F 
R

9
- 
9 
8

---

Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 9/98

HGS-1 SALVAGE SUCCESSFUL AFTER MOON FLY-BYS

A total loss. That's how it first looked when on 25 December 1997 the upper stage of a Proton launch rocket failed. The upper stage engine, which was designed to burn for 120 seconds, shut off after just one second. The result: The communications satellite AsiaSat-3 stranded in a highly elliptical, useless orbit (350 x 36,000 km) with an inclination of 51 degrees.

The possibility of correcting the satellites orbit by using the built in apogee engine was soon rejected as the entire fuel supply would have to be used to acomplish this.

According to American media reports three weeks after AsiaSat-3 stranded in orbit, two space experts approached Hughes Space & Communications, the American manufacturer of the satellite. Their idea was to send the probe on a journey into space. In an area where the gravitational forces of earth, sun and moon are balanced, big orbital changes can be achieved with very little fuel.

However, Hughes rejected this alternative as well, because it would have led the satellite millions of kilometres away from earth and out of save tracking and communication distance.

This idea, however, was the catalyst for the successful plan, to change the orbit of the satellite by using the gravitational forces of the moon. Hughes Global Service (HGS), a subsidiary of Hughes Space, took over the task.

Meanwhile, a consortium of 27 insurance companies had paid AsiaSat the insurance claim of $200 million. HGS came to an agreement with the insurance companies, took over the title of the satellite and re-named it to HGS-1. In return HGS was to take over the marketing of the satellite service if HGS-1 was successfully brought into a useable earth orbit. The agreement also sees that the financial gains from any future marketing have to be shared with the insurance companies.

The exact flight path and times when the HGS-1's main engine, a Marquardt R4D-11,300 (490 N thrust), was to be fired for the journey around the moon had yet to be calculated. For this, HGS used the Satellite Tool Kit software from Analytical Graphics (AGI) in Malvern, Pennsylvania. Interestingly this program is available and can be downloaded free of charge and without user restrictions from the AGI web site (http://www.stk.com).

On 10 April a series of twelve firings of the HGS-1 main engine was initiated. The furthest point of the orbit was extended to 321,000 km. The last of the twelve firings was the trans lunar injection and sent HGS-1 towards the moon. The journey took almost six days.

On 13 May HGS-1 passed the moon at a distance of about 6,200km. The moon's gravitational force and its course inclination towards earth had the desired effect in that the HGS-1 flew a three-dimensional figure eight around the moon before it went on course towards earth again.

As a result of the darting about in space, the HGS-1's inclination to the earth's equator was reduced from 51 to 18 degrees. Also, the satellites perigee was increased from 350 to 42,061 km.

Encouraged by the success of the first flyby, HGS decided to send the satellite around the moon for a second time. Even an inclination of 18 degrees meant that the satellite would have a strong North-South drift in a geo-synchronous Earth orbit. This inclination requires an extremely exact (and expensive) ground antenna to track the satellite. However, only few potential customers, the US military for example, own such aerials. A further reduction of the inclination was going to make the use of simpler ground antennae possible and thus increase the number of potential customers.

The second flyby required only a little more fuel, because HGS-1's orbit already had the correct apogee. On 6 June HGS-1 passed the moon for a second time, this time at a height of 43,000km. After this manoeuver, the inclination was reduced by a further four degrees. Another four to six degrees were achieved by several firings of the engine on the final leg of HGS-1 to Earth. The first firing lasted 46 minutes and reduced the orbit around earth to 36,000 x 82,000 km. After two further manoeuvres on 17 June HGS-1 was stabilised in a geo-synchronous Earth orbit at a height of 36,000 km. The satellite currently has an inclination only about eight to nine degrees.

The success of this mission depended very much on the design of the satellite. It was possible to keep HGS-1 in a spinstabilized state. The satellite turned ten times per minute around its own axis. During the salvage manouever, the electronic payload was switched off. The solar panels were kept in a stowed position. Only the visible outer panels generated energy. The restartable liquid fuel engine was also an important factor in the unique mission: HGS-1 is the first commercial satellite that flew around the moon.

During its moon mission HGS-1 used up most of its fuel supply. However, according to Hughes there is enough left to keep the satellite on its East-West position for the predicted life time of 15 years.

As mentioned before the North-South drift caused by the remaining inclination can be coped with the help of a ground antenna. There is no customer yet for the revitalized satellite. Hughes is optimistic that it won't take long.

From page 50 of FLUG REVUE 9/98