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#920
by
ChrisGebhardt
on 21 May, 2017 17:32
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Feature Article - by Chris Gebhardt:
https://www.nasaspaceflight.com/2017/05/rocket-labs-electron-inaugural-flight-new-zealand/
The flow chart is quite bad, Rutherford have clearly two electric motors, one for each pump. And such thing as "electric engine" doesn't exist.
...
The company's own website calls it an "electric Rutherford engine."
Now you just try to trigger mah autism. No they don't: "Rutherford is an oxygen/kerosene pump fed engine specifically designed in-house for Electron using an entirely new propulsion cycle. Its unique high-performance electric propellant pumps reduce mass and replace hardware with software." [1]"Rutherford is the first oxygen/kerosene engine to use 3D printing for all primary components." [2]
Then whole chart's title should be "Electric engine", not some part name. Also my car has an electric fuel pump, but it's still not an electric car... Also pumps don't have common shaft (so no fear with leaking seals between oxygen/kerosene sides).
"An engine is a device that burns or otherwise consumes fuel, changing its chemical composition, whereas a motor is a device driven by electricity, air, or hydraulic pressure, which does not change the chemical composition of its energy source." [3]
1. https://www.rocketlabusa.com/electron/ "A New Propulsion Cycle"
2. https://www.rocketlabusa.com/electron/ "3D Printing"
3. "Engine", McGraw-Hill Concise Encyclopedia of Science and Technology, Third Edition, Sybil P. Parker, ed. McGraw-Hill, Inc., 1994, p. 714.
;P
Yes, they do. Third bullet point.
https://www.rocketlabusa.com/latest/rocket-lab-reveals-first-battery-powered-rocket-for-commercial-launches-to-space/
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#921
by
HVM
on 21 May, 2017 17:40
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They also call their electric pump to a turbopump (no turbines there), in Youtube. So what they'll know?
>;-P
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#922
by
ringsider
on 21 May, 2017 18:06
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First real indication that the Electron could evolve into an RLV. With cubesats as its main payload, halving payload to recover booster is not big deal as long as cost per kg to orbit drops. They will still need a low cost expendable for heavier payloads.
If long term plans are a RLV then move to methane engines would make sense.
This is speculation out of left field, but if you look at how much extra room there is on the bottom of Electron, I don't think it'd be unreasonable for them to have an upgraded RLV with stretched tanks and larger engines, using the same thrust structure.
The problem is not fuel, it's battery power. The batteries run out of power to drive the fuel pumps. Turbopumps don't have that issue, so bigger tanks solve the problem. But for Electron you would need bigger tanks and a bigger battery pack.
Just based on looking at their payload user guide it's pretty obvious this vehicle has zero margin as it is.
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#923
by
gosnold
on 21 May, 2017 18:28
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Why did they go with electric engines? Does it have better performance than turbopumps for this class of vehicles, or is it because it is easier to develop?
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#924
by
Robotbeat
on 21 May, 2017 18:54
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Why did they go with electric engines? Does it have better performance than turbopumps for this class of vehicles, or is it because it is easier to develop?
The latter. Also likely has some control advantages.
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#925
by
savuporo
on 21 May, 2017 19:02
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Why did they go with electric engines? Does it have better performance than turbopumps for this class of vehicles, or is it because it is easier to develop?
The latter. Also likely has some control advantages.
We actually don't know about performance, as this is a fairly small engine. Turbines have their scaling laws, and cryocooled electric motors may deliver fairly interesting results here.
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#926
by
ringsider
on 21 May, 2017 19:04
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Why did they go with electric engines? Does it have better performance than turbopumps for this class of vehicles, or is it because it is easier to develop?
I think it looks easier to develop, but it has limitations as well e.g. power density, added mass.
Notice they focus on the efficiency of the motors but not on the power density of the batteries? That's because they are massively disadvantaged by that compared to an old school turbopump, so the e-pumps have to be near 100% efficient to make the entire solution work even close to what a TPA would offer.
As for margin just overlay a couple of orbital insertion performance charts for similar vehicles and their lack of margin is blindingly obvious.
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#927
by
Robotbeat
on 21 May, 2017 19:21
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Why did they go with electric engines? Does it have better performance than turbopumps for this class of vehicles, or is it because it is easier to develop?
The latter. Also likely has some control advantages.
We actually don't know about performance, as this is a fairly small engine. Turbines have their scaling laws, and cryocooled electric motors may deliver fairly interesting results here.
We know enough to know that gas generator turbopumps are better, though electropumps can have sufficient performance. The fact you don't need starter cartridges and the usual starting complexity could mean an overall win on mass for a small rocket, but gas generators definitely have higher specific power.
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#928
by
TrevorMonty
on 21 May, 2017 19:41
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They are trading extra weight of batteries and motors for easier development cycle. Engine is more efficient as all fuel is used for thrust, instead of some being burnt for pumping fuel. That extract fuel saving helps to off set extra weight of batteries. Every year or two vehicle reduces weight by changing to latest battery technology for very little R&D cost.
For RLV the batteries shouldn't change, they are sized for fuel capacity of LV.
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#929
by
Davidthefat
on 21 May, 2017 19:56
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First real indication that the Electron could evolve into an RLV. With cubesats as its main payload, halving payload to recover booster is not big deal as long as cost per kg to orbit drops. They will still need a low cost expendable for heavier payloads.
If long term plans are a RLV then move to methane engines would make sense.
This is speculation out of left field, but if you look at how much extra room there is on the bottom of Electron, I don't think it'd be unreasonable for them to have an upgraded RLV with stretched tanks and larger engines, using the same thrust structure.
The problem is not fuel, it's battery power. The batteries run out of power to drive the fuel pumps. Turbopumps don't have that issue, so bigger tanks solve the problem. But for Electron you would need bigger tanks and a bigger battery pack.
Just based on looking at their payload user guide it's pretty obvious this vehicle has zero margin as it is.
Where'd you get a PUG?
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#930
by
hkultala
on 21 May, 2017 20:07
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They are trading extra weight of batteries and motors for easier development cycle. Engine is more efficient as all fuel is used for thrust, instead of some being burnt for pumping fuel. That extract fuel saving helps to off set extra weight of batteries.
The weight of the batteries should still be many times higher than the amount of fuel spent by the gas generator in a comparable gg cycle engine, AND the full mass of the batteries is in the craft during full duration of the flight, unlike the fuel used by gas generator which is leaving the vehicle.
So, performance-wise, gas generator is much better. But they are not aiming for high performance, they are aiming for simplicity, low cost and maybe also reliability through that simplicity.
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#931
by
ringsider
on 21 May, 2017 20:11
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Where'd you get a PUG?
I requested one and they sent it.
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#932
by
envy887
on 21 May, 2017 20:14
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Just a friendly question/critique for Chris G... what is the utility of converting ISP to minutes and seconds?
Also noticed that kg/lb dual units are used for mass figures, but thrust is only in lbs.
Sorry, used to a tech writer! (Not a very good one, really.) Thanks!
For ISP, I put the seconds (which is normal) and the minute equivalent because people have complained in the past that they'd prefer the minute equivalent so they can have a "very clear sense of how long that is."
Thrust only in pounds is just something I missed going back to convert throughout. As I write, I list the figures as I find them. Sometimes the figures are metric, other times not (as in this calculation for lbs). I go back and put the other one in to cover all basis. I just missed the lbs conversion to kg for thrust here. I'll go back in now that launch has slipped and put in the conversion to kg.
Specific impulse had no physically significant relation to time, despite having units of seconds. Listing it in minutes is entirely unnecessary (and anyone who wants to see it in minutes has no idea what it means). If you want a meaningful metric, convert specific impulse to exhaust velocity in m/s by multiplying by 9.81 m/s^2.
Thrust is not measured in lbs or kg. It should be listed in lbf and kN.
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#933
by
LouScheffer
on 21 May, 2017 20:43
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Specific impulse had no physically significant relation to time, despite having units of seconds.
Well, ISP tells how many seconds an engine can support a mass against gravity, using the same mass of fuel. So a bigger number is better in a way that is very relevant for rockets.
Listing it in minutes is entirely unnecessary (and anyone who wants to see it in minutes has no idea what it means).
I agree completely. Has anyone here *ever* seen ISP listed in minutes?? I never have...
If you want a meaningful metric, convert specific impulse to exhaust velocity in m/s by multiplying by 9.81 m/s^2.
The two are both equivalent since they only differ by a scaling factor. In terms of meaningfull-ness, "how many seconds can you support your own weight" might be more intuitive than velocities well beyond usual human experience.
Thrust is not measured in lbs or kg. It should be listed in lbf and kN.
Agreed.
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#934
by
strangequark
on 21 May, 2017 21:03
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Specific impulse had no physically significant relation to time, despite having units of seconds. Listing it in minutes is entirely unnecessary (and anyone who wants to see it in minutes has no idea what it means). If you want a meaningful metric, convert specific impulse to exhaust velocity in m/s by multiplying by 9.81 m/s^2.
Thrust is not measured in lbs or kg. It should be listed in lbf and kN.
I'm against measuring Isp in minutes, it's just weird. However, it's not quite correct to say it has no physical relationship to time.
You can consider it as the number of seconds that it will take to consume a given number of pounds of propellant while continuously delivering that same number of pounds of thrust.
That is, the upper stage Rutherford engine, which provides 5000lbf of thrust and 327 seconds of Isp, would consume 5000lbm of propellant after firing for 327 seconds of burn time.
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#935
by
Craftyatom
on 21 May, 2017 21:26
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Specific impulse had no physically significant relation to time, despite having units of seconds. Listing it in minutes is entirely unnecessary (and anyone who wants to see it in minutes has no idea what it means). If you want a meaningful metric, convert specific impulse to exhaust velocity in m/s by multiplying by 9.81 m/s^2.
Thrust is not measured in lbs or kg. It should be listed in lbf and kN.
I'm against measuring Isp in minutes, it's just weird. However, it's not quite correct to say it has no physical relationship to time.
You can consider it as the number of seconds that it will take to consume a given number of pounds of propellant while continuously delivering that same number of pounds of thrust.
That is, the upper stage Rutherford engine, which provides 5000lbf of thrust and 327 seconds of Isp, would consume 5000lbm of propellant after firing for 327 seconds of burn time.
I often heard it quoted as "how long a rocket with said engine could hover above Earth's surface" - given, of course, that the engine could throttle all the way down to just above 0% with no change in ISP. Same thing, just different explanations.
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#936
by
CameronD
on 21 May, 2017 23:43
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#937
by
Steven Pietrobon
on 21 May, 2017 23:50
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Thrust only in pounds is just something I missed going back to convert throughout. As I write, I list the figures as I find them. Sometimes the figures are metric, other times not (as in this calculation for lbs). I go back and put the other one in to cover all basis. I just missed the lbs conversion to kg for thrust here. I'll go back in now that launch has slipped and put in the conversion to kg.
Ahhh, Noooo! Thrust in metric should be in Newtons (N) or kilo Newtons (kN).
Specific impulse had no physically significant relation to time, despite having units of seconds.
Yes it does. Its the time taken under 1 g of acceleration to reach the exhaust speed of the engine.
Well, ISP tells how many seconds an engine can support a mass against gravity, using the same mass of fuel. So a bigger number is better in a way that is very relevant for rockets.
I don't think that is correct. From the rocket equation
dV = g*Isp*ln(mi/mf).
In your case mi/mf = 2 which gives
dV = g*Isp*ln(2).
If acceleration is constant at 1 g then
dV = integral from 0 to T g dt = g*T
Equating the two implies T = Isp*ln(2) which is not correct. If you want T = Isp, then mi/mf = e = 2.718.... That is you need about 1.718 units of propellant to hold 1 unit of final mass for T = Isp seconds under 1 g of acceleration.
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#938
by
QuantumG
on 22 May, 2017 00:08
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Ya know we could start a new thread.
mdot = thrust / (isp * g)
isp = thrust / g / mdot
As such, isp is: for an ideal rocket, how long can we maintain that mass-force?
The reason isp is popular in the US (whereas exhaust velocity is more popular in Europe/Russia) is because non-metric people prefer to talk in terms of mass-force.
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#939
by
MATTBLAK
on 22 May, 2017 00:13
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I'm excited by this project. And virtually in my backyard, no less! Well... A six hour drive from my backyard
Though when it comes to N.Z. media coverage of it; there is a
non-charming, naive and parochial attitude to Space industry and Space Exploration in Kiwi media. Some, like Duncan Garner on 'The A.M. Show' is reasonably upbeat and excited by it. But if you listen to Talkback Radio in this country - the producers seem to let on air more than the usual share of ''
Oh; but isn't all a waste of money when there are people living in cardboard boxes in this country, and Earthquake damaged Cathedrals still to rebuild". They seem to be blissfully unaware of the existence of Commercial Space Ventures - regardless of large or small Government subsidies...
And I even heard one R-word (
you know the one) pipe up with "
I see that New Zealand is going to join the 'Fake Space club' like the Yanks and the Russkies with their model rockets and computer effects..."
There are people in my country who are very,
very punchable w4nkers indeed...
Though when it comes to N.Z. media coverage of it; there is a non-charming, naive and parochial attitude to Space industry and Space Exploration in Kiwi media. Some, like Duncan Garner on 'The A.M. Show' is reasonably upbeat and excited by it. But if you listen to Talkback Radio in this country - the producers seem to let on air more than the usual share of ''Oh; but isn't all a waste of money when there are people living in cardboard boxes in this country, and Earthquake damaged Cathedrals still to rebuild". They seem to be blissfully unaware of the existence of Commercial Space Ventures - regardless of large or small Government subsidies...
