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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.