A satellite that is placed into geosynchronous transfer orbit must perform its own circularisation. An HS-601 consumed 1,175 kilograms of propellant doing this, and hence by the time that it reached its operating station its mass was just 1,800 kilograms.105 While the Block-DM stage of the Proton-K could put 2,600 kilograms directly into geostationary orbit, constraints (such as the delayed shedding of the aerodynamic shroud) requested by the suppliers of western satellites effectively reduced its capacity to 2,100 kilograms. Nevertheless, the fact that the Block-DM could make the circularisation burn meant that the Proton-K could readily deliver an HS-601. In the competitive launch vehicle market, however, this was not good enough. In September 1995, ILS announced that it would introduce the Proton-M in 199 8.106,J07 This would be capable of either inserting 7,600 kilograms into geosynchronous transfer orbit - which would be a 50 per cent increase over the Proton-K - or placing 4,500 kilograms into geostationary orbit. By being able to carry two HS-601 satellites, it would be able compete with the (yet to fly) Ariane V.
A Proton-K lifts off on 6 September 1996 with Intelsat 3F2 for ILS.
Although the first stage was to be improved, the key would be the Briz-M multiple-start hypergolic upper stage that was to supersede the Energiya Block-DM. This was a modified Briz-K stage of the Rokot - a launcher based on the SS-19 intercontinental-range ballistic missile that was was being marketed by Khrunichev and Daimler-Benz Aerospace to put payloads of up to 1 tonne into low orbit with launches from Plesetsk.108 The Briz-M comprised a cylindrical motor section in the middle of a toroidal tankage that could be jettisoned. It was controlled by a closed-loop, triple-redundant guidance system that could be commanded in flight.109,11° An advantage of this stage was that it used the same propellants as the Proton, which would simplify preparations.111,m The enhancements to the main stages were to be the introduction of a digital control system (adapted from that of the Zenit), the uprating of the performance of the RD-253 first-stage engines by 7 per cent, the modification of the feed system to reduce the residual propellant wastage, and the reinforcement of the second and third stages to carry a heavier payload.113 A future option was to establish a facility at a near-equatorial latitude to enable the Proton to compete on equal terms with the Ariane V. However, because this would involve the construction of a wholly new site, such an option, if it were to be pursued, would more likely be in Florida in order to exploit the existing infrastructure.114
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