| RD-8 (РД-8) | |
| Country Of Origin: | Soviet Union |
| First Date: | April 13, 1985 |
| Designer: | Yuzhnoye Design Bureau |
| Manufacturer: | Yuzhmash |
| Purpose: | Second Stage Vernier |
| Associated: | Zenit |
| Successor: | RD-809M |
| Status: | In Production |
| Type: | liquid |
| Oxidiser: | LOX |
| Fuel: | RG-1 |
| Mixture Ratio: | 2.4 |
| Cycle: | Staged Combustion |
| Combustion Chamber: | 4 |
| Thrust(Vac): | 78.45kN |
| Chamber Pressure: | 7.747MPa |
| Specific Impulse Vacuum: | 342isp |
| Burn Time: | 1100 s |
| Gimbal: | ±33° |
| Length: | 1500mm |
| Diameter: | 4000mm |
| Dry Weight: | 380kg (840lb) |
| Used In: | Zenit family second stage |
The RD-8 (Russian: Russian: '''РД-8''' and GRAU Index: 11D513) is a Soviet / Ukrainian liquid propellant rocket engine burning LOX and RG-1 (a rocket grade kerosene) in an oxidizer rich staged combustion cycle. It has a four combustion chambers that provide thrust vector control by gimbaling each of the nozzles in a single axis ±33°. It was designed in Dnipropetrovsk by the Yuzhnoye Design Bureau as the vernier thruster of the Zenit (GRAU: 11K77) second stage. As such, it has always been paired with the RD-120 engine for main propulsion.
It can only be started once, and as a high altitude engine it has a thrust of 78.45kN and a specific impulse of 342isp. It is the first ever steering engine to use the staged combustion cycle, and as such is the basis for a family of planned engines for the Mayak launch vehicle family.
The engine itself is built like a hollow cylinder, with a cylindrical space in the center so the RD-120 nozzle can pass through.
Whilst Yuzhnoye's propulsion experience had been mostly on hypergolic propellants engines, like RD-855 or RD-861, they are considered too toxic for current ecological standards. While Yuzhnoye still offer to develop hypergolic propulsion, such as RD-843 for the Vega's AVUM stage or the Tsyklon-4 project, Yuzhnoye selected a more environmentally friendly LOX and kerosene propellant for the Mayak launch vehicle family.
Not only had Yuzhnoye mastered the most complex cycle for the propellant (oxidizer rich staged combustion cycle) with the RD-8, but they had worked closely with NPO Energomash during the RD-120 program. The manufacturing is done at its sister company of Yuzhmash in Dnipropetrovsk, and the RD-120 thrust augmentation project of 2001 to 2003 had been a mixed project between the three companies.
On the base of this experience, a family of derivatives engines were proposed. While the RD-801 and RD-810 are really just based on the general technology, the other members of the family are related enough that they reuse many components of the RD-8. One characteristic of this family is the limitation of keeping the preburner output temperature below 500C.