more electric thruster efficiency questions.

[SpaceMouse]

Please excuse my crude physics. Since both VASIMR and ION drives work on the principle of accelerating propellant magnetically, presumably you can improve efficiency by increasing the length of it / adding bigger more magnets for more acceleration? I'm probably gonna make a VASIMR and i like doing engines capable of driving things bigger than probes. Presumably i should run on more logic than 'make it bigger'.

Thanks!

Edited November 20, 2016 by SpaceMouse

[Tullius]

Sure, accelerating your propellant more gives better thrust for a given amount of propellant.

The problem is just to achieve twice the thrust by accelerating your propellant to twice the speed you need to put 4 times as much energy into it.

So by accelerating your propellant more, you need less propellant for the same dv, but the main problem with ion drives is the ridiculous amount of electricity you need. So you might want to reduce propellant efficiency to get more thrust out of the same electricity, i.e. the same area of solar panels. Saving 1kg of propellants, but needing 2 kg of solar panels more instead, does not achieve anything.

Edit: And this is also one of the advantages of VASIMR: It can switch between low thrust, high propellant efficiency and high thrust, low propellant efficiency.

Edited November 20, 2016 by Tullius

[p1t1o]

May there also be side problems associated with increasing the strength of your magnetic field by an arbitrary amount? ie: effects on sensitive instruments, magnetic interactions with the exhaust plume, space dust etc?

[Streetwind]

15 hours ago, SpaceMouse said:

Please excuse my crude physics. Since both VASIMR and ION drives work on the principle of accelerating propellant magnetically, presumably you can improve efficiency by increasing the length of it / adding bigger more magnets for more acceleration? I'm probably gonna make a VASIMR and i like doing engines capable of driving things bigger than probes. Presumably i should run on more logic than 'make it bigger'.

Thanks!

ESA already put that idea to the test... well, sort of, anyway. Their Dual Stage 4-Grid thruster concept study uses, as the name implies, multiple acceleration stages and grids. The results looked fairly promising; based on the subscale tests conducted, a full scale model of 20 cm diameter would be expected to put out 2.5 N worth of thrust at a staggering 210 km/s exhaust velocity (19,300s Isp). That makes it the highest efficiency plasma thruster ever tested.

And then ESA took the project and shelved it. Some extra work was done on it by a student team who figured they could slim it down to just three grids without affecting performance, but there's no intent to bring it to production at this time. Because, as Tullius already said, power is an issue. In order to produce the performance described above, such a thruster would consume 250 kW worth of power. I mean, harnessing that much power inside a single 20cm diameter engine is quite impressive, considering electric engines are normally size-limited (to input more power, the engine must be made bigger, and the scaling factor is very impractical). But you would need solar arrays larger than those on the ISS in order to supply just one of these! For comparison, the NSTAR ion drive currently in use on the Dawn spacecraft takes 2.1 kW.

Additionally, as you input more power, it's difficult to get that power to produce thrust. You can't just increase the propellant flow to match and hope it sorts itself out. Instead, most of the power tends to go towards increasing Isp instead, as this example illustrates quite well. But just Isp alone doesn't make a better engine. There is a relationship between payload mass, expected dV needs, the specific power of your power production solution, and mission duration which I always forget the details of... but it basically says "for these given parameters, an engine with this specific impulse is ideal". And for the missions we fly today, that relationship usually spits out numbers between 3,000s and 6,000s. That means that a hypothetical ion drive with nearly 20,000s would be way out of scope for all but the most exotic of research missions - those going way out to really distant destinations, like the Kuiper belt and Sednoids. Problem: even your giant ISS-sized solar array won't help you out there, and there's not enough plutonium in the entire world to make an RTG big enough (by more than an order of magnitude difference). You'd need a true nuclear reactor. And we don't have one.

So basically, this kind of engine cannot be powered right now, and even if it could be powered, it would have next to no usage scenario.

This is also why the VASIMR is different, and being actively developed. It promises to push 100 kW into something that ends up with "only" 5,000s specific impulse. This means that a.) it's extracting quite a bit of thrust despite the high power input, as opposed to only producing Isp; and b.) it's smack-dab in that Isp range than real mission planners are actually interested in. It still has a problem with power supply (the engine produces rotation torque, so it needs to always come paired in 2x100 kW dual units), but at least solar could be an option.

Edited November 21, 2016 by Streetwind

[wumpus]

One huge issue with VASIMR is that it uses hydrogen as a fuel.  This makes "long, slow, efficient" burns less efficient if all the hydrogen escapes before it is used.  Ion systems tend to use Xenon, which has the unfortunate property of being unbelievably rare.  It *could* be replaces with argon (more common than CO₂, but that would cut the efficiency  by roughly half).

I'm pretty sure NASA launched a probe with three ion engines, but I suspect that the goal was reliability not more power (they are typically limited by the energy source, not the engine).  For any large probe/cargo run, I'd assume that you can either scale up an ion engine or simply spam plenty of them Kerbal-style.  The real catch is you have to provide power for all those engines, and that won't be light/cheap.

Don't forget modern gravity tricks (not just slingshots, try fuzzy boundries), such things come into play when using extremely slow spacecraft (unfortunately you can't really use Obereth unless you have something like VASIMR).

[magnemoe]

18 minutes ago, wumpus said:

One huge issue with VASIMR is that it uses hydrogen as a fuel.  This makes "long, slow, efficient" burns less efficient if all the hydrogen escapes before it is used.  Ion systems tend to use Xenon, which has the unfortunate property of being unbelievably rare.  It *could* be replaces with argon (more common than CO₂, but that would cut the efficiency  by roughly half).

I'm pretty sure NASA launched a probe with three ion engines, but I suspect that the goal was reliability not more power (they are typically limited by the energy source, not the engine).  For any large probe/cargo run, I'd assume that you can either scale up an ion engine or simply spam plenty of them Kerbal-style.  The real catch is you have to provide power for all those engines, and that won't be light/cheap.

Don't forget modern gravity tricks (not just slingshots, try fuzzy boundries), such things come into play when using extremely slow spacecraft (unfortunately you can't really use Obereth unless you have something like VASIMR).

You could use other gasses in vasmir, downside is that heavier molecyles give lower ISP, they give more trust however.
For ion engines an issue is that you need energy to remove the electron to generate ions, this is why xenon is nice, also you want trust more than ISP in an ion engine. 
Now you also have to remove electrons in vasmir however you do this by generating plasma so you keep most of the energy. 

[Technical Ben]

9 hours ago, Streetwind said:

Spoiler

ESA already put that idea to the test... well, sort of, anyway. Their Dual Stage 4-Grid thruster concept study uses, as the name implies, multiple acceleration stages and grids. The results looked fairly promising; based on the subscale tests conducted, a full scale model of 20 cm diameter would be expected to put out 2.5 N worth of thrust at a staggering 210 km/s exhaust velocity (19,300s Isp). That makes it the highest efficiency plasma thruster ever tested.

And then ESA took the project and shelved it. Some extra work was done on it by a student team who figured they could slim it down to just three grids without affecting performance, but there's no intent to bring it to production at this time. Because, as Tullius already said, power is an issue. In order to produce the performance described above, such a thruster would consume 250 kW worth of power. I mean, harnessing that much power inside a single 20cm diameter engine is quite impressive, considering electric engines are normally size-limited (to input more power, the engine must be made bigger, and the scaling factor is very impractical). But you would need solar arrays larger than those on the ISS in order to supply just one of these! For comparison, the NSTAR ion drive currently in use on the Dawn spacecraft takes 2.1 kW.

Additionally, as you input more power, it's difficult to get that power to produce thrust. You can't just increase the propellant flow to match and hope it sorts itself out. Instead, most of the power tends to go towards increasing Isp instead, as this example illustrates quite well. But just Isp alone doesn't make a better engine. There is a relationship between payload mass, expected dV needs, the specific power of your power production solution, and mission duration which I always forget the details of... but it basically says "for these given parameters, an engine with this specific impulse is ideal". And for the missions we fly today, that relationship usually spits out numbers between 3,000s and 6,000s. That means that a hypothetical ion drive with nearly 20,000s would be way out of scope for all but the most exotic of research missions - those going way out to really distant destinations, like the Kuiper belt and Sednoids. Problem: even your giant ISS-sized solar array won't help you out there, and there's not enough plutonium in the entire world to make an RTG big enough (by more than an order of magnitude difference). You'd need a true nuclear reactor. And we don't have one.

So basically, this kind of engine cannot be powered right now, and even if it could be powered, it would have next to no usage scenario.

This is also why the VASIMR is different, and being actively developed. It promises to push 100 kW into something that ends up with "only" 5,000s specific impulse. This means that a.) it's extracting quite a bit of thrust despite the high power input, as opposed to only producing Isp; and b.) it's smack-dab in that Isp range than real mission planners are actually interested in. It still has a problem with power supply (the engine produces rotation torque, so it needs to always come paired in 2x100 kW dual units), but at least solar could be an option.

 

Thanks for that info. In my si-fi (loosely based on real life) ship designs I've had quad Vasmirs on as a interplanetary tug... did not know they produced rotational forces... so good thing I did not put an odd number on them!

Edited November 21, 2016 by Technical Ben

[todofwar]

I always like thinking of chemical rockets as being the moral equivalent of ion engines that dump empty batteries as soon as they empty, and also throw them out the back fast enough to give even more umph. 

[Streetwind]

4 hours ago, wumpus said:

I'm pretty sure NASA launched a probe with three ion engines, but I suspect that the goal was reliability not more power (they are typically limited by the energy source, not the engine).

That's correct. Dawn carries three NSTAR thrusters, but only runs one at a time. They are even angled in different directions, and while they all gimbal, I doubt that more than two could realistically fire together anyways, even if the power was there (out in the asteroid belt, it isn't). Dawn was built in part to push endurance limits, so the designers thought it prudent to include redundancy.

Almost all electric engines suffer from electrode erosion. Gridded thrusters like the NSTAR for example constantly see the acceleration grids get hit by the very ions they accelerate, which damages them over time. When Dawn was built no one could predict with a small enough confidence interval how long exactly a single thruster grid might survive. Another bonus in the VASIMR's book: that one's an electrodeless thruster and therefore doesn't suffer this erosion issue. Electrodeless plasma thrusters are the hip thing nowadays and Ad Astra aren't the only people designing one.

Incidentally, it was wise foresight to include multiple engines, because it turned out that Dawn needed them. It had to switch engines twice. Amusingly, neither of these events was because of a burned-out grid... those seem to have survived much better than expected. But, in 2014 while enroute to Ceres, Dawn lost an engine controller to a cosmic ray strike. I'm not sure if it was recovered later, but Dawn continued onwards with a different engine anyways. Then, in 2015 while lowering its orbit around Ceres, that engine's mechanical gimbal system developed a problem, and they switched to the third engine.

Edited November 21, 2016 by Streetwind

[FleshJeb]

14 hours ago, Streetwind said:

But just Isp alone doesn't make a better engine. There is a relationship between payload mass, expected dV needs, the specific power of your power production solution, and mission duration which I always forget the details of... but it basically says "for these given parameters, an engine with this specific impulse is ideal".

Relevant:

https://en.wikipedia.org/wiki/Spacecraft_propulsion#Energy

Minimum energy Ve = 0.6275*dV

[Bill Phil]

Let's use the KE equation for a moment. First we can actually make it a power equation. Instead of KE = .5mv^2, it becomes W=.5mv^2 / t. This lets us solve for force. F=mv/t. 2W/v = mv/t. This means that 2W/v = F. So, what does that actually mean? Well, thrust is inversely proportional to exhaust velocity. The lower velocity is for a given amount of power, the higher the thrust.

[wumpus]

46 minutes ago, FleshJeb said:

Relevant:

https://en.wikipedia.org/wiki/Spacecraft_propulsion#Energy

Minimum energy Ve = 0.6275*dV

I'm willing to believe that for anything past the moon (and not limited by life support), chemical rockets are rarely the answer.  Once you get to 4 digits, don't be surprised if ISP isn't so important (unless you are constrained by the price/availability of Xenon).

7 hours ago, magnemoe said:

You could use other gasses in vasmir, downside is that heavier molecyles give lower ISP, they give more trust however.

This can be pretty bad.  Using NERVAs with water would reduce ISP from >=800 to no more than hydrolox (and likely worse as that assumes hydrolox temperatures in the nozzle (which is presently the limit of our technology, then how do we contain the reactor that heats the water up hotter?)).

Presumably you could get better than chemical ISPs out of helium (and presumably the James Webb telescope will pioneer holding liquid He indefinitely), but expect a big, big hit.  But it is the only known high power (compared to everything else with a high ISP.  Don't compare it to a real rocket) interplanetary engine.

[SpaceMouse]

Posted a pic on my dev thread. Should anyone be interested.

 

[SpaceMouse]

Thanks for all the input everyone, i wish i got more of that when i post stuff. Lol. Conversely, i'd think you could pump some oxidizer into the (hydrogen) plasma exaust for a extra thrust boost. Might do that as a additional feature too.

Haven't really heard helium discussed as a VASIMR propellant before. Would kinda think it would be impractical based on its rarity.

Edited November 23, 2016 by SpaceMouse

[magnemoe]

7 hours ago, SpaceMouse said:

Thanks for all the input everyone, i wish i got more of that when i post stuff. Lol. Conversely, i'd think you could pump some oxidizer into the (hydrogen) plasma exaust for a extra thrust boost. Might do that as a additional feature too.

Haven't really heard helium discussed as a VASIMR propellant before. Would kinda think it would be impractical based on its rarity.

Bad idea, you are better of using an separate chemical engine.  no nozzle and the hydrogen atoms might even move so fast they have issues reacting. 
Some nuclear thermal designs has this option, its an nice way to increase trust, however it drops your ISP down close to an normal chemical engine as the oxygen atom is so much heavier. Still you have an nozzle so you can use it,