|Fig. 1: Image of the Orion Project propulsion device (Source: Wikimedia Commons)|
The External Pulsed Plasma Propulsion Engine is a concept that has been around since the late 1950s. It was one of the original solutions to thruster materials overheating. The inner walls of rocket engines get very hot, so some of the rocket fuel must be used to cool the walls using convection currents. Without that cooling, the rocket walls would disintegrate or vaporize. This results in inefficient rockets - which compounds as an issue with the extra financial costs associated with more fuel to continuously cool the inner walls of the thrusters. 
The External Pulsed Plasma Propulsion Engine (EPPP) and its underlying principles have recently been rediscovered from work done by nuclear physicists in the early 60s as a part of project ORION. This engine is one of the most promising methods of propulsion for travelling past mars on manned missions. Existing modern-day engines based on fusion principles, antimatter, or beamed energy propulsion have significant scientific issues that need to be addressed before a serious prototype can be developed. [1,2] However, the EPPP is significantly cheaper and, more importantly, does not require major scientific breakthroughs to be realized as an actual prototype. 
The concept behind this engine is fairly simple. The limiting factor of traditional rockets is that any thrust that gets produced needs to be inside a confinement area. However, the issue is that any material that makes up the confinement walls will reach its melting temperature. In Fig. 1, you can see the Orion Project's initial design based on a thrust propulsion system using controlled nuclear detonations. In the EPPP design, everything revolves around impulse. In the EPPP engine, everyday materials can work as confinement container materials. How can this be? Because of the principle of impulse. All materials have very high temperature tolerances for very short times. But thermal gradients take time to cause heat flow.  So even if you increase the temperature of the containment chamber very very rapidly, heat will not be absorbed by the material instantly. It takes some time (on the order of milliseconds) to begin melting. You therefore engineer the impulse to occur on the order of nanoseconds - a time insufficient to cause breakdown of the material of the containment chambers. This then eliminates the need to use fuel to cool the walls. Providing high thrust over short durations also uses mass more efficiently than traditional rocket propulsion methods do. Thrust is produced by ejecting and detonating small, fission-driven, pulse units at the aft end of the vehicle.  The EPPP engine produces a shell of ionized particles with an extremely high axial velocity. Thus, this concept of "riding on a plasma wave" is appropriately termed External Pulsed Plasma Propulsion. 
The Orion Mission had one specific purpose - to develop technology that would allow humans to travel beyond mars safely. The mission was assigned to General Atomics. At this time (1958), NASA had not been created. NASA most likely wouldn't have taken this mission because it lacked the requisite nuclear weapons background. The Orion Mission had recruited some of the best physicists at the time. Many of them worked on solving some very difficult problems relating to opacity and ablation. Ablation was particularly important; the work done solving the ablation problem was fundamental to the creation of the modern day of the EPPP engine. Ablation requires figuring out how long a specific material can endure specific impulses before melting. It was also necessary to design absorbing plates to convert thrust into impulse, reduce the temperature of the overall system, and shield the crew from radiation. These absorbers had to reduce the g force experienced by the crew down to 2g.  After solving this problem, the project's next steps were to begin development of a prototype. However, politics intervened. The Orion mission lasted 7 years, from 1958 to 1965. During this time, the nuclear arms race was heating up and funding was being diverted to military and war applications. The Orion project was then shut down once test ban treaties were signed by the world's governments. The negotiating parties wouldn't make exceptions for the Orion project, the propulsion system of which was based on nuclear bomb detonations. 
The EPPP engine is one of the most viable technologies we currently have towards long distance space travel. With the costs required to build this engine lowering, NASA has begun testing experimental hardware and publishing more studies of the principles of the EPPP. With more understanding and collaboration between space agencies, a manned mission to planets past mars may not only be recognized as a possibility but as feasible.
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 J. A. Bonometti and P. J. Morton, "External Pulsed Plasma Propulsion (EPPP) Analysis Maturation," American Institute for Aeronautics and Astronautics, AIAA-2000-3610, 24 Jul 00.
 A. Micks, "A Survey of Nuclear Propulsion Technologies for Space Applications," Physics 241, Stanford University, Winter 2013.
 A. Klein, "Nuclear Pulse Propulsion," Physics 241, Stanford University, Winter 2012.
 J. A. Bonometti, P. J. Morton, and G. R. Schmidt, "External Pulsed Plasma Propulsion and Its Potential For the Near Future," AIP Conf. Proc. 504, 1236 (2000).
 G. Dyson, Project Orion: The True Story of the Atomic Spaceship (Henry Holt, 2002).