23-03-2005 00:46									#1
	Nuclear Thermal Propulsion Support Apolyton or Terrorists Win

As some of you may be familiar with the rocket equation and nuclear-powered submarines, for instance, I am interested in hearing your views on the possibility of using nuclear thermal propulsion instead of chemical propulsion. The incoming head of NASA is a big proponent of nuclear space, so there may be some activity in the area in the coming years.

The specific impulse (fuel efficiency) of a solid core nuclear engine with LH2 as a propellant is expected to be some 800-900 seconds. Even though this is only twice the specific impulse of LH2/LOX, which is used today in various rockets, a solid nuclear engine would make it "easy" to have a reusable rocket with only a single stage (or at least a stage and half) and would increase a lot of the safety margins that drive the extreme expense of spaceflight.

We have some experience with nuke subs, and that has been very positive to my mind, even though I don't know too much about nuke subs. We also have some experience with nuclear engines destined for space uses. The Kiwi solid core reactors and NRX engines were built and tested. 1,100 MW generated. The Phoebus engine generated 4,000 MW (i.e., the most powerful reactor ever built). A follow on viewgraph in the NERVA program was an engine of about 1,000 MW to be mated with a Saturn V upper stage.

So what say you? Is this just a huge boondoggle to the aerospace contractors and inherently unsafe otherwise, or is there substance here?

http://en.wikipedia.org/wiki/Nuclear_thermal_rocket

Last edited by DanS on 23-03-2005 at 01:14

	23-03-2005 01:04									#2
	Support Apolyton buy from Amazon

I thought the exhaust was radioactive... it was my understanding that this was the reason this method was not practical.

	23-03-2005 01:28									#3
	Support Apolyton buy from Amazon

Some higher performance designs may be dirty, but not the solid-core designs, as I understand. The only release of radioactive materials would be if the reactor blew up inside the atmosphere for one reason or another. There are precautions to take against that, of course.

	23-03-2005 01:32									#4
	Full PM-box? Change here!

Well as long as it doesn't spew radioactive garbage into the atmosphere, I would agree that this should be researched.

	23-03-2005 01:34									#5
	Got spare money?

My thesis was in a related area (Aeronautics and Astronautics from MIT). This is very feasible, however it is not applicable in all cases.

1) Nuclear power plants are quite heavy, and thus very expensive (from a spaceship point of view in addition to an economical point of view). If the ship is unmanned, it would be possible to reduce the shielding (and thus weight and therefore, cost).
2) Nuclear propulsion is only suitable in space. Unless you are willing to have radioactive exhaust, you cannot operate this type of engine in an atmosphere.

Therefore, nuclear propulsion would be perfect for very long unmanned space trips (insterstellar probes anyone?).

	23-03-2005 03:10									#6
	Suffering from ads?

quote: Originally posted by DanS Some higher performance designs may be dirty, but not the solid-core designs, as I understand.

From what I remember, Sava is partly correct. The problem is the nuclear space ban, which prohibits nuclear explosions in space, and thus nuclear thrust.

IIRC, the fastest method theoretically for in system flight fight now is a nuclear sail, which fires a bomb towrads a huge sail, sploding it, and using that to push the sail (and thus the ship, around.

The method to which I believe you are refering uses a nuke plant to heat up exhaust, which is used as propulsion.

	23-03-2005 03:46									#7
	Get a bigger avatar today!

quote: From what I remember, Sava is partly correct. The problem is the nuclear space ban, which prohibits nuclear explosions in space, and thus nuclear thrust.

The nuclear ban prohibits nuclear weapons in space, and the US has interpreted that to mean that propulsion methods like the nuclear sail (pulsed nuclear detonation) are disallowed.

However, the U.S., Russia, and the Ukraine have interpreted the ban to allow for most nuclear propulsion methods other than the nuclear sail.

	23-03-2005 03:54									#8
	Lose 30 kilos (of popups)

quote: Originally posted by pchang My thesis was in a related area (Aeronautics and Astronautics from MIT).

quote: 1) Nuclear power plants are quite heavy, and thus very expensive (from a spaceship point of view in addition to an economical point of view). If the ship is unmanned, it would be possible to reduce the shielding (and thus weight and therefore, cost).

I don't like the sound of that with regard to cost. As I recall, Griffin, the proposed head of NASA is thinking about nuclear propulsion for manned trips to Mars. He also wants a rocket similar to the Saturn V, so maybe size and weight have a little wiggle room from his perspective.

quote: 2) Nuclear propulsion is only suitable in space. Unless you are willing to have radioactive exhaust, you cannot operate this type of engine in an atmosphere. Therefore, nuclear propulsion would be perfect for very long unmanned space trips (insterstellar probes anyone?).

I was under the impression that some types of nuclear engines do not have radioactive results. Am I incorrect?

With regard to interstellar travel, there's an article floating around that goes through all of the options for it. I always find need to refer to it, so I should hunt it down on the internet.

	23-03-2005 04:01									#9
	Re: Nuclear Thermal Propulsion Lose 30 kilos (of popups)

quote: Originally posted by DanS As some of you may be familiar with the rocket equation and nuclear-powered submarines, for instance, I am interested in hearing your views on the possibility of using nuclear thermal propulsion instead of chemical propulsion. The incoming head of NASA is a big proponent of nuclear space, so there may be some activity in the area in the coming years. The specific impulse (fuel efficiency) of a solid core nuclear engine with LH2 as a propellant is expected to be some 800-900 seconds. Even though this is only twice the specific impulse of LH2/LOX, which is used today in various rockets, a solid nuclear engine would make it "easy" to have a reusable rocket with only a single stage (or at least a stage and half) and would increase a lot of the safety margins that drive the extreme expense of spaceflight. We have some experience with nuke subs, and that has been very positive to my mind, even though I don't know too much about nuke subs. We also have some experience with nuclear engines destined for space uses. The Kiwi solid core reactors and NRX engines were built and tested. 1,100 MW generated. The Phoebus engine generated 4,000 MW (i.e., the most powerful reactor ever built). A follow on viewgraph in the NERVA program was an engine of about 1,000 MW to be mated with a Saturn V upper stage. So what say you? Is this just a huge boondoggle to the aerospace contractors and inherently unsafe otherwise, or is there substance here? http://en.wikipedia.org/wiki/Nuclear_thermal_rocket

We have to go nuclear if we want to go outward from the sun.

	23-03-2005 04:21									#10
	Support Apolyton, buy Alpha Centauri

Nuclear propulsion that does not produce radioactive exhaust uses electricity and thermal energy to ionize particles (hydrogen is best) and electromagnetically accelerate them out the back at near relativistic speeds. This type of propulsion cannot operate in an atmosphere. It provides a low amount of thrust but is very fuel efficient. It is the only way to travel in space for long periods of time and still have fuel left at the end (unless you can refuel somehow along the way - EM ramjets and scooping from gas giants are two ways).

Nuclear propulsion that does produce radioactive exhaust can heat up fuel directly in the reactor core and acts like a supercharger for chemical rockets. This can operate in an atmosphere but is very dirty and unsuitable for use on any planet that one would want to do something with later. This type of rocket can produce high thrust, but is only marginally more fuel efficient than conventional chemical rockets.

And no, it is not necessary to use nuclear propulsion to escape the solar system.

Last edited by pchang on 23-03-2005 at 04:33

	23-03-2005 10:03									#11
	Support Apolyton, buy Alpha Centauri

I'm guessing he means that solar power becomes much less as you go away from the sun and you would need increasingly large collection systems for solar energy. But nuclear isn't necessary, even if advantageous in most situations.

quote: Nuclear propulsion that does not produce radioactive exhaust uses electricity and thermal energy to ionize particles (hydrogen is best) and electromagnetically accelerate them out the back at near relativistic speeds. This type of propulsion cannot operate in an atmosphere.

Is this because the thrust/weight of these engines is < 1? I know of the ion engines and Hall Effect Thrusters in this category, but do you include solid-core nuclear thermal engines in this category as well, even though they are low efficiency? (As I understand, they also have a thrust/weight slightly lower than 1.)

Quite simply, do you know if solid-core nuclear thermal engines spew radioactivity?

	23-03-2005 10:33									#12
	Remove this text

The nuclear core designs I studied all create radioactive exhaust.

The ion or magneto plasma dynamic engines (MPD) I described cannot operate in an atmosphere because the density of the gas will not allow the formation of a plasma. Engines that rely upon having the working gas be a plasma can only operate in near vacuum conditions. However, MPD engines are capable of generating a fair amount of thrust (but then again, in general, they go through fuel faster than ion engines).

BTW - these things are generally classified as electric propulsion and MPDs are the state of the art (in my opinion).

	23-03-2005 10:34									#13
	Support Apolyton, buy GURPS/ Alpha Centauri

Yea its the solar one, also I don't see Chemical rockets being in the price range for the stuff we want to do.

	23-03-2005 17:55									#14
	Re: Nuclear Thermal Propulsion Support Apolyton buy from Amazon

quote: Originally posted by DanS The specific impulse (fuel efficiency) of a solid core nuclear engine with LH2 as a propellant is expected to be some 800-900 seconds. Even though this is only twice the specific impulse of LH2/LOX,

So, the heat energy transferred to the H2 and O2 is equal to the energy produced by the chemical reaction of 2H2 + O2 = 2H2O

In order to prevent the exhaust from being radioactive, we need a material or materials that:
1) Can stand the high temperature needed to transfer this amount of heat to the fuel in a relative short period of time (length of pass the fuel takes through the reactor can be quite long, but the mass flow rate would be very high)
2) Absorb/block the radiation generated by the core
3) Not be too chemically reactive to H2 and O2 at these high temperatures
4) Not be so massive as to weigh the rocket down

Please keep in mind that conventional nuclear reactors are only run at temperatures hot enough to create superheated steam (300F - 400F or so).

	23-03-2005 19:33									#15
	Increase the size of your Attachments

I doubt this will happen because the eco-nuts with start b*tching, remember how crazy the eco-nuts were when Cassini was launched because it's electronics were powered by plutonium?

	23-03-2005 23:52									#16
	Support Apolyton buy from Amazon

At a certain point, when education doesn't work, you have to just not let the eco-nuts bother you when they are in the distinct minority opinion. I think that some parts of our government are successful in doing so, including NASA, DoD, and the DoEnergy. NASA successfully launched Cassini, after all.

quote: Please keep in mind that conventional nuclear reactors are only run at temperatures hot enough to create superheated steam (300F - 400F or so).

That hadn't occured to me. You're right in that this chamber temperature is at least an order of magnitude lower than most rocket engines.

quote: The ion or magneto plasma dynamic engines (MPD) I described cannot operate in an atmosphere because the density of the gas will not allow the formation of a plasma.

I did not know that. That makes a lot of things clear to me. So we'll be using chemical engines at least as a first stage from Earth for a long, long time.

quote: BTW - these things are generally classified as electric propulsion and MPDs are the state of the art (in my opinion).

I haven't really checked out MPDs, although I have checked out ion thrusters and Hall Effect thrusters because they are being used nowadays with fairly high frequency (especially the HETs since the late 90s). I assume by MPDs, you mean like that proposed in the VASIMR?

http://spaceflight.nasa.gov/shuttle...spl/vasimr.html

I think VASIMR has been canceled, or at least curtailed.

As an aside, NASA seems to be upping the power available from its proposed space nuclear reactor (Prometheus) being developed by the Department of Energy. It started at 50 KW. But now I'm starting to hear numbers like 100 KW or 200 KW. This might be useful in order to speed up propulsion, although I would prefer that they speed up development.

	24-03-2005 03:28									#17
	Support Apolyton, buy Civilization 2

Yes. VASIMR is an example of an MPD (magneto plasma dynamic) engine.