A theoretical study on throttle ranges of O/F controllable hybrid rocket propulsion systems

Abstract

The characteristics of several O/F control methods for hybrid rocket propulsion have been discussed and theoretically analyzed from the physical properties of propellants and fuel regression behavior. In this research, comparisons have been made among different oxidizer injection methods of Altering-intensity Swirling Oxidizer Flow Type (A-SOFT), Aft-chamber Oxidizer Injection Method (AOIM), and Swirling-AOIM for the throttle range with a constant O/F, design restrictions of the fuel grain, penalties on the adoption of the methods, and suitable scales of the engine. Theoretical analysis on regression rates has revealed that A-SOFT has upper and lower limits of throttle while maintaining a constant O/F whereas AOIM does not have any lower limit, and Swirling-AOIM covers both the throttle ranges. The designing restriction of the fuel grain derived from the regression rate behavior has indicated that A-SOFT using paraffin and oxygen has a potential to maintain 50-100% throttle range over a burn. The penalties for the adoption of these O/F control methods have also been discussed from the aspects of the increase in the complexity of the system, structural mass, and pressure drop at the injector for the methods using gaseous injection. The pressure drop has quantitatively been evaluated by relating the available swirl strength with the cross-sectional area and gaseous oxidizer mass flux at the injector. This analysis has revealed 5 times difference in the available swirl strength between the gaseous oxygen and the decomposed gas of 90% hydrogen peroxide. The sizing of the 1st stage of the satellite launcher has revealed that A-SOFT and Swirling-AOIM are suitable for small-scale engines with a propellant mass of 10 0 -10 2 [ton] using paraffin and liquid oxygen whereas AOIM and Swirling-AOIM are suitable for engines with paraffin and 90% hydrogen peroxide.

Variations and Control of Thrust and Mixture Ratio in Hybrid Rocket Motors

“…In [43] it is claimed that a swirled AOIM will combine both regions of operation; however, with the current hypothesis of same maximum oxidizer flux limit, the swirled AOIM will behave exactly as an axial AOIM, except for a shorter grain length and potentially better baseline combustion efficiency and stability, with better throttling and worse time behaviour compared to the A-SOFT.…”

“…1 AOIM and A-SOFT configurations. Figure taken from Ozawa and Shimada [43] star's shapes used in solid propulsion for neutral burning, the neutral point is n = 0 and the hybrid thrust behaviour is always regressive.…”

“…Another alternative way to control the mixture ratio in a hybrid rocket motor has been proposed by Shimada et al [38][39][40][41][42][43] and is called A-SOFT. The basic idea is that the mixture ratio shift can be compensated acting in real time on the a value of the fuel regression rate.…”

“…In [43], Ozawa has drawn a graph showing the regions of applicability of the two technologies. However, the graph considered the same grain dimensions, i.e.…”

“…The chamber pressure and the mixture ratio are thus determined by the imposed oxidizer mass flow and the corresponding indirectly obtained fuel mass flow: (2) G ox =ṁ oxp ∕A p Several ideas have been proposed to be able to control the mixture ratio both in time and during throttling, like the addition of an aft-injection of further oxidizer, called AOIM [2,37], and the recently proposed Altering-intensity Swirling Oxidizer Flow Type (A-SOFT) configuration by Shimada et al [38][39][40][41][42][43], see Fig. 1.…”