Section 10 Microgravity Environment of Ground-based ...€¦ · Microgravity Environment of Ground-based ... Section 10 Microgravity Environment of Ground-based Facilities ... Shuttle

March 2003 MEIT-2003 / Section 10 / Page 1

Microgravity Environment of Ground-based Facilities and Non-orbital Flight Platforms

Section 10


Richard DeLombardAcceleration Measurement Discipline Scientist

NASA Glenn Research Center



Acceleration measurements for experiments• Experiments in microgravity are disturbed by accelerations

(e.g. vibrations, shocks, gravity gradient, linear motion)

• Experiments in ground laboratories are disturbed by accelerations also

• Gravity (very pervasive!)• Elevator motions in laboratory building • Traffic nearby building (e.g. street, loading dock) • Air conditioning equipment (e.g. compressor, fans, etc.)• Clumsy lab assistants

• Accelerations should be measured during experiment ground operations - not just during orbital operations



10-2 10-1 100 101 10210-7

10-6

10-5

10-4

10-3

10-2

10-1One-Third Octave Band Comparison

Frequency (Hz)

ISS REQs GRC 2.2 GRC ZGF JAMIC Mir 1996 Shuttle IML-2 KC-135 Bolted KC-135 Free-Float Sounding Rocket Bremen (Drop Axis Only)

RM

S A

ccel

erat

ion

(gR

MS)

Residual acceleration for various microgravity facilities

(from Ross, 2001)



Methods of creating ‘zero-g’ or microgravity• Center of Earth’s mass (ge ~ 0 m/s2)

• Impractical location for experiment operations

• Very distant from Earth or other celestial body (ge= 10-6 m/s2)• Impractical location for experiment operations

• Free fall • Zero horizontal velocity ------> drop tower (ge = 9.8 m/s2)• 400 kph horizontal velocity ------> aircraft (ge = 9.8 m/s2)• 30,000 kph horizontal velocity ------> orbital (ge ~ 9 m/s2)• Where ge is the acceleration due to Earth’s gravitational pull

• The reduced gravity features comes from free fall, not the absolute reduction or elimination of Earth’s gravitational acceleration!



Ground-based facilities with zero horizontal velocity• Seismic mass / vibration isolation

• Not free-fall but vibrationally quiet- Still 1-g environment

• Isolated floor mass • Vibration isolation platform

• Drop tower• Carrier containing experiment is dropped• Experiments may be complex

• Drop tube• Sample material only is dropped • Most often sample is molten metal drops



Ground Facilities with zero horizontal velocity

2.2 Second Drop TowerNASA Glenn

Drag shield being assembled for an experiment drop

Seismic MassPlum Brook Station

Base of huge vacuum chamber (illustrative of method to utilize

vibration-quiet laboratory conditions)



SPF Seismic Mass Characterization

• Figure 10-1 illustrates the conditions existing on a large mass of concrete

• Concrete foundation of world’s largest vacuum chamber • The X-axis was vertical • a = F/m implies low levels of acceleration with large value of

mass with nominal forces from ground and wind

March 2003


• 1-g condition• Gravity effects are apparent when a retarding force

disturbs free fall• Beaker exerts a force to stop water from falling • Floor exerts a force on people (felt as their weight)

• Microgravity condition in a free fall• Gravity effects are not apparent in free fall • Beaker falls with the fluid

- beaker is no longer exerting a retarding force on water- sedimentation and buoyancy are reduced- surface tension & capillary forces are ‘revealed’

• Acapulco cliff divers feel weightless during their free-fall to the ocean

Free-fall vs. 1-g1-g

µg



• Drop towers attempt to minimize external forces• Air drag is a large external force

- Steady force which gradually increases with increasing velocity• Several mechanisms are used to counteract air drag

- Drag shield - Experiment package surrounded by free falling container

- Vacuum operation- Evacuate air from the chamber in which the experiment is dropped

- Drag force compensation- Apply compensating force (thrust) to experiment carrier

• Keys for a ‘quiet’ drop - Smooth release mechanism to minimize initial transient vibration- Structural relaxation depends on design of carrier and

experiment - Dynamically balance moving experiment and carrier

components

Drop Towers & Tubes

March 2003


• NASA GRC 2.2 Second Drop Tower uses a drag shield

• Capture in an air bag

Drag Shield

Drag Shield

Experiment package

Velocity

March 2003


NASA GRC 2.2 Second Drop Tower

Experiment rig assemblyDrag shield preparation



Vacuum Operation• Vacuum drop towers include:

• Zero Gravity Research Facility at NASA GRC- Capture in foam pellet container

• ZARM facility at University of Bremen, Germany - Capture in foam pellet container

ZARM tower

exteriorExperiment capture in Zero Gravity Research Facility



Drag Force Compensation• Japan Microgravity Center

• Inner & outer capsule (i.e. drag shield)- Vacuum drawn between inner & outer capsules

• Acceleration added to outer capsule for drag compensation- Cold-gas jet

• Capture accomplished with air pressure then mechanical brake

JAMIC



Drop Tower Comparison• NASA GRC 2.2 Second Drop Tower

• 2.2 seconds 24.1 m 10-4 g

• ZARM Drop Tower• 4.74 seconds 123 m 10- 5 g

• NASA GRC Zero Gravity Research Facility• 5.18 seconds 145 m 10-5 g

• Japan Microgravity Center • 10 seconds 490 m 10-5 g

ZARM



Acceleration Environment Features of Drop Towers• Release

• Step change transition from 1-g to sub-milli-g level • Transition occurs over very short time that the mechanism

actually releases carrier

• Vibrations from release mechanism• The release transition is similar to ringing a bell

- Step change causes (unwanted) vibration in experiment carrier- The ‘bell ringing’ is damped by carrier and experiment mechanical design

• May persist for major portion of microgravity time

• Figure 10-2



Acceleration Environment Features of Drop Towers• Vibrations from experiment equipment operation, such as:

• Camera shutters• Film transport• Solenoid and relay actions• Pumps• Motor-driven fluid mixers• Figure 10-3

• High level of deceleration at capture • Levels depend on capture mechanism and final velocity • Figures 10-2 and 10-4



Non-orbital flight platforms (~ 200 mph horizontal velocity)

• KC-135 aircraft (NASA)• Operated by NASA Johnson Space Center• Each parabola provides 15-20 seconds of reduced gravity

environment• Periodic free-fall interspersed with high-g pull-out• Approximately 40-50 parabolas per flight (campaign)

• Terrier-Black Brant sounding rocket• Achieves free-fall conditions on the order of 500 seconds after

motor burn-out • One of several types of sounding rockets



Aircraft Facilities

SAMS-FF / KC-135Linear acceleration sensors and fiber optic gyro sensor

Parabolic Aircraft Rating System

Linear acceleration sensors and software processing

KC-135



SAMS-FF / Sounding RocketLinear acceleration sensors and

fiber optic gyro sensor

Sounding Rockets

Terrier-Orion



KC-135 Environment Characterization

• Figure 10-5 illustrates the KC-135 overall environment over multiple parabolas during a typical campaign

• Figure 10-6 is a detailed plot of the KC-135 environment during the reduced gravity portion of the parabola

• Figure 10-7 is a plot of KC-135 parabola recorded in support of SAL experiment. Shows free-float of SAL test equipment and timelines the activity within the parabola

• Figure 10-8 is a detailed plot of the free-float period of the parabola



Sounding Rocket Environment Characterization

• Terrier-Black sounding rocket DARTFire flight timeline is shown in the graphic in Figure 10-9

• Figure 10-10 illustrates the acceleration vector magnitude for the time period when the sampling rate was 25 samples per second

• environment measured to be less than 30 micro-g root sum square (RSS) for the time interval analyzed

• Figure 10-11 is the RSS power spectral density for the time period when the sampling rate was 25 samples per second

• frequency domain characteristics track known disturbance sources from the DARTFire equipment

- Intensified Multispectral Imager filter wheel operates at 5 Hz- Infrared Imager filter wheel operates at 1 Hz



References• Zero Gravity Research Facility

• http://microgravity.grc.nasa.gov/zero-g/index.html

• 2.2 Second Drop Tower• http://microgravity.grc.nasa.gov/drop2/

• ZARM Drop Tower• http://www.zarm.uni-bremen.de/main.htm

• ZARM Drop Tower Bremen - Users Manual, Version 28, April 2000• JAMIC Drop Tower

• http://www.jamic.co.jp/ENG/JAMIC/3.html

• Microgravity Carrier Summary• http://microgravity.msfc.nasa.gov/NASA_Carrier_User_Guide.pdf

• Ross, H. D. (2001) Microgravity Combustion, Academic Press



Figure 10-1: Ground Testing – SPF data



1.400

-1 .400

-1 .200

-1 .000

-0 .800

-0 .600

-0 .400

-0 .200

0 .000

0 .200

0 .400

0 .600

0 .800

1 .000

1 .200

9 .05 .0 5 .5 6 .0 6 .5 7 .0 7 .5 8 .0 8 .5

accel data (g)

Data from the vertical axis in NASA GRC 2.2 Second Drop Tower facility.

0 .0 0 5

- 0 .0 0 5

- 0 .0 0 4

- 0 .0 0 3

- 0 .0 0 2

- 0 .0 0 1

0 .0 0 0

0 .0 0 1

0 .0 0 2

0 .0 0 3

0 .0 0 4

7 .55 .0 5 .5 6 .0 6 .5 7 .0

accel data (g) Low-g portion of dropComplete drop

1-g

0-g

Figure 10-2: Acceleration level for drop tower test

5-mg

0-mg

Time (sec)

Time (sec)

Acc

eler

atio

n le

vel (

g)

Acc

eler

atio

n le

vel (

g)



PSD of Residual Acceleration,Bremen Drop Tower, Weight of Capsule: 500kg

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

Freq / Hz

PSD / 10-12 g02 / Hz

Figure 10-3: Power Spectral Density plot during drop (ZARM)(note: release disturbances not included)

Equipment operation



Deceleration Forces at Bremen Drop Towerz-axis

-10

0

10

20

30

40

50

4,7 4,8 4,9 5 5,1 5,2 5,3 5,4 5,5

time [s]

g0

Figure 10-4: Deceleration at capture (ZARM)

Peak > 40-g (!)

Figure 10-5

−1

−0.5 0

0.5 1

1.5 2

2.5 3

Original Mean=−5.27e−02 gRMS Value=5.27e−02 g

X−Axis Acceleration (g)

−1

−0.5 0

0.5 1

1.5 2

2.5 3

Original Mean=6.83e−03 gRMS Value=9.71e−03 g

Y−Axis Acceleration (g)

050

100150

200250

300−

1

−0.5 0

0.5 1

1.5 2

2.5 3Original Mean=1.09e+00 gRMS Value=8.06e−01 g

Z−Axis Acceleration (g)

Tim

e (seconds)

Multiple K

C−

135 Parabolas Without D

e−M

eaningM

ET

Start at 055/15:09:59.992

Head C

, 5.0 Hz

fs=25.0 sam

ples per secondK

C−

135K

C−

135 Coordinates

T=

300.0 seconds

MA

TL

AB

: 13−A

ug−1998, 10:35 am

Figure 10-6

02

46

810

1214

1618

20−

0.4

−0.2 0

0.2

0.4

Tim

e (sec)

X−Axis (g)

Original Mean = −2.87e−02 g

RMS Value = 8.75e−02 g

Head SA

MS−

FF DA

TA

, 209.6 Hz

fs= 800 sam

ples per seconduSE

G M

issionSA

MS−

FF Coordinates

MA

TL

AB

: 15−Jun−

1998, 04:41 pm

ME

T Start at 02/27/1998 10:35:52

uSEG

Mission: R

educed Gravity Portion of K

C−

135 Parabola Without D

e−M

eaning

02

46

810

1214

1618

20−

0.4

−0.2 0

0.2

0.4

Tim

e (sec)

Y−Axis (g)

Original Mean = 2.01e−03 g


02

46

810

1214

1618

20−

0.4

−0.2 0

0.2

0.4

Tim

e (sec)

Z−Axis (g)

Original Mean = 5.24e−04 g


FREE-FLOATDATE: 04-27-99, FILE #0031

SAMS-FF, 26.2 Hzfs=100 samples per second

KC-135-FREE FLOATKC-135 Coordinates

T=25.0 seconds

MA TLA B: 1 0 -Jan -2 0 0 0 , 0 7 :1 1 am

0 5 10 15 20 25-1.5

-1

-0.5

0

0.5

1

1.5

Time (seconds)

Acc

eler

atio

n (g

)

pickup rig from floor

test conductor adjustments

release

bump

capture

reposition rig

place rig back on floor mounts

free-float

X-AxisY-AxisZ-Axis

Figure 10-7

SAMS-FF Data Recorded in Support of SAL Experiment Showing Free-Float Interval

Figure 10-8

12 12.5 13 13.5 14 14.5 15 15.5 16−1.5

−1

−0.5

0

0.5

1

1.5x 10

−4Enhancement of Free Float Interval for Z−Axis

Data Set #31

Time (Seconds)

Acc

eler

atio

n (g

)

Head SAMS−FF, 50 Hzfs=100 samples per second

KC−135 Free FloatApril 27, 1999

Figure 8−7: Enhancement of the Free Float Period for the Z−Axis

Figure 10-9

TS

H

RM

S

y

(µg

)z

(µ

g)

376405

243x

(µ

g)

50 MIL

ES

TS

H

RM

S

y

(µg

)z

(µ

g)

89

91

37

58

22

x

(µg

)

TS

H

RM

S

y

(µg

)z

(µ

g)

58

16

82

38

x

(µg

)

200 MIL

ES

150 MIL

ES

200 SA

MP

LE

S P

ER

SE

CO

ND

0 MIL

ES

SA

MS

-FF

DA

RT

Fire

RM

S V

alues fo

r Each

TS

H S

amp

ling

Rate

100 MIL

ES

400 SA

MP

LE

S P

ER

SE

CO

ND

TS

H

RM

S

y

(µg

)z

(µ

g)

13

13

10

x

(µg

)

TS

H

RM

S

y

(µg

)

11

5

x

(µg

)z

(µ

g)

4

Figure 10-10

020

4060

80100

120140

160

0

0.5 1

1.5 2

2.5 3x 10

−5

Tim

e (sec)

Acceleration Vector Magnitude (g)

Head SA

MS−

FF DA

TA

, 6.55 Hz

fs= 25 sam

ples per second

DA

RT

Fire Mission

SAM

S−FF C

oordinates

MA

TL

AB

: 11−A

ug−1998, 06:02 pm

ME

T Start at 000/00:02:05.027

Acceleration V

ector Magnitude for Fs=

25

Figure 10-11

02

46

810

12

10−

14

10−

13

10−

12

10−

11

10−

10

10−

9

10−

8

RSS PSD Value (g2/Hz)

Frequency (Hz)

Head SA

MS−

FF DA

TA

, 6.55 Hz

fs= 25 sam

ples per second

dF= 0.048828 H

z

DA

RT

Fire Mission

SAM

S−FF C

oordinates

T=

179.988 sec

MA

TL

AB

: 11−A

ug−1998, 06:05 pm

ME

T Start at 000/00:02:05.027, H

anning k=8

RSS Pow

er Spectral Density for Fs=

25

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