Author Topic: RocketLab Electron Smallsat Launcher  (Read 738501 times)

Offline ArbitraryConstant

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Re: RocketLab Electron Smallsat Launcher
« Reply #200 on: 04/21/2015 06:45 pm »
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.
« Last Edit: 04/21/2015 06:46 pm by ArbitraryConstant »

Offline ArbitraryConstant

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Re: RocketLab Electron Smallsat Launcher
« Reply #201 on: 04/21/2015 10:08 pm »
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.

Offline Robotbeat

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Re: RocketLab Electron Smallsat Launcher
« Reply #202 on: 04/22/2015 01:02 am »
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.
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Offline Elmar Moelzer

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Re: RocketLab Electron Smallsat Launcher
« Reply #203 on: 04/22/2015 03:25 am »
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.

Offline QuantumG

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Re: RocketLab Electron Smallsat Launcher
« Reply #204 on: 04/22/2015 03:28 am »
I expect you could make the same argument for an alcohol fuel cell.. and it'd probably run on kero.
Human spaceflight is basically just LARPing now.

Offline ArbitraryConstant

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Re: RocketLab Electron Smallsat Launcher
« Reply #205 on: 04/22/2015 03:42 am »
IIRC, hydrogen fuel cells have a higher power density than batteries as well
Don't think this is true.

Offline Burninate

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Re: RocketLab Electron Smallsat Launcher
« Reply #206 on: 04/22/2015 03:46 am »
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.

Offline TrevorMonty

Re: RocketLab Electron Smallsat Launcher
« Reply #207 on: 04/22/2015 03:48 am »
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.

Offline sanman

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Re: RocketLab Electron Smallsat Launcher
« Reply #208 on: 04/22/2015 04:11 am »
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?

Offline jongoff

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Re: RocketLab Electron Smallsat Launcher
« Reply #209 on: 04/22/2015 05:58 am »
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

Offline Elmar Moelzer

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Re: RocketLab Electron Smallsat Launcher
« Reply #210 on: 04/22/2015 06:11 am »
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.
« Last Edit: 04/22/2015 06:11 am by Elmar Moelzer »

Offline Elmar Moelzer

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Re: RocketLab Electron Smallsat Launcher
« Reply #211 on: 04/22/2015 06:12 am »
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!

Offline sanman

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Re: RocketLab Electron Smallsat Launcher
« Reply #212 on: 04/22/2015 06:31 am »
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.

Offline Patchouli

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Re: RocketLab Electron Smallsat Launcher
« Reply #213 on: 04/22/2015 07:27 am »
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.


« Last Edit: 04/22/2015 08:05 am by Patchouli »

Offline sanman

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Re: RocketLab Electron Smallsat Launcher
« Reply #214 on: 04/22/2015 08:14 am »
Actually glass cockpits predate main stream tablets like the ipad by several decades  first appearing in the early 1980s.

Heh, okay - I was just remembering that when Musk presented Dragon 2, it looked like the big screens on the inside were just large tablets. It seems like consumer tablet technology is currently evolving much faster than any glass cockpit systems created by aerospace people - but that's just the kinetics of a larger and more active market.

Quote
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.

Does anyone remember a discovery called "Thermopower Wave"? The extremely high power demand of an electric turbopump sounds like a good fit for it:

http://www.technologyreview.com/news/530346/progress-on-a-powerful-new-way-to-generate-electricity/

http://www.pbs.org/wgbh/nova/next/tech/carbon-nanotube-fuses-unleash-surge-electrons-called-thermopower-wave/

http://www.nature.com/nmat/journal/v9/n5/abs/nmat2714.html

http://physicsworld.com/cws/article/multimedia/2013/feb/19/what-is-a-thermopower-wave
« Last Edit: 04/22/2015 08:27 am by sanman »

Offline R7

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Re: RocketLab Electron Smallsat Launcher
« Reply #215 on: 04/22/2015 11:32 am »
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.
« Last Edit: 04/22/2015 11:36 am by R7 »
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Offline notsorandom

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Re: RocketLab Electron Smallsat Launcher
« Reply #216 on: 04/22/2015 01:28 pm »
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.

Offline Burninate

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Re: RocketLab Electron Smallsat Launcher
« Reply #217 on: 04/22/2015 01:36 pm »
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.
« Last Edit: 04/22/2015 01:42 pm by Burninate »

Offline Patchouli

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Re: RocketLab Electron Smallsat Launcher
« Reply #218 on: 04/22/2015 04:47 pm »

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.

« Last Edit: 04/22/2015 05:02 pm by Patchouli »

Offline ArbitraryConstant

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Re: RocketLab Electron Smallsat Launcher
« Reply #219 on: 04/22/2015 05:01 pm »
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.

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