Arif Karabeyoglu

Scale-Up Tests of High Regression Rate Paraffin-Based Hybrid Rocket Fuels

Recent research at Stanford University has led to the identification of a class of paraffin-based fuels that burn at surface regression rates that are three to four times that of conventional hybrid fuels. The approach involves the use of materials that form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid-gas interface substantially increases the rate of fuel mass transfer, leading to much higher surface regression rates than can be achieved with conventional polymeric fuels. Thus, high regression rate is a natural attribute of the fuel material, and the use of oxidizing additives or other regression rate enhancement schemes is not required. The high regression rate hybrid removes the need for a complex multiport grain, and most applications up to large boosters can be designed with a single port configuration. The fuel contains no toxic or hazardous components and can be shipped by commercial freight as a nonhazardous commodity. At the present time, grains up to 0.19 m [19.1 cm (7.5 in.)] in diameter and 1.14 m [114.8 cm (45.2 in.)] long are fabricated in a general-purpose laboratory at Stanford University. To further demonstrate the feasibility of this approach, a series of scale-up tests with gaseous oxygen have been carried out using a new Hybrid Combustion Facility (HCF) at NASA Ames Research Center. Data from these tests are in agreement with the small-scale, low-pressure, and low mass flux laboratory tests at Stanford University and confirm the high regression rate behavior of the fuels at chamber pressures and mass fluxes representative of commercial applications.

SCALE-UP TESTS OF HIGH REGRESSION RATE LIQUEFYING HYBRID ROCKET FUELS

Recent research at Stanford University has led to the identification of a class of paraffin-based fuels that burn at surface regression rates that are 3 to 4 times that of conventional hybrid fuels. The approach involves the use of materials that form a thin, hydro-dynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid-gas interface can substantially increase the rate of fuel mass transfer leading to much higher surface regression rates than can be achieved with conventional polymeric fuels. Thus, a high regression rate is a natural attribute of the fuel material and the use of oxidizing additives or other regression rate enhancement schemes is not required. The high regression rate hybrid removes the need for a complex multi-port grain and most applications up to large boosters can be designed with a single port configuration. The fuel contains no toxic or hazardous components and can be shipped by commercial freight as a non-hazardous commodity. At the present time, grains up to 8.4 inches in diameter and 45 inches long are fabricated in a general-purpose laboratory at Stanford University. To further demonstrate the feasibility of this approach, a series of scale-up tests with gaseous oxygen have been carried out using a new Hybrid Combustion Facility (HCF) at NASA Ames Research Center. The data from these tests are in agreement with the small scale, low pressure and low mass flux laboratory tests at Stanford and confirm the high regression rate behavior of the fuels at chamber pressures and mass fluxes representative of commercial applications.

High Performance Hybrid Upper Stage Motor

Hybrid rocket propulsion is a tipping point technology in the sense that a small, short term investment could have game changing consequences in developing green, safe, affordable and high performance systems needed in future space missions. In order to demonstrate the advantages of hybrids most effectively, the effort should be concentrated on improving the Technology Readiness Level (TRL) of the technology for a carefully selected class of missions. Arguably upper stage motors used in small launch vehicles constitute a perfect platform for this purpose due their relatively small scale and high performance requirements. The advanced hybrid rockets that are being developed by SPG are believed to have the capability to deliver high performances desirable for upper stages, while retaining the cost, environmental and simplicity advantages of the classical hybrids. In order to demonstrate the performance capabilities of advanced hybrid rockets, a design study has been conducted to replace the Orion 38 solid rocket motor with a LOX/paraffin-based system. The LOX hybrids delivering the same level of total impulse as Orion 38 system are determined to be 15-18% lighter. It has been shown that switching to higher performance hybrid upper stages could lead to payload increases up to 40% for a typical launch vehicle. The additional cost, environmental, safety, stop/restart/throttling advantages are expected to make the hybrids desirable alternatives to the existing upper stage systems.

Modeling of N2O Decomposition Events

This paper addresses the safety issues associated with the oxidizer nitrous oxide (N2O) with emphasis on propulsion systems. Even though N2O is a widely used energetic material, the number of decomposition related accidents are quite limited due to its abnormally slow decomposition kinetics. However hazards do exist especially in propulsion systems where large quantities of N2O are stored at room temperature in thin walled vessels. Moreover the closely coupled combustion chamber is a significant source for ignition which does not naturally exist in other applications. A detailed kinetics model for the N2O decomposition process is presented. It is shown that a simplified single step first order kinetics model accurately captures the decomposition process at pressures larger than 40 atm. With use of the kinetics data, it has been shown that, at the same pressure and temperature, the N2O decomposition rate is six orders of magnitude slower than the decomposition of hydrogen peroxide (H2O2), making it a much safer propellant. Models for homogenous and local thermal ignition are also presented. It is shown that the estimated minimum ignition energy for pure N2O is approximately 450 mJ which is three orders of magnitude larger than the ignition energy for a stoichiometric CH4/air mixture. Small concentrations of diluents (i.e. N2, O2 or He) further increase the ignition energy making the mixture extremely difficult to ignite at dilution levels higher than 30%. The results of a model developed to predict the pressure rise in a closed vessel subject to decomposition is presented to demonstrate the significant hazard that exists in the N2O tank. The model predicts a 20 fold increase in pressure over a time period of many seconds for tanks that are in the range of 1-3 meters in length. Finally, a list of safety related recommendations unique to N2O operations have been included. The general conclusion is that despite its potential decomposition hazard, if handled properly, N2O is one of the safest oxidizers being used in rocket propulsion systems.

Time-resolved fuel-grain port diameter measurement in hybrid rockets

A novel technique is presented for determining the instantaneous spatially averaged port diameter of solid-fuel grains in hybrid rocket motors. This technique requires measurement of the frequency of the Helmholtz oscillation of the motor and is based on the principle that this frequency is inversely proportional to the square root of the chamber volume. This technique was applied to a hybrid rocket motor burning paraffin wax with gaseous oxygen. The calculated variation of port diameter agreed well with the correlation for average regression rate, determined from mass loss during operation. A major advantage is that the only instrumentation required for implementing this technique is a high-speed pressure transducer or a photomultiplier tube.

Investigation of Feed System Coupled Low Frequency Combustion Instabilities in Hybrid Rockets

In this paper, the transient combustion theory has been extended to hybrid rockets using liquid oxidizers with feed systems characterized by finite response times. Models for generic hybrid feed systems have been developed and these have been coupled to the combustion chamber dynamics with lag times introduced to model the delays associated with oxidizer vaporization and fuel gasification processes. This study has been limited to a simplified behavior for the combustion chamber that only includes the filling/emptying dynamics. The set of Ordinary Differential Equations that represent the system behavior have been linearized and nondimensionalized. Using the technique of Laplace Transformation, transfer functions for the feed coupled system with and with out flow isolation elements have been developed. The model has been used to investigate the stability behavior of the feed coupled system and to develop stability criteria in terms of the practical operational parameters for liquid fed hybrid motors. The oxidizer vaporization delay is treated as an input parameter which can be adjusted to match the observed oscillation frequency to the model prediction. As a practical application of the model, the estimated vaporization delay can be used to evaluate the atomization characteristics of injectors and pre-combustion chamber designs for hybrid rocket motors.

Peregrine Hybrid Rocket Motor Development

To further develop and demonstrate the applicability of liquefying-fuel hybrid rocket technology to low-cost launch applications, a small team of engineers is developing a medium-scale liquefying-fuel hybrid sounding rocket using storable propellants (paraffin wax and N2O) that will propel a 5 kg payload to the edge of space. This rocket, known as the Peregrine Sounding Rocket, is being developed by engineers from NASA Ames, Stanford University, Space Propulsion Group Inc. (SPG, Sunnyvale, CA) and NASA Wallops, with a launch from Wallops anticipated at some point in the future. Results of ground testing performed using a heavy-weight configuration of the motor show that stable and efficient combustion has been achieved.

DESIGN OF AN ORBITAL HYBRID ROCKET VEHICLE LAUNCHED FROM CANBERRA AIR PLATFORM

A two stage small orbital vehicle, which is launched from the Canberra air platform, has been designed using paraffin-based hybrid rocket propulsion for both stages. Following the release from the Canberra aircraft, the rocket vehicle is designed to be ignited at 45,000 ft with an initial trajectory angle of 60 degrees with respect to the local horizon. Using an in house design and optimization code, a payload capability of 31 kg has been demonstrated for a 500 km sun-synchronous orbit. The safety and low cost aspects of the paraffin hybrids coupled with the operational simplicity and cost effectiveness of the Canberra air platform are believed to be the right ingredients for an affordable and responsive launch system solution that is lacking in todays launch market.

Development of Ammonia Based Fuels for Environmentally Friendly Power Generation

Ammonia’s high hydrogen density makes it a very promising green energy storage and carrier medium. In fact, among practical fuels, ammonia has the highest hydrogen density including hydrogen itself both in the cryogenic and also in the compressed gas storage modes. Unlike many other practical fuels, ammonia (NH3) molecule is free of carbon atoms, which leads to zero CO2 emissions during its combustion. The fact that ammonia is already a widely produced and used commodity with well established distribution and handling procedures would allow for its smooth transition as an alternative fuel. The two major disadvantages of the ammonia fuel: 1) low energy density compared to hydrocarbons and 2) toxicity, have hindered its development as a transportation system fuel. Note that both of these issues are not key drivers in power generation systems, making this mode an ideal entry point for the ammonia fuel. SPG Inc. has been developing technologies to burn ammonia in the gas turbine systems with the primary objective of minimizing the NOx and NH3 emissions. The first phase of the project involves establishing the fundamentals of ammonia combustion in gas turbine combustors. As an important part of this phase, testing with ammonia fuel in a simulated gas turbine combustor has already been started.

Status Update Report for the Peregrine Sounding Rocket Project: Part III

The Peregrine Sounding Rocket Program is a joint program of NASA-Ames, NASAWallops, Stanford University and Space Propulsion Group, Inc. to develop and fly a high performance sounding rocket based on liquefying fuel hybrid rocket technology. The program was kicked off in November of 2006 and initial ground testing of the propulsion system begain in July 2008. Two virtually identical vehicles capable of lofting a 5kg payload to 100km will be constructed and flown out of the NASA Sounding Rocket Facility at Wallops Island. The propellants utilized are nitrous oxide and paraffin, a high regression rate liquefying fuel initially developed at Stanford University. The goal of the Peregrine program is to demonstrate the operational maturity of liquefying hybrid propulsion systems for space applications and their potential to reduce propulsion system costs. This is the third in a series of three annual papers outlining the Peregrine project and providing status updates. The majority of this (JPC 2009) paper will focus on the results of the propulsion system ground test program and the detailed design of the vehicle.