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

[PB666]

The problems of such space trips in BEO space for several months have been much discussed, from bone and muscle loss, to radiation damage, to the recently discovered eye damage. In fact William Gerstenmaier, head of NASA's human spaceflight division, has said the NASA Mars mission architectures, and also Bob Zubrin's, that might take 900 days total round trip are unworkable:

Yes, NASA really is reconsidering the moon, and here’s why that’s important.

Posted on April 6, 2015 | BY ERIC BERGER

http://blog.mysanantonio.com/newswatch/2015/04/yes-nasa-really-is-reconsidering-the-moon-and-heres-why-thats-important/

Gerstenmaier suggested, as have many others outside of NASA, that using lunar derived fuel in orbital propellant depots would make Mars missions easier

I dont have a problem with going to the moon, byt frankly the idea in the article iis dumb. I work with water under a vacuum, it sublimates real easy, the flask at room temperature in about thirty minutes are covered with ice and if you isulate them the temperature is around -30C. I actually freze the flask to get them going by twirling them in an alcohol gel in frozen carbon dioxide. Even at that starting condition in about 6 hours all the water is gone.The amount of ice you would recover at the moons poles would not be enough to brush your teeth with for how many hours of work? and it certainly could not pay for a mission to the poles.

You can pressurized hydrogen to 3000 PSI and in space at space temperatures, keeping them at L2 or shielded and the sun you are pressurized damn near to liquid hydrogen, same is true for oxygen. just buy one or two of those 100 ton payload rockets from the russians and start parking fuel at L2 when the time comes to go, liquify the hydrogen and oxygen and fill up the tank. 900 days is too long, but it will take 1.8 years to get back no matter what. All you need to do is to create a station at L2, manned with one engineer. Once your spaceship is almost ready bring your Mars crew and send the engineer home.

Seriously, mining water on the moon for space flight, this cannot be a serious plan, the moon is a desert that puts atacama to shame.

L

[SargeRho]

Seriously, mining water on the moon for space flight, this cannot be a serious plan, the moon is a desert that puts atacama to shame.

It's more like mining Antartica for Ice. There is plenty of water ice in the polar craters.

[AngelLestat]

What is the mass of a terrestrial 1 GW reactor? Include all heat sinks (say seawater passed through a heat exchanger, plus the entire mass of air that the seawater might also interact with. And the earth, for conductive heat loss).

The issue is power per unit mass, not the ability to make X power on earth with no mass limitations. Spaceflight is about payload fraction.

Why you need 1 gw? some vasimr designs goes from 100kw to 50 mw. If you dont need to carry humans, 100kw is enoght.

Furthermore nuclear space reactors does not need to be heavy, you dont need the concrete or heavy shielding, you can put the crew very far.

The mass of the reactor+heat engine is just a 5% or 15% the radiator mass. A reactor for 100 mw might weight 300kg.

There are ship designs already made by nasa with details of each component. That give us an idea.

Sandia nuclear lab is working on a Brayton-cycle nuclear plant. This is the thermodynamic cycle used for jet engines and gas turbine power plants:

http://www.bizjournals.com/albuquerque/print-edition/2012/05/04/sandias-heat-producing-turbine-boosts.html

The researchers expect to boost efficiency to 50%, where it's commonly at 30%. Could this efficiency be made even higher? With a jet or rocket engine you get higher efficiency by expanding the exhaust out to a lower pressure, at most out to vacuum. Could this work for these plants or gas turbine plants in general?

For Earth bound plants you would need large nozzles but more importantly this large nozzle would have to exhaust out into a vacuum chamber. Getting a large vacuum chamber would be difficult and expensive on Earth, because of the thick heavy walls that would be needed to hold out the air pressure.

But if you expand the exhaust out, then you lost the critical co2, or you will need to compress it again.

About this new method, not sure if that increment in efficiency is keeping the radiation area or not.

Actually, the pumps are the achiles heel of any such system. See, "The Problem" is we are discussing electrical systems, and electrical systems need electricity. And the only ways we know how to generate electricity (thermocouples, photovoltaics, heat engines and alternators, whatever you want to consider) all produce more waste heat than electricity.

electric motors are the things with lower waste heat, we are talking of 93% efficiency.

The real bottlenecks with nuclear vasimr propulsion, is the core temperature and the radiator temperature.

That is what set the efficiency of the whole system.

and those say the weight of the drive system is dominated by the weight of the cooling system. Which uses lots of pumps and fluid engineering. And let me tell you, those haven't changed much since the last century.

But the weight of the cooling system it depends the amount of heat you need to radiate, but you will lost just 7% of the energy generated, that is nothing compared with the heat engine, which you might lost the 50% or 70% of the energy.

Because you can cheat a bit with the ideal gas law by compressing and expanding gasses to change their temperature (at a cost in energy)

You dont gain nothing doing that in space. what are the benefics?

but you really can't go too far without seriously staging your system, which adds complexity and mass really quick

Of course, more steps and you add more radiators and mass.

So how do you keep your fancy superconductors and control electronics cool when you run your fancy droplet radiator at 1600ºK with liquid lithium?

I guess other metals are better due the heat capacity. About your superconductors will use a different radiator, if all radiators are on the same plane, then they dont transmit heat between.

Yeah, a sad fact: all those exotic radiator concepts that give orders of magnitude less mass than what spacecraft use today? They are only really capable of taking heat out of high-temperature machinery like, say, a bulky fission reactor. Which is why spacecraft use today what they use, and why VASIMIR is even deader than the analysis we have done here imply.

That has not sense, first nasa is interested in vasimr.

Second, if nasa choice to use radiators from 1950 for the space station, is their choice, but it does not mean that we can not include a liquid radiation which is nothing from other world.

Just some holes and one collector.

You might be wondering now how is it that commsats and probes and such can run ion engines without issues. The answer is simple: they are already designed to handle the kilowatts of waste heat their solar panels generate, whether they use the small fraction (~15-20%) of power that gets turned into electricity, or not. And kilowats is a long way from drive systems that have to be in the gigawatt range to fulfil that absurd "39 days to Mars" really-high-energy trajectory with a ship in the hundreds of tonnes. Hundreds of tonnes vs Gigawatts in heat... really, that can't go well in those proportions.

You dont need gigawatts as I already mention.

Also space PV reach 45% of efficiency. ANd I also mention this option for travels to moon, venus or near asteroids.

This is in-silico analysis, and its a fail. Solar weather changes at 500,000 miles per hour, if the solar flux changes the computer would not have a chance to collect water vapor molecules already in motion and they would be lost.

What they did is an accurate software to model the movement of doplets not matter the circumstance, they will be able to make the corrections needed to collected.

So that detail of liquid radiators is not longer a problem.

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

The real benefic of a nuclear ship vasimr, is that once you have it, you can do any mission you want to any place.

You want to go mars... you can and it will be very cheap, go and back, then you want to go to jupiter and drill europa ice.. you can do it, fast and cheap..

The same for any location you will need. Is your cargo ship multipurpose.

So the most important is to achieve a core temperature very high, maybe with magnetic confinement and fission melted materials mix and carbon base walls, it will be possible to have a reactor temperature of 2000k, then it will be possible to reduce the radiator temperature to 1000 or 1200 k, which give us 50% efficiency in the heat engine.

[SargeRho]

A 300kg 100 MW reactor would have an energy density of 333.3... kw/kg, which as far as I know is several hundred times greater than the best we can do currently.

[PB666]

What they did is an accurate software to model the movement of doplets not matter the circumstance, they will be able to make the corrections needed to collected.

So that detail of liquid radiators is not longer a problem.

I think not. For this to work water needs to turn to vapor as it is being released cooling the surface it is released form which means its a single molecule which also means quantum mechanics is somewhat applicable. The ship it is released from is accelerating, and the medium the water droplet it is released from contains UV, X rays and solar-wind, sometimes densely charged solar winds. If any of these things hit your little water vapor molecule, its gone, goodbye, adios, sayonara.

The problem can be limited by injecting into a pot, the problem is that the energy released is attenuated by any attempt to contain the resulting stream of particles.

[tater]

Why you need 1 gw? some vasimr designs goes from 100kw to 50 mw. If you dont need to carry humans, 100kw is enoght.

Furthermore nuclear space reactors does not need to be heavy, you dont need the concrete or heavy shielding, you can put the crew very far.

The mass of the reactor+heat engine is just a 5% or 15% the radiator mass. A reactor for 100 mw might weight 300kg.

There are ship designs already made by nasa with details of each component. That give us an idea.

I said 1 gW because YOU DID. You used that as an example. So I asked about the mass for that SYSTEM (including all parts of the earth it uses for heat rejection). Scaling this to the required power would be a lower limit on reactor mass.

Space reactors != terrestrial reactors. Making up numbers based upon terrestrial reactors and stuff you think might be right is not the same thing as engineering space power systems. Look at actual proposals for space power reactors, instead. ALL have power densities above what the 39 nonsense assumes (1kg/kW, vs 20, or 50 kg/kW).

[Kryten]

I think not. For this to work water needs to turn to vapor as it is being released cooling the surface it is released form which means its a single molecule which also means quantum mechanics is somewhat applicable. The ship it is released from is accelerating, and the medium the water droplet it is released from contains UV, X rays and solar-wind, sometimes densely charged solar winds. If any of these things hit your little water vapor molecule, its gone, goodbye, adios, sayonara.

The problem can be limited by injecting into a pot, the problem is that the energy released is attenuated by any attempt to contain the resulting stream of particles.

All the droplet radiator concepts I've seen use liquid metals, I've no idea where he's got this idea of using water from.

[Kibble]

I don't understand droplet radiators - isn't liquid just a gas condensed by surrounding gas (like, the atmosphere)? And so in space, where there's no pressure, wouldn't the liquid droplets just expand away like gas likes to in a vacuum?

[tater]

I know the guys here at ISNPS have been working on pebble-bed reactors for a lunar surface application (or Mars). One aspect for those is refueling, which is pretty easy/safe in pebble beds (drop balls in the top, take balls off the bottom (assuming gravity ).

SP-100 was to be over 5 tons. It was 2 MWt, but only 100kWe. I have not read of any currently testing concepts that have power densities anywhere near what is required.

OP links to, well, nothing that supports his claim.

It might be possible at some point, but it's not now.

[AngelLestat]

A 300kg 100 MW reactor would have an energy density of 333.3... kw/kg, which as far as I know is several hundred times greater than the best we can do currently.

Its mentioned here:

http://www.sciencedirect.com/science/article/pii/S1738573315001540

Search for "reactor total mass".

I think not. For this to work water needs to turn to vapor as it is being released cooling the surface it is released form which means its a single molecule which also means quantum mechanics is somewhat applicable. The ship it is released from is accelerating, and the medium the water droplet it is released from contains UV, X rays and solar-wind, sometimes densely charged solar winds. If any of these things hit your little water vapor molecule, its gone, goodbye, adios, sayonara.

The problem can be limited by injecting into a pot, the problem is that the energy released is attenuated by any attempt to contain the resulting stream of particles.

But those liquid doplet radiators does not use water! XD

They use metals which remains liquid at very high temperature and in vaccum.

[tater]

I don't understand droplet radiators - isn't liquid just a gas condensed by surrounding gas (like, the atmosphere)? And so in space, where there's no pressure, wouldn't the liquid droplets just expand away like gas likes to in a vacuum?

The idea is to spray coolant droplets into space, then collect them after some travel time (during which time the individual droplets are radiating instead of having to conduct to a radiator, which then does the radiating to space. The issue for a transportation application would be collecting the droplets at the other side while under acceleration I think.

[AngelLestat]

I said 1 gW because YOU DID. You used that as an example. So I asked about the mass for that SYSTEM (including all parts of the earth it uses for heat rejection). Scaling this to the required power would be a lower limit on reactor mass.

Space reactors != terrestrial reactors. Making up numbers based upon terrestrial reactors and stuff you think might be right is not the same thing as engineering space power systems. Look at actual proposals for space power reactors, instead. ALL have power densities above what the 39 nonsense assumes (1kg/kW, vs 20, or 50 kg/kW).

See the link that I provide SargerRho

[Kibble]

The idea is to spray coolant droplets into space, then collect them after some travel time (during which time the individual droplets are radiating instead of having to conduct to a radiator, which then does the radiating to space. The issue for a transportation application would be collecting the droplets at the other side while under acceleration I think.

But...why don't the droplets just boil away?

[tater]

Its mentioned here:

http://www.sciencedirect.com/science/article/pii/S1738573315001540

Search for "reactor total mass".

You complain about derailing the thread, then post a bimodal concept? A reactor that gets to reject heat via a propellant is a rocket motor. That's easier to do (from a cooling standpoint). It's an analog to a terrestrial reactor throwing seawater out the back. Also, that link says it only makes 100 kWt (thermal) for use in electric generation, so kWe will be a fraction of that (25 kWe, maybe). Doesn't matter, it's a rocket, anyway (and probably better than VASIMR)

Seems to me (at first glance) that regarding radiation of heat, another issue is which heat. We have the actual reactor, then we have the heat after electric conversion has cooled the working fluid. The reactor turns out to be easier in some respects, because of higher temps (blackbody radiation will go like t^4). Lower temp means bigger radiators. If you take a lot of thermal energy (super high efficiency conversion to electricity), then the temp will drop, and the remaining heat must be rejected with substantially larger radiators.

Edited September 4, 2015 by tater

[shynung]

But...why don't the droplets just boil away?

Surface tension, and by extension fluid cohesion. In the vacuum of space, that's the only thing that can keep a liquid from boiling.

[tater]

But...why don't the droplets just boil away?

Choice of materials, I presume. I've heard of it, and don't know much about it, just that it's a shower of some working fluid, and a collector. Might be a metal.

[AngelLestat]

You complain about derailing the thread, then post a bimodal concept? A reactor that gets to reject heat via a propellant is a rocket motor. That's easier to do (from a cooling standpoint). It's an analog to a terrestrial reactor throwing seawater out the back. Also, that link says it only makes 100 kWt (thermal) for use in electric generation, so kWe will be a fraction of that (25 kWe, maybe). Doesn't matter, it's a rocket, anyway (and probably better than VASIMR)

Try to read, is the only that I am asking you..

In the link is all explained.

Reactor power (at electric power mode) 100 MWth (100 kWth)

Avg. fuel power density 14.7 MWth/L

Number of fuel elements 37

Pitch to diameter ratio 2

Fuel type, U enrichment, and mass (235U mass) (U, Zr, Nb)C, 93 wt%235U/U, & 40∼50 kg (4 kg for 7LiH & 9 kg for ZrH1.8)

Moderator type and mass 13 kg for 7LiH

93 kg for ZrH1.8

Reflector (PV) type and mass (including control drums) Be – Be – C/C & 97 kg

Total reactor mass 180 kg for 7LiH

268 kg for ZrH1.8

Reactor diameter and height (core diameter and height) 50 & 46 cm (39.2 & 39.2 cm)

The true reactor thermal power output is 100 megawatts!!

It does not matter much if this reactor is used as rocket and the colling system is the same proppelent, because I am not saying that the reactor + cooling system will weight just that, I am saying that the reactor will weight that.

When it said 100kw electric, is because that rocket design will also need electricity, so it has an auxiliar generator that only uses 100kw. But the reactor power is 100 MW.

First, we dont need 100 Mw, maybe with just 50Mw is enoght for a manned mission, lets imagine that it will weight a bit more for X reasons.. 50 Mw - 1 Ton, this still is nothing compare to the mass you need for the radiators. So the reactor is very light (ask to anyone), the main issue here is the radiator mass.

[shynung]

Try to read, is the only that I am asking you..

In the link is all explained.

Reactor power (at electric power mode) 100 MWth (100 kWth)

Avg. fuel power density 14.7 MWth/L

Number of fuel elements 37

Pitch to diameter ratio 2

Fuel type, U enrichment, and mass (235U mass) (U, Zr, Nb)C, 93 wt%235U/U, & 40∼50 kg (4 kg for 7LiH & 9 kg for ZrH1.8)

Moderator type and mass 13 kg for 7LiH

93 kg for ZrH1.8

Reflector (PV) type and mass (including control drums) Be – Be – C/C & 97 kg

Total reactor mass 180 kg for 7LiH

268 kg for ZrH1.8

Reactor diameter and height (core diameter and height) 50 & 46 cm (39.2 & 39.2 cm)

The true reactor thermal power output is 100 megawatts!!

It does not matter much if this reactor is used as rocket and the colling system is the same proppelent, because I am not saying that the reactor + cooling system will weight just that, I am saying that the reactor will weight that.

When it said 100kw electric, is because that rocket design will also need electricity, so it has an auxiliar generator that only uses 100kw. But the reactor power is 100 MW.

First, we dont need 100 Mw, maybe with just 50Mw is enoght for a manned mission, lets imagine that it will weight a bit more for X reasons.. 50 Mw - 1 Ton, this still is nothing compare to the mass you need for the radiators. So the reactor is very light (ask to anyone), the main issue here is the radiator mass.

So what you say is, Forget VASIMR. We go there by nuclear thermal rocket.

[AngelLestat]

So what you say is, Forget VASIMR. We go there by nuclear thermal rocket.

I just use that study to know how much it will weight a reactor..

A reactor is a reactor.. how is cool it may be different, for vasimr we need huge radiators, but for rocket you use the same proppelent.

The only difference will be with a molten salt rocket, that is different.

[tater]

I just use that study to know how much it will weight a reactor..

A reactor is a reactor.. how is cool it may be different, for vasimr we need huge radiators, but for rocket you use the same proppelent.

The only difference will be with a molten salt rocket, that is different.

You entirely missed 2 points. One, if your rocket/reactor is better than VASIMR, then it's off topic. Two, a reactor is not a reactor (meaning all are not interchangeable). Heat rejection is a HUGE factor in space reactor design, and the example posted gets around this by using propellant as coolant.

Do you really not understand this? It's making a crap-ton of heat---and throwing that heat out the back... highly excited (hot) atoms leaving the back at high velocity. Radiator mass ALONE is a problem. Assume a magic rector that makes the required 200 kWe and is 1kg. If the required radiators are 5kg/kW, and VASIMR requires 1kg/kW, we're still off by a factor of 5.

Reactor design doesn't happen in a vacuum (heheh). They are designed for specific roles. A design that maintains proper temperature via dumping tons of coolant through the middle, and out to space is not optimized for closed-cycle operations. Part of the reason it can be so small is that cooling is not a real concern. You can fit a nuclear bomb in a small backpack (W54) that is only ~20kg. How many Watts does that produce? No need for cooling, it's a BOMB! If you want that energy some other way, then you need to think about removing waste heat, hence all the talk about pumps, and cooling. If you make energy, to must radiate it.

[Rune]

The problems of such space trips in BEO space for several months have been much discussed, from bone and muscle loss, to radiation damage, to the recently discovered eye damage. In fact William Gerstenmaier, head of NASA's human spaceflight division, has said the NASA Mars mission architectures, and also Bob Zubrin's, that might take 900 days total round trip are unworkable:

Yes, NASA really is reconsidering the moon, and here’s why that’s important.

Posted on April 6, 2015 | BY ERIC BERGER

http://blog.mysanantonio.com/newswatch/2015/04/yes-nasa-really-is-reconsidering-the-moon-and-heres-why-thats-important/

Gerstenmaier suggested, as have many others outside of NASA, that using lunar derived fuel in orbital propellant depots would make Mars missions easier and cheaper. It could also be done by using plasma propulsion such as VASIMR to shorten the flight time.

Bob Clark

Gernstenmaier is a politician, and he has said many things. Don't change the fact that packing radiation shielding and a thether system for artificial gravity is a fraction of the mass of an electrical engine. And I would seriously cuestion the sanity (or honesty) of someone that tells you that sitting next to a Gigawatt-class nuclear reactor is the best way of reducing the radiation dose going to Mars.

Why you need 1 gw? some vasimr designs goes from 100kw to 50 mw. If you dont need to carry humans, 100kw is enoght.

Furthermore nuclear space reactors does not need to be heavy, you dont need the concrete or heavy shielding, you can put the crew very far.

The mass of the reactor+heat engine is just a 5% or 15% the radiator mass. A reactor for 100 mw might weight 300kg.

There are ship designs already made by nasa with details of each component. That give us an idea.

To achieve the "39 days to Mars" trajectory that started this whole thing, you need jet powers on the order of Gigawatts. Period. Unless your payload weights grams, of course, but we are discussing manned flights here.

electric motors are the things with lower waste heat, we are talking of 93% efficiency.

The real bottlenecks with nuclear vasimr propulsion, is the core temperature and the radiator temperature.

That is what set the efficiency of the whole system.

A shame you need electricity to run electric motors, produced at a peak 50% efficiency. A shame, too, that all your ship will try to stay at the radiator operating temperature. How well do you fare at 1600ºK? I mean, I'm from Spain and we have hot summers, but even I would be a tad too warm.

But the weight of the cooling system it depends the amount of heat you need to radiate, but you will lost just 7% of the energy generated, that is nothing compared with the heat engine, which you might lost the 50% or 70% of the energy.

I don't get that. Are you stating my point, that the weight of the radiator system dominates the whole thing? For a given thrust, you get a given jet power (the energy in your exhaust). And if you pick an engine, then you have a given efficiency in heat-to-thrust conversion, so you also get a set waste heat to get rid off. For nuclear-electric drives, that means at least more than half your total wattage output is waste heat.

You dont gain nothing doing that in space. what are the benefics?

You can get some parts of your ship to stay under the temperature of your radiator. Just like your fridge keeps part of your house slightly colder than the rest, using a boopload of energy, a convection radiator, and a complex fluid compression/expansion system. You kind of need to do it if touching the radiator would melt the crew's hands... even if they were robots.

I guess other metals are better due the heat capacity. About your superconductors will use a different radiator, if all radiators are on the same plane, then they dont transmit heat between.

I'm using Lithium because it is one the best at moving amazing quantities of heat from point A to B with the minimum mass. But you touch a very good point here, the "secondary" radiators. I know they would not bother the high-temperature radiators... they would be too big to notice them, even if they only reject a fraction of the heat. It's funny how people forget that "secondary" system, which is likely to weight more than the fancy high-temperature radiators that handle the nuclear core. I mean, anything with a magnetic nozzle (and VASIMIR and every other high power, high Isp engine has one) means superconductors working at spitting distance from the main reaction chamber. In VASIMIR's case, about 40% of the the electrical power going to the engine gets wasted as low level heat. That's after you have had >50% losses in thermal-to-electric conversion, so you are dealing with heat on sensitive electronics and high-power magnets that needs to be rejecter at a MUCH lower temperature. Hence, your "secondary" radiators are likely your main radiator wings.

That has not sense, first nasa is interested in vasimr.

Second, if nasa choice to use radiators from 1950 for the space station, is their choice, but it does not mean that we can not include a liquid radiation which is nothing from other world.

Just some holes and one collector.

NASA is "interested", yeah. Shorta. That is tricky, they have actually built themselves the ion engines they wanted, Chang-Diaz just got a few technology development grants to work on the side on his own company. But I'll grant that. In any case, NASA is interested in a 200kW VASIMIR, to get an awesome payload fraction in a very expensive future probe, or do a crazy mission with incredible dV requirements (like putting a telescope at the sun's gravity lensing point). Certainly no NASA engineer believes they will ever use a VASIMIR to send a crew to Mars in 39 days, I hope.

You dont need gigawatts as I already mention.

Also space PV reach 45% of efficiency. ANd I also mention this option for travels to moon, venus or near asteroids.

Meaning 65% of the energy is low-level heat deposited in your solar panels. Are you going to keep them at 1600ºK with liquid lithium? Because those would be some amazing photodiodes. Also, show me the operational system that gets 45%, those are lab results on very particular conditions, at best.

The real benefic of a nuclear ship vasimr, is that once you have it, you can do any mission you want to any place.

You want to go mars... you can and it will be very cheap, go and back, then you want to go to jupiter and drill europa ice.. you can do it, fast and cheap..

The same for any location you will need. Is your cargo ship multipurpose.

So the most important is to achieve a core temperature very high, maybe with magnetic confinement and fission melted materials mix and carbon base walls, it will be possible to have a reactor temperature of 2000k, then it will be possible to reduce the radiator temperature to 1000 or 1200 k, which give us 50% efficiency in the heat engine.

The ship you are describing would be such a "battlestar galactica", with such a pathetic TWR, that not only could you fund several Mars programs for what it would cost to build, the astronauts would probably get more radiation from the reactor and the extended flight time through the Van Allen belts, that what you would save by reducing the trip to Mars to 39 days. Not that you would reduce the trip time to Mars to 39 days in any case, of course. It might end up being longer, depending on TWR.

Rune. I'm more and more a fan low Isp engines, and open cycles, the more I know about the subject.

- - - Updated - - -

So what you say is, Forget VASIMR. We go there by nuclear thermal rocket.

That would make sense.

Rune. There was a lot to read.

Edited September 4, 2015 by Rune

[AngelLestat]

You entirely missed 2 points. One, if your rocket/reactor is better than VASIMR, then it's off topic. Two, a reactor is not a reactor (meaning all are not interchangeable). Heat rejection is a HUGE factor in space reactor design, and the example posted gets around this by using propellant as coolant.

Is not better!! a nuclear thermal rocket only has 1000 ISP, it may have 2000 secs as maximun with complex designs.

Vasimr has from 5000 to 50000 seconds, so even if you need to use a huge radiator, it worth it.

Do you really not understand this? It's making a crap-ton of heat---and throwing that heat out the back... highly excited (hot) atoms leaving the back at high velocity. Radiator mass ALONE is a problem. Assume a magic rector that makes the required 200 kWe and is 1kg. If the required radiators are 5kg/kW, and VASIMR requires 1kg/kW, we're still off by a factor of 5.

There are a lot of radiators paper that I already post, that reach 1kw/kg or 2kw/kg.

Also vasimr does not require any specific radiator mass, if is lower, it will work better and it will be cheaper.

Magic reactor? a reactor is a reactor.. You have nuclear submarines or any other kind of artifact using big or small reactors. Where is the magic?? Depending the amount of fission material that you have plus the moderator substance + control rods (that act as a brake) it will determine the amount of heat you release.

That´s all. Then you need to choose how to cool it, in a rocket you may use the same proppelent, if you want to use the heat to produce electricity, then you will use a pump or other methods inside the reactor to move that heat in a close cycle to a heat exchanger that will be connected to the cycle fluid from your radiator.

This is space, so you have a lot of tricks that you can use as magnetic confining to reduce the mass of the reactor or rise its working temperature.

So instead release that heat with the proppelent, you connect it to a heat exchanger in a close cycle. That is the only difference.. But you dont need to include the radiator mass in the reactor mass.

Reactor design doesn't happen in a vacuum (heheh). They are designed for specific roles. A design that maintains proper temperature via dumping tons of coolant through the middle, and out to space is not optimized for closed-cycle operations.

So? This is like said that is not possible to eat in space because spoons dont work there.

Any time that you have a different task, the nuclear reactor will be specific designed to that task, if you use critic co2 or helium or water as main working fluid, what kind of fission material or mix you use, if it needs control rods or not, all that is designed for each reactor..

Is like design a new ICE engine depending the car or truck it will use it, you dont need to shoot you in your head to avoid that. Is not magic.. it almost happen in each new car model we see.

[SargeRho]

*hijacks thread*

Let's use something that doesn't have the same kind of problems as VASIMR then - such as a Z-Pinch Fusion Rocket as laid out in this article: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120002875.pdf

The craft does, however, weigh 600 tons, of which 88 are fuel. Nearly 20000 seconds of specific impulse, too.

Edited September 4, 2015 by SargeRho

[shynung]

Is not better!! a nuclear thermal rocket only has 1000 ISP, it may have 2000 secs as maximun with complex designs.

Vasimr has from 5000 to 50000 seconds, so even if you need to use a huge radiator, it worth it.

But the thrust is... close to negligible. Gas-core nuclear thermal rockets can get 1500 secs, with decent thrust.

Also, nuclear pulse rocket. And nuclear salt water rocket. They can get big specific impulses too, but with much more thrust to bear.

But you dont need to include the radiator mass in the reactor mass.

One must. Because without radiators, reactors are useless. Nuclear thermal rockets happen to not need a radiator because it doesn't convert thermal energy into electricity first; it dumps the heat straight into propellant instead.

By comparison, nuclear electric rockets need radiators, because they feed on electricity rather than straight thermal energy. To generate electricity from heat, a cold spot is required, because the heat engines (be it Brayton turbines, Stirling engine, or thermocouples) that drives the electric generator work on temperature differences. This is the radiators' role, radiating heat from itself to act as a cold spot for the heat engines to dump heat onto.

In short, reactors meant for straight thermal energy (thermal rockets, or nuclear smelters) need no radiators by default. Reactors meant for electrical energy will require radiators. So, for nuclear electric rocket, radiator mass must be taken into account.

Edited September 4, 2015 by shynung

[Rune]

Also, nuclear pulse rocket. And nuclear salt water rocket. They can get big specific impulses too, but with much more thrust to bear.

Wow there! Hold on to your horses. Gas core is already difficult enough to cool, and 1,500s overkill enough for most of the solar system. Zubrin's NSWR is, shall we say, worrying (you know how the fuel is actually stored in tubes because if you let it pool it goes critical, right?), and Orion is expensive as boop, not to mention a problem to launch. I had in mind something more in the scale of the RD-0410, a very efficient design you can ground-test easily and can be used for a wide variety of applications.

Rune. I'm all for unmanned electric tugs, though. After we have the other stuff sorted out, to improve cargo economics.