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#1280
by
mrhuggy
on 03 Feb, 2018 21:06
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The better batteries are 240wh/kg so 175kg for 42kwh. But these are for normal discharge rates of 2-3C. Electron would need 20-30C, so expect lot heavier battery with lower wh/kg.
Amongst the things that I do, I do MultiCopter (drone) racing, 400g flying machines with 0-60 in tenths of a second.
High perfomance lipos (lithium polymer battries) and deliver an explosive ammout of energy (literally at times) for instance I have 236gm battries that can deliver 2741w in 180s (1330mh 22.2v 6s 95c). If you wanted more longer time you can get 30000mh battries with 25c at 22.2v for arround 3kg. If you needed a higher voltage you can use mutiple battries in parallel.
Looking at the needs of the rocket I would think they are using simmlar lipos.
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#1281
by
john smith 19
on 03 Feb, 2018 23:16
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The startup sequence is challenging at any size. 
My understanding is that as you scale turbines down the mechanical tolerances for good efficiency are extremely challenging. The result is that the turbine can't scale down as much as the rest of the engine and the thrust to weight ratio ends up being quite poor. Hence an electric pumped engine has advantages because while there's a mass penalty, it's at least a cheap mass penalty with low development costs. Also as we see with Electron's upper stage, it's possible to jettison some of the battery mass.
Clearances are an issue, as is the boundary layer, which does not scale down so much, so at a certain size basically the whole gap between blades is boundary layer. These are the issues that have encouraged people to look at (and build) positive displacement pumps for the job below about 5000 lbs.
With turbo driven turbopumps there's a certain amount of interconnectedness, with the pace at which the pump can change depending on the drive system. There seemed to be an idea that this would not be the case with staged combustion, but the problems developing the SSME start sequence disagree.
Having the pump being driven by batteries shifts this problem to a power electronics problem, which have made huge strides since the 70's onwards.
BTW Regarding upgrades this is called "Electron," IIRC Rutherford was from NZ and I'd imagine they might be planning a bigger launcher called "Proton," probably not quite 1800x bigger than an Electron however.
. Electron is 1.2m in diameter. Road transportable LVs are about 12-14 feet in dia, so a 3x increase in diameter would be viable before they'd have to consider special transport arrangements.
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#1282
by
ArbitraryConstant
on 04 Feb, 2018 00:31
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Watch those batteries when you charge them and even after you charge them, as I sure you know. I investigate a lot of LiPo fires, especially of the hobby batteries that have little mechanical protection from physical damage.
I'm curious what they do for thermal management, or whether it's necessary when they don't care about cycle life.
Better batteries would also be needed if restart was a goal one day.
Why? A battery that can restart is just a battery.
I don't see Rocket Lab rushing to develop a bigger rocket. Cubesat was the goal, and Electron more than meets that goal.
I think incremental performance gains would allow them to reach more LEO payloads, more in the Orbcomm OG2 to Iridium size range. That seems worthwhile if they can do it without taking on a bigger cost structure.
The nature of their propulsion stack strikes me as well positioned to do that, and they've already bumped the numbers on the engines, significantly better ISP and more thrust. That kinda makes me think they can already launch more than they advertise.
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#1283
by
ArbitraryConstant
on 04 Feb, 2018 06:44
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Better batteries would also be needed if restart was a goal one day.
Why? A battery that can restart is just a battery.
Presumably, they are using up the battery charge for the single burn. They would need more Coulombs to do a restart and second burn, thus either more batteries or more charge capacity. Better batteries would have higher charge density, or more charge per kg.
Think about it though. The propellant and the batteries will be sized so they're roughly in proportion so neither wastes mass unnecessarily. Pump energy for a given volume and pressure gradient should be roughly constant whether it's one burn or several. Hence if they're using up the battery for a single burn, they're also using up all the propellant. And it's not true that adding propellant is the only way to do multiple burns, that's not how rockets work. It's also possible to use a smaller payload so there's propellant remaining after reaching the initial parking orbit. In an electric pumped rocket, propellant remaining is the same thing as battery charge remaining, if Rocket Lab is doing their jobs both are depleted at the same time.
I mean, I'm ignoring whatever the startup power draw is, and whatever battery energy is consumed when coasting, let's assume that's minimal for the sake of argument. The point of improved batteries wouldn't be to add energy, it would be to reduce dry mass. What would increase the need for battery energy is increasing combustion chamber pressure again to increase ISP or add propellant mass.
I would think what they need for multiple burns is what anyone else needs, ullage thrusters and so forth.
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#1284
by
Lar
on 04 Feb, 2018 15:28
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Startup draw is large, we are led to believe. So multiple starts, even for the same amount of propellant consumed by(and thus pumped into) the engines, may consume more total energy than a single start.
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#1285
by
msat
on 04 Feb, 2018 15:35
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Inrush current limiting during motor startup isn't some uncharted territory, especially is motors using electronically controlled commutation. I can't help but to doubt that this would be the primary issue precluding RL from being able to do a restart. Even using capacitors to handle high rush current which are charged by the batteries at a lower rate wouldn't be out of the realm of possibility.
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#1286
by
ArbitraryConstant
on 04 Feb, 2018 17:46
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Startup draw is large, we are led to believe. So multiple starts, even for the same amount of propellant consumed by(and thus pumped into) the engines, may consume more total energy than a single start.
Inrush current can be more than steady state but time at the higher draw is short, doesn't seem like it impacts the total mission energy budget very much. Batteries are also fairly good at dealing with this as long as the fuse/breakers take inrush current into account. In Rocket Lab's case it can be further mitigated because the entire thing is computer controlled and the engine's start box is undoubtedly extremely forgiving.
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#1287
by
Lar
on 04 Feb, 2018 18:09
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Not saying it can't be handled, just that a start up might use more energy than the equivalent steady state time, for less propellants moved. More start ups mean slightly more energy requirements overall. Not sure how slightly.
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#1288
by
msat
on 04 Feb, 2018 18:31
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Maybe someone who understands turbopump hydrodynamics can chime in and correct me, but I suspect generating your peak torque on the impeller from a standstill would be undesirable from a flow perspective, nevermind a mechanical one. The ramp up possibly isn't even as fast as it technically could be, and so startup current would have to be limited anyway.
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#1289
by
Space Ghost 1962
on 04 Feb, 2018 18:36
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Keep in mind also that the 3D printed engines likely have just enough material (plus margin) for the mission.
Had wondered with the aborted start in December if all nine were replaced, because they were consumed past margin (you could do acceptance testing by making lots not intended for flight with more margin).
The engine concept leads itself to performance enhancement by trimming losses in these ways. (You could also design the batteries with an advantage for a certain discharge curve and limited life to trim weight more.)
Maybe someone who understands turbopump hydrodynamics can chime in and correct me, but I suspect generating your peak torque on the impeller from a standstill would be undesirable from a flow perspective, never mind a mechanical one. The ramp up possibly isn't even as fast as it technically could be, and so startup current would have to be limited anyway.
Yes.
BTW, with modern electric vehicles we've had to design torque management into the motor/transmission/other - the early ones had so much torque they would shear axles/other. You'd have a control system that models current vs torque to match.
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#1290
by
ArbitraryConstant
on 04 Feb, 2018 18:46
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Keep in mind also that the 3D printed engines likely have just enough material (plus margin) for the mission.
I did see a comment that they are likely eroding as we can see sparks, which makes sense given the materials properties of 3D printing (likely tiny voids in the material that lead to hotspots on the surface in the combustion chamber?), but there too it seems like they would fab the engines with adequate burn time to fully consume the propellant, so also not a barrier to a ~3 burn mission.
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#1291
by
john smith 19
on 04 Feb, 2018 19:00
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Had wondered with the aborted start in December if all nine were replaced, because they were consumed past margin (you could do acceptance testing by making lots not intended for flight with more margin).
The engine concept leads itself to performance enhancement by trimming losses in these ways. (You could also design the batteries with an advantage for a certain discharge curve and limited life to trim weight more.)
SOP for rocket design seems to start with conservative IE Large design margins, then gradually reduce those margins to lower structural and engine weight as operating experience is gained.
Now they have a first full launch I'd presume RL will be studying all the launch data and looking at where things weren't just in the nominal range, but
narrowly in the nominal range
Let's see what happens with flight 3.
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#1292
by
CameronD
on 04 Feb, 2018 21:26
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BTW Regarding upgrades this is called "Electron," IIRC Rutherford was from NZ and I'd imagine they might be planning a bigger launcher called "Proton," probably not quite 1800x bigger than an Electron however. . Electron is 1.2m in diameter. Road transportable LVs are about 12-14 feet in dia, so a 3x increase in diameter would be viable before they'd have to consider special transport arrangements.
Methinks you are seriously under-estimating New Zealand's road network.
From what we've seen already, they have a little room to move but not much. There are many good reasons why Electron is the size and configuration it currently is and although up to a 2x increase might just possibly be doable, maybe,... 3x?!? No. Don't think so.
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#1293
by
speedevil
on 05 Feb, 2018 11:00
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Startup draw is large, we are led to believe. So multiple starts, even for the same amount of propellant consumed by(and thus pumped into) the engines, may consume more total energy than a single start.
By who?
It's an impeller, working against
~800PSI , or 6MPa.
The device consumes ~100kW per engine static. The rotor mass is of the order of a kilo.
It's going to take very little time to come up to speed, and is going to be dominated utterly by the fluid pumping load.
Also, on shutdown, some or most of the startup energy will be recovered.
'soft start' if you're concerned about instantaneous loads when at zero speed is pretty much inherent in this class of motor driver.
The only way I can imagine a large surge power would be if you try to valve off the outlet while bringing the impeller up to speed - but that seems unlikely to me for many reasons. (boiling LOX, ...)
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#1294
by
Space Ghost 1962
on 05 Feb, 2018 17:00
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One other consideration - running the turbo dry/tail off. If you sense the motor current load and if the sump is dry. you can cut/reverse current, and get a precision burn down to the last drop, without fear of turbo blowing (if you new/inferred turbo pressure, you could even stretch a marginal/failing one by accepting an lesser iSP and compensating in the GNC too).
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#1295
by
ArbitraryConstant
on 05 Feb, 2018 19:42
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One other consideration - running the turbo dry/tail off. If you sense the motor current load and if the sump is dry. you can cut/reverse current, and get a precision burn down to the last drop, without fear of turbo blowing (if you new/inferred turbo pressure, you could even stretch a marginal/failing one by accepting an lesser iSP and compensating in the GNC too).
Something else I noticed, it looks like a very quick startup sequence after staging. On another vehicle like Falcon 9 you see maybe 3 seconds before the exhaust settles down. With Election there's no visible startup like that and acceleration as visible from moving away from the first stage begins effectively immediately.
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#1296
by
acsawdey
on 05 Feb, 2018 20:18
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One other consideration - running the turbo dry/tail off. If you sense the motor current load and if the sump is dry. you can cut/reverse current, and get a precision burn down to the last drop, without fear of turbo blowing (if you new/inferred turbo pressure, you could even stretch a marginal/failing one by accepting an lesser iSP and compensating in the GNC too).
Something else I noticed, it looks like a very quick startup sequence after staging. On another vehicle like Falcon 9 you see maybe 3 seconds before the exhaust settles down. With Election there's no visible startup like that and acceleration as visible from moving away from the first stage begins effectively immediately.
You can see this on the first stage too, it appears to go from first visible flames to upward acceleration in about 1 second or so, much faster than Falcon 9 launches. I think in both cases we can chalk it up to precise speed control of the electric pumps.
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#1297
by
whitelancer64
on 05 Feb, 2018 20:20
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One other consideration - running the turbo dry/tail off. If you sense the motor current load and if the sump is dry. you can cut/reverse current, and get a precision burn down to the last drop, without fear of turbo blowing (if you new/inferred turbo pressure, you could even stretch a marginal/failing one by accepting an lesser iSP and compensating in the GNC too).
Something else I noticed, it looks like a very quick startup sequence after staging. On another vehicle like Falcon 9 you see maybe 3 seconds before the exhaust settles down. With Election there's no visible startup like that and acceleration as visible from moving away from the first stage begins effectively immediately.
The faster you light your 2nd stage, the less dV losses you get from gravity. Falcon 9 2nd stage is large and powerful enough to absorb some losses. Not so much leeway for a small launcher.
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#1298
by
ArbitraryConstant
on 05 Feb, 2018 21:10
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The faster you light your 2nd stage, the less dV losses you get from gravity. Falcon 9 2nd stage is large and powerful enough to absorb some losses. Not so much leeway for a small launcher.
I get that it's beneficial to start the engine as soon as possible, but regardless it looks like the startup sequence is significantly quicker.
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#1299
by
msat
on 05 Feb, 2018 21:52
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That would seem to make sense as there's no inter-dependency loop such as when you have a turbine energized by a gas generator fed by a pump powered by the turbine. Not only is an electric pump startup significantly simplified over a GG-based pump, the electric motor can generate higher torque at lower RPM than a gas turbine with equivalent power output in its primary operating range.
The more we discuss it, the more convinced I am that electric pumps really are the way to go for this class of launcher, especially for any company without fat gov-backed funding.
