39 days to Mars possible now with nuclear-powered VASIMR.

[tater]

please, dont come out with trash stadistics. This is not even the topic for that, this has nothing to do with normal nuclear thermal plants.

You disagreeing doesn't make it untrue. The meaningful statistic for safety wrt power production is deaths per kWh. For the whole world total, deaths per tWh. Coal and hydro are terrible (the latter largely from a huge dam disaster in China---and actually chinese coal is also pretty bad). Solar and wind kill very few people, but the power they produce is no more than noise, so a tiny handful of deaths wrecks their mortality rate. Note that they have very low rates, under 1 death per tWh. Nukes are safer because they make so very much power, with relatively few issues. If we had a Chernobyl every few years, they'd start looking less safe (they'd still beat coal since nuke power beats coal by 3 orders of magnitude).

Space reactors are not a problem. I'd imaging they'd want as few points of failure as possible, so I'd expect pebble bed designs, anyway, which are incredibly safe.

[vger]

As many said, If you need to produce a lot of power in space, you need to take into account all the extra mass you need to radiate waste heat. Then you need to keep your crew far from the reactor. And your rocket launch may have extra security measures because you need to launch a nuclear reactor (that of course will not be operative in the moment of launch, which reduce by a lot the damage that can made).

If you put the reactor in the right place, how much shielding do you really need? Assuming it's way in the back, you only need to shield it in the direction of the rest of the ship. In any other direction, the radiation can just shoot out into space and nobody has to worry about it. Unless someone else wants to dock with the ship...

And I don't think anyone is going to care about extra security measures. As far as the other difficulties involved, the cost of extra security measures would be a non-factor by comparison.

Another thing about this. Assuming some of these would be used for return trip missions, I would hope this would be the sort of thing that could be recycled. Returning to Earth, shut the reactor down and leave it in a stable orbit (or dock it with the ISS, whatever). Then when you want to run another long-range mission, just send another ship to pick it up and turn it on again.

The heat, I'm not so sure about. But couldn't that even be used advantageously to get some extra speed?

[PB666]

If you put the reactor in the right place, how much shielding do you really need? Assuming it's way in the back, you only need to shield it in the direction of the rest of the ship. In any other direction, the radiation can just shoot out into space and nobody has to worry about it. Unless someone else wants to dock with the ship...

And I don't think anyone is going to care about extra security measures. As far as the other difficulties involved, the cost of extra security measures would be a non-factor by comparison.

Another thing about this. Assuming some of these would be used for return trip missions, I would hope this would be the sort of thing that could be recycled. Returning to Earth, shut the reactor down and leave it in a stable orbit (or dock it with the ISS, whatever). Then when you want to run another long-range mission, just send another ship to pick it up and turn it on again.

The heat, I'm not so sure about. But couldn't that even be used advantageously to get some extra speed?

You don't just shut them down, once they are on, they are more or less on for a month, maybe not subcritical but still producing alot of heat.

[Thomassino]

Who are those people, ive never heard of them. Even in Germany i dont know anyone saying anything about nuclear power in space, even when Curiosity was launched in the aftermath of Fukushima...

http://m.space.com/1929-nasa-pluto-mission-draws-dozen-protesters.html

http://www.space4peace.org/

I mean, yes, there is a slight chance that launch of a reactor, or RTG-powered craft could go wrong, but come on people! How do you want to power a Pluto probe anyway?

[peadar1987]

Those Bluefin off the coast of Florida are just waiting to ingest any uranium pellets that fall from the heavens. Its psychological issue, people would be afraid at just the hint that there was a reactor breach. Lead shielding wont kill you, I have worked around it for 30 years, uh... uh-oh..... damn [ker-plonk].

That was kind of my point. The LD50 for lead is about 70mg/kg (http://www.ila-lead.org/UserFiles/File/factbook/chapter7.pdf). The LD50 for Uranium is about 140-150 mg/kg (http://www.who.int/ionizing_radiation/pub_meet/en/Depluranium4.pdf)

Neither is going to kill you, but if a rocket carrying both explodes, the lead is actually the thing people should be worrying about, if they worry about anything.

[Nothalogh]

Nuclear-powered VASIMR won't be tested in space anytime soon IMHO, because some people are not comfortable with putting nuclear devices into space (they can hardly stand RTGs).

Those people need to be systematically ignored

[YNM]

The reason why people is worrying RTG is for their plutonium, which is nowhere near uranium.

We're talking about space don't we ? Just forget falling panels - just think of the panel glinting. In the same manner, don't mind Chernobyl-like accident (or those illegal handlings in Brazil), just think of some fried-out rover computer, which never happened out of nuclear radiation (except the Sun, which is a different thing).

Back on topic: as others have said, the problem will be energy density. 10 kW/kg is almost equal to heating 10 kg of water spread in an area ~9 m^2 (more like stretch of wet road than a pond) using daylight - while the numbers looks safe, you still have a lot of problem because fission (even fusion to a point) of nucleus radiates off subatomic particles and high-energy photons, unlike the Sun which radiates visible and IR photons out of it's surface. Also, wet road dries fast at clear day - you should consider that happening in a smaller area. You're going to need a good radiator because conduction and convection is a no-go.

Edited August 30, 2015 by YNM

[lajoswinkler]

Nuclear-powered VASIMR won't be tested in space anytime soon IMHO, because some people are not comfortable with putting nuclear devices into space (they can hardly stand RTGs).

I doubt that will be the case. The last kerfuffle about RTGs was with Cassini and it was mild. The society is way more connected now, and also seems to care less about some things.

How is a nuclear fission plant safer than a solar cell array ?

When the total life cycle is considered (and there's lots to consider; hence the energy economics study) nuclear fission wins by far because of its ginormous energy density and total output. Per unit of energy released, it causes the least damage to the environment. All power plants cause damage in some way.

Even without that in mind, fission and solar electrical are not mutually replaceable, whatever the lying hippies say.

They belong to totally different categories of energy production. Fission is sluggish and provides the base load, solar electrical is fast acting and can fill up peak loads... sometimes.

[jwenting]

How is a nuclear fission plant safer than a solar cell array ?

Over the entire energy cycle, it is. The environmental impact and risk to workers (accidents, deaths) is lower. That risk is mainly in the production of fuels and components, and in construction, but it's real.

- - - Updated - - -

Those people need to be systematically ignored

sadly they're the people who assign the budgets and sign the treaties banning nuclear "weapons" from space which makes putting full scale reactors in space all but impossible.

- - - Updated - - -

The reason why people is worrying RTG is for their plutonium, which is nowhere near uranium.

Which is ultimately silly. Plutonium isn't a doomsday device, whether in an RTG or any form ecxept maybe a ball of weapons grade material with an explosive lens packed around it and correctly primed...

Of course both Plutonium and Uranium are rather toxic in their oxidised form and salts, but the same is true for a lot of things and proper containment procedures (which have been known and in place for decades) make those a non-issue (any spill due to an RTG crashing down to earth would either spread the material over such a large area as to be non-problematic or keep it contained enough for a crew in with respirators and a bucket to safely remove it).

[Thomassino]

I doubt that will be the case. The last kerfuffle about RTGs was with Cassini and it was mild. The society is way more connected now, and also seems to care less about some things.

Let's hope you are right. But RTGs and nuclear reactors are relatively different things and reactors in space can unfortunately cause next kerfuffle (brilliant word ) and public hatred.

[YNM]

The reason why people is worrying RTG is for their plutonium, which is nowhere near uranium.

Which is ultimately silly. Plutonium isn't a doomsday device, whether in an RTG or any form ecxept maybe a ball of weapons grade material with an explosive lens packed around it and correctly primed...

Of course both Plutonium and Uranium are rather toxic in their oxidised form and salts, but the same is true for a lot of things and proper containment procedures (which have been known and in place for decades) make those a non-issue (any spill due to an RTG crashing down to earth would either spread the material over such a large area as to be non-problematic or keep it contained enough for a crew in with respirators and a bucket to safely remove it).

"... (any spill due to an RTG crashing down to earth would either spread the material over such a large area as to be non-problematic or keep it contained enough for a crew in with respirators and a bucket to safely remove it)."

Really ? Really ? REEALLLY ? Then what were those canadians doing, considering their super-empty neighborhood ? Granted it's a reactor, but that would bypass the RTG problem.

Rest of the post : I agree with you. I just pointed out what are people concerned about, for me as long we're not reckless while handling it it's fine, very fine.

Edited August 30, 2015 by YNM

[lajoswinkler]

"... (any spill due to an RTG crashing down to earth would either spread the material over such a large area as to be non-problematic or keep it contained enough for a crew in with respirators and a bucket to safely remove it)."

Really ? Really ? REEALLLY ? Then what were those canadians doing, considering their super-empty neighborhood ? Granted it's a reactor, but that would bypass the RTG problem.

Rest of the post : I agree with you. I just pointed out what are people concerned about, for me as long we're not reckless while handling it it's fine, very fine.

That was a reactor core that worked at one point in time, therefore it had fission products like cesium-137 which is soluble in water and, as all alkali metal ions, very mobile in the environment.

Reactor that's about to get launched is poorly radioactive.

You'd be surprised how easy nonfissioned uranium and plutonium in oxide (ceramic) form are manageable when dispersed.

[AngelLestat]

You disagreeing doesn't make it untrue. The meaningful statistic for safety wrt power production is deaths per kWh. For the whole world total, deaths per tWh. Coal and hydro are terrible (the latter largely from a huge dam disaster in China---and actually chinese coal is also pretty bad). Solar and wind kill very few people, but the power they produce is no more than noise, so a tiny handful of deaths wrecks their mortality rate. Note that they have very low rates, under 1 death per tWh. Nukes are safer because they make so very much power, with relatively few issues. If we had a Chernobyl every few years, they'd start looking less safe (they'd still beat coal since nuke power beats coal by 3 orders of magnitude).

Space reactors are not a problem. I'd imaging they'd want as few points of failure as possible, so I'd expect pebble bed designs, anyway, which are incredibly safe.

As I said.. this is not the place to have this discussion, we already had this discussion in 3 different threads.

If you want.. make a new thread and I will prove you there how that stadistic source is full of trash.

If you put the reactor in the right place, how much shielding do you really need? Assuming it's way in the back, you only need to shield it in the direction of the rest of the ship. In any other direction, the radiation can just shoot out into space and nobody has to worry about it. Unless someone else wants to dock with the ship...

And I don't think anyone is going to care about extra security measures. As far as the other difficulties involved, the cost of extra security measures would be a non-factor by comparison.

Another thing about this. Assuming some of these would be used for return trip missions, I would hope this would be the sort of thing that could be recycled. Returning to Earth, shut the reactor down and leave it in a stable orbit (or dock it with the ISS, whatever). Then when you want to run another long-range mission, just send another ship to pick it up and turn it on again.

The heat, I'm not so sure about. But couldn't that even be used advantageously to get some extra speed?

You can have a combined cycle to produce extra power from the waste heat (but it does not reduce your radiatior area or mass, in fact is increased)

Not sure what it would be the ultimate design solution for a Vasimir-nuclear ship (in case vasimir works as is expected).

If you plan to have many trips to mars go and back, it might have sense.

Maybe engines on front with a very long "cable" that might act also as radiator with a liquid heat carrier might work. After acceleration the cable can be used as artificial gravity between the reactor-engine and the crew cabin.

A magnetic field in the crew cabin may divert the radioactive particles from the exhaust.

[Exoscientist]

...with a VASIMR drive several years away from being flight ready, and a nuclear reactor which does not exist, not even in engineering blueprints - because it is physically impossible to achieve your numbers. I'm sorry Mr. Clark, but your article is pure clickbait with no substance. Please try to be more thorough in your research, and less sensational. If your math turns out an unbelievable result, it's usually best to actually disbelieve it and look for your error instead of taking it for truth.

The idea of using nuclear reactors for space applications isn't new. And it's not as simple as you think it is. For example, you conveniently ignore the mass of the radiators required to cool the reactor, which is not trivial - in fact, the majority of the reactor's output is going to be waste heat. The NTR does not require radiators, because the fuel carries the heat away. If you're producing power, the heat needs to go somewhere. You did not notice this because you went straight from "the reactor produces heat" to "I am converting mechanical into electrical energy". But where does that mechanical energy come from? You forgot the heat engine, the part that converts thermal energy into mechanical energy. And that part is massively inefficient, which causes the huge amount of waste heat.

Some 15 years ago, NASA construted a test reactor called the SAFE-400 ("Safe Affordable Fission Engine"). It produced just 400 kW of thermal power, and it had a built-in dynamo driven by an equally built-in brayton cycle turbine - the best heat engine that physics knows how to build today. It's rated electrical output was 100 kW. In other words, 25% of the reactor's output was usable, and 75% was waste heat. At 520 kg weight, the reactor had a specific power of 192 W/kg... and that is presumably measured without the mass of the radiators required to deal with the 300 kW waste heat, since such a thing was not part of the experiment setup according to photos.

Upscaling the reactor would yield better mass ratios and increase the specific power. Perhaps a fancy cryocooled generator could further improve things, but probably not by a lot, since a.) that is not where the major losses are in the first place and b.) cryocoolers are heavy and require even more radiators. However, to get anywhere close to your dream of 10 kW/kg, the reactor would need to become 52 times as efficient as it was in NASA's lab... and then it would need to become even more efficient, because that still ignores the weight of the radiators. Unfortunately I need to tell you: this is not going to happen. Even if the SAFE-400 had absolutely zero losses, it would still only have posted ~770 W/kg. And you are never going to get a lossless conversion. 300 W/kg? Sure, may work on a large-scale reactor. Maybe even 500 W/kg in the future, if such reactors were being actively developed, which they are not. 10-100 kW/kg? Not today, not ever, not with nuclear fission.

SAFE-400 is not at the maximum efficiency possible. The conversion from thermal to kinetic via rocket exhaust is above 90% for rocket engines. For example, for the space shuttle main engines I think it's 97%. Rocket engines are more efficient then jet engines or gas turbines largely because of the higher temperatures, jet engines and gas turbines typically being only around 35% efficient. For one thing, you don't have nitrogen ameliorating the combustion temperature in rocket engines.

However, for our purposes of driving an electric generator the best comparison would be to a component of a rocket engine that would drive a shaft. This would be the turbopumps. The space shuttle LH2 turbopumps have extraordinary weight efficiency for converting the thermal energy of the rocket combustion to mechanical energy, in the range of 150,000 w/kg (!). Also important though is the energy conversion efficiency. For the turbopumps it's about 80%. However, other turbopumps have been known to get above 90%. Then first of all for our nuclear power system, we want the energy conversion to be at least as efficient as the shuttle turbopumps. Likely we could then even push the efficiency to the 90% and above range. We can sacrifice some of the weight efficiency since we only need the specific power to be at ca. 1,000 watt/kg for the VASIMR system. That is, we could allow our turbines to be heavier to get the higher conversion efficiency.

Note also I mentioned the conversion efficiency for electric motors and generators from mechanical to electrical can be in the 95% range. We want the overall conversion efficiency to be high to reduce the need for radiators. Large radiators were needed before because of the low conversion efficiency to electric. For instance if the conversion efficiency is only 5%, which some of these systems had, then 95% of the thermal energy has to be radiated away as heat. For multimegawatt and gigawatt systems this would be very large radiators for rejecting actually almost all the generated power. With efficient energy conversion you would be able to get the radiators to be only 1/10th the size.

On this page there's a table listing the mass for various types of waste heat radiators:

Radiator for 250,000 kilowatts waste heat.

http://www.projectrho.com/public_html/rocket/basicdesign.php#id--Heat_Radiators

If the waste heat is only 1/10th this high, requiring a radiator 1/10th the mass, this would still give us a quite high specific power for our system.

Bob Clark

[peadar1987]

SAFE-400 is not at the maximum efficiency possible. The conversion from thermal to kinetic via rocket exhaust is above 90% for rocket engines. For example, for the space shuttle main engines I think it's 97%. Rocket engines are more efficient then jet engines or gas turbines largely because of the higher temperatures, jet engines and gas turbines typically being only around 35% efficient. For one thing, you don't have nitrogen ameliorating the combustion temperature in rocket engines.

However, for our purposes of driving an electric generator the best comparison would be to a component of a rocket engine that would drive a shaft. This would be the turbopumps. The space shuttle LH2 turbopumps have extraordinary weight efficiency for converting the thermal energy of the rocket combustion to mechanical energy, in the range of 150,000 w/kg (!). Also important though is the energy conversion efficiency. For the turbopumps it's about 80%. However, other turbopumps have been known to get above 90%. Then first of all for our nuclear power system, we want the energy conversion to be at least as efficient as the shuttle turbopumps. Likely we could then even push the efficiency to the 90% and above range. We can sacrifice some of the weight efficiency since we only need the specific power to be at ca. 1,000 watt/kg for the VASIMR system. That is, we could allow our turbines to be heavier to get the higher conversion efficiency.

Note also I mentioned the conversion efficiency for electric motors and generators from mechanical to electrical can be in the 95% range. We want the overall conversion efficiency to be high to reduce the need for radiators. Large radiators were needed before because of the low conversion efficiency to electric. For instance if the conversion efficiency is only 5%, which some of these systems had, then 95% of the thermal energy has to be radiated away as heat. For multimegawatt and gigawatt systems this would be very large radiators for rejecting actually almost all the generated power. With efficient energy conversion you would be able to get the radiators to be only 1/10th the size.

On this page there's a table listing the mass for various types of waste heat radiators:

Radiator for 250,000 kilowatts waste heat.

http://www.projectrho.com/public_html/rocket/basicdesign.php#id--Heat_Radiators

If the waste heat is only 1/10th this high, requiring a radiator 1/10th the mass, this would still give us a quite high specific power for our system.

Bob Clark

That's not how it works. All heat engines are limited by the Carnot efficiency of the cycle, given by ETA=1-(TSink/TSource). This is a hard law of thermodynamics and there is no way around it, save by avoiding using a heat engine altogether (fuel cells, for example, can achieve higher efficiencies).

You might be thinking of the propulsive efficiency of a rocket engine, which is not the same as the thermal efficiency. Propulsive efficiency can, in theory, approach 100%.

The "efficiency" you see quoted for turbopumps isn't for their ability to convert heat in the exhaust into mechanical work. It is what is known as the "isentropic efficiency", which is how they perform their pressurisation process compared to a thermodynamically ideal model.

For a nuclear reactor operating at, say, 1500K (which is extremely high, and will require a very high flow rate of coolant, which costs mass), you can easily calculate the Carnot efficiency of the power conversion cycle for a range of heat sink temperatures. For a heat sink of 500K, the Carnot efficiency is 67%. For a heat sink of 300K, the Carnot efficiency is 80%.

Now, the Carnot efficiency isn't a limit you can ever actually reach, only approach. And the closer to the Carnot efficiency you get, the more cumbersome and heavy your equipment becomes. The best that is usually achieved in practice is with a combined cycle gas turbine, which uses a gas turbine running on the Brayton cycle, with a bottoming Rankine cycle powered by the Brayton cycle's exhaust. These achieve thermal efficiencies of about 50% from a 1400K heat source, or in other words, about 60% of the Carnot efficiency.

The real kicker is that the lower your heat sink temperature, the bigger your radiators have to be, so if you try and save mass by cranking up the Carnot efficiency by dropping the heat sink temperature, the less effectively your radiators work. Radiative heat transfer scales with the fourth power of temperature, so if you drop the heat sink temperature from 500K to 300K, you more or less triple the size of the radiators for a given heat rejection rate, while only reducing the heat you need to reject by about 60%.

---------------------------------------------

So anyway, Atomic Rockets have a section on this, and it turns out that if you want to minimise radiator mass while maximising power, your heat sink temperature needs to be about 80% of your heat source temperature. Go higher and you lose thermal efficiency, meaning you need to radiate more heat, go lower and your radiators don't radiate heat as well.

This means that the Carnot efficiency of your cycle will only be 20%. Real world efficiency, if we're being really generous, will be maybe 14%, if we install heavy, efficient heat exchangers and expanders.

If our reactor is producing 335MWth, about 47MW will actually be converted into useful electrical power for the VASIMRs. The remaining ~290MW will need to be rejected through the radiators. The Atomic Rockets link you used previously tells us that this will require about 22,000kg of radiator.

So now we're already up to about 24,000kg of generation system for 47MW of electrical power. That's only 2000W/kg. And that's just a reactor core and radiators, remember. No mention of generators, switchgear, heat exchangers, expanders, pumps, shielding, working fluid, or conversion losses. Maybe, with heavy, heavy optimisation of every possible element of the cycle, we can get to the magical 1,000W/kg, but it will be very far from easy.

Edited August 30, 2015 by peadar1987

[AngelLestat]

Peadar, that highly depend on the radiatior mass efficiency.

What kind of material and system are you using for your hypothetic radiator?

There are new materials and techniques that can increase that by a lot.

[peadar1987]

Peadar, that highly depend on the radiatior mass efficiency.

What kind of material and system are you using for your hypothetic radiator?

There are new materials and techniques that can increase that by a lot.

A Marangoni Flow Radiator, as per Atomic Rockets.

[Nothalogh]

The cold hard truth is that all major funding allocated to NASA since 2004 should have been for building and fielding NERVAs.

They've been static tested, they work flawlessly, they only need to be built and put to work

[Exoscientist]

That's not how it works. All heat engines are limited by the Carnot efficiency of the cycle, given by ETA=1-(TSink/TSource). This is a hard law of thermodynamics and there is no way around it, save by avoiding using a heat engine altogether (fuel cells, for example, can achieve higher efficiencies).

You might be thinking of the propulsive efficiency of a rocket engine, which is not the same as the thermal efficiency. Propulsive efficiency can, in theory, approach 100%.

The "efficiency" you see quoted for turbopumps isn't for their ability to convert heat in the exhaust into mechanical work. It is what is known as the "isentropic efficiency", which is how they perform their pressurisation process compared to a thermodynamically ideal model.

The efficiency of a rocket engine really is that high as calculated by their thermal energy produced compared to the kinetic energy of the exhaust stream. It is hard to believe for people used to the ca. 35% efficiency of piston engines and gas turbine engines. But the 80% nitrogen content of air, which does not contribute to the combustion, both lowers the combustion temperature and slows the exhaust velocity. That plus the many moving parts in the internal combustion engine lowers the efficiency to that extent. Rocket engines use pure O2 or other efficient oxidizer, thus giving high combustion temperature, and have no moving parts so they can achieve high thermal efficiency. And in any case for the actually produced nuclear thermal engines their thermal efficiency as measured by their exhaust velocity was proven to exceed 90%.

For turbopumps their efficiency is calculated by comparing the energy input compared to the energy required to compress their fluids to the measured extent. Since there are some losses involved in the compression process, the efficiency of the power put into the compressor is actually higher than the quoted efficiency. That is, for our purposes of driving an electric generator driveshaft the efficiency would be higher.

I'm not familiar with the calculation of the ideal temperature of a radiator but given the thermal efficiency and weight efficiency of a rocket engine, the radiator could be 100 times the weight of the engine and you would still have the ca. 1,000 watts per kilo overall specific power required. Considering the low weights in that table of radiator mass, that's not likely.

Bob Clark

[fredinno]

Similar to space travel, in a sense.

Bus crashes, kills 30 people, barely makes the news.

Space shuttle explodes, carrying 7 people and communications satellites, and everyone flips their .....

Never mind that the space shuttle is technically safer than any bus.

- - - Updated - - -

Nuclear is incredibly safe in deaths per unit power produced. Safer than wind or solar. Solar is often placed on roofs, and one fall can kill their average. The nuke mortality includes Chernobyl. Being worried about nuclear power is like being worried about flying while driving to the airport (more dangerous than the flight by a wide margin).

It's been known that Nuclear is technically the safest energy source you can find ( even if you count in Uranium mine deaths, and construction deaths) due to its immense safety margins. People probably are afraid of it, due to not really knowing how it works- nearly every other practical energy source has an obvious power source- nuclear does not. It uses something called "nuclear fission" only "scientists" and "nerds" understand.

I wonder how people will react to nuclear fusion (or LFTR) when it comes online, since it has the same basic problem. Either

1. It will make energy so cheap anyways, it will become the primary energy source of developed nations due to pure economics, resulting in a much better quality of life, and a new economic boom.

2. It will be so heavily regulated that it will never take off due to irrational fears, and the economic boom will never take place.

If the 2nd happens, I don't want to live on this planet anymore. Oh, wait...

- - - Updated - - -

Nuclear is incredibly safe in deaths per unit power produced. Safer than wind or solar. Solar is often placed on roofs, and one fall can kill their average. The nuke mortality includes Chernobyl. Being worried about nuclear power is like being worried about flying while driving to the airport (more dangerous than the flight by a wide margin).

It's been known that Nuclear is technically the safest energy source you can find ( even if you count in Uranium mine deaths, and construction deaths) due to its immense safety margins. People probably are afraid of it, due to not really knowing how it works- nearly every other practical energy source has an obvious power source- nuclear does not. It uses something called "nuclear fission" only "scientists" and "nerds" understand.

I wonder how people will react to nuclear fusion (or LFTR) when it comes online, since it has the same basic problem. Either

1. It will make energy so cheap anyways, it will become the primary energy source of developed nations due to pure economics, resulting in a much better quality of life, and a new economic boom.

2. It will be so heavily regulated that it will never take off due to irrational fears, and the economic boom will never take place.

If the 2nd happens, I don't want to live on this planet anymore. Oh, wait...

[AngelLestat]

A Marangoni Flow Radiator, as per Atomic Rockets.

Not sure why the mass density for all radiators is so high... it said 24kg/m2 or similar for other kinds of radiators.

Meanwhile Nasa is looking for 2 to 4 kg/m2 which is totally coherent, and there is some papers that achieve 2.5 just using carbon fibers.. nothing special.

Think about it, why it needs 24kg/m2??? That weights more than 1 m2 of heavy mosaic, meanwhile a very thin carbon layer can do the same work.

The key here is to maximize the core temperature in the reactor, I guess 1500 celcius is possible.

[peadar1987]

The efficiency of a rocket engine really is that high as calculated by their thermal energy produced compared to the kinetic energy of the exhaust stream. It is hard to believe for people used to the ca. 35% efficiency of piston engines and gas turbine engines. But the 80% nitrogen content of air, which does not contribute to the combustion, both lowers the combustion temperature and slows the exhaust velocity. That plus the many moving parts in the internal combustion engine lowers the efficiency to that extent. Rocket engines use pure O2 or other efficient oxidizer, thus giving high combustion temperature, and have no moving parts so they can achieve high thermal efficiency. And in any case for the actually produced nuclear thermal engines their thermal efficiency as measured by their exhaust velocity was proven to exceed 90%.

Not true. You can't surpass the Carnot limit with a heat engine. A rocket engine is still a heat engine.

I'll crunch the numbers for you if you like.

The Space Shuttle Main Engine has a combustion temperature of 3500K and a chamber pressure of 20.64MPa (from wikipedia).

Plugging these values into the NIST REFPROP 9.0 fluid properties calculator for water vapour, this gives us an enthalpy of 12500kJ/kg.

The exhaust velocity in vacuum is 4.4km/s. This gives a kinetic energy of 9,680kJ/kg.

Thermal efficiency of 77.44% in vacuum. Dropping to 51.8% at sea level, as the exhaust velocity drops to 3.6km/s.

This is because the chamber temperature is extremely high. It can afford to be as the chamber walls are regeneratively cooled, so the 3500K temperature never actually reaches them, it just exists in the centre of the chamber. For a nuclear thermal rocket, and especially for a nuclear power plant, you're limited by the material properties of the engine and the fuel. Uranium dioxide, for example, melts at 3100K. Not softens, actually melts. You can't run a nuclear reactor at those temperatures. NERVA ran at about 2400K.

Look at it this way: If the SSME has a thermal efficiency of 95%, then why was the exhaust hot?

If you disagree with me, show me the maths. Or at least give me a source. Don't just say "it's been proven", because it looks like you're fundamentally misunderstanding quite a few things about thermodynamics here, and I'd like to be able to set things straight.

For turbopumps their efficiency is calculated by comparing the energy input compared to the energy required to compress their fluids to the measured extent. Since there are some losses involved in the compression process, the efficiency of the power put into the compressor is actually higher than the quoted efficiency. That is, for our purposes of driving an electric generator driveshaft the efficiency would be higher.

You can't just scale up a turbopump and use it to recover 90% of the energy from rocket exhaust. Thermodynamics does not work that way. If you're skimming a tiny amount of extremely high-temperature gas off to run a compressor, you can run with very small losses, but the more you take out of the stream, the more its temperature and pressure drop, and the harder it is to extract energy. Extracting further energy becomes uneconomical fairly soon, and then you run into the Carnot limit, which, I will reiterate, cannot be surpassed by a heat engine.

I'm not familiar with the calculation of the ideal temperature of a radiator but given the thermal efficiency and weight efficiency of a rocket engine, the radiator could be 100 times the weight of the engine and you would still have the ca. 1,000 watts per kilo overall specific power required. Considering the low weights in that table of radiator mass, that's not likely.

Bob Clark

You're still making several erroneous assumptions. Firstly that rocket engines have a thermal efficiency of 95%+ (they don't), and secondly, that a rocket engine is an appropriate cycle to use for electrical power generation (it isn't).

If you replace these assumptions with more realistic ones, as detailed in my first post, the thermal efficiency of the reactor drops from 95% to about 14%, with a corresponding increase in required radiator mass.

- - - Updated - - -

Not sure why the mass density for all radiators is so high... it said 24kg/m2 or similar for other kinds of radiators.

Meanwhile Nasa is looking for 2 to 4 kg/m2 which is totally coherent, and there is some papers that achieve 2.5 just using carbon fibers.. nothing special.

Think about it, why it needs 24kg/m2??? That weights more than 1 m2 of heavy mosaic, meanwhile a very thin carbon layer can do the same work.

Possibly because the effectiveness of a radiator goes up if it's more complicated. NASA's radiators are fer less dense, but also radiate far less heat per unit area. I'm not an expert in radiative heat transfer, so I'm just going to have to work with what I have.

The key here is to maximize the core temperature in the reactor, I guess 1500 celcius is possible.

Actually, core temperature doesn't matter (strangely enough). You'll always get the same answer for Carnot efficiency. I can't link to the relevant secton, but I'm going to try copying it in:

It is surprising but there is an optimum temperature ratio at which to run a starship heat exchanger (or similar power source) to achieve maximum free power with a minimum of radiator area. The only assumptions necesary are that the power source obeys the laws of thermodynamics and that the starship may only get rid of waste heat by radiating.

Let us assume that we have a heat engine as a power source with a relative efficiency of η, and an absolute efficiency is η times the Carnot efficiency ε. We can write the available free power, F, as:

F = Qηε = Qη(1 - T1/T2)

where Q is the rate of heat flow into the exchanger and T1 and T2 are the temperatures of the cold and hot sides of the engine, respectively. The waste heat, H, released into the starship is Q - F, or:

H = Q(1 - η + ηT1/T2)

H = F (1 - η + ηT1/T2)/η(1 - T1/T2)

To simplify, we will measure temperature in units of T2 and let T1 be called just T. After dividing through by η the amount of waste heat associated with a given free power F is then:

H = F (η-1 - 1 + T) / (1 -T)

Now this waste heat must be radiated away from the ship. The power radiated by a black body at temperature T and with area A is given by the Stephan-Boltzmann Law:

P = AÃÆ’T4

with ÃÆ’ a constant depending on the choice of units. Setting these equal to each other gives:

A = F (η-1 - 1 + T) / ÃÆ’(T4 - T5)

Now we can ask what value of T will give the minimum radiator area. Taking the derivative of A with respect to T and setting it equal to zero gives:

(T4 - T5) - (4T3 - 5T4)(η-1 - 1 + T) = 0

Or, dividing by T3 and expanding:

T - T2 - 4η-1 - 4 + 4T + 5Tη-1 - 5T + 5T2 = 0

After collecting terms, we have:

4T2 + (5η-1 - 8)T + 4(η-1 - 1) = 0

or, dividing through by 4:

T2 + (5/4η-1 - 2)T + (η-1 - 1) = 0

We write η-1 as γ then the solution to the above quadratic can be written:

T = 1 - 5/8γ + 1/8 sqrt(25γ2 - 16γ)

In the special case where the exchanger runs at maximum theoretical efficiency, η = γ = 1 and the equation above gives T = 3/4. This means that the cold side of the heat engine is at 75% of the temperature of the hot side.

This is horribly inefficient as a Carnot heat engine goes, but if the radiator temperature drops, the surface area (and thus mass) must increase, because of the T4 behavior of the radiation law  colder radiators dump heat much less efficiently. This function is fairly flat  as η drops from 1 to a more plausible 0.1, T changes from 3/4 to 4/5

Alternatively just go to here and search for "optimum radiator temperature".

Edited August 31, 2015 by peadar1987

[Albert VDS]

Isn't a 39 days to Mars a riskier trip than a 6 month trip? Because a 6 month trip can give a free return to Earth if something goes wrong en route.

[NuclearNut]

The best thermal power plants on earth run at about 40% thermal efficiency. This is approaching a hard limit imposed by the Carnot efficiency of the cycle. The other 60% of the energy needs to be dumped into the environment. If you're dumping multiple megawatts of heat into space, there's no way around it, you need heavy radiators.

It's also worth noting that the source you cited is only for the engine. It doesn't include any shielding at all. What it also doesn't include is some sort of working fluid for the cycle. No pump, no heat exchangers, no expander.

A power plant for electricity generation is a significantly more heavy and complex machine than something that just heats up fuel and spits it out the back.

Actually, it appears that the average Combined Cycle power station gets around 60%.

Nuclear reactors only get around 40% because of the low coolant temperatures, around a mere 300 degrees celsius. This is due to the fact that water will boil even if pressurized above around 370 degrees. Thus nuclear reactors run well below that margin to prevent coolant boiling in the reactor core and corrosion of the fuel rods which are supposed to last three to four years. This is of course assuming you are using a PWR. If you are using a BWR you have a slightly different problem, the coolant is the moderator, and when it boils it becomes a lot less effective at moderating, thus more boiling shuts down the reactor, creating a strong negative feedback loop in the reactor core and preventing the temperature from getting that high.

Of course this is assuming you are using water as a coolant, and you are not using something like the currently being researched SCWR. Now for a reactor on a spaceship you would not want water as a coolant, moderator or anything. You would probably be using a gas cooled fast reactor running the carnot cycle, or a liquid metal cooled fast reactor using a thermocouple. If you want to get really fancy (though I do not know how well this would work, with the neutrons actively altering the material) you could integrate the thermocouple directly into the fuel rods, with the only coolant being liquid metal going to the radiators located on the cold side of the hollow thermocouple integrated fuel rods. Mind, thermocouples would not be efficient, but they would save on mass compared to a giant turbine, and in this sanario fuel is most certainly not a concern, though again, I have a feeling that the neutrons would be a problem, damaging the thermocouple over time.

EDIT:

Looking over the article they seem a tad optimistic. Before we begin to fret over if people will oppose the launch, we must look at getting an extremely light reactor that is easy to control, can change power output quite a bit very quickly, needs no maintenance throughout the trip, and most importantly has large margins of safety. Tests in a vacuum chamber lasting a few hours may be good, and definitely provide some ideas, however they do not provide the same experience running one for a few years under varying conditions and loads would. While knowledge regarding the gross points of the design are easily obtainable it is probable that the finer points, such as corrosion of some obscure but essential part, take time to identify.

Edited August 31, 2015 by NuclearNut

[peadar1987]

Actually, it appears that the average Combined Cycle power station gets around 60%.

This is true (I actually do mention CCGTs in a subsequent post, but I was in nuclear mode at this point). The OP was assuming you could get 90-95% thermal efficiency out of the reactor in power generation mode, with minimal ancillaries, which is never going to happen. See my two most recent posts for something a little more in-depth.

Nuclear reactors only get around 40% because of the low coolant temperatures, around a mere 300 degrees celsius. This is due to the fact that water will boil even if pressurized above around 370 degrees. Thus nuclear reactors run well below that margin to prevent coolant boiling in the reactor core and corrosion of the fuel rods which are supposed to last three to four years. This is of course assuming you are using a PWR. If you are using a BWR you have a slightly different problem, the coolant is the moderator, and when it boils it becomes a lot less effective at moderating, thus more boiling shuts down the reactor, creating a strong negative feedback loop in the reactor core and preventing the temperature from getting that high.

The AGRs I worked on had a channel gas outlet temperature out 648 celsius. They used supercritical CO2 as their coolant. Really nice design, and the thermal efficiency is about 41% (except for Hunterston B, because they sucked a load of sea water into the reactor core by accident and gubbed it up with salt!)

Of course this is assuming you are using water as a coolant, and you are not using something like the currently being researched SCWR. Now for a reactor on a spaceship you would not want water as a coolant, moderator or anything. You would probably be using a gas cooled fast reactor running the carnot cycle, or a liquid metal cooled fast reactor using a thermocouple. If you want to get really fancy (though I do not know how well this would work, with the neutrons actively altering the material) you could integrate the thermocouple directly into the fuel rods, with the only coolant being liquid metal going to the radiators located on the cold side of the hollow thermocouple integrated fuel rods. Mind, thermocouples would not be efficient, but they would save on mass compared to a giant turbine, and in this sanario fuel is most certainly not a concern, though again, I have a feeling that the neutrons would be a problem, damaging the thermocouple over time.

All of the UK's reactor fleet have thermocouples in the reactor core, and although they do fail, it's pretty rare.

The main problem isn't that it is hard to maximise the efficiency of the reactor though, it is the trade-off between the thermal cycle efficiency and the radiator efficiency, which means you're not going to break 20% efficiency using a heat engine without needing a monster radiator which will completely destroy your specific power anyway. Thermocouples don't have the same fundamental limit as far as I know, but in practice they are even less efficient.

EDIT:

Looking over the article they seem a tad optimistic. Before we begin to fret over if people will oppose the launch, we must look at getting an extremely light reactor that is easy to control, can change power output quite a bit very quickly, needs no maintenance throughout the trip, and most importantly has large margins of safety. Tests in a vacuum chamber lasting a few hours may be good, and definitely provide some ideas, however they do not provide the same experience running one for a few years under varying conditions and loads would. While knowledge regarding the gross points of the design are easily obtainable it is probable that the finer points, such as corrosion of some obscure but essential part, take time to identify.

And the OP is misinformed about even the gross points of the design.