VIBRATION ISOLATOR FOR CRYOGENIC MACHINES

The ESA METEOSAT Third Generation satellites are geostationary satellites with scientific payload requiring high pointing stability: Flexible Combined Imager, Lightning Imager, Infrared Sounder, UV instrument

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ABSTRACT

The ESA METEOSAT Third Generation satellites are geostationary satellites with scientific payload requiring high pointing stability: Flexible Combined Imager, Lightning Imager, Infrared Sounder, UV instrument. Because of the observation of the Earth in the infrared range, cryogenic machines are on board and generate micro vibrations of the order of few Newton. The stringent stability requirements lead to mitigate these vibration sources with isolators. Passive isolator are the simplest ones and the most reliable. In the frame of the ESA MTG programme, Thales Alenia Space and the SMAC company have developed an isolator based on a highly damped space qualified elastomer. This isolator has been designed to both withstand the launch loads with limited dynamic amplification and to efficiently mitigate the micro vibrations due to the cryogenic machines during orbital operation. The architecture of this isolator will be presented and its performances will be highlighted in this paper.

 

INTRODUCTION

Both MTG-I and MTG-S satellites are equipped with cryogenic coolers to maintain their infrared focal planes and detectors at their low operating temperature. The working gas used for the thermos dynamical cycle is compressed with twin reciprocating pistons compressors with linear electrical motors. Even if the pistons are accurately balanced, the compressors may generate perturbing forces up to few Newton due to slight variations in elastic bearings, magnetic fields, electromagnetic coils and unbalanced pressure fields. For reliability reasons, two cryogenic machines are mounted side by side on a common supporting frame also holding the compressors radiator and the two cold fingers. The cryogenic cooler system (CCS) weights around 40 kg and generates micro vibrations that perturb the scientific instruments on-board of the satellites: Flexible Combined Imager (FCI), Lightning Imager (LI) and Sounder leading to the need of dynamically isolate this cryogenic equipment. For reliability reasons, a passive isolator has been developed with elastic suspension elements (ESE). Metallic springs have not been selected to avoid the need of a launch lock [1]. A set of six ESE is needed to meet the micro vibration mitigation requirements. These ESE are of two different types with different stiffness. The main challenge in this development is to design ESE able to withstand the high launch loads without any locking mechanism and to perform satisfactory microvibration mitigation to ensure the stability of the line of sight of the scientific instruments. These two constraints have opposite keys parameters:

– The ESE shall be stiff with high damping capabilities to meet the launch requirements (limited differential displacement to avoid breaking the heat pipes, thermal breads and bellows, low dynamic amplification to limit the acceleration of the cryogenic machines and limited stiffness variations to allow reliable operation). The frequency of the first natural mode of the isolator shall not be lower than 35 Hz under qualifying loads at instrument level.

– The ESE shall be soft with low damping capabilities to meet the micro vibration performances that aim at reducing as much as possible the transmissibility of the perturbing forces to the rest of the satellite. A low stiffness permits to have a low cutting frequency and a satisfactory mitigation performance. And a low damping permits to limit the sensitivity of the ESE to temperature variations that would affect their elastic modulus. The frequency of the first two in plane natural modes of the isolator shall not be larger than 42 Hz to guarantee a sufficient mitigation of the micro vibration.

With these opposite constraints, the track to the solution is narrow. In this article, we will present at first the design for the micro vibration with the general architecture of the isolator. In a second part, the launch capabilities of the ESE will be presented and the last part will put into light the performances met both for the micro vibrations and for the launch.

 

MICRO VIBRATION ARCHITECTURE

Thales Alenia Space and the SMAC company have developed an isolator for large pulse tube cooler (LPTC, Fig. 1) based on elastomeric suspension elements. The selected elastomeric material is the SMACTANE SP from the SMAC company, already selected for the Elastic Stops of the Reaction Wheel Isolator (RWI) developed by Thales Alenia Space with the SMAC company for the MTG-I and MTG-S satellites. This material has a high damping capability well suited for the CCS isolator w.r.t the launch loads. Because of accommodation constraints, the ESE cannot be at their optimum position to meet the best micro vibration mitigation performances and shall be placed under the CCS between its bottom plate and the instrument supporting panel. Because of this, a large offset exists between the centre of inertia (CoI) of the CCS and the ESE plane leading the natural modes of the isolator to have frequencies widely spread.

Fig. 1. Large pulse tube cooler on its transport frame

The only modes that really need to be adjusted are the first two modes of the isolator that are mainly lateral rocking modes. The Fig. 2 presents the schematic of the isolator’s architecture with the location of the ESE on a rectangular pattern. It can be noticed that the dimension L2 is almost twice larger than the dimension L1. So if the six ESE were identical, the first two lateral rocking modes along the X and Y axis would be at very different frequencies leading to unbalanced mitigation performances in the X,Y plane. To have frequencies as close as possible for these two modes, two types of ESEare necessary as shown in Fig. 2.

Fig. 2. Schematic of the isolator architecture

The ESE type 2 are around 3 times stiffer than the ESE type 1 to compensate the smaller L1 dimension with regards to the L2 dimension. The 4 ESE type 1 work with a large lever arm when the CCS is excited along the X axis: situation where the 2 ESE type 2 have a small contribution due to their null lever arm in that direction. But when the CCS is excited along the Y axis, the ESE type 1 work with a small arm lever and in that situation the ESE type 2 contribute efficiently to the global stiffness of the isolator as they have the same lever arm than the ESE type 1. The stiffness of both types of ESE is finely adjusted to minimize the frequency difference between the first two lateral rocking modes. The modes finally spread this way:

Mode  Frequency   Comment 1          39.9 Hz         X rocking + Y translation 2          40.5 Hz         Y rocking + X translation 3           165 Hz          Z translation 4           260 Hz         X translation + Y rocking 5           280 Hz          Y translation + X rocking

Table 1 : synthesis of the performances of the Thales Alenia Space CCS isolator

Obviously, an isolator with its suspension elements placed around the CoI and along the main inertia axis at correct distances w.r.t CoI would have better performances but is more difficult to accommodate in the limited space allowed for the CCS in the MTG-I satellite. The transmissibility obtained in the micro vibration configuration are presented in the Fig. 3.

Fig. 3. Transmissibility of the CCS isolator (blue: in plane X transmissibility, red: in plane Y transmissibility, green: out of plane Z transmissibility)

The stability of the micro vibration performances of the isolator w.r.t the thermal environment is obtained with heaters and thermal sensors mounted on the ESE.

 

3. LAUNCH BEHAVIOUR OF THE ISOLATOR

Because the elastomeric material have mechanical properties that vary with their excitation frequency, their strain and their temperature and because the ESE dissipate into heat a large amount of the mechanical energy that they receive, the design of suspension elements with this class of material that shall sustain the high level mechanical loads can become rapidly complex. The elastomeric material selected to produce the ESE.

Fig. 4. ESE type 1 of the CCS isolator

Three set of dynamic loads are defined to meet the various need of the MTG-I program. 1. Qualification loads are the highest loads that shall withstand the ESE without any damages and degradation of their performances but no other requirements are attached to these loads.

2. FCI instrument level loads (FCI characterization) are defined as close as possible to what the ESE will really experienced once mounted in the FCI instrument. A resonance frequency stability requirement is attached to this load: the lowest resonance frequency shall remain higher than 35 Hz.

3. System level loads are defined as close as possible to what the ESE will experience during the satellite qualification test. The requirements attached to this load definition are the same than for the FCI characterization. These tests results aim at providing accurate finite element models of the ESE to have reliable predictions for each situation. The load profiles are presented in the Fig. 5.

Fig. 5. MTG-I CCS isolator dynamic loads

One can notice that the system characterization load (yellow curve) is significantly lower than the FCI characterization ones and obviously largely lower than the qualification ones. Because the qualification loads are significantly high and because the mass of the CCS is important (around 40 kg), the ESE have been designed with the largest possible amount of rubber (the bigger, the better) and with the highest allowable stiffness to limit their strain during vibration tests.

Fig. 6. MTG-I CCS isolator dynamic response (blue) under X axis sine qualification load (red).

The Fig. 6 presents the acceleration at the CoI of the suspended CCS under X axis qualification loads and makes appear the frequency ranges where the isolator amplify moderately the base acceleration (from 5 Hz up to 43 Hz) and where the ESE mitigate the load transmitted to the CCS (from 43 Hz and up to 100 Hz). The transmissibility of the isolator varies with the applied loads (see Fig. 7).

Fig. 7. MTG-I CCS isolator X axis transmissibility depending on the dynamic load (blue: low level load, red: system characterization load, green: FCI characterization load, purple: qualification load).

The non-linear behaviour of the CCS isolator w.r.t the applied dynamic load affects both its resonance frequency and its damping: the higher the loads, the lower the resonance frequency and the damping. Fig. 7 put also into light that the system characterization load leads to a dynamic behaviour of the CCS isolator very close to the low level one (equivalent to the micro vibration behaviour) with a moderate resonance frequency shift (lower than 2 Hz) and with a very moderate damping variation (lower than 5%).

Fig. 8. MTG-I CCS isolator dynamic response (blue) under Y axis sine qualification load (red).

The Fig. 8 presents the dynamic response of the CCS isolator under Y axis qualification loads and demonstrates that the CCS isolator has the same dynamic behaviour in both in plane directions as it was expected from the design choices. The Fig. 9 presents the Y axis transmissibility of the CCS isolator depending on the applied dynamic loads. When the comparison is made with the Fig. 7, it can be noticed that the non-linear behaviour in the Y direction is lower than in the X direction. This is due to the fact that in the Y direction the 6 ESE that constitute the CCS isolator all work at the same time (meaning that the stiffer ESE type 2 are solicited) when in the X direction, the main part of the load is sustained only by the 4 ESE type 1.

Fig. 9. MTG-I CCS isolator Y axis transmissibility depending on the dynamic load (blue: low level load, red: system characterization load, green: FCI characterization load, purple: qualification load).

Thanks to these test results, it has been possible to derive a finite element model of the ESE type 1 and ESE type 2 that accurately reproduce the dynamic behaviour of the CCS isolator (Fig. 10).

Fig. 10. MTG-I CCS isolator X axis transmissibility (blue: measurement, red: FEM).

Random loads have also been applied to the CCS isolator (Fig. 11) that lead to a dynamic behaviour very much similar to the one obtained under low level loads demonstrating that the random loads are not sizing loads for the CCS isolator. Shock loads have not been applied to the CCS isolator as it is not sensitive to shock, being itself a shock isolator as well.

Fig. 11. MTG-I CCS isolator X axis (blue) and Y axis (red) Power Spectral Density (PSD) of the acceleration of its CoI under X and Y axis qualification random loads respectively.

The Structural and Thermal Model (STM) of the CCS isolator has already been tested and the Proto Flight Models are the ones to be tested and validated in 2018.

 

4. CONCLUSION

Two types of Elastomeric Suspension Elements (ESE) have been developed to make the CCS isolator with a set of 4 ESE type 1 and 2 ESE type 2 per isolator. Thanks to this choice, it has been possible to obtain identical dynamic behaviours in both in plane directions to have identical in plane mitigation efficiency. Taking advantage of the allocated volume, it has been possible to design strong enough ESE able to sustain the qualification loads without the need of a launch lock. The fine stiffness adjustments of both types of ESE permitted to meet the micro vibration mitigation requirements and the choice of highly damped SMACTANE SP elastomer permitted to meet the qualification loads requirements.The ESE have also been designed to limit as much as possible the constraint imposed by their integration in the instrument and the satellite.

 

5. ACKNOWLEDGEMENT

The authors would like to thank the MTG program leaders at Thales Alenia Space for having granted the ESE development as well as the ESA people in charge of the mechanical supervision of the MTG program for their positive point of view about the ESE concept.

 

6. REFERENCES

1. Carte, G. & Demerville, T., ECSSMET 2016, Vibration isolators for space applications, various configurations.

 

Gilles CARTE, THALES ALENIA SPACE Tony DEMERVILLE, SMAC André SMOLDERS, THALES ALENIA SPACE