.5 to 2.5.

Depending on the purpose of use, the type of propulsion engine varies, and therefore propellants studied for infrared signature of exhaust plume are different for all types, mixture ratio, and size. In military field, there are many recent studies on the IR signature of HTPB [5], [10] based propellant with weak afterburning and NEPE (Nitrate Ester Polyester) based propellant with strong afterburning, which do not show visible primary and secondary smoke [21], [22], [23], [24], [25]. However, only a few studies have been conducted on afterburning and infrared signatures relative to flight condition and motor size.

In his study, a numerical analysis was performed for infrared signature and flow structure of HTPB (weak afterburning) and NEPE (strong afterburning) based propellant plume and confirms the effect on the infrared signature according to flight condition and motor size.

Ludwig's model was used to predict the infrared signatures. The flow structure of rocket plume was analyzed using computational fluid dynamics and ray tracing methods. This was applied to obtain the thermodynamic properties of the medium in which the ray travels and which the results used as input values to Ludwig’s model. It was assumed that the ray proceeds in a lateral direction. To verify the numerical method, static firing experiments using small rocket motors and radiometric FTIR (Fourier Transform Infrared) were conducted. The experimental and numerical results were compared. To confirm the effect of flight condition, the radiation signature of the plume was predicted depending on altitude and flight Mach number. To confirm the size effect, the motor size was changed while maintaining its shape.