[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

AO-40 Special Bulletin

SB SAT @ AMSAT $ANS-075.01

BID: $ANS-075.01

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

Approximately twelve 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 it's 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.


Robin Haighton VE3FRH 
President AMSAT-NA 

[ANS thanks AMSAT-NA for this information]

Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org