Abstract
Hybrid rocket engines are promising propulsion systems which present appealing features such as safety, low cost, and environmental friendliness. On the other hand, certain issues hamper the development hoped for. The present paper discusses approaches addressing improvements to one of the most important among these issues: low fuel regression rate. To highlight the consequence of such an issue and to better understand the concepts proposed, fundamentals are summarized. Two approaches are presented (multiport grain and high mixture ratio) which aim at reducing negative effects without enhancing regression rate. Furthermore, fuel material changes and nonconventional geometries of grain and/or injector are presented as methods to increase fuel regression rate. Although most of these approaches are still at the laboratory or concept scale, many of them are promising.
Highlights
Hybrid rocket engines (HREs) are chemical rockets which present interesting advantages over liquid rocket engines (LREs) and solid rocket motors (SRMs) and can provide a safe and affordable option for many applications. They have recently come to the fore as they have been chosen to power the second stage of the Tier One, the winner of the $10 million X-Prize [1]
Where r is the fuel regression rate, ρF is the fuel density, q is the total heat flux received by the fuel grain, and ΔHv,eff is the thermal energy required to change a unit mass of solid fuel into gas
The liquid oxygen (LOX)/HTPB propellant combination presents higher maximum values of c∗, but this maximum is reached for small values of α: large grains are required and the propulsion system mass penalty is higher than propellant savings
Summary
Hybrid rocket engines (HREs) are chemical rockets which present interesting advantages over liquid rocket engines (LREs) and solid rocket motors (SRMs) and can provide a safe and affordable option for many applications. This limit is usually set by the physical phenomenon of heat transfer from the diffusion flame to the fuel surface.
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