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NBSIR 74-604 Ta )

Space Shuttle Pogo Pressure

Measuring System

A Progress Report

John S. Hilten and Paul S. Lederer

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

December 20, 1974

Progress Report Covering Period

December 15, 1973 to June 15, 1974

Prepared for NASA George C. Marshall Space Flight Center

Huntsville, Alabama 35812

NBSIR 74-604

SPACE SHUTTLE POGO PRESSURE

MEASURING SYSTEM

A PROGRESS REPORT

John S. Hilten and Paul S. Lederer

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

December 20, 1974

Progress Report Covering Period

December 15, 1973 to June 15, 1974

This is a progress report. The work is incomplete and is continuing.

Results and conclusions are not necessarily those that will be included

in the final report. Performance Test Data were obtained from one or two samples of several transducer types, and do not necessarily re- present the characteristics of all transducers of a given type.

Prepared for NASA George C. Marshall Space Flight Center

Huntsville, AlaDama 35812

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

ABSTRACT

! This progress report describes two methods for the dynamic calibration

of pogo pressure transducers used to measure oscillatory pressures

generated in the propulsion system of the space shuttle. The requirements are for the generation of a known (5% or better) sinusoidal pressure perturbation of 140 kPa (approximately 20 psi ) peak-to-peak at

bias pressures up to 55 MPa (approximately 8000 psi) over a frequency

range of from 1 Hz to 100 Hz. Rotating a mercury-filled column in a vertical plane at frequencies below 10 Hz generates peak-to-peak

pressures of 48 kPa (7.0 psi); vibrating the same mercury-filled column

in a vertical plane continues the frequency response from 10 Hz to

200 Hz at peak-to-peak pressures ranging from 21 kPa to 145 kPa

(approximately 3 psi to 21 psi). A method for vacuum filling of the

column is described.

KEY WORDS

Calibration; differential pressure; dynamic; high pressure; orbiter

vehicle; pogo; pressure; sinusoidal space shuttle; transducer.

Space Shuttle Pogo Pressure Measuring System

Progress Report for the Period

from December 15, 1973 to June 15, 1974

to the

George C. Marshall Space Flight Center

NASA Order #H-92100A

NBS Project 4253448

Prepared by

John S. Hilten

and

Paul S. Lederer

1 . Introduction

The objectives of this project include (1) the development of a dynamic pressure calibration method for pogo pressure transducers

used to measure oscillatory pressures generated in the propulsion

system of the space shuttle and (2) the procurement and evaluation of

several commercial transducers to determine their suitability for the

measurement of such pogo pressure perturbations. Specified is a fullscale range of 140 kPa peak-to-peak at bias pressures up to 55 MPa over a frequency range of from 1 Hz to 100 Hz*. Transducers used for

shuttle pogo measurements will be required to operate in vibration

environments ranging from 5 Hz (0.8-cm double-amplitude displacement)

to 3 kHz (140 peak-to-peak acceleration) and in the shuttle propulsion system temperature range of from -253°C to about 480°C. Vibration

levels and temperatures to be experienced by each transducer will be

determined by its site in the system; sites have not yet been selected.

Since exact information on the temperature environment for pogo transducers is not available, transducers have been procured with a major

consideration being performance over as wide a portion possible of the

temperature range given above, and efforts have been directed toward

developing calibration methods suitable for use at the most convenient

temperature, laboratory ambient (the NBS Laboratories at Boulder,

under a separate contract, have developed some preliminary techniques

for determining transducer suitability at cryogenic temperatures).

As methods for transducer calibration using liquid-filled calibration

tubes were available, these methods v.ere chosen as starting points for

the present work. The use of a liquid-filled tube as opposed to a gas-filled tube does not affect the performance of flush-diaphragm

* 1 kPa = 0.145 psi; 1 MPa = 145 psi

.
type pressure transducers; the performance of differential types may

be different for gaseous than for liquid media. Whether a given

transducer in the shuttle propulsion system is responding to gas

pressure, to liquid pressure, or to the pressure of a mixture of

phases will depend on both its detailed location in the oxidizer or propellant flow stream and on the flow conditions at any given moment

of time.

2. Dynamic Calibration of a Piezoelectric Pressure Transducer Using

the Liquid-Filled Calibration Tube on the Windmill Apparatus and

on the Vibration Generator

A test was described in the previous report [1]** in which the windmill

tube (a liquid-filled calibration tube mounted on the windmill apparatus)

v/as first partially filled with mercury; filling was completed with oil

to prevent mercury contamination of the oil piston-gage system connected

to the tube during the pressuri zation cycle, Ihe use of two liquids

with significantly different compressibilities resulted in a calibration

tube that behaved in part as an open tube and in part as a closed tube.

A conclusion was that the tube should be filled with mercury only.

Figure 1 is a schematic of the apparatus used to fill an evacuated

calibration tube with mercury; valve sequencing is indicated in the

valve position chart. Following the mercury fill', the tube was pressurized to 55 MPa by means of the oil piston gage. The closed

liquid-filled calibration tube was mounted on the windmill apparatus

and measurements of transducer output were made at 17 rotational speeds

corresponding to frequencies ranging from 1.93 Hz to 6.25 Hz; the

dynamic pressure level for all tests was 48 kPa peak-to-peak. The

liquid-filled calibration tube was then removed from the windmill

apparatus and attached to the vibration exciter with the transducer

under test at the upper end of the tube. Calibrations were carried

out from 10 Hz to 200 Hz at dynamic pressure levels ranging from

21 kPa to 145 kPa peak-to-peak. For the tube used, an acceleration of

± 3 g-p produces pressure pulses of 134 kPa peak-to-peak. Extrapolations

to the region of 8 Hz of the two calibration curves obtained from these

tests agreed to within 1%. The tube was then mounted on the vibration

exciter in an inverted position with the transducer at the lower end

of the tube, and a set of 10-200 Hz frequency-response measurements

was performed as described above, ihese two sets of 10-200 Hz measurements were then averaged, as described in the previous report [1], to

determine a geometry correction factor of 6% for the data shown in

figure 2. With the factor of 6% applied, the manufacturer's calibration

was within 2% of the NBS dynamic calibration. Some uncertainty in

this comparison is created by the fact that in the manufacturer's

calibration and in the NBS calibration different combination power

supply-amplifiers were used; as reported previously, choice of power

supply may affect the calibration significantly (up to 5%) [1].

Figure 2 shows the frequency response for transducer A-1 in terms of

response ratio, defined as the ratio of (1) transducer output at the ** Figures in brackets refer to references, section 7.

2
given frequency to (2) the average transducer output over that portion

of the frequency range in which the output is considered to be flat,

for A-1, 20-50 Hz. At the low-frequency end of the range, response is

down about 15% at 2 Hz from unity response ratio.

3. Construction of New Windmill Calibrator

As reported, the present windmill calibrator was modified from the

prototype Earth's field dynamic calibrator for acceleroemters [2] by

removing the turntable and replacing it with a 76-cm tube and mount.

The new apparatus is intended to extend the frequency range downward

to below 1 Hz and upward to approximately 15 Hz. The new design

incorporates two variable-speed motors; the slower of these rotates

the tube at frequencies from below 1 Hz to about 4 Hz; the other,

at frequencies from about 2 Hz to as much as 15 Hz. Chief factors

limiting maximum rotational speed are the degree of balance achieved

and the amount of friction present. Mercury slip rings are used to

reduce friction and noise. Completion of the calibrator is subject

to component delivery. As of July 1, 1974 the air bearings, two motors,

and drive belts have been received; shaft and shaft-support housing

have been fabricated. Pulleys for the drive system and mercury slip

rings have not been received although the orders for these items were placed over 5 months ago. The manufacturers have been contacted

repeatedly; delivery is now scheduled within 40 days.

4. Transducer Repair and Procurement

Two piezoelectric pressure transducers F-1 and F-2 have been procured

with a range of from 0 MPa to 69 MPa, a sensitivity of about 0.65 mV/kPa,

and a time constant of 2500 s. Two power supply-amplifiers for use with

these transducers (one line powered, S-4, and one battery powered, S-5)

have also been received.

As previously reported, transducer A-4 has been returned to the manu- facturer for warranty repair. To provide improved low-frequency

response the manufacturer was instructed to modify the instrument

increase the time constant to 0.5 s. The new designation for the

Modified A-4 is G-1 . This change also reduces the sensitivity of the

instrument from 4.4 mV/kPa to 1 . 5 mV/kPa.

During tests, transducer C-2 was damaged in an unknown way; it was also returned to the manufacturer for repair and conversion to a 0.5-s time constant. The new designation for the modified C-2 is

G-2.

5. Calibration of Strain Gage Differential Pressure Transducer

The sponsor requested that a specific manufacturer be given the

opportunity to supply a di fferential - type transducer for evaluation.

On being contacted, this manufacturer indicated that considerations

of time and cost precluded the development of a special design for

3
pogo evaluation, but that he would modify an existing industrial high

line-pressure, low di fferential -pressure transducer constructed with

two diaphragms separated by an oil-filled cavity. Accordingly, the

manufacturer supplied a metallic strain gage transducer consisting of

'two pressure chambers, connected through an adjustable leak-rate valve

and separated by a diaphragm which responds to the pressure difference

between chambers."

A 25.4-cm brass pipe was screwed into the pressure fitting of the transducer to form a vertical calibration tube; both pressure chambers as well as the 25.4-cm pipe were filled with oil using a vacuum filling

technique and the transducer-calibrator was mounted on the vibration

exciter in a vertical position. A hose was attached to the top of the

pipe, and a bellows was used to apply a pneumatic pressure of first

+10 kPa and then -10 kPa for a static calibration. The static calibration resulted in only about 4% of the output predicted by the manufacturer's calibration sheet. An explanation may be that when the chambers

of the pressure transducer are filled v/ith oil and when the transducer

by-pass valve connecting the two chambers is closed, the presence of the

nearly incompressible oil in the bottom chamber of the transducer changes

the effective spring constant of the pressure-sensing diaphragm to

result in a stiffer system. When the oil was removed from the transducer

and the static calibration again performed, the sensitivity observed

was approximately that given by the manufacturer; this observation tends

to confirm the explanation given above.

A few dynamic calibration measurements of the response of the transducer were performed using the vibration exciter. These measurements also

showed a greatly reduced sensitivity factor from that claimed by the

manufacturer.

It should be noted that the manufacturer supplied a transducer more

suitable for gaseous than for liquid media. The test results indicate

in themselves only the unsui tabi 1 i ty of the design for liquid-media

pogo measurements. However, experience with pressure transducer evaluation, including analysis of design principles, suggests that behavior

of the acoustic filter -- the by-pass line and adjustable leak-rate

valve -- will vary with changes in the ambient temperature and media

density and render this design unsuitable for flight rating. No

further testing of transducers with acoustic filters is planned.

6. Future Plans

Tasks for the Reporting Period June 15, 1974 to December 15, 1974 are

the following:

(1) Completion and testing of windmill apparatus.

(2) Calibration of transducers F-1 , F-2, G-1 , and G-2. These transducers will first be calibrated using the windmill apparatus and

then calibrated using the vibration exciter. Calibration will

4
be at full bias pressure and with a dynamic component varying from

48 kPa to 145 kPa peak-to-peak. The data will cover the frequency

range of from 0.5 Hz to 100 Hz.

Two-plane vibration sensitivity tests of transducers F-1 , F-2,

G-1, and G-2 over the frequency range of from 5 Hz to 3 kHz.

A mathematical analysis of the closed tube amplitude-halving and

frequency-doubling phenomena will be developed and included in

the final report.

References

Hilten, J.S., and Lederer, P.S., Space Shuttle Pogo Pressure

Measuring System, Progress Report for the period September 15, 1973

to December 15, 1973, NBSIR.

Hilten, J.S., Accel erometer Calibration with the Earth's Field

Dynamic Calibrator, NBS Tech. Note 517 (March 1970).
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BIBLIOGRAPHIC DATA

SHEET

1. PUHl.K A riON OK KF.POKT ND. NBSIR 74-604 CR)2. (tov 't Ac( (. s.sion No.

3. Recipient's Accession No.

4. riTLI-: AND .SIMMTTIJ'; SPACE SHUTTLE POGO PRESSURE MEASURING SYSTEM

A PROGRESS REPORT

5. Pu hi ic at ion Date 9/15/74

6. Performing Organization Code

7. AU'[T10R(S)

John S. Hilten and Paul S. Lederer

8» i'crformin^ ^^rgiin. Report No. NBSIR 74-604 (R)

9. l'i£KFORMlN(i ORC> AN IZATION NAMF AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. l-'roiect/ Task/Work Unit No. 4253448

11. Tontract /Grant No.

^ASA ORDER #H92100A

12. Spcinsoring Organization Name and C^oniplete Addres.s (Strevt, City, State, ZIP)

NASA G. C. Marshall Space Flight Center

Huntsville, Alabama 35812

13. 1 ype of Report & Period Covered

12/15/73 - 6/15/74

14. Sponsoring Agency Cc>de

15. siippi.f:mf:ntary notfs

16. .ABSTRAC T (A 200-word or /e.s.s factual summary of most si^idcant information. If document includes a significant

bibliography or literature survey, mention it here.)

This progress report describes two methods for the dynamic calibration of pogo

pressure transducers used to measure oscillatory pressures generated in the

propulsion system of the space shuttle. The requirements are for the generation

of a known (5% or better) sinusoidal pressure perturbation of 140 kPa (approximately 20 psi) peak-to-peak at bias pressures up to 55 mPa (approximately

8000 psi) over a frequency range of from 1 Hz to 100 Hz. Rotating a mercuryfilled column 1n a vertical plane at frequencies below 10 Hz generates peak-topeak pressures of 48 kPa (7.0 psi); vibrating the same mercury-filled column

in a vertical plane continues the frequency response from 10 Hz to 200 Hz at

peak-to-peak pressures ranging from 21 kPa to 145 kPa (approximately 3 psi to

21 psi). A method for vacuum filling of the column is described,

7. KF;Y words (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Calibration; differential pressure; dynamic; high pressure; orbiter vehicle; pogo;

pressure; sinusoidal; space shuttle; transducer.

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