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Propulsion Systems

Posted by: Monroe - Wed Sep 24, 2008 12:11 pm
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Space Station Commander
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Post    Posted on: Thu Mar 19, 2009 6:07 pm
Goddard's work. See what I mean? Back to basic's sounds good For an N-Prize motor\engine (in this case). I know nothing about this but draw the same conclusions. This is what we need to look at! Stop nuking the problem here! You know what I mean? I'm really interested here but we are pretty sure about how we want to do what we have so far. We will be switching over and changing gears at some point. Somebody needs to build something! My favorite Inventor of all time is TESLA but Goddard must have know something right? Thats were I would start if I were going to build a liquid rocket. We all to often forget the simplisity of the beginings of every thing we build. I don't know jack and I admit it! But we sure can find out and thats what makes the difference you have to build it! You have to make mistakes and you have to learn from them. We have to start somewhere right. NOBODY is going to figure this out and just plop down the solution without an act from god and I think he is busy right now. Should'nt you be?

Monroe

I'm sorry Guy's all I'm saying is YOUR ON THE RIGHT TRACK! keep going!

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Post    Posted on: Fri Mar 20, 2009 4:33 am
Andrew, I am completely with you till you got to the sheet aluminum construction. I'd LOVE to hear more. Is is a free piston system?

I had read peripherally about Goddard's early work with positive displacement pumps but need a better reference, can you make a suggestion? From what little I've read I assumed he switched to turbo-pumps because they better matched his scale, but since we are attempting a task that is radically different than his, all assumptions must be reassessed for the different application.

My worry with piston pumps is pulsation and complexity. I assume you'd have to do at least three pistons phased evenly. My idea in this vein is a three piston radial pump with spring loaded one way valves. You could then add a buffering chamber to even out the pulses, but that would also add the possibility that varying pressure in the combustion chamber will alter the flow rates leading to resonance issues. That is why I think it has to be vane or screw pumps for their lack of pulsation.

That is unless you are doing a free piston version of a Flometrics type design, but then my worry is the inefficient energy usage of the pressurizing gas and the pressure fluctuation as successive pistons are transitioned.


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Post    Posted on: Fri Mar 20, 2009 6:14 am
MisterQED: You don't like the aluminium sheet construction?

It's a hybrid of what currently exists, it's not linked piston and crankshaft like the XCOR design, it's not free piston like the Lawrence Livermore National Labs design and it's not pistonless like the flometrics design. It has two opposed fluid cylinders with a central gas cylinder and linked pistons, with a gas charged accumulator to damp out pressure oscillations during chamber switching. I want to get it to at least a static testing stage so I can actually test the effect the pressure fluctuations have on the motor.

The info I got on Goddard's pump work is from "History of Liquid Propellant Rocket Engines" by Sutton. It says he abandoned vane, piston and gear pumps because of internal leakage problems and the unpredictable vaporisation of LOX in them. First problem is neatly side-stepped by piston pumps using zero-leakage soft seals, second problem is solved by not pumping cryogens.


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Post    Posted on: Fri Mar 20, 2009 7:17 am
Calculate the fluctuation and simulate it with a silonoid or something in a simple pressure system (find out how much fluctuation you can get away with) then think about building the pump. Build a nozzle and injector first you can experiment with and then build your pump. "Just a suggestion" get a motor running (not childs play) and then work on your pump! pumps can be changed and redesigned around a running motor then make it into an engine? Make sence?

Monroe

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Post    Posted on: Fri Mar 20, 2009 7:43 am
Monroe, made the motor quite a while ago but it never got properly finished. It's a purely static test affair, aluminium with a graphite nozzle, water jacket cooling, nitrous oxide and ethanol propellants pressurised with nitrogen. I stuffed up when milling the injector so when I get around to fixing that and rigging up the servo-controlled plug valves I'll get stuck into testing it. Still I'm fairly busy so it's not likely to be any time soon.


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Post    Posted on: Fri Mar 20, 2009 8:01 am
Thats good news! How about some pictures or something? Do you have a website? If not there's room for one overhere! Anyway that's what I want to hear! Besides a motor at Mach that is! Get'er runin dang it! We don't need good motors colecting dust! Blow the dust off of it and take a fue pictures at least!

Monroe

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Post    Posted on: Fri Mar 20, 2009 8:05 am
//MisterQED: You don't like the aluminum sheet construction?//
No, I meant I didn't understand it. If I did I think I'd LOVE it. I don't understand how you can seal a bore if the aluminum tube isn't seamless.

I will have to get that book, thanks for the reference.

I don't expect to be pumping LOX, and I think I have different solutions for the other problems.

Contact me if you want to want to join a team.

MisterQED
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Post    Posted on: Fri Mar 20, 2009 9:34 am
MisterQED thanks for the offer but I don't really have time, I'm happy to give out info on anything I know/do to anyone who wants to know though, the more people playing with rockets the better.

I probably wasn't clear, the pump and motor are made out of machined aluminium parts, the primary structure of the rocket was to be constructed of sheet metal.

The key points driving the design of my fuselage were:

- Simple manufacture, sheet metal and pop rivets, no welding etc.
- High strength to weight ratio, designed with a reasonable safety factor.
- Cheap materials, ultra-common 5000 series aluminium alloy instead of much better aircraft grades (etc 7075, 2024, 6061).
- Producible without significant metal-working machinery (no lathes or mills).

Basically it's a post-buckle skin, stringers and rib design. The stringers are folded u sections of 5005-H34 0.6mm thick aluminium sheet, 6 for the fuselage based on my load cases. The skin is 0.6mm thick sheet of the same alloy (the skin could be thinner, 0.4mm or less, however such thicknesses aren't as readily available). Ribs are spaced around every 80cm along the length to prevent global buckling of the stringers.

I started a design log of this module when I was working on it last year so I have all the equations and design rationale in ink for when I get back to it eventually. For an idea on the efficiency of such a structure, the weight of a 2.75m long 400mm diameter module structure only (neglecting the weight of ribs) is only 7.239Kg (most of which is superfluous skin weight as mentioned earlier) and can withstand a 55000N axial limit load with a margin of safety of ~1.8. That's around 5 tonnes of force on a ~8Kg aluminium column made of 0.6mm thick sheet metal.


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Post    Posted on: Fri Mar 20, 2009 6:37 pm
We have some machinery now not so much for building the body of the rocket as for the engine and guidance systems and such. If you like I could prob. make that injector for ya if you sent me a drawing. But you would have to blow the dust off that old project!

Monroe

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Post    Posted on: Fri Mar 20, 2009 11:48 pm
//MisterQED thanks for the offer but I don't really have time,//
Who does? My job has me running 15 hours a day, most of my annos are made from my phone between tasks so most of my progress so far has been in researching a realistic plan.
//I'm happy to give out info on anything I know/do to anyone//
Well then do you have any software simulation suggestions? Have you used GMAT? I may just write one myself. I'd really like to gather together all the pieces for anyone to ballpark a design to see if it could get to orbit.

//the more people playing with rockets the better.//
I completely agree. What I want out of this competition is a demystification of rocketry. It's been done for low altitude model rockets and I think it's possible to do for larger ones too. Or at least illustrate the effects of choices made along the way even if it isn't completely accurate.

//the pump and motor are made out ther together of machined aluminium parts//
That's what I figured, but I'm still a little vague on the details of the pump operation. Could you give me some more details?

I get the advantages of thin skin construction, but since most of the rocket has to contain liquids or pressurized gasses, doesn't it make sense just to use the pressure to maintain the rocket shape like a soda can?

//I started a design log of this module when I was working on it last year so I have all the equations and design rationale in ink for when I get back to it eventually.//
What did you use for the calculations?

MisterQED
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Post    Posted on: Sat Mar 21, 2009 12:07 am
Guy's
I'd love to get some discussion going about GMAT and FreeFlyer or even Orbiter in the guidance and telemitry section if we could. Matlab seems to be THE software for high end compatiabilty. By the way SolidWorks is what we use for CAD if anyone needs some work done.

Monroe

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Post    Posted on: Sun Mar 22, 2009 10:08 pm
MisterQED: I just used a very simple simulator code I wrote in MATLAB, I'm looking to write a much more accruate one soon in which I could simulate the guidance/control systems of the rocket and their effect on the flightpath too. I haven't used GMAT.

If you want I could send you a copy of my thesis? It's fairly big (around 3Mb) not sure if your email account would accept it.

Pressure stabalised construction is interesting and certainly can make for an extremely light structure however the reasons I didn't choose it are:

- You'd need to source some large diameter, very very thin seamless aluminium tube to make the body from.
- You'd have to keep the tanks pressurised a lot of the time which makes ground handling more difficult/dangerous if your safety factors aren't high.
- It would be more vaunerable to hanger-damage, you'd have to be very careful about how you store, move and transport it.

If you could pull it off then you'd end up with a more efficient structure, I was just more interested in something robust and simple, like a space tractor.

For my calculations I did a lot by hand and in excel, the flight simulations had to be done in matlab but the structural design can be done by hand in most cases. All the equations I used are written down in the design book too.

Monroe: I'll send you an email, do you want a copy of the thesis too?


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Post    Posted on: Sun Mar 22, 2009 10:38 pm
Most defiantly! Seeing some other work would help me learn about how accurate my own calculations are. So I need to start using Matlab anyway and I have it so here we go I guess. I can send you some of mine as well when I get it going. Great Idea! I plan to share what we learn you know. Is this ok with you?

Monroe

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Post    Posted on: Sun Mar 22, 2009 10:43 pm
Sure, I won't be back to my home computer for another day or so now but I'll send you some of my old code then. My new code won't be done for a while, it's going to be pretty tricky I think.


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Post    Posted on: Sun Mar 22, 2009 10:58 pm
Hey if we help each other out here it should'nt be as tough. I have to get into Matlab and get busy anyway because FreeFlyer uses it.

Monroe

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