a compendium of tech stuff

Jan 3, 2012

On 8:09 PM by Lalith Varun   No comments


  1. INTRODUCTION
  2. MATERIALS FOR REUSABLE AEROSPACE VEHICLES


INTRODUCTION

          The cost of launching spacecrafts using expendable vehicles is very high and efforts are being made to reduce these costs significantly. This reduction in costs can be achieved either by cutting down the overall weight and cost of the vehicle by selecting suitable low cost materials and optimize the design of the components or by opting for reusable vehicles so that the initial high development costs can be recovered over number of flights.
          The main issues pertaining to reusable aerospace vehicles are a) recovery and reflight, b) maintainability, c) reusable materials, d) thermal management etc. The intense aero-thermal loads to which the vehicle is subjected during its flight, reentry and stringent mass budget make selection of materials for reusable aerospace vehicles a challenging task.
          The main considerations for the development of Reusable Launch Vehicle's (RLV) are
1. they can bring down the launch cost substantially as compared to that of expendable vehicles.
2. their design having in-built abort and emergency landing capabilities would fructify mission success probability and overall safety.
3. retrieval of payloads for overhaul and reuse, in-orbit servicing of space systems and re-fueling of satellites are possible.
4. they can avoid debris in orbit and loss of pricey materials, and can cut down environmental pollution.

MATERIALS FOR REUSABLE AEROSPACE VEHICLES
          The materials for reusable aerospace vehicles can be classified as
1. Air-frame materials
2. Thermal protection materials

AIR FRAME MATERIALS

          Sizeable cost savings can be obtained by minimizing the overall weight and part count. Air frame weight reduction can be normally achieved by the use of lighter materials and using efficient structural designs.
Concisely, materials for constructing air-frame should have high
1. stiffness
2. stress corrosion resistance
3. fracture toughness
4. fatigue strength
5. creep resistance
6. ease of fabrication and repair.

Aluminium Alloys
          Most commonly used Aluminium alloys are AA2024 and AA7075. Modifications to the base alloy composition resulted in higher fracture toughness alloys such as AA7175 and AA7475. AA7150, AA7055 and AA2524 have higher compression yield strength, corrosion resistance and fatigue crack growth resistance.

Composite Materials
          They have very high strength and resistance to corrosion and fatigue. Their properties can be tailored to meet the specific needs and they can be formed to complex shapes. High strength composites such as Carbon Fibre Reinforced Polymer (CFRP) and Graphite-Epoxy are used for making Air-frame structures.

THERMAL PROTECTION SYSTEMS

          The factors which lead to heating of the external surface of the reusable vehicle are aerodynamic heating and the thermal properties of the materials used in making the air-frames. The aerodynamic heating is dependent on flight profile of the vehicle, i.e. the angle of attack, mach number, body geometry, pressure etc. The thermal properties such as emissivity, absorptivity, catalycity and conductivity of the external surface decides the heat load acting on the reusable vehicle.

Reinforced Carbon-Carbon (C-C) composites have operating range of -150K to about 2000K
Carbon/Silicon Carbide Ceramic Matrix Composites (C/SiC) can operate up to 1800K
Silicon Carbide/Silicon Carbide Ceramic Matrix Composites (SiC/SiC) are resistant up to 1600K
Alumina borosilicate (ABS)-silica ceramic tiles can withstand temperatures up to 1600K
Toughened Unified Fibre Insulation (TUFI) tiles can operate up to 3000K
Carbon aerogels up to 3200K and tiles coated with carbides of hafnium, zirconium and titanium can withstand temperatures as high as 3250K

          Since the prime requisite of the materials selected for fabrication is re-usability, the testing and quality requirements have to be very rigorous and precise. The development of reusable aerospace vehicles can result in lower launch costs of satellites as compared to the use of expendable launch vehicles.


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