Space Batteries

[PB666]

Over the past few days we have been having 2 threads (actually more if we count the last month or so) discussing non-nuclear high ISP space engines. So that I am going to through in some food for thought.

The key element for any storage fuel is energy density available on its expulsion from the accelerated system.  The more energy, the higher the exhaust velocity which we should all know by now is ~10 times the ISP.

But the most mass efficient engines are the most difficult to use with humans. 1) they either deal unpredictable amounts of ionizing radiation 2) they don't carry energy of their own.

The most efficient ION drives would take literally decades to burn through their fuel. There is actually nothing stopping engineers from having an ION drive that accelerates ions to 0.9c, the only problem is the power required, the mass of the accelerator, and of course the power conversion equation, which at 0.9c has to use lorentz transformations.

So why am I posting about batteries, how would they help.

As per the argument with Camacha last year (who did not like the links I was posting), I pointed out that it takes technological improvements in alot of areas to take what we have (space craft traveling outward from our sun in interstellar space at 16 km/sec) and make them go crazy faster. Essentially what you are doing is crossmultiply improvements (1.2 x 1.2 x 1.2 x 1.2) = 2.07. So that if you can improve your technologies in each area by 20% and it takes 4 areas to complete an operation you can increase your capability by 100%. So instead of leaving at 16 km/sec you are leaving at 32 km/sec. For the last 40 years we have been basically stagnate at this capability, but the science that currently exists allows us to break this and move on.

http://www.sciencemag.org/news/2016/05/how-build-better-battery-through-nanotechnology

So lets think about this.

1. If you want to kick an ION drive out of LEO without wasting fuel spiraling out (terribly inefficient you can lose as much as 50% of your fuel mass, more if you have to fight drag for endless periods)

2. If you couple an ION drive or possibly Cannae drive to waste energy from an NTG. The problem is that NTGs don't generate that much power and are sometimes used for experiments, the other problem is that you might want to kick on the ION drives as the approach the periapsis of some planet you are oberth-ing around.

Let me make the first few premises. The mass of an ION drive is hideously low compared to NTGs and Solar panels. But there is a certain load density it about 200 KW per output meter you cannot go over, and safely its about 100 KW.

So basically a high output ION drive system needs to 'roll' out in space, we could make the system about 1/10th to 1/5th the mass of the solar panels and the would still be underutilized.

So what SEP and NEP space craft need are batteries with higher energy storage densities.

What do we need solar electric power for, quite simple, we should not launch tugs into space that deplete their resources and float around the earth, moon, sun forever. What we really need are ships that sip fuel and run multiple missions.

The problem is that SEP does not work well in fast decaying orbits, these are the orbits in which you would want them to refuel themselves and capture fuel and supplies from Earth and carry them into clean space. This takes care of so many problems at once, it gets rid of the space junk problem (fuel launches decay back into earth quickly) and since the tug is recycled it can carry fuel out, transfer and then dump the container in LEO before it picks up another, and the ship itself is not wasted. It can carry fuel to just about anywhere in the solar system using ION drives to leave earths orbit and potentially Cannae drives in transfer orbits. But to get out of LEO in orbits that decay within a years time, you really need an efficient battery, because the ION drive needs to both fight drag and increase velocity at its periapsis. So that it needs to be able to push into an eccentric orbit quickly, then keep kicking itself out at its  periapsis until it apoapsis is at its target radius.

To optimize this you need more ION drives and a battery that can store Solar and/or Nuclear electric power.

As mentioned in the other post to take true advantage from the oberth effect while traveling around the sun you really need to approach the planet from above (relative to the sun), in intering the planets Hill radius and approach a safe distance from the planet you are borrowing thermodynamic energy, converting it into kinetic energy and while going the very fastest (at the periapsis heading prograde around the star) you add dV to that. At that perfect point you need to add as much dV as possible at that moment (the fraction of the KE you have to pay back on exit is alot less than you paid when dV is applied along the prograde at peak velocity). This of course is not possible, but most of the energy added will come on the dark side of the planet where direct SEP will not work. Some ION drives allow the lowering of ISP for greater thrust but there is a practical limitation to the efficient mass of ION drives, but the typical onboard batteries will not suffice to sustain operation of ION drives in complete darkness, the only real way to take advantage of the Oberth effect with SEP is to have highly powerful and mass efficient batteries.

Just to make these points because some of the arguements get silly. Improving on a well made wheel is quite difficult, this is the nature of space now, dV improvments are not a simple example of improvement in one area, it requires improvement in many areas to get substantial improvements in performance. This is not limited to engineering and technology, but ultimately, better understandings of space and physics.

 

 

[wumpus]

I think you need to define your terms better.  To me, the term "battery" means the things in my phone, that which stores the energy of a Tesla, and those trusty old things that made some of my toys expensive to play with when I was a boy.

Chemical batteries are a lost cause when it comes to rocketry.  The only possible case for them I could imagine would be for use in electric-powered spacecraft on the initial parts of the journey when the Earth tends to eclipse the Sun (i.e. night) a large chunk of the time.  But the whole point of this is that the spacecraft is solar powered and the batteries are solar-charged.  Using "batteries" as a power source is something else.

Even if you somehow had a superior battery (as a power source), I would still assume that converting it into a fuel cell would make more sense (you certainly don't want to accelerate the "used up" portions and need to eject them).  Note that this means your "ion accelerator" is certainly unlikely to be efficient at .9c (or even .009c), but will likely want to accelerate n grams of fuel cell exhaust to whatever speed n grams of fuel cell fuel can accelerate it to.   While I wouldn't be terribly surprised that with great expense and complexity this system might be more efficient than a chemical rocket, it is certainly likely to fall behind existing RTG + ion systems.

One thing you have completely failed to mention is something I will call "heat efficiency".  My concept of "heat efficiency" is rather unlike any normal idea of efficiency (it doesn't care about things like fuel or mass) but simply the location of the build up of waste heat.  While a chemical rocket is inefficient in just about every way, nearly all of its tremendous heat is thrown out the exhaust bell, never to be seen again.  This is important: to produce 1W of power we can expect to heat our system by >1W.  Yet in the case of chemical rockets, that "system" has the exhaust fuel containing most of the heat increase and the rest of the rocket containing very little.  Note that a brief visual check of the ISS shows that the heat radiators are within an order of magnitude of the size of the solar panels.  This means that whatever your engines used must have a greater efficiency (i.e. the power used goes into the reaction mass) than an ion engine to get greater than an order of magnitude more thrust/kg of an ion engine.  This doesn't help your argument at all since you will need a few more orders of magnitude of thrust to begin to use the Oberth effect and avoid the slow spiral trajectories of ion powered spacecraft.

To effectively use the Oberth effect (for leaving Earth), I'm guessing that you need to be able to achieve thousands of m/s of delta-v (1k for Mars, 5k for Neptune) in roughly 30 minutes (you might get away with longer with higher perigee, but that defeats your purpose).  I wouldn't be that shocked if rocket built to do such a thing wound up carrying multiple kg of expendable coolant for every kg of reaction mass, but that seems odd (although it would put reusable batteries back in play as you wouldn't necessarily care as much about losing the mass the instant it becomes dry weight).

[PB666]

19 minutes ago, wumpus said:

I think you need to define your terms better.  To me, the term "battery" means the things in my phone, that which stores the energy of a Tesla, and those trusty old things that made some of my toys expensive to play with when I was a boy.

Chemical batteries are a lost cause when it comes to rocketry.  The only possible case for them I could imagine would be for use in electric-powered spacecraft on the initial parts of the journey when the Earth tends to eclipse the Sun (i.e. night) a large chunk of the time.  But the whole point of this is that the spacecraft is solar powered and the batteries are solar-charged.  Using "batteries" as a power source is something else.

Even if you somehow had a superior battery (as a power source), I would still assume that converting it into a fuel cell would make more sense (you certainly don't want to accelerate the "used up" portions and need to eject them).  Note that this means your "ion accelerator" is certainly unlikely to be efficient at .9c (or even .009c), but will likely want to accelerate n grams of fuel cell exhaust to whatever speed n grams of fuel cell fuel can accelerate it to.   While I wouldn't be terribly surprised that with great expense and complexity this system might be more efficient than a chemical rocket, it is certainly likely to fall behind existing RTG + ion systems.

One thing you have completely failed to mention is something I will call "heat efficiency".  My concept of "heat efficiency" is rather unlike any normal idea of efficiency (it doesn't care about things like fuel or mass) but simply the location of the build up of waste heat.  While a chemical rocket is inefficient in just about every way, nearly all of its tremendous heat is thrown out the exhaust bell, never to be seen again.  This is important: to produce 1W of power we can expect to heat our system by >1W.  Yet in the case of chemical rockets, that "system" has the exhaust fuel containing most of the heat increase and the rest of the rocket containing very little.  Note that a brief visual check of the ISS shows that the heat radiators are within an order of magnitude of the size of the solar panels.  This means that whatever your engines used must have a greater efficiency (i.e. the power used goes into the reaction mass) than an ion engine to get greater than an order of magnitude more thrust/kg of an ion engine.  This doesn't help your argument at all since you will need a few more orders of magnitude of thrust to begin to use the Oberth effect and avoid the slow spiral trajectories of ion powered spacecraft.

To effectively use the Oberth effect (for leaving Earth), I'm guessing that you need to be able to achieve thousands of m/s of delta-v (1k for Mars, 5k for Neptune) in roughly 30 minutes (you might get away with longer with higher perigee, but that defeats your purpose).  I wouldn't be that shocked if rocket built to do such a thing wound up carrying multiple kg of expendable coolant for every kg of reaction mass, but that seems odd (although it would put reusable batteries back in play as you wouldn't necessarily care as much about losing the mass the instant it becomes dry weight).

Fuel cells 'burn' what they make eventually you have to find a way to store and reconvert back the products or you have carried their mass for no particularly good reason. Conversion of water to hydrogen and oxygen s terribly inefficient.

Read what i wrote, using an ION drive at 0.9c would only be used in fusion powered craft, it can be done, 99.99% of the time it would be unwisely done.

Heat efficiency is the issue, but if you read the link they are work on reducing heat, and if your read my past threads on solar power, this is a major limitation of solar power if you keep building bigger and bigger panels, you either have to add weight for power transformers and heat radiators or end up using bare cable copper and operating at near melting temperatures over much of the craft. In this sense batteries definitely are a bonus because they mean fewer panels, shorter wires and less amperage. Secondarily advantagous because with really big panel arrays, the heat radiators will interfere, were as with batteries you can radiate isolated from the insolating areas of the panels.

You need alot of dV, that is true, and what's important is that ION drives need to be able to run at the mass efficiency ISP when entering a gravity well, that means more output at lower ISP. Definitely a problem is SEP and NEP craft. But then an efficient ION craft once out of the gravity well has alot of time and dV to steer itself again into another gravity well as to repeat the process. The point about ION drives and gravity wells, you have to be patient, which means neccesarily they may take longer to get to that destination sector, but they will be going faster once they get there.

Third you don't really care if you expend all the power in one cycle, if the battery does not overheat, you have all the time after the fact to let it cool and recharge. And so what you damage the battery so it can only get 3/4ths the job in the future, you have pump that ship up as fast as it can go.

Apparently you are not getting the juxt, I am not talking about what you think right now is the technology because obviously you seemed to be stuck in that, I'm talking about where the technology might be in 0 to 15 years. ION drives are a thing, we can expect more use of them in space, so that the new technologies that make them work better.

 

[wumpus]

No, I didn't read the link.  None of my calculations depended on a battery, you could have an unobtanium battery that magically stored power and you will still be limited by the radiators and ion engines.  Thanks to cell phones, batteries are basically a measurable chunk of the GNP and anybody with a thinner/more powerful/faster charging (less so with the last) can expect to charge a significant margin to Apple/Samsung and others.  Make a battery cheaper/faster charging and you can expect to get a similar sale to GM/Nissan (and Tesla might be interested in buying your tech).  You could spend all you time reading the latest puff piece on somebody's latest battery tech, only to find out some other tech beat it to market. 

Essentially you have described a system that might be able to get to LM2 slightly more efficiently, but may or may not get there faster.  Going past LM2 would be pointless for such a craft, at that point ditch the batteries and go pure solar.

At LEO, you would have a distinct disadvantage over a pure solar ion ship.  They both would be firing during the day and the battery-powered system would simply have extra mass in the form of the battery and likely the radiators.

At some point in higher orbit the battery-backed system would have an advantage (we hope), due to much lower times being eclipsed and the ability to store an entire orbit's worth of power in the batteries.  The big question is will it catch up before the orbit is big enough that it becomes way more than the batteries can store (if this isn't a limit than the engines certainly will be.  And understand that the size of the required radiators are going to become at least as large as the pure-solar solar panels).  I would be somewhat surprised if this got to LM2 faster, but it is certainly possible (and should be more efficient).

By the time you get to LM2 (or LTO), there are pretty much two ways to go.  To use the Obereth effect, you need to dump several thousand delta-v in roughly 30 minutes or so (1k for Mars, 5k for Neptune).  A vastly more likely situation is that you will be taking months if not years to get to LM2, in which case you might as well rely on gravity tricks to get you to where you need in the solar system (don't forget to supply the needed capture delta-v).  Going this route likely means that you are better off losing the unneeded mass of the batteries and extra radiators.

Finally, there really isn't any way of getting around the time issues.  Adding a battery (and radiators, don't forget them) to an ion engine means that it will always produce less delta-v/month, just that the delta-v/month will get you farther because of the Obereth effect (assuming you can even use it without getting tossed into escape velocity).  For the ion engines that are even in the powerpoint stages, you will eventually be better off using gravity tricks to get anywhere.  And yes, all of this is still true with magical unobtanium batteries (although at LEO they would go equal speeds, if and only if you had magical unobtanium radiators as well).

But it might be worth it to build an LEO-(LM2 and/or LTO) space tug.

[PB666]

Quote

No, I didn't read the link.  None of my calculations depended on a battery, you could have an unobtanium battery that magically stored power and you will still be limited by the radiators and ion engines. 

That says alot right there. The radiators are not a theorectial limitation except when considering the non expelled heat of the ION thruster, However given the fact that ION thrusters can handle about 200 KW per meter of thruster. So lets just saw we have a 1.2 megawatt of power, we have 6 square meters of thruster exhaust area, we set the drive at a perfect eff of 1000 isp, exhaust vel of 10000 at 80% eff. What then have

So lets calculate  2 x 1000000/ 10000 that gives 100 N of thrust, not good, but then lets consider the target such as venus, So now lets argue that you have the unobtanium battery, you have normal thrust of SEP uptonprojected termination at which poin you are behind mercury traveling from ? to 14000 m/s, drifting, but in the approach with SEP you managed to add over the 2 m km inside the hill sphere an average of 2 km/s or 1,000,000 sec at 2N , thats lousy N is 2,000,000 m/s*m at 10,000 ISP, lets say the space craft weighs 100 tons. Thats 20 m/s Then for 2000s you are 100N, thats another 2 m/sec. so you got 14022 m/s and going out you add another  20 m/s, this seems pretty miserable, we only picked up 41 m/s, so lets see, we enetered the hill sphere at 1000 meter per second, we would have had m*196000000/2 , at peak we had ~ 14032 ( Translating after gain energy back) looks really bad, but how bad was it At peak you had m*196,896400/2, on exit we have 500^2 + 896400 = m*1396400.

I have to repeat that there are ION drives of ~70 to 83 percent efficient depending on v-pot that are rated to about 35 kw per 0.25 sq. meter face in space. The lowest efficiences are at the highest powers but these are only used for 2000 sec, and those are rated at ISPs of 19500. 

Now how bad did we do, we entered venus's hill radius with 1000, we left with 1671, we gained 671 dV and spent at an average of about 9999 sec ISP, thats pretty friggen good if you have all the time in the world tonspend, but can you imagine what that would do if you slid into jupiters hill spere with only 500 m/s. During the interem we can use the high ISP. You got anything that can match that? Again you said i could have an unobtainium battery, i made the problem hard, i set my craft at 100t, heavier than the falcon-9 second stage and dragon payload with payload. How much fuel can i carry? 

So what would be next, I would do what rosetta did, steering myself around the inner solar system, using these 9999 isps to get 700, 500, 300, 250 dV wherever i could I might even use jupiter to steer me into a mercurian intecept before i used that to throw me a one more shot through venus and then out of the system, using as many kicks on high insolance planets. I might even tune that ION drive to its highest rated ISP as I approached Mercury, taking advantage of the high level of radiation. 

 

 

[PB666]

http://www.lifescience.net/news/946/new-lithium-oxygen-battery-greatly-improves-energy/#.V5dFTaRTw-t.reddit

 

[wumpus]

While I keep having visions of the eventual elliptic orbit and week-long-charging cycle (out to LTI), I'd guess that a two-three hour charge cycle combined with a 15 minute discharge cycle would probably work best (and the charging cycle would have to deal with "day/night" issues).  I also suspect you will spend even more time in the Van Allen Belts (especially in the outer belts, as the craft struggles to escape with a fixed amount of delta-v per orbit).

The biggest problem with this discussion is the idea that you can increase thrust by an order of magnitude without losing efficiency (well, no more than 50%).  That seems to be counter to all available engineering on electric-powered spacecraft.  VASIMR makes a whole lot of noise about this idea, but seems to willing to handwave away *all* problems, many of which would greatly change current practice elsewhere (long-term storage of hydrogen, cooling in space at massive power levels, and just how the Isp varies with power.  None of them seem to be mentioned).

[magnemoe]

The problem is that batteries is unlikely to hold more energy than good rocket fuel and oxidizer even non cryogenic ones, they only require the tanks not the battery infrastructure. 
Beamed power this includes termal or PV cells powered by lasers changes this still dont trust in shadow is an good idea. 

Yes you will use batteries, one obvious use is to power systems while in shadow of earth,moon, or other bodies other is peak use like pulsed fusion 

[wumpus]

The other issue is cooling.  You might wind needing to pump a significant amount of heat out to your radiators (which would then be getting considerably hotter than your spacecraft) during battery use (I'm assuming they are used for the Oberth effect, as initialing described above.  Other issues wouldn't seriously effect the engineering).

Oddly enough, the NASA research suggests using capacitors for pulsed fusion (I haven't had time to read the paper, so I don't know if Fe-based batteries are an option).  I had no idea anyone who was working on such a thing used actual hardware (for at least fusion tests, I doubt they really bothered where the electricity came from).  Plenty of radiators, though.

http://www.nasa.gov/sites/default/files/files/Adams_2013_PhI_PuFF_inProgress.pdf  Note: this assumes a "five day burn" to Mars (for a 90 day transit).  Presumably with the efficiencies of a Mangallayan maneuver you might be able to turn it closer to a "two day burn" that takes 10+ days to get to Earth Escape velocity (followed by a one day burn with no further efficiency gains).  You can take all the time you want for the first 3000m/s to escape velocity, but if you can't do the last 1000m/s in one burn, you don't get the Oberth effect.

I'd also expect that for large values of power, "batteries" would become "similar battery chemistry" flowing through a fuel cell in a way that can be recharged.  Presumably this would optimize all the packaging constraints of keeping the electrodes next to each other and whatever materials  need to separate them.  Considering the speed battery technology is moving (nothing like billions of people who all want a thinner cell phone to drive the tech), it might take awhile for this to be an option (so far fuel cell tech keeps getting passed by battery tech.  Once battery tech levels off, I suspect that fuel cells will be developed with superior J/kg.  While this might take awhile, I'm not holding my breath for pulsed fusion either.