Transcript mechanical
Spacecraft Structure • • Requirements – Mass, Strength, Stiffness, Cleanliness (Particulate/Magnetic) Constraints – Fields of view (instruments, thrusters, motors) – Thermal control – Center of mass – Ratio of moments of inertia – Propulsion lever arm – Constraints of launch vehicle shroud – Constraints of sun and earth vector – Component Collocation – Launch Vehicle Environment (loads, shock, acoustics) • Steady state • Transient – Random Vibration – Acoustic vibration – Testing Design Approach • Accessibility – Test, Repair • Harness and Cables – Routing • Standard Approaches? – Attempts at “generic” spacecraft have failed • Building blocks – Boxes, Truss (tube), Cylinders, X-agons – Materials: Metals, Honeycomb, Composites Launch Vehicle Payload Planners Guide http://www.boeing.com/defense-space/space/delta/docs/DELTA_II_PPG_2000.PDF Spacecraft • Spacecraft are generally classified by: – Attitude Control System • • • • • None Gravity Gradient Spinner Despun (Momentum Biased) 3-Axis Stabilized – Mission they are performing • • • • • Flyby Orbiter Probe Lander Penetrator None Sputnik 1957 Gravity Gradient LDEF Deployed on orbit on April 7, 1984 by the Shuttle Challenger. Circular orbit Altitude of 275 nautical miles Inclination of 28.4 degrees. Attitude control of the LDEF spacecraft was achieved with gravity gradient and inertial distribution to maintain three-axis stability in orbit. Spinner SNOE Circular orbit 580 km altitude 97.75 degrees inclination sun synchronous precession, 26 Feb. 1998. Despun (Momentum Biased) OSO-8 Launched on 21 June 1975. 550 km circular orbit 33 degree inclination scanning and pointing mode. spin period was 10.7 seconds. rotating cylindrical base section non-spinning upper section 3-Axis Stabilized TERRA (EOS AM) 12/18/1999 3-Axis Stabilized TERRA (EOS AM) Solar Array Flyby Voyager 1977 Jupiter Saturn Uranus Neptune Cassini Orbiter Prob e Viking Lander Space Environment and Effects – Earth • Planet Mass • Iron Core • Atmosphere – Sun • Mass • Solar Activity – Planets • Planet environment • Proximity to sun Spacecraft Systems Computers Batteries Propellant Electronics Instruments Windows Mirrors Mechanisms Detectors Environment and Effects Earth Magnetosphere Radiation Belts Trapped Protons and Electrons Changing Magnetic Field Atmosphere South Atlantic Anomaly Solar Wind Magnetopause Environment and Effects - Sun UV Wavelength Solar Wind CMEs - Protons, Neutrons and X rays Coronal Mass Ejection Space Environment from the Spacecraft Perspective • • • • Thermal: Solar, IR, S/C Dissipatoin Solar UV Charged Particles – Solar Wind, Sun Materials Outgassing and Contamination • Magnetics • Spacecraft Charging • Atomic Oxygen Thermal • Total radiant-heat energy emitted from a surface is proportional to the fourth power of its absolute temperature. E = T4 • Direct Solar Exposure • Eclipse • Orbit transition • Changing response of spacecraft • Temperature of planet body Solar UV Flux Short wavelength energy Reactions Changes material properties Optical Properties of Surfaces Glass darkens Polymerize hydrocarbons Outgassing and Contamination http://epims.gsfc.nasa.gov/og/ Earth vs Space: Pressure, water, oil, unpolymerized materials Fingerprint Total Mass Loss (TML) <1% Collected Volatile Condensible Materials (CVCM) < 0.1% Effects of Radiation • • • • Single Events Upsets Latchup Long term exposure (Total Dose) Electronics Degredation – Threshold Levels and Timing – Semiconductors – Holes Magnetic Earth Axis 11 Dipole (bar magnet) •Rotating magnetic field •Magnetometer •Fixed and Changing Fields on Instruments •Non-magnetic materials Spacecraft Charging • Spacecraft moving through a plasma • Plasma density • Debye length • Field around spacecraft • Photo-electric emission – Photons hit surface, release electrons • Insulators on spacecraft surfaces – Near earth ~1-2 volts – 10 Earth Radii (10RE) ~10,000 Volts Atomic Oxygen • In low Earth orbits, satellites encounter the very low density residual atmosphere. At orbital altitudes, this is composed primarily of oxygen in an atomic state. • A satellite moves through the atomic oxygen (AO) at a velocity of about 7.5 km/sec. Although the density of AO is relatively low, the flux is high (speed x density x surface area). • Highly reactive atomic oxygen can produce serious erosion of surfaces through oxidation. Thermal cycling of surfaces, which go in and out of the earth's shadow frequently in this orbit, can remove the oxidized layer from the surface. Aurora Australis or Southern Lights, in the 80 - 120 km altitude region Charged plasma glow around shuttle Excitation of atomic oxygen in the upper atmosphere by the van Allen Radiation Belts Glow Radiation Environment • • • • Radiation Belts around the Earth Electrons Protons Cosmic Rays Debris and Micrometeorites • Humans • Damage to Spacecraft and Systems