Re: ion/plasma/nuclear drives

[Keith Watt <kilo@xxxxxxxxxxxxxxxxx>]

On Sunday, December 22, 2002, at 01:28 AM, Dark DAVE wrote:

> In what circumstance would one pick a plasma drive on top of an ion drive? You make it sound like as tho they have almost equal performance specifications.

Remembering there are different "flavors" of each,  *in general* the ion drive will have a higher Isp and a lower thrust than a plasma drive.  But there is a lot more to engineering than two numbers.  You have to look at your specific mission (and by that I mean the necessary deltaV, allowed thrust time, payload mass, launch vehicle -- you'll never use either of these systems to escape a gravity well, reliability, cost, etc.).  When you plot all of these variables, there will generally "windows" for which one drive is better than another.


> What about a plasma drive? I was also under the impression (from reading some where) that you could integrate the plasma drive with a nuclear plant and use the strong magnetic field and charged propelant exiting the exhaust vent to generate eletrical power for the space craft.

Yes, you could, but this is -not- a plasma drive.  It's just a more efficient nuclear fission torch.  The amount of thrust contributed by the electromagnetic stream of the plasma is pretty small compared to the thrust of the fission exhaust.  It's just a trick to tweak a little more efficiency out of the drive. That said, the engineering studies I recall seeing (can't put my hands on 'em at the moment, so this is just my memory) seemed to indicate that the extra mass from the accelerator coils negated any extra efficiency you got from the drive.  I could be wrong about that, though.


> Which gave me an idea, could a nuclear powered plasma drive be used to generate lots of eletrical energy for the ion drives in a ion/plasma drive hybrid type space craft? Or is it not possible to integrate nuclear technology with the plasma drive? I cant seem to remember where i saw this documentary talking about a plasma type drive that could also generate eletrical power because it deals with charged particles and magnetic fields. I was pretty certain they were also talking about using a nuclear core to integrate with it so the nuclear energy can be converted to electricity for the space craft. perhaps you can clear this up for me if this all sounds familiar or not.

(Keeping in mind that you are using the term "plasma drive" when you mean something else...)  You can provide electrical power any way you want.  You should be aware, though, that a nuclear power plant does not generate electricity directly using plasma.  It generates electricity the old fashioned way: the radiation heats water to steam, which spins a turbine.  Small "pocket" reactors have been on the books for years and we may have actually built one, though I've never seen one so I can't swear to it.  Regardless, current nuclear reactors produce electricity through turbines, not through plasma directly.  You -could- conceivably pass a plasma through coils to generate electricity (basically a plasma drive in reverse), but I question that this is more efficient than a turbine.  Remember, higher heat and coils means more mass.  That's not a problem for ground-based installations, but it's a killer for space flight!

By the way, many electric drives get their power from an RTG, not solar panels.  A good RTG can provide you with plenty of power, they are just a bit heavy (not as heavy as a real reactor, though!)

Also consider that any time you use a nuclear plant, you've got to add significant heavy shielding.  It doesn't matter if it is a manned mission or not (electronics are in many ways even more sensitive to radiation than humans).  Generally a nuclear torch's thrust should outweigh the disadvantage, just don't forget to include this shielding mass in your design!  


> Oh one last thing, i did email some one in the deep space 1 project, cant remember who it was, but, he or she said that a nuclear and ion drive hybrid has already been considered but at the time of the email Mr Clinton was the president of USA so there wasnt much consideration going into it then. Would this make any sense to you?

It all depends on what you mean by "nuclear/ion hybrid"  That doesn't really tell me much, I'm afraid.  Keep in mind, too, that just because we "consider" things at NASA doesn't mean we have the capability of building it! :)

BTW, I hope I don't seem to negative here.  I think your project to simulate a nuclear torch drive is great.  I just don't want people in the community starting to think that ion, plasma, or really any electric drive is the "super high thrust" drive of the future.  They are very low thrust, but very high-efficiency drives and are extremely valuable for long-duration missions.  You can get to the outer planets without having to wait for the planetary alignments for slingshots.  I'm actually a big proponent of mag sails, so you won't hear me disparaging low-thrust propulsion!


> >I don't think that is what you really want to use for the missions
> >you have in mind. If you have an engineering background, I'd be
> >happy to refer you to some papers that might help you design your
> >nuclear fission drive in Orbiter, just let me know!
> AFFIRMATIVE! Send them my way! J

Sure thing:

Angelo and Butler (1985). Space Nuclear Power
Bhattacharyya (1988). CERMET Reactor Orbit Transfer Vehicle Concept, AFAL-TR-88-033, Edwards/USAF Astronautical Laboratory
Clark (1991). A Historical Collection of Papers on Thermal Nuclear Propulsion, AIAA
NASA (1990). NASA Thermal Nuclear Propulsion Workshop Notebook.

This last one is especially good.  Very informative.  A good general reference on all forms of propulsion (which also references the above) is Humble, Henry, and Larson (1995). Space Propulsion Analysis and Design.  This one is much more easily obtained than the others.  Be aware that all of these assume you have a strong engineering background.  

Okay, I've said my piece, so I'll get back to my other projects now, sorry for the interruption.  We now return you to your regularly scheduled Orbiter discussion, already in progress.  

Good luck!
Kilo