-
#200
by
ArbitraryConstant
on 21 Apr, 2015 18:45
-
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.
I agree as far as it goes, but even though the analysis needs to take that into account, it's not like we've used that as a reason to never talk about ISP for pressure fed even though it looks like that has a larger dry mass penalty. There's lots of dimensions to optimizing a rocket. Pressure fed and even solid upper stages are in use in spite of being even worse than electric.
What's interesting is that it may not be that much worse than turbopumps, at much lower cost, and significantly better than other extant technologies. That means it can credibly offer improvement in at least some uses.
-
#201
by
ArbitraryConstant
on 21 Apr, 2015 22:08
-
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.
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.
-
#202
by
Robotbeat
on 22 Apr, 2015 01:02
-
Superpumps are not going to ever beat the highest performance turbopump-based cycle, Ed is right.
But of course, a superpump should be easier to develop and fits in well with a fully electrically actuated rocket.
-
#203
by
Elmar Moelzer
on 22 Apr, 2015 03:25
-
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.
-
#204
by
QuantumG
on 22 Apr, 2015 03:28
-
I expect you could make the same argument for an alcohol fuel cell.. and it'd probably run on kero.
-
#205
by
ArbitraryConstant
on 22 Apr, 2015 03:42
-
IIRC, hydrogen fuel cells have a higher power density than batteries as well
Don't think this is true.
-
#206
by
Burninate
on 22 Apr, 2015 03:46
-
How do electric pumps compare to turbopumps and pressure-fed engines as far as high-frequency throttle response? Hydrazine RCS thrusters still need a suitably agile replacement.
-
#207
by
TrevorMonty
on 22 Apr, 2015 03:48
-
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.
-
#208
by
sanman
on 22 Apr, 2015 04:11
-
Batteries are a lot cheaper and simple. Plus they get performance increase for free as battery technology improves.
Cellphone Batteries => Electric Rockets
Tablets => Glass Cockpit
Any other consumer technologies that can improve aerospace?
-
#209
by
jongoff
on 22 Apr, 2015 05:58
-
IIRC, hydrogen fuel cells have a higher power density than batteries as well
Don'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
-
#210
by
Elmar Moelzer
on 22 Apr, 2015 06:11
-
I 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.
IIRC, hydrogen fuel cells have a higher power density than batteries as well
Don'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.
-
#211
by
Elmar Moelzer
on 22 Apr, 2015 06:12
-
IIRC, hydrogen fuel cells have a higher power density than batteries as well
Don'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
Ahh, ok. That is interesting! Thanks Jon!
-
#212
by
sanman
on 22 Apr, 2015 06:31
-
Well, the whole appeal of fuel cells over batteries is their higher energy density, with that energy density depending on what fuel is used.
Usually, the higher the fuel cell's operating temperature, the higher its power output due to faster reaction kinetics. So solid oxide fuel cells, which operate at much higher temperatures than proton exchange membrane, would usually give higher power output.
-
#213
by
Patchouli
on 22 Apr, 2015 07:27
-
Batteries are a lot cheaper and simple. Plus they get performance increase for free as battery technology improves.
Cellphone Batteries => Electric Rockets
Tablets => Glass Cockpit
Any 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.
-
#214
by
sanman
on 22 Apr, 2015 08:14
-
-
#215
by
R7
on 22 Apr, 2015 11:32
-
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.
Yes, comparing modern battery and electric motors against 60s TP with additional water circuit shows heavy bias. There's a crude error too, TP mass is calculated knowing pump power requirement and using pump specific power (22kW/kg) and turbine specific power (18kW/kg) separately. Combined specific power is 9.9kW/kg while their reference NASA SP-8107 shows 17.6kW/kg (10.7hp/lbm) for the entire TPA. LR87 TPA is geared which adds mass further. Single shaft F-1 TPA shows 27kW/kg in the same reference. Modern TPA kW/kg numbers should have three digits before decimal separator like SSME had. Turbine efficiency seems lowballed too, causing higher gg mass penalty than it should be.
-
#216
by
notsorandom
on 22 Apr, 2015 13:28
-
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.
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.
Expander is a great cycle. It is simple and efficient. There are some major drawbacks though. It has to use cryogenic propellant, either fuel or oxidizer. The colder the better which means LH2 has pretty much been the only one worth using this cycle for. The other drawback is limited thrust these engines can provide. As the thrust increases the heat exchange that drives the cycle becomes less effective.
Hydrogen is a more expensive fuel to use than RP-1 so I can understand why they chose not to use it. I would bet that the extra difficulties in designing and building an LH2 pump outweigh the simplicity of designing an expander cycle. Based on the DC-X which was powered by four RL-10s a small team can effectively use a hydrogen expander rocket. However they were able to use off the shelf engines and didn't have to design and build it themselves.
-
#217
by
Burninate
on 22 Apr, 2015 13:36
-
I 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.
IIRC, hydrogen fuel cells have a higher power density than batteries as well
Don'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.
-
#218
by
Patchouli
on 22 Apr, 2015 16:47
-
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.
Another improvement to battery technology is renewed interest in silver zinc chemistry due to recent improvements to the technology.
They're safer due to a water based electrolyte that is not flammable and now that mercury is no longer needed are more environmentally friendly.
Wh per kg is somewhat worse about the same as mid range LiPos but Wh per liter is much better.
-
#219
by
ArbitraryConstant
on 22 Apr, 2015 17:01
-
IIRC, hydrogen fuel cells have a higher power density than batteries as well
Don't think this is true.
A quick google search reveals a 8 to 10X energy density of fuel cells versus batteries.
You said
power density not
energy density, which are different things.