NASA CR-184707 

SPACE SHUTTLE MAIN ENGINE 

RADIO FREQUENCY EMISSIONS 

A.W. Rester 

E.L. Valenti 

L.R. Smith 

NOVEMBER 1988 

(NASA-CB-184707) SPACE SHUTTLE HAIN EYGINB 

8ADIO FREQUENCY EHISSIONS Final Report, €lay 

- Sep: 1988 (Sverdrup Technology) CSCL 23 p 218 Unclas 

N89-26896 

C3/20 0219597 
NASA CR-184707 

SPACE SHUTTLE MAIN ENGINE RADIO FREQUENCY EMISSIONS 

A.W. Rester 

E.L. Valenti 

L.R. Smith 

November 1988 

Sverdrup Technology, Inc. 

SSC Group 

Stenni s Space Center, MS 39529-6000 

Prepared for the 

National Aeronautics and Space Administration 

John C. Stennis Space Center 

Under NASA Contract No. NAS13-290 
Disclaimer 

Distribution of this report is provided in the interest of information 

exchange. 

tion that prepared it. 

or recommendation for use by the Federal Government. 

Responsibility for the contents resides in the author or organizaMention of trade names does not constitute endorsement 

ii 
CONTENTS 

SUMMARY .................................................................. 

INTRODUCTION ............................................................. 

Background .......................................................... Objectives .......................................................... 

EXPERIMENTAL METHODS ..................................................... 

Approach ............................................................ Apparatus ........................................................... Experiment Description .............................................. SSME Description .................................................... Test Stand/Instal lation ............................................. 

RESULTS .................................................................. 

CONCLUSIONS .............................................................. 

RECOMMENDATIONS .......................................................... 

APPENDIX.. COMPONENTS AND SETTINGS FOR THE SPECTRUM ANALYZER USED IN THE 

LOCAL VIDEO RECORDING AND REMOTE SITE MONITORING SYSTEMS ....... 

FIGURES 

1 . Antenna Location. RFE Measurement Systems ........................... 

2 . RFE Measurement Systems ............................................. 

3 . Engine 0211. Test 901.582. Thrust Profile ........................... 

4 . Comparison of Spectral Waveforms Before and After Test Firing 

as Measured on August 29. 1988 ...................................... 

5 . Six Sequential Video Displays of P1 ume Emission Spectral Signatures 

Before. During. and After SSME Test Firing .......................... 

TABLE 

RFE from the SSME Exhaust Plume. Expressed in Terms of Spectral 

Line Amplitude (in microvolts) from 25 to 43 MHz .................... 

iv 

1 

1 

1 

2 

2 

2 

5 

6 

6 

7 

13 

14 

15 

8 

9 

12 

iii 
SUMMARY 

Severa approaches to develop a diagnostics system for monitoring the 

operational health of the Space Shuttle Main Engine (SSME) are being evaluated. The ultimate goal is to provide protection for the SSME as well as to 

improve ground and flight test techniques. One scenario with potential is to 

measure Radio Frequency Emissions (RFE), if present, in the exhaust plume and 

correlate the data to engine health. 

step in evaluating this approach. 

The work reported herein is the first 

An RFE detection system was designed, the equipment leased, and the components integrated and checked out to conduct a quick-look investigation of RFE 

in the SSME exhaust plume. The system was installed on the A-1 Test Stand at 

Stennis Space Center, and data were successfully acquired during SSME firings 

from May 3 to September 15, 1988. The experiments indicated that emitted radiation in the RF spectrum (20-470 MHz) definitely exists in the SSME exhaust 

plume, and is of such magnitude that it can be distinguished from background 

noise during the firing. A1 though additional efforts are necessary to assess 

the merit of this approach as a health monitoring technique; the potential is 

significant, and additional studies are recommended. 

iv 
SPACE SHUTTLE MAIN ENGINE RADIO FREQUENCY EMISSIONS 

INTRODUCTION 

Background 

Widespread efforts are currently underway to develop an engine diagnostics system to monitor the Space Shuttle Main Engine (SSME) (and subsequent 

engines) during operation in order to detect component degradation. Theoretically, the development of such a system, combined with a feedback control 

system to shut down the engine, can be utilized to avoid catastrophic engine 

fail ure. Several approaches are being eval uated to accompl ish this goal, and 

those with the most potential will be pursued to a greater extent. 

scenario with apparent potential is to measure the emitted radiation in the 

radio frequency (RF) spectrum in the SSME exhaust plume and to correlate the 

data to engine health. Since the plume is a cool plasma with ions accelerating at various rates, the characteristics of these products of combustion 

quite possibly are related to the normal or abnormal status of engine operation. 

in the plume as noise or electromagnetic interference (EMI), no known studies 

have been previously conducted or measurements taken in the RF spectrum to 

support this approach. 

under TWR No. CB-NEMI, using Center Director Discretionary Funding (CDDF), 

at the request of the Technical Operations Division of the Center Operations 

Office of NASA. 

Inc., SSC Group, Technical Support Services Contractor at the SSC, Tektronix, 

and Rhode & Schwarz. 

One 

Although it is generally known that electromagnetic energy is indicated 

The work reported herein was conducted at the Stennis Space Center (SSC) 

The results of the test were obtained by Sverdrup Technology, 

The data presented were acquired at the A-1 Test Stand during the period 

of May 3 to September 15, 1988. 

and the Sverdrup Project Manager was L.R. Smith. 

plishing the work were Dr. S.T. Wu (NASA) and E.L. Valenti, A.W. Rester, and 

J.C. Lau (Sverdrup). 

The NASA Technical Monitor was D.J. Chenevert 

Key personnel in accomObjectives 

The objectives of the efforts reported herein were to perform a quickThe long-range 

look baseline experiment to confirm that RF signatures exist and to document 

the measurement of the emissions in the SSME exhaust plume. 

objective is to establish a relationship between RF signatures and engine 

heal th. 

1 
EXPERIMENTAL METHODS 

Approach 

To accomplish the quick-look basel ine experiment, an industry survey 

was conducted to determine the most suitable, off-the-shelf hardware currently 

available to measure radio frequency emissions (RFE). From the results of the 

survey, the desired components were selected and either purchased or leased 

depending on cost and anticipated future use. 

checked out, installed on the A-1 Test Stand at Stennis Space Center, and 

operated during Space Shuttle Main Engine (SSME) static test firings. The 

data acquired were analyzed and evaluated to determine the existence of RFE 

and also to begin formulating a perspective of the potential use of RFE as an 

engine health monitoring technique. 

Then the system was assembled, 

Apparatus 

Basically, three components or subsystems were required to perform 

the experiment: 

1. An RFE field-strength receiver (antenna) 

2. An electromagnetic (EM) wave spectrum analyzer 

3. Associated computer and recording systems to process and store 

the data. 

Since no known previous studies on RFE in the SSME exhaust plume were 

available for reference, it was necessary to select a system to cover and 

fast-sweep a wide frequency range in the spectrum, i.e., from essentially 

0 to 1000 MHz in real time. Two instrumentation systems were used. 

0 A regular VHF/UHF TV antenna (already on hand and covering an 

approximate range of 20-800 MHz) was used in all four of the 

experiments discussed herein. 

A calibrated, log periodic antenna (covering an approximate range of 

200-1000 MHz) was also purchased, installed, and used for the latter 

two experiments as part of a second, independent detection system. 

0 

The initial system, referred to as the Local Video Recording System, was contained totally at the A-1 Test Stand, and was set up, turned on, and left running before personnel were evacuated for the SSME firings. The second, or 

Remote Site Monitoring System, utilized a telephone interconnect and additional computer systems to allow monitoring and adjustment of frequency settings (on the spectrum analyzer) from a protected area (Building 4301) during 

the actual static firings. A separate spectrum analyzer was coupled to each 

antenna and its respective computer system to provide system independence. 

Schematics of the two system configurations are shown in Figures 1 and 2, 

with the respective listing of equipment, model numbers, and other pertinent 

information listed in the Appendix. 

2 
Antenna 

Level 4. As1 Test Stand 

Figure 1. Anntenna Location, RFE Measurement Systems 

Monitoring System 

Antenna Location Local Video Remote Site 

a = Distance from nozzle exit 0.0 2.5 

to center of antenna (ft) 

b = Distance from nozzle center 24.5 26.0 

line to antenna (11) 

c = Antenna viewing angle- 15.0 

degrees from horizontal 

0.0 

3 
A-l Test Stand 

Antenna (Radio Shack) 

Building 1100 

Spectrum 

Analyzer, MOD 

TEK 2756P 

Camcorder 

IBM PC with 

miimage Processing 

a. Local Video Recording System 

A-l Test Stand 

Antenna (Log Periodic) 

Spectrum 

Analyzer, MOD I TEK 2755P I 

IBM PC with GPlB 

and Remote Site 

Software 

Building 4301 

Printer 

rL_I PC with Host 

Site Software 

b. Remote Site Monitoring System 

Figure 2. RFE Measurement Systems 

4 
Experiment Description 

The experiments were conducted during the period May 3 to September 15, 

1988. Several baseline measurements were obtained during SSME static firings 

from May 3 to August 29, 1988, during which the system was checked out and personnel became familiar with the equipment. However, the primary test, which 

was investigated with a full-up system following resolution of early problems, 

was conducted on September 15, 1988 (Test No. 901-582). The duration of this 

firing was 50 sec and it occurred at approximately 7:OO p.m. 

this experiment are presented in the Conclusion along with data from the last 

baseline experiment. 

Figure 3 for the September 15 test. 

The data from 

The SSME Run Schedule/Thrust Profile is shown in 

100 - 

80 - L. E 

- 

0) 

60- 

ii 

: 

40 - 

C/O at 

20 40 

100% 50 

* 104% * 

100% t 

10 20 30 40 50 

Time (sec) 

Figure 3. Engine 0211, Test 901-582, Thrust Profile 

The RFE propagated in the exhaust plume were received by the antenna and 

transmitted to the spectrum analyzer where the signals were converted to a 

function of frequency and voltage. 

corder (for the Local Video Recording System) and after the firing was processed on an IBM PC located in Building 1100, or transmitted through the 

telephone interconnect and processed in near real time on the IBM PC in 

Building 4301 (for the Remote Site Monitoring System). 

analyzed and evaluated to determine the presence of EM energy and whether or 

not it could be distinguished from background noise. 

The output was then recorded on the camThe data were then 

5 
SSME Description 

The SSME is a liquid propellant (LOX/LH2), pump-fed, regeneratively cooled 

rocket engine that develops 393,000 lb of thrust (104% of rated power level) 

at sea level conditions. The SSME has a nozzle area ratio of 77 and, with an 

oxidizer/fuel ratio of 6 (a total mass flowrate of 1081 lbm/sec), operates at 

3126-psia combustion chamber pressure and 3000 'F chamber temperature. SSME 

S/N 0211 was undergoing production testing at the A-1 Test Stand when the 

experiments discussed herein were conducted. 

Test Stand/Instal lation 

The A-1 Test Stand was designed and built to accomplish static testing 

The RFE Detection System was installed on the fourth level of 

of the Saturn V, S-IC engine at ambient conditions. 

Apollo Program, the test was modified to accomplish similar testing of the 

SSME engine. 

the A-1 Stand, and the RF antennas were positioned near the SSME exhaust 

nozzle to receive the emitted radio waves. 

Following the end of the 

6 
RESULTS 

Several baseline RFE measurements (using max-hold, and later remotely 

control led sweep-frequency techniques) were conducted from May 3, 1988 to 

August 29, 1988. In addition to establishing a baseline, these initial 

operations provided the necessary checkout period required to identify 

inherent problems with the equipment and techniques used, and also allowed 

the personnel to become familiar with the systems operation. 

tests are discussed below: 

These early 

1. May 3, 1988--(760-sec SSME test firing). Using the max-hold technique with the Tektronix 2756P Spectrum Analyzer, and by comparing 

the recorded spectral waveforms before, during, and after the test 

firing; it was found that the exhaust plume did radiate EM energy 

in several spectral regions: 20 to 100, 140 to 160, 400 to 420, and 

460 to 470 MHz. The maximum amplitude differences were more than 

20 dBm from the regular noise level. However, since the max-hold 

technique was used, no real-time amplitude variations on the 

dynamics of spectral signatures were recorded. 

2. July 20 and 21, 1988--Rhode & Schwarz, designers and manufacturers 

of EM1 and RFI instrumentation, were contacted for the purpose of 

using their spectrum analyzer FSA in place of the Tektronix 2756P 

to conduct a demonstration measurement of RFE from the SSME exhaust 

plume during test firings on July 20 and 21. 

similar to those obtained from the previous measurements, which 

utilized the Tektronix Model 2756P Spectrum Analyzer. The two sets 

of measurements confirmed that electromagnetic energy is radiated in 

several spectral regions of the SSME exhaust plume. 

The results were 

3. August 29, 1988--(1040-sec SSME test firing). Two Tektronix spectrum analyzers were used to measure RFE: Model 2755P with max-hold, 

remote-si te control techniques, and Model 2756P with run-time di splay of spectral waveforms using a video camcorder to record realtime amplitude variations or the dynamics of spectral signatures 

from the scope. The video recording failed due to low battery 

power, but the offsite system did record a very prominent RF radiation in the spectral range of 20 to 40 MHz (as shown in Figure 4). 

Following the baseline experiments, data were then acquired that yielded 

the following resu'l ts: 

1. September 15, 1988--(50-sec SSME test firing). The encouraging 

results obtained on August 29 indicated the need for additional measurements with spectral bandwidth set at 4 to 44 MHz. 

problem previously encountered, an AC adapter was used to power the 

camcorder. The tape recorded the dynamics of spectral signatures 

from the scope, and the recorder images were sequentially "framegrabbed." The RFE from the SSME exhaust plume expressed in terms of 

spectral line amplitude variations, in microvolts, from 25 to 43 MHz 

are shown in the table. There are significant amplitude variations 

both in 29 to 31 MHz and 34 to 38 MHz spectral bands. Figure 5 also 

illustrates these variations. 

To resolve the 

7 
Figure 4. Comparison of Spectral Waveforms Before and After Test Firing 

as Measured on August 29, 1988 

8 
OR4GINAL PAGE 

BLACK AND WHLTE PHOTOGRAPH 

Figure 5. Six Sequential Video Displays of Plume Emission Spectral Signatures 

Before, During, and After SSME Test Firing 

9 
ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 

Figure 5. Six Sequential Video Displays of Plume Emission Spectral Signatures 

Before, During, and After SSME Test Firing (Continued) 

10 
ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 

Figure 5. Six Sequential Video Displays of Plume Emission Spectral Signatures 

Before, During, and After SSME Test Firing (Continued) 

11 
Table. RFE from the SSME Exhaust Plume, Expressed in Terms of Spectral 

Line Amplitude (in microvolts) from 25 to 43 MHz. 

Data File ID* 

Baseline before test 

RFEBTF 07 

Data during test 

RFEDTF 32 

RFEDTF 35 

RFEDTF 42 

RFEDTF 53 

RFEDTF 57 

RFEDTF 60 

RFEDTF 64 

RFEDTF 68 

Baseline after test 

RFEATF 97 

Frequency 

(MHz) 

- 25 - 29 - 30 - 31 - 32 ------ 34 36 38 40 41 42 43 

60 -- 90 - 95 131 118 41 72 32 30 31 28 

60 - 110 100 -- 130 102 48 75 30 31 25 26 

50 70 91 93 141 102 - 60 91 32 33 20 37 

60 - 110 -- -- 124 120 40 80 32 29 30 29 

65 100 100 -- 120 86 35 80 28 26 25 25 

60 70 70 90 131 - 145 55 90 36 37 28 25 

70 60 - 115 -- 120 132 50 - 100 35 33 30 29 

68 86 79 87 122 140 56 95 -- 40 -- 32 

61 -- 80 -- 122 55 -- 24 -- -- -- -- 

*Data File ID Code: 

RFE = Radio Frequency Emissions 

ATF = After Test Firing 

BTF = Before Test Firing 

DTF = During Test Firing 

07, 32, etc. = Video frame numbers 

12 
CON CLU SI ON S 

Measurements of RFE from the SSME exhaust plume using a spectrum analyzer 

system identified several spectral bands of electromagnetic radiation. The use 

of a video camcorder to record the run-time spectral waveform captured the 

dynamics of the emission spectral signatures. These preliminary results indicate that future efforts should definitely be pursued as a potential means of 

engine health monitoring or anomaly condition detection. The approach taken 

was innovative and the results were significant. To maintain a low-cost initiative, the main measurement system (spectrum analyzer and computer systems) 

was leased. Since the A-1 Test Stand was inaccessible during the test firing, 

a remote-site interface system was implemented using a host computer and a 

remote-site control computer. To capture the real-time emission spectral signature variation, a video camcorder was used to record the real-time 

em i s s i on s pe c t r a 1 wave form s . 

The following conclusions were developed: 

1. The small data sets obtained to date are qualitative, but not 

concl usi ve. 

2. The efforts should be continued in order to acquire additional data 

using the same techniques. 

13 
RECOMMENDATIONS 

To further these study efforts, the following items are recommended: 

1. Obtain the same measurements at SSC's Diagnostic Test Facility (DTF) 

investigating different spectral regions, i.e., from 100 KHz to 

50 MHz. The DTF thruster should emit electromagnetic energy in much 

lower frequency regions because of temperature, pressure, firing 

duration, and thrust. 

2. The spectrum analyzer system was leased for six months (through 

January 1989). To acquire additional data, a one-year extension of 

this lease is recommended. 

3. In addition to the lease extension, the current measurement system 

should be upgraded to include: 

a. A new antenna with calibrated operating frequency ranging from 

10 to 200 MHz. 

b. Modification of the programmable part of the spectrum analyzer 

to be capable of storing the spectral waveform data to computer 

random access memory, cache, or expand memory in real time. 

(This will replace the use of a camcorder to record the realtime spectral waveforms, and the digital ly stored spectral data 

are much better for the study of plume emission spectral 

signatures. 

c. Direct measurement of radiation field strength, using RFI 

receivers (manufactured by Rhode & Schwarz, Electro-Metrics, 

and others). Direct measurement is recommended for future 

investigation, since the technique provides better accuracy 

and faster data acquisition when emission spectral bands are 

1 oca ted . 

14 
AP P END I X 

COMPONENTS AND SETTINGS FOR THE SPECTRUM ANALYZER 

USED IN THE 

LOCAL VIDEO RECORDING AND REMOTE SITE MONITORING SYSTEMS 

15 
APPEND I X 

COMPONENTS AND SETTINGS FOR THE SPECTRUM ANALYZER 

USED IN THE 

LOCAL VIDEO RECORDING AND REMOTE SITE MONITORING SYSTEMS 

Components of the Local Video Recording System 

~ ~~ ~~ ~~~~ ~ 

Component Description 

Log periodic antenna Electro-Metrics Model No. LPA-30 

Antenna cable rg-59Iu; 75 ft 

Spectrum analyzer Tektronix model no. 2756p 

TV camcorder Panasonic model no. PV 330 

VCR Realistic HQ PLL model no. 41 

PC with image processing card and software IBM PCIAT; Targa/AT&T-24 

Settings for the Spectrum Analyzer Used in the Local Video Recording System 

Parameter 

Center frequency 

Span division 

Reference level 

Vertical display 

RF attenuation 

Frequency range 

Reference oscillator 

Video fi 1 ter 

Resolution bandwidth 

Setting 

24 MHz 

4 MHz 

(-1 63 dBm 

20 V/div 

0 dB 

0-1.8 

I NT 

30 kHz 

1 MHz 

Pa ramet er 

Linear vertical display 

Auto resol ution bandwidth 

Video filter wide 

Marker threshold auto 

Grating illumination 

Readout 

Digital storage view A 

Triggering free run 

Ti me/d i vi s ion 

Setting 

ON 

ON 

ON 

ON 

ON 

ON 

ON 

ON 

AUTO 

16 
Components of the Remote Site Monitoring System 

Component 

VHF/UHF antenna 

Antenna cab1 e 

Spectrum analyzer 

PC with GPIB Card and 

Remote-Si te Software 

Tel ephone/Modem System 

PC with Host-Site Software 

Printer 

Description 

Radio Shack model no. VU-75 

RG-59-U; 75 ft 

Tektronix model no. 2755P 

IBM PC/AT; IBM GPIB card; Tektronix model 

No. S26RM01 V.1.00 RSM (remote site) program 

disk, and V.1.30 RSM (remote site) system disk; 

Norton-Lambert Close-up (customer), Tektronix 

Version, V1.10 

Intecom Corp. model no. IBX; ITE 128 DOBlA 

(operated at 9600 baud) 

IBM PC/AT; Tektronix model no. S26RM01 V.1.00 RSM 

host-si te disk; Norton-Lambert Close-up 

(support) , Tektronix Version, V1.10 

Epson FX 286E 

~ ~~ ~ 

Settings for the Spectrum Analyzer Used in the Remote Site Monitoring System 

Parameter 

Center frequency 

Pan division 

Reference level 

Vertical display 

RF attenuation 

Frequency range 

Reference oscillator 

Video filter 

Resolution bandwidth 

Settinq 

200, 400, 

600, 800 MHz 

40 MHz 

(-1 23 dBm 

10 dB 

10 dB 

0-1.8 

I NT 

30 kHz 

1 MHz 

Parameter 

10 dB/Di v. vertical di splay 

Auto resol uti on bandwidth 

Video filter wide 

Marker t hreshol d auto 

Grating illumination 

reado ut 

Digital storage view a 

Triggering free run 

Time/division 

Setting 

ON 

ON 

ON 

ON 

ON 

on 

on 

ON 

AUTO 

17 
Report Documentation Page 

1. Report No. 2. Government Accession No. 

NASA CR-1847G7 

4. Title and Subtitle 

17. Key Words (Suggested by Authorts)) 

RF Emissions 

Rocket exhaust plume 

Electromagnetic interference 

Spectrum analyzer 

Space Shuttle Main Engine Radio Frequency Emissions 

18. Distribution Statement 

Unclassi fied--Unl imited 

Subject Category 20 

7. Author(s1 

A. W. Rester, E. L. Valenti, and L. R. Smith 

19. Security Classif. (of this report] 20. Security Classif. (of this page) 

Uncl ass i f i ed Unclassified 

9. Performing Organization Name and Address 

Sverdrup Technology, Inc. 

Stennis Space Center, MS 39529 

21. No. of pages 22. Price 

24 A02 

2. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Stennis Space Center, MS 39529 

3. Recipient's Catalog No. 

5. Report Date 

November 1988 

6. Performing Organization Code 

8. Performing Organization Report No. 

10. Work Unit No. 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Final Contractor Report 

May-September 1988 

14. Sponsoring Agency Code 

5. Supplementary Notes 

NASA Technical Monitor - D. J. Chenevert 

NASA Investigator - Dr. S. Wu