NASA TM-2001-210253

A Wind Tunnel Study on the Mars Pathfinder (MPF)

Lander Descent Pressure Sensor

J. Francisco Soriano, Rachael V. Coquilla, Gregory R. Wilson, Alvin Seiff, and Tomas Rivell

National Aeronautics and Space Administration

John F. Kennedy Space Center, Kennedy Space Center, Florida 32899-0001

June 2001

https://ntrs.nasa.gov/search.jsp?R=20010043986 2018-05-14T14:57:54+00:00Z
A Wind Tunnel Study on the Mars Pathfinder (MPF)

Lander Descent Pressure Sensor

J. Francisco Soriano 1, Rachael V. Coquilla 2, Gregory R. Wilson 3, Alvin Seiff 4, and Tomas

Rivell 5

1Aerospace Engineer and McNair Scholar. NASA Ames Research Center, Moffett Field,

California. Now at John F. Kennedy Space Center, Florida.

2Graduate Research Associate. Mechanical and Aeronautical Engineering Department,

University of California, Davis.

3Faculty Research Associate, Jet Propulsion Laboratory, Pasadena, California.

4Senior Research Associate, San Jose State University Foundation, San Jose, California.

SNASA Ames Research Center, Moffett Field, California.

ABSTRACT

The primary focus of this study was to determine the accuracy of the Mars Pathfinder lander

local pressure readings in accordance with the actual ambient atmospheric pressures of Mars

during parachute descent. In order to obtain good measurements, the plane of the lander pressure

sensor opening should ideally be situated so that it is parallel to the freestream. However, due to

two unfavorable conditions, the sensor was positioned in locations where correction factors are

required. One of these disadvantages is due to the fact that the parachute attachment point rotated

the lander's center of gravity forcing the location of the pressure sensor opening to be off tangent

to the freestream. The second and most troublesome factor was that the lander descends with

slight oscillations that could vary the amplitude of the sensor readings. In order to accurately map

the correction factors required at each sensor position, an experiment simulating the lander

descent was conducted in the Martian Surface Wind Tunnel at NASA Ames Research Center.

Using a 1/5 scale model at Earth ambient pressures, the test settings provided the necessary

Reynolds number conditions in which the actual lander was possibly subjected to during the

descent. In the analysis and results of this experiment, the readings from the lander sensor were

converted to the form of pressure coefficients. With a contour map of pressure coefficients at

each lander oscillatory position, this report will provide a guideline to determine the correction

factors required for the Mars Pathfinder lander descent pressure sensor readings.
INTRODUCTION

On July4, 1997the MarsPathfinderlanderenteredthe Martian atmosphere.Onecomponent

of its sciencepackagecarriedinstrumentationaimedto take readingsof the ambientpressures

duringthedescentphase.Althoughmeasurementswereinitiatedimmediatelyuponentry,thekey

set of readingsthat portray the Mars atmospherewereperformedimmediatelyafter parachute

deploymentandheatshieldejectionuntil just prior to airbaginflation (seeFigure 1).

Figure 1: Mars Pathfinder Lander Parachute Descent Phase

/

Triangular Window Opening _ _ 7

_ _ _d_ Petal

over

Base Panel

Lander Note: Airbags not shown

One of the concerns in the configuration of the spacecraft's atmospheric pressure sensor was

to determine how its readings were affected by its position within the lander and with respect to

the descent freestream. The sensor is centered and flushed within the triangular window opening

formed by the junction of the lander's base and by the two adjacent petals in which one holds the

Sojourner Truth rover and the other carries the ASI/MET mast (see Figure 2). Unfortunately, this

location of the pressure sensor is also diverted by the rotation of the lander with respect to its

parachute. The parachute attachment point is located at the top edge midpoint of the rover petal

which forces the center of gravity of the lander to rotate approximately 14 degrees about this

attachment point (see Figure 2). Ideally, in order to read ambient pressures, the plane of the
sensoropening shouldbe parallel to the flow. Due to the rotation of the lander, the sensor

openingis situatedoffset from freestream.Additionally, the lander was subjectedto varying

oscillationsduring thedescentforcingthe sensoropeningto bepositionedin the turbulentwake

of thelandersideandbaseplatformsor evenupto facingagainstthefreestream.Ultimately, with

these unfavorable pressure sensor locations, there were times when the readings drew away from

the actual atmospheric pressure of Mars.

Figure 2: Model of the Mars Pathfinder Lander

TOP VIEW

Parachute Attachment Point

Lander Pressure/

Sensor Inlet --_ Lander's

ASI/MET ._,_/f/_t _ Center of Gravity

Mast Wa_ I I

Sojourner Truth Rover Petal

In order to find the correction factors at the various sensor positions, a wind tunnel study,

simulating the oscillatory Martian descent, was conducted using a 1/5 scale model of the Mars

Pathfinder lander. This experiment was performed at the Martian Surface Wind Tunnel which is

housed inside a low pressure chamber facility at NASA Ames Research Center, Moffett Field,

California. The chamber was originally developed for low pressure acoustic and structural tests

on rockets and was later reassigned to accommodate an atmospheric and planetary aeolian wind

tunnel. Although the experiment for this study was conducted at Earth atmospheric pressures, it

still provided a comparable simulation by duplicating the Reynolds numbers experienced during

the descent.

3
TEST CONDITIONS

Conditions to be Simulated

From a previous study which diagrammed the internal and extemal flow of the Pathfinder

lander's Martian descent [Rivell et al, I997], the dynamic conditions inside the wind tunnel were

reconstructed for this experiment. According to this investigation, upon parachute deployment

and heat shield removal, at approximately 6.0 km above the Martian surface, the lander was

predicted to experience a Mach number of 0.48. Based on the full-scale length of one of the side

petals' long edges, the Reynolds number is calculated to be 11.3 x 104. During the middle of the

descent, variations in velocity and density readings also resulted in acquiring Reynolds numbers

ranging from 12.0 x 10 4 to as low as 8.9 x 10 4.

For the wind tunnel experiment, the freestream velocity was varied to meet the descent

Reynolds numbers which were calculated based on the model's side petal edge height.

P

Re- o_V_I and p_-

RA_T_

Re = Measure of the ratio of inertia forces to viscous forces in a flow

Voo = Freestream velocity (m/s)

! = Model's side petal edge height (m)

= Dynamic viscosity (Ns/m a)

P_o = Test section pressure (mb)

RAIR = Universal Gas constant for air ( 8.31441 x 103J kmoi "1K "1)

To_ = Inside chamber temperature (K)

poo= Inside chamber density (kg/m 3)

To find the density, both the ambient temperature and pressure were directly measured and were

used along with the universal gas constant for air at Earth sea level standard temperature and

pressure.

Although the Mach numbers obtained during the experiment were much lower than those of

the actual descent (< 0.02), duplicating the Mach numbers was, in fact, negligible for the
experiment.Both the descentand experimentMach numberswereconsidered to be in the low

subsonic speed regime, therefore, compressibility effects were minimal.

Martian Surface Wind Tunnel

Currently managed by Arizona State University, the Martian Surface Wind Tunnel is an open

circuit, atmospheric and surface wind tunnel of approximately 13 meters in length with a 1.26 m

x 0.96 m test section. It is placed inside a 4,000 m 3 chamber where experiments can be conducted

under Earth or Mars atmospheric pressures (see Figure 3). Winds inside the tunnel can be

developed in two different methods. When the chamber is maintained at Earth pressures, an

electrically powered axial fan at the end of the tunnel is used, creating a maximum velocity of

about 12 m/s inside the test section. At low pressure runs, winds are developed by ejecting high

velocity air in the rear diffuser section creating a low pressure area downstream of the test section

which, in turn, forms a suction through the tunnel. With this method and at a Mars simulated

pressure of 4 mb, the tunnel can obtain test section velocities of approximately 100 m/s. In

addition, carbon dioxide gas can also be pumped into the chamber to further closely portray the

Martian atmosphere.

.Fimare 3: Mars Surface Wind Tunnel at NASA Ames

Research Center

Test Section

(0.9 m high x 1.2 m wide) Injectors

 ow:rai hie

IIIill'Ilia

Contoured Inlet Section Fan /

In order to obtain several key atmospheric measurements for each experimental run, the wind

tunnel is also equipped with various analog instrumentation. Using a Setra model 270 barometer,

ambient pressure is measured through a 1.53 mm diameter orifice from the test section ceiling.
Ambient temperatureis measuredwith a typeT thermocouple.For velocity calculations,thetest

sectiondifferentialpressureis sensedby apitot tubeequippedwith a Setramodel239differential

pressuretransducer.Boththe thermocoupleandthepitot tubearealsosuspended13cm from the

test sectionceiling, a positionin which theceiling boundarylayer andthe wake from thelander

modeldonot interferewith theirindividualreadings.

Mars Pathfinder Lander Model

In this experiment a 1/5 scale model of the Mars Pathfinder lander was used. According to

previous studies [RivelI et al, 1997], the use of this model inside the Martian Surface Wind

Tunnel was verified through potential flow calculations which indicated that the wall turbulence

inside the test section would not interfere with the flowfield developed around the model. Based

on the scale edge length of one of the side petals and on the maximum test section velocities that

can be obtained at both Earth and Mars pressures, this size model can also easily duplicate the

Reynolds numbers achieved during the actual Martian descent.

Made of wood, styrene, and transparent acrylic plastic, the model duplicates not only the

external size features of the Mars Pathfinder lander but also the approximate size and shape of its

major internal structural components, such as the main instrumentation package on the base

panel and the Sojourner Truth rover attached to one of the side petals. By recognizing these

internal features, the model can also create the general pattern of the lander's internal flow. To

secure the model inside the center of the test section, it was mounted to a movable aluminum

brace which is connected to the outside center of the side petal that houses no scientific

measuring equipment. The brace was then attached onto a heavy lead pedestal with the model's

base platform initially facing the incoming flow (see Figure 4).
Figure 4: Model of the Mars Pathinder Lander

Triangular I / PROFILE VIEW

I

Window _ _0d_/

Opening __-_---_

__ Lander Pressure Sensor

Freestream ....... l"_" ....... ] .... Angles of Attack were

Ill /J ,_ measured from this

I II / _ - position where the base

L__ _X_ of the lander is

\ / \ XI ] perpendicular to the

Base Lander _ " ___oming flow

Lead Pedestal _ I I i [ I Aluminum Brace

I : I

Experimental Setup

Within the triangular window formed by the rover petal, the ASI/MET mast petal, and the

lander base platform, a scaled down version of the atmospheric pressure sensor inlet tube was

also added to the model. Based on the spacecraft's design, this particular tube was made only

with a single opening in order to attain direct ambient pressure readings. The inlet was connected

to a highly sensitive Setra 239 differential pressure transducer. Since this instrument requires

readings from two ports, the single-opening lander sensor was connected to the transducer's

dynamic port, and a test section ceiling orifice, which is also connected to the barometer, was

applied to its reference port. The size of the model dictated a lander sensor opening of 0.2 mm

inside diameter. However, with this small opening, the Setra 239 transducer tends to produce

uncontrollable pressure fluctuations. Through several tests using various size tubing, starting

with the desired scale size, the errors were minimized with a 0.93 mm inside diameter tubing

representing a 0.7 scale model. Although this size tubing does not correspond to the scale of the

lander model, the distribution of the readings provides the same average as that of the 0.2 scale

model. The 0.7 scale tubing simply refines the readings acquired from the smaller tubing.
Therefore, for this experiment a 0.93 mm inside diameter stainless steel tube was conformed to

represent the Mars Pathfinder descent pressure sensor (see Figure 5).

Figure 5: Model of the Mars Pathfinder Lander

Top View

is perpendicular to the

Lander Base

tl tT

Sideslip angles were measured

from this position where the lander

incoming flow.

Lander

Pressure

Sensor

location

within

Triangular

Window

Opening.

Freestream

To simulate the Martian descent, the Reynolds numbers for the experiment were varied from

5.0 x 10 4 to 13.0 x 104 at increments of 1.0 x 104. With this resolution, a satisfactory set of test

runs were conducted at nine different Reynolds numbers. The oscillatory positions were also

duplicated by first defining the zero angle from the model and then by applying single

longitudinal and lateral rotations. Hence, the reference position was established to be where the

lander base platform is perpendicular to the freestream. From this position the top edge of the

lander base also defines one side of the triangular window where the lander pressure sensor is

situated (see Figure 4). According to the model, the plane of this triangular window was found to

be rotated 20 ° from the lander base platform. By defining the reference position with respect to

the base platform and by knowing the location of the sensor window from the base, the position

of the lander pressure sensor opening can be determined.
From the zero referenceposition, a longitudinal rotation or angleof attack was measured

using a metal squareand protractoragainstthe baseplatform. Positive anglesof attack were

designatedwhenthe platformis rotatedupward.To preseta lateralrotation or sideslipangle,a

sheetof paper,preciselydraftedwith the desiredangularpositionsfrom thefreestreamdirection,

wascenteredandsecuredontothetestsectionfloor. Thelandermodel,attachedto its stand,was

thencenteredon top of this sheet.Its leadpedestalwasthenmarkeddepictingthepositionwhen

the landerbaseplatform is perpendicularto the incoming flow. By positioning the pedestal's

markson a desiredangularpositiondefinedonthesheet,thelateralrotation of the model'sbase

with respectto the freestreamcanbedetermined.The positivesideslipangleswerealsoassigned

whenthebaseplatform is rotatedclockwiseandawayfrom theincomingflow.

In decidingthe rangeof anglesof attack and sideslip anglesto be tested,two factors were

considered.Onepertainedto therotation of thelander'scenterof gravity due to the positionof

theparachuteattachmentpoint.Theother accountedfor thepossiblelanderdescentoscillations.

As previously stated,sincethe parachuteattachmentpoint is slightly offset from the lander's

centerof gravity,thelanderwasassumedto restat approximately14° abouttheattachmentpoint.

Fromthemodel's zeroreferenceposition,this rotationis characterizedas+7° angleof attackand

+12° sideslip(seeFigure2).Basedon thepredicteddescentvelocity andatmosphericconditions,

thelanderwasalsopresumedto oscillateat anamplitudeof + 20 ° from its equilibrium position.

Therefore, as shown in Table 1, the sensor positions to be tested were preset using the selected

rotation angles measured in degrees.

Data Acquisition

To cover all possible positions in which the lander sensor could oscillate into, the model was

first set at a particular angle of attack and then rotated laterally at every sideslip angle. For each

angle of attack and sideslip angle position, nine Reynolds number runs were performed. To

acquire the readings from all the instruments simultaneously, the Martian Surface Wind Tunnel

is outfitted with an automated data acquisition system called LabVIEW, which is also equipped

with a local signal conditioning device. Operated through a 100 MHz Pentium processor using

Windows '95 and 32.0 MB of RAM, is capable of acquiring voltage readings at rates of up to

1000 Hz, averaging 500 samples per data point. LabVIEW also has the capability to conduct

'1
several simultaneous calculations for each set of data points. Therefore, the acquisition rates

primarily depend on the amount of calculations the program performs. Although LabVIEW is

capable of carrying out higher sampling rates, the data acquisition was set to 500 Hz, averaging

100 samples per data point over a period of 30 seconds. The lower sample rates were decided

primarily due to the amount of calculations that LabVIEW performed.

For this experiment, the program determined the wind tunnel temperature in degree Celsius;

the wind tunnel barometric pressure, freestream differential pressure, and lander sensor

differential pressure in millibars; the freestream velocity in meters per second; the atmospheric

density in kilograms per cubic meter; and the Reynolds number. Despite the lower acquisition

rate, LabVIEW was still able to provide a set of 300 data points (means) for each 30 second run.
RESULTS

With LabV/EW's signal conditioning device, the wind tunnel instruments were maintained

with the variations shown in Table 2 and Figures 4-5. Due to wind tunnel vibrations made by the

axial fan and to other minor disturbances, the tolerances for the differential pressure readings

varied from a minimum of + 0.009 mb to the maximum values stated in Table 2. Although the

readings from the freestream pitot tube became more erratic with increasing velocity, the test

section speeds were still maintained within a maximum variation of + 0.05 rn/s. Since the lander

model pressure sensor was equipped with the more precise and stable transducer, its readings

only amplified to a maximum of + 0.062 mb.

Although the Reynolds numbers were easily duplicated, the dynamic pressures from the actual

descent were of a greater magnitude than those depicted during the wind tunnel experiment.

Therefore, in order for the pressure measurements from the lander sensor to be more applicable

to any conditions in the low subsonic regime, they were converted in the form of coefficients of

pressure, Cp.

Ap

Cpq=

Ap = PL - P_ = Differential pressure (mb)

q_ = (1/2) 9_ VJ = Dynamic pressure (mb)

PL = Lander pressure (mb)

Poo = Ambient pressure (mb)

A coefficient of pressure is a dimensionless, similarity parameter used to describe the local

surface conditions as a function of the freestream dynamic conditions. Consequently, due to its

location within the lander, the pressure sensor inlet was actually sampling local "surface"

pressure rather than ambient pressure. Therefore, with the differential pressure (Ap) readings

from the sensor and the dynamic pressure (q=) calculations from the test section atmospheric

instruments, the sensor pressure coefficient can be determined for each designated lander rotation

from the freestream. As long as the Mach number for this approximately blunt-shaped spacecraft
doesnot exceeda rangewherecompressibilityfactors aremagnified (Moo- 0.8), the pressure

coefficientspredictedaroundthemodelcanbereferencedto describethe conditionsaroundthe

actuallanderduringits descent.

In this reporttheexperimentalresultsarepresentedin two ways.The first setshowstheeffect

of modelrotationsagainsta rangeof test sectionReynoldsnumbers(Figures6-15.)To simplify

the plots, angleof attackwasdesignatedas c_and sideslip angleas 1_.The next set of results

(Figures 16-24)displaysthepredictedcontour mapsof Cpfor eachReynoldsnumbertestedat

various landerrotationsfrom its equilibrium position. Finally, Figures 25-27, displayplots of

averagedifferential pressurereadings,averagestandarddeviations, and averagevelocity as

functions of Reynoldsnumbersto confirm the legitimacy and accuracyof the data for each

experimentalrun.

ANALYSIS

As previously stated in the experimental setup, the lander pressure sensor opening was located

at an additional 20 ° away from the freestream for each angle of attack that the model is rotated.

This sensor position is designated in the graphs as c_2. Through preliminary observations of

Figures 6 through 15, the plots revealed concentrations of pressure coefficients approaching one

at an initial c_ of -6 °, placing the sensor opening at an c_2 of 14 ° (See Figure 13). As the angle of

attack was decreased, the distributions became more uniform with Reynolds number at a

particular C Ofor each sideslip position, but yet still closer to one. In particular, these longitudinal

rotations completely exposed the lander sensor opening to the incoming flow. As the sensor

opening became more perpendicular to the flow, its Cp values became closer to the value of one

which implied that a stagnation point develops at the tip of the sensor.

t7
Fiqure 13: Cp vs Re for cx= -6 deg (cxz= 14 deg)

1

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

-1

A

v

A •

v • ¢.

40,000 60,000 80,000 100,000 120,000 140,000

Re

--_ Sideslip

-=- Sideslip

-*- Sideslip

-_-- Sideslip

Sideslip

---e- Sideslip

-=- Sideslip

Sideslip

-._- Sideslip

Sideslip

_-Sideslip

= 35 deg

= 30 deg

= 25 deg

= 20 deg

= 12 deg

= 10 deg

= 6 deg

= 3 deg

= 0 deg

= -5 deg

= -10 deg

Over the same observations, clusters of negative Cp were also found to develop at an early c_ of

0 ° in the lower 13 angles (see Figure 12). In these positions the plane of the sensor opening was

still faced against the incoming flow, therefore, the pressure coefficients should have stayed

positive. By contrast, positive pressures were not attained until 13 was increased beyond +6 ° .

Looking at the approach of the freestream towards the triangular opening which houses the

sensor, these premature negative pressure readings were actually caused by the wake from the

lander base platform. As 13 increased, the edge of the platform was also rotated away from the

freestream which, in turn, reduced the size of the resulting wake and eventually recovered the

presumed positive Cp.

13
Fiqure 12: Cp vs Re for = 0 deg = 20 deg)

1

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

-1

40,000

v.

I

m _ m

60,000

I

80,000 100,000 120,000 140,000

Re

--_ Sideslip = 35 deg

--=-Sideslip = 30 deg

--_ Sideslip = 25 deg

....... Sideslip = 20 deg

--_ Sideslip = 12 deg

-e-Sideslip = 10 deg

-m-Sideslip = 6 deg

--o-Sideslip = 3 deg

....._=_-Sideslip = 0 deg

Sideslip = -5 deg

...._ Sideslip = -10 deg

Results (Figure 12) also introduced a variation in pressures at sideslip angles of _+10°.

According to the externally symmetrical shape of the model, as it is attached on its stand, these

particular lateral rotations should have reported the same pressure readings. Instead, the readings

fell within an average pressure coefficient difference of 0.4, where the higher pressure was

attained at 13= +10 °. This difference was possibly due to the presence of the rover structure on

one of the side petals. As the freestream reached and entered the window opening, the rover

model constricted the flow allowing most of it to escape into one side of the triangular window.

As the sideslip was increased from zero, the rover structure forced the entry velocity to increase,

thereby allowing the wake from the lander to decrease faster. When 13was decreased, the rover

structure was not obstructing the entry, hence, the flow only slowly entered the window

maintaining a constant size wake and allowing the pressure sensor to acquire negative C v

By increasing the angle of attack, the pressure coefficients for each sideslip angle slowly turned

negative. At an ot of 20 ° (Figure 7), the Cp distribution began to stabilize at only negative values
which were alsodistinctfor eachsideslipangle.Until then,thepressuresvariedfrom positiveto

negativewith sideslipangleand,in somecases,with Reynoldsnumber.The variations with Re

were possibly due to the amount of flow entering the triangular window opening. At slower

velocity, most of the flow was tripped by the edge of the base platform producing a larger wake

over the window and negative pressures from the sensor. As velocity increased, the larger

pressure difference between outside and inside the lander created a greater suction through the

window opening. This suction decreased the size of the base platform's wake and also began to

increase the pressure sensor reading. In Figure 10, this phenomena was greatly revealed at an o_ of

7 ° and 13of 12 ° where the sensor read from negative to positive pressures.

Fiqure 10: Cp vs Re for = 7 deg = 27 deg)

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

-1

i _ -&

40,000 60,000 80,000 I00,000 120,000 140,000

Re

-4- Sideslip = 35 deg

--=-Sideslip = 30 deg

-*-- Sideslip = 25 deg

...._- Sideslip = 20 deg

--_ Sideslip = 12 deg

-e-Sideslip = 10 deg

-a--Sideslip -- 6 deg

-e- Sideslip = 3 deg

--,- Sideslip = 0 deg

--*- Sideslip = -5 deg

-,_- Sideslip = -10 deg

Through further evaluations Cp also showed to be Reynolds number independent with most of

the tested lander oscillatory positions. However, there were certain situations where the Cp

drastically changed with Reynolds number. Unfortunately, the most significant occurrence can be

seen at the nominal or equilibrium position of ot = 7 ° and 13 = 12 °, where the Cp increased from

"i5
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O_

I_,

II

(xl

v

0")

o,,I

II

n,'

00

c_

¢,o

c_

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c_

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o

d_ I I iI i_ i ,

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¢Xl ooo

c_oooo

o" ooooo
A

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I!

n,,

i

"O "D "D "D "D "D

_ 0 u_ 0 0_1 0 _3 "0 "0 0

II II II II II II II II II II II

•i ,i ,_ ,i ,i ,I ,i Qi ii ,I iI

• i ,m ii m_ ,I I_ II Im D_ mm II

.mIII

.If-
30

25

20

15

_, 10

b.

_ 5-

_ 0

-5

-10 -

-15 -

-20

-15

Figure 16: Contours of Cp for Re = 50,000

l I I I I ] I I p b

-10 -5 0 5 10 15 20 25 30 35

J3(degrees)

40

..... Cp=l.0

---m---Cp=0.9

Cp=0.8

--:p---Cp=0.7

_Cp=0.6

---e--Cp=0.5

_Cp=0.4

---a---Cp=0.2

_---e---Cp=0.1

--@--Cp=0

- - o - -Cp = -0.1

- - _r - - Cp = -0.2

: " Cp = -0.3

- c_ Cp = -0.4

- c> -Cp=-0.5

• Nominal Position

-------+1- 3 deg Oscillation

= - - +1-5 deg Oscillation

-'- - +/- 8 deg Oscillation

--+1-11 deg Oscillation

-- --=+1- 15 deg Oscillation

30

25 1-

20 T

15t-

_-10+

_,54-

_ OT

-5 t-

-10 t_

-15 t-

-20

-15

Figure 17: Contours of Cp for Re = 60,000

A".... A .... _--&:_..:-&-& ....... A ;- -_ O

" ' "

Q ",_.. .K o-,,D'T--a-o

".n-'._ O:'-a.-,"a.'_.,", b 6_ _ n

: [ /."/'_',--. '_.. !, \

r_ [] [] .i+r_ ••..5;;;_,u_._,+ _ _.. • ;

o : ,: .

"" '_' Il t l ¢ _ " " I I

I I q t I r I ' t I I

-10 -5 0 5 10 15 20 25 30 35

IB(degrees)

4O

----¢_--Cp=1.0

---B---Cp=0.9

Cp=0.8

---e_Cp=0.7

---_--Cp=0.6

---e---Cp=0.5

----e_Cp=0.4

---o---Cp = 0.2

---e--- Cp = 0.1

-4_--Cp=0

- _ - -Cp =-0.1

- _ - -Cp =-0.2

- c- - -Cp = -0.3

- <> - -Cp =-0.4

Cp = -0.6

• Nominal Position

•---'_=+/- 3 deg Oscillation

= = = +1- 5 deg Oscillation

_" - +/- 8 deg Oscillation

...............+1- 11 deg Oscillation

--" "--+/- 15 deg Oscillation
_oo

0

0

0

II

Q,

0

0

C

0

0

, I

' ' , +, ". -.,, ,', , _ , ._ ,, u\

, , _ ,,. "...._,..,...,.' ., _ ,_ :

a _ ". ,_n o',•', r, _/- \ ', _ ?

.... ;, ' ' i -

.."i ..."__-_..._,,Ii\ i !

," ; .... ',' ' _ i i

.','" ..-" _ :,i , _ ', 1

0

I I I I I ' I '" I I I

IO 0 _ 0 to 0 IO 0 IO

I I

0

,eg

u_

0

u_

0

0

|

0

I

I

0

N

!

L_
i

(qtu) dV
(quJ) uo!le!AaO pJepuels

z_
i

0

0

0

0

0

0

0

0

0

0 0

0 0

(s/w) _I!_oI_A

0 0

0 0

0

0

0

0

0

0

0

0
approximately -0.22 at Re = 5.0 x 104 to about 0.12 at Re = 13.0 x 10 4. In order to account for

these inconsistencies, Cp contour plots were drafted for each Reynolds number experimental run.

These contour plots not only display the slight Reynolds number dependency but also predict the

Cp that may generate in a particular region of oscillation.

In Figures 16 through 24 the possible lander oscillations are represented by five concentric

circles around the nominal position, while approximate areas of constant Cp are shown using the

various contour lines. Through a general examination, areas of negative pressure were found to

be in regions of high c_ which are depicted by dotted contour lines, and areas of positive pressure

were located in regions of lower _, identified by solid contour lines. These plots provide a better

illustration of the physical interpretation for the Mars Pathfinder lander parachute descent since

one can extract from these contour plots, a reasonable range of Cp that the lander sensor observes

at particular spacecraft attitudes. Although there are locations of insufficient data points, such as

at 13= 15 °, these values can be interpolated from the surrounding calculated Co values to further

improve the resolution.

APPLICATIONS

At moderate oscillations of about +3 ° to +8 ° , the pressure sensor is more likely to be

measuring a range of Cp closer to zero, indicating a small pressure correction. However, when the

oscillations exceeded these amplitudes, major corrections may be required since at a certain

region of oscillation Cp reaches a value closer to one. By previous definition when Cp is zero, the

lander sensor reading is equal to the ambient pressure (i.e. Ap = PL - P_ = 0), therefore, no

correction is required. For sensor positions where Cp is steered away from zero, measurements

from the Mars Pathfinder's science instruments and the values of Cp can be used to correct the

discrepancies in the descent pressure readings. During its descent Mars Pathfinder employed six

sensitive servo accelerometers not only to directly measure spacecraft deceleration, velocity,

attitude, and altitude but also to derive the atmospheric density at certain altitudes. Atmospheric

temperature was also directly measured using specially designed sensors depicting the same

concept as hot wire anemometers [Seiffet al, 1997]. With the temperature and density readings

and the ideal gas law, a preliminary ambient pressure can be calculated. By also knowing the

r.3
attitudeof the spacecraft,thepredictedCpvaluefrom thepressuresensorcanbedeterminedand

correspondedin thecontourplotsprovidedin theresultssection.

With the readingsfrom the scienceinstrumentsand thepredictedvalueof Cp,a correction

factor can be determinedby meansof the following example.Assuming a sampleMartian

descentconditionof M_ = 0.4 andy = 1.28andapreliminaryambientpressureof P== 4 rob,the

dynamicpressure(q=) is calculatedto be 0.41 rob. By applying an extremeof Cp= -2.5, Ap

becomes -0.256 mb which corresponds to a 25.6% pressure difference between the lander sensor

surface pressure reading and the preliminary ambient pressure. Therefore, using the Cp values

from the experiment and the spacecraft's accelerometer data, the discrepancies in the pressure

sensor can be corrected.

Table 1: Mars Pathfinder Lander Model Rotation Angles

Angle of Attack

-16

-10

-6

0

+4

+7

+10

+14

+20

+27

Sideslip Angle

-10

-5

0

+3

+6

+10

+12

+20

+25

+30

+35

Table 2" Wind Tunnel Instrument Accuracy

Instrument

Type T Thermocouple

Measured Parameter

Ambient Temperature

Accuracy

+ 0.4 °C

Setra Model 270 Barometer Ambient Pressure + 0.006 mb

Setra Model 239 Transducer Freestream Differential Pressure maximum + 0.005 mb

Setra Model 239 Transducer Lander Sensor Differential Pressure maximum + 0.008 mb
REFERENCES

_Seiff, A., Tillman, J. E., Murphy, J. R., Schofield, J. T., Crisp, D., Barnes, J. R., LaBaw, C.,

Mahoney, C., Mihalov, J. D., Wilson, G. R., and Haberle, R., "The Atmosphere structure and

meteorology instrument of the Mars Pathfinder lander," Journal of Geophysical Research

(Planets), Vol. 102, No. E2, 1997, pp. 4045-4056.

2Rivell, T., Seiff, A., Jovic, S., Wilson, G. R., Maa, S., and Castellano, T. P., "Wind-Tunnel

Test of Flow Through Mars Pathfinder Lander," Journal of Spacecraft and Rockets, Vol. 34, No.

3, 1997, pp. 265-271.

3White, B. R., "AIAA-87-0291 A Low-Density Boundary-Layer Wind Tunnel Facility," AIAA

25 thAerospace Sciences Meeting, January 12-15, 1987/Reno, Nevada.

4Anderson, John D., Jr., Fundamentals of Aerodynamics, 2nd. Ed., McGraw-Hill, Inc., 1991.