Energy From Thorium Discussion Forum

It is currently Oct 15, 2018 11:04 pm

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 27 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Nuclear power into space
PostPosted: Jan 01, 2014 11:56 pm 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 575
Location: Iowa, USA
For some time now I've been soaking up political commentary wherever I could find it. In doing so I came across a rather prolific political common tater Bill Whittle. Along with his commentary on politics he mixes in things like airplanes, movie making, science fiction, and space travel. It's space travel that I want to focus on here, and I suppose a little bit of science fiction. Bill Whittle has announced a project he's working on for a movie called Aurora, the link to his web page on it is here:
https://www.billwhittle.com/channels/aurora

In the preview of his movie Bill Whittle claims everything will be physically possible, or "no magic" as he puts it. A vital component to this story is a nuclear rocket design that saves the day. In my searching around the internet for more on another interest of mine, energy, I came across a video with Kirk Sorensen giving a throw away comment on how nuclear rockets are a very bad idea. Having seen numerous videos from both Mr. Whittle and Mr. Sorensen I believe that both men are educated and intelligent enough that both coming to such opposing views on nuclear rockets seemed like quite the dilemma for me. They can't both be right.

I e-mailed Mr. Sorensen and he gave me a link to some of his work on the topic:
http://selenianboondocks.com/category/r ... gn-theory/

Mr. Sorensen, if you are reading this I loved reading your work but physics class was a long time ago for me. So, I come here seeking clarification on some things.

After looking into this further the idea of nuclear rockets as both a good idea and a bad idea are not in opposition. That is because nuclear rockets are a good idea in space, just a bad idea for getting there. While physics class was a long time ago I do understand thrust to weight ratio, or T/W. I also think I have a good idea on what specific impulse, Isp, means. In order to get into space a rocket has to have enough T/W to lift itself off the ground and reach escape velocity. There must also be enough Isp that the rocket isn't so big, and therefore expensive, that someone can't afford to throw it into space.

NASA did a lot of work on nuclear rockets some time ago. There seemed to be plenty of people that thought nuclear rockets were the way to get into space. It sounds like there still are plenty of people at NASA that think that nuclear rockets are the future of heavy lift rockets. There might be life in nuclear rockets yet.

Now, Mr. Sorensen, I didn't come to your forum to doubt your analysis of nuclear thermal rockets as a means to get into Earth orbit. You've convinced me. What I'm wondering about is if there might be another technology that would have better performance than nuclear thermal rockets.

One idea I came across is a nuclear-chemical hybrid rocket. It would be not much different than a nuclear thermal rocket but instead of just one tank of liquid hydrogen there would also be a tank of liquid oxygen. The theory being that the heat of the combustion added to that of the reactor would allow for T/W and Isp that either alone could not achieve.

Another idea that was considered, and led to MSR research, is nuclear jet engines. Very bad idea for manned flight as it releases significant radiation and any shielding sufficient to protect a crew makes it too heavy to fly. However if there was no crew then that would not matter. It would still be useful for launching materials into orbit as a first stage, it needs air as the working fluid so a non-air breathing second stage would be required. Ignore the political implications of an aircraft that would irradiate anyone unfortunate enough to be near it in flight, does the physics work?

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Jan 02, 2014 11:23 am 
Offline

Joined: Dec 24, 2011 12:43 pm
Posts: 219
Location: Newport Beach, CA
Kurt Sellner wrote:
Another idea that was considered, and led to MSR research, is nuclear jet engines. Very bad idea for manned flight as it releases significant radiation and any shielding sufficient to protect a crew makes it too heavy to fly. However if there was no crew then that would not matter. It would still be useful for launching materials into orbit as a first stage, it needs air as the working fluid so a non-air breathing second stage would be required. Ignore the political implications of an aircraft that would irradiate anyone unfortunate enough to be near it in flight, does the physics work?


The physics unquestionably work, but that's not the question that demonstrates why nuclear rockets and/or nuclear jet engines are a 'bad idea'. Strictly speaking they are not 'bad ideas' but they are a poor alternative to chemical rockets or jet engines. Even for unmanned flight, where the weight of extra shielding can be saved, nuclear reactors require much more "overhead" in terms of the weight and complexity of the reactor.

You will always be better off using the reactor as the 'fuel depot', not the engine. This way the reactor can run as efficiently as possible and produce conventional fuels (likely liquid hydrogen and oxygen, but kerosene and any number of hydrocarbons are possible). If you're already out of Earth's gravity well, and your travel is limited to the inner solar system, you still don't need a full blown reactor with you (at least for propulsion). RTGs or chemical rockets, ideally sourced from reactors/materials already in space, would be the workhorse engines for getting from A to B. You'd want to bring along a reactor if you had long periods of habitation, in which case your needs would more appropriately match the capabilities of a reactor. Nuclear reactors are good at providing a very consistent amount of energy, they're less stellar at providing a short burst of power (nuclear bombs are extremely good at this though).


Top
 Profile  
 
PostPosted: Jan 02, 2014 8:18 pm 
Offline

Joined: Dec 16, 2011 7:27 am
Posts: 262
There was the radical Project Orion nuclear pulse rocket that would have used small fission bombs to give high thrust and specific impulse.

http://en.wikipedia.org/wiki/Project_Or ... propulsion)


Top
 Profile  
 
PostPosted: Jan 02, 2014 11:23 pm 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 575
Location: Iowa, USA
DougC wrote:
There was the radical Project Orion nuclear pulse rocket that would have used small fission bombs to give high thrust and specific impulse.

http://en.wikipedia.org/wiki/Project_Or ... propulsion)


I am aware of that project. I'll draw a line between nuclear jet engines, which irradiate only people within a short line of sight distance, and atmospheric nuclear detonations, which will spread radioactive material onto anyone downwind. I realize this might be a fuzzy line but I'll try to draw it anyway.

I did say that we could assume that political considerations could be ignored but creating nuclear bombs, even if they are not intended as weapons, and intentionally detonating them within the atmosphere is just something where the politics cannot be ignored. It is difficult to imagine a situation where this would be tolerated by any nation.

I remember some time ago of someone that came up with a plan for a nuclear rocket for single stage to orbit. I believe the design was a nuclear light bulb.
http://en.wikipedia.org/wiki/Nuclear_lightbulb

The claim was that the design would fit within the physical parameters of a Saturn V, making use of existing infrastructure possible. The claim was also that it could carry enough propellant for reaching orbit, turning around, and making a powered vertical landing. And that's not all, it would have enough propellant to do it twice. That is one powerful rocket.

Another idea that intrigued me was a nuclear rocket design where the core could also be used to generate power for the spacecraft. As I recall a spacecraft would be designed so that the nuclear core would "idle" at low power and the heat used to drive a closed cycle heat engine of some sort, a Stirling engine perhaps. When the crew needed to adjust course the core would be brought up to full power and propellant would be injected, heated up, and propelled out in the desired direction. Sounds like a great idea for a lengthy space mission.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Jan 03, 2014 3:26 pm 
Offline

Joined: Dec 14, 2006 1:01 pm
Posts: 379
A wonderfully sane way to use nuclear power for launch is to have it power solid-state lasers, and focus them on the rocket's engine. In that way, you leave most of your engine on the ground, because only the heat is delivered. The crucial issue here is the efficiency of the lasers, which has been growing by leaps and bounds lately, more or less in synchrony with LED developments.

There's some good references at http://en.wikipedia.org/wiki/Laser_propulsion


Top
 Profile  
 
PostPosted: Jan 03, 2014 4:04 pm 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 575
Location: Iowa, USA
With laser propulsion I'd think that blooming would be a problem, the laser energy getting eaten up by heating the air in between the laser and the spacecraft. Directed energy weapons have this problem. Well, with directed energy weapons it may not be too much of a problem Limited range may actually be a feature, don't want to shoot down your own satellites if you miss.

I'm not saying laser propulsion is a bad idea, just tossing out one of the more obvious problems it may have.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Jan 03, 2014 4:09 pm 
Offline
User avatar

Joined: Nov 30, 2006 3:30 pm
Posts: 3819
Location: Alabama
Kurt Sellner wrote:
Mr. Sorensen, if you are reading this I loved reading your work but physics class was a long time ago for me. So, I come here seeking clarification on some things.


I just wanted to acknowledge receipt of your request.


Top
 Profile  
 
PostPosted: Jan 03, 2014 6:11 pm 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1553
Myself I am a fan of dusty fission fragment rockets.
Variable ISP thanks to injecting a noble gas into the exhaust, and no major heat dispersal problems because you can dump heat at high temperatures.


Need very high performance reentrant moderators, though, which basically limits you to 13C and Deuterium AUIU.


Top
 Profile  
 
PostPosted: Jan 05, 2014 2:27 pm 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 575
Location: Iowa, USA
Kirk Sorensen wrote:
Kurt Sellner wrote:
Mr. Sorensen, if you are reading this I loved reading your work but physics class was a long time ago for me. So, I come here seeking clarification on some things.


I just wanted to acknowledge receipt of your request.


Thank you, sir.

I realize you are a busy person so I understand that you might not have the time to answer all my questions. I'll try to break my questions down to a handful of yes/no questions for you, which you can expand upon if you wish.

In your articles you have shown through your calculations that use of nuclear thermal rockets is a bad idea for getting from the surface of Earth to orbit, does the same apply to all applications of nuclear thermal rockets? Is there a place for that technology anywhere?

Are the problems of nuclear thermal rockets just a limitation of our current technology or is there something inherent to nuclear thermal rocket physics that is the limitation. My understanding so far is that the physics just won't make it worthwhile.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Jan 06, 2014 12:10 am 
Offline
User avatar

Joined: Nov 30, 2006 3:30 pm
Posts: 3819
Location: Alabama
I spent a great deal of time doing the derivations and explaining their implications, and then even more posting them and making them available. If you truly wish to understand their implications then you need to spend some time of your own. I've done about all I can at this point.


Top
 Profile  
 
PostPosted: Mar 23, 2014 9:52 pm 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 575
Location: Iowa, USA
I recently saw Pacific Rim on DVD recently which compelled me to revive this thread. Putting aside all the other poor displays of physics in the movie there was one scene that brought this thread to mind. The next paragraph contains spoilers, skip it if you must.

In the movie the gigantic nuclear powered robot "Gipsy Danger" is fighting the gigantic alien beast, and the beast grabs the robot and flies up high in the sky. The robot produces a sword and kills the beast. It's now falling at a considerable rate and is ordered to "vent the core" or something which produces a blast from what I had assumed to be the air intake on the robot's chest. Of course this slows the descent of the robot enough that our heroes survive to fight another day. This improvised use of the nuclear power plant has effectively created a nuclear rocket, which is why I thought of this thread.

Kirk Sorensen wrote:
I spent a great deal of time doing the derivations and explaining their implications, and then even more posting them and making them available. If you truly wish to understand their implications then you need to spend some time of your own. I've done about all I can at this point.


I appreciate your making your calculations available and I appreciate you taking the time to reply. I came back to this thread not necessarily looking for answers but to clarify the questions I gave since I believe I worded them poorly before. Mostly I'm just thinking out loud and to point out the bad physics that we see in movies all the time. I suspect these questions have either been asked and answered before or don't really have a cut and dried answer. So, I'll rephrase my earlier questions and invite all to speculate with me.

Mr. Sorensen's calculations take into account the weight and strength of materials that are currently typical of modern spacecraft. This leads me to wonder if there is the potential for some different materials that might be used that could make the math work out differently in favor of a nuclear thermal rocket. Mr. Sorensen points out that a NTR might be feasible if the weight of the material needed to construct the craft could be reduced to 1/3rd of what he has computed as necessary.

Of course such a spacecraft could get to orbit if we applied enough unobtanium in its construction. I'm not looking for something purely theoretical. I'm think that instead of using steel to make the rocket we use carbon nanotubes or something. Maybe we don't even have to get that exotic to lower the weight. As I'm not a materials scientist I'm not even sure where to start looking. Now, if we have such strong and lightweight materials available then it might just make chemical rockets look even better, making NTRs look relatively worse by comparison.

Supposing we did find the material that could make this NTR fly it'd probably end up being a Hughes Hercules, something that would work but performs poorly in comparison to existing spacecraft.

The other question I was pondering is if we had a variation on this theme. A pure NTR would not leave the launchpad but what if we made some improvements? A variation that I mentioned earlier is to replace the liquid hydrogen working fluid with a mix of liquid hydrogen and liquid oxygen. There appears to be little doubt that such a chemical-nuclear hybrid rocket would be able to lift itself off a launchpad, the question is more of if the added weight, cost, and complexity is offset by the added power the nuclear reactor would provide.

Is there some other variation on this theme that might be a better idea? This steps away from the question in my original post on if the physics work and into the question on if it would be an improvement over what technologies we already have. We've already discussed numerous technologies in this thread so perhaps we've mentioned them all.

It appears nuclear propulsion research is limited to upper stages of a multistage rocket and interplanetary travel. I'm just left to wonder if there might be some leap in material science that would make nuclear propulsion favored for lifting a spacecraft off a launchpad. I'm quite certain that even if we resolve the physics there will be political obstacles to limit their use. My original premise was to discard with the politics but once we determine if the physics work then things like politics and economics must be considered.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Mar 24, 2014 5:13 am 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1553
The cheaper option might just be to strap giant, and cheap, SRBs to the main stage and use them to provide additional thrust during the crucial early minutes of launch.

Or just go entirely insane and dump mercury into the engine bell.....


Top
 Profile  
 
PostPosted: Mar 25, 2014 8:12 pm 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 575
Location: Iowa, USA
E Ireland wrote:
The cheaper option might just be to strap giant, and cheap, SRBs to the main stage and use them to provide additional thrust during the crucial early minutes of launch.


That might work, a 1.5 stage system like that used in the Space Shuttle. Unless the nuclear rocket is making a significant contribution to the total thrust it's less of a rocket and more of payload.

E Ireland wrote:
Or just go entirely insane and dump mercury into the engine bell.....


Excuse my ignorance but I'm not sure I follow. I assume that by using mercury the added mass of the material over hydrogen adds efficiency to the engine. If that's the case I'm sure that there's all kinds of crazy stuff we could consider, depending on how hot the engine can get. We just need to get something that boils at the exhaust temperature.

If we're going there then why not consider open cycle nuclear rockets? We'll just expel the fission products out the engine exhaust. We'll just salt the earth below our flight path with a thin layer of radioactive iodine. That'd be about as much fun as mercury.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Mar 25, 2014 8:19 pm 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1553
The specific impulse of a rocket is inversely proportional to the square root of the formula mass of the materials in the exhaust - so a material like mercury (being the heaviest easily pumpable material that will survive into the exhaust) will have a low ISP.

One of the less obvious implications of this is that reducing the ISP of an engine in this manner will increase the thrust if all else remains equal.

So in the early stages of a launch you would dump mercury or some other heavy element (I have seen Iron in the form of Iron pentacarbonyl proposed for chemical rockets) into the exhaust to massively increase your thrust as the expense of lost ISP - the idea being that the reduced 'gravity drag' from the faster acceleration of the rocket will more than cancel out the reduction in performance from the loss of specific impulse [ISP].


Top
 Profile  
 
PostPosted: Mar 26, 2014 9:25 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5045
Mercury is surprisingly rare. This makes any large scale application very difficult.

Iron in rocket fuel seems like a pretty good alternative. I suppose you could even use depleted uranium.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 27 posts ]  Go to page 1, 2  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group