Monday, January 16, 2012

Scott Fintel Correspondences

I have been discussing several aspects of my project with a notable expert in the field of experimental rocketry. Mr. Fintel is the founder of the Iowa Amateur Rocketry Group (IARG) and has given me permission to post our conversations and cite him as a resource in my final paper. I want to thank him again for agreeing to help me on this project.

Here are our conversations to date:


Tuesday January 3, 2012

Dear Mr. Fintel,

Hi, my name is Sam Otto and I am a senior at Winchester Thurston School. Besides being an avid amateur rocketry enthusiast, I am presently working on a year-long research project involving the rate of fuel regression in hybrid rocket motors. My extensive literature review has identified liquefying hydrocarbon fuels that form a hydrodynamically unstable melt layer under high boundary layer shear stresses can vastly increase the burn rate and delivered thrust. Theory dictates that decreased heats of fusion in these fuels cause an increase in burn rate, however, the relationship has not be experimentally established to my knowledge. I plan to examine the burn rate behavior of n-paraffins with varying melting points. The oxidizer nitrous oxide will be used in all trials, being injected through the annular combustion port.

I am searching for someone to serve as a guide, helping me trouble-shoot unforeseen problems along the way. If interested, I would like to further discuss my envisioned procedure and the design of the test motor with you along with any background that might pertain to my research. I am specifically interested in safety issues that may arise during my experiment.

Please take a look at my research blog <http://aerospacewt.blogspot.com> that covers the background research I have done. I also use the blog to describe my separate research in computational fluid dynamics. Thanks.

Sincerely,
Sam

Wednesday January 4, 2012

Hi Sam,

Let me preface my thoughts by saying, yes, I'd be glad help in whatever capacity I can.

That said. I have to let you know I'm not a big fan of wax as a hybrid fuel. It's simply a practical matter of installed total impulse in a rocket. As my amateur hybrid projects became larger, it soon became clear to me that high regression rate fuels would become the limiting factor in total installed impulse, the fuel would simply run out too soon. The inherent design of amateur rockets (long and skinny) necessitates the use of a low regression rate fuel for large N2O loads. While wax is a viable and good performing fuel, it has some other drawbacks too. It's low melting point can cause issues if launched in hot weather, as can it's brittle nature in low temperatures.

HTPB is probably the best overall hybrid fuel. It has good performance, is stable over a wide temperature range and has a moderate regression rate that can be tailored somewhat to specific burn requirements. But here's the real problem with trying to characterize any fuel. Regression rate is highly dependent on injector design, fuel geometry and combustion chamber design. So you can't really characterize a fuel per se, what you'll be characterizing is a motor design.

Please feel free to throw your thoughts and ideas at me.

Cheers,

Scott Fintel

Thursday January 5, 2012

Dear Mr. Fintel,
Thanks again for your help! I hadn't considered the practical issues associated with the delivered impulse. For the sake of experimentation, it may be interesting to attempt to address these problems with the goal of maximum performance -- an engineering challenge rather than a specific scientific set of trials. The problem with impulse seems to come in two parts. First is the rate at which heat is transferred through the paraffin. Evidently, large thermal fluxes into the fuel are permitted given the high temperature gradients towards the core. Rapid melting occurs and the motor burns increasingly fuel rich throughout the duration.
My first thoughts in tackling this problem involve lowering the optimal O/F ratio of the fuel and increasing its density specific impulse. Aluminum is tricky and inefficient due to the formation of aluminum oxide unless it is in nano-powder form. Of course, this is not an option. Magnesium powder could be added to the fuel with similar effects without the formation of limiting oxide coatings. Secondly, the fuel itself must act more like an insulator while maintaining its low melt viscosity. Maybe this could be achieved with the addition of polymeric fibers like cellulose, nylon, or polyester. The fibers may also improve the fuel's mechanical properties. Another option, yet not optimal due to reductions in fuel density, would be the addition of microballoons to the fuel. Let me know what you think.
Sincerely,
Sam

Friday January 13, 2012

Dear Mr. Fintel,

Hey, I'm just following up on the email I sent you last week. Thank you again for offering to help me. I really appreciate it. I plan on characterizing these fuels only within the context of a specific motor. For this reason, I do not expect that my results will have any kind of predictive capacity in other motor designs. Holding geometry constant, at best I intend to draw a comparison between various fuels, suggesting possible trends and solutions for future motors.

My documentation suggests that some of the problems you mentioned might be remedied with the addition of a polymer and/or a light metal to the fuel. Higher oxidizer flow rates and more effective mixing may also improve the Isp. Let me know what your thoughts are on the addition of magnesium and/or cellulose fibers to the paraffin fuel. Also, I posted my design for the oxidizer feed system and a possible forward closure design for the motor. The injector is for a RATTWorks k-240, supporting a higher flow rate than my motor would demand if it used HTPB. I may still have to enlarge the orifice given the high projected burn rates. Let me know what you think. Also, I'm wondering about the pre-combustion chamber. I don't like the design I have posted since the graphite section seems to "float" inside the liner. How might I accommodate both a pre-heater grain and a graphite or ablative substance within the pre-combustion area? Any suggestions would be greatly appreciated.

<http://aerospacewt.blogspot.com/>

Sincerely,
Sam

Friday January 13, 2012

Hello Sam,

You could certainly add magnesium or aluminum fuel to your paraffin, either would work and in theory increase Isp a significant amount. In practice, neither are going to increase Isp very much, if at all. With metal fuels you need residence time in the combustion chamber for complete combustion of the metal fuel. Small motors just don't have enough residence time, thus Isp falls well short of expectations.

You may want to add something to the paraffin as an opacifier. If your fuel grain is translucent you may have thermal energy radiated through the fuel leading to fuel melting from the inside out. Probably not a problem in a small motor with a short burn like you're building. But it is something to think about. A small amount of metal fuel would work as an opacifier, or even some powdered charcoal/lamp black.

Cotton, yes that sounds like a good additive. How about a fuel grain of wound cotton twine soaked in paraffin, or even cotton cloth wound over a mandrel and soaked in paraffin?

As for the pre-combustion chamber/pre-heater grain. I made a pre-heater grain with a thermal liner made from cast epoxy with graphite powder added. I glued the thermal liner to the fuel with high temp RTV, this worked fine with little ablation. There's no reason a graphite thermal liner wouldn't work, the only concern might be that the brittle graphite could break, but I'm uncertain as to how likely that would be.

It looks like you've really done your research, I'd say you're well on your way to a successful motor!

Cheers,

Scott

Sunday January 15, 2012

Mr. Fintel,

I would like to post our correspondences on the blog and also cite you as a source in my final paper. Is that okay with you? Anyway, I completed the feed system (minus the forward closure) and a test stand for the motor. Pictures are on the blog. Thanks again for helping me.

sincerely,
Sam

Sunday January 15, 2012

Hi Sam,

Yeah, that's fine.
I'll check out the updates.

Cheers,

Scott

Sunday January 15, 2012

Sam,

Something I wanted to mention before you get to the testing phase. Have you looked into filling the "run" tank? It can be a little tricky. I know I wasted a lot of N2O getting the process figured out. You need to fill the run tank with liquid N2O of course, but that's easier said than done sometimes.

Cheers,

Scott

Monday January 16, 2012

Mr. Fintel,

I have only looked into it briefly. It seemed to me that the easiest way to do the filling was with a venting port, however I have not yet attempted it. Any tips and tricks you could give me are greatly appreciated. After all, there is no need to reinvent the wheel.

Thanks,
Sam  


Monday January 15, 2012

Sam,
 
Make sure you are drawing liquid from the fill tank, that means you need to know if the tank has a dip tube in it or not and how that dip tube is oriented. Most 15-20 pound N2O tanks intended for cars have a dip tube designed for the tank to lay horizontally with the bottom tilted slightly down and one side down as well. Usually the tanks have that marked on the outside somewhere (or on the valve). The tank I originally used didn't have a dip tube, where I'd thought it did. I lost a lot of N2O until I e-mailed the company and they said it didn't have a dip tube at all. So I had to make a stand to hold the tank upside down, then it worked a lot better!
 
A vent at the top  of your flight tank makes filling faster and let's you know when the tank it full of liquid. But make sure it is a very small valve, a needle valve works best and even then, just crack it ever so slightly. The flight tank should not be filled all the way, you need some ullage space for gas on top of the liquid. I used a short dip tube coming off the top vent, when the liquid level hit the short dip tube I stopped filling. A minimum of 10% tank space should be ullage.
 
You can fill even faster if you pre-chill your flight tank, just cool it down in a freezer for an hour or so before filling. But you'll want to let it all warm up to ambient temperature before firing the motor or you'll have very low N2O pressure. That's the basics, let me know if you have any questions.
 
Cheers,
 
Scott
  

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