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2:07 am
December 6, 2009
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Luke Maurits
| | Adelaide, Australia | |
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I think a sensible early course of action for the propulsion part of OHKLA would be to build something like this setup (by Copenhagen Suborbitals), only perhaps a bit smaller at first:
 
It looks like they have a fixed oxidiser tank here, permanently connected to the engine, which they must refill as needed. We could probably, at least in our early stages, go a bit simpler and just attach small, cheap N2O bottles via a suitable hose as needed. The combustion chamber itself would be permanent and used for many test burns. Since mass is unimportant it could just be a big heavy steel pipe – easy to find and machine. Later on we can try doing fancy composite engines or whatever if we like.
I think we will want to experiment with different nozzle materials to find something that strikes a good balance between cheap, easy to make and able to stand the heat. If we could have a system whereby we could attach/detach nozzles to the end that would be awesome. The biggest concern would be making the connection absolutely pressure-tight.
For fuel grain, here's an idea that I'd like feedback on. Is everyone familiar with the large cardboard tubes you can buy from post offices to send posters in, etc? If we made the inner diameter of the combustion chamber just a little larger than the outer diameter of one of those, we could cast fuel into one of those tubes and then just slide it in prior to testing. The cardboard will burn cleanly and easily at the end of the test. This system would be nice and cheap and easy. Buying a big pile of hurricane candles, gently melting them down and pouring them into a coardboard post tube would be a very easy way to make basic paraffin motors. I don't know if it would be easier to drill out the port afterward or cast it with a pipe or pole the size of the port positioned in the middle of the tube and then pulling that pipe/pole out afterward – it might be hard to make sure the pipe/pole doesn't get stuck in there and rip up the paraffin wax too much on its way out.
I may be overlooking something because I don't have much actual physical building experience, but it seems to me that we could knock up a small version of something like this pretty quickly and easily. A paraffin wax fuel grain could be quick and cheap, too. The one thing I don't know much about is how we would want to do ignition, but I don't think that could be too hard.
What do people think?
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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6:50 am
December 6, 2009
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Luke Maurits
| | Adelaide, Australia | |
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| posts 1483 | |
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Here is a (terrible) photo from Rocket Labs' website showing a smaller (seemingly benchtop) version of the same kind of setup above, on a scale that we should probably experiment with first.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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8:32 am
December 6, 2009
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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Post edited 3:21 pm – December 6, 2009 by Rocket-To-The-Moon
I was thinking about how to build this last night. I'll be home for 20 days over Christmas so I will have access to my Dad's well equipped workshop. My plan for a first generation test motor is to take a fairly large precast candle and then drill out a single central port. I'll lathe the wax to the appropriate diameter so that it fits snugly inside the combustion chamber. At the top of the chamber I am thinking about using a rosebud gas torch tip for the oxidizer distribution. The oxidizer will be gaseous oxygen since we have it readily available on the farm. The nozzle will probably be the hardest part to both design and construct. We have an old lathe (leather drive belt old…) that I can use to make the nozzle.
My plan for ignition is rudimentary and needs refinement. I'll just shove a sparkler down the port and lite it. Then I will slowly increase the oxygen pressure.
I leave for home on Friday and my parents only have very slow dial-up so I need inputs before I leave. I specifically need help with the nozzle design.
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Main Workgroups: Propulsion & Spacecraft Engineering
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9:49 am
December 6, 2009
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Luke Maurits
| | Adelaide, Australia | |
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| posts 1483 | |
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Post edited 3:54 pm – December 6, 2009 by Luke Maurits
Wow, it's exciting to think we'll actually have built a simple version of this so soon!
I probably can't be of much specific help with regards to how to machine nozzles because I'm not much of a hands-on-person, but this Wikipedia article does a decent job of explaining how modern rocket engine nozzles work. If you're not already familiar with the basic operating theory it would be a good place to start.
EDT: This article too.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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10:01 am
December 6, 2009
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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I'm somewhat familiar with the basic principles of how the nozzle works but not so much with how to optimize it for a specific rocket (mass flow, pressure, ect). I'll be reading up on this today. I doubt that I will be able to procure a block of graphite so I imagine that it will end up being a steel nozzle.
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Main Workgroups: Propulsion & Spacecraft Engineering
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11:28 am
December 6, 2009
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brmj
| | Rochester, New York, United States | |
| Member | posts 402 | |
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Rocket-To-The-Moon said:
I was thinking about how to build this last night. I'll be home for 20 days over Christmas so I will have access to my Dad's well equipped workshop. My plan for a first generation test motor is to take a fairly large precast candle and then drill out a single central port. I'll lathe the wax to the appropriate diameter so that it fits snugly inside the combustion chamber. At the top of the chamber I am thinking about using a rosebud gas torch tip for the oxidizer distribution. The oxidizer will be gaseous oxygen since we have it readily available on the farm. The nozzle will probably be the hardest part to both design and construct. We have an old lathe (leather drive belt old…) that I can use to make the nozzle.
My plan for ignition is rudimentary and needs refinement. I'll just shove a sparkler down the port and lite it. Then I will slowly increase the oxygen pressure.
I leave for home on Friday and my parents only have very slow dial-up so I need inputs before I leave. I specifically need help with the nozzle design.
I don't think your candle is likely to be long enough, if you go with a single-port design. Single port hybrid rockets typically want a relatively large lentgh to diameter ratio. Consider casting something yourself, or perhaps just stacking several candles.
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Main work groups: Propulsion (booster), Spacecraft Engineering, Computer Systems, Navigation and Guidance (software)
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1:16 pm
December 6, 2009
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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brmj said:Consider casting something yourself, or perhaps just stacking several candles.
I think that stacking will be my preferred method. Is there a rule of thumb to use for diamater:length?
I'm not sure how big to start out with. These guys have a fairly small one that looks quite impressive. I would like to have something in the 10cm diamete range eventually, but I'm not sure if I will be able to get enough oxygen flow for something that large.
I think that I will start off with something around 1.5" diameter just to get a feel for things.
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Main Workgroups: Propulsion & Spacecraft Engineering
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2:44 am
December 9, 2009
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Luke Maurits
| | Adelaide, Australia | |
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| posts 1483 | |
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How are you feeling about the nozzle, Rocket? You're leaving in a few days and it would be great if you didn't have any major unanswered questions before you ventured into the Land of Dialup.
Will you have the right equipment to machine a nozzle out of steel at your father's workshop? How much control will you have over the shape of the nozzle (I can imagine using a lathe shape the outer surface of a nozzle but I don't know how you would hollow out the result)? Are you confident the steel won't melt through? I've seen a few YouTube videos of very small homebuilt hybrids where steel nozzles have melted fairly quickly.
I know this is very short notice, but do you think you could try an acrylic fuel grain too? I have started to see more and more of these on the web. I remember my highschool workshop used to have quite large (in diameter) lengths of cylindrical acrylic, if your father has a drill press it would probably be easy to put a port through one of these. I don't know how easy acrylic cylinders are to find or what they cost, though, it was just a thought.
I'm really looking forward to hearing how your experiments go!
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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3:00 am
December 9, 2009
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brmj
| | Rochester, New York, United States | |
| Member | posts 402 | |
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Rocket-To-The-Moon said:
brmj said:Consider casting something yourself, or perhaps just stacking several candles.
I think that stacking will be my preferred method. Is there a rule of thumb to use for diamater:length?
I'm not sure how big to start out with. These guys have a fairly small one that looks quite impressive. I would like to have something in the 10cm diamete range eventually, but I'm not sure if I will be able to get enough oxygen flow for something that large.
I think that I will start off with something around 1.5" diameter just to get a feel for things.
I've been unable to find a rule of thumb for single port hybrid rockets, just vauge statements about a low diameter to length ratio being necessary, and a statement somewhere else that for any hybrid engine whatsoever, longer fuel grain = more stable combustion. I suspect that if you try, say, something in the 1/10 to 1/15 range for that ratio, it will work well enough for early testing purposes. This is based on a statement elsewhere giving 1/7 as an acceptible, perhaps somewhat higher end number for a multi-port design, and the widely documented fact that single port designs should be skinnyer than multi-port designs.
Once you have the dimensions you want, perhaps encluding port diameter, make sure to post it. I have a good stash of, amoung other things, powdered magnesium at home, and may make a few fuel grains over the break for testing performance additives if I have time and people think it advisable.
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Main work groups: Propulsion (booster), Spacecraft Engineering, Computer Systems, Navigation and Guidance (software)
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3:06 am
December 9, 2009
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Luke Maurits
| | Adelaide, Australia | |
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| posts 1483 | |
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Rocket-To-The-Moon said:
Is there a rule of thumb to use for diamater:length?
This page says:
The disadvantage of the single port configuration however, is that it generally requires long length to diameter ratios compared to a multi-port configuration. The multi-port configuration can be made quite short and compact, with length to diameter ratios of between 3-7.
From this I guess for a single port design, one wants length:diameter to be more like 8-12? For 1.5" diameter engine this gives a length of between 12" = 1' and 18" = 1.5'.
This page describes a patent (boo! hiss!) for a hybrid rocket engine with what is essentially a single annular (as opposed to cylindrical) port. It says:
With reference to FIG. 1, the solid fuel grain has a length L and a diameter D. It is generally desirable to maximize the ratio of the length L to the diameter D in order to improve performance of the rocket motor 100.
Which suggests the higher the better. I don't know if that is a peculiarity of their annular port or if it is true in general.
I haven't been able to find much good general advice about this sort of thing online (but see later in this post for how we may be able to change this).
If you want to experiment with fairly thin grains for now, between 1 and 2 inches in diameter, it seems to me like making the length between 1 and 2 feet should be an okay starting point. You can experiment with longer lengths if you have enough time and material, but since I'm assuming you'll have no way of accurately measuring produced thrust you may not be able to learn much from doing this anyway.
(As something of an aside, single port hybrid engines working better with a high length:diameter is a good argument for my proposal elsewhere of making our modular boosters consist of a few sub-engines. Having a single very wide engine taking up the entire module would, as I currently understand things, result in a booster which has a very large burn time (since hybrids burn radially from the inside out) and relatively low thrust. Ideally we would like to push things in the other direction.)
I found out today that my university library has just acquired a book entitled "Fundamentals of hybrid rocket combustion and propulsion", published in 2007! It's so new that they still have to do stuff bind it, put a magnetic theft sensor in it, etc. before it hits the shelves (apparently they put books in the search database before they do this, beats me). They were good enough to let me reserve it in advance so as soon as it is ready for borrowing I'll get an email about it and will have a few days to pick it up, during which time nobody else can borrow it. Hopefully once I get that book I'll be able to offer some more advice on what sort of diameter:length ratios we should experiment with, what port shapes and sizes should be, and that sort of thing.
As we acquire more of this sort of knowledge, a good thing for CSTART to do may be to produce short introductory books on these topics that we release under a Creative Commons license, so that people who don't have access to university libraries can still educate themselves on rocketry and space related concerns. Maybe something like this could even go into our mission statement?
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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3:10 am
December 9, 2009
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Luke Maurits
| | Adelaide, Australia | |
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| posts 1483 | |
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brmj said:
I have a good stash of, amoung other things, powdered magnesium at home, and may make a few fuel grains over the break for testing performance additives if I have time and people think it advisable.
I think this would be a good idea, it seems relatively common to add things like this to base fuels to enhance performance. One of the big appeals to me of using paraffin is that it's so easy to melt and recast at home that it would be a snap to stir in things like magnesium powder and have them evenly distributed throughout the grain.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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5:25 pm
December 9, 2009
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Rocket-To-The-Moon
| | Altus, Oklahoma, USA | |
| Member | posts 685 | |
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Post edited 11:48 pm – December 9, 2009 by Rocket-To-The-Moon
I should have the tools necessary to make a steel nozzle. However I'm fairly certain that it will melt after (during) the first burn.
I just need to find something acceptably strong to make the combustion chamber from. There used to be a scrap yard in town that would allow you to look around and buy stuff from the scrap heap by the pound. A lot of it was industrial scrap that was already almost an end product. My Dad bought thousands of pounds of steel over the years and used it to make a wide array of things. Unfortunately due to legal concerns they don't allow people to walk through the yard anymore.
I will be sure to take video of this assuming that it turns out.
This would be a good project for several of us to tackle on our own so that we can all get firsthand experience. I just talked to my sister and she tells me that we actually have a block of paraffin wax in our basement.
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Main Workgroups: Propulsion & Spacecraft Engineering
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1:42 am
December 11, 2009
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rpulkrabek
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I'm not sure if this belongs in this discussion, but I have once did an experiment at my university measuring the thrust of a model rocket engine. This is more of an FYI for everyone, but also, I think we should consider this method for measurements. Obviously an engine such as one to be used in OHKLA is far more powerful than a model rocket, but this is a good way to get on track. I have wanted for a long time to acquire the equipment used to measure these things. I am still hoping to get them some time in the future.
Below is a short description my lab partner and I wrote. I believe this was towards the end of the semester and were given a task to explain how to improve the class instead of analyzing in more detail the rocket engine, which is the reason for the lack of description. I also don't know what happened to the data. I have to see if I have that backed up somewhere. Basically, we used a strain gage and oscilloscope to measure deflection. Read below if you are interested in the method we used.
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Introduction
Prior to predicting the trajectory of a rocket engine, the thrust history of the model rocket engine should be measured. This can be done in a controlled situation. The data will be determined in volts, from an oscilloscope, which can be converted from a known calibration factor. The experimental data can be used to predict the trajectory of a vehicle using the same engine.
Objective
The main objective in this experiment is to measure thrust history of a specific model rocket engine. Once the thrust history is known, it will be used to predict the trajectory of a vehicle using this engine.
Experimental Setup
The lab station is equipped with a thrust measurement test cell which includes an exhaust duct, a fracture enclosure, a strain gauge equipped thruster mount, a calibration weight pulley and a rocket igniter control pad. The thruster mount is connected to a weight via a pulley for calibration purposes. The strain gauge is connected to an oscilloscope, for diagnostics. The thruster mount is enclosed and connected to the exhaust duct.
Experimental Procedure
To calibrate the thrust stand, a known mass of 504 g is fixed to a string which is attached to the stand via a pulley. The strain gage that is connected to the oscilloscope measures the difference in voltage of the loaded and unloaded states of the stand, of 26.5 mV. With this voltage measurement and the force of the mass due to gravity (4.944 N), a calibration factor can be calculated to determine the appropriate settings on the oscilloscope to measure the thrust history of a 15 N of thrust rocket. The calibration factor and oscilloscope range can be determined as follows:
Calibration Factor= 26.5mV/4.944N = 5.36mV/N (Eqn. 1)
Range= Calibration Factor *15N of thrust = 80.39mV (Eqn. 2)
With the correct information entered to the oscilloscope, the rocket can be set up. The rocket is mounted to the thrust stand by a hose clamp with its nozzle facing to the exhaust duct. The igniter is inserted into the engine. The cell is then closed and the power leads are connected to the power supply. Once all of the procedures are completed, the rocket is ignited and the thrust history data is acquired with the oscilloscope.
Experimental Data and Figures
SORRY, I DON'T KNOW WHERE THIS WENT
Summary of Findings
SORRY, I DON'T KNOW WHERE THIS WENT
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