Design of Orbits and Spacecraft Systems Engineering Scott Schoneman 13 November 03
Introduction Why did satellites or spacecraft in the space run in this orbit, not in that orbit? How do we design the orbit to achieve some special scientific goals? Are there any difficulties in the mission?
Mission Design Low Earth Orbit (LEO) Earth or Space Observation International Space Station Support Rendezvous and Servicing Geosynchronous Orbit (GEO) Communication Satellites Weather Satellites Earth and Space Observation High Eccentricity Orbit (HEO) Lunar and Deep Space Lunar Inner and Outer Planetary Sun Observing
Perturbations: Reality is More Complicated Than Two Body Motion
Orbit Perturbations Non-spherical Earth gravity effects (i.e J-22 Effects ) Earth Earth is an Oblate Spheriod Not a Sphere Atmospheric Drag: Even in Space! Third bodies Other Other effects Solar Solar Radiation pressure Relativistic Effects
Applications of J2 Effects Sun-synchronous Orbits The regression of nodes matches the Sun s s longitude motion (360 deg/365 days = 0.9863 deg/day) Keep passing over locations at same time of day, same lighting conditions Useful for Earth or Sun observation (TRACE, Weather satellites) Frozen Orbits At the critical inclination, the precession of Apsides is zero Used for high-eccentricity communications satellites (Molniya-3)
Sun-synchronous Orbit
Third-Body Effects z Third body effects for Earth-orbiters are primarily due to the Sun and Moon z Affects GEOS more than LEOS z Points where the gravity and orbital motion cancel each other are called the Lagrange points z Sun-Earth L1 has been the destination for several Sun-science missions ( SOHO, ACE, Genesis, others planned) z Sun-Earth L2 (WSO/UV) z Sun-Earth L3 maybe
Orbit of SOHO (Halo Orbit)
Question Why don t t we design a satellite around the Sun like GEOS? 1.Bad space environment may be damage the instruments (High Temperature) 2. Connection with the Earth will be influenced by strong radiation of the Sun, distances, and sun shadow
Lagrange Points Application Genesis Mission: NASA/JPL Mission to collect solar wind samples from outside Earth s s magnetosphere Launched: 8 August 2001 Returning: Sept 2004
Third-Body Effects: Slingshot A way of taking orbital energy from one body ( a planet ) and giving it to another ( a spacecraft ) Used extensively for outer planet missions (Pioneer 10/11, Voyager, Galileo, Cassini) Analogous to Hitting a Baseball: Same Speed, Different Direction departing suncentric velocity spacecraft departing planet planet s orbit velocity hyperbolic flyby (relative to planet) spacecraft incoming to planet incoming suncentric velocity
Hohmann Transfer Hohmann transfer is the most efficient transfer (requires the least V) between 2 orbit assuming: Only 2 burns allowed Circular initial and final orbits Initial Circular Parking Orbit Perform first burn to transfer to an elliptical orbit which just touches both circular orbits Perform second burn to transfer to final circular GEO orbit GTO orbit GEO orbit
Earth-Mars Transfer A (nearly) Hohmann transfer to Mars Mars at Spacecraft Arrival Mars at Spacecraft Departure
Applications of Drag Aerobraking / aerocapture Instead of using a rocket, dip into the atmosphere Lower existing orbit: aerobraking Brake into orbit: aerocapture Aerobraking to control orbit first demonstrated with Magellan mission to Venus Used extensively by Mars Global Surveyor Of Of course, all landing missions to bodies with an atmosphere use drag to slow down from orbital speed (Shuttle, Apollo return to Earth, Mars/Venus landers)
Ballistic Reentry Suborbital Reentry Dynamics: Coming Back to Earth Reentry Vehicles Orbital Mercury and Gemini Skip Entry Apollo Gliding Entry Shuttle
Systems Engineering Looking at the Big Picture Requirements: What Does the Satellite Need to Do? When? Where? How? Juggling All The Pieces Mission Design: Orbits, etc. Instruments and Payloads Electronics and Power Communications Mass Attitude Control Propulsion Cost and Schedule
Spacecraft Integration and Test
GPS Satellites Constellation of 24 satellites in 12,000 nm orbits First GPS satellite launched in 1978 Full constellation achieved in 1994. 10 Year Lifetime Replacements are constantly being built and launched into orbit. Weight: ~2,000 pounds Size: ~17 feet across with the solar panels extended. Transmitter power is only 50 watts or less.