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To All Members of AMSAT-NA,
The following statement is addressed to those members of AMSAT-NA who have requested an explanation of the December 2000 incident that took place on P3D. This statement has been prepared and developed by Robin Haighton, VE3FRH, President of AMSAT-NA, with input and review from other AMSAT-NA members.
As you are aware, Phase 3D was launched on November 16, 2001, into an almost perfect geosynchronous transfer orbit (GTO) by an Arianespace launch vehicle (AR-507) from Kourou, French Guiana. Within a few hours of launch excellent telemetry was being received from the 2-meter beacon, and Amateur Radio stations worldwide started downloading data with great accuracy, due in-part to the strong signal strength. The original plan was to use the 70-cm beacon, however, for reasons not yet known, the 70-cm transmitter signal was not heard.
Before the satellite could be regularly used for general Amateur Radio communications, it was necessary to carry out orbital changes, stabilize the satellite, open the solar panels, etc. The first changes to the orientation of the satellite were carried out using the onboard magnetorquing system--this worked well and after relatively few orbits the attitude of (now) AO-40 was 270/0, and ready for operation of the 400-newton motor. Among the many components which comprise this motor system, there are several valves which control pressurizing helium and fuel. During construction of the satellite it was noted that one of the helium valves had a tendency to "stick" when operated. Both of these valves were sent back to the manufacturer for inspection and repair. Both valves were inspected and one valve was repaired, followed by return and re-installation into the system.
On the first attempt to fire the 400-N propulsion system, it failed to operate, possibly due to a sticking helium valve. Before the second attempt, it was determined that the fuel tanks could be pressurized (by helium) to their correct pressure over a five minute period, and although this was about one-tenth of the normal helium flow rate, it was still adequate for the planned three minute motor burn.
On the second attempt to fire the 400-N motor, all systems appeared to respond correctly at first. At the three minute mark the internal timer transmitted a signal for the main solenoid valve to close, which should have shut off the fuel to the motor. Telemetry shows that the signal was sent and received, but the motor did not shut off for two or three more minutes, placing AO-40 into a higher apogee orbit than was planned at that time.
To understand how this may have happened, it is necessary to be aware that the fuel for the 400-N motor is made up of two components, hydrazine (MMH) and nitrogen tetraoxide (N2O4), with each component contained in two separate tanks, both of which could be pressurized by helium. Helium could also be applied to the solenoid motor valve, the output of which operates the two fuel valves which start (and stop) the fuel flow. These valves are actually part of the 400-N motor and are located inside the motor itself.
On the solenoid motor valve there is an evacuation port that allows excess helium at the output port of the valve to escape when closing the valve. It is believed that this evacuation port was blocked and that the output port remained pressurized beyond the three-minute mark of the motor operation--thus the motor continued to burn for an extended period of time.
Between the fuel tanks and the 400-newton motor there are fuel isolation valves which are pressure-operated by the helium system. When the pressure in the helium manifold had been reduced to approximately 6 Bar (100 PSI), the fuel isolation valves closed and prevented any additional fuel from entering the motor, stopping the burn. At this time it is possible that the main motor valves were still open, due to the trapped pressurized helium that had not vented at the solenoid motor valve.
Approximately 12 minutes after the motor shut down, a second anomaly occurred. This was detected when the motor solenoid valve changed from closed to open, possibly caused by fuel migrating in the lines between the isolation valve and the 400-N motor. The motor could have also "burped" or "popped" as the fuel mixed and then ignited.
High pressure helium (180 Bar) is fed to the motor system via a high-pressure on/off valve and a regulator valve--reducing the pressure to a nominal 15 Bar level. It is then fed to the low-pressure helium manifold. Because of the longer 400-N burn, a program for testing the high pressure helium valve was written to "cycle" the valve (to insure proper functioning) and uploaded to AO-40.
On December 11, 2000, while cycling the helium valve, a sudden loss of signal from AO-40 occurred. It is believed that during this exercise the system became pressurized and that a leakage of fuel was the end result. Initial thoughts were that the spacecraft was completely dead and that chances of recovery were remote, with the possibility that AO-40 was in multiple pieces. However, with help from NORAD, it was determined that the satellite was in one piece and still in its known orbit, with a possibility of some recovery. At least two automatic resets passed without hearing from the spacecraft. It was decided to try and hear the general beacon on the S-band (2.4 GHz) transmitter. On Christmas Day 2000 the second attempt to activate the S-band transmitter was successful, and since that day downlink telemetry has been recovered on a regular basis.
The following items have been found to be working; the 2-meter, 70-cm and 1.2 GHz receivers, the S-2 (2.4 GHz) transmitter, the magnetorquing system, the YACE camera, IHU-2 and the high-gain antennas. The following items are believed not to be working; the 2-meter and 70-cm transmitters and the omni-directional antennas.
At the time of this bulletin (March 16, 2001) we still do not know the status of the arc-jet motor--which is an important item, needed to position the satellite for future use. We do know that the satellite has lost mass, and we attribute this to the loss of bi-propellant fuel from the 400-N motor. The satellite spin rate had increased as the overall weight decreased, but by using the magnetorquing system the spin rate is now nearly down to a usable 5 RPM. In addition, the heat pipe system (which became unusable at the higher spin rates) has now become effective again.
Soon AO-40 will be able to be re-orientated so that the high-gain antennas will face the Earth, and the Arc-jet motor will be tested. Following the re-orientation it will be possible to test the remaining systems on board the spacecraft and to determine which systems and bands will be available for future operations and under what conditions.
As we all learn more about the status of the satellite, additional bulletins will be posted on AMSAT-BB, and placed on the AMSAT-NA, AMSAT-DL and AMSAT-UK web sites. Meanwhile, all those involved in the recovery of AO-40 are to be congratulated for their skills and perseverance, and may their hard work continue to bring us an operational satellite.
73,
Robin Haighton VE3FRH
President, AMSAT-NA
