Stefan
I think "Project Orion" was as much as "have the bomb, must find a peaceful civilian use to justify funding" as anything else. (Project Plowshare is another example that comes to mind.
https://www.llnl.gov/str/Hacker.html)
I have worked professionally in the high performance propulsion community for more than 3 decades, so I am aware of the various propulsion concepts that have be proposed over the years. High Isp is only one requirement for a thruster system. Equally important items are high propulsive efficiency and high thrust.
Many satellites have reentered with their high Isp thrusters blasting full bore because they can't develop enough thrust to maintain the necessary delta V to overcome atmospheric drag as their orbits decay.
In the case of the pulsed nuclear detonation rocket, thrust may not be the issue but the propulsive efficiency is horrible at low velocities. You have maximum propulsive efficiency when the exhaust gas velocity equals the vehicle velocity. If you have a 10 KT explosion, the plasma velocity is 100 km/s.
https://en.wikipedia.org/wiki/Project_Orion If you are moving at 10 km/s, this give a propulsive efficiency of 10%. This is akin to spinning your wheels at a drag strip because your tires can't couple the energy to the road. Additionally most of the vehicle mass is not payload, it's shielding and reaction mass, totally worthless to the mission.
Nuclear propulsion definately has a place in deep space exploration, however it needs to be part of a combined cycle energy system. The heat from fission has to be used to generate steam for vehicle electrical power as well as to heat the propellant gases that generate thrust to propel the spacecraft. Large amounts of water need to be present to provide shielding from deep space radiation, to provide a volume to grow aquatic plants and animals for food, process wastes, absorb CO2 and generate oxygen, and provide a thermal mass for temperature stability. Water provides the highest density storage for both hydrogen and oxygen at the same time, and in a non-cryogenic form. With a nuclear power plant, the water can be broken down into oxygen for breathing and hydrogen for a propellant. The reaction mass fraction of this type of propulsion system is substantially less than that of a Dyson drive system, alowing for a higher useful vehicle payload mass fraction.
Nuclear thermal rocket motors are possible with today's technology, and are a more practical and useful propulsion system for deep space exploration vehicles than the Dyson drive for which none of the technology has been developed, or is likely to be developed in the next several centuries, if ever. The real thermomechanical issues of repetitive nuclear detonation plasmas impinging on surfaces has not been studied, and the perceptions on survivability in the original concept are likely wrong based on ongoing research.
Bob Krech
