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#240 by ArbitraryConstant on 24 Apr, 2015 15:22
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I was merely countering the idea that battery specific energy has not markedly improved over the introduction and widespread use of lithium ion batteries. Specific energy has about doubled and could possibly nearly double again (eventually), though my guess is that most of the new improvements will come from switching to another chemistry.
Yeah, that about lines up with my understanding. Other chemistries like lithium sulfur potentially get into kwh/kg territory but have to nail down side reactions that limit cell life, though I think rockets can tolerate lower cell life than cars, even if they want to be reusable.
Lithium sulfur has lower instantaneous power as the above paper calls out, currently needs a burn time over 600 seconds to break even, though you'd expect that to improve over time.
For plain old lithium ion (or polymer) though, my take is that if it's marginal now when you save a ton of money on other complexity, a factor of 2, or even 5% per year for a few years, is actually pretty substantial. In an industry where RL-10 can be king of the hill well into its 5th decade, 5% a year is huge. -
#241 by ArbitraryConstant on 25 Apr, 2015 04:45
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Trying to take a stab at the second stage ISP they're getting based on the pictures on the website...
http://www.rocketlabusa.com/static/images/first-stage-hi.png
http://www.rocketlabusa.com/static/images/second-stage-hi.png
Scaling the second stage picture so the combustion chamber is the same number of pixels across as the first stage, I get 160 and 365 pixels across for the nozzle so the expansion ratio from that would be something like 5.5x larger.
Taking a stab at sea level expansion ratio, looks like something like 12. Which would suggest a chamber pressure on the low side, like maybe 800 psi or less. That makes sense if there's a large mass penalty for increasing pump power.
Getting to upper stage ISP from this info is over my head. -
#242 by Katana on 26 Apr, 2015 06:41
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Trying to take a stab at the second stage ISP they're getting based on the pictures on the website...
http://www.rocketlabusa.com/static/images/first-stage-hi.png
http://www.rocketlabusa.com/static/images/second-stage-hi.png
Two large battery packs? much heavier than engine. May also be more heavy than 2nd stage tank. -
#243 by Robotbeat on 26 Apr, 2015 13:31
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You do realize that good LiPos can get upwards of 5-10kW/kg, right? That's much, much better than the engine in your car, I guarantee it.
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#244 by ArbitraryConstant on 26 Apr, 2015 15:33
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Two large battery packs?
How do you know that?
If all non-fuel stuff is represented there presumably there's a pressurized pod for avionics. -
#245 by Katana on 26 Apr, 2015 21:14
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You do realize that good LiPos can get upwards of 5-10kW/kg, right? That's much, much better than the engine in your car, I guarantee it.
http://www.a123systems.com/prismatic-cell-ahp14.htm
A123 best 4kw/kg plus
36kwe 9kg, 100kwe 25kg.
Which is correct? Engine 36kee but first stage 1Mwe from different news -
#246 by Katana on 26 Apr, 2015 21:19
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Two large battery packs?
How do you know that?
If all non-fuel stuff is represented there presumably there's a pressurized pod for avionics.
Should be so, compared to calculated weight. -
#247 by acsawdey on 26 Apr, 2015 23:38
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Trying to take a stab at the second stage ISP they're getting based on the pictures on the website...
http://www.rocketlabusa.com/static/images/first-stage-hi.png
http://www.rocketlabusa.com/static/images/second-stage-hi.png
Scaling the second stage picture so the combustion chamber is the same number of pixels across as the first stage, I get 160 and 365 pixels across for the nozzle so the expansion ratio from that would be something like 5.5x larger.
Taking a stab at sea level expansion ratio, looks like something like 12. Which would suggest a chamber pressure on the low side, like maybe 800 psi or less. That makes sense if there's a large mass penalty for increasing pump power.
Getting to upper stage ISP from this info is over my head.
On the subject of pump power, would it make sense to run the first stage engines on a profile such you gradually throttle back as atmospheric pressure and vehicle weight reduces? What's the exit pressure that goes with your 800 psi and a 12:1 nozzle?
Normally under-expanded exhaust just means you're not getting the most out of your propellant, but for this engine it also means you spent too much electrical power (and battery weight) running at that chamber pressure. There must be a point at which the weight penalty for additional battery power to run at a higher chamber pressure isn't offset by reduced gravity loss from the increased thrust.
If those two pods are the battery packs for the second stage, where did they put all the batteries for the first stage? At the top of the first stage to help aerodynamic stability (move cg up closer to cp as tanks empty)? Supposing that the 2nd stage burn was 3x as long as the first stage, you'd still need 3x more batteries on the first stage.
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#248 by ArbitraryConstant on 27 Apr, 2015 01:03
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On the subject of pump power, would it make sense to run the first stage engines on a profile such you gradually throttle back as atmospheric pressure and vehicle weight reduces?
I see what you're talking about, this can probably be answered with a spreadsheet but not a spreadsheet I have.
What's the exit pressure that goes with your 800 psi and a 12:1 nozzle?
I'm not actually sure, I'm basically scaling from Merlin 1C. 12:1 would be 66 psi or so, but isn't there a Bernoulli thing going on here as well since the exhaust velocity is large?
Your question reminded me of a picture that was posted of a Rutherford test. Googled some pictures... here's a comparison to Merlin:
https://qzprod.files.wordpress.com/2015/04/rutherford-test.jpg
http://33.media.tumblr.com/tumblr_m67uwzl0N01rti11fo1_1280.jpg
Merlin is a little under-expanded as can be seen from the exhaust continuing to expand, but Rutherford actually pinches in a little which suggests it's a little over-expanded. This can be gotten away with to an extent if care is taken with the nozzle design, I think SSME did this. That might suggest an even lower chamber pressure, like maybe 500 psi. That's still much higher than pressure fed, the examples I've looked at are <200 psi.Normally under-expanded exhaust just means you're not getting the most out of your propellant, but for this engine it also means you spent too much electrical power (and battery weight) running at that chamber pressure. There must be a point at which the weight penalty for additional battery power to run at a higher chamber pressure isn't offset by reduced gravity loss from the increased thrust.
Yup, can be traded with nozzle mass as well. This point is well taken.If those two pods are the battery packs for the second stage, where did they put all the batteries for the first stage?
Inside the carbon fiber body somewhere? The exterior is too crowded on the first stage and it needs to be aerodynamic for supersonic flight.Supposing that the 2nd stage burn was 3x as long as the first stage, you'd still need 3x more batteries on the first stage.
3x per engine you mean? Agreed, but there's a trade there, see the linked paper. Since the discharge time is longer it may allow a different chemistry with better specific energy and worse specific power. -
#249 by Katana on 27 Apr, 2015 01:21
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1/3 per engine, 9 engines, 3x total
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#250 by acsawdey on 27 Apr, 2015 01:48
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What's the exit pressure that goes with your 800 psi and a 12:1 nozzle?
I'm not actually sure, I'm basically scaling from Merlin 1C. 12:1 would be 66 psi or so, but isn't there a Bernoulli thing going on here as well since the exhaust velocity is large?
Your question reminded me of a picture that was posted of a Rutherford test. Googled some pictures... here's a comparison to Merlin:
https://qzprod.files.wordpress.com/2015/04/rutherford-test.jpg
http://33.media.tumblr.com/tumblr_m67uwzl0N01rti11fo1_1280.jpg
Merlin is a little under-expanded as can be seen from the exhaust continuing to expand, but Rutherford actually pinches in a little which suggests it's a little over-expanded. This can be gotten away with to an extent if care is taken with the nozzle design, I think SSME did this. That might suggest an even lower chamber pressure, like maybe 500 psi. That's still much higher than pressure fed, the examples I've looked at are <200 psi.
I suspect this model is a bit of an educational toy: http://exploration.grc.nasa.gov/education/rocket/ienzl.html
But if you turn the knobs to make it look like rutherford (kerolox, Pc 800 psi, 12:1 expansion, scale throat area to 3.81 in^2 for 4600 lbf sea level thrust) you get exhaust pressure about 7psi, which goes with your over-expanded observation.
This model only gives 270/309 Isp split for the 1st stage engine though. I guess we conclude that the 327 Isp on their web site is for the vacuum version? If you increase the area ratio to 39 in vacuum, this model gives 327 and 5300lbf for the same chamber pressure. -
#251 by ArbitraryConstant on 27 Apr, 2015 02:08
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This model only gives 270/309 Isp split for the 1st stage engine though. I guess we conclude that the 327 Isp on their web site is for the vacuum version? If you increase the area ratio to 39 in vacuum, this model gives 327 and 5300lbf for the same chamber pressure.
Not sure about this. From how the page is worded I got the impression their specs were for the sea level engine in vacuum.
http://www.rocketlabusa.com/about-us/propulsion/rutherford/ - "The Rutherford Engine is an electric turbo-pumped LOX/RP-1 engine specifically designed for the Electron launch vehicle, capable of 4,600 lbf thrust and with an ISP of 327 s."
4600 * 9 is pretty close to the vac thrust given for the first stage of 40000 lbf, so that makes me think 327s is for the first stage in vac, but this is definitely worth questioning. -
#252 by R7 on 27 Apr, 2015 07:30
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Merlin is a little under-expanded as can be seen from the exhaust continuing to expand,
At SL, no. Over-expanded like all booster engines.Quotebut Rutherford actually pinches in a little which suggests it's a little over-expanded. This can be gotten away with to an extent if care is taken with the nozzle design, I think SSME did this. That might suggest an even lower chamber pressure, like maybe 500 psi.
You can over-expand at SL without nozzle tricks to a certain limit (~0.5atm), the limit depends on the propellants used. SSME nozzle lip tricks pushed that limit even further.
The data on Rocketlabs page seems a bit self-contradictory;QuoteElectron’s first stage is powered by multiple Rutherford engines with a total peak thrust of 146.6kN, enough to lift a fully laden double decker bus off the ground.
• Lift off thrust: 152kN (34,500lbf)
• Peak thrust: 183kN (41,500lbf)
If the 327s is achieved with the higher value denoting first stage vacuum thrust then SL Isp is 272s assuming same throttle setting. The drop is quite high suggesting low chamber pressure and higher area ratio.
I can't get anything close to 327s vacuum performance with 800psi and 12:1 in RPA. 269s/310s when real losses are estimated into the figures.
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#253 by acsawdey on 27 Apr, 2015 13:49
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I can't get anything close to 327s vacuum performance with 800psi and 12:1 in RPA. 269s/310s when real losses are estimated into the figures.
It seems to me that the vacuum ISP will largely depend on the expansion ratio if you're not talking about changing fuels. The toy model was pretty close to RPA for this, it seems as it got 270/309. I had to push the expansion ratio up to 39:1 to get 327 vacuum ISP. So, either the 12:1 is wrong, or the 327 ISP is for the vacuum version with the bigger bell.
The expansion ratio you can get away with (SSME tricks notwithstanding) seems to be largely governed by by the chamber pressure -- higher chamber pressure means you can have more expansion and still have 0.5atm exit pressure. However they can't be playing too many tricks with that as they quote a vacuum thrust that's higher than the takeoff thrust. So they can't have dropped the chamber pressure much. -
#254 by ArbitraryConstant on 27 Apr, 2015 17:12
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At SL, no. Over-expanded like all booster engines.
Interesting, thanks.I can't get anything close to 327s vacuum performance with 800psi and 12:1 in RPA. 269s/310s when real losses are estimated into the figures.
Interesting. I think it's even harder to argue for a really high chamber pressure so that may have to be considered ISP with the vac nozzle. -
#255 by Steven Pietrobon on 02 May, 2015 05:35
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$4.9M for 100 kg to SSO is $49,000 a kg. That's not cheap!
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#256 by a_langwich on 02 May, 2015 07:18
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$4.9M for 100 kg to SSO is $49,000 a kg. That's not cheap!
The small launchers usually aren't trying to compete on a $/kg basis, but on overall launch cost. As a secondary payload, your launch schedule is competely at the mercy of the primary. And the primary has veto power over major characteristics of your design that might introduce risk to them (see, for example, the unwillingness of any primary payload to host a Lunar X-prize competitor as a secondary because of the risks).
So it makes sense that at least some customers would be happy to have a launch to themselves, and would pay the extra price for that.
An automobile costs more $/people-km than a bus, but people still ride around in cars because they can provide similar flexibility in schedule and specific destination, right?
The other argument is that if your spacecraft fits in 100 kg, $4.9M is a lot cheaper than $61M for a Falcon 9 or $40M for a Pegasus.
How many people fit into this market is the open question, and the answer may change over the next few years in unpredictable ways. -
#257 by Kryten on 01 Jul, 2015 14:15
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A launchsite has been selected; Birdling's Flat, Caterbury, South Island.Quote
Beck said preparations were under way to submit resource consent applications to Christchurch City Council for the launch site.
Doesn't sound like 2015 launch is feasible if applications are still going through, but it's still being given on their site. No date is given in the article itself.
[..]
Initially Rocket Lab would launch one rocket a month, but aimed to increase frequency to one a week using a number of sites. Beck said. -
#258 by BowShock on 01 Jul, 2015 14:18
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Looks like they've selected a launch site; near Christchurch. The article indicates the launch vehicle will also be produced in Christchurch - not Auckland. Seems like an ideal location with wide open flight azimuths.
http://www.nzherald.co.nz/business/news/article.cfm?c_id=3&objectid=11473933QuoteThe company's chief executive, Peter Beck said the area met all the firm's requirements; a sparse population, a launch path over the ocean and proximity to a city where the 18m tall Electron Rockets can be built.
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#259 by gin455res on 01 Jul, 2015 17:51
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At SL, no. Over-expanded like all booster engines.
Interesting, thanks.I can't get anything close to 327s vacuum performance with 800psi and 12:1 in RPA. 269s/310s when real losses are estimated into the figures.
Interesting. I think it's even harder to argue for a really high chamber pressure so that may have to be considered ISP with the vac nozzle.
I take it the energy added by pumping is so small it wouldn't add even one second of isp?
