To Mars in 39 days?

[Urwumpe]

..., and for electromagnetic radiation (x-rays, gamma rays) lead is a good, dense choice.

More important, Gamma radiation is less dangerous as other radiations, because it has a lower chance to interact with you - what passes meters of concrete doesn't really notice a human. It requires a lot of gamma ray flux to harm humans, and the radiation drops by the inverse square law automatically.

I've heard that thermoelectric conversion is seriously inefficient (something like 1%). Is this true?

Not for nuclear reactors, I remember it getting more effective with higher power densities. 10-20% are possible with thermionic converters.

[ame="http://en.wikipedia.org/wiki/Thermionic_converter"]Thermionic converter - Wikipedia, the free encyclopedia[/ame]

A radioisotopethermalgenerator that just uses decay heat, can't power such a converter well, a nuclear fission reactor has no problems.

 

[Sky Captain]

The best place to go would be info on the alpha (mass to power) of various nuclear reactor designs.

According to Atomic Rockets, most reactors have an alpha of around 20 kg/kW, and an experimental 1 MW reactor had an alpha of just under 0.5 kg/kW.

Nuclear reactors can get far better than 0,5 kg/kw. NERVA prototypes reached power of 4GW and according to wikipedia
[ame="http://en.wikipedia.org/wiki/NERVA"]NERVA - Wikipedia, the free encyclopedia[/ame]
a NERVA rocket stage would have dry mass of 34000 kg, with power of 4 GW it would be ~0,0085 kg/kW. And that was 40 years old technology.

 

[Urwumpe]

Sky Captain: Nuclear thermal rockets are something different than nuclear reactors for producing electricity.

 

[Sky Captain]

But NERVA only lacked the machinery to generate electricity. Suppose you use the same core and run some sort of coolant through it to power a turbine and turn generator. Would the recquired systems really add so much weight that you would go from 34 ton reactor core to 4000+ ton electricity generating power reactor?

 

[jedidia]

Would the recquired systems really add so much weight that you would go from 34 ton reactor core to 4000+ ton electricity generating power reactor?

It depends on how much power you want, I think. To make full use of the power output (minus efficiency of course), you'll need to plug quite a generator in there, yes (and the cooling is also a problem... the nice thing about NERVA is that it blows most of its heat out of the exhaust).

To just tab the power a bit to have enough for your onboard systems (as was the plan of a NERVA variant), you could go with a pretty small generator, but the plan for a VASIMR is to use the whole power output of the reactor to power a thruster, which is a completely different kind of animal.

It can still pay off though, because a VASIMR is a MUCH more efficient thruster than NERVA.

 

[Urwumpe]

But NERVA only lacked the machinery to generate electricity.

:facepalm:

Suppose you use the same core and run some sort of coolant through it to power a turbine and turn generator. Would the recquired systems really add so much weight that you would go from 34 ton reactor core to 4000+ ton electricity generating power reactor?

NERVA is only supposed to run for a few minutes, not months. NERVA is optimized for thermal power, NERVA is open core (=fuel is directly ejected after heating). You can tap electrical power of a nuclear thermal rocket engine by Brayton cycle, but at much lower energy levels and effectivity.

 

[T.Neo]

Ah, open cycle cooling. Pity it doesn't work if you have a low mass flow.

It can still pay off though, because a VASIMR is a MUCH more efficient thruster than NERVA.

Except... except... it isn't high thrust! :shifty:

 

[Sky Captain]

:facepalm:



NERVA is only supposed to run for a few minutes, not months. NERVA is optimized for thermal power, NERVA is open core (=fuel is directly ejected after heating). You can tap electrical power of a nuclear thermal rocket engine by Brayton cycle, but at much lower energy levels and effectivity.

Since NERVA proved it is possible to build a reactor core with very high power to mass ratio I want to know which systems make the mass budget skyrocket when you want to get electricity from reactor. A reactor core that generates 4 GW of heat and is designed to run continously would be heavier, but it certainly won't weigh thousands of tons. A gas turbine can reach very high power to mass ratio I have heard a 500 MW gas turbine could weigh only few tons. electrical generators also can be much better than 1 - 2 kg/kw, high temperature radiators could be ~0,02 kg/kw. If MHD generator is used a weight could be saved even more.

 

[Urwumpe]

A gas turbine can reach very high power to mass ratio I have heard a 500 MW gas turbine could weigh only few tons.

if you only need to let the turbine run for minutes, that is no problem - if you want to let the turbine run for months, you need lower temperatures, lower rotation speeds, less vibrations, less metal fatigue, etc. Even on Earth, you can't just disassemble a nuclear reactor turbine every few hours for a check, in space, you want to let the reactor run ideally without EVA/IVA maintenance.

The reason why nuclear reactors weight so much is simply for making them robust enough. The 10 MW nuclear reactor of the Nautilus added about 1500 tons displacement to the XXI submarine, that the Nautilus was based on. If you removed most batteries, electric engines and diesels for this, this would mean about 2000 tons of reactor mass.

It was of course, a first prototype, but as you can see, came with a lot of mass for little power. The S9G reactor of today is a bit heavier, about twice as powerful, but really gained lifetime: 33 years of service now.

 

[dougkeenan]

Where is the addon?

 

[EnDSchultz]

I also just got that issue of PopSci. Was surprised when I saw the cover, and I thought I had seen that space ship somewhere before... Guess where?!

http://"http://orbithangar.com/searchid.php?ID=3212[ame="http://orbithangar.com/searchid.php?ID=3212"]USS Bekuo[/ame]

 

[tblaxland]

Staff note: Off-topic posts regarding intercontinental trips moved to Random Comments thread.