Open-cycle Nuclear Propulsion Systems

[shynung]

For the sake of the argument, let's disregard all political factors in this discussion. Let's talk only about technical matters of the subject.

Whenever I think about nuclear propulsion systems, two designs often came to my mind: Nuclear Thermal Rocket or Nuclear Pulse Propulsion, both having been studied as project NERVA and Orion, respectively.

Later, I thought of a different design. This one would work similar to NTR systems, in which nuclear fuel is reacted to heat a different propellant(say, hydrogen). However, instead of a typical nuclear reactor, it had a simpler combustion chamber with no means to retain the fissile material. Instead, products of the nuclear reaction is ejected, being part of the exhaust, hence open-cycle.

What are the possible advantages and disadvantages of such propulsion systems? How would it perform in comparison to NERVA and Orion systems? Also, what technical challenges the propulsion system might impose?

[Soda Popinski]

The two I can think of off the top of my head (I've been playing this game way too much), are the gas core fission rocket (basis of the Discovery One's rocket in 2001) and the Spherical Torus Fusion engine. I believe both can either run in open or closed cycle. The open cycle has the advantage of being more efficient, but the disadvantage of spewing out radioactive particles with the propellant. Probably fine for deep space, as the space ship is going the other way.

For NASA's take on an entire ship based on the Spehrical Torus Fusion engine, take a look at the "Discovery Two"

http://www.ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050160960_2005161052.pdf

For an open cycle solid fission system, I would expect it to be more efficient than a NERVA. I'm not sure what the moderator would be, as I don't know if hydrogen propellant would slow down neutrons enough. Maybe water could be used as the propellant and nuclear moderator, similar to a salt water nuclear rocket.

Edit: I believe a salt water nuclear rocket is by definition an open cycle system.

I somehow doubt it would be as efficient as an Orion drive, but at least it wouldn't have to be as impossibly large.

Edited February 9, 2014 by Soda Popinski

[Idobox]

It's called a nuclear salt-water rocket and it's awesome, but deadly.

http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

[Headhunter09]

The technical feasibility of a NSWR is questionable. Unlike some other nuclear propulsion systems, nobody is quite sure how to go about constructing a NSWR.

[shynung]

The Wikipedia article referred to this journal:http://path-2.narod.ru/design/base_e/nswr.pdf

The design outlines a system with at least 10,000 seconds of I_SP with comparable thrust to chemical systems, which is quite impressive.

Regarding technical challenges, isn't it enough to simply use a shielded combustion chamber? The system uses liquid-based fuel (mostly water), so I believe even turbopumps would still work.

[DerekL1963]

The technical feasibility of a NSWR is questionable. Unlike some other nuclear propulsion systems, nobody is quite sure how to go about constructing a NSWR.

Not only is nobody sure how to build a NSWR, nobody has even looked at doing do, there haven't even been any significant studies made or modeling done. It's about as close to complete vaporware as an engine can be. It pretty much only exists in Zubrin's proposal, and that proposal is at a level that's pretty much just a glorified BOTE.

[Idobox]

the NSWR reactor isn't vaporware, it's a concept. And although the engineering is not trivial, we can have reasonable expectations on its performances. In terms of design, it's reasonably close to liquid fuel reactors envisaged for the IV generation of nuclear power plants.

And nobody is going to invest serious time and money into it if we can't even fly a NERVA. That doesn't mean it's bogus or overhyped, just that it's a very good engineering solution to a given problem plagued by terrible environmental and political issues.

[DerekL1963]

the NSWR reactor isn't vaporware, it's a concept.

I was being generous as "concepts" are even lower on the scale.

And although the engineering is not trivial, we can have reasonable expectations on its performances. In terms of design, it's reasonably close to liquid fuel reactors envisaged for the IV generation of nuclear power plants.

Absent any serious engineering, no we can't have reasonable expectations on it's performance because we don't have any useful information on which to base those expectations. (Let alone to refer to is as "awesome" as you did.) Not to mention it's precisely nothing like a liquid fueled power reactor - liquid fueled power reactors maintain their fuel in a liquid state, it never enters a vapor phase. (And the vapor phase is critically important as it's likely a fair portion of the heating will occur in that phase, as well as driving the design of any nozzle cooling. You also need to make sure the propellant doesn't condense inside the reactor, etc...) Nor do they store their fuel in a liquid state (it's stored as a solid), etc... etc...

And nobody is going to invest serious time and money into it if we can't even fly a NERVA. That doesn't mean it's bogus or overhyped, just that it's a very good engineering solution to a given problem plagued by terrible environmental and political issues.

Claiming that something about which practically nothing is known is a "very good engineering solution" is the very definition of hype. (And it's a hype that's all too common in the mostly engineering illiterate space fan/supporter community.)