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7:33 pm
September 30, 2010
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brmj
| | Rochester, New York, United States | |
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Post edited 7:38 pm – September 30, 2010 by brmj
There has been some discussion recently about designing and building a small static testing rocket. I have taken the liberty of beginning to work on this.
First, we are going to need to decade if we want to go with the propellants we have planned for the full scale, or perhaps substitute a gaseous oxygen oxidizer to save money while we are still early on in testing. In any case, I think the previously decided upon fuel is a no-brainer. For the size, I think we ought to pick one that closely matches readily available cylinders of our fuel, to simplify fuel grain production. Bonus points if metal suitable for the combustion chamber walls is readily available in close to the right diameter. McMaster-Carr, for example, offers steel and aluminum tubing and HDPE cylinders in a number of sizes. The largest inner diameter for steel tubing they offer that matches an HDPE diameter is 3.5 inches. The largest for aluminum is 4 inches. Both of these strike me as entirely reasonable fuel grain diameters for a small rocket to be tested on the ground. Surprisingly, the listed costs for steel and aluminum tube of the same dimensions are virtually identical. However, I strongly suspect steel is a better idea at this stage unless we really want to practice working with aluminum, since steel is a bit easier to work with and is stronger for the same dimensions, allowing us to get by with a thinner combustion chamber. Various companies offer used high pressure steel pipe, which may be another nice combustion chamber material, but the ones I have looked at don't offer inner diameters that match the available HDPE diameters as nicely.
Since this is going to be used exclusively on the ground, the oxidizer tank can be any commercially available tank suitable for the oxidizer and the pressures it will be used at. For the same reason, the design of the nozzle probably doesn't really have to be optimal, as long as it works well enough for us to get a good idea of what our engine is accomplishing.
I see no reason not to use a single, circular port geometry or, at worst, seven circular port geometry if we are building a rocket that small. Added complexity is bad at this stage. For the igniter, I suspect a little Estes-style solid rocket engine or similar stuck down the port may even work, as long as it wouldn't get stuck in the nozzle. Other than that, I'm not sure. I may have to ask the Project Meteor guys about this.
What does everyone think? Is this a step in the right direction?
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Main work groups: Propulsion (booster), Spacecraft Engineering, Computer Systems, Navigation and Guidance (software)
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12:08 am
October 1, 2010
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rpulkrabek
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Post edited 12:13 am – October 1, 2010 by rpulkrabek
For the testing rig, we have starting discussing it here.
I am thinking we should try to keep the scaled tests as close to the actual design as possible, meaning, we should use N20 as the oxidizer and HDPE as the fuel grain. Ultimately, I would like to do a scaled test of something like 1/3. We have narrowed down the total rocket diameter to be about 25-30cm. A 1/3 scale of that would be about 10cm which is approximately equal to 4 inches. So, I agree with you; 4 inches would be a good start.
brmj said:
However, I strongly suspect steel is a better idea at this stage unless we really want to practice working with aluminum, since steel is a bit easier to work with and is stronger for the same dimensions, allowing us to get by with a thinner combustion chamber.
This is a bit inaccurate. Steel has a greater ultimate tensile strength, meaning, it can handle more stress before it breaks. The interesting thing, though, is that the aluminium that we chose as candidate (6061 T6) has a higher yield tensile strength than the steel we chose as a candidate (304 stainless). Yield tensile strength is the measure of how much stress the material can handle before it permanently deforms. We decided to have the material that can handle more stress before it deforms, instead of before it breaks. Using the yield stress as the limiting factor, Aluminium is stronger and can there for be thinner, and then much less massive. We've done the calculations to approximate mass. You can see the results here.brmj said:
Since this is going to be used exclusively on the ground, the oxidizer tank can be any commercially available tank suitable for the oxidizer and the pressures it will be used at. For the same reason, the design of the nozzle probably doesn't really have to be optimal, as long as it works well enough for us to get a good idea of what our engine is accomplishing.
Yes, the tank can be of any shape. But, at some point, I would really like to test that our calculated thickness (which includes a safety factor) can withstand the inside pressure. As for the nozzle, this is something that I really would like to test. I want to see how CFD compares to real world results. Also, changing it's geometry a bit will affect the thrust quite a bit. Think if we create a test stand similar to Copenhagen Suborbitals, like this:
 
brmj said:
I see no reason not to use a single, circular port geometry or, at worst, seven circular port geometry if we are building a rocket that small. Added complexity is bad at this stage. For the igniter, I suspect a little Estes-style solid rocket engine or similar stuck down the port may even work, as long as it wouldn't get stuck in the nozzle. Other than that, I'm not sure. I may have to ask the Project Meteor guys about this.
I am not sure about the fuel grain geometry either. Perhaps this is something we could then determine what is best during the experiments. We may even be able to cast our own fuel grains.
brmj said:
What does everyone think? Is this a step in the right direction?
Absolutely :)
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12:42 am
October 1, 2010
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brmj
| | Rochester, New York, United States | |
| Member | posts 402 | |
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All right, then. Considering your corrections about the strength issue and the benifits inherent in keepingthings as similar as possible, I am now in favor of using aluminum for the chamber walls from the begining.
rpulkrabek said:
Yes, the tank can be of any shape. But, at some point, I would really
like to test that our calculated thickness (which includes a safety
factor) can withstand the inside pressure. As for the nozzle, this is
something that I really would like to test. I want to see how CFD
compares to real world results. Also, changing it's geometry a bit will
affect the thrust quite a bit. Think if we create a test stand similar
to Copenhagen Suborbitals, like this:
At this stage, testing the tank probably ought to be seperate from testng the rest of the rocket, to confine our tests to a minimum number of variables. I would suggest starting to test the tank by welding one up and pressure-testing it with air, and working up from there. As for the nozzels, I think it would be really neat if the entire nozzel end of the combustion chamber could come off, allowing us to try out different nozzels and reload fuel grans more easilly. Dealing with the pressures such a fitting would experience could be an issue, but I suspect someone has already solved that problem. As for the test stand, that is pretty much ideal as far as I can tell.
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Main work groups: Propulsion (booster), Spacecraft Engineering, Computer Systems, Navigation and Guidance (software)
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1:28 am
October 1, 2010
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rpulkrabek
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Post edited 1:29 am – October 1, 2010 by rpulkrabek
Yes, I like the idea of keeping the variables confined. At first we can use any sort of chamber shape. After that, we can work our way up to determine the optimum chamber construction.
For the nozzle, I have been dreaming up a design that looks like this, which is inline with your suggestion:
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10:10 pm
October 2, 2010
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Luke Maurits
| | Adelaide, Australia | |
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It's really good to see enthusiasm rising again for getting to work on a test rig.
The question of which parts of our rig will match the actual rocket and which won't (e.g. N2O vs GOX, steel vs aluminium, etc.) should, I think, primarily be answered by first answering one question: what do we want to learn from the test rig?
For instance, if we really want to learn about length to diameter ratios for the fuel grain or about different port geometries, then replacing N2O with GOX is probably not okay – it changes the reactions that go on in the engine, changing things like regression rates, optimal O:F ratios, etc. So if we go with GOX to save money (and I'm not saying we shouldn't, necessarily) we should be aware from the start that we will learn basically nothing about fuel grain design (except we'll get practice with any lathing, drilling, etc. involved). As a quick aside, there was some mention above about casting our fuel grains: IIRC, one of the arguments for going with PE fuel in the first place was that we'd never need to do this. You can just buy solid rods off the shelf in a wide range of diameters. This is way easier than casting anything.
Perhaps a good place to start here would be to make a matrix, with the columns corresponding to "things we want to learn about or experiment with" and the rows corresponding to "things we can change between the test rig and real rocket". Then we can put ticks and crosses throughought the matrix depending no whether or not changing a certain factor will make it difficult or impossible for us to test certain things out. Then we can decide which things we want to learn/test most, and the matrix will basically tell us which parts of the test rig we can use cheaper, simpler things for and which things we can't.
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Main CLLARE workgroups: Mission Planning, Navigation and Guidance. I do maths, physics, C, Python and Java.
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12:48 pm
October 21, 2010
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biollante
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Any update on how the nozzle and fuel grain design are coming along?
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12:48 pm
October 21, 2010
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biollante
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Post edited 7:15 pm – October 22, 2010 by biollante
-Double Post-
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10:03 am
October 22, 2010
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rpulkrabek
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| Member | posts 349 | |
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biollante said:
Any update on how the nozzle and fuel grain design are coming along?
The basic design is there. We will just have to determine an overall diameter, which we can then determine the nozzle geometry. Determining the nozzle geometry is something that I could do. This will also help determine how much thrust we'll have.
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