Updated Wednesday, 21-Jan-2004 10:03:27 EST
Working in Space
Manned maneuvering unit (MMU) -- rocket pack attached to space suit
- EVA (extravehicular activity) -- space walks
- Space suit required
- Must protect astronaut from radiation, small debris, micrometeors
- Multiple layers of various flexible materials give added strength and act as barriers against small high-speed objects
- Some layers also provide a certain amount of radiation shielding
- Present suits are relatively flexible, but some experimental hard suits are being tested
- Hard suits made up of several rigid shells (torso, arm and leg pieces) with stronger, but stiffer, flexible joints
- Hard suits are difficult to store (they don't fold up for storage), but can provide better protection from debris, micrometeors and radiation
- Must provide self-contained life support (including breathable air and temperature control)
- Present suits have internal pressure of 5 pounds/sq. in. -- this is the maximum that a flexible suit can contain. Lower pressure requires special preparations (see Living in Space)
- Hard suits could maintain internal pressures closer to standard (10 to 14 pounds/sq. in.)
- Hard suits not as flexible -- harder for astronauts to bend or twist at waist, move arms and legs.
- About 7 hrs of oxygen under average exertion, plus additional 30 min. emergency tank
- Velcro patches for attaching tools (so they won't float away)
- Tether to attach astronaut to Space Shuttle (or Mir or other spacecraft)
- Good safety feature, but sometimes gets in the way
- Communications package (radio link to Shuttle)
- Permits rotation (pitch, roll, and yaw) and translation (up/down, front/back, and left/right)
- Astronaut can manually control these motions, or put system into autopilot mode so that it holds his/her in constant position while working on a satellite, etc.
- Carries about six hours worth of propellant (under average usage) -- propellant can be recharged on Shuttle (it's just compressed nitrogen gas)
- Can produce speeds of 0.5 to 1.5 ft/sec with respect to the Shuttle (remember, astronaut is orbiting along with Shuttle, and so moves at 7.5 km/sec with respect to Earth!)
- Satellite services
- Satellite enclosed in protective cradle in Shuttle cargo bay
- Spin-stabilized satellites released using one of two means:
- Turntable -- starts satellite spinning before spring mechanism releases satellite into orbit (used for satellites with tall cylindrical shape -- like a drink can)
- "Frisbee" device -- spins and ejects satellite all in one motion (used for satellites with short cylindrical shapes -- like a Frisbee or tuna fish can)
- Other satellites (3-axis stabilized with thrusters, or dual-spinners) are released using the robotic arm (remote manipulator system), which simply moves them out of the cargo bay and releases them
- Shuttle's maximum altitude is 600 miles, so most satellites need a rocket booster to move them into higher orbits to complete their missions
- Typically use a Hohmann transfer
- Satellite must carry either a) two solid chemical rocket engines, using one to leave the Shuttle orbit and the other to change speed and enter the final orbit, or b) a single liquid propellant engine that can be fired once to leave Shuttle orbit, then shut down and restarted at a later time to enter the final orbit.
- Satellites in GEO cannot be retrieved by Shuttle (it can't reach that distance from Earth)
- Feb. 1984 Shuttle flight STS-10 deployed two satellites: WESTAR VI and PALAPA B-2
- Both used solid engines called PAM's (Payload Assist Modules), which failed to fire correctly, leaving the satellites in low-Earth orbits
- Nov. 1984 STS-14 mission recovered both satellites and returned them to Earth
- Why recover them? Most satellites are insured and the owner can recoup some of the loss even if satellite not recovered. But cost of repairing and relaunching is much less than building new satellite.
- Shuttle rendezvous with satellites (for repair in orbit or retrieval for Earth-return)
- Cannot launch directly from ground to satellite position because of targeting inaccuracies (this mostly has to do with the Shuttle's speed relative to the satellite which must be very low to avoid collision and damage).
- Instead, launch Shuttle into same orbit as satellite (the target) and then adjust Shuttle's motion so that it rendezvous with sat.
- On Earth, if we are driving behind a car and wish to catch up to it, we increase our speed, but in orbit it's a little different. If we (Shuttle) are behind the satellite in its orbit, then we speed up by firing our thruster in reverse; this causes us to momentarily slow down and move into an orbit that is slightly smaller and thus has a shorter period. With the shorter period, we make a complete pass around Earth slightly faster than the satellite does and so after a few orbits, we catch up to the satellite, fire our thrusters again to move back onto the satellite's orbit and then use careful, small thruster firings to maneuver next to the satellite. If we had simply fired our large thrusters until we caught up with the satellite, it would have taken an enormous amount of propellant!
- Example -- Shuttle in 290 km orbit, period = 90.31 min, speed = 7.732 km/sec
Satellite in 300 km orbit, period = 90.51 min, speed = 7.726 km/sec
If satellite leads Shuttle by 15 degrees (around the circle), then how long does or how many orbits for Shuttle to catch it?
a = separation angle = 15 degrees in this problem
R1 = 360 deg/90.31 min = 3.986 deg/min (Shuttle rate)
R2 = 360 deg/90.51 min = 3.977 deg/min (satellite rate)
Rr = R1 - R2 = 3.986 deg/min - 3.977 deg/min = 0.009 deg/min (relative rate)
Time to rendezvous = a/Rr
= 15 deg /0.009 deg./min. = 1667 min. = approx. 18 orbits.
- Repair (in orbit)
- First accomplished on Solar Max satellite, April 1984
- Astronauts replaced three blown fuses
- $235 million for original satellite and launch, $55 million to repair
- Most notable repair was Hubble Space Telescope lens correction
Copyright © 1998, Robert G. Melton
Updated Wednesday, 21-Jan-2004 10:03:27 EST