AO-40
Orbit ManeuversAO-40
Orbit ManeuversThe following article appeared in the May-July issue of The AMSAT-DL Journal and in the July/August issue of The AMSAT Journal.
Peter Guelzow, DB2OS (db2os@amsat.org)
Early in April Viktor Kudielka, OE1VKW, James Miller, G3RUH, Peter Guelzow, DB2OS, and Project Leader Prof. Karl Meinzer met at the headquarters of AMSAT-DL in Marburg. The purpose of the meeting was to discuss the approach and necessary orbital maneuvers after the launch of Phase 3D on board the Ariane 5. At the same time there was also a meeting with DASA (now Astrium) to discuss details of the fueling of the satellite with propellants for the 400 Newton motor and the arc-jet propulsion system.
The planned highly-elliptical 16-hour orbit of Phase 3D is especially susceptible to irregularities caused by the gravity pull of the sun and the moon. Viktor Kudielka has already performed preliminary studies on how to achieve the planned orbit with the limited resources available. Of particular concern was how to achieve an orbit that would be stable for many years and not lead to the same fate as previously befell OSCAR-13.
Kudielka's calculations were based on a successful nominal insertion into a geostationary transfer orbit (GTO) with an apogee of 37,786 km, a perigee of 560 km and an inclination of 7 degrees. Several other factors that depend on the exact time of the launch can only be considered after launch.
At the time of the meeting a question was received from Arianespace as to whether a modified GTO was acceptable to us. This GTO could lead to a maximum apogee of 39,122 km, which is 3,340 km higher than the nominal value, but it might also be 1,649 km lower than the nominal value. The perigee of 590 km would also be higher; however the inclination would be reduced to 6.5 degrees. The idea here is obviously is to fully utilize the resource of the rocket to increase the weight of the payload and save on fuel that the satellite needs to take along to get to and maintain the final orbit for a long lifespan. If it were not for the most "unfavorable" orbit, this modified orbit would also be of advantage to us. Subsequently we discussed in detail the risks and advantages of this approach. In principle, we could live even with the most unfavorable GTO offered, but the available propellants would be used up faster and long-lasting maneuvers using the arcjet would be required to save fuel. This is not nice, but we could live with it, and there is, of course, the chance for a quite favorable orbit as well.
The fully fueled AMSAT Phase 3D weighs in at 650 kg (unfueled 402.3 kg). Fuel consists of 196.7 kg MMH (monomethyl-hydrazine) and N2O4 (nitrogen-tetroxyide) for the 400 N (Newton) rocket motor and 51 kg of ammonia (NH3) for the arcjet. In addition 15.7 liters of helium at a pressure of 200 bar is also carried along to pressurize the propulsion systems in orbit. The bi-propellant 400 N motor provides an ISP (specific impulse) of 306 sec and a delta-V of 1150 m/sec; the 0.1 N arcjet with an ISP of 408 to 510 sec produces an additional delta-V of approximately 450 m/sec. Using these data, Viktor Kudielka explored a number of possibilities for maneuvering Phase 3D into its final orbit. To report about these would, however, go beyond this article. The study [available in PostScript format] contains further information and outlines the strategy used to select the orbit for Phase 3D.
Considering all the advantages and disadvantages after a nominal launch we face the following situation:
Assume a launch with a resulting GTO of 6.5 degrees inclination, a perigee of 590 km and an apogee of either 39,122 km (optimal) or 34,122 km (minimal). Three hours after separation from the last rocket stage the onboard computer automatically activates the 70cm telemetry beacon (general beacon, GB). Because of the uncertainty of the spacecraft attitude, omni antennas are employed. At this point in time (see picture) the spacecraft will be over Asia and our command stations in Australia (VK5AGR) and New Zealand (ZL1AOX) will examine the telemetry and establish a connection with the onboard computer to check out the command links. In Europe, Phase 3D will be heard only 4 to 5 hours after launch. In the days following, several important systems will be activated and checked out. The middle telemetry beacon (MB) for the IHU-2 (internal house keeping unit) will be turned on at times; likewise the 9600 baud downlink of the IHU-2 will be activated to relay to the ground camera pictures of the separation as well as data from the acceleration sensors and the microphone. Subsequently RUDAK will be turned on and software loaded to activate the two GPS receivers and to perform first radiation measurements with the CEDEX (Cosmic-Ray Energy Deposition Experiment). After the position sensors and the magnetic position-regulating system have been checked out and calibrated preparations for operating the arcjet can begin.
Depending on the obtainable ISP of the arcjet and the apogee reached by the firing of Ariane 5's second stage the apogee will be raised by the arcjet, through intermittent firings over a period of 270 days, to a range of 60,000 to 70,000 km. The argument of perigee will drift during this time from 178 degrees to about 270 degrees. The arcjet will be fired from one to two hours during perigee. The firings will be automatically computer-controlled since no command station can "see" the satellite during this time. In the past automatic computer control was used for AO-10 and AO-13 even in cases when the satellite was in good view. During this nine-month period there are approximately four months when an unfavorable sun angle will not permit firing of the arcjet. Depending on the available electrical energy, the satellite will then for the first time be available for radio amateurs. After the 270 days there will be another 5 to 6 weeks needed for additional maneuvers with the 400 N motor as described below.
The 400 N motor will raise the inclination to about 63 degrees. This burn takes place at apogee and could simultaneously adjust the perigee to 4000 km. However, raising of the perigee is unnecessary since, caused by perturbations, the perigee will raise to 10,000 km or more in the course of time (10 years).
After the orbital parameters have been accurately determined and further calculations performed, another firing of the 400 N motor will take place to "fine adjust" the inclination. This maneuver will also take place at apogee a few days after the first firing. The desired inclination is 63 degrees +/- 2 degrees depending on the right ascension of the ascending node (RAAN). The goal is a slow drift of the argument of perigee to around 252 degrees as determined by the selected inclination. Thus, over a period of 20 years, the argument of perigee will change from 315 degrees to 270 degrees to 225 degrees. A more stable argument of perigee cannot be obtained under our limitations of the ratio of mass of the satellite to the mass of the propellants.
Finally a further firing of the 400 N motor will reduce the apogee to 47,700 km to obtain the desired 16-hour orbit.
After extensive tests of the position-regulating system and the momentum wheels the spin mode of Phase 3D will be converted to three-axis stabilization. Now the solar panels, which until this time have still been wrapped around the satellite, can be unfolded and full electrical power to operate several high power transmitters will become available.
Provided everything works as planned, Phase 3D will reach a final and stable orbit in less than 12 months and not as previously assumed in almost two years. As mentioned above, Phase 3D can be used during part of this time. However, signals will not compare to the expected strengths achievable with full power and 3-axis stabilization, which will permit use of the gain antennas pointing towards Earth. At the initial argument of perigee of 315 degrees (which corresponds to an AP of 225 degrees ) stations located at the 50th parallel north will experience a visibility of 15 hours per day while stations at the southern 40th parallel still have access for 5 hours per day.
Following the sequence of firings above, the intermediate apogee of 60,000 to 75,000 km appears especially unusual to say the least. Signals will be weak corresponding to this distance; however, this intermediate situation is for a good cause. On getting from A to B, one has a choice. Using "full throttle" one arrives in the shortest period of time. However, a lot of fuel is used (air-drag increases with the 3rd power of speed) and one is in danger of an accident. Going more slowly, less fuel is used, but the trip takes longer. On the other hand one gets to the destination relaxed and safe. Of course, for Phase 3D there is no air resistance! But Viktor Kudielka demonstrated with his calculations that similar rules apply to space and a proper strategy, even with limited resources, can lead to the desired results. Aside from reaching the final stable orbit it was important for the planning that Phase 3D be in no danger should a motor prematurely fail or even should a motor fail during a maneuver. To get to our goal faster, only the removal of several modules to reduce weight would have been an option. But, who wants to make that selection? Better safe than sorry.
Viktor, OE1VKW has provided the following data sets for Phase 3D:
Start: 8-1-2000 23:00:00 UT
Apogee: 39122 km, Perigee 590 km
P3D 001 1 99001U 00099Z 00214.97200000 .00000000 00000-0 00000-0 0 016 2 99001 6.5000 109.0000 7340000 176.0000 0.0000 2.04300000 09
Start: 8-1-2000 23:00:00 UT
Apogee: 34122 km, Perigee 590 km
P3D 002 1 99002U 00099Z 00214.97200000 .00000000 00000-0 00000-0 0 017 2 99002 6.5000 109.0000 7060000 176.0000 0.0000 2.37400000 06
Date: 7 January 2002 (Orbit #600)
P3D 003 1 99003U 00099Z 02007.00000000 .00000000 00000-0 00000-0 0 034 2 99003 63.5000 327.0000 7830000 352.0000 0.0000 1.50000000 6009
Date: 1 July 2013 (Orbit #6900):
P3D 004 1 99004U 00099Z 13182.00000000 .00000000 00000-0 00000-0 0 042 2 99004 74.0000 87.0000 6000000 315.0000 0.0000 1.50000000 69006
Thanks to Peter Guelzow, DB2OS, and Frank Sperber, DL6DBN, for allowing us to post this article, and to Gerd Schrick, WB8IFM, for translating. Comments on this HTML version to kb5mu@amsat.org.
