Madhav Sonkusare

Solid Propellant Research

A reusable threaded-closure motor hardware built as a platform for novel propellant research. Nitrate based propellants that calculate 40% to 75% above KNSB in specific impulse were made.

RolePropulsion Lead

  • +40% to +75% Isp over KNSB, calculated
  • Ø73 × 220 mm aluminium casing
  • Proof-tested at 1.5 × MEOP
Solid Propellant Research, rendered in section

Inside the research motor

Forward closure

A knurled aluminium closure that threads straight into the casing, so the motor can be taken apart for the next propellant. A port through it takes the chamber pressure transducer, and a carbon-fibre disc shields its face from the flame.

Thread
Metric, threaded in
Engagement
10 mm
Port
Pressure transducer

Propellant grain

One grain, 150 mm long, cast in an oven straight into a carbon-fibre liner round a coring rod. The liner is the casting mould and then the heat shield for the casing. To try another formulation, a new lined grain goes into the same hardware.

Grain
150 mm long
Liner
Carbon fibre
Baseline
KNSB · 65 : 35

Casing

A 220 mm tube of aluminium, sized by the ASME pressure-vessel code. By calculation it bursts at 18.98 MPa, and the propellants were expected to reach 5 MPa at most. Before the firing it held water at 1.5 times its maximum expected operating pressure.

Material
Aluminium
Size
Ø73 × 220 mm
Wall
5.2 mm

Nozzle

The nozzle is a separate stainless steel insert. A second knurled closure, the retainer, threads into the aft end of the casing and holds it in, sealed by a silicone O-ring.

Insert
Stainless steel
Seal
Silicone O-ring
Retainer
Threads into casing
  1. Forward closure. A knurled aluminium closure that threads straight into the casing, so the motor can be taken apart for the next propellant. A port through it takes the chamber pressure transducer, and a carbon-fibre disc shields its face from the flame.
  2. Propellant grain. One grain, 150 mm long, cast in an oven straight into a carbon-fibre liner round a coring rod. The liner is the casting mould and then the heat shield for the casing. To try another formulation, a new lined grain goes into the same hardware.
  3. Casing. A 220 mm tube of aluminium, sized by the ASME pressure-vessel code. By calculation it bursts at 18.98 MPa, and the propellants were expected to reach 5 MPa at most. Before the firing it held water at 1.5 times its maximum expected operating pressure.
  4. Nozzle. The nozzle is a separate stainless steel insert. A second knurled closure, the retainer, threads into the aft end of the casing and holds it in, sealed by a silicone O-ring.
Section view of the research motor, forward end at the top

  1. Forward closure

    A knurled aluminium closure that threads straight into the casing, so the motor can be taken apart for the next propellant. A port through it takes the chamber pressure transducer, and a carbon-fibre disc shields its face from the flame.

  2. Propellant grain

    One grain, 150 mm long, cast in an oven straight into a carbon-fibre liner round a coring rod. The liner is the casting mould and then the heat shield for the casing. To try another formulation, a new lined grain goes into the same hardware.

  3. Casing

    A 220 mm tube of aluminium, sized by the ASME pressure-vessel code. By calculation it bursts at 18.98 MPa, and the propellants were expected to reach 5 MPa at most. Before the firing it held water at 1.5 times its maximum expected operating pressure.

  4. Nozzle

    The nozzle is a separate stainless steel insert. A second knurled closure, the retainer, threads into the aft end of the casing and holds it in, sealed by a silicone O-ring.

Static fire

Static fire, 26 March 2025 · Slow-motion

The problem

Every motor the team had fired used KNSB. We wanted to try nitrate propellants that calculate well above it, on one reusable motor instead of a new motor for each.

Specific impulse · calculatedSame hardware, same propellant mass
  • KNSBbaseline120 s565 N·s, J class
  • Formula 1+40%167.9 s792 N·s, J class
  • Formula 2+75%209.5 s980 N·s, J class

Design

A 220 mm aluminium tube with a threaded, knurled closure at each end. To change propellant, unscrew both, clean the parts and load a new grain.

  • Ø73 × 220 mm · 5.2 mm wall
  • Silicone O-ring seals
  • Burst 18.98 MPa · design 12.65 MPa
  • MEOP 8.44 MPa
  • Safety factor 2.53 at 5 MPa
  • Thread engagement 10 mm
  • Port for a pressure transducer
FIG. 01RESEARCH MOTOR · EXPLODED
Pressure port1Forward closure2O-ring3Carbon headliner4Cast grain5Carbon-fibre liner6Aluminium casing, 5.2 mm wall7Nozzle insert8O-ring9Nozzle retainer10
  1. Pressure port
  2. Forward closure
  3. O-ring
  4. Carbon headliner
  5. Cast grain
  6. Carbon-fibre liner
  7. Aluminium casing, 5.2 mm wall
  8. Nozzle insert
  9. O-ring
  10. Nozzle retainer
Grain and liner cut in half · CAD dimensions

Casting

Lots of casting tests were done with both formulations to ensure safe, good-quality grains. Grains were cast in an oven at 120 °C for 30 to 35 minutes, straight into carbon-fibre liners, then centred, compacted and checked for voids.

A black carbon-fibre tube standing on a red base on a workbench, with a coiled spring and a metal plate pressing down on its top.
A grain in its carbon-fibre liner under a spring-loaded plate, 20 March 2025
A hand holding a grey cast propellant grain end-on, showing a rough, uneven central bore.
The bore of a cast grain, 21 March 2025, rough in places

Testing

Before firing

Proof test

Water at 1.5 × MEOP

  • Both closures sealed
  • Checked for leaks and deformation
26 March 2025

Static fire

Smooth burn

  • Formula 1 propellant
  • Slow ignition
  • No structural failure
Engineering drawing of the casing set up for the water proof test, a section view with the volume filled with water, a pressure transducer fitted to the left closure and a pump connection on the right closure.
Proof test set-up: pressure transducer on the left, pump connection on the right
A tall aluminium-extrusion frame standing inside a steel-mesh cage on bare ground, with a lower platform for the motor and its load cell.
The upright test stand in its cage, 26 March 2025
A gloved hand holding the forward closure, its face covered in black char and a grey crust, with the red O-ring still in place.
Forward closure face after the firing, 26 March 2025
An aluminium casing lying on a table with three blue temperature-indicator labels along it, and a black carbon-fibre liner lying below it with bits of char beside it.
Casing and the liner beside it, after the firing

Gallery

Preparing the O-rings on a closure before assembly

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