OHKLA Design/Fuel grain geometry
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This document is part of the OHKLA Design Tasks tree.
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[edit] Summary
The purpose of this design task is to select the solid fuel grain geometry for the OHKLA hybrid rocket engine. This involves selecting a diameter and length for the grain as well as a number of combustion ports and the combustion port shapes.
[edit] Background
The geometry of a hybrid rocket engine's fuel grain - the ratio between its diameter and its height, how many combustion ports it has and the shape and arrangement of the combustion ports - impacts on the engine's mass flow rate. Low mass flow rates result in engines which burn for a long time but provide low thrust, whereas high mass flow rates result in engines which burn for a short time but which provide high thrust. In an application such as OHKLA, quick-burning high-engine thrusts are optimal, so this task is concerned with choosing a geometry which maximises mass flow rate, subject to other constraints. However, there are a number of other factors to consider, pertaining to simplicity and reliability, which mass flow rates need to be balanced against.
[edit] Features/Considerations
[edit] Ease of grain casting
[edit] Mass ratio
[edit] Grain integrity
[edit] Mass flow rate
A high mass flow rate (and hence high thrust) is very important for the success of the OHKLA project. The higher the mass flow rate, the better. However, this should not be the only factor taken into consideration when choosing a geometry. The CSTART Design Philosophy explicitly states that "Simplicity is more important than performance".
The mass flow rate of a geometry is proportional to the total surface area that is exposed for combustion. Thus, cylindrical combustion ports with large radii result in higher mass flow than cylindrical ports with small radii, and several parallel ports of equal size will result in higher mass flow than an individual port. More complicated combustion port shapes can expose greater surface areas for combustion without taking up larger cross-sectional areas of the grain (consider the well-known Koch snowflake curve as an example of this).
