COMPARISON OF THE HUYGENS MISSION AND THE SM2 TEST FLIGHT FOR HUYGENS COMPARISON OF THE HUYGENS MISSION AND THE SM2 TEST FLIGHT FOR HUYGENS ATTITUDE RECONSTRUCTIONATTITUDE RECONSTRUCTION

ENGINEERINGENGINEERING

PROPERTIESPROPERTIES Huygens Flight Module

SM2 Drop Test

Inner & Outer Structure ~ Identical

( no HASI booms on SM2 )

mass Identical ~ 300 kg

balance “similar“ moments of inertia

Descent Control SubSystem Identical

mechanisms Identical

spin vanes Identical design

3° inclination 2.2° inclination

main parachute Identical DGB double gap

stabilizer parachute

DGB

double gap

DGB

single gap

recovery parachute (no swivel)

A. Sarlette(1), M. Pérez-Ayúcar, O. Witasse, J.-P. LebretonPlanetary Missions Division, Research and Scientific Support Department, ESTEC-ESA, Noordwijk, The Netherlands.

(1) Stagiaire from February 1 to April 29; student at Liège University, Belgium Email: alain.sarlette@student.ulg.ac.be, mperez@rssd.esa.int, owitasse@rssd.esa.int, jplebret@rssd.esa.int

AABSTRACTBSTRACTThe Huygens probe is the ESA’s main contribution to the Cassini/Huygens mission, carried out jointly by NASA, ESA and ASI. It was designed to descend into the atmosphere of Titan on January 14, 2005, providing surface images and scientific data

to study the ground and the atmosphere of Saturn’s largest moon.

In the framework of the reconstruction of the probe’s motions during the descent based on the engineering data, additional information was needed to investigate the attitude and an anomaly in the spin direction.

Two years before the launch of the Cassini/Huygens spacecraft, in May 1995, a test probe called SM2 (Special Model 2) was dropped in the Earth’s atmosphere from the balloon launch site of Kiruna, Sweden, to verify proper operation during the descent and especially the parachute deployments sequence. It featured a flight standard structure and DCSS (Descent Control SubSystem) and, unlike the Huygens probe, was fully instrumented to monitor the orientation of the descent module (3-axes

accelerometers and gyroscopes).

On this poster, we describe how a comparison between the SM2 test flight and the Huygens mission provides some information about the Huygens probe’s behaviour. After briefly discussing the spin direction, we focus on the tip and tilt.

The final conclusions of this comparison are still of qualitative nature, but the results are a starting point for better interpretation of the engineering data in terms of attitude to derive the probe’s orientation.THE HUYGENS MISSION

The Huygens probe is the ESA’s main contribution to the Cassini/Huygens mission, carried out jointly by NASA, ESA and ASI. It was designed to descend into the atmosphere of Titan on January 14, 2005, providing surface

images and scientific data to study the ground and the atmosphere of Saturn’s largest moon

THE SM2 TEST FLIGHT

Two years before the launch of the Cassini/Huygens spacecraft, in May 1995, a test probe called SM2 (Special Model 2) was dropped in the Earth’s atmosphere from the balloon launch site of Kiruna, Sweden, to verify proper

operation during the descent and especially the parachute deployments sequence.

FLIGHT CHARACTERISTICS FLIGHT CHARACTERISTICS

INSTRUMENTS FOR INSTRUMENTS FOR

ATTITUDE & AZIMUTHATTITUDE & AZIMUTH

RECONSTRUCTIONRECONSTRUCTION

Measurement Resolution

Time Resolution

Huygens Flight Data

CASU (vertical acc.) ± 0.2 m/s2 1 Hz

RASU (radial acc.) ± 0.0024 m/s2 4 Hz

On HASI, SSP, DISR (see Scientific Instruments´ teams)

SM2 Drop Test Data

X-accelerometer (vertical) ± 0.2 m/s2 200 Hz

Y-acc. & Z-acc. (horizontal) ± 0.15 m/s2 200 Hz

X-gyroscope (spin) ± 0.1 °/s 40 Hz

Y-gyro & Z-gyro (horizontal)

[High / Low range]

± 2.4 ± 0.24 °/s40 Hz

Video Recordings Up & Down (separation phases)

PROBE CHARACTERISTICSPROBE CHARACTERISTICS

1. Huygens mission spin profile 1. Huygens mission spin profile (cfr. M.Pérez et al., “Huygens attitude

reconstruction based on engineering parameters”)

~ 7.5 rpm through entry phase (spin induced at Cassini ejection) Spin annihilation at direction reversal

2. SM2 test flight spin rate 2. SM2 test flight spin rate ( X-axis gyroscope )

Spin movement starts after front shield release (spin vanes active)Spin movement starts after front shield release (spin vanes active) Constant direction under main chute and stabilizer chute, Constant direction under main chute and stabilizer chute, no annihilationno annihilation Oscillations under recovery chute (no swivel Oscillations under recovery chute (no swivel suspension cable suspension cable acts as torsional spring) around acts as torsional spring) around -1.4 rpm-1.4 rpm ( = parachute spin rate?) ( = parachute spin rate?)

SPIN RATESPIN RATE

Main Chute

Stabilizer Chute

Stabilizer Chute

Main Chute

Recovery Chute

Under Balloon & Gondola

Pilot & Main Chute Deployments

Stabilizer Chute Deployment

Recovery Chute Deployment

Landing

Front Shield Release

SPIN DIRECTIONSPIN DIRECTIONAfter the spin direction anomaly was confirmed on Huygens, the SM2 spin direction was also investigated

1. According to X-gyroscope, SM2 was spinning clockwise as seen from above (see positive gyroscope measurement and axes definitions on the right)

2. On video recording, sun ray is moving clockwise (up-looking) probe is moving counterclockwise (up-looking), which is clockwise seen from above, confirming the result:

SM2 Acc. and Gyro axes definitionsSM2 Acc. and Gyro axes definitions

SM2 up-looking video camera – Main chute releaseSM2 up-looking video camera – Main chute releaseConsecutive pictures a) – d) cover an interval of about 4s

Spin direction anomaly present on SM2

Check of spin vane Check of spin vane orientation on the SM2 orientation on the SM2 probe: probe: seems to induce counterclockwise torque

SIMULATIONS to fit SIMULATIONS to fit CASU / RASUCASU / RASU

Based on the identified frequencies of about 1 Hz -- which should correspond to attitude motions according to the SM2 comparison.

The effect on the Huygens accelerometers of pendulum-like and coning-like probe motions was simulated and compared to the mission measurements.

Variables:Variables: Amplitude Amplitude Fix point height Fix point height ll

[ discarding what happens above [ discarding what happens above l l ]]

Results:Results: Most plausible fixed point:

~ 10 cm above accelerometers Most plausible aperture angles:

- Main chute: 0 – 4°

- Beginning stabilizer chute: 4 – 9°

- End stabilizer chute: 1 – 5°

PLANETPLANET

PROPERTIESPROPERTIES Huygens

(Titan)

SM2

(Earth)

Gravity 1.354 m/s2 9.81 m/s2

Atmosphere

Temperatures 70 – 200 K ~ 270 K

Surface pressure 1.5 bar 1 bar

Typical vsound 200 m/s 330 m/s

Typical scale factor 20 km

(nearly no validity)

8 – 9 km

FLIGHTFLIGHT

PROPERTIESPROPERTIES Huygens Mission Flight

SM2 Drop Test

Main chute phase duration 15 mn 6 mn

altitudes 155 – 111 km 34 – 14.6 km

Stab. chute phase duration 2 h 13 mn 3 mn 30 s

altitudes 111 – 0 km 14.6 – 4.8 km

Typical Re ~ 107 ~ 106

Qdyn Descent initiation 316 Pa 300 Pa

Front shield release 34 Pa 90 Pa

Mach Descent initiation 1.465 0.9

Front shield release 0.4 0.4

Main Main chutechute

Stabilizer Stabilizer ChuteChute

SM2 Test FlightSM2 Test Flight 2.1 Hz 2.6 Hz

Huygens missionHuygens mission 0.75 – 0.8 Hz 1 Hz

FREQUENCY DOMAIN FREQUENCY DOMAIN COMPARISONCOMPARISON

SM2 Z-axis accelerometer spectrumSM2 Z-axis accelerometer spectrum

Huygens RASU spectrumHuygens RASU spectrum

Similarities:Similarities:

Calm descent under main chute, noisy under stabilizer chute Dominant horizontal frequency lines:

Differences:Differences:

Varying line decreasing from 1 Hz to 0.6 Hz under stabilizer on Huygens, absent on SM2 Progressive damping of oscillations under stabilizer on Huygens, no damping on SM2

( but SM2 stabilizer phase much shorter)

SM2 gyroscopes SM2 gyroscopes confirm that the SM2 confirm that the SM2 frequency lines are reflecting attitude motionsfrequency lines are reflecting attitude motions

Frequency comparison:Frequency comparison:

fSM2 / fHuygens = 2.77 (Main) ; 2.6 (Stabilizer)

~ ( g Earth / g Titan )1/2 = 2.7

It is likely that the same type of It is likely that the same type of movement was observed for the SM2 test movement was observed for the SM2 test

flight and for the Huygens missionflight and for the Huygens mission

Still to be explained: Still to be explained: - parachute influence- parachute influence- presence of decreasing frequency line on RASU - presence of decreasing frequency line on RASU

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