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Launches
a. What Are the Major Phases of Launch Preparation?
The Rocket Scientist sat with Ron Capozzi, Senior Member of our Engineering Staff, and asked the following questions:
1) What key steps must happen to launch a satellite?
From our filing for an orbital slot with the FCC to the actual launch can often take years of preparation. Even prior to that, rounds of sales and marketing decisions have to be made. In parallel, since Americom also operates its satellites once launched, the Telemetry, Tracking and Control teams must develop any unique TT&C programming that is required, and train to "fly" the satellite. In addition, insurance must be obtained and the satellite itself must be contracted for, as well as for the launch services, including the launch vehicle that will be required. All this transpires in sequence until one day the satellite arrives at the launch site to be readied for its big day.
2) How does a satellite get to the launch site and what happens after it arrives?
Typically, a satellite is shipped by truck or airplane within a special container. GE-5, for example, arrived at the ArianeSpace launch complex in French Guiana via a 747 about four weeks prior to launch. It was then transferred in its shipping container to the Payload Processing Center where it was placed in a clean room and the container's sensors were examined to determine if temperature, humidity and shock stayed within acceptable limits during shipment.
3) Does the spacecraft itself get tested?
Absolutely. Each subsystem is tested to assure the satellite still tests just as it did prior to its shipment. If not, and depending on the defect, the satellite is either recalibrated on-site and launch preparations continue, or the satellite is returned to the factory for re-work.
4) What comes next?
The satellite's on-board fuel tanks are filled with hydrazine, a fuel which is used both to circularize the satellite's orbit initially and also provide fuel for station keeping maneuvers during its life. The satellite is next mated to a launch vehicle adapter where it is enclosed within its launch vehicle fairing. The fairing- clad satellite is then transported to the launch pad where it is affixed atop the erected launch vehicle to await the countdown. The satellite is now about a week from launch.
During this final week, pad checks are made to assure the reliability of launch vehicle communications and Command & Telemetry links. Finally, about two days prior to launch, the fueling of the launch vehicle commences so it can be completed just before launch. Then, as was the case with GE-5, all that is left to do was to enjoy the spectacle of a successful launch.
b. How Does a Satellite Reach its Orbital Slot?
The Rocket Scientist sat with Sue Hubert of Spacecraft Engineering to learn how a satellite is inserted into its orbital slot:
1) We know a typical communications satellite "flies" in a geosynchronous circular orbit 22,300 miles above the equator. Why?
A satellite positioned 22,300 miles over the equator has an orbit period that exactly matches the earth's rotation speed. This makes the satellite appear from earth to stand still in space. If a satellite did not travel in this geosynchronous orbit, the satellite would not remain in a constant location in the sky and at best, would require tracking antennas to up and downlink its signals.
2) Following launch, what has to happen to get it there?
Every launch is a little different, but in general, a rocket-launched satellite will go through a series of "burns" before and after detaching from its launch vehicle. Typically, the launch vehicle will have several stages. The first stage engines, often aided by strap-on boosters, permits the entire launch configuration to escape the gravitational hold of earth. Following first stage burnout, the first stage separates, leaving the second stage liquid fuel engines to carry the remaining third stage and satellite further into space.
Following separation of the second stage, the third stage fires to accelerate the spacecraft and places the satellite in a highly elliptical orbit 200 kilometers at its closest point to earth and 35,786 kilometers at its furthermost point. After separating from the third stage, the satellite remains in the elliptical orbit for approximately five days. During this time the satellite's subsystems are checked. Then, so that the satellite's tracking, telemetry and control systems can be powered and operated, the satellite is steered into a favorable attitude where the sun can efficiently strike an operable portion of the still undeployed solar arrays. Once accomplished, a series of apogee engine burns places the satellite into a favorable longitude and circularizes the orbit at 22,300 miles over the equator.
3) Is the satellite operational at that point?
Not yet. After being placed in the circular orbit, several critical events must occur. The foremost of these is the successful deployment of the satellite's solar arrays and antennas. These enable the satellite to be fully powered-up and pass communication signals to and from the spacecraft. Next, "earthlock" must be achieved via on-board infrared earth sensors that sense the heat of the earth against the cold of space and orient the spacecraft antenna at the earth. Finally, ranging frequencies are sent up from the ground to determine the exact location of the satellite so that corrective thruster burns can steer the satellite into its orbital slot. Then, after testing the transponders, the satellite is declared operational and open for business.
c. Sample Launch: GE-4
The Rocket Scientist asked Henry White, Principal Engineer, Spacecraft Engineering, to describe the launch of GE-4 on board an Arianespace Ariane 44LP rocket.
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