"Pulse detonation is a hot topic in combustion research," says Gabriel Roy of the Office of Naval Research. "Compared with gas turbines, the PDE has a much simpler configuration. It has the capability of going from subsonic to supersonic using less fuel, and it's thermodynamically more efficient. But there are big engineering issues—thermal fatigue, noise. It's very challenging research."
The concept behind the PDE is deceptively simple. In short, there are two kinds of combustion: the old, familiar, slow kind of burning, called deflagration, and another, much more energetic process called detonation, which is a different animal entirely. Imagine a tube, closed at one end and filled with a mixture of fuel and air. A spark ignites the fuel at the closed end, and a combustion reaction propagates down the tube. In deflagration—even in "fast flame" situations ordinarily called explosions—that reaction moves at tens of meters per second at most. But in detonation, a supersonic shock wave slams down the tube at thousands of meters per second, close to Mach 5, compressing and igniting fuel and air almost instantaneously in a narrow, high-pressure, heat-release zone.

Source:Popular Science | After Combustion: Detonation!
Text by Jim Kelly.

other types:

Laser Propulsion Research

Here's how it works: the rear of the lightcraft is a highly polished parabolic mirror, designed to focus laser beams into it's engine. The only "fuel" this ring-shaped engine requires is ordinary air. When the air inside the chamber is hit by the highly focused laser beam, it bursts into super-heated, luminous plasma, with a momentary temperature as high as thirty thousand degrees, kelvin -- almost five and a half times as hot as the surface of the sun!
This plasma produces the thrust needed to lift the ship into the air. The lightcraft is currently powered by a ground based ten kilowatt carbon dioxide laser, which tracks and fires laser pulses into the base of the craft.
The lightcraft technology was created by Dr. Leik Myrabo (leek my-raboh), in conjunction with the United States Airforce and NASA. The laser powered craft was scheduled for a series of test flights at the White Sands missile range in New Mexico. The concept worked in the lab simulations, but how well would the light-powered craft fly under real life conditions?
The first challenge was how to get the craft to function without a guide wire keeping it in laser range. The researchers took a tip from its top-like shape, and temporarily turned it into a gyroscope! "In order to keep it stable and the laser boost, we have spun it up like a child's top to about 3000 revolutions per minute".

Dr. Myrabo has high hopes for light powered technology.
"Beamed energy propulsion has the possibility of becoming a future way of getting around the planet instead of jumbo jets. I believe that we may be able to have vehicles come to your house to pick you up, pick up a couple of other people in their neighborhoods, go up to a beam boost station and blast off around the planet, being at the farthest point of the planet in 45 minutes. So this could revolutionize the way we travel here on earth."

may achieve exhaust velocities of more than 100 km/s

Electric Thrusters
An electric thruster is a device that uses electrostatic force as the primary means of propulsion.

Electron Bombardment Thrusters
An electron bombardment thruster is a type of electric thruster. This thruster uses electrons to generate ions. When a fast moving, energetic electron collides with a propellant atom, it knocks out a second electron, thereby converting the atom to a positive ion. This positive ion can then be used for propulsion.

Ion Thrusters
An ion thruster is a type of electron bombardment thruster. Pictured above, the ion thruster uses a cathode to generate electrons. These electrons fly into the thruster's discharge chamber and collide with propellant atoms. These atoms are then ionized. After the propellant atoms are ionized, they see a large negative potential formed by the ion optics (or grids). Since opposite charges attract, the ions race toward the optics at speeds as high as 10,000 m/s (10 times faster than chemical rockets). After the ions pass through the optics they collide with electrons emitted by the neutralizer. These electrons combine with the ions to form neutral atoms. This is done because the ions would eventually be re-attracted to the grids and spacecraft due to the negative optics.

Text source: NASA Glen Research Center
Images source: DS1 Archives

Ion Propulsion
- Ion propulsion is a technology that involves ionizing a gas to propel a craft. Instead of a spacecraft being propelled with standard chemicals, the gas xenon (which is like neon or helium, but heavier) is given an electrical charge, or ionized. It is then electrically accelerated to a speed of about 30 km/second. When xenon ions are emitted at such high speed as exhaust from a spacecraft, they push the spacecraft in the opposite direction.
- The ion propulsion system (IPS), provided by NSTAR (NASA SEP Technology Application Readiness), uses a hollow cathode to produce electrons to collisionally ionize xenon. The Xe+ is electrostatically accelerated through a potential of up to 1280 V and emitted from the 30-cm thruster through a molybdenum grid. A separate electron beam is emitted to produce a neutral plasma beam. The power processing unit (PPU) of the IPS can accept as much as 2.5 kW, corresponding to a peak thruster operating power of 2.3 kW and a thrust of 92 mN. Throttling is achieved by balancing thruster and Xe feed system parameters at lower power levels, and at the lowest thruster power, 500 W, the thrust is 20 mN. The specific impulse decreases from 3100 s at high power to 1900 s at the minimum throttle level.

- The DS1 ion propulsion engine only produces 92 mN (milli Newtons) of thrust which is roughly equivalent to the weight (on Earth) of a couple drops of water. (Deep Space One is a spacecraft that launched on October 24, 1998.)
- Ion engines are vastly different from chemical (solid, liquid) engines in that they are low thrust engines which can run for extended periods of time. The length of use of chemical engines is usually from seconds to days while the length of use of ion engines can be anywhere from days to months.

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