IEEE SEMINAR REPORT ON

FREQUENCY –DEPENDENT FDTD SIMULATION OF THE

INTERACTION OF MICROWAVES WITH ROCKET

PLUMEBY

Kiyoshi Kimefunchi , Ikkoh Funaki (IEEE MEMBER )And Takashi abe

TABLE OF CONTENTS1) Abstract

2) Introduction

3) Experiment setup detail

4) Experiment working

5) Experiment results

6) Frequency dependent FDTD

7) Calculation result

8) Conclusion

9) References

ABSTRACTThe ionized exhaust plumes of solid rocket motors

may interfere with RF transmission under certain flight conditions.

To understand the important physical processes involved,

we measured microwave attenuation and phase delay due to the exhaust plume during sea-level static firing tests for a full-scale solid propellant rocket motor.

The measured data were compared with the results of a detailed simulation performed using the frequency-dependent finite-difference time-domain ((FD)^2TD)

INTRODUCTIONExhaust plumes from solid propellant rocket motors

interfere with microwave transmission, as shown in Fig. a, and such interference can result in failure of telecommunications ,which could affect telemetry, command procedures, or the radar link between a vehicle and the ground-based antennas.

Hence, we investigated the microwave transmission behavior during sea-level static firing tests conducted for a full-scale solid rocket motor.

We measure microwave attenuation and phase delays at three frequency by a simple antenna setup as shown in fig b

Fig a:Plume RF interaction in rocket flight.

Fig b:Antenna setup in the experiment.

Experiment Setup Detail

• The microwave attenuation experiments were conducted during static firing tests on a full-scale solid rocket motor.

• Three microwave frequencies generally used in space communications were selected: S-band, 2.3 GHz; C-band, 5.6

GHz; and X-band, 8.5 GHz. • Horn-type antennas were adopted for both the transmitter and

receiver in order to achieve high directivity. • The motor was positioned horizontally, and the antennas were

placed on the motor test site facing each other across the motor plume, as shown in Fig. c.

• The heights of the antennas were adjusted such that they were positioned along the center of the axis of the motor nozzle.

Fig c :Microwave attenuations and S-band phase delay measurement system in the experiment

Working of experiment

The microwaves with each of the three frequencies were received and finally down converted to 10 kHz and then recorded on a digital data recorder with a sampling rate of 48 kHz.

The attenuation was evaluated by comparing the received microwave levels before and during the motor firing.

For the phase delay measurement, the S-band signal from the oscillator was divided and its phase was periodically varied by using a phase shifter.

The phase-delayed signal was combined with the signal received by the receiver antenna and the synthesized signal was recorded.

The phase delay data were obtained by analyzing the waveform or hum of the recorded signal.

Here attenuation is defined as (1-(V/Vo)^2 )

Experiment results

Fig. d. Experimental results: microwave attenuations, phase delay and chamberpressure (S-band, 2.3 GHz; C-band, 5.6 GHz; X-band, 8.5 GHz).

It can be seen in Fig. d that low-frequency microwaves are highly attenuated during the initial 90 s.

This frequency dependence of attenuation is in accordance with the characteristics attributable to the plane wave-slab plasma interaction theory ,

There is an apparent relationship between the voltage ratios and the chamber pressure because the plasma density distribution in the plume changes with decreasing chamber

For approximately 20 s before the end of the firing (90–110 s in Fig. d), it is observed that large attenuations and large phase shift appear and that the frequency dependence of attenuation changes.

Frequency dependent FDTD and its model

To analyze microwave transmission through such plasma, we have developed the (FD)^2 TD method calculation code . The electric field update equation for the plasma region is expressed as

• Where Ve is the electron collision frequency, ωp is the electron plasma frequency, and ∆t is the time step size. The two plasma properties (ω and Ve ) are required for the (FD) TD simulation

• Voltage ratio is calculated as follows:

where ν is the microwave frequency, c is the speed of light, and d is the distance the wave travels in the plasma

• Input plasma properties for calculation of V/Vo is given table 1

The attenuation of each frequency changes dynamically

with time. hence, the firing period is subdivided into three

periods:• the early period (10–20 s), the middle period

(40–50 s) and the last period (95–105 s). The measured voltage ratios were averaged for each period before substituting to in order to reduce the received noise mainly generated by the flow turbulence in the plume.The estimated plasma properties in each period are summarized in Table I.

CALCULATION RESULTSThe calculations were conducted for the 3 frequencies

(S-, C-,and X-band) and 4 periods (before firing, early period, middle period, and last period). These 12 FDTD simulations were separately performed from the start of the microwave radiation at the transmitter through the receiving at the receiver.

The origin of x-axis corresponds to the start of microwave radiation from the transmitter and the microwave reaches at the receiver about 48 ns. The received levels keep constant after receiving.

The voltage ratios is found using these constant values

Various graph b/w bands Ey distribution for various period are shown below:

For early firing period

For middle firing period

For Last firing period

Fig. y-band Ey distribution on the transmitter-plume core-receiver line.

Fig. X-band Ey distribution on the transmitter-plume core-receiver line.

CONCLUSIONFor estimating plume plasma properties and understanding the

essence of rocket plume RF interference, experiments involving the interaction between rocket exhaust and microwave transmission were conducted during a full-scale solid rocket firing test. Microwave transmission analysis using the (FD)^2 TD approach was applied. The attenuation predicted by the (FD) ^2TD calculations agrees well with the experimental results. The (FD) ^2TD results indicated some interactions such as microwave attenuation through the plume and diffraction around the plume. Furthermore, the (FD) ^2TD calculations revealed that when a higher plasma density is achieved in the plume, microwaves hardly penetrate the plume and bypass it, in other words, the diffraction effect becomes dominant. The diffraction effect changes the microwave path length to the receiver and causes the phase delay observed in both the experimental data and the numerical (FD) ^2TD calculation results. In the low plasma density case, however, the penetration path becomes dominant and the plane wave-slab plasma interaction theory is useful for estimating the attenuation.

Fig Comparison of the voltage ratio between the experimental results and the FDTD results.

Fig. Comparison of the phase advance according to the plane wave-slabplasma interaction theory and the FDTD results.

REFERENCES[1] T. Abe, K. Fujita, H. Ogawa, and I. Funaki, “Microwave telemetry breakdown

caused by rocket plume,” in Proc. 31st AIAA Plasma dynamics and Lasers Conf., Denver, 2000, pp. 2000–2484.

[2] D. E. McIver, Jr., “The radio frequency signal attenuation problem of rocket exhaust,” in Proce. NASA Conf. on Communicating Through Plasmas of Atmospheric Entry and Rocket Exhaust, 1964, pp. 167–179, Langley Research Center.

[3] W. A.Wood and J. E. DeMore, “Microwave attenuation characteristics of solid propellant rocket exhaust products,” in Proc. AIAA 6th Solid Propellant Rocket Conf., WA, 1965, pp. 65–183.

[4] F. A.Vicente, E. C. Taylor, and R. W. Phelps, “Analysis of flame effects on measured electromagnetic propagation date,” J. Spacecraft, vol. 4, no. 8, pp. 1069–1075, 1967.

[5] F. P. Boynton and P. S. Pajasekhar, “Plume RF Interference Calculations for Space Shuttle,” 1978, NASA-CR-161099.

[6] L. D. Smoot, “Causes of ionization in rocket exhausts,” J. Spacecraft, vol. 12, no. 3, pp. 179–183, 1975.

[7] L. D. Smoot, “Rocket exhaust plume radar attenuation and amplitude/phase noise,” J. Spacecraft, vol. 4, no. 6, p. 774, 1975.

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