Use of Nuclear power in space exploration.

shashankac655

I read that nuclear power when used for propulsion of a vehicle can be very efficient for example nuclear powered submarines and aircraft carriers can run over twenty years without refueling and can travel enormously long distances with the fission of a few kilograms of uranium,but why is this source of power not used for space exploration,is not possible to use nuclear energy to send spacecrafts(maybe with human crew),at much greater speeds and to much far away destinations??

tiny-tim

hi shashankac655!

the main problem is that submarines have water outside for the propellers to push against, and aircraft have air outside either for the for the propellers to push against, or for the jet engines to suck in and expel

but there's almost nothing in space, so no matter how powerful or efficient a spaceship's engine is, it needs to bring stuff with it to chuck out the back!

this stuff is not needed in nuclear-powered vessels on Earth, which is why they can be so efficient

(and fuel is preferred to household trash, builders rubble, etc, in rockets, because fuel can power itself)

shashankac655

How does a conventional spacecraft move in space ?? it also requires some stuff to be ejected out to move forward right??....the stuff that ejected out consists of superheated gases forced out of a tiny hole to create thrust right??..if this is the way spacecrafts move,we can use nuclear power to create extremely high temperatures and pressures of the gas that is to be forced out,can this make the spacecraft faster??

Since space doesn't have any medium to slow the craft down,single big push or impulse may be enough to travel certain a distance and if it slows down due some reason like gravitational force due to other planets or asteroids something else,another big push can set it in a high velocity again. This kind of a spacecraft may be very bulky,but can it be economical??

Borek

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

Unrest

Sorry, I prefer a rocket that expels builder's rubble. Preferably over the neighbor's yard.

shashankac655

Thanks Borek, has NASA completely forgotten about NERVA is it completely dead???.....
I am waiting for China to do something big (may be a manned lunar mission),big enough to make the US government to seriously consider a manned Mars mission,this will be the dawn of a new space race!!!

russ_watters

My guess would be because ion thrusters have a higher specific impulse (mass efficiency) for the tasks such things are needed right now. They can also be nuclear powered, though...

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

Also, politics no doubt played a role. NASA takes a lot of heat when they launch nuclear reactors into space.

enigma

What you're proposing is called a nuclear thermal rocket. It's much more efficient than chemical rockets because it can use pure hydrogen (more efficient) and it can get hotter than the combustion temp of hydrogen/oxygen. The downside is you need to launch a nuclear engine on top of 600,000 lbs of explosive material built by the lowest bidder.

68658

Once in space you can use the nuclear reactor as a method of powering a hyper strong laser at a forward mounted solar sail. Even more effective once upto its opperating speed of abou 0.5c.

Unrest

That doesn't really make sense. Why would you use a laser for a solar sail? A concentrated beam leaves most of the sail unused and requires a hyper-laser-proof patch where it's being hit.

But more imporatantly, conservation of momentum says you might as well fire the laser backwards and not have any solar sail. Same effect but easier.

Furthermore, photons aren't very "efficient" at providing thrust. Where did you get 0.5c from, and why does going faster make it more "effective"?


Or are you talking about a laser on Earth, shining on the spacecraft somewhere in space?

68658

NO NO NO. Through realtavistic physics photons can exert a force when reflected but the force generated by fireing a laser in the oppisite direction is neal to non (i read some where its like 300 Mw for 1 newton of thrust useing photon ejection out the rear of a vessel). Second off i under simplified useing beam divergence you can illuminate the intire sail, it just has to be far enough away, so optical systems to increase beam divergence could be used. And as fore effectiveness i used the wrong wording, my point was once upto 0.5c the craft can merely coast. As for the maxium speed a light sail can go faster it depends on how strong the light is and how long the light at that intensity is hsined on the sial. So i say 0.5c because its incredably fast and due to time dialtions radial curve upwards and 0.5c outside observence on the trips duration would only be about 3 earth days to 1 of the ship traveling (i hope i dont have said dilation backwards so dont take that part to seriusly).

Unrest

Hmm. I'm still deeply suspicious. If you look at the space ship with sail as a black box, then it's simply another space ship with photons coming out the back. Why does it matter if the sail looks like a conventional solar sail, or if it's just mirrors inside the laser device itself? The photons are still photons regardless of where they came from.

I missed the obvious idea of spreading the beam out. But if you do that, why do you need a laser? Why not any other light source?

Borek

It won't work, just like it is not possible to propel sailboat with a fan blowing into sail.

You really should start with basic physics before fantasizing about nuclear and high energies.

Unrest

No. It would work. If the sail is a mirror (which is a good idea anyway) then the light ends up being reflected backward. But I don't see any advantage over just pointing the laser backwards in the first place.

Borek

You are correct about physics, but I doubt reflection is what OP had on mind.

68658

Its not mirrors, mirrors dont reflect light the abosrb large quantities of it, these being the ones you would see youre reflection in. Im proposing highly reflective surface like milar, which reflet 97% + of the light that hits it. The idea of haveing a laser on bored the ship, is to sercumvent the issues of beam propulsion; because typical light sail propulsion becomes less effecient the further from its light source. And a laser would be better than most other lighting techniques because it would be a single very strong light source spread over the intire sial through amplified bem divergrence.

Unrest

That's still a mirror. And it doesn't matter if you have 95% or 99.9% of the light reflected, an extra couple of percent efficiency isn't going to make or break such a different design.


I'm not sure if you're trying to say something but lacking suitable words, or just making it up. What you wrote has no meaning. Any light source can be made into a wide beam. What's the advantage of a single source if you then spread it out? You might as well have more/bigger sources and then you can get even more power without overheating them. What are the issues of beam propulsion? How does reflecting the beam circumvent them? Be aware that a laser can have mirrors internally anyway. Just about every directed light source has internal mirrors. How are they different from a solar sail? 95% vs 98% isn't a significant difference.

If you're talking about the double momentum imparted to a solar sail by having the light reflected vs just absorbed, then this doesn't take advantage of that because the forward-pointing laser will retard the craft's motion, the light needs to be reflected to compensate for that.