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From GA-5009 Volume 3 Nuclear Pulse Space Vehicle Study Vol. III Conceptual Vehicle Designs and Operational Systems.
Operation of the nuclear pulse engine can be briefly described as follows: Nuclear pulse units are ejected successively, falling behind the vehicle to the specified 25 meter detonation point. As the pulse unit nears the detonation point it is armed by a signal from the pulse unit firing computer located in the body of the propulsion module, at the detonation point they are exploded. The pulse units are designed in such a manner that when they are exploded a large fraction of their mass is propelled toward a heavy circular disc (the pusher) which forms the base of the vehicle. Interaction of the high-velocity propellant with the pusher drives the pusher upward, and the shock-absorber system attached to the pusher transfers the momentum to the upper sections of the vehicle at accelerations tolerable to sensitive payloads.
Special Thanks to Winchell Chung of Atomic Rockets. His text and Pusher-Cycle diagram which served as reference for this render.
20-Meter Orion Jupiter Moon Landing Mission
10-Meter Mars Capture Mission Orion
Starting in 1958, Project Orion ran until 1965. Research was performed at General Atomic. The program was funded under ARPA (now DARPA). Some of the leading scientists of the time contributed to the project. Freeman Dyson theoretical physicist and mathematician, famous for his work in quantum electrodynamics, solid-state physics, astronomy and nuclear engineering was responsible for validating the physics of the propulsion system.
Theoretical physicist and prominent nuclear weapon designer Ted Taylor was responsible for the pulse unit design.
By the time the program ended in 1965 the physics and engineering challenges involved in construction and operation of such spacecraft were well understood, all that remained was the material engineering, building and testing full scale propulsion modules under actual operating conditions.
At the core of the system is the nuclear pulse unit. This is a shaped charge nuclear device. More details will be included in a future post, this is an ongoing series. Briefly: Efficient directional explosives maximize the momentum transfer, leading to specific impulses in the range of 6,000 seconds, or about thirteen times that of the Space Shuttle Main Engine. With refinements a theoretical maximum of 100,000 seconds (1 MN•s/kg) might be possible. Thrusts were in the millions of tons, allowing spacecraft larger than 8 × 106 tons to be built with 1958 materials.
The General Atomic reference design (much larger than these, later, and smaller spacecraft) was to be constructed of steel using submarine-style construction techniques, a 4,000 ton spacecraft with a crew of 100. This low-tech single-stage reference design would reach Mars and back in four weeks from the Earth's surface – compared to 12 months for NASA's current chemically powered reference mission. The same spacecraft could visit Saturn's moons in seven-months – compared to chemically powered missions of about nine years.
There is literally no comparison between Orion and NERVA. The individual who told you otherwise is quite mistaken. That individual, Mhvost, has been banned from my pages after I caught him posting misleading information on several posts.
The information below is from the Atomic Rockets Engine List entries for NERVA and Orion (fission) it is accurate. I have included the links so you can check the numbers yourself.
NERVA had a Specific Impulse 825 s. Thrust 49,000N
Orion Fission had a Specific Impulse of 4,383 s. Thrust 263,000N
Flight test footage of the C4 powered scale model Orion can be found here in an excerpt from the BBC documentary To Mars by A-Bomb. Orion Flight test footage
I also highly recommend the entire seven-part documentary, part one here: To Mars by A-Bomb
Orion remains as a candidate for future development. There is no other propulsion system of equal capability that is so immediately within our technological abilities.