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Molniya COMSAT Details

THE MOLNIYA COMMUNICATIONS SATELLITES

By Geoffrey E. Perry*

1971-1975

The Molniya communications satellites are placed in highly elliptical orbits

(eccentricity—0.74) with perigee in the southern hemisphere. The orbital period is some 3.3 minutes less than 12 hours so that, despite the very small precession of the orbital plane and the motion of the Earth around the Sun, the ground-track repeats itself, albeit a few minutes earlier each day. Such an orbit, with repetitive ground tracks, may be said to be stabilized.

Initially all Molniya satellites were launched from Tyuratam into orbits inclined at

65.0° to the Equator. Dr. E. R. Allan, of the RAE, has shown that the 4th Molniya 1 was the first to use a stabilized ground-track. (1) The 5th Molniya 1 was not stabilized and the 6th and 7th only partly stabilized. Thereafter the correction engine has been used to stabilize all Molniya ground-tracks.

The initial method of achieving a stabilized ground-track was to place the satellite

into an intermediate orbit with period of the order of 700 to 710 minutes. This resulted in an eastward drift of ground-track of between 3° and 8° per day. When the ground track had drifted to the desired position, a firing of the correction engine at perigee raised apogee to produce the stabilized orbit. The times elapsing between launch and stabilization ranged from 8 to 20 days.

The launch of the 13th Molniya 1 signaled a move to the Plesetsk launch site and orbital inclinations of 65.4°. A similar stabilization technique was employed. The change of site was a transfer of routine launches from Tyuratam rather than to reduce the time delay between launch and stabilization of ground track. The 15th Molniya 1 was not ground-track-stabilized until the 60th revolution, 30 days after launch.

All Molniya 2 satellites have originated from Plesetsk, as have the four Molniya 3's, and only four later Molniya 1's have been launched from Tyuratam and these during winter months.

The 7th Molniya 2 satellite and all subsequent Plesetsk-launched Molniya's have flown at an orbital inclination of 62.9°. The change in inclination resulted in a new stabilization pattern. The satellite is placed into an intermediate orbit with a period of between 730 and 740 minutes producing a westerly drift of ground-track of around 10° per day. This has permitted stabilization by lowering apogee within 4 or 5 days of the launch.

The early history of the Molniya satellites has been described by Allan. (2) who pointed out that the 8th, 9th, and 10th Molniya 1's constituted a three-satellite system with orbital planes spaced at approximately 120° intervals providing a complete 24-hour coverage of the U.S.S.R. Each satellite repeated the ground track of its predecessor some 8 hours later in time. He also speculated that the 11 th and 12 th Molniya 1's might be the pioneers of a four-satellite system with orbital planes spaced 90° apart and satellites following each other at 6 hour intervals.

In 1974, Philip Perkins and Geoffrey Perry, of Kettering Grammar School , developed a simple graphical method for monitoring the operational status of Molniya satellites in the Orbita system. Using data supplied by the Goddard Space Flight Center, they plotted the values of the right ascension of the ascending nodes against date and obtained a series of four straight lines spaced at 90° intervals, confirming Allan's hypothesis. (3) From this they were able to trace the history of replacement of the Molniya satellites as they reached the ends of their active lives. Labeling the four groups of satellites from A through D, the replacement history of the Molniya 1's is seen.

The 1st Molniya 2 was positioned approximately mid-way between groups A and D of the Molniya 1's and, when the 2nd Molniya 2 was positioned mid-way between groups A and B, the possibility arose that the Molniya 2's were a supplementary system to fill gaps in the existing system. However the 3rd Molniya 2, between groups B and C, was spaced 120° away from the 2nd; i.e., nearer to group C. Although these first three Molniya 2's did not join the Molniya 1 groups, thereafter, all Molniya 2 launches have added their payloads to those groups. The sequence of replacement is given in the following table:

When the Molniya 3 satellites were introduced they joined groups C, D, A and B in turn over a 13 month period. The orbit of the fourth and latest in the series was stabilized on January 1, 1976 . The close proximity of the members of each group can be seen from the times and longitudes of their ascending nodes as of January 2, 1976 .

Consideration of these times and longitudes provides an explanation for the replacement of the 13th Molniya 2 by the 14th after only two months. Presumably launched to ensure optimum communications during the Apollo-Soyuz mission in the following week, the orbit was never properly stabilized. The orbital period was lowered to 718.6 minutes but no further correction was made. Consequently, although still in the same orbital plane as the other members of Group A, it fell further and further behind in time and consequently drifted off. station in a westerly direction. By January 2, 1976 it was crossing the Equator 198 minutes after its replacement and at 7°E. By then, luni-solar perturbations had reduced the period to 717 • 5 minutes and the ground-track was temporarily stabilized, although in the wrong position.

As has already been mentioned in another section of this report, certain satellites in the Kosmos series have flown in orbits similar to those of the Molniya satellites. With the exception of Kosmos 41 which was, most probably, an engineering test as part of the R & D leading to the establishment of the Orbita system, it was at one time tempting to regard such satellites as Molniya failures. Whilst it would appear from consideration of orbital plane spacing that the 6th Molniya 1 replaced Kosmos 174 within 33 days of its launch (4) and that Kosmos 260 could have been a replacement for the 8th Molniya 1, such an assumption is not valid in the cases of Kosmos satellites launched since 1972. Perry pointed out that the orbital plane of Kosmos 706 does not coincide with any of the four groups.(5) Neither do the planes of Kosmos 520, 606, and 665, although the planes of Kosmos 520 and 606 are close enough to each other for one to have been a replacement for the other.

References:

(A) SOVIET SPACE PROGRAMS, 1971-75, OVERVIEW, FACILITIES AND HARDWARE MANNED AND UNMANNED FLIGHT PROGRAMS, BIOASTRONAUTICS CIVIL AND MILITARY APPLICATIONS PROJECTIONS OF FUTURE PLANS, STAFF REPORT , THE COMMITTEE ON AERONAUTICAL AND SPACE .SCIENCES, UNITED STATES SENATE, BY THE SCIENCE POLICY RESEARCH DIVISION CONGRESSIONAL RESEARCH SERVICE, THE LIBRARY OF CONGRESS, VOLUME – I, AUGUST 30, 1976, GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976.

2. Ibid.

3. Perkins, P. J. and G. E. Perry, Flight International, London . 107, 79, 16 January 1975 .

4. Flight International London , 92, 628, 12 October 1967 .

5. Perry, G. E., Fight International, London , 107, 686, 24 April 1975 .

* Mr. Perry is senior science master at the Kettering Grammar School , Kettering , England .

* Allan. R. R., The operation of Molniya communications satellites, Tech. Report 69266, R.A.E., November 1969.