Airframe

Spring 2020 - Fall 2021

Boston University Rocket Propulsion Group (BURPG) designwork as Airframe Lead.

Abstract:

I'm extremely proud of my time at BURPG and I credit a lot of my design mindset and detailed eye, CAD skills, and team skills to hundreds of hours of working on this rocket and being involved in BURPG in general. And because I've had multiple experiences like Airframe, I believe I have the potential to be a great candidate on most any design team. For more design work on BURPG see Heat Exchanger.

This design came from scratch, with few design constraints, and those constraints that existed were always changing because Airframe had to yield to other in-progress designs. As with any team project, this was an exercise in communication, but my system was especially reliant on great commmunication because it interfaced with more systems than any other. I had frequent meetings and checkups with other teams to ensure that my designs were in line with theirs.

This project was by far the most involved project I've ever had to work on. On top of this, because this was all self guided work by students, with no assistance from faculty, we were all "thrown in the deep end". Our senior leadership however, was extremely dedicated and upon graduation founded Inversion Space.

See my Critical Design Review below!

CDR

Fig. 1. Shown above is a Solidworks rendering of the Airframe panels and struts. The panels have been rendered translucent.

Fig. 2. Shown here is the entire rocket, Pursuit.

Fig. 3. Airframe assembly in context. On the right in red is the Nose Cone, and on the left with several radial mounting holes is the Oxidizer Dome. Between the two, housing the Fluids Components is the Airframe.

Design:

When I stepped into this project, there were extremely few constraints to work with. Because this design relies so heavily on the other systems to interface to, the design had to yield to the other teams' constant changes. The systems I had to work with were Fluids Components, Avionics Bay / Controls, Separation Team, Telemetry, and Oxidizer Dome.

The first aspect of this design to complete became the general geometry of the struts and panels.

Internal Interfacing, Struts and Panels:

The interface between the panels and struts was relatively straightforward: the panels would only be acting as a cover for the components housed in the Airframe, namely Avionics Bay and Fluids Components. The overall length of the struts and panels was driven by a rough estimate of how much distance was needed between the top of the Avionics Bay and the parachute that would be packed partially in the Nose Cone and the Airframe. Respective to internal interfacing, all the struts had to do was hold the panels.

The number of bolts per strut-panel interface was largely determined by cost. More bolts would mean more cost, so by quick estimate, each strut would hold 8 bolts on each side (as shown in Figure 7). This number of bolts would easily hold the lightweight aluminum panels. This number was be subject to change, as the Separation Team estimated they would need to increase the length of the Airframe to fit more parachute.

The geometry of the airframe has two purposes, mount the panels and external interfaces with Ox Dome and Separation Ring (See "External Interfacing"). The assembly would be four struts equidistant from each other, holding the panels between each strut.

The stock would have to be channeled out and milled to size from stock that meets the largest rectangular measurements, at least 1" x 2.75".

The material of the struts was chosen to be AL-6061-T6511 due to its relative cost, availability, and relatively robust material properties when compared to mass.

Fig. 4. Selected stock of AL-6061-T6511.

Fig. 5. Material properties of AL-6061-T6 from MatWeb.

Fig. 6. Cross section of the struts with measurements for material stock to be bought.

Fig. 7. The left side is where the struts interface with the Oxidizer Dome at the bottom of the Airframe assembly.

The right side is where the struts interface with the Separation Ring.

All that was left was the panels. Omitting external interfacing, the only thing to design here was the material type and thickness. The lighest and most cost effective material choice would be a foil or sheet of Aluminum. The material type and thickness was driven by consultation from Dr. Anna Thornton, who instructed my ME358 - Manufacturing Processes class, BU Engineering Product Innovation Center (EPIC) Lab Manager Joe Estano, and consultation from my former bosses Reza Bahadorinia and James Koger. Whichever stock met the minimum thickness and hardness recommended by Dr. Thornton, was cheapest and readily available to buy, and was also able to be rolled to diameter at EPIC, was chosen. Because AL-6061-T6 was a lot cheaper and by many metrics, stronger, it was the clear choice between the two. Others that were vetted early due to relative cost.

The final material type and thickness was AL-6061-T6 at 0.16" thickness.

Fig. 8. Material properties of AL-6061-T6 from MatWeb.

Fig. 9. Material properties of AL-3003-H14 from MatWeb.

Fig. 10. Final material choice AL-6061-T6.

Fig. 11. Another alloy option, AL-3003-H14, that was discarded due to cost.

External Interfacing, Struts to Ox Dome and Separation Ring:

With the internal interfacing more or less complete, the next step would be to interface with other systems.

Firstly, the struts needed to interface with Ox Dome below and the Separation Ring above (see Figure 4). The Ox Dome Team Lead and I came up with an interface that would key into the Ox Dome to restrict lateral movement (movement in the XY Plane), and eliminate the need for bolts in this case.

Fig. 13. Ox Dome interface: Ox Dome in silver and Panels and Struts in blue-gray.

Fig. 12. Isometric view of the Ox Dome interface.

Fig. 14. Strut (solid) keying into the Ox Dome (transparent).

Fig. 15. Top view dimensions of the Ox Dome key

Fig. 16. Front view dimensions of the Ox Dome key. (1" x 1.5")

To restrict movement in the Z-Direction, a bolt would connect straight through both as shown above. This bolt would need to hold the weight of the rocket below it, as during the Separation Phase, the parachute would connect to the top of the Airframe and strain the interfaces above and below the Airframe. Calculations for bolts in double shear were performed with the weight of the rocket below it, both "plate" thicknesses, the ultimate stress of the bolt, and Factor of Safety 2. During launch, with good manufacturing, the interface should bottom out the and bolt should see no stress. This eliminates the need to calculate for forces on the bolt during launch.

After iterating through bolts on McMaster-Carr, the final selection was: 316 SS Shoulder Screw, 5/16" Shoulder Diameter, 1/2" Shoulder Length, 1/4"-20. The shoulder on the bolt would hold the brunt of the shear stress with good manufacturing. Assuming Mc-MasterCarr listed the ultimate stress of the bolt as the weakest point on the bolt (where the threads meet the shoulder), the shoulder should hold up bounds better. If not, then the design is still met the weight requirement a by large, comfortable margin.

Fig. 17. Estimate Rocket weight given all existing geometries and material types.

Fig. 18. Bolt in double shear calculations. The Ultimate Stress was given by the McMaster-Carr page for the bolt in Fig. 17.

Fig. 19. Final bolt choice from McMaster-Carr.

The Separation Ring interface was based on the Ox Dome interface with a few minor tweaks, because they serve a similar purpose. The depth of the key was lessened to meet the size of the Separation Ring. At this point in time, the Separation Ring was largely incomplete, with no CAD to show for. But the dimensions of the Separation Ring interface slot would meet the dimensions of the key. The same type of bolt would be used, and the calculations would be stable, assuming the Airframe itself does not lengthen and add more weight (recall the Airframe length was driven by the amount of space to pack a parachute). If need be, there would have been calculations to confirm this bolt choice.

Fig. 20. Isometric view of Separation Ring interface. Note the smaller thickness of this key compared to the Ox Dome key.

Fig. 21. Front view of the Separation Ring key.

Fig. 22. Top view of the Separation Ring key.

External Interfacing, Panels to Avionics Bay, Telemetry, and Fluids:

With the struts and panels in place, all that is left is interfacing with Avionics Bay, Telemetry, and Fluids Components.

Avionics Bay and Telemetry are the systems that deliver controls signals and telemetry data, respectively for during and after launch and landing. Fluids is responsible for liquid and gas exchanges related to the liquid bi-propellant fuel and is housed inside the Airframe. These three systems combined allow for remote control of the rocket in case an in-flight correction needs to be made; exhaust gasses would be stored and expelled for axial correction.

Fig. 23. Fluids components are housed in the Airframe. Not shown: Avionics Bay

In the assembly phase, the struts would be assembled to the Ox Dome first, then Fluids, Av Bay and Telemetry, then Panels last.

For the Fluids interfacing, there would need to have been outlets for gas where the gasses would be expelled for controls. This design was driven by the final design and placement of the fluids components, which at this point had not been finalized. But these could easily come last in the manufacturing phase.

During the flight phase, telemetry data would need to be able to make its way out of the closed panels. There were two design routes to choose, mount antennae inside the panels and manufacture acrylic panels (so as to not impede signals). Or mount the antennae on the outside, flush to the panels. The second choice was less expensive and more straightforward. This decision was driven by the existing material of the panels, and was safer than buying panels of acrylic and manufacturing them with improper tools.

During the disassembly phase, the panel cutouts (shown below) serve as easy access to the Avionics Bay. These panel cutouts were also driven by the size of rollers in EPIC; I had to be sure to buy stock that could be rolled in-house to save on manufacturing costs. Anything under 12" could be rolled. This amount of cutouts also drove the number of bolts needed for the panels, results in 26 bolts per panel, minus the number of bolts to mount to Separation Ring (which had not been designed yet). Also not previously mentioned, there are panel bolts on the Ox Dome as well (shown below).

Fig. 24. Cutouts of the panels.

Fig. 25. Transparent view of the Av Bay mounted to the struts behind the panels.