NASA space ship

Thucydides wrote:Since we are talking about using a NTR as a rocket engine and water as remass, the sources of energy and vacuum for desalinating the water seem pretty self evident.

This is certainly possible but uses energy from a stored, mobile source in an effectively stationary application (unless you purify your water while already moving).

Thucydides wrote:Finally, if you really want to combine the "oomph" of a NTR with "dirty" water, I would simply inject the water into the reaction stream of a dusty fission fragment reactor.

This sounds similar to Zubrin's nuclear salt-water rocket

With a nuclear reactor providing energy, purifying water could be done either stationary or on the move. I suspect that it would be easier to do so while stationary (purifying the water as part of the extraction process) since you can leave the stuff you don't want behind. On the move means you will be spilling concentrated brine over the side (so to speak), which might cause some complications.

The Space Patrol might also be a bit annoyed at giant popsicles of salt water drifting through the space lanes (or even clouds of ice crystals), simply because they will be a navigation hazard to any spacecraft moving at interplanetary velocity, and have the potential to cause a great deal of damage to unwary spacecraft.

Tenshinai wrote:

If you wish, there is no reason you could not use salt water if you had designed the systems to handle the salt load. Wear is going to be a problem depending on the velocities involved and same goes for salt "balls" running through if you do not have a turbidity trap or 3.

How many steam engines through history do you know about that ran on saltwater?

I know, messed up the quotes.

Many civilian freighters had the ability to use salt water over the years. Before all terminals had available tap water and before ubiquitous on-board desalination plants. They used partially desalinated ocean water. In modern times, complete on-board desalination is cheaper than providing the contaminant traps etc. Optimal? No. Doable, Yes. A bit more maintenance required as the crew had to usually manually clean the traps.

Relax wrote:
I know, messed up the quotes.

Many civilian freighters had the ability to use salt water over the years. Before all terminals had available tap water and before ubiquitous on-board desalination plants. They used partially desalinated ocean water. In modern times, complete on-board desalination is cheaper than providing the contaminant traps etc. Optimal? No. Doable, Yes. A bit more maintenance required as the crew had to usually manually clean the traps.

Yup. Now try to get that to work in vacuum, severe cold and heat, without any easy access to individual parts. That´s the big pitfall, it´s not impossible at all to get around, it´s getting around it when you are highly constrained by being in space that messes things up utterly.

For example, most(all?) containment traps like that wont even work properly outside of normal gravity. This is why space isn´t easy to deal with, it messes up all the normal rules.

Doing things in space will be much different than doing things here on Earth. Purifying water or even metallic elements might only need a concentrated source of sunlight and a giant foil bag to contain the materials to be processed, and a magnetic coil to separate elements.

Controlling the amount of heat applied to the foil bag could allow very fine control of the purity of the elements being separated, and using the magnetic coil as a form of mass spectrometer could allow the escaping vapours or plasma (depending on how hot you run the thing) to be separated and condense on cold plates.

Since we are talking about water, separating water from brine should be rather easy using a variation of this method. Of course, you could also forego the separation and simply use the mirrors to "raise steam" and make a simple steam rocket. This should work well from Mars inwards (farther out and the mirrors will need to be improbably large; this will add a lot of mass and complications to the focusing and control of the device).

For some consideration on where to go to, here is a blog about using the voids inside asteroids to create habitable spaces: http://gravitationalballoon.blogspot.com.au

Lots of interesting speculations and worked examples.