Is it reasonable to build real world bigger ion engines?

[juvilado]

Hi, I suggested in "suggestions" to implement a bigger xenon tank. Some ppl argued that until no bigger ion engines are implemented, the tank would be useless or at least with situational use. My question is: Is it possible in real world to build bigger ion engines? Would they be efficient? If the answer is 'yes' then it could be good idea to include them in the main KSP.

[KerikBalm]

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

Yes, its reasonable, but its really only reasonable if you have a nuclear power source.

Solar power is too weak.

The mass of panels you'd need realtive to the power output strongly favors a nuclear reactor

[Nuke]

engines aren't the limit, its power. you can throw out hundreds of square meters of solar panels and still only be able to scrape along with engines that can only put out only a few newtons (if that much). then you got to haul than monstrosity and its support hardware everywhere you go. then you also only get enough juice out of it in the inner solar system where none of the cool stuff is. im sure at some point the panels get more massive than a fission reactor would be at similar power level. its not that we dont know how to build space rated reactors, its just everyone seems to be a negative nancy when it comes to putting reactors on rockets. fusion reactor might work if you can make it work (we cant at this point) and assuming its not a massive reactor design like a tokamak, which is too massive to be useful. for the time being the most we can hope for is an rtg and a battery/capacitor bank. things will get interesting when we can take a 1+ MW power supply with us wherever we go.

[TheGatesofLogic]

In addition fusion releases less energy per nuclide than fission, and requires much heavier components, which makes its power density inherently smaller than fission reactors, even if it was workable right now.

[Kryten]

Another issue is ion engines ultimately wear out; that's why most missions carry a few, even though they only use one at a time. Scale up the engines, and including spares becomes much harder.

[KerikBalm]

In addition fusion releases less energy per nuclide than fission,

That is sort of irrelevant, what is relevant is the rate of fission * energy released by each fission event vs rate of fusion * energy released by each fusion event.

And... except for H-bombs, we can't get decent fusion rates.

Yes, this is main part, to get any decent sort of reaction rate, you'd need a massive reactor.

Fission power on the other hand can be made to have a nearly unlimited energy output once you reach critical mass. The output is not limited by how high you can take the reation rate per say, but rather how much power output your reactor can handle before it destroys itself/ how much of that energy can actually be harnessed by the machinery.

It is in harnessing the energy, that some fusion designs may have an advantage over fission.

Some fusion designs would allow for direct power conversion from the high eneryg plasma. Fission requires a brayton heat cycle.

[NERVAfan]

I think solar electric propulsion for higher thrusts might be more feasible if you had large areas of very light thin film panels mounted on a very light structure. JAXA proposed a mission like this as a follow-on to the IKAROS solar sail, but I haven't heard anything new since so don't know if it is still planned.

[Streetwind]

Not really. With solar panels you're limited by area more than by weight. For example, the ISS with all its panels combined is rated at a peak output of 180 kW. That is a lot already for solar power. And yet, we have ion and plasma thrusters that you could hold in your hands that consume way more than that. It seems a bit ridiculous to equip your cupboard-sized space probe with solar panels sized 1.5 times the surface area of those on the ISS, just to get a few netwons of thrust, no? And besides, if you head out to Mars, your solar panel output drops sharply, and once you pass the asteroid belt, you're pretty much not going to power anything with solar anymore. Rosetta for example was put into hibernation on its final swing out to Jupiter's orbit because the sizable solar panels couldn't even keep the onboard computers running, much less an ion drive.

Solar electric propulsion is useful for the inner solar system - Earth, Moon, Venus, Mercury. Anything beyond that becomes impractical very quickly. We really need something with significantly higher power density for Mars and beyond, even if it makes for heavier spacecraft.

@juvilado: if you want to play with bigger xenon tanks and engines, try the Near Future Technologies mods - particularly the Propulsion pack. All the engines in it are inspired by realworld developments from around the world. Although you may find that the Electrical and/or Solar packs are a good addition to that, since otherwise you may run into the same issues that currently keep these engines from flying in the real world - you'll struggle to power them.

Edited September 29, 2014 by Streetwind

[magnemoe]

is not heat buildup an issue with reactors? to get 200 kW power you need to get rid of a megawatt of heat?

[Streetwind]

It is, yes. But you still need a lot less radiator area for 200 kW of nuclear power than you need panel area for 200 kW of solar power (and in fact, excessively large solar panels may also require heat radiators).

[Guest]

Some electric thrusters have grids, that erode, and some are grid-less, using induction heating and magnetic acceleration ... there is a nice article at wikipedia :

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

[NERVAfan]

Not really. With solar panels you're limited by area more than by weight. For example, the ISS with all its panels combined is rated at a peak output of 180 kW. That is a lot already for solar power. And yet, we have ion and plasma thrusters that you could hold in your hands that consume way more than that. It seems a bit ridiculous to equip your cupboard-sized space probe with solar panels sized 1.5 times the surface area of those on the ISS, just to get a few netwons of thrust, no?

Seem ridiculous, maybe... but if you can deploy the huge area by spinning like IKAROS's solar sail, is it necessarily a problem?

And besides, if you head out to Mars, your solar panel output drops sharply,

It's not that bad at Mars's orbit; about 40-50% the power at Earth's orbit, I think. Dawn is using solar-electric propulsion at Ceres and Vesta, which are significantly farther out than Mars.

and once you pass the asteroid belt, you're pretty much not going to power anything with solar anymore.

Well, as I say, JAXA was talking about a solar sail/ion mission to Jupiter and the Jupiter Trojan asteroids.

[78stonewobble]

Hmm .... Well... According to wikipedia a Nimitz class carrier is powered by 2 A4W nuclear reactors rated at 550 thermal MW's or enough steam to produce an unspecified amount of electrical power and 140.000 shaft horsepower (104 MW).

What could an ion thruster do with one of these pup... uhm. presumably a helluva lot larger and heavier than puppies?

Designed to run for 20 years without refuelling and a service life of 50 years.

[Streetwind]

Seem ridiculous, maybe... but if you can deploy the huge area by spinning like IKAROS's solar sail, is it necessarily a problem?

It's not that bad at Mars's orbit; about 40-50% the power at Earth's orbit, I think. Dawn is using solar-electric propulsion at Ceres and Vesta, which are significantly farther out than Mars.

It can work, yes. But it's usually not done, because in space travel, the one thing you have in large quantities is time. To take the Dawn probe you cite as an example, it's using a 10kW/1AU solar array to drive a 2kW NSTAR gridded ion thruster delivering 0.09N worth of thrust at 3100 Isp (it has three for redundancy, but fires only one at a time). So basically it experiences up to 80% reduction in solar power efficiency out there in the belt, and the power requirements of the engine are tiny. Dawn can get away with it because it has incredible amounts of time; it can thrust for years. So why bother with a ginormous power solution if you can just wait a little longer and do it for less money and less complexity?

In order for Dawn to properly operate one of the engines we're talking about here - for example Ad Astra's VF-200 VASIMR (200kW, variable but typically 5N, 5000s) or ESA's DS4G ion thruster (250kW, 2.5N, 19300s) - while out there in the belt, it would need to carry solar panels rated in excess of a megawatt of power at 1 AU. You could potentially implement something like that from a pure technical feasibility standpoint, but I believe we all agree that it's absolute crazy-talk from a cost and complexity standpoint

It's only when considering time limited usage scenarios - for example manned missions, or NASA's asteroid redirect mission which is supposed to use solar electric propulsion - that you really need to up the power. For instance, the same VF-200 mentioned above has been theorized for a solar electric lunar ferry/tug. Equipped with 200kW worth of panel area, it could ferry payloads between Earth and Moon orbits, with full reusability. You kind of need the (for electric propulsion standards) high-thrust magnetoplasma rocket for this application because you're trying to propel payload in addition to the craft itself, and because you don't want to take months for a trip a chemical engine could do in three days.

[Justin Kerbice]

Maybe the main issue with power generators is the use of the old concept of alternator. All major electricity production systems use it (nuclear & coil power plants), dam, wind turbine and internal combustion engine.

Even if nuclear reactions can create an incredible amount of energy, heat have to be converted, and that's conversion process need heavy machinery which prevent them to be sent into space (as long as cost is an issue).

So, it is likely possible the currently available technologies based on alternator create a boundary, or a wall, human kind will not be able to break until something new, something better, come. Because it is only possible to tweak the alternator to improve it (better/lighter materials, magnetic field to decrease friction, ...) but it still remain basically what it is.

It's kind of the planes before jet engine was created, they are limited in sizes, capabilities, range, altitude, and after the jet engine come, a brand new set of planes appears quite fast.

[Idobox]

Using a nuclear reactor to power an ion engine seems terribly wasteful, when you could use a NTR instead. Sure, you get a lower ISP, but you save a lot of mass too. Anybody knows a proper study on the subject?

[Streetwind]

Ion engines weigh next to nothing. You can lift and hold one no problem. They're smaller than you think - the nozzle skirt of an NTR alone is probably heavier.

The thing that makes a real mass difference between the NTR and the electric propulsion system is the thermodynamic heat engine and power management infrastructure used to create electric power from the reactor in the first place, because fission produces mostly just heat. Now I'm not sure how much extra mass that costs, but considering designs exist that use the same components that an NTR would use as part of their heat engine, the difference is probably not very large.

For example, since there is no government nuclear space development program in the US for political reasons right now, NASA's been doing it on the side using spare change. The SAFE-400 is actually a very state of the art design, compared to a lot of the legacy hardware that was tested in space before it become unfashionable to do so. A 50cm by 30cm box weighing just over 500kg - that's the whole unit, reactor and heat engine and everything. And that tiny thing outputs 100 kW electric power. Compare that to the NSTAR ion thruster's 2 kW energy need... or, two of those reactors could run the VF-200 at full bore and weigh just a little over a ton while doing so (plus a bit extra for the power processing infrastructure). That is an insane power density right there, in a form factor that's suitable for the smallest probes. 195 W per kg, independent of your distance to the sun - while the high-voltage blanket solar arrays on the ISS are rated for 32 W/kg. Curiosity's RTG is around 2.5 W/kg.

The question of lifetime remains. Since the SAFE reactor series is a discretionary "hobby project" outside of the lab's regular tasks, you don't hear a whole lot about it, but it is alleged that the 400 series has run at least a few years continuously. It has also been used as a power source for an electric engine in a test stand run.

Now I don't really know what it takes to build a NTR, but I do know that they need some extreme core temperatures and thermal throughput. The SAFE-400 only has 400 kW of thermal power, that's not a lot to work with for heating a hydrogen stream to really high temperatures (even handwaving the fact that you probably would need to build the reactor heavier in order to sustain the really high core temperatures necessary to make NTRs work at all).

Hmm .... Well... According to wikipedia a Nimitz class carrier is powered by 2 A4W nuclear reactors rated at 550 thermal MW's or enough steam to produce an unspecified amount of electrical power and 140.000 shaft horsepower (104 MW).

What could an ion thruster do with one of these pup... uhm. presumably a helluva lot larger and heavier than puppies?

Those things are so heavy you cannot even launch one of them on any currently existing launch vehicle (much less a spacecraft built around them). They're completey impractical for anything involving space.

But yes, three digit megawatts of electrical power could produce an enormous plasma rocket. Probably big and powerful enough to be considered a weapon (see: Jon's Law / the Kzinti Lesson).

Edited September 30, 2014 by Streetwind

[Nuke]

Hmm .... Well... According to wikipedia a Nimitz class carrier is powered by 2 A4W nuclear reactors rated at 550 thermal MW's or enough steam to produce an unspecified amount of electrical power and 140.000 shaft horsepower (104 MW).

What could an ion thruster do with one of these pup... uhm. presumably a helluva lot larger and heavier than puppies?

Designed to run for 20 years without refuelling and a service life of 50 years.

a naval reactor and a space reactor are two completely different beasts. a ship has a ready supply of coolant at its disposal. a space craft can only get rid of heat through radiation. the reactors that have been flown were only a few hundred kw. can we do better? i think so. but im curious how much better.

[78stonewobble]

Well the only reason to look at naval nuclear reactors is that they're pretty compact by necessity.

Though obviously it would take on orbit assembly and that ie. a nuclear thermal rocket would probably be better. Those however have their own problems. Wouldn't necessarily want to use one in low orbit.

[magnemoe]

Well the only reason to look at naval nuclear reactors is that they're pretty compact by necessity.

Though obviously it would take on orbit assembly and that ie. a nuclear thermal rocket would probably be better. Those however have their own problems. Wouldn't necessarily want to use one in low orbit.

Nuclear thermal would have far more trust however lower ISP as in 8-900 versus 5-20.000 for vasmir or ion.

[Nuke]

thing about nuclear thermal is it can run on a wide range of propellants. which might make it useful on isru capable spacecraft. you could refuel at pretty much every icy body in the solar system, and you also have the thrust to land at some of the higher gravity bodies. then its a matter of drilling, crushing, melting ice, filtering the liquid and feeding that into your propellant tanks. isp wont be great but propellant will be plentiful. ultimately we want a space equivalent of a gas turbine engine, which can run on everything from cooking oil to booze.

there are versatile nuclear electric solutions too, mpd can run on straight hydrogen, arcjets can run on ammonia, hydrogen, etc. having a reactor on board means you have the energy to drive endothermic chemical reactions to allow you to reprocess whatever resources you may find into usable propellant. low thrust does limit what you can land on though. you could do a massively thorough kuiper belt survey with just a few isru capable probes.

Edited October 1, 2014 by Nuke

[shynung]

Until the nuclear fuel run out, that is.

Not that it matters much. Naval reactors only need to be refueled every few decades or so.

[Nuke]

then isru can move into the realm of uranium processing.

[Idobox]

Ion engines weigh next to nothing. You can lift and hold one no problem. They're smaller than you think - the nozzle skirt of an NTR alone is probably heavier.

The thing that makes a real mass difference between the NTR and the electric propulsion system is the thermodynamic heat engine and power management infrastructure used to create electric power from the reactor in the first place, because fission produces mostly just heat. Now I'm not sure how much extra mass that costs, but considering designs exist that use the same components that an NTR would use as part of their heat engine, the difference is probably not very large.

The problem is that a space based nuclear generator gets maybe 10% efficiency at best, because it can't get rid of the heat easily, and the insane mass of the cooling and power conversion systems. If we take the SNAP8 as an example, it weighs 4.5t, produces 600kW thermal but only 35kW electric. For comparison, NERVA had an empty mass of 35t for 1100MW, 2000 times more powerful but only 7 times heavier.

I couldn't find how much the actual SNAP8 reactor core weighs, but it has 6.5kg of uranium, and 12 control rods weighing 12kg each, in a steel core that's 23cm in diameter by 53cm height. The coolant alone is more than 400kg, and the heat exchangers and piping probably account for most of the mass, so I'd be surprised if the reactor itself was heavier than 200kg. On the other hand, you have to take the turbopump and and nozzle.

Let's do some math, boiling and heating 1kg of H2 to 2500K takes 18MJ, with 600kWt available, it means a flow of 32.5g/s. I doubt the pump and nozzle required to handle 32g/s of hydrogen have masses in a the hundreds of kg.

If we assume a 600kW NTR would weight 1t, 8.3km/s ISP, a 1t payload and a 6km/s deltaV budget, we would need a wet mass of 4.1t.

Using the SNAP8 and a ion thruster of negligible mass and 100km/s ISP, we have a dry mass of 5.5t, and a wet mass of 5.85t.

For the NTR mission wet mass to be heavier than the ion mission dry mass, you need a delta V budget of 8.3km/s.

All in all, there must be a niche for nuclear powered ion thrusters, but given the weight of a nuclear power generator compared to a NTR, it ill be only for very heavy payloads and massive deltaV budgets, when dividing reaction mass by 100 compensate the mass of the cooling and power conversion systems.

[-Velocity-]

then isru can move into the realm of uranium processing.

It takes A LOT of processing to make uranium fuel. Now you want to do that on another planet or an asteroid? That's tough.

If you're going to make a fission reactor that is to be refueled by in-situ mining operations, you'd probably be A LOT smarter to utilize thorium, and a thorium reactor. In fact, there are a lot of thorium proponents out there that say in general, a thorium-based fuel cycle is vastly superior to a uranium fuel cycle (in terms of safety, lower quantities of nuclear waste, nuclear proliferation, and abundancy of fuel), and that the only reason we don't have thorium reactors instead of uranium reactors is because thorium reactors cannot easily be used to breed fissile material for nuclear weapons. Creating nuclear weapons was a priority back in the 40s and 50s when we had to chose whether to develop uranium reactors or thorium reactors.

I'm sure that someone out there has pointed out flaws in some or more of the arguments of the thorium reactor proponents. The only one I am aware of is that thorium reactors actually could be used to breed uranium-233 (and the fissioning of U-233 is where the majority of a thorium reactors energy output comes from), which could in fact be used as a nuclear bomb... but I don't know how easily that could be done.

Regardless of how superior thorium reactors are over uranium reactors, the point remains that if you're mining asteroids for your nuclear fuel, it makes a hell of a lot more sense to mine thorium than uranium. Thorium-232 composes almost all of naturally occurring thorium, and it's your fuel in a thorium reactor. Uranium-235, the fuel required for uranium reactors comprises only like 0.7% of all naturally occurring uranium (the majority of natural uranium being uranium-238), requiring you to create massive uranium enrichment plants in order to acquire usable fuel.

Edited October 1, 2014 by |Velocity|