highaltitude.log.20090625

[00:03] <Laurenceb> http://www.dunnspace.com/self_pressurized_rockets.htm#Technology1
[00:04] <Laurenceb> thats interesting, basically what I'm thinking of, but with propane
[00:04] <Laurenceb> it takes 1.5Kg of propane to pressurise a 35Kg stage :(
[00:04] <Laurenceb> He is about 1.4Kg total with tank
[00:06] <SpeedEvil> :/
[00:07] <Laurenceb> http://www77.wolframalpha.com/input/?i=nitrous+oxide
[00:07] <Laurenceb> nitrous works, but very heavy
[00:08] <Laurenceb> as heavy as propane in fact
[00:08] <Laurenceb> so youd have nitrous in a tank on the balloon, then unload liquid N2O into a small slightly insulated tank inside the ullage space of the main tank
[00:09] <Laurenceb> have a small pump that pumps nitrous into the H2O2 to raise the pressure
[00:10] <Laurenceb> apply the same principle to liquid methane and the kerosene as a heat source, and its quite light, but methane isnt easily storable on the balloon
[00:11] <SpeedEvil> teeny dewars aren't horribly massy
[00:11] <SpeedEvil> if you strip off the normal padding, and just have the glass bit
[00:11] <Laurenceb> yeah, but sourcing liquid methane is a pain
[00:11] <Laurenceb> guess you could produce it with LN2
[00:11] <SpeedEvil> Umm.
[00:12] <SpeedEvil> You could just insulate a cylinder, and rapidly vent 2/3 of it
[00:13] <Laurenceb> interesting idea
[00:13] <Laurenceb> dont think it work like that
[00:14] <Laurenceb> http://en.wikipedia.org/wiki/Adiabatic_process
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[00:15] <SpeedEvil> umm - how not?
[00:16] <SpeedEvil> you vent 2/3 of the stuff (neglecting supercriticality) then the pressure is 1/3, and the temperature follows the gas laws
[00:18] <Laurenceb> yes
[00:18] <Laurenceb> nvm I havent worked it out
[00:27] <Laurenceb> a lithium borohydride powder/water slurry passed through a stainless coil would work
[00:27] <Laurenceb> unfortunately its react at room temperature as well
[00:28] Nick change: kingj -> KingJ
[00:32] <SpeedEvil> wrap it in gelatin or something?
[00:33] <Laurenceb> yeah
[00:33] <Laurenceb> if there was a way to produce small coated particles
[00:34] <Laurenceb> its so much work tho and you cant be sure its going to work :-/
[00:37] <Laurenceb> I like the liquid gas approach
[00:47] <SpeedEvil> We need to get that 'molecular gastronomy' guy involved.
[00:48] <Laurenceb> hmm found a paper where they try steam + hollow glass beads coated in LiBH4
[00:48] <Laurenceb> it works quite well but at high temperature the LiBO2 forms a coating over the surface
[00:49] <Laurenceb> and stop the reaction
[00:50] <Laurenceb> maybe have sperate LiBH4 and water, then mix just before launch
[00:50] <Laurenceb> not sure if the mix will flow well, its got to be 45% solids by volume
[00:51] <SpeedEvil> Or might you get a coaating of LiBO2 on the surface
[00:51] <SpeedEvil> of the pipe
[00:52] <Laurenceb> yeah
[00:52] <Laurenceb> hmm you could use molten LiBH4 at 300C
[00:53] <Laurenceb> its stable at that temperature
[00:54] <Laurenceb> but all the hardware required for that one will add tons of mass
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[07:28] <jcoxon> morning all
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[10:11] <mc-> Laurenceb, was looking at the dunnspace link you found. ISP only slightly increases as you increase chamber pressure. Blowdown pressurizing would be good enough, IMO
[10:12] <SpeedEvil> It also helps with reaction rate in the chamber
[10:12] <SpeedEvil> AIUI
[10:12] <mc-> higher pressure gives faster reaction rate?
[10:13] <SpeedEvil> AIUI, yes, it improves reaction rate of teh propellant in the chamber
[10:13] <mc-> is reaction rate is proportional to surface area?
[10:13] <SpeedEvil> no
[10:13] <SpeedEvil> the evaporated, or injected propellant or whatever
[10:14] <SpeedEvil> It takes some time for the propellant to react 'completely' - for the oxidiser to burn the fuel - in most cases there is a fraction of unburned fuel.
[10:14] <mc-> ok, so you might need a longer chamber to combust
[10:15] <SpeedEvil> The higher the temperature and pressure, and the longer the time it stays in the chamber, the less unreacted propellant goes out the back wasting ISP
[10:15] <SpeedEvil> Also - lower pressure means - for a constant expansion ratio - a larger diameter engine
[10:16] <mc-> hmm, does lower pressure also mean a smaller nozzle is needed?
[10:17] <SpeedEvil> If you want to expand to a given target pressure - yes.
[10:17] <SpeedEvil> (lower pressure = larger throat per unit thrust)
[10:18] <mc-> I think Laurence's helium ullage pressurization is a good idea - and if ISP only drops slightly then it seems a good tradeoff for a much simpler design.
[10:19] <SpeedEvil> perhaps - I'm unsure if he's done tradeoff sums on ullage pressure vs weight
[10:20] <SpeedEvil> The higher pressure also indirectly helps with the reaction rate
[10:20] <mc-> http://engineeringtv.com/blogs/etv/archive/2009/06/24/starfighter-integrated-repeater-payload.aspx - commercial use of balloons
[10:20] <SpeedEvil> (ideally) volume of a droplet of a spray depends on the cube root of the energy applied to its formation.
[10:21] <SpeedEvil> If you apply more pressure to a spray, you get finer droplets - which react faster
[10:21] <SpeedEvil> this is the injector pressure drop
[10:21] <mc-> good point
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[10:24] <SpeedEvil> cube root?
[10:24] <SpeedEvil> anyway - surface area is proportional to input energy
[10:35] <mc-> also as the pressure drops, there is less propellant entering the chamber, so it might counteract the slower reaction rate, to maintain a constant ISP.
[10:37] <SpeedEvil> err - no.
[10:38] <SpeedEvil> For a constant thrust, there is a given amount of propellant (pretty much) - with a lower chamber pressure, and a constant flow, there is less propellant in the chamber, and it has less time to react
[10:43] <mc-> if there's less propellant entering the chamber, wouldn't the thrust reduce?
[10:43] <SpeedEvil> there is a constant amount of propellant entering the chamber - per second.
[10:44] <SpeedEvil> So there is a constant amount leaving
[10:44] <SpeedEvil> The pressure of the chamber * the volume = the amount of propellant in the chamber
[10:44] <SpeedEvil> The residence time in the chamber is inversely proportional to pressure - assuming the chamber is constant volume
[10:45] <mc-> ah, even if the tank pressure reduces, the chamber pressure remains constant
[10:46] <SpeedEvil> no - the chamber pressure is the tank pressure minus the injector pressure drop.
[10:47] <SpeedEvil> I suspect I'm not being very clear.
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[10:51] <mc-> so if tank+chamber pressure reduces, does the flow rate remain constant?
[10:52] <mc-> you said flow rate is constant above
[10:52] <mc-> but I guess ISP drops, if it's not at the designed chamber pressure
[10:53] <SpeedEvil> Flow rate is constant - for a constant thrust level.
[10:53] <SpeedEvil> (neglecting changes in ISP)
[10:54] <SpeedEvil> If the pressure reduces, then the ISP reduces, then you need to increase flow if you want to keep a given thrust
[10:58] <mc-> assuming you don't need a constant thrust, why does the graph on dunnspace show the ISP only slightly drops as chamber pressure reduces?
[10:58] <SpeedEvil> ISP is only one metric that you need to keep in mind.
[10:58] <SpeedEvil> you also need engine diameter - which constrains the maximum expansion you can do
[10:59] <SpeedEvil> and adequate chamber size.
[10:59] <SpeedEvil> Lots of things detract from 'ideal' ISP - which isusually computed on various assumptions.
[10:59] <SpeedEvil> assumptions for a given chamber pressure
[10:59] <SpeedEvil> namely - for example - that it all reacts
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[11:00] <SpeedEvil> If 60% of the propellant goes out the back unburned, then your performaance is nowhere near optimal, and this theoretical ISP/chamber pressure graph doen't apply
[11:00] <mc-> so it's best to keep close to the designed pressure
[11:01] <SpeedEvil> There are other problems - like it may not undergo stable combustion at low thrust rates.
[11:02] <SpeedEvil> It may not be possible to make a 1Kg H2O2/kerosene engine with a 10PSI injector drop, for example, with a sensible sized chamber, for example.
[11:08] <mc-> one advantage of gas propellants is that a choke can be used to regulate the flow
[11:09] <mc-> but perhaps keeping the tank pressure constant is not too hard
[11:13] <SpeedEvil> gaseous propellants are nice - but storage is a bitch
[11:13] <SpeedEvil> or evaporation
[11:13] <SpeedEvil> They also react _lots_ faster than liquid particles.
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[13:15] <Futurity> Hi
[13:15] <Futurity> I stumbled across a recent balloon launch map the other day that seemed to end in the sea
[13:15] <Futurity> was the payload lost
[13:34] <SpeedEvil> yes
[13:34] Action: SpeedEvil pokes NeptuneKingOfTheSea.
[13:57] <Futurity> oh dear
[13:58] <Futurity> and expensive payload (HD cameras etc) or a simple one?
[14:01] <SpeedEvil> As I understand it not very expensive - but it had schools stuff on
[14:01] <SpeedEvil> as in a class puts stuff on, and sees what happens to it at altitude
[14:04] Nick change: kingj -> KingJ
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[14:42] <Futurity> that's a big shame
[14:43] <Futurity> i have a 7 year old son
[14:43] <Futurity> and I know he'd be upset about this
[14:43] <Futurity> mind you it would also teach him that things in life do go wrong
[14:43] <Futurity> upset if he had experiments on board I mean
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[14:47] <mc-> I also have a 7yrold son
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[14:54] <gordonjcp> ah well
[14:55] <gordonjcp> there's still a valuable lesson to be learnt from the whole thing ditching in the sea
[14:56] <SpeedEvil> Add a solar powered prop.
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[20:04] <Laurenceb> hi
[20:05] <Fighter1405> Hey
[20:06] <Laurenceb> hi there
[20:07] Action: Laurenceb readsa the logs
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[20:14] <Laurenceb> SpeedEvil: ISP is more important than the mass increase caused by increasing pressures
[20:14] <Laurenceb> up to some limit
[20:15] <Laurenceb> then, if your designed for that pressure, its a big waste if your losing pressure through the flight, as you still have to carry the dry mass of the stronger tanks
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[20:18] <Laurenceb> SpeedEvil: its almost as bad as using ullage space gas pressurisation
[20:23] <SpeedEvil> yes, I know.
[20:23] <SpeedEvil> I just diddn't know if you'd done the actual sums
[20:24] <SpeedEvil> the tradeoffs on optimal pressure for a He system, for example?
[20:24] <Laurenceb> not exactly
[20:24] <Laurenceb> I just guesstimated
[20:24] <SpeedEvil> yave you compared 2000,1000,500 forex?
[20:25] <Laurenceb> yes
[20:25] <Laurenceb> above 500psi its deminishing returns
[20:25] <Laurenceb> I think liquid methane is the way to go
[20:26] <SpeedEvil> esp if you can work out a way to burn it
[20:26] <SpeedEvil> or even exhaust it slowly in a cold gas thruster
[20:27] <Laurenceb> yeah, but jet vanes seem to work well
[20:27] <Laurenceb> if you use stainless in the end of the nozzle it works ok
[20:27] <Laurenceb> one very important thing is nozzle size
[20:27] <Laurenceb> theres a lot to be optimised there
[20:27] <rjharrison> Hi all
[20:27] <Laurenceb> hi
[20:28] <rjharrison> How is it going for every one
[20:28] <Laurenceb> cool
[20:28] <rjharrison> Ohh that reminds me I have a mosfet question
[20:28] <Laurenceb> SpeedEvil: http://www.machinemart.co.uk/shop/categories/search/air-compressor-pumps-pumpmotor-units use on of those with propane, and you can make a methane liquifier
[20:28] <Hiena> Hi all! Any last words, before i start the Big Badass Selfexciting Induction Furnace Coil?
[20:29] <SpeedEvil> Hiena: Keep fireextinguisher nearby.
[20:29] <SpeedEvil> Hiena: And keep extinguisher away from actual coil
[20:29] <SpeedEvil> 'big' = what?
[20:29] <SpeedEvil> hmm
[20:29] <rjharrison> When firing by powering the gate from the avr it seems to stay on even when I set the pin off. If I connect a big R to ground from the pin then it seems to power off ok. I'm not understanding why this is
[20:29] <Hiena> Thought it would be a fine core.
[20:30] <Laurenceb> rjharrison: you need to make the pin active low
[20:30] <SpeedEvil> rjharrison: are you perhaps not enabling pulldowns on the AVR, or disabling them?
[20:30] <Laurenceb> gate impadance is infinite
[20:31] <SpeedEvil> In practice, the gate can stay charged for hours
[20:31] <rjharrison> Oh ok I'll have a look
[20:31] <SpeedEvil> if it's disconnected
[20:31] <Laurenceb> its how flash memory works
[20:31] <Hiena> SpeedEvil, 24 winding from a 10mm brake line, power input 360V and 16 amps, limited for 6Amps.
[20:31] <SpeedEvil> Hiena: Ah - a ssmall one then
[20:31] <SpeedEvil> Hiena: and wwhy brake, not copper?
[20:32] <Hiena> It is a copper, and has a nice fittings for a coolants.
[20:32] <rjharrison> At the moment when I power the pin PORTC |= PC4 it turns the LED on powered byt the mosfet
[20:32] <SpeedEvil> Oh
[20:32] <SpeedEvil> Brake lines IME are steel
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[20:32] <rjharrison> At the moment when I depower the pin PORTC &= ~PC4 it leaves LED on
[20:32] <SpeedEvil> rjharrison: Do you have the port setup as able to pull low
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[20:33] <rjharrison> no
[20:33] <SpeedEvil> you need to do that then
[20:33] <rjharrison> And if I do then that will solve the problem
[20:33] <SpeedEvil> yes
[20:33] Nick change: Mogwie_ -> Mogwie
[20:33] <rjharrison> It's like it want to drain the power from the gate but can't
[20:33] <rjharrison> In my simle non electrical way
[20:33] <rjharrison> simple
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[20:35] <SpeedEvil> just ser the port up so it pulls low, and it'll work
[20:36] <rjharrison> In fact that is what I am doing with the external R isn't it SpeedEvil
[20:36] <rjharrison> With the R connected to ground
[20:37] <rjharrison> got to put the kids to bed
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[20:39] <SpeedEvil> pretty much
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[20:57] <Hiena> Hmmm...Well, time to remove the protections, and doing some calculations.
[20:58] <Hiena> What i really don't get the low number of the windings in the inductance furnace.
[21:00] <Hiena> The most use a primer coil up to 6 windings, but they had to use one hell of capacitor to balance it.
[21:01] <Hiena> Also, the current at the secondary coil (the melted material) depends on the ratio of the two side.
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[21:14] <SpeedEvil> why - were you expecting hundreds of turns?
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[21:44] <Laurenceb> SpeedEvil: I worked out the characteristic lenght of the combustion chamber, it only needs to be ~10cm long
[21:47] <SpeedEvil> at 2000psi?
[21:47] <SpeedEvil> it gets bigger as you decrease pressure?
[21:49] <Laurenceb> apparently not
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[21:49] <Laurenceb> you should have injector pressure loss of ~20% of chamber pressure
[21:49] <Laurenceb> but characteristic lenght doesnt change
[21:50] <Laurenceb> of course that assumes your injectors work ok
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[21:59] <Laurenceb> well - within a factor of two or so of existing stuff
[22:00] <SpeedEvil> surely droplet size is directly set by pressure drop on the injector?
[22:01] <Laurenceb> nope
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[22:01] <Laurenceb> its more complicated, you have to consider injector size
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[22:01] <SpeedEvil> In that for a given pressure drop, you can make only so small a droplet
[22:02] <SpeedEvil> the limiting droplet size is set by the energy in the surface tension
[22:02] <Laurenceb> hmm
[22:02] <Laurenceb> yeah I guess
[22:03] <SpeedEvil> http://en.wikipedia.org/wiki/Surface_tension
[22:03] <SpeedEvil> see the table
[22:04] <SpeedEvil> 1um droplet has a 1.4 bar internal pressure
[22:06] <Laurenceb> not too bad
[22:06] <Laurenceb> so a 1.4bar injector pressure could create a minimum size of 1um?
[22:08] <Laurenceb> space propulsion analysis and design says you need between 15 and 25% overpressure between the chamber and the injectors
[22:09] <SpeedEvil> I'm unsure if that's what it means.
[22:10] <Laurenceb> what? the droplet size?
[22:10] <SpeedEvil> http://www.spray-cas.com/pdfs/dropsize.pdf
[22:10] <SpeedEvil> yes
[22:14] <Laurenceb> doesnt have any figures
[22:14] <SpeedEvil> hmm - not as useful as it first looked
[22:16] <SpeedEvil> http://journals.pepublishing.com/content/7m27265vj37520kx/
[22:16] <SpeedEvil> probably not representative of injectors though
[22:17] <SpeedEvil> http://books.google.com/books?id=JyNcYiY11T8C&pg=PA211&lpg=PA211&dq="drop+size"++"surface+tension"+spray+pressure&source=bl&ots=-KEZDDmrIn&sig=5np00Rg03TSuAlG7U7J4zQqv2as&hl=en&ei=6-hDSoC9GNmrjAes2PGdDw&sa=X&oi=book_result&ct=result&resnum=4
[22:17] <SpeedEvil> aha
[22:17] <SpeedEvil> figures
[22:19] <SpeedEvil> one useful formula, with a magic constant
[22:21] <SpeedEvil> (surface tension * constant
[22:21] <SpeedEvil> cuberoot((surface tension * constant)/pressure-drop)
[22:21] <SpeedEvil> = mean drop size
[22:21] <Laurenceb> yeah
[22:22] <Laurenceb> and that graph gives ~0.5mm
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[22:22] <Laurenceb> now I guess you could compare with some rocket motors
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[22:27] <Laurenceb> F-1 engine gives ~0.4mm
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[22:31] <SpeedEvil> I may be wrong, but isn't the F1 slightly larger?
[22:32] <SpeedEvil> (than your proposed design)
[22:32] <SpeedEvil> does the charachtersinc length you're computing - from where - take into account droplet size?
[22:32] <Laurenceb> i think it does
[22:33] <Laurenceb> its from space proplusion elements
[22:34] <SpeedEvil> Unless it's explicit in a formula - 5mm vs 5um droplets are going to differ
[22:34] <SpeedEvil> I need to find my e-copy of those.
[22:34] <SpeedEvil> Or get driving license sorted, and check them out of library
[22:36] <Laurenceb> I guess its mainly based on gas flow
[22:36] <Laurenceb> droplets my fly through the chamber too fast
[22:37] <Laurenceb> guess impinging nozzles help a lot wwith that
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[22:46] <Laurenceb> http://www.b3tards.com/u/e24c5f2c5477ca531ba2/francis.jpg
[22:47] <SpeedEvil> :)
[23:11] Nick change: G8KHW -> RocketBoy
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[23:17] <Laurenceb> http://i60.photobucket.com/albums/h28/lordbofingham/McPosh.jpg <- lol
[23:18] <Laurenceb> http://www2.b3ta.com/host/creative/64347/1218064062/willonebewantingfries.jpg
[23:26] Nick change: Fighter1405 -> Fighter|sleep
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[23:45] <Laurenceb> http://news.bbc.co.uk/ <- omg no its the end of the world
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[23:47] <SpeedEvil> Wasn't that R.E.M?
[00:00] --- Fri Jun 26 2009