Heat Exchanger

Spring 2020 - Fall 2021

Boston University Rocket Propulsion Group (BURPG) designwork on the Heat Exchanger Team.

Fig. 1. Shown above is a Solidworks rendering of the Heat Exchanger rendered transparent to show internal components. Above it in red, white, and silver is the Gas Generator.

Fig. 2. Shown above is the an unrendered version of the Heat Exchanger without the Gas Generator.

Abstract:

This project marks my very first endeavor as an engineer, where I learned CAD (Solidworks), DFM, manufacturing, and so much more. Taking on this work felt like learning to swim for the first time, but with great mentors, I was able to be a productive teammate and accelerate my skillset more than I ever thought possible.

Through this experience and others during my time at BURPG, I feel as though I have been equipped to be candidate for most any design team. For more designwork related to aerospace and BURPG, see Airframe.

I stepped into this project during my first semester of college, with a team of two other first year students lead by mentors Elysse Lescarbeau (Class of 2021) and Trevor Melsheimer (Class of 2022). To divide the work, I was tasked with designing the geometry of the pressure vessel and interfacing with the internal components of the Heat Exchanger.

See both our Preliminary Design Review and Delta PDR Below.

PDR

Delta PDR

Design:

The goal of the Heat Exchanger was to provide efficient heat transfer from the gas generator to pressurant gas N2, reducing the number of COPVs needed on the rocket.

The constraints for my design were as follows: 11.5" diameter, 4" - 6" height, 100psig, >800°F shell, >1300 °F top / bottom .

Preliminary Design Review Stage:

The first step to creating this pressure vessel was material selection. After some cost analysis, the team settled on using AISI 1080 steel for the shell, and 304SS for the top and bottom covers. Despite being heavier than say any aluminum it would be easier to machine and weld / braze, and they exceeded our temp requirements.

Fig. 3 Iso view of Pressure Vessel V1. Note the radial bolts for mounting purposes.

Fig. 4 Top view with transparent top cover.

The pressure vessel would be at minimum, 0.1" thick, which is calculated using Von Mises equations for hoop / longitudal stress (fig. 5). The rounded edges were created to allow a .25" radius end mill bit to machine the top and bottom plate covers to the middle shell section. The shell and the top / bottom covers would be sealed with O-rings so as to maintain pressure (100psig) with clamping force from bolts.

Fig. 5 Von Mises Calculations.

Delta PDR Stage:

A critical issue with manufacturing this pressure vessel arose during our Preliminary Design Review. How would we manufacture the middle section, effectively a shell with flanges, without wasting money on a large stock of material, or welding / brazing a sheet to a machined flange?

This discussion led to the decision to get rid of the flanges all together and braze the shell to top and bottom covers.

Fig. 6 Pressure Vessel V2.

Additionally, the geometry of the pressure vessel was more fine tuned to spec, resulting in a 0.05" (1.25 SF) minimum for the wall thickness, allowing us to save weight and cost. Along with a now 8" O.D. (7.9" I.D.). Note that the top cover of the pressure vessel still needed to be 0.25" to allow for radial mounting bolts.

Challenges Moving Forward:

The biggest challenge to this project was manufacturing. Once all materials were sourced and ready to be purchased, the team began to doubt the idea that this could actually happen succesfully. There were a lot of if's.

The coils inside the pressure vessel needed to be bent with special tools to maintain heat transfer within margin. The manifolds had to line up precisely with holes cut from the covers of the pressure vessel. And the pressure vessel actually had to hold under pressure, hinging on the skill of our brazing in-house (to save cost). To achieve the high quality result we needed, it would be costly. Even more costly than just making space for more COPVs, which defeated the whole purpose of this project.

So in the end, this project was discarded.

What I Learned:

Despite having our project discarded by upper leadership, we all felt proud of our work. For me, as my first real endeavor as an engineer, I learned an incredible amount. This project marked by first experience with CAD, DFM, prototyping, materials research, cost analyses, BOMs, and so many more core engineering concepts.

I could not be more grateful because little did I know, these concepts would come back throughout all of college. I was extremely lucky to be introduced to these concepts early, and would love to give a special thanks to my mentors Elysse Lescarbeau and Trevor Melsheimer.