As was the case with virtually all parts of the rocket, the initial design called for an aluminum tubular shell to encase the rocket body, consisting of the injection nozzle, pre-combustion chamber, fuel grain, post-combustion chamber and nozzle. As data was received from rocket testing, from the steel rocket team, and the grad student, along with our own fine element analysis using ANSYS, we determined that alone, there would be no way that an aluminum, or composite for that matter, shell would be able to withstand the temperatures that it would see. Immediately clear were a couple of alternatives: provide insulation, or use a more robust material for the engine shell.
Our research into other hybrid rocket teams showed that our fuel grain material, HTPB, works very well as an insulator. What those teams did in the past was to make an oversized fuel grain and then not burn all of it so as to take advantage of this property and be able to use lighter and less heat resistant shells. As we worked with the steel rocket team to solidify the final design for the entire rocket, our two teams agreed to expand the outer shell to a five inch outer diameter with a 1/16” wall thickness and then fill the rest of the space with an oversized fuel grain, with an OD of 4 7/8”. While at this point it is still unknown how much of the fuel grain will in fact be consumed during a 50 second burn, we believe that it will not be enough to negate the insulation properties of the HTPB.
Another way to provide insulation for the shell is in the form of high temperature ceramics. Though we searched through multiple vendors of ceramic insulation, we ended up choosing the Cotronics Corporation because we already had a relationship with them for rocket testing materials as well as a previous order that our team had placed, some high temperature adhesive. This insulation, “Wrap-it” ceramic sheets (part number 372UHT), can be molded around the nozzle and post-combustion chamber where its extremely low thermal conductivity (0.5 BTU-in/HR-Ft2-°F at 1000°F) and high temperature melting point(3000°F sustained service, 3600°F melting) are needed at this most critical point in the structure. Able to form around 90° corners and other sharp points, these sheets also provide a fair bit of structural reinforcement around the nozzle, preventing it from overheating the shell and then breaking away during flight.
For the shell itself, it was clear that an aluminum alloy, with such a low melting temperature and a sharp drop in strength with almost any increase in temperature, would not be able to be used, at least to house the engine core. During our look at aluminum, however, we looked at 6061 aerospace grade and 7075, also aerospace grade, but stronger than 6061. From there, we turned our attentions to titanium, as it has all-around better properties than aluminum, save cost and weight, though proportionally, the weight is not as much of an issue. Since titanium is more than twice as strong and just about twice as heavy as aluminum, along with having about 1000K higher of a melting point than aluminum, titanium was an outstanding way to approach this problem. The only problem, and a big problem, was cost and availability. The best titanium for this application is grade 5, or Ti6Al4V, which stands for Titanium with 6% aluminum and 4% Vanadium. Unfortunately, it also seems to be the rarest form as well, as we had multiple companies that had never heard of grade 5 actually coming in tubular form and one other that told us that it would be a 14 week lead time, without even hearing the specifications of the tube.This lead us to searching for other alloys of titanium that would work almost as well as grade 5. We found Ti3Al2.5V, Ti3Al2.5Sn and Ti6Al2Nb1Ta1Mo are not nearly as strong as grade 5 (much closer to aluminum) and just as heavy as grade 5 however, at least the melting temperature was still up in the 1600°C range.
Another possibility, though unfavorable, would be to use steel, though of a much less thickness than currently used on the steel rocket, which was made to withstand unknown pressures that could potentially be seen through testing. While the heaviest of the three options, it would be by far the easiest to obtain, machine and use for the application, as we have so much more experience with it. Another benefit is its continued strength when exposed to heat.