Model of the Fregat-SB, as configured for the Fobos-Grunt
Fregat (Russian: Фрегат, frigate) is a type of rocket stage developed by NPO Lavochkin in the 1990s. Its main engine is a liquid propellant rocket that uses UDMH and N2O4 as propellants.
The dry mass of a Fregat (as of October 2006) is 924 kg; with fuel the mass is up to 5350 kg. Current modification of the main engine has a specific impulse of 332 s and a thrust of 2018 kg. The reaction control system includes 12 engines, each having 5 kg thrust, 225 s Isp, using pure hydrazine, with mass up to 42 kg. Height: 1,500 mm, Diameter 3,350 mm, Fueled Mass: 6,415-6,535 kg., Firing Duration: 1350 seconds
The stage can be restarted multiple times, which has been used to place payloads such as the GIOVE-B satellite into their planned orbits.
NPO Lavochkin created many interplanetary probes in the past, 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 launch modern space probes into interplanetary trajectories (e.g., Venus Express and Mars Express).
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 which makes increased payload capability possible. Fregat-SB was launched for the first time on 20 January 2011, when it lifted the Elektro-L weather satellite into geosynchronous orbit.
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