Testing the Waters

Four down, seven to go. But what an incredible month it has been.

I'm going to do my best to write posts every other week about the work I'm doing but I have a lot of packed days and weekends coming up. So if it ends up being just once a month, my apologies. Your patience will be rewarded soon enough. Now, for the important stuff!

My research is some of the coolest stuff I've ever been able to do and I haven't even gotten to the coolest part of my research! For those unaware, I am participating in the Summer Undergraduate Research Fellowship (SURF) at Purdue University this summer. I am working in Zucrow Labs, a part of the School of Aeronautical and Astronautical Engineering, where I will be comparing the physical and thermodynamic properties of cast hybrid rocket fuel grains and printed fuel grains. In layman's terms, I will be 3D printing rocket fuel and studying how it burns. Doesn't get much cooler than that in my book!

There are really two ways in which my research is really special (other than they're actually letting me do it). 1st) the ability to 3D print hybrid rocket fuel grains means it is now dramatically easier to make complicated fuel grains. 2nd) I'm able to print higher solids loading than anyone before.

The port of a hybrid rocket fuel grain is the channel that the oxidizer (normally some form of oxygen) and flame travel through.

In the case of this rendering of the Bloodhound land speed record cars rocket (yes it's a car with a rocket), the flame through the center makes it pretty clear where the port is. It looks like this rocket has a single, straight, circular port. Since only the fuel in contact with the fire is being combusted, and the more fuel combusting the more powerful it is, it makes sense to design the port to have as high a surface area as possible. This is why it's very common to see hybrid and solid propellant rockets with star shaped ports.

Star/gear port design

Strange, honeycomb structure port design

Traditionally, these ports are made by special molds or maybe even by machining the fuel. This introduces a level of complexity, cost, and risk to the process. With 3D printing, I simply design my file in a CAD program, hit print, then let it do its thing. No additional labor costs, reduced risk from the lack of machining, all around an easier process.

So only being easier is great but it's just not a game changer. What makes 3D printing fuel grains a game changer is being able to swirl your port.

The pictures above show opaque sections of the printed material. These sections are the hollow ports that the oxidizer will travel through, making copious amounts of fire and plentiful thrust along the way. Now, why is spiraling the fuel grain so important? Basically, it forces the flames to interact more with the surface of the fuel.

In this incredibly complicated diagram, the most important area is the red stripe down the middle. The "gas phase reaction" and "diffusion flame region". This is where the flame lives. It's the intersection between the evaporating fuel and the oxidizer. A regular, straight port fuel grain relies on convection and diffusion to carry fuel to the oxidizer and oxidizer to the fuel. This stuff is also being shot out of the back of the engine at incredible speeds so some of the fuel and oxidizers will never get a chance to mingle and combust. They'll simply exit the engine all hot and bothered. It's rather lonely and sad when you think about it...

Anyways, that's really bad for your efficiency because you're wasting fuel and oxidizer. That could have provided more thrust and oomph to your mission.

The spiraling fuel grain ramps up the convection dramatically. One paper I read saw a more than 200% increase in burn rate with the use of a spiral structure. This is because the particles, oxidizer and fuel alike, want to travel in a straight line. Their velocity points in one, straight lined direction and they don't want to deviate from that path. That path just so happens to be into the wall of the spiraling port a little bit ahead. This means that your fuel and oxidizer are swirling inside of the fuel port and have a higher chance of being combusted because of it.

That's why this is so cool. Being able to 3D print these fuel grains can substantially improve their performance and efficiency.

I haven't even gotten to the truly incredible part yet.

The pictures I showed earlier of the spiral fuel ports were exciting but also rather boring.

The thing is, there's nothing special about the fuel itself in these engines. It was just a plain, boring plastic or rubber. Nothing exciting in it.

To be fair, pure HTPB rubber has been used a number of times in hybrid rocket motors and it works very well. The real fun comes when you start mixing stuff into the fuel, energetic stuff.

Metals, when exposed to a source of oxygen (such as the air), oxidize. Rust is the classic example. Raw iron was left exposed, reacted with the oxygen, and formed iron oxide commonly known as rust. Now, when things oxidize, most release energy. Sometimes a lot of energy. One of the most popular, in rocketry at least, is aluminum. Aluminum is relatively easy to work with, decently cheap and has one of the best enthalpies of formation in the business (it releases the most energy when it oxidizes).

So, why not just add it to the plastic/rubber and print it? That doesn't sound so hard. Technically, it doesn't sound hard at all. In practice, however...

HTPB rubber is a very runny liquid before it cures. This makes it easy to pour into molds and easy to work with. As you add aluminum to your HTPB, the viscosity increases. A lot. Printing high viscosity things is difficult because it either requires a big whole or a lot of force. If you use a big hole, well you're not going to be making anything to technical and high-resolution then. If you use a lot of force, there's a lot more danger if something goes wrong. This means that, traditionally, people haven't been able to print anything with a solids loading of more than 20-40% (in the business, solids loading simply means the amount of the mixture that's solid so it's basically the concentration). You're really limited on the performance of your rocket if you can only put so much aluminum into your fuel.

This is where Dr. Gunduz and his special printer comes in. Unfortunately, I'm not able to say how it works because of patent filings and intellectual property so I'll simply say what he told me when I first met him:


Why is Dr. Gunduz's printer so special you ask?

Say you want to print some Aluminum and HTPB fuel. For his printer to work properly, you have to have a solids loading of at least 70-75%. It works really well around 80%. Once you hit 85% or 90% you're basically working with flammable sand and it's just a nightmare anyways. That's anywhere from a 100% to 400% increase in the amount of aluminum you can put in your fuel compared to a normal 3D printer. That's huge.

So that's the importance/significance of my project. As for what I will actually be doing, I will be printing, and casting, small pellets of high solids loading fuel and using an opposed flow burner to measure the burn rate (also known as regression rate). Opposed flow burners are pretty cool because we can directly measure how both oxygen flow rates and different fuel compositions affect the regression rate. Then, I will compare the results of the printed pellets to that of the cast pellets and see if there's any difference. Hopefully, there isn't that dramatic of a difference. If there is, hopefully, it burns better than the cast method. This would show that there is no performance downside to printing the fuel grain.

So, what have I done so far?

Unfortunately, the printer isn't quite ready for me but it should be set up so I can use it by some time next week. Instead, I've been reading a lot of papers to learn about my niche in this field. I've also mixed up a few fuel samples to familiarize myself with the process and ratios.

The first sample I mixed up was 80% ammonium perchlorate (AP) and 20% HTPB as a binder and fuel. AP is a very common oxidizer is rocket fuels. Having the oxidizer mixed into the fuel allows it to burn faster than if it had to rely on the oxygen in the air around it. This can be seen in how vigorously it burns.

Next, I made some pure HTPB and an aluminized HTPB sample.

The above images are of the pure HTPB. The sample in the first picture we were able to degas. This is accomplished by putting the sample in a vacuum for a little while to pull any trapped gasses out. When we made the next sample, something was wrong with our degasser so we weren't able to. It's impressive how such a simple step can change the final state of the sample. All of those little bubbles will create an uneven burn that, if it was a much larger sample, could cause some real issues. Another thing that I found impressive was just how flexible the HTPB was. HTPB is just a form of rubber so I don't know why I was so amazed by its flexibility but it withstood my bends very well.

This sample is 80% aluminum-20% HTPB. If my memory serves me, this was mixed the same day as the larger HTPB sample so we weren't able to degas it. This can be seen in all the little holes along the sample. As can be seen in the last image, the aluminum addition makes the sample a lot stiffer and responds very differently to small bends.

We haven't burned any of the neat or aluminized HTPB samples. Frankly, they likely wouldn't be that exciting because it would be reliant on pulling in oxygen from the atmosphere. If we loaded it with some AP as well, now that would be fun.

This fuel sample is 40% Ammonia Borane (AB) and 60% Epoxy. The epoxy is a creation of my graduate student mentor because AB really likes to react and decompose with traditional binders. Thus, he had to experiment and play around with some different combinations until he found something stable. It doesn't normally bulge out of the tube like this but we still weren't able to degas samples so there were some air pockets inside that expanded and pushed it out. AB's chemical formula is H3N-BH3 so it releases a lot of hydrogen as it decomposes and combusts. This hydrogen will also combust with the oxygen to form water and release a significant amount of energy in the process. This, along with boron burning pretty colors, means that AB loaded fuels are rather exciting to burn..

Aside from my research, I've had a fantastic time so far. The students that I'm living with are amazing people and there's rarely a dull moment. We've had an awesome trip to Indianapolis, have an epic weekend in Chicago planned, and I even got to check something off my bucket list and go to the Indy 500 (that'll have it's own post soon enough).

I cannot believe that its only been three weeks and I have a full two months left. My research is promising to pick up soon and I'll be able to collect real data and set more stuff on fire (in the name of science, of course).