Log 01 · Nozzle Design & Material Selection

← ZA-125 Rocket Motor

Project	ZA-125 Experimental Rocket Motor	Entry	01
Class date	Tue 20 Oct 2026	Phase	Design → Fabrication
Author	Arturo	Instructor	S. Gurgel
Subsystem	Nozzle & motor case	Status	In progress

01 Goal / Objectives

Goal: Design and begin fabricating a two-piece nozzle (graphite throat insert in an aluminum carrier) for the ZA-125 motor, and set the pressure the motor case must be designed to.

Simulate motor performance in OpenMotor with the first nozzle iteration.
Use the predicted peak chamber pressure to set the motor case design pressure.
Choose a nozzle material using a weighted decision matrix.
Design the two-piece nozzle and generate CNC toolpaths for the carrier.
Make a drilling guide for the radial bolts that retain the nozzle.

02 Criteria / Constraints

Table 1 · Nozzle and case requirements
Type	Requirement	Reason
Constraint	Machinable on a 3-axis CNC mill	Available shop equipment
Constraint	Not brittle at the radial bolt holes	Nozzle is retained by radial bolts through the case
Criterion	High-temperature resistance at the throat	Throat sees the highest heat flux
Criterion	Wear (erosion) resistant	Throat diameter must stay constant to hold chamber pressure
Criterion	Reusable across multiple firings	Lower cost per test
Constraint	Safe: case rated at least 2× max expected pressure	Margin for uncontrolled variables such as propellant casting quality

03 Analysis

The motor was modeled in OpenMotor with the first nozzle iteration (Fig. 1). The simulation (Fig. 2) shows chamber pressure rising across the burn and peaking just before burnout. The maximum pressure should stay below, but close to, 800 psi. The motor case wall must therefore be at least double the thickness required to hold 800 psi, giving a 2× safety factor.

Table 2 · Nozzle iteration 1 inputs (OpenMotor)
Parameter	Value
Throat diameter, Dt	4.000 mm
Exit diameter, De	10.710 mm
Expansion ratio, ε = Ae/At	7.169
Divergence half-angle	16.7°
Convergence half-angle	51.4°
Throat length	1.814 mm
Nozzle efficiency	0.90

Table 3 · Simulated performance (read from Fig. 2)
Result	Value
Burn time	≈ 2.7 s
Peak chamber pressure	≈ 660 psi
Peak thrust	≈ 168 N
Kn (start → end)	≈ 160 → 337
Design pressure limit	800 psi
Case design pressure (2× limit)	1,600 psi

04 Material Decision Matrix

Two nozzle concepts were compared: an aluminum carrier with a graphite throat insert, and a nozzle machined entirely from graphite. Each criterion was scored 1–4, higher is better (Fig. 8).

Table 4 · Nozzle material decision matrix
Criterion	Aluminum + graphite throat	Full graphite
Complexity	4	1
Risk during firing	2	4
Reusability	3	1
Machinability	3	2.5
Total	12	8.5

Table 5 · Selected concept: aluminum carrier + graphite throat
Pros	Cons
Machinable	Aluminum will erode over repeated uses
Throat insert is replaceable
Cost savings
Aluminum is not brittle at the carrier bolts

05 Work Log

Table 6 · Tasks completed
Where	Task	Result	Status
Before class	OpenMotor analysis with nozzle iteration 1	Design dimensions, angles and expected performance (Figs. 1, 2, 5)	Done
Class	Researched nozzle materials against Table 1	Two candidate concepts identified	Done
Class	Designed two-piece nozzle from reference design	CAD section views (Figs. 3, 4, 6)	Done
Class	Built decision matrix: aluminum + graphite vs. full graphite	Aluminum + graphite selected, 12 vs. 8.5 (Table 4)	Done
Class	CNC toolpaths for aluminum nozzle carrier	Fusion CAM setup complete (Fig. 9)	Done
Class	Drilling guide for radial nozzle bolts	Add result	In progress
Home	Machined test graphite nozzle	Used to check achievable tolerances in graphite	Done
Home	Started machining aluminum carrier	Tolerances checked with micrometers and metrology instruments	In progress

06 Materials & Purchasing

Table 7 · Bill of materials for this stage
Item	Use	Source	Status
Motor casing tube	Pressure vessel	Amazon	To buy
Phenolic liner	Insulates case from propellant grain	McMaster-Carr 6394A36	To buy
Graphite ingot	Nozzle throat insert	Add source	Bought
6061-T6 aluminum stock	Nozzle carrier	Add source	Bought

07 Figures

OpenMotor nozzle input panel — **Fig. 1**OpenMotor nozzle inputs for iteration 1. Throat 4.00 mm, exit 10.71 mm, expansion ratio 7.169.

Graph of Kn, chamber pressure and thrust over time — **Fig. 2**Simulated Kn, chamber pressure (psi) and thrust (N) vs. time. Pressure peaks near 660 psi at about 2.7 s.

Section view of full motor assembly — **Fig. 3**Section view of the full motor: casing, phenolic liner, propellant chamber and two-piece nozzle.

Section view of the nozzle seated in the motor case — **Fig. 4**Nozzle seated in the motor case, showing the graphite throat insert inside the aluminum carrier.

Fig. 5Dimensioned nozzle profile sketch (mm). 35.00 mm outer diameter, R0.50 throat radius.

Section view of the two-piece nozzle — **Fig. 6**Two-piece nozzle assembly, section view. The graphite insert carries the converging section and throat.

Reference nozzle with a graphite insert in a metal housing — **Fig. 7**Reference design: graphite throat insert pressed into a metal nozzle housing.

Handwritten decision matrix for nozzle material — **Fig. 8**Hand-drawn concept sketch and decision matrix for nozzle material (typed up in Table 4).

Fusion CAM toolpaths on the nozzle carrier model — **Fig. 9**Fusion CAM toolpaths for the 6061-T6 nozzle carrier: adaptive clearing, 2D contours, bores and scallop finish.

08 Results & Next Steps

The aluminum carrier with a graphite throat scored highest (12 vs. 8.5) and was selected. Stock material has been bought, toolpaths are complete and machining of the carrier has started. The motor case will be designed for 1,600 psi, twice the 800 psi limit.

Finish machining the aluminum carrier and record measured dimensions.
Machine the graphite throat insert and test its fit in the carrier.
Finish the drilling guide for the radial bolts.
Order the motor casing and phenolic liner.
Calculate the required case wall thickness for 1,600 psi.

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