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2:20 am
July 25, 2010
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rpulkrabek
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Luke Maurits said:
Did you have any particular plastics in mind?
No, I don't necessarily know of any plastics that fulfill the requirements of the top of my head. My usual go-to plastic is ABS. Mostly because it is a durable plastic and it is quite cheap and easy to acquire.
We definitely need to do some investigation to see if it could withstand the forces involved, as well as the heat transfer. I would just like to keep this as an option, at this point. I was just not sure if there was something I completely overlooked.
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7:10 am
July 27, 2010
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rpulkrabek
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Luke Maurits said:
- The tank and chamber should be made of 6061 Al T6.
- The engine should have a maximum outer diameter between 25 and 30 cm (anything narrower is going to have an absurdly high fineness ratio).
- The total length of the engine should not exceed 5m (the 175 kg propellant engine I used to break the Karman line in OR is 4.3 m, so we know this is possible)
- The total propellant mass should be 180 kg (this is more than enough with our current design (which can probably be made lighter) and has the nice property that a 8:1 O:F ratio results in 160 kg of N2O and 20 kg of PE, i.e. we have nice round numbers for both propellants).
- The inner diameter of the chamber should be such that it is easy to buy PE rods of that diameter to use as grains.
- The average thrust or maximum burn time should be SOMETHING WE STILL NEED TO FIGURE OUT
How should these decisions be handled? A vote? In my opinion, the first 2 could be finalized, although, it's best to get everyone's opinion too.
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8:27 am
July 27, 2010
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Luke Maurits
| | Adelaide, Australia | |
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I'm not sure I think a vote makes a huge amount of sense here. For one thing, they're not particularly subjective questions, and can largely be made on the basis of merit. For another thing, who else do we want to get the opinion of? Basically everyone other than us has vanished. I feel like there's a bit too much work which has gone in to getting us to the point where you and I are convinced that around 30 cm is right, for us to be able to expect the community in general to just read up on it and vote.
I basically agree that the first two could be finalised without much more discussion. We definitely shouldn't vote on 6061 Al T6: this is obviously the correct choice on the basis of all our research and modelling. I am happy to lock that one in right now. As for choosing the outer diameter of the engine / the diameter of the aeroshell: I feel like this is the only point above that we would really need to make a hard choice on before we could move forward with meaningful extra work, so we should try to make a choice on it soonest. I think maybe the main driver for this decision should be material availability. Is there a measurement somewhere between 25 and 30 cm such that findings things like thin-walled aluminium tubing or tubing of strong engineering plastics in that diameter is relatively easy? If so, we should just choose that.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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7:01 am
July 28, 2010
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rpulkrabek
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You're right. Where did everyone go? I think if we continue to work and continue to make decisions, more and more people will become interested. Perhaps more reddit submissions, tweets and blog posts will give more excitement.
I am also happy to lock down the material choice for the oxidizer tank and combustion chamber. Maybe we can put a green check mark on this in the wiki and create the blog posts and such with a comment that describes our decision and to provide details if there are any disagreements. With regards to the diameters, I agree with you that we should focus to about 30cm diameter, but to try to relate this to what is readily available, if, for example, we can only get 29cm diameter.
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11:14 pm
July 29, 2010
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rpulkrabek
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Post edited 4:57 am – July 30, 2010 by rpulkrabek
Luke Maurits said:
Did you have any particular plastics in mind?
I was thinking about this recently. If the combustion temperature and heat transfer from Al to ABS (or whatever plastic) is too high, we could simply implement some sort of insulation, such polystyrene foam (styrofoam).
There is one other worry that I have. Will there be heat generation from atmospheric reentry? Or, could it be so, that the drogue parachute will prevent this?
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1:56 am
July 30, 2010
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rpulkrabek
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I solved a simple heat transfer problem, with pen and paper (no FEA), where there was a 30cm inside diameter aluminium tube with a 2mm thickness and a 3mm thick abs tube on the outside of the aluminium tube. This is to represent the aerostructure covering the combustion chamber. I set the inside temperature to 1000 deg C and the outside temperature to 25 deg C. I am not sure of the combustion temperature, though. This was purely a guess. For one thing, 6061 Al has a melting temp of about 600 deg C! Something needs to be done about that.
My calculations went on to show that the temperature at the point where the Al connects with the abs is equal to 999.3 deg C. Not much changed from the 1000 deg inside temp. This would definitely melt the abs tube, which has a melting point of about 105 deg C! I suppose this should have been obvious in the first place for such a thin piece of metal. Well, as one of my old professors once said, "Common sense seems to come to us once we have already done something."
Something clearly needs to be done, if the combustion temperature is to be this high. I have noticed recently that other rockets are using phenolic insulation on the inside of the combustion chamber. I think we should look more into this.
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3:20 am
August 10, 2010
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Luke Maurits
| | Adelaide, Australia | |
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Thanks for doing that little bit of analysis, rpulkrabek. When you say you set the inside temperature to 1000 C, exactly what do you mean? You set the inside surface of the combustion chamber to that temperature? Or you set the temperature of the air inside the chamber to that temperature and let heat transfer to the inside surface via conduction? Or something else?
It's worth noting that for the majority of a burn, there is some PE fuel between the actual combustion region and the inner wall of the combustion chamber and that will act as insulation. Combustion will only actually happen directly adjacent to the inner wall at the very end of combustion when the PE grain has been almost completely burned out – and, in fact, we could design the grain geometry so that oxidiser runs out and combustion ends before this happens. I have no idea how good an insulator HDPE is. Could you model the transfer of heat from the combustion region through the PE and then through the Al and then to the ABS easily using some software?
I've also seen the phenolic insulation a lot and agree it can't hurt to look into that more.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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11:31 pm
September 6, 2010
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rpulkrabek
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Post edited 11:32 pm – September 6, 2010 by rpulkrabek
Sorry for my absence recently. I was enjoying my summer holiday/wedding. I am back now and will gradually resume working with OHKLA. I see that there have been quite many posts too. It's nice to see things moving forward.
Luke Maurits said:
Thanks for doing that little bit of analysis, rpulkrabek. When you say you set the inside temperature to 1000 C, exactly what do you mean?
I set the inside of the aluminium combustion chamber to be 1000 C. I would have rather set the fuel to be at that temperature, but I would have needed to know the convection coefficient, which, I believe, is determined experimentally. In any case, I feel that the high temperature of combustion is too much for aluminium.
My gut feeling is that it is best to move forward with using phenolic insulation. I don't know much about it, but I think we will be able to put a thin layer between the HDPE and Aluminium. I think this thin layer of phenolic insulation will be much less massive and take up much less volume than planning to leave unburned HDPE. But, I don't know for sure. I would like to find out more about phenolic insulation first.
With that said, can anyone provide information about phenolic insulation? How is it used? How expensive is it? How efficient is it? How much heat can it withstand?
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