Editing Hybrid-propellant rocket (section)

===Grain manufacturing methods===
====Cast====
Hybrid rocket fuel grains can be manufactured via casting techniques, since they are typically a plastic or a rubber. Complex geometries, which are driven by the need for higher fuel mass flow rates, makes casting fuel grains for hybrid rockets expensive and time-consuming due in part to equipment costs. On a larger scale, cast grains must be supported by internal webbing, so that large chunks of fuel do not impact or even potentially block the nozzle. Grain defects are also an issue in larger grains. Traditional fuels that are cast are [[hydroxyl-terminated polybutadiene]] (HTPB) and paraffin waxes.<ref name="stratasys">{{cite web |title=Hybrid rocket engines use additive manufacturing to combine the advantages of solid and liquid propellants |website=Stratasys |url=http://www.stratasys.com/resources/case-studies/aerospace/rocket-crafters |access-date=December 19, 2016 |archive-date=July 5, 2017 |archive-url=https://web.archive.org/web/20170705085309/http://www.stratasys.com/resources/case-studies/aerospace/rocket-crafters |url-status=dead }}</ref>

====Additive manufacturing====
[[File:3D Printed Hybrid Rocket Fuel Grain.jpg|thumb|A transparent portable education demonstrator 3D-printed hybrid rocket fuel grain with dual helical fuel ports, a post-combustion chamber, and a [[de Laval nozzle]], shown prior to hot fire test.]]
Additive manufacturing is currently being used to create grain structures that were otherwise not possible to manufacture. Helical ports have been shown to increase fuel regression rates while also increasing volumetric efficiency.<ref>{{cite thesis |last=Walker |first=Sean |year=2015 |title=High regression rate hybrid rocket fuel grains with helical port structures |page=40 |degree=M.S. |department=Aerospace Engineering |publisher=Utah State University (USU) |place=Logan, UT |via=Digital Commons, Merrill-Cazier Library, USU |bibcode=2016PhDT.........6W |url=http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=5645&context=etd}}</ref> An example of material used for a hybrid rocket fuel is [[acrylonitrile butadiene styrene]] (ABS). The printed material is also typically enhanced with additives to improve rocket performance.<ref name="stratasys"/> Recent work at the University of Tennessee Knoxville has shown that, due to the increased surface area, the use of powdered fuels (i.e. graphite, coal, aluminum) encased in a 3D printed, [[acrylonitrile butadiene styrene|ABS]] matrix can significantly increase the fuel burn rate and thrust level as compared to traditional polymer grains.<ref name=":0">The Use of a 3-D Printed, Polymer Matrix Containing Pulverized Fuel in a Hybrid Rocket, James Evans Lyne, A. Brigham, R. Savery, K. Karcher, J. Pyron, L. Adams, G. Reagan, H. Furches, D. Sola, L. Melendez, and C. Keck, AIAA Paper 2018-4597, 2018 Propulsion and Energy Forum, Cincinnati, Ohio.</ref><ref name=":1">The Development of a Powder-Filled, ABS Matrix for Use as Fuel in a Hybrid Rocket Motor, Teague Aarant, Jared Bass, Timothy Grizzel, Seth Holladay, Matthew McVey, William Putthoff, Angus Shaw, Peter Tarle, Robert Nickel, Caroline Littel, and James Evans Lyne, AIAA Paper 2019-4417, 2019 Propulsion and Energy Forum.</ref>