Combustion chamber model

Combustion Chamber :

The combustion chamber is the most critical component of the rocket because a failure could potentially be catastrophic. The purpose of the chamber is to contain the high temperature and high pressure gas resulting from the solid fuel combusting and direct it through the convergent divergent nozzle. The convergent divergent nozzle attached at the bottom of the chamber is used to convert the thermal energy in the chamber to kinetic energy, thus producing thrust.


Design Considerations:

The combustion chamber is typically defined by its characteristic length which is an indication of the chamber residence time of the reacting propellants, it is defined by:

For small combustion chambers , the contraction ratio of the chamber should be in the range from 3 to 5. The contraction ratio is essentially the ratio of the chamber diameter to the CDN throat diameter. The convergent section of the CDN should be about 1/10 of the volume of the cylindrical portion of the chamber.

The wall thickness of the chamber must:

• Be able to withstand the internal pressure and temperature within the chamber.
• If a cooling jacket is requires, it must be sufficiently thick enough for welding.
• If no cooling jacket is used, it should be thick enough to transfer heat fro the interior surface to the exterior surface efficiently without failing.

The working stress in the chamber wall can be estimated as:

If we solve this equation for the wall thickness, we can calculate the minimum wall thickness needed. This equation can also be used to calculate the thickness of the cooling jacket, if needed.

FEA of the combustion chamber.

Material:

We decided to construct the combustion chamber out of ANSI 1020 cold rolled steel. Not only do the properties of the 1020 steel exceed our design conditions, but being in a refinery rich environment, 1020 steel of all shapes and sizes were readily available. 1020 steel is a low to medium type carbon steel with a low concentration of manganese. It has a nominal carbon content of 0.20% with approximately 0.50% manganese. The low-carbon steel is good because it is easy to handle (good formability, machinable, and weldable) and fairly inexpensive. These steels typically have a good combination of strength and ductility, and may be hardened and carburized. Below are the properties of ANSI 1020 Cold rolled steel as listed at matweb.com.

AISI 1020 Steel, cold rolled

Physical Properties	Metric	English	Comments
Density	7.87 g/cc	0.284 lb/in³
Mechanical Properties
Hardness, Brinell	121	121
Hardness, Knoop	140	140	Converted from Brinell hardness.
Hardness, Rockwell B	68	68	Converted from Brinell hardness.
Hardness, Vickers	126	126	Converted from Brinell hardness.
Tensile Strength, Ultimate	420 MPa	60900 psi
Tensile Strength, Yield	350 MPa	50800 psi
Elongation at Break	15 %	15 %	In 50 mm
Reduction of Area	40 %	40 %
Modulus of Elasticity	205 GPa	29700 ksi	Typical for steel
Bulk Modulus	140 GPa	20300 ksi	Typical for steel
Poisson's Ratio	0.29	0.29
Machinability	65 %	65 %	Based on AISI 1212 steel. as 100% machinability
Shear Modulus	80 GPa	11600 ksi	Typical for steel
Thermal Properties
CTE, linear 20°C	11.7 µm/m-°C	6.5 µin/in-°F	0-100ºC
CTE, linear 250°C	12.8 µm/m-°C	7.11 µin/in-°F	0-300°C (68-570°F)
CTE, linear 500°C	13.9 µm/m-°C	7.72 µin/in-°F	0-500°C (68-930°F)
Specific Heat Capacity	0.486 J/g-°C	0.116 BTU/lb-°F	condition unknown; 50-100°C (122-212°F)
Specific Heat Capacity at Elevated Temperature	0.519 J/g-°C	0.124 BTU/lb-°F	condition unknown; 150-200°C (302-390°F)
Specific Heat Capacity at Elevated Temperature	0.599 J/g-°C	0.143 BTU/lb-°F	condition unknown; 350-400°C (662-752°F)
Thermal Conductivity	51.9 W/m-K	360 BTU-in/hr-ft²-°F	Typical steel

Construction:

The chamber consists of three parts the bottom flange, chamber wall, and top flange as shown below. All of the parts will be machined from ANSI 1020 cold rolled steel and will be welded together. The top flange will secure the chamber to the rocket fuselage and the bottom flange will be have holes drilled in it, allowing the convergent divergent nozzle mounting flange to be bolt mounted into place. The flanges are 1/8 in 1020 steel plate and the chamber wall is 1020 3.5 in. outer diameter tube 0.1275 in. thick. So far, the tube has been cut and ground to length and the flanges have been rough cut, but not yet complete. In between the CDN mounting flange and the combustion chamber bottom flange, and the combustion chamber top flange and fuselage mounting flange are high temperature Garlock gaskets.

Exploded view of the combustion chamber.

Scrap piece of the chamber wall.

Disassembled combustion chamber.

Side view.

Side view.

Top view.

CDN mount side view.

Garlock Hi-Temp Style 9800
Material:	Carbon Fiber with a SBR Binder
Fluid Services:	Steam, Water, Inert Gases
Minimum Temperature:	-100 °F (-75 °C)
Continuous Operating Temperature:	+650 °F (+340 °C)
Maximum Temperature:	+900 °F (+450 °C)
Maximum Pressure:	2000 psig (138 bar)
P x T Max.:	700,000 (25,000) for 1/16 in.

Test Method	Physical Properties	Notes	Style 9800
ASTM F-37	Sealability:	ASTM Fuel A (Isooctane) Gasket load, 500 psi Pressure, 9.8 psig	0.1 ml/hr Leakage
		Nitrogen Gasket Load, 3000 psi	0.1 ml/hr Leakage
DIN 3535	Permeation:		0.001 cc/min. Leakage
ASTM F-36	Recovery:		55 %
ASTM F-36	Compressibility:		7 - 17 %
ASTM F-38	Creep Relaxation:	22 hrs. at 212 °F	15 %
ASTM F-146	5 hr. Fluid Resistance	ASTM #1 Oil at 300 °F
		Thickness Increase	0 - 10 %
		Weight Increase	20 %
		ASTM IRM 903 Oil at 300 °F
		Thickness Increase	15 - 40 %
		Weight Increase	60 %
		ASTM Fuel A at 75 - 85 °F
		Thickness Increase	0 - 10 %
		Weight Increase	20 %
		ASTM Fuel B at 75 - 85 °F
		Thickness Increase	5 - 20 %
		Weight Increase	20 %
ASTM F-152	Tensile Strength	Across Grain	1500 psi
	Density		105 Lbs./Ft3