Figure 1. Artist's impression of the descent of ESA's Huygens Probe through Titan's atmosphere
The Huygens Probe was drop-tested at the ESRANGE Balloon Launch Site in Kiruna, Sweden, on 14 May 1995, to simulate its spectacular journey through Titan's atmosphere. The Probe's descent to the surface of Saturn's largest moon is scheduled for 27 November 2004 (Fig. 1). The development and verification programme included this demonstration, with a full-scale model, of the Probe's essential characteristics during its free fall and the Titan parachute descent.
Figure 1. Artist's impression of the descent of ESA's Huygens Probe through Titan's atmosphere
A special full-sized model of the Huygens Probe, designated SM2, was assembled for this test. It was attached to a gondola and lifted to an altitude of approximately 38 km by a stratospheric balloon. The probe was then separated by ground command from the gondola for the parachute descent test.
The specific objectives of the drop test were to demonstrate:
and to provide the data needed to correlate predictions with flight results.
Mechanical design
The external shape of the SM2 Probe (Fig. 2) is almost identical to that of the flight model that will descend into Titan's
atmosphere. The inner structure is built to flight-standard, as are the back-cover and front-
shield release mechanisms and the descent control sub-system. A special bracket was
added to interface the Probe and the gondola to allow a pyrotechnic separation, with
umbilical separation provided by a lanyard.
Figure 2. The Huygens Probe and gondola assembly prior to supsension from the auxiliary balloon
Electrical design
The electrical system was specially designed to
meet the objectives of the drop test and was powered by NiCd rechargeable batteries. The
data acquisition and telemetry system managed the interfaces with the various sensors,
providing signal conversion and conditioning of the sensor outputs, formatting of the data
into PCM frames, and transmitting the data (at 38 400 bit/s) to ground via the gondola.
It simultaneously stored the data in a solid-state recorder (16 Mbyte) onboard the Probe.
The S-band transmitter (2 W), utilising the telemetry link between the Probe and the gondola, operated at a frequency of 2.2515 GHz and had a link range of up to 18 km at a 55 degree angle. The Probe could be commanded from the ground via the gondola up to the moment of separation; thereafter, it operated autonomously under time-tagged commands from the Pyro Timing and Firing Unit.
The Probe's instrumentation determined altitude and horizontal position, vertical and horizontal velocity and accelerations, roll, yaw and pitch axis rate, and also handled onboard event timing, visual imaging and housekeeping data.
The following instruments were used for data registration during the descent:
together with the following equipment
All sensors and data acquisition and telemetry equipment were off-the-shelf items. The PDU, PTFU and CPS were custom-made.
Redundancy
As a compromise between programme risk and
available budget, a limited redundancy scheme was applied. In particular:
The electrical testing of the various units was performed by the unit suppliers and later by Fokker Space & Systems (FSS) as work-bench tests prior to integration. The Probe and balloon gondola were interated by FSS's engineers in the company's clean room at Schiphol (NL). The radio- frequency and mass-property tests were performed at ESTEC in Noordwijk (NL). Finally, just prior to its shipment to ESRANGE (S), a system functional test was conducted on the combined gondola/Probe assembly by FSS at Schiphol, which included a pyro separation test.
The Flight Acceptance Review was successfully completed at FSS on 4/5 April 1995, and the test campaign started on 17 April at the Swedish Space Corporation's sounding- rocket and balloon launch facility at ESRANGE. All tests performed at Schiphol were repeated in Kiruna after the intervening shipment and reassembly. An end-to-end test with the CNES ground station was also performed at the launch site.
A complete dress rehearsal for the drop test was successfully conducted on 12 May during which operation of the gondola, Probe and ground station was checked whilst commanding the Probe just as during the real flight. A Launch Acceptance Review held at ESRANGE on 13 May showed that all systems were ready for launch.
Flight and recovery
The Huygens Probe System Drop Test took
place on 14 May 1995, with the following flight scenario:
Figure 3. Auxiliary and main-balloon filling on the 'launch pad'
Figure 4. Probe/gondola ascent at the moment of auxiliary balloon release
Figure 5. Main balloon and Probe/gondola ascent after auxiliary balloon release
Figure 6. The descent module after landing
Figure 8. Recovery of the gondola by helicopter
The weather forecast was acceptable and the wind directions in the troposphere and stratosphere, determined with a sounding balloon on the evening of 13 May, confirmed that the wind pattern would guarantee a safe landing for all items descending without a parachute, within the ESRANGE safety zone.
The batteries were fully charged and the Probe was warmed-up overnight to approximately 30 degC in order to keep it within temperature limits during the very cold checkout environment on the launch pad and the almost 3 h ascent phase, during which it would be largely inactive. The gondola lifted-off at 8.15 a.m. local time (Figure 3 & Figure 4) and two in-flight checkouts were performed during the ascent phase, one 1 hour prior to Probe release and one to confirm healthy status at release.
The Probe was released from the gondola at 11.09 a.m. local time at an altitude of 37.4 km. All of the Probe's systems functioned perfectly during the descent, the descent module landing at 11.27 a.m. (Fig. 6). The Probe was operational after landing and good data were transmitted to ground via the gondola, which had a much longer descent time.
All housekeeping data were recorded and distributed to the analysis teams. The film cameras operated flawlessly and the quick-look analysis revealed that the parachutes and mechanisms had operated according to plan. The Probe and the gondola (Fig. 7>) were both recovered and flown back by helicopter to the launch site (Fig. 8) the same day; the front shield and back cover were located the same day, but were recovered the following day.
Figure 7. The gondola after landing
The post-flight parachute analyses were conducted by Martin Baker Ltd. and the Probe and system analyses by Aerospatiale. Analyses of the whole sequence of events (parachute deployments/inflations, back-cover and front-shield separations) were based on the film material shot, which was of excellent quality. The other sensors (GPS, gyroscopes, accelerometers) were used for the drag-coefficient and Probe stability analyses.
All three flight parachutes - pilot, main and stabiliser - deployed cleanly and inflated very positively. Pilot- and main-chute inflation occurred at Mach 0.8, at a dynamic pressure representative of conditions on Titan. The parachute deployment and inflation times were as predicted, and the drag coefficients were also within specification.
Detailed analysis of the film taken confirmed that there was no post-separation contact between the Probe and its back cover or front shield. The back-cover separation was slightly faster than predicted, due to a lower Probe wake recirculation (subsonic) compared with the predicted Titan-entry case (supersonic). The front shield's separation showed good correlation between predictions and flight results.
The stability analysis confirmed that the main parachute, operating in the Earth's upper stratosphere, provided very positive damping of Probe oscillations, which is consistent with predictions for Titan's atmosphere.
During the descent phase under the stabiliser chute in the lower stratosphere and troposphere, there was no obvious damping, which is in contradiction with predictions for the Titan atmosphere. Additional helicopter drop tests were therefore conducted and their results demonstrated that this undamped motion was not due to an interaction between the descent module's wake and the stabiliser chute. Further analysis of the wind conditions during the original drop test revealed that during the descent through the lower stratosphere and troposphere the wind had reached speeds of 50 m/s and there was a high wind gradient. It was the resulting turbulence continuously exciting the Probe and stabiliser chute that was causing the undamped motion. Conditions on Titan are such that there will be sufficient damping to stabilise the Probe within specified limits.
The spin analysis showed good agreement between predictions and test results for the descent under the main parachute. Lower than expected spin rates during the descent under the stabiliser chute were recorded. The Probe will nevertheless meet all of the scientific objectives, even under worst-case assumptions.
This highly successful Huygens Probe System Drop Test allowed a complete flight sequence test to be conducted with a full-scale Probe model.
Most importantly, it demonstrated:
Although the individual parachutes and mechanisms had already been tested extensively at subsystem and system level, the SM2 Probe Drop Test demonstrated the descent sequence under realistic dynamic conditions for the first time and has thereby provided the necessary total confidence regarding the flight-model Probe's successful descent into Titan's atmosphere in November 2004.
The considerable experience of the French national space agency, CNES, was of major help in procuring and operating the balloon and the ground station, and in selecting the launch site. The CNES/Air sur l'Adour balloon team was led by P. Faucon.
The Drop Test was conducted by a combined team drawn from the ESA Huygens Project, Aerospatiale, Fokker and Martin Baker, CNES/Air sur l'Adour and ESRANGE.
The authors would like to take this opportunity to express their appreciation for the excellent spirit of cooperation between the participants and the efforts of the ESRANGE Launch Site Team, led by S. Kemi and B. Sjöholm.
ESA Bulletin Nr. 85.