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Silica Tile System Development: A Key Technology Element in Spacecapsule Recovery Experiment

By:

A Subramoniam, PD-SRE and N.C. Bhat, APD-SRE,
Indian Space Research Organisation (ISRO)

Spacecapsule Recovery Experiment (SRE) has heralded a new era to Indian Space Research program by successfully re-entering into earth's atmosphere and splashing in sea at predicted impact zone. With SRE, India and ISRO in particular demonstrated the capability in bringing back orbiting spacecraft and recover it at the desired location. For SRE, the de-boost starts over Mexico in southnorth pass of SRE. The flight covers almost half the globe and splash down occurs at Bay of Bengal in north south pass en route north pole. The flight from 100 km down to about 30 km is very critical. The module enters 100 km at a speed of 29000 km/hr (Mach 30) and experience aero-breaking and drag, which results in generation of hot plasma layers surrounding the capsule body. The peak heating of 258 W/cm2 at the stagnation region occurs at 50 km at Mach 20. To prevent capsule body from such a hostile environment SRE heat shield is configured with Carbon Phenolic Ablative and Silica Tile systems over composite base structure. It is worth mentioning that without meticulous development and realisation of Silica tile system by ISRO, it would not have been possible to conduct the re-entry experiment successfully.

figure1


SRE re-entered in earth's atmosphere on 22nd January 2007. Figure 1 shows re-entry operation sequence and its timeline. At 9:00 IST de-boost command is activated over Mexico. Its de-boost performance was as expected. The re-entry started at 100 Km over Lucknow (India). A minute laterdue to severe heating and formation of plasma, capsule went to RF blackout phase. This was very critical phase (of about two minutes) when maximum heating and aerodynamic breaking has to take place and capsule to sustain the hostile re-entry environment. At the same time due to RF blackout no ground communication can be established with spacecraft. Figure 2 shows a typical heating profile on the capsule.

figure2


Development of highly insulative low-density Silica tiles and its attachment systems were one of the major challenges. The ceramic silica tile system required development of a number of materials like Silica tiles, strain isolation pad, gap fillers, special emissivity coatings and high temperature adhesives.

The silica tile system is designed to meet,

1. Low Thermal Conductivity to keep back wall temperature with acceptable limits.

2. The tile density must as low as possible (< 0.4 gm/cc) so as the mass of TPS shall not increase the total SRE mass. It is critical in Silica tile case as it covers almost 90% of external surface of heat shield.

3. The SRE Spacecraft was designed to stay in space for 90 days. Hence, all these new TPS materials have to meet the deep vacuum specifications and vacuum degassing constraints.

4. Further RF transparency of tiles protecting the antenna has to be ensured even in hot condition.

The Silica tile is made of amorphous silica fibres, which is moulded in to the form of block and impregnated with ceramic binder. These blocks are dried and sintered to achieve the properties required. The blocks are later machined to suit the outer surface of the base structure. These tiles can with stand temperature upto 1400oC and very low thermal conductivity so as to keep back-wall temperature within acceptable limit of 120C. The first flight of SRE has about 264 Nos of tiles of 14 different types.

The Silica Thermal protection system is bonded on to the composite base structure by means of high temperature adhesive. A Strain isolation pad is used to isolate the structural deformations and differential coefficients of thermal expansion. The SIP is located between the tile and the base structure. Figure 3 shows Silica tile system.

figure3

Tests were done in Kinetic heating simulation, plasma arc jet, rocket exhaust, thermo vac cycling, thermal shock, acoustics etc, for qualifying TPS. While basic laboratory level processes were developed in ISRO, the industries were enabled to set up the facilities for large-scale production. A large number of samples were characterized in thermovac facilities to ensure the space environment compatibility of Silica tiles and Silica tile system.

figure4

The Flight results of SRE suggest that thermal response of Silica tiles were better than expected as shown in Figure 4. The tile surfaces were apparently intact after splash down except for some minor damages during recovery operations. Figure 5 compares the pre-launch SRE tile surfaces with recovered SRE Spacecraft. The spacecraft body was floating in seawater for about 2 hours prior to its recovery, which caused minor surface cracks on tile surfaces and seawater deposits on tile surfaces.

figure5

With the success of Spacecapsule Recovery experiment, ISRO has demonstrated the capabilities for development of critical technologies needed like providing stable platform for microgravity experiments, Navigation Guidance & Control for system for end to end mission, parachute and floatation systems for terminal phase and new materials developments like Silica tile system. It is prudent to recall here the unstinted efforts by Advance Material and Ceramics Group of VSSC, Trivandrum in realisation of the Silica tiles.


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