Rocket Propulsion [top]
Rocket propulsion is different from other forms of accelerating an object because the entire acceleration system is contained within the object. A runner makes use of the ground to accelerate; by pushing off of it with her foot. A car uses the road to accelerate; by the friction of wheels against the asphalt. A jet uses the air to accelerate; by pulling air through a turbine. A rocket makes itself go entirely by expelling its own mass backwards. A rocket does not need anything to push against or pull on. A sailor throwing cannonballs off the back of a ship is using the same principle; his craft will accelerate forward with each throw.
There are several types of rocket propulsion. The most common and well-known is called chemical or conventional rocket propulsion. A chemical rocket relies on a highly explosive chemical reaction inside a tube to force fuel mass downward and the rocket up. NASA's Space Shuttle and the Saturn rockets used in the Apollo missions are all entirely chemical systems. The liftoff of Apollo 15 is shown above right.
Another type is electric propulsion. An electric rocket uses electrical power to accelerate non-volatile propellant out the back of the engine.
Thrust and Specific Impulse [top]
The two most important characteristics of a rocket engine are its thrust and its specific impulse. Thrust is a measure of how much force the rocket can exert. In order to take off from Earth's surface, a rocket's thrust must be greater than the force of gravity upon it. In engineering terms, the thrust-to-weight ratio must be greater than one.
Specific impulse can be thought of as a rocket's "gas mileage". It measures the amount of speed obtainable from a unit mass of propellant. It is related to the speed of the rocket's exhaust.
Chemical rockets are useful for their extremely high thrust. For example, a single engine of the Saturn-V rocket has 6,800,000 N of thrust at 300 s specific impulse. Currently, chemical rockets are the only propulsion technology that has been used to reach Earth orbit. No, as yet conceived of, electric thruster has a thrust-to-weight ratio greater than one. However, electric thrusters have much greater specific impulse than conventional rockets, and can therefore ultimately propel a spacecraft to much greater speeds using much less propellant mass once the craft has reached Earth orbit.
The table below compares the thrust and specific impulse of several different propulsion systems.
|Specific Impulse (s)||Propellant|
|PPS-1350 Hall Thruster
|NSTAR Ion Engine
(Deep Space 1)
|NEXT Ion Engine
|VASIMR® VX-200||200||5||5000||Ar (Optional:
D, N, Xe)
VASIMR® Engine [top]
The VAriable Specific Impulse Magnetoplasma Rocket (VASIMR®) engine is a new type of electric thruster with many unique advantages. In a VASIMR® engine, gas such as argon, xenon, or hydrogen is injected into a tube surrounded by a magnet and a series of two radio wave (RF) antennas (called "couplers" in this context). The couplers turn cold gas into superheated plasma and the expanding magnetic field at the end of the rocket (the magnetic nozzle) converts the plasma particles' thermal motion into directed flow.
Principles of Operation [top]
The primary purpose of the first RF coupler is to convert gas into plasma by ionizing it, or knocking an electron loose from each gas atom. It is known as the helicon section, because its coupler is shaped in such a way that it can ionize gas by launching helical waves through the gas. Helicon antennae are a common method of generating plasma. See the Further Reading section for more about helicons.
After the helicon section, the gas is now a "cold plasma", even though its temperature approaches that of the surface of the Sun. The soup of electrons and the atoms they were stripped from (ions) is primed for acceleration in the second stage. Where there were neutral gas atoms, there are now ions and electrons, which are charged, and such moving charged particles interact with magnetic fields. The magnetic field can be visualized as lines passing through the rocket with ions orbiting around each line.
The second RF coupler is called the Ion Cyclotron Heating (ICH) section. ICH is a technique used in fusion experiments to heat plasma to temperatures on the order of those in the Sun's core. The radio waves hit ions and electrons along their orbits around field lines at resonance, similar to a person pushing another on a swing, resulting in accelerated motion and higher temperature. The VASIMR® ICH section produces plasma hotter than one million degrees Kelvin, or two hundred times the temperature of the Sun's surface.
Thermal motion of ions around field lines is mostly perpendicular to the rocket's direction of travel, however, and can't help its propulsion. The rocket is dependent on its magnetic nozzle to convert the ions orbital momentum into useful linear momentum. As the magnetic field lines expand, the spiral paths of the ions around their field lines elongate, resulting in ion speeds on the order of 100,000 mph (50,000 m/s).
VASIMR® Engine Compared to Other Electric Thrusters [top]
The VASIMR® engine has three important features that distinguish it from other plasma propulsion systems:
VASIMR® engine has the ability to vary its exhaust parameters (thrust and specific impulse) in order to optimally match mission requirements, which results in the lowest trip time with the highest delivered payload for a given fuel load.
VASIMR® thruster uses electromagnetic (RF) waves to create and energize the plasma within its core. In this way, VASIMR® engine has no physical material electrodes in contact with the hot plasma. This results in greater reliability and longer life and enables a much higher power density than competing ion thruster and plasma rocket designs.
VASIMR® thruster is able to process a large amount of power, meaning that it can then generate a larger amount of thrust. This larger thrust capability promises to make VASIMR® engine useful for moving large payloads around in low Earth orbit, transferring payloads from the Earth to the Moon, and transferring payloads from the Earth to the outer solar system. VASIMR® technology is also highly scalable, meaning that higher power versions can be easily designed, making human missions using electric propulsion a reality.
Power Sources [top]
One of the key challenges in developing VASIMR® engine is supplying power to it. A high-power electric thruster requires a lot of electricity, and generating that in space may require some engineering innovations. Below is a discussion of two options.
Solar power can be efficiently used for near-Earth VASIMR® missions, such as drag compensation for space stations, lunar cargo transport and in-space refueling. Recent advances in solar array technology promises increases in solar power utilization by an order of magnitude.
A nuclear reactor has a very large amount of energy per unit mass, in fact a reactor core has the highest energy density of any useful energy source on earth. This high energy density and scalability make nuclear reactors an ideal power source in space. A nuclear electric powered spacecraft could dramatically shorten human transit times between planets (less than 3 months to Mars) and propel robotic cargo missions with a very large payload mass fraction. Trip times and payload mass are major limitations of conventional and nuclear thermal rockets because of their inherently low specific impulse (less than 1000 seconds). A VASIMR® propelled, nuclear powered spacecraft promises to make fast human missions a reality.