Hybrid rocket fuel residuals - an overlooked topic

Abstract

Rocket propulsion performance is characterized by parameters as thrust, specific impulse, and propellant mass ratio. The propellant mass ratio is affected by propellant residuals, which is often overlooked in hybrid rocket design. Such residual is the mass of propellant which cannot be used for propulsion purposes and is left in the vehicle after performing its mission objective. In case of liquid and solid rocket design it is state-of-the art that propellant residuals are small so that they do not affect the usage of both propulsion types for high performance missions with high ideal velocity changes. In authors' view the residual topic is not recognized by its importance and impact on performance in published hybrid rocket studies, which motivates this work. When hybrid rockets are concerned, due to their specific working characteristics the propellant residual size may have implications on their usability for missions with high ideal velocity requirements. For this reason, residual minimization might be another important design challenge for this rocket propulsion type, beside enhancement of fuel regression rate and combustion efficiency. An overview of the different origins and types of hybrid residuals is presented and reviewed for its relevance. Three different types of residuals are identified: the first part is related to the oxidizer that remains in the tank and in the feed lines. The second part can be mainly attributed to fuel grain design and spatial regression behaviour. The third part of residuals results from uncertainties in important design parameters of the hybrid rocket propulsion system, which determine thrust, pressure and O/F-mixture (and therefore this part of residuals). These design parameters, which are used to model oxidizer injection and fuel regression behaviour as well as nozzle mass flow are identified and introduced in the governing equations of hybrid motor operation to investigate the effect of uncertainties on the rocket velocity change bandwidth, which can be delivered by the specific hybrid rocket propulsion unit. This investigation is done by numerical simulations and in addition by existing analytical solutions. NomenclatureA = area a, b, d, e = characteristic velocity equation constants a, α, n = regression rate law coefficients c* = characteristic velocity = thrust coefficient Cd = discharge coefficient D = diameter Propulsion and Energy Forum 2 = throat erosion rate g = gravity value G = mass flux H 2 O 2 = Hydrogen Peroxide I = impulse k = structural index K = mass flow (oxidizer or fuel) parameter L = length = mass flow m = mass LOX = Liquid Oxygen N 2 O = Nitrous Oxide O/F = oxidizer to fuel ratio (text) p = pressure r = port radius = regression rate R = diameter ratio Re = reliability SSTO = Single Stage To Orbit t = time T = thrust = injection parameter = expansion ratio = oxidizer to fuel ratio (equations) = efficiency ΔV = velocity increment Δp = pressure drop = density subscripts 0 = initial or reference b = burning c = combustion chamber exit = nozzle exit f = fu...

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

“…The worst case for a defined throttling level capability is to consider that the motor could possibly operate the whole burning time both at maximum or minimum thrust. For the typical high optimal mixture ratio occurring in hybrid propulsion the best residuals mitigation is to design the system to always consume all the oxidizer and to have a fuel margin for any situation [47]. Therefore, the design mixture ratio should be the minimum one, which is reached at minimum thrust.…”

“…Another source of residuals is related to unexpected mixture ratio shift due to manufacturing tolerances, parameters uncertainties and motor performance variations from burn to burn [47].…”

“…However, in this case a good design including residual management and a consistent manufacturing/quality control could limit the penalty to very low values [47] and so a mixture ratio control seems not worthy.…”

Genetic Algorithm Optimization of a Cost Competitive Hybrid Rocket Booster

“…Also, the smaller ports should yield fuel regression rates closer to the base burnrate being used. American Institute of Aeronautics and Astronautics 45 . Lockheed Martin's solution for the fuel residual problem is hybrids 29 was to direct the oxidizer to 'burn the motor preferentially from the center row out.'…”

Modeling and CFD Simulation of Regression Rate in Hybrid Rocket Motors

“…Several aspects of hybrid rockets make them an attractive technology for next-gen propulsion, such as simplicity, low cost, safety, reliability, environmental friendliness, thrust modulation and the ability to be restarted [1][2][3][4][5][6]. Nevertheless, sufficient maturity has not yet been attained and nowadays, no system has reached full operational status, even if this could happen in the near future [7][8][9][10][11][12][13]. In this frame, Computational Fluid Dynamics (CFD) is a powerful tool that can help develop a high-performance solution and increase the TRL of hybrid rocket motors in a cost-effective way [14].…”