Page History

...

Design Process

To design the chamber and nozzle, the specifications we already know include the chamber pressure, OF mixture ratio, chamber temperature, and mass flow, as these are design choices based on the tank and overall constraints, as well as the properties of the propellants. To find T0, R, and gamma, we used NASA CEA. For other values,

Chamber Geometry

For parameters such as contraction angle, expansion angle,

...

characteristic length, and contraction ratio, constraints were chosen based on literature or previous projects.

For instance, 15 degrees for an a 15-degree, fairly gradual expansion angle in a conical nozzle is standard and known to be close to near the ideal value. For our design, We selected a conical shaped nozzle was the best choice, as nozzle design because it provides almost as good the same performance as the an ideal bell nozzle , but it is while being much easier to machine. The ideal contraction angle is less specific in what is idealwell-defined, but values between 30 -and 45 degrees is normalare common. Since Given our relatively wide chamber is fairly wide compared to the throat (large contraction area ratio), we chose opted for a more gradual angle to promote better gas flow. However, but this choice is not critical.

The charactaristic characteristic length is defined as the total volume of the chamber (from the injector to the throat) divided by the area of the throat, which is basically throat area. It serves as a metric for the chamber’s overall size of the chamber. This value needs to must be high enough to allow enough sufficient space for the gas to mix and fully combust in the chamber. If this value mixing and complete combustion. If the characteristic length is too small, we have non combusted propellant being uncombusted propellant may be expelled through the nozzle, resulting in lost potential energy and less efficient combustion. If reduced efficiency. Conversely, if the chamber is too large, than the fully combusted gases spend more time remain in the chamber longer, increasing the heat stress on the chamber components. The ideal value is usually found emperically, so there are ranges of accepted values Ideal characteristic lengths are often determined empirically, with accepted values varying for different propellant combinations. We observed other For nitrous-ethanol projects with , values typically range between 20" and 300," which is a broad range to choose from. At this point, we basically chose 20” and 300”. We selected a chamber length that would give roughly provided a 2 to :1 length-to-diameter aspect ratio in , making the chamber . This made the chamber roughly approximately 6 inches long from the injector to the start of contraction. This gave yielded a charactaristic characteristic length of 57.8 ". Furhermore, the inches, providing enough volume while not adding unecessary mass. Notably, extremely large values closer to 200 or 300 " inches were on found in engines not intended for flight.

The that did not go on a rocket, so it is unreasonable to assume that the amount of mass involved in such a large chamber is reasonable for a flight application. Lastly, the contraction ratio is defined as the ratio of the chamber cross-sectional area of the chaber to the throat area of the throat. If this ratio is too large, than the faceplate of the injector has injector faceplate is exposed to excessive thermal loads due to a large area exposed to the hot gases of the chamber, resulting in large thermal stresses. Also, this can create gas flow problems in the chamber, and the stagnant gases remaining in the sides of the chamber further cause thermal problems. If the contraction area ratio is too small, we run out of room fitting the injector we want, especially in our design with impinging triplets. Additionally, there is a point where you can no longer assume that the velocity in the chamber is small (near M=surface area being in contact with hot gases. Additionally, a large contraction ratio can cause gas flow problems and result in stagnant gases near the chamber walls, exacerbating thermal issues. Conversely, if the contraction ratio is too small, there may be insufficient space to accommodate the desired injector design. This is particularly important in our design, which uses impinging triplets. Furthermore, a small contraction ratio can invalidate the assumption that chamber velocity remains near zero (M ≈ 0) compared to the throat velocity, and it reduces combustion efficiency. From the chart below, you can see that typical values are available data, typical contraction ratios range from as low as 2 for large engines and up to as high as 10 for very small engines. For PolarisInitially, initially we had a very large contraction ratio to fit in the injector we wanted, and we determined that getting the value down to 10 was along the limits of what was possible. Essentially, the our preferred injector design. However, we managed to reduce this ratio to around 10, which was near the upper feasible limit. The smallest chamber diameter we could accomplish achieve was 3.25 ," and then we inches. We slightly increased the mass flow to attain a larger throat diameter that made our which brought the contraction ratio less than 10. This gave us a mass flow rate of 1.18 kg/sbelow 10.

Image AddedChart of typical contraction area ratio values.

Nozzle Geometry

The sizing of the nozzle throat is critical to maintain choked flow throughout the burn. 

Materials

Most of the considerations for materials with regards to the chamber and nozzle are based on the thermal loads each part will be exposed to. For instance, the nozzle will experience the highest temperatures, up to 3000 k, and we want it to degrade as little as possible. The clear material choice in this case is fine-grained (isomolded) graphite, as it is fairly easy for us to obtain, machine, and it basically never melts. It is very common to use graphite at least for the throat of the nozzle. Since our design is fairly small, to keep things simple the majority of the contraction and expansion parts of the nozzle are also graphite. For the structural wall of the chamber, we chose aluminum 6061 since it is easy to machine, strong, and light. However, aluminum can't survive the high temperatures expected in the chamber, so we have a paper phenolic liner along the chamber wall, which will ablate and keep the aluminum cool. 

Reuse Considerations

One important aspect of this design is that we hope to fire this engine up to 4 times with minimal part replacements between each firing.

Sealing

As we expect the chamber to reach pressures of 445 psi, we need to seal all leak paths to prevent the hot propellant gases from escaping.

Thermal and CFD Analysis

To ensure that all parts involved do not melt and simulate the flow through the nozzle, we used Ansys Fluent to model the flow through the nozzle.. Image Removed

References:

1. https://ocw-mit-edu.ezproxyberklee.flo.org/courses/16-512-rocket-propulsion-fall-2005/resources/lecture_3/ (Exit pressure)

...