As with everything, all good things must come to an end. SURF is no exception. In the beginning of August I wrapped up one of the most memorable summers of my life. Most people don't believe me when I tell them I spent my summer 3D printing rocket fuel. To be honest, it still can't believe how it all happened.
Seeing as it's been a while since I updated you on the progress of my research, it only seems right that I tell you how it all ended.
The end goal of my project chained throughout the course of the summer. In the beginning, the goal was to prove that 3D printed fuel burned no differently than cast fuel. Due to time restrictions and not having access to certain machines, in the end I set out to prove that the fuel could, in fact, be printed. Unfortunately, I still cannot talk about how the printer I was using works so forgive me for having to be in intentionally vague.
For those just joining this journey and those who have understandably forgotten the vernacular and specifics, I will happily fill those gaps.
In rocket propulsion, hybrid rocket systems are a very interesting middle ground between liquid and solid rocket systems. Hybrid rockets have their fuel and oxidizer in different physical states. Typically, there is a solid fuel grain and a liquid or gaseous oxidizer. My project was printing this solid fuel grain.
There are two easy ways to improve the performance of hybrid rockets. The first is by making a complicated fuel port shape. This creates more burning area and thus burns more fuel. Additive manufacturing promises to make the process of creating these complicated port geometries more efficiently. Another means of increasing performance is by adding energetic materials into the fuel such as aluminum or magnesium. These additives release more energy when they combust than the regular fuel so it's easy to see how they can increase the performance. Typically, the concentration of solid particles in cast fuels is less than 40%. To get the viscosity of the material to where it could support itself during printing, I used 85% concentration of aluminum fuel.
3D Printing Rocket Fuel
Well, it worked. After weeks of fine tuning and dozens of articles read, I was able to print fuel samples. Significant creep in the samples was noticed. This can be seen in the sagging and bulging of the sample as its being printed. There area a number of explanations for this. It could be that the concentraion of aluminum was just a little too low. Maybe 86% or 86.5% is perfect. Maybe I need to use two differently particle sizes to get optimal packing. Maybe the binder I was using just isn't the right binder. More research is needed to perfect this system. But that's beside the point. I was able to 3D print a highly viscous hybrid fuel grain sample.
This image shows that one of the samples did not experience the creep that became prevalent in later tests. What was different about this particular test is beyond me.
The printed sample on the right in the above 2 images is the same sample that was shown being printed previously. The first of these images compares the surface finish of the samples. The creep was an issue and caused the cross-section diameter of the samples to increase by about 10% but it also smoothed out the sample considerably. The creep filled all of the little gaps between the print lines and resulted in a fully dense interior that is comparable to that of the cast samples. This fully dense interior and comparable surface finish tells me that the printing process has no effect on the physical characteristics of the fuel samples and, thus, shouldn't effect the burning characteristics. If printing the fuel didn't effect how it burns then there would be only be performance benefits and minuscule downsides to using printing as a manufacturing technique.
Opposed Flow Burner
Proving that rocket fuel can be printed is one thing. But what if it doesn't burn the same way as cast fuel? What if it burns slower? What if small voids cause localized pressure increases that result in micro-explosions? These are things we need to know.
As far as I'm aware, there are two styles of testing for rockets. Small scale and full motor tests. Full motor tests are far more complicated and expensive but are a requirement to prove that the systems works in a real rocket application. Thus, they are only done after small scale tests or when small scale tests won't cut it.
For our small scale test we used an Opposed Flow Burner. In the opposed flow, we have our fuel sample at the base and pump an oxidizer, pure oxygen in our case, down onto it. This allows us to measure the rate at which the fuel sample burns and study the flame structure coming out the sides of the stand to see any instabilities or anything exciting.
In a previous study done at Purdue, they found that 10% concentration Aluminum-HTPB fuel burned SLOWER than pure HTPB. This contradicts what actually happens in rocket motors. What they believe was happening was the aluminum was absorbing a bunch of heat from the HTPB being burned but wasn't reaching its combustion temperature until it was released from the sample and entering the flame zone. There it would finally combust but was to far from the surface to add any energy to the burning surface. Basically, because the sample is so small, there isn't enough time for the aluminum to reach its combustion temperature and add energy to the combustion reaction. In full motor tests there is much more time and adding aluminum to the fuel does increase the burning rate.
If they saw that with 10% concentration, 85% concentration would take it to a whole new level.
We were able to perform a few burn tests at the end of my internship and the fuel samples did very strange things. Instead of the aluminum burning at all, it just melted. The aluminum melted, formed little balls of molten aluminum, then would be blown off the surface without ever actually burning. As with the previous issues, there could be a number of factors causing this. It's possible that our particle size was too big or there wasn't enough binder around the aluminum to act as a catalyst for the aluminum combustion. Not to mention that our samples were just a bit too big for the stand so they couldn't slide upwards as they burned and that caused a new slew of issues. Regardless, as it normally goes in these early projects, more research is needed.
The last part of the SURF program is a big symposium where every student presents their work to the other students and to a panel of judges. Prizes are awarded to the best presentations in both the oral and poster categories at a banquet the following day.
I'm very proud of my poster and the research I was able to do this summer. Though I did not win any awards, I had a blast presenting my project and had great feedback. Also, how can you not have a great time when you get to talk about 3D printing rocket fuel in space suspenders?
In conclusion, this is without a doubt one of the best things I have ever been able to do. The connections that I made, the research I was able to accomplish, and the memories I made have more it one of the best summers I've ever had. I cannot wait to see what other projects are done on the magical printer I used and hope to conduct my graduate school studies and research at Purdue once I finish my time at Olivet.