For the past several months, the topic of rocket engines has been prominent in the space media. Most recently, Russia has threatened to stop selling its rocket engines to the U.S. for use on the Atlas V launch vehicle.

This is in response to sanctions placed on Russian due to its activities in eastern Ukraine. For those of you who are not "rocket scientists" a brief primer is presented here on why rocket engines are fundamental to space flight and how the Russian actions may affect U.S. launch vehicle capabilities.

It was a Russian scientist, Konstantin Eduardovich Tsiolkovsky, who contributed to a basic understanding of rocket engines for space flight. Tsiolkovsky, a leading Russian developer of early astronautical theories, was born in 1857 and died in 1935.

We generally think of three founding fathers of modern astronautics and rocketry: Hermann Oberth; Robert Goddard and Tsiolkovsky. However, Konstantin is credited with deriving the fundamental and underlying relationship that tells us how propellant mass and change-in-velocity of a space vehicle are connected.

This is usually referred to as the rocket equation. Although not precisely accurate, it does offer a quick and insightful calculation of propellant mass requirements for given space mission objectives. And, it is particularly effective for large and small chemical rocket engines such as those in use on launch vehicles and satellites.

Unlike a jet engine, a rocket engine is a device that imparts acceleration to itself and the vehicle to which it is attached by expelling part of its mass and without the help of the local environment. Jet engines are used on vehicles that carry fuel but require the availability of oxygen from the surrounding environment, while a rocket must carry its own oxidizer and fuel.

This characteristic allows the production of thrust outside of the atmosphere. Thus, a typical rocket engine creates thrust by expelling part of its mass at high speed. Simply put, the constrained performance of a conventional rocket engine is characterized by the rocket equation.

Launch vehicles carry and employ chemical rocket engines, which take advantage of the energy release resulting from chemical reactions of the two propellants as they mix and burn. The resulting thermal energy is converted into kinetic energy as the products of combustion pass through a converging-diverging nozzle and are exhausted into space at high velocity. Many spacecraft also use chemical rockets for in-orbit maneuvering.

Some satellites now incorporate more-exotic propulsive devises such as electric thrust-producing motors. Nevertheless, thrust is still produced by expelling part of the vehicle's mass.

It is worth noting that the rocket equation is usually associated with Tsiolkovsky, who did independently derive and published it in his 1903 work. But,theequation apparently had been derived earlier by a British mathematician, William Moore, in 1813. Nevertheless, it was Tsiolkovsky who first applied it to space flight.

The Russians have been developing very reliable liquid rocket engines since the 1950s and these have proven cost-effective and efficient for many launch vehicles. One engine, in particular, has attracted attention for use on the U.S. Atlas V launch vehicle. This is the RD-180, developed in the 1990s by NPO Energomash. Roots of this engine technology go back to the 1960s, when the USSR was trying to develop a lunar launch vehicle.

In 1974, all work on the rocket was shut down and all documents and hardware were to be destroyed. However, a bureaucrat secretly stored the engine hardware and technology in a warehouse. Eventually, the U.S. learned of this, and the RD-180 technology came to America.

Today, Russia provides existing engine hardware to Pratt and Whitney and this is then prepared and tested for use on the Atlas V. The RD-180 is a critical element in the Atlas V. Replacing the engine would require the expensive development of a new engine, result in significant design changes and recertification of the launch vehicle, and would likely cause launch schedule delays for important payloads.