Testing a nuclear propulsion reactor in 1960s. (Physics Today, May)
The recent Physics Today (May, Nuclear Fission Technologies for Space Exploration, p. 25), reminded me that there were once aspirations to use nuclear propulsion for manned space flight. As early as the 1950s propulsion systems based on nuclear fission were being designed for rockets. U.S. programs included Project Rover which ran from 1955 to 1973 and NERVA (Nuclear Engine for Rocket Vehicle Applications) which ran from 1961 to 1973. On one occasion (in 165) a nuclear powered system (SNAP 10A) actually reached orbit and last 43 days - until the non-nuclear components went dead.
Indeed, the basic design of a nuclear thermal propulsion (NTP) system is shown in Fig. 1 (from the same PT issue) and a photo of the test firing of an NPT reactor is shown in Fig. 2 from the same article.
We also learn "multiple NPT programs were initiated over the last seven decades." Note that NPT represent one type of nuclear fission propulsion system. Basically, it uses the heat generated in a fission reactor core to convert a liquid propellant into gas - which is then expanded through a nozzle to provide thrust in the exhaust. The other (NEP) or nuclear electric propulsion system uses fission to generate electricity which then ionizes a gaseous component, providing thrust and acceleration.
Compared with the chemical propulsion used in rockets (like Space X's 'Starship') both the NTP and NEP systems provide significantly higher specific impulse, Isp . This is defined as the momentum transferred to the rocket per unit weight of propellant flow and usually expressed in seconds. In a 2016 post on rocketry:
I showed how the total impulse could be computed for a small model rocket such s the type I used to launch in the early 60s (the post also showed the design):
Let the total impulse of the engine be 10 pound-seconds, and the burn time of the engine be 2 seconds. Then the force F or thrust is: (10 lb-sec)/ 2 sec x (16 oz/ 1 lb) = 80 oz.
Of course, this just illuminates the general principle at work to get thrust. What is omitted (and needed to get Isp ) is how effectively the propellant is converted into thrust. Whereas in the model rocket example we are only interested in the total thrust delivered over the burn time (2 sec.). Thus, the greater the specific impulse the less propellant needed for a given space mission. Translated into practical terms, it means nuclear propulsion could reduce trips to Mars by 25% or more, e.g. from 16 months down to 12 months. And also enable payloads of significantly greater mass. As the PT article notes:
"For a human Mars mission, the target Isp is approximately 900 seconds, roughly twice as much as achievable with chemical systems. For hydrogen propellant - the leading option for a Mars mission because of its low molecular weight - that corresponds to a temperature of approximately 2700 K when the propellant exists the reactor."
The author goes on to note the biggest impediment to this scenario is the development of reactor and fuel components that can withstand rapidly heating of cryogenic liquid H2 to 2700 K. Another challenge is the long duration storage of cryogenic H2.
In the end I am confident all these challenges could be met to provide a nuclear propulsion mission to Mars, say by 2035. IF (especially NASA) can recover from the disastrous, reckless cuts at the hands of the current cabal of Philistines occupying the seats of power. This lot are not only intent on destroying our scientific advantages- but driving away the critical talent necessary to make the nuclear propulsion space missions possible. (NASA's space nuclear propulsion project is responsible for all the agency's current work related to NTP and NEP systems.)
Make no mistake that several of today's efforts are aimed at developing the technologies that will enable NTP or NEP for fast transit missions to the Moon, Mars and the outer planets. That includes the power production to support permanent outposts on their surfaces.
If.......
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