LARES MISSIONCCR MOUNTING RING THERMAL ANALYSIS WITH SOL400

Presented By: Ing. Francesco PasseggioJune 12, 2012

Authors: A. Paolozzi1 I. Ciufolini2

G. Caputo3, F. Passeggio3

1Sapienza Università di Roma, Scuola di Ingegneria Aerospaziale, Via Salaria 851, 00138 Roma, Italy2University of Salento

2OMPM srl Via Fontana 5 84012 Angri (SA), Italy

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The LARES Satellite for testing general relativity

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• Measure the twist of spacetime induced by the Earth Rotation (Lense-Thirring effect).

• The node of LARES orbit will be dragged, by the Earth rotation (because of Lense-Thirring effect) by a small but measurable amount.

LAGEOS 2

LAGEOS

LARES

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LARES Satellite

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• To minimize the non gravitational perturbations LARES satellite will be made of tungsten alloy with a density of about 18.000 kg/m3.

• 92 Cube Corner Reflectors.

• one single piece of tungsten alloy reduce thermal thrust.

Mass = 387 kgDiam. = 364 mm

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LARES The satellite of records

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1. The orbiting object with the highest mean density in the solar system2. Made out from the heaviest tungsten alloy piece ever manufactured3. Satellite designed, built, tested and launched in 4 year and 1 week 4. Very high machining and tolerance challenges'5. Most of the design and testing work performed inside a University

a. Engineering - School of Aerospace Engineering, Sapienzab. Physics – Salento Universityc. University - proposers but also heavy involvement in all activities from manufacturing to launchd. Satellite successfully tracked by 34 laser stations all over the worlde. Tracking can continue for decades and probably for hundreds of years

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The equilibrium temperature in orbit is dominated by thermo-optical surface properties that depend both on the material chosen (tungsten alloy) and the surface finish (a thermal test was performed on breadboards having different surface roughness).

The α/ε_LARES is about 4.3 – 8.2, this determine an equilibrium temperature in orbit of about 100°C, higher than the one of the same category aluminum satellites (Lageos I) that is about 20°C.

The potential problems created by the high temperature are:• High temperature on satellite body induces high gradients on CCR

– Distortion on Far Field Diffraction Pattern (FFDP), i.e. Distortion of the energy distribution on the ground station of the return laser pulse

• High temperature on satellite body induce high temperature on CCR PCTFE mounting rings

– Plastic deformation of PCTFE rings– Verify possible contact between the CCR and the mounting rings

due to high Linear expansion coefficient of PCTFE. In case of contact verify is pressures are acceptable

LARESThermal performances

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The figure shows the CCR assy with the indication of the nominal clearances in mmTo validate the design of the PCTFE rings, before the thermal tests, was decided to make an FEM thermal

analysis with MSC Nastran using the new SOL400 solution that could give better results for the following reason:

• Capability to simulate more realistic contact boundary condition using rigid body contact surfaces.• Capability to manage both linear and non linear thermal solution.

CCR Rings Thermal analysis

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The FE model contains only the PCTFE rings, these are modelled with TET10 elements that give the best performances with non linear analysis.

The non linearity taken into account are related to the PCTFE material, for which the elastic modulus and the Linear expansion coefficient vary with the temperature, see graph and table.

FE model description

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The big advantage given by sol 400 is the possibility to define contact surfaces as boundary condition, much more effective and easier to use respect the linear gap contact methodology.

The rings are constrained between the following surfaces:• Bottom, simulating the bottom of the CCR cavity• Upper, simulating the retainer ring.• Side, simulating the side edges of the CCR cavity• Screw, simulating the nominal diameter of the screw

FE model description

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Conservatively the max temperature considered for the analysis is 125 °C, corresponding to a ∆ of 100 °C.

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The plot of the deformations shows a good behaviour of the solution.The rings tend to increase in diameter except in the zones were there are the screws, in which the material

tend to expand.Remarkable is also the crippling effect in the upper mounting ring well represented by the non linear

solution, the maximum displacement towards the CCR is about .4 mm, at 125 °C, so higher than the gap, suggesting to make a thermo-optical test in the thermovacuum chamber, available at Scuola diIngegneria Aerospaziale, CRPSM and DIAEE Astronautical Area.

FE model deformation plots

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The stress plots at 125 °C, bring to some important consideration:• The crippling in the upper ring could result in a permanent deformation.• Except from some “mesh” induced stress concentration, the max stress is under the rupture

strength.

FE model Stress plots

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Numerical results suggested to make a qualification thermo-optical test in the thermovacuum chamber, available at Scuola di Ingegneria Aerospaziale, CRPSM and DIAEE Astronautical Area.

Qualification test was successful, so no modifications was introduced in the original design.LARES satellite was launched on the 13° of February 2012 at 7 am Kourou local time.Orbital injection occurred 55 minutes after the launch.Satellite is performing exceptionally well, laser return are very strong and due to its challenging design is

showing a drag free behavior better than active and very expensive satellites

Conclusions

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