Driving Nozzle Performance
(Below are important performance values for our engine. We want to design the geometry of our engine to optimize these values)
Isp - specific impulse
Specific impulse is a quantity effectively measuring the efficiency of an engine through the relation of thrust and mass flow rate; from the equation, this efficiency may be viewed as the impulse obtained per unit mass of propellant.
F=thrust
ṁ=mass flow rate
g0=9.8 m/s2
ṁcc=combustion chamber mass flow rate
Ve=exit velocity
Ae=exit area
Pe=exit pressure
Pa=ambient pressure
Assuming a higher efficiency is more desirable, our goal is to maximize our Isp, thus maximizing efficiency. In the case of our engine, we may make a few simplifications in our model of Isp. Because our nozzle is matched, we may eliminate the 2nd term of the thrust equation thus allowing us to also eliminate the mass flow term under the key assumption that all propellants flow through the combustion chamber. This leaves us with the following model:
Ve=exit velocity
g0=9.8 m/s2
It is important to note the discrepancy between calculated Isp values and those achieved in practice. While our model assumes ideal combustion and lossless flow, these factors and others account for the lower efficiency observed in practice compared to calculations.
Our goal of efficiency through the optimization of Isp may be best achieved through an increase in exit velocity as seen by our model. By observation of the exit velocity equation, desirable Isp may be obtained through a high combustion temperature although limitations in materials may limit this to a point.
C* - characteristic velocity
C* is a measure of the combustion performance in the engine (measured in m/s)--the larger the value, the more efficient the engine will be.
C* is related to the chemical properties of the fuel and oxidizer within the combustion chamber (different combinations of fuel and oxidizer will affect its value). The most typical way to increase C* with a given fuel and oxidizer is to increase the combustion temperature, which is done by altering the ratio of fuel and oxidizer used. However, other considerations taken into account such as max temperature for the combustion chamber wall material force non-ideal ratios to be used in practice. C* can be found both theoretically and experimentally with the equations below.
C*=characteristic velocity
R=gas constant
gc=acceleration due to gravity
Tc=combustion chamber temperature
𝛾=specific heat ratio of propellant mixture
C*=characteristic velocity
At=throat area
Pc=chamber pressure
gc=acceleration due to gravity
ṁ=mass flow rate (of gas)
Cf - thrust coefficient
Cf is a measure of the thrust amplification by the engine’s nozzle, essentially being a measure of how efficient the nozzle is. The nozzle expands the exhaust gas to lower pressures and higher velocities, increasing thrust, and Cf is a measure of how well it does that.
Cf depends on the chemical characteristics of the fuel and oxidizer, the expansion ratio of the nozzle exit area and throat area, as well as the different pressures within and outside of the engine. The expansion ratio alters the ratio of exit to chamber pressure, thus changing Cf itself. To have the best possible Cf, an engine should have a very high chamber pressure which the nozzle turns into a low exit pressure matching the ambient pressure around the nozzle.
Cf=thrust coefficient
𝛾=specific heat ratio of propellant mixture
Pe=exit pressure
Pc=chamber pressure
𝜺=expansion ratio (exit area/throat area)
Pa=ambient pressure
Combustion Chamber Geometry
The combustion chamber lies after the injector face and before the nozzle throat. Propellant mixing and combustion occur in the chamber, and it’s geometry has a huge impact on the above mentioned performance values. We will look at different aspects and choices for combustion chamber design, and how they affect our overall engine performance
ts - Propellant Stay Time
The combustion chamber functions most notably as a point for propellant mixture and combustion, ideally at a high efficiency. Propellant stay time is the time required of propellants within the combustion chamber for complete mixing and combustion and is reliant on many factors. These factors include propellant combination, the state of injected propellants (liquid, gas), chamber geometry, initial conditions from injector design, and rate-limiting factors. Propellant stay time is related to chamber volume by the following equation:
Vc=chamber volume
Wtc=propellant mass flow rate
V=average specific volume
ts=propellant stay time
L* - Characteristic Length
L* is the combustion chamber volume divided by the throat area. Ideally, it is a measure of how long a combustion chamber should be so that the rocket fuel can completely combust while still inside of the combustion chamber. As a result of different chemicals having different combustion properties, an ideal L* varies with the type of fuel and oxidizer being used. Having a characteristic length too small can prevent complete combustion of fuel before exiting the chamber, reducing its effectiveness. However, having a characteristic length too large can increase engine weight, cost of materials, increase surface area requiring cooling, and increase frictional losses within the chamber. When designing an engine, an ideal L* for a certain fuel can be found off of charts based off of industry practice and experiments.
Shape, Length, and Diameter
After determining the chamber volume, factors of shape, length, and diameter are all important considerations, each with inherent complications for engine performance. Beginning with shape, a factor of great importance is the surface to volume ratio, a correlate of thermal and structural parameters. While a spherical shape maximizes the surface to volume ratio leading to a smaller cooling surface and lighter weight due to material reduction, it presents difficulties in manufacturing and is not ideal in other respects. Another chamber shape is a cylinder which is more practical in terms of manufacturing and though not ideal, presents satisfactory surface to volume ratio.
Assuming chamber volume and shape (cylinder) are defined, design choice comes down to a long chamber with a small diameter or a short chamber with a large diameter. A long chamber with small diameter results in high pressure losses as well as concerns of heat transfer with a longer propellant stay time in addition to restrictions on injector design. A short chamber with large diameter limits combustion efficiency as the mixing and combustion zone is reduced and ideal propellant stay time may not be satisfied. Considering the size of our engine and trends of other engines, a chamber of shorter length and larger diameter is most beneficial.
Contraction Ratio
The contraction ratio is the ratio of the cylindrical cross-sectional area of the chamber to the cross-sectional area of the throat of the nozzle. Larger engines typically have a low contraction ratio with a longer chamber length, and smaller engines typically have a larger contraction ratio with a smaller chamber length to have a large enough L* for complete fuel combustion. Along with L*, the contraction ratio governs how wide vs. how long the chamber will be. If the ratio is too small, the chamber might be needlessly long while increasing weight and cost, and if the ratio is too big the chamber might be either impractically wide or not allow for adequate gas flow.