It really doesn't make sense to compare Isp of a battery-pumped engine with that of a turbopumped engine. The energy source for the turbopump goes into the exhaust, tending to decrease the Isp. That makes it look bad compared to the battery-pumped engine if you just look at Isp.
Note that this is a comparison with gas generator turbopump engines, which loose several percentage points of efficiency by tossing the turbopump exhaust overboard. Staged combustion turbopump engines would likely outperform electric pump engines.
IIRC, hydrogen fuel cells have a higher power density than batteries as well
Not sure if this has been mentioned before, but I can't help thinking that with a hydrolox instead of a kerolox engine, they could have skipped the batteries all together and just used some of the LOX and LH2 from the main fuel tanks in a fuel cell to drive the pump (IIRC, hydrogen fuel cells have a higher power density than batteries as well). That would probably improve the overall system weight of electric pumps quite a bit.
Batteries are a lot cheaper and simple. Plus they get performance increase for free as battery technology improves.
Quote from: Elmar Moelzer on 04/22/2015 03:25 amIIRC, hydrogen fuel cells have a higher power density than batteries as wellDon't think this is true.
I expect you could make the same argument for an alcohol fuel cell.. and it'd probably run on kero.
Quote from: ArbitraryConstant on 04/22/2015 03:42 amQuote from: Elmar Moelzer on 04/22/2015 03:25 amIIRC, hydrogen fuel cells have a higher power density than batteries as wellDon't think this is true.Yeah, IIRC higher energy density, much worse power density. At least that's what I understood from Frank Zegler's discussion on IVF on the other thread.~Jon
Quote from: TrevorMonty on 04/22/2015 03:48 amBatteries are a lot cheaper and simple. Plus they get performance increase for free as battery technology improves.Cellphone Batteries => Electric RocketsTablets => Glass CockpitAny other consumer technologies that can improve aerospace?
Actually glass cockpits predate main stream tablets like the ipad by several decades first appearing in the early 1980s.
As for battery technology the energy density of the silver zinc technology previously used in aerospace is not much lower then modern lithium ion batteries. The biggest problem with the older technology is historically it used mercury and had a very limited number of recharge cycles.Interestingly now that those two issues have been solved the technology is now being looked at again as a safer alternative to lithium ion.
He may have artificially hobbled the turbopump. He's assuming a relatively unsophisticated turbopump that uses decomposition of MMH for it's gas source and further requires a cooling water supply that is injected to keep the turbine inlet temperatures in check. A gas generator running on LOX/RP-1 might do better, as might one that has higher temperature metallurgy.
Quote from: edkyle99 on 04/21/2015 05:20 pmNote that this is a comparison with gas generator turbopump engines, which loose several percentage points of efficiency by tossing the turbopump exhaust overboard. Staged combustion turbopump engines would likely outperform electric pump engines. True of expander as well. But, the costs associated with this are much higher. Faced with developing an engine like that or even just buying RL-10, Orbital went with a solid stage. Electric seems much more economical than any turbopump, especially at small size; Rutherford is pretty close to Kestrel in thrust. What's impressive isn't just decent performance, but that level of performance with such a cheap engine and quick development program. They did it on startup money.
Quote from: QuantumG on 04/22/2015 03:28 amI expect you could make the same argument for an alcohol fuel cell.. and it'd probably run on kero.True, but I am not aware of their energy density and efficiency compared to batteries and hydrogen fuel cells. If the thing weights more than the battery, there is no point to it.Quote from: ArbitraryConstant on 04/22/2015 03:42 amQuote from: Elmar Moelzer on 04/22/2015 03:25 amIIRC, hydrogen fuel cells have a higher power density than batteries as wellDon't think this is true.A quick google search reveals a 8 to 10X energy density of fuel cells versus batteries. Now it is true that batteries have been improving significantly. So that is a valid argument.Either way, these types of engines might improve with time and fuel types.
This is likely hydrogen fuel cells at some given pressure reacting with atmospheric oxygen through a PEM, and being said to contain lots of energy per unit mass of hydrogen. That's not a valid number on a spacecraft where there is no atmosphere.Aside from that:Fuel cells and flow batteries dis-aggregate the association between discharge rate and capacity that exists with batteries. It is possible to design one that uses all its energy storage arbitrarily fast, at the expense of increased total system mass without increased energy storage.AFAICT, the only big improvements on lithium ion batteries in the last ten years have been increasing the safe charge & discharge rate by a factor of five or ten in LiPos with new anode & cathode chemistries. This has been basically irrelevant for battery life, and affects only high-power applications, and possibly but not definitely charging rates.
Quote from: ArbitraryConstant on 04/22/2015 03:42 amQuote from: Elmar Moelzer on 04/22/2015 03:25 amIIRC, hydrogen fuel cells have a higher power density than batteries as wellDon't think this is true.A quick google search reveals a 8 to 10X energy density of fuel cells versus batteries.