Guidance and control of an earth to orbit vehicle with optimum transition from airbreathing to concurrent airbreathing/rocket propulsion

S = vehicle reference area An efficient performance analysis method (suitable for PC operation) is developed to evaluate potential airbreathing/ rocket propulsion systems for advanced technology single-stage-to-orbit (SSTO) launch vehicles. Evaluated are tradeoffs between airbreathing (AB), rocket, and concurrent airbreathing/rocketpropulsion inminimizing fuel consumption for a given ascent flight trajectory. Many mission, flight, and vehicle related requirements and constraints distance down range from launch point thrust flight velocity combined AB/rocket fuel flow-rate airbreather air flow-rate fuel flow-rate (airbreather) propellant flow-rate (rocket) vehicle weight weight fraction angle of attack factor in exponential altitude density relation are satisfied in the process. With the y = flight path angle analysis method, several modes of airbreathA = denotes an increment ing/rocket propulsion are compared to a r = time constant in guidance law baseline "airbreather alone" propulsion @ = fuel-to-air equivalence ratio system in terms of fuel required to attain p = air density orbital velocity. The optimal switch point (staging) is identified for the transition from airbreathing to rocket propulsion. Nomenclature Acronyms nm = nautical mile AB = airbreather ABLV = airbreathing-launch-vehicle CQ = engine throttle ratio HTOHL =horizontal-takeoff-horizontal-landing C, = coefficient of lift SSTO = single-stage-to-orbit D = drag force VTOVL = vertical-takeoff-vertical-landing FAsT,,,= stoichiometric fuel-air ratio gravity constant controller transfer function vehicle transfer function height above sea level specific impulse (airbreather) specific impulse (rocket) gain in guidance law lift force vehicle mass specific excess power dynamic pressure radius from Earth's center Earth radius Introduction The idea of a single-stage-to-orbit (SSTO) vehicle has long been considered by the Aerospace community. Studies of airbreathing (AB) propulsion in combination with some form of rocket propulsion have appeared in several research papers. AB propulsion for the atmospheric assent portion of an Earth to orbit trajectory was first proposed by Goddard in 1932'. The rational for such a proposal comes from the fact that AB engines provide about 8-20 times higher effective specific impulse 'Graduate Research Assistant, Member AIAA values than pure rockets at altitudes below *Ass3ciate Professor, Member AIAA 100,000 ft. Present-day (multiple stag^) 3professor, ~ssociate Fellow A I M rocket launch vehicles consume nearly 80% of '~esearch Engineer their available propellant mass during climb and acceleration through the atmosphere to an altitude of about 190,000 feet and 7500 ft/sec velocity. For some specific vehicle types, a large proportion of this rocket propellant mass can be saved by using a more efficient AB propulsion system until such an altitude is achieved that the rocket begins to attain the greater efficiency. Kramer and ~uhler* evaluated this problem for a If stage VTOVL vehicle and found that airbreathing propulsion is a feasible approach. The concept of a SSTO vehicle using two fuels and rocket propulsion was first proposed by salkeld3, by operating separate engines in series. Later the dual-fuel engine concept was introduced to reduce the mass of the hydrogen The parallel burn of hydrogen and hydrocarbon engines has been proposed as an alternative to achieve good performance without requiring a dual-fuel engine6-'. Integral rocket/ ramjet propulsion has been evaluated as the theoretical performance of the ramjet propulsion system is much better than either solid or liquid fueled rocket motors9-". This system allows for a high booster propellant-packaging efficiency which results in a considerable reduction in size of the vehicle. Optimizing the performance of both AB and combination AB/rocket systems for vehicle delivery to low Earth orbit has been examined with some systems showing considerable promise'2-15. Optimizing flight path trajectories, reducing vehicle size, minimizing fuel expended, etc. all have an effect on the vehicle operating costs, payload delivery to orbit and mission flexibility. This paper considers optimizing AB/rocket propulsion for a generic SSTOHTOHL Hypersonic vehicle. The primary parameter to be evaluated is when the transition from AB to rocket propulsion should take place. Additionally, the form of transition must be considered, whether switching from AB to rocket is most efficient or whether the AB engine should be utilized as long as possible by burning the AB/rocket engines concurrently. This paper attempts to answer these questions for a generic Hypersonic vehicle as given in ref. 16. In order to screen different conceptual aerospace propulsion systems, the total system performance must be evaluated. This analysis combines the flight mechanics, aerodynamic characteristics, structural factors, and subsystem weight breakdown with the propulsion features. A numerical simulation must be accomplished and an optimization performed to achieve a favorable design. This paper addresses these issues for the given aerospace vehicle, modeled using a two degree of freedom trajectory simulation, suitable for operation on a PC. The present methodology integrates the equations of motion and tracks velocity, altitude, range, flight path angle and weight. A robust guidance system has been developed to control the "stick and throttlem settings to achieve an optimum flight path for maximum payload into orbit. Once an optimum flight path is chosen (e.g. via the ItEnergy Method", see N. ~enugo~al'~ et al), it is then necessary to evaluate the form of propulsion or combination which will achieve the greatest payload delivery to low Earth orbit. By choosing a SSTO vehicle to study for this problem, both engine and structure weights will be increased over that required for a vehicle utilizing only one form of propulsion. In spite of this constraint, significant increases in deliverable payload may be accomplished with proper use of AB/rocket propulsion.