Fregat

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Fregat
Phobos Grunt base section model.jpg
Model of the Fregat-SB, as configured for the Fobos-Grunt mission's sample return vehicle
Manufacturer NPO Lavochkin
Country of origin Russia
Used on Soyuz, Zenit
General Characteristics
Height 1.5 m (4 ft 11 in)[1]
Diameter 3.35 m (11.0 ft)
Mass 6,500 kg (14,300 lb)
Propellant mass 5,350 kg (11,790 lb)
Engine details
Engines 1 S5.92
Thrust 20 kN (2,000 kgf)
Specific impulse 327 seconds (3.21 km/s)
Burn time 1,350 seconds[1]
Fuel N2O4/UDMH


Fregat (Russian: Фрегат, frigate) is an upper stage developed by NPO Lavochkin in the 1990s, which is used in some Soyuz and Zenit rockets. Its liquid propellant engine uses UDMH and N2O4.

Capabilities[edit]

The stage can be restarted up to 25 times,[1] which has been used to place payloads such as the GIOVE-B satellite into their planned orbits.[2]

Uses[edit]

NPO Lavochkin has built many interplanetary probes, and the Fregat stage follows their design traditions. The main part of the stage is six intersecting spheres placed on a single plane, four of which contain propellants. The remaining two contain the control equipment. The main engine is placed between the spheres, so Fregat is a tightly-packed stage with a diameter much larger than its height.

Fregat is a versatile spacecraft: for example, in addition to orbital insertion, it can be used as an escape stage to send modern space probes into interplanetary trajectories (e.g. Venus Express and Mars Express).

Fregat-SB[edit]

Fregat stages are currently used as the fourth stage of some Soyuz-FG launch vehicles. A version called Fregat-SB can be used with Zenit-2SB rocket. This version has a block of drop-off tanks ("SBB" or Сбрасываемый Блок Баков in Russian) which makes increased payload capability possible. The torus-shaped SBB weighs 360 kilograms (790 lb) and contains up to 3,050 kilograms (6,720 lb) of propellant. The total dry weight of the Fregat-SB (including SBB) is 1,410 kilograms (3,110 lb) and the maximum propellant carrying capacity is 10,150 kilograms (22,380 lb). [3]

Fregat-SB was launched for the first time on 20 January 2011, when it lifted the Elektro-L weather satellite into geosynchronous orbit.[4]

Fregat-M/Fregat-MT[edit]

Like the Fregat-SB, the Fregat-M/Fregat-MT configurations have detachable tanks on an additional block assembly which could be jettisoned when their task was complete. Additionally, the Fregat-M/Fregat-MT tanks have ball-shaped additions on the tops of the tanks. These additions increase the load capability of the propellant from 5,350 kilograms (11,790 lb) to 6,640 kilograms (14,640 lb), without causing any other changes to the physical dimensions of the vehicle.[5]

August 2014 failure[edit]

The Arianespace-operated flight of a Fregat MT ended in failure on 22 August 2014 after the vehicle deposited two EU/ESA Galileo navigation satellites into the wrong orbit. The lift off at 1227 GMT from the Sinnamary launch site near Kourou, French Guiana, appeared to go well. However, a failure was only apparent later when, after the second firing of the Fregat MT upper stage had taken place, the satellites were detected as being in the wrong orbit.[6]

The Independent Inquiry Board formed to analyze the causes of the "anomaly" announced its definitive conclusions on October 7, 2014 following a meeting at Arianespace headquarters in Evry, near Paris.[7]

The failure occurred during the flight of the Fregat fourth stage. It occurred about 35 minutes after liftoff, at the beginning of the ballistic phase preceding the second ignition of this stage.

The scenario that led to an error in the orbital injection of the satellites was precisely reconstructed, as follows:

  • The orbital error resulted from an error in the thrust orientation of the main engine on the Fregat stage during its second powered phase.
  • This orientation error was the result of the loss of inertial reference for the stage.
  • This loss occurred when the stage's inertial system operated outside its authorized operating envelope, an excursion that was caused by the failure of two of Fregat's attitude control thrusters during the preceding ballistic phase.
  • This failure was due to a temporary interruption of the joint hydrazine propellant supply to these thrusters.
  • The interruption in the flow was caused by freezing of the hydrazine.
  • The freezing resulted from the proximity of hydrazine and cold helium feed lines, these lines being connected by the same support structure, which acted as a thermal bridge.
  • Ambiguities in the design documents allowed the installation of this type of thermal "bridge" between the two lines. In fact, such bridges have also been seen on other Fregat stages now under production at NPO Lavochkin.
  • The design ambiguity is the result of not taking into account the relevant thermal transfers during the thermal analyses of the stage system design.

The root cause of the failure of flight VS09 is therefore a shortcoming in the system thermal analysis performed during stage design, and not an operator error during stage assembly.[7]

References[edit]

External links[edit]