Accelerometers Data Elaboration during Separation and Atmosferic Descent of the Huygens Mock Up Probe in 2002 Stratospheric Balloon Launch Campaign

Abstract

A mock up of the probe descending in Titan atmosphere as part of the Huygens Cassini Mission has been successfully launched and recovered on May 30th 2002 after a stratospheric balloon launch from the Italian Space Agency Base “Luigi Broglio” in Trapani, Sicily. To simulate Huygens mission at Titan, the probe has been lifted at 32 km altitude and then released to perform a 45 minutes parachute decelerated descent. An integrated data acquisition and instrument control system, based on PC architecture and soft-real-time application, sampled a total of 77 sensor channels during ascent, drift and descent phase. Among them probe hosts HASI accelerometer subsystem which has been developed mainly by Open University and contains one servo accelerometer sensitive to the mock-up spin axis and a three-axial piezoresistive accelerometer unit. This work analyses accelerometers data to investigate the dynamic behaviour of the probe after separation from the balloon, parachute opening and atmospheric descent. The work ends with a final study on temperature influence on piezo accelerometers measurements, where piezo unit values are checked with piezo temperature logs and servo accelerometer values in order to obtain an esteem of accelerometer unit performance with different temperature. HUYGENS MOCK UP PROBE The 1:1 scale mock-up of the Huygens probe has been completely developed at CISAS University of Padova. For a detailed description on probe mechanical and thermal configuration please refer to [1]. To be noted is that the overall external dimensions of the mock-up are 1304mm x 631 mm for a total weight close to 100 kg. Fig. 1 3D impression of Huygens mock up HASI ACC package is accommodated in the payload area in a position close to the centre of mass of the probe as shown in figure 2. HASSI ACC consists of: (1)One Servo ( highly sensitive ) accelerometer. (2) Three Piezo resistive ( PZR ) accelerometers, with one accelerometer mounted in each of the three orthogonal axes. (3) Two temperature sensors. The x-axis servo accelerometer’s output is conditioned and amplified by two noninverting amplifiers, one with gain of 1 and 54th International Astronautical Congress of the International Astronautical Federation, the International Academy of Astronautics, and the International Institute of Space Law 29 September 3 October 2003, Bremen, Germany IAC-03-I.2.05 Copyright © 2003 by the International Astronautical Federation. All rights reserved. 2 the other with gain 10. The ACC is incorporated in a Wheastone bridge; a small output voltage dependent on the acceleration is produced when an external voltage is applied. Temperature variations of both servo and piezo units are measured by an AD 590 sensor fixed on the mounting structure. Fig. 2 HASI ACC accommodation in the payload area HASI ACC sensors are sampled by data acquisition system at 100 Hz rate while ACC temperatures are monitored at 0.8 Hz rate. THE BALLOON FLIGHT The balloon flight started on May 30 when a RAVEN 98.862 m3 volume stratospheric balloon inflated with helium gas lifted the whole launch train off the ground. The train consisted in the Huygens probe, the Italian Space Agency gondola, hosting ground communication system for telecommand and telemetry and above them an IRVIN 24 meter diameter parachute. The length of the flight train from the top of the parachute to the ASI gondola is around 70 meter. The probe was raised to almost 32 km altitude with a 2 hour ascent , then drifted for almost one hour and a half at this altitude to reach the position for best calculated descent location and then started a 40 minutes descent. Figure 3 shows the ground track of the flight starting from Trapani, where the three different plot colours illustrate the three different flight phases: ascent, floating and descent. Fig. 3 Ground track of May 30 flight (red: ascent, green: floating, blue: descent) Figure 4 shows the altitude profile versus time reconstructed by GPS data.