Russia’s Proton rocket will make its first flight in almost three years on Thursday, as it carries the Elektro-L No.5 weather satellite into orbit. The launch, which is expected to be the last time Proton flies with a Blok-DM upper stage, is scheduled for 08:52 UTC from Site 81/24 at the Baikonur Cosmodrome in Kazakhstan.
Proton, once a workhorse of the Soviet and later Russian space programs, is now very much in the twilight years of its career. First flown in 1965, the rocket has made over 400 launches in various configurations; however, since 2020, it has carried out only a handful of missions as Russia transitions to its replacement, the Angara. Prior to the Elektro-L No.5 launch, Proton’s last flight came in March 2023, when it deployed an Olymp-K spy satellite for the Russian security services. This is the longest gap between launches in Proton’s history, with 2024 marking the first calendar year since its introduction without a single Proton launch.
Thursday’s launch is currently expected to be the last Proton mission with the Blok DM-03 upper stage, and by extension, the wider Blok-D family. Proton’s remaining launches will use either the Briz-M or fly with no upper stage. Variants of the Blok DM-03 remain in production for use with other rockets such as Angara, but Thursday’s launch will mark the end of a relationship between a rocket and a series of upper stages that began in 1967 and enabled many of the Soviet Union’s interplanetary missions and communications satellite launches.
Elektro-L No.5 weather satellite and OChR secondary payload
The primary payload for this launch is Elektro-L No.5, a geostationary weather satellite to be operated by Russia’s federal space agency, Roscosmos. This is the fifth Elektro-L satellite to be launched, following earlier members of the series deployed in 2011, 2015, 2019, and 2023. The first two satellites were launched by Zenit rockets, and subsequent launches used Proton rockets.
A previous Elektro-L satellite, Elektro-L No.4, undergoing launch preparations. (Credit: Roscosmos)
Elektro-L satellites form the geostationary component of Russia’s fleet of weather satellites, complementing the low-Earth orbit (LEO) Meteor series. Operating in geostationary orbit allows each satellite to maintain a constant view of the same part of the Earth’s surface. Constructed by NPO Lavochkin, the satellite has a mass of just over 2,000 kg and is expected to operate for at least 10 years.
The satellite’s primary instrument — Multispectral Scanner – Geostationary (MSU-GS) — images the full disk of the Earth in ten different spectral bands: three visible-light and seven infrared. It has a resolution of up to one kilometer in the visible-light and shorter-wavelength infrared bands, dropping to four kilometers at longer infrared wavelengths. A secondary instrument package, Helio-Geophysical Instrument Complex on Elektro-L (GGAK-E), uses a suite of sensors to measure solar radiation and magnetic fields, helping to forecast space weather and its effects on the Earth.
The Elektro-L satellites also carry two communications payloads: the Onboard Data Sampling System (ODSS) transmits data between Moscow and remote monitoring stations in the far East, while a Geostationary Search and Rescue System (GEOSAR) package relays emergency signals as part of the international COSPAS-SARSAT network.
Proton-M/DM-03 at the launch pad for the Elektro-L No.5 mission. (Credit: Roscosmos)
A second satellite, OChR, is launching alongside Elektro-L No.5 as a secondary payload. No details of this co-passenger have been announced, but its name appears to be an abbreviation of the Russian for “orbit spectrum,” which suggests the satellite will be used as a placeholder to secure Russia’s allocation of communications frequencies or a slot in geostationary orbit intended to be used by a delayed mission in danger of missing its deadline to use them.
Proton-M and Blok-DM upper stage
The Proton-M/DM-03 rocket that will launch Elektro-L No.5 is a four-stage vehicle, consisting of a three-stage Proton-M and a Blok DM-03 upper stage to carry the satellite to its final orbit. Proton-M was first flown in 2001 as an upgraded version of the earlier Proton-K, whose design dated back to the 1960s. Vladimir Chelomei designed the then two-stage UR-500 as an exceptionally large ballistic missile, as part of the Universalnaya Raketa (UR, or Universal Rocket) series developed by his design bureau.
While the UR-500 was too large to be practical as a missile, it was adapted to carry four heavy scientific satellites, the Proton, into orbit between 1965 and 1966. Taking its name from these satellites, the Proton-K first flew in 1967, with modifications to its second stage and a new third stage added to optimize it for orbital missions. The majority of Protons have flown in four-stage configurations, with an upper stage enabling launches to higher or more complex orbits than the core vehicle could reach on its own. When flown without an upper stage, Proton has been used to carry some of the Soviet Union and Russia’s heaviest payloads, including space stations and large space station modules.
Elektro-L No.5 is delivered to Baikonur ahead of its launch. (Credit: Roscosmos)
Two families of upper stages have been used in conjunction with Proton: Blok-D and Briz-M. The Blok-D family uses kerosene and liquid oxygen as propellants, while the Briz-M uses storable hypergolic propellants, as Proton does. Briz-M was first flown in 1999 and has been used on the majority of Proton launches since the mid-2000s, while Blok-D made its debut on the first Proton-K launch in 1967, with Thursday’s launch expected to be its last on Proton, in the form of the significantly-upgraded Blok DM-03.
Blok-D was developed as the fifth stage of the Soviet Union’s N1 moon rocket, and its earliest flights on Proton were in support of the lunar program: tests with the 7K-L1 spacecraft, a stripped-down Soyuz which would have carried cosmonauts on a flyby of the Moon. The Proton-K/Blok-D combination was also used to launch Lunokhod rovers and sample-return missions to the Moon, as well as probes to Mars and Venus.
In 1974, the introduction of the Blok-DM, a Blok-D upgraded and optimised for Earth orbit missions, gave Russia access to geostationary orbit (GEO) for the first time, with Proton beginning to deploy communications and military satellites to GEO over the following years. Shortly after the introduction of the Blok-DM, an upgraded Blok D-1 replaced the Blok-D for interplanetary work.
Blok DM-03 during pre-launch processing for the Elektro-L No.5 mission. (Credit: Roscosmos)
An upgraded Blok DM-2 was introduced in 1982 to enable Proton’s deployment of Uragan satellites for the Soviet GLONASS navigation system, with the DM-2 eventually replacing the Blok-DM for geostationary missions. A corresponding Blok D-2 was used to launch the twin Fobos missions to Mars’s moon Phobos in 1988, and Russia’s ill-fated 1996 Mars mission. In the 1990s, a version of the Blok DM-2 was adapted to use synthetic kerosene as the Blok DM-2M, while the Blok DM-5 was optimised for LEO missions.
Marketed by International Launch Services (ILS), Proton was at the forefront of Russia’s efforts to compete for commercial satellite launches in the 1990s following the fall of the Soviet Union, with Blok-DM variants used before the introduction of the Briz-M. Confusingly, ILS adopted its own designation system for the Blok-DM, which is usually written without a hyphen. The designation Blok DM1 corresponded to the Blok DM-2; DM2 corresponded to the DM-5; and DM3 and DM4 were used for different configurations of the Blok DM-2M.
Once Briz-M was in service, the Blok DM-2 continued to be used for a decreasing number of military launches with older satellites – in particular, Uragan and Uragan-M GLONASS satellites, which were carried in groups of three. The final Blok DM-2 launch took place in 2012 with a US-KMO missile detection satellite, Kosmos 2479.
Proton is integrated ahead of its launch of the fifth Elektro-L satellite. (Credit: Roscosmos)
The Blok DM-03 was developed as a successor to the Blok DM-2 on Proton, but was also designed for use with other rockets — including Zenit and Angara. It suffered an inauspicious start to its career, with the first two Proton-M/DM-03 launches failing to reach orbit, though in both cases this was due to mistakes during pre-launch preparations rather than faulty rockets. The maiden flight took place in December 2010, but technicians familiar with the fueling procedures for the smaller Blok DM-2 loaded an incorrect amount of propellant into the vehicle, rendering it too heavy to make orbit on the programmed flight profile.
The second launch came in July 2013. Accelerometers in Proton’s first stage had been installed incorrectly, causing the onboard computers to receive faulty data on the vehicle’s speed and orientation. The rocket went out of control seconds after liftoff and crashed back to Earth a few miles from the launch pad. Since then, Proton-M has flown five consecutive successful missions with the Blok DM-03, with the Elektro-L No.5 mission marking the eighth flight of this configuration overall.
With the end of Blok-DM’s service on Proton, the stage will continue to fly atop the Angara rocket under the designations Persei and Orion, and may also feature on the future Soyuz-5 rocket, which is expected to make its maiden flight later this year.
Proton arriving at Site 81/24. (Credit: Roscosmos)
Proton’s launch and mission timeline
Thursday’s launch will take place from Site 81/24 at the Baikonur Cosmodrome in Kazakhstan. Site 81 was the first Proton launch complex built at Baikonur, and originally consisted of two pads, although pad 81/23 has not been used since 2005. A second two-pad complex was built at Site 200 in the 1970s, and, like Site 81, it has one pad operational while the other has been decommissioned. It is unclear how much longer Pad 24 will remain operational, as it was reported to be at the end of its operational life in 2019, but has continued to host launches since then.
The launch will begin with the ignition of the six RD-276 engines powering Proton’s first stage. These burn unsymmetrical dimethylhydrazine (UDMH) oxidized by dinitrogen tetroxide, the same propellant mix used on the first three stages of the rocket. The first stage will burn for the first two minutes of flight, as the rocket lifts off and begins its climb through the atmosphere.
Shortly before the first stage burns out, the second stage will ignite its four engines. Proton uses a “hot staging” separation sequence, where the second stage ignites while the first stage is still firing. This ensures the rocket is always accelerating, simplifying the process of delivering propellant to the second stage engines. Once the second stage is up and burning, the spent first stage can be shut down and jettisoned.
Proton’s first stage engines, seen during rollout to the launch pad. (Credit: Roscosmos)
Proton’s second stage engines consist of three RD-0210s and an RD-0211 — the latter being an RD-0210 with additional equipment to provide pressurization to the propellant tanks. The second stage burn is expected to last a little under three and a half minutes, after which it will hand over to the third stage and separate, with Proton’s payload fairing separating a few seconds into the burn. The third stage’s RD-0212 propulsion unit will fire for about four minutes, with its main engine shutting down a few seconds before its vernier (steering) engines and separation from Blok DM-03.
Proton-M will place the Blok DM-03 and Elektro-L No.5 onto a suborbital trajectory. It will coast for about six minutes before Blok DM-03’s RD-58M engine ignites for the first of three burns. Lasting just over a minute, this burn will establish an initial parking orbit in which the rocket and payload will remain for about an hour. At about T+73:00 minutes after launch, Blok-DM’s second burn will begin, with the stage firing for eight minutes to reach geostationary transfer orbit. The mission will then enter an extended coast phase before a third burn begins at T+06:30:00 hours. This will last about four minutes, placing the satellite into geostationary orbit.
Elektro-L No.5 will separate from Blok DM-03 just under eight minutes after the end of the third burn, at T+06:37:49 hours. OChR will separate eight minutes later, marking the end of Blok DM-03’s mission.
The Elektro-L launch ends a span of almost three years since the last Proton launch, which took place on March 12, 2023, the longest gap between launches in Proton’s career. It is currently uncertain when the rocket’s next launch will take place, but several more missions are planned before its retirement.
(Lead image: Proton being transported to the launch pad for the Elektro-L No.5 mission. Credit: Roscosmos)
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