Fatboy: ABLATE-O6000

Overview

The ABLATE Fatboy O6000 was the third iteration of case-bonded motors we’ve designed. It took what failed in the previous ABLATE-P9000 firing and improved upon it with a higher efficiency propellant formulation, revised forward seal geometry, and improved ignition system. Although the firing resulted in a forward-end thermal failure, it provided more data on how to design and construct these case-bonded motors.

Primary goals for this project:

  1. Revise the forward-seal geometry and hold together until burn-out.

  2. Improve the spin casting process by using a solvent.

  3. Test a new head-end ignition mounting fixture.

The overall motor design is shown in the CAD model below:

This page will discuss the motor architecture and design philosophy, as well as document the manufacturing process and report lessons learned.

Spin Casting

Similar to the Fatboy P9000, this motor utilized a spin cast insulative layer to protect the casing from hot combustion gases. The insulator was HTPB-based and filled with silica, carbon black, and Kevlar fibers to improve insulative properties and erosion characteristics. You can read more about the spin caster by visiting this page. Pictures of this process are shown below:

Insulator top-view.

Insulator head-view.

The spin casting process was heavily modified for this test motor. Previously, the insulation mix was poured into the casing as-is and spread out by rotating the case until the surface was fully covered. However, this poses several limitations—namely, the insulator mix must have a low viscosity (which is counter-productive to an effective ablative composition) and the thickness must be great enough to allow for flow to occur. In low web thicknesses, the mix will stop flowing due to its adherence to the inner casing surface. To circumvent this issue, the mix was solvenated using hexane. This lowers the viscosity of the insulator mix by several orders of magnitude. However, a new problem arises: the hexane must be fully removed from the system before the mix cures. To accomplish this, the casing was spun cast under vacuum for several hours before the pump was disengaged:

Spin cast vacuum setup.

Machined vacuum closures.

Using this process, an insulator with more solids loading by mass and thinner web thickness than ever before was able to be produced.

Grain Geometry & Propellant Casting

The propellant grain geometry was directly scaled down from the ABLATE-P9000. It consisted of a forward, 8-fin, high aspect ratio finocyl that transitions to a long cylindrical core, which then terminates in a long taper. Like the P9000, the mandrel was composed of two parts. The cylindrical and taper sections were joined together. The cylindrical part was formed by a polished aluminum tube and the taper section was printed using a high-precision FDM printer. The forward finocyl was all aluminum, with laser-cut fins tacked onto a central aluminum tube. Pictures documenting this process are shown below:

Polished aluminum mandrel with taper section.

Finocyl mandrel.

Cast finocyl section. Voids from casting are trimmed and filled with insulator.

Motor Hardware

The nozzle assembly was modified from the P9000. Instead of a 3-2-1 aluminum-phenolic-graphite assembly, a full diameter LE-grade phenolic nozzle with a graphite insert was used. The use of aluminum in this assembly was minimized to save weight and prevent heat transfer into the casing. A small epoxy-reinforced aluminum retaining wedge was used to provide mechanical interlocking with the casing. EP-420NS structural epoxy was added to reinforce the phenolic overhang.

Machined nozzle assembly.

Nozzle test-fit in the casing.

Completed nozzle assembly.

The forward closure geometry was also modified from the P9000. To improve the sealing characteristics, it incorporated a 70° taper that interfaces with the forward inhibitor. For ignition, the flanged igniter assembly was replaced with a snap ring-retained article. A printed pellet basket filled with MTV pyrogen provided the motivation for ignition.

Forward closure installed in casing.

Loaded pyrogen basket.

Testing and Results

The outcome of this motor was another thermal failure at the forward end. Again, this provided invaluable data that can only be gained by firing full scale case-bonded rocket motors. Coming from the ABLATE-P9000, the expected failure mode was the breach of the butt joint between the forward closure and forward inhibitor. To solve this, a 70° taper was incorporated to provide a positive sealing geometry between the two elements. In reality, this may have only been one of two issues present with the design. The other unknown issue was the effect of long, thin fins in the upper finocyl geometry on surrounding ablative insulators. As discovered in this static test motor, the fins resulted in a high, localized mass flux of hot combustion gas that directly impinge on the forward inhibitor. This resulted in quick erosion of the inhibitor in those areas which exposed the aluminum forward closure. The gas then ate through the closure, completely breaching the pressure vessel. Shown below is the thrust-pressure curve of this test motor:

Delivered thrust/pressure trace.

Once again, the performance of the propellant formula and geometry matched expectations. The formula solids loading was increased to 88% by mass to boost performance, and the same forward finocyl geometry was employed. It reached a steady state pressure of 1200 psi and provided a relatively neutral burn profile for the duration of the nominal firing. The pictures below show the post-failure forward closure:

From this recovered component, it is clear that the forward seal held up great thanks to the 70° taper incorporated in the forward inhibitor. However, the inhibitor was eroded completely by the hot gas jets from the forward finocyl. Wear tracks are visible in the non-consumed areas. It is likely that the part of the inhibitor that failed contained a defect which allowed for a path of least resistance for the gas to travel.

Since this motor was intended to be a static test article for a flight in September of 2024, the forward seal was modified. To protect the forward inhibitor and closure from the hot gas, a propellant “top hat” was used—the fins were filled with propellant at the top 0.25” of the finocyl section. The propellant can then serve as an insulator with a known regression rate. It is sized such that the fin web thickness burns out before the top hat fully regresses, so the mass flux concentration is eliminated. The image below shows the revised forward seal setup:

Propellant tophat which replaces the upper section of the finocyl geometry.

Unfortunately, due to time constraints, this revised motor will not be static test fired before the September flight. What happens in flight will give more information about how this geometry interacts with the insulator setup and provide more invaluable data for future motors. To learn more about the flight vehicle, visit this page. Stay tuned!