Abstract
The feasibility of 3D-printed molds for complex solid fuel block geometries of hybrid rocket engines is investigated. Additively produced molds offer more degrees of freedom in designing an optimized but easy to manufacture mold. The solid fuel used for this demonstration was hydroxyl-terminated polybutadiene (HTPB). Polyvinyl alcohol (PVA) was chosen as the mold material due to its good dissolving characteristics. It is shown that conventional and complex geometries can be produced reliably with the presented methods. In addition to the manufacturing process, this article presents several engine tests with different fuel grain geometries, including a short overview of the test bed, the engine and first tests.
Highlights
A hybrid rocket engine uses a combination of a solid and a liquid propellant
Due to the used pure, and partly clear, hydroxyl-terminated polybutadiene (HTPB), the radiation could heat deeper into the fuel block, creating a liquid layer that could be moved due to the flow momentum. This issue was resolved through adding a small percentage (0.5%) of carbon black during the solid fuel block manufacturing process
It was shown that it is possible to print dissolving molds that enable the production of complex flow-optimized shapes. These geometries allow an increase in the regression rate of solid fuel in hybrid rocket engines, enabling them to compensate partly for a major disadvantage; further tests are necessary to evaluate this in depths
Summary
A hybrid rocket engine uses a combination of a solid and a liquid propellant It unites some advantages of liquid rocket engines (e.g., possibility to control the burn time) and those of solid rocket motors (e.g., simple combustion chamber design); one major inherent disadvantage of most hybrid engines is a low regression rate of the fuel grain, which means that only a moderate thrust can be achieved. In order to achieve a higher regression rate with common fuel block manufacturing, multiple grains with different geometries or geometrical orientations can be stacked. To demonstrate the possibilities of an additively manufactured disposable mold, a flow-optimized geometry was developed and produced. Polyvinyl alcohol (PVA) was selected as print material because of its good dissolving properties in water This approach was chosen because it offers the capability to produce any relevant kind of shape. In accordance to the literature, curing of HTPB with isophorone diisocyanate (IPDI) as the curative and dibutylzinn-dilaurat (DBTDL) as catalyst is performed at 60 ◦C for seven days [13,14]
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