RD-701

The RD-701 (Russian: Раке́тный дви́гатель 701, Rocket Engine 701) was a liquid-fuel rocket engine proposed by Energomash, Russia (USSR at that time). It was briefly proposed to propel the reusable MAKS space plane, but the project was cancelled shortly before the end of USSR. The RD-701 would have been a tripropellant engine that used a staged combustion cycle with afterburning of oxidizer-rich hot turbine gas. The RD-701 would have had two modes. Mode 1 used three components: LOX as an oxidizer and a fuel mixture of RP-1 / LH2 which is used in the lower atmosphere. Mode 2 would have also used LOX, with LH2 as fuel in vacuum where atmospheric influence is negligible.

RD-701 (РД-701)

Country of origin	USSR/Russia
Date	late 1980s
Manufacturer	Energomash
Successor	RD-704
Status	Experimental
Liquid-fuel engine
Propellant	Mode 1: LOX / LH2&RP-1; Mode 2: LOX / LH2
Cycle	staged combustion
Performance
Thrust, vacuum	Mode 1: 900,000 lbf (4 MN) Mode 2: 357,000 lbf (1.6 MN)
Thrust, sea-level	Mode 1: 714,000 lbf (3.2 MN)[1]
Chamber pressure	Mode 1: 30 MPa (4,400 psi) Mode 2: 15 MPa (2,200 psi)
Specific impulse, vacuum	Mode 1: 415 s (4.07 km/s) Mode 2: 460 s (4.5 km/s)
Specific impulse, sea-level	Mode 1: 330 s (3.2 km/s)
Dimensions
Diameter	94 in (2,400 mm) (1 nozzle)
Dry mass	4,240 lb (1,923 kg)[1]

The use of less dense fuel components at maximum efficiency conditions allows minimizing the volume of fuel tanks and subsequently their mass down to 30%. The RD-701 proposal was developed into another proposal, the RD-704, which featured a single combustion chamber.

Contrary to information presented in some English-language sources, the RD-701 was never constructed or tested, although a smaller test-stand engine was built and tested as part of development.^[2]

Specifications

The 'RD-701' is a two chamber engine. It can operate in two modes. In the first, it burns both kerosene and hydrogen, granting it greater thrust and impulse density, allowing for smaller, lighter tanks. In the second, it burns only hydrogen, allowing for greater specific impulse. The RD-701 has two preburners per combustion chamber, both of which run oxygen rich. One is used to pump kerosene and oxygen, the other is used to pump hydrogen and oxygen.^[3]

In mode one, both burn kerosene. Liquid hydrogen is combined with kerosene and oxygen rich preburner exhaust in the combustion chamber. In this mode, it produces 4 MN of thrust, with a vacuum specific impulse of 415s, using 73.7 kg/s (12.6% of propellant mass) of kerosene, 29.5 kg/s (6% of propellant mass) of hydrogen, and 388.4 kg/s (81.4% of propellant mass) of oxygen.

In mode two, the preburner used to pump kerosene is shut down, and the other is switched to burning hydrogen. A small amount of kerosene is still used to ensure oxygen is effectively atomised. Gases from the preburner combined with hydrogen in the combustion chamber. In this mode it burns 27.4 kg/s of hydrogen, and 148 kg/s of oxygen. Thrust reduces to 1.6 MN, and specific impulse increases to 460s.^[4]

See also

• MAKS space plane
• Staged combustion cycle
• RD-170 - RP-1/LOX Russian engine
• RD-0120 - LH2/LOX Russian engine