Fatboy: ABLATE-P9000

Overview

The Fatboy P9000 was the second iteration of case-bonded motors we have constructed. Improving on what was learned by designing and manufacturing the Reliant Robin N3200, this motor used a revised grain geometry, insulator formulation, and hardware design. Although the test firing ended abruptly with a thermal failure, it provided invaluable data and it will be iterated upon in the next design and test fire.

Four primary goals were set for this project:

  1. Full scale test of a higher performance Fatboy variant at 87% solids loading by mass.

  2. Test a new thin wall casing with aggressive closure retention.

  3. Test a revised forward finocyl grain geometry.

  4. Test new insertable, head-end igniter hardware.

The overall motor design is shown below in the CAD cross section 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 Reliant Robin N3200, this motor utilized a spin cast insulator 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 spin casting.

Finished spin cast insulator.

Grain Geometry & Propellant Casting

The propellant grain geometry was a revised design from the Reliant Robin N3200. It retained the forward finocyl design, but increased the number of fins from 6 to 8, and increased their overall aspect ratio. This geometry gives the motor a more neutral burn while using a shorter finocyl section. The mandrel was composed of two pieces: the aft cylindrical and conical section, and the finocyl section. The cylindrical section was removed from the aft side of the motor and the finocyl section was removed from the forward side. Pictures documenting this process are shown below:

P9000 mandrel setup.

Cast Fatboy propellant.

Motor Hardware

The first new component this motor introduced was the use of a 3-2-1 carrier, composed of an aluminum outer shell, XX phenolic middle shell, and central isomolded graphite nozzle. This type of design achieves non-erosive nozzle characteristics while still managing heat transfer to the aluminum motor casing, which cannot withstand even moderate temperatures. Shown below is the completed nozzle assembly, fresh off the lathe:

Completed P9000 nozzle assembly.

The forward closure was also radically different from what has been used before. To improve motor ignition time, a pyrogen basket is mounted to the forward end of the motor. However, this basket needs to be kept separate from the motor in transport to prevent an inadvertent ignition. To quickly install the igniter at the field, the basket was affixed to a machined aluminum flange that bolts to the forward closure with 14x 1/4”-20 bolts. Save for a broken tap, this design worked great. Pictures of this are shown below:

Installed forward closure awaiting igniter installation.

Igniter flange installed on the field.

Testing and Results

Although the ultimate fate of this motor was a thermal failure initiated at the forward end, several notable goals were still met. First off, the motor geometry was verified. The pressure spike at that occurred at the start of the N3200 burn was all but completely eliminated. Additionally, the closure retaining mechanism was verified. To improve performance, the retaining pins were placed very close to the edge of the casing. They held strong at 950 psi with no signs of yielding. Shown below is the delivered thrust/pressure trace as well as some photos of the melted motor hardware.

Delivered thrust/pressure trace.

Thermal failure closer up.

Site of the thermal failure.

Head-on view of the thermal failure.

The thrust curve is very promising! The initial pressure spike has been heavily mitigated. This is important with thinner wall casings—the margins for overpressurization are lower, so anomalous spikes far above maximum expected operating pressure are unacceptable. From the failure photos, it seems that the gas blow-by occurred at the seal between the forward closure and the forward inhibitor, shown in the following CAD mockup:

Suspected site of the thermal failure.

Since it was a flat butt-joint, there are no positive sealing mechanisms that discourage gas from entering. This is opposed to the nozzle-end, which incorporates a 70° taper. With the tapered geometry, as more pressure is applied the seal performance improves. The problems with the butt-joint were only amplified with the long protruding fins. These gave the gas a very small distance to travel to reach the casing. On future iterations, a positive-pressure taper will be incorporated at the forward end.

Overall, this motor was very successful in certain ways, and has room to improve on others! This motor provided important data that will be used to aid future decisions.