THEORETICAL ANALYSIS AND EXPERIMENTAL VERIFICATION OF A MONOPROPELLANT DRIVEN FREE PISTON HYDRAULIC PUMP 1

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1 Proceedins of IMECE 3 23 ASME International Mechanical Enineerin Conress Washinton, D.C., November 15 21, 23 IMECE THEORETICAL ANALYSIS AND EXPERIMENTAL VERIFICATION OF A MONOPROPELLANT DRIVEN FREE PISTON HYDRAULIC PUMP 1 Timothy G. McGee 2 Justin W. Raade 3 H. Kazerooni 4 Human Enineerin Laboratory Department of Mechanical Enineerin University of California-Berkeley Berkeley, CA 9472 ABSTRACT The authors present a novel power supply for mobile robotic systems. A monopropellant (e.. hydroen peroxide) decomposes into hih temperature ases, which drive a free piston hydraulic pump (FPHP). The elimination of fuel/oxidizer mixin allows the desin of simple, lihtweiht systems capable of operation in oxyen free environments. A thermodynamic analysis has been performed, and an experimental FPHP has been built and tested. The prototype successfully pumped hydraulic fluid, althouh the flow rate was limited by the off-the-shelf components used. 1 INTRODUCTION The limitation of current power sources is one of the dominant bottlenecks preventin the more widespread appearance of fully autonomous field robotics. These robotic systems include any automated mobile platforms such as walkin machines, robotic fish, or any similar system that must maintain eneretic autonomy in non-laboratory environments. To overcome the power supply problem in past research efforts, researchers have typically either used a lare number of batteries to demonstrate the system performance for a short time in the field, or they used an umbilical cord to power their system from a lare stationary power supply. Thus, in order to achieve true eneretic autonomy for many mobile robotic systems, new advances in power source technoloy are still required. 1 Funded in part by ONR Grant N Graduate Student, funded by NDSEG Fellowship 3 Graduate Student, funded by NSF Fellowship 4 Professor 1 Most human scale and smaller robotic systems have power requirements ranin from 1W to 2W. The dominant traditional power supplies in this rane are electric batteries, fuel cells, and small internal combustion (IC) enines, such as model airplane enines. These power supplies have sinificant drawbacks, however. The low enery density of batteries prevents them from bein applicable for any proloned period of time. Althouh fuel cells do have larer enery density than batteries, they lack hih power density and cannot create bursts of power quickly. Electric actuators are also much larer and bulkier than hydraulic or pneumatic actuators for comparable power outputs. While the hih enery density of asoline is desirable, all hydrocarbon enines require elaborate systems for air compression and inition in addition to many movin parts such as crank shafts and pistons. Small IC enines must also run at extremely hih speeds in order to achieve ood power densities. Thus, ear reduction systems are required to connect these enines to pumps, addin complexity to the system. Hydrocarbon enines are also limited by their dependence on the oxyen in air, restrictin underwater and space applications. Given these limitations of more traditional power sources, the use of monopropellant technoloy for mobile robotic power supplies has promisin potential. Monopropellants refer to a class of eneretic liquids, such as hih concentration hydroen peroxide and hydrazine, which decompose upon contactin a solid catalyst surface and release heat: Monopropellant + catalyst as products + heat (1) The enery produced by this reaction can be harnessed by allowin the expandin hot ases to perform work on a piston or turbine, just as the combustion products of an IC enine are 1 Copyriht 23 by ASME

2 used to perform work. Since the monopropellant reaction does not require an oxidizer, fuel/oxidizer mixin is eliminated. This allows the desin of simple, lihtweiht systems with increased power and enery densities, and operation in oxyen free environments, such as underwater or space. Unlike IC enines, monopropellant driven enines do not require a compression stae. This eliminates idlin when there is no load on the system, and allows a monopropellant power supply to produce power on demand by producin discrete enine strokes. The ability to control individual strokes of the enine also provides more flexibility for the overall control stratey for the power supply. Furthermore, hydroen peroxide, one of the available monopropellants, decomposes into steam and oxyen, which are nontoxic to humans. Monopropellants have a successful history of applications. They have most often been used as a rocket propellant in spacecraft includin the Mercury spacecraft, satellite attitude control, and an experimental Personal Rocket Belt [1]. Monopropellants have also been successfully used to power turbine driven hydraulic pumps for the X-15 Rocket Plane [2] and NASA Space Shuttle [3]. While no literature was found on a detailed study of the use of monopropellants for small scaled robotics applications outside of the recent past, a NASA sponsored technoloy study from 1967 mentions the possibility of usin the hot as from monopropellant decomposition to power human scaled robotics [4]. More recently, there have been some renewed investiations into the development of monopropellant power supplies. Amendola and Petillo outline the benefits of usin various monopropellants, includin hydroen peroxide, to drive a piston enine in their 21 patent [5]. Also, a team from Vanderbilt University has recently done some extensive testin usin decomposed hydroen peroxide to directly power hot as cylinders [6]. The two main approaches for monopropellant robotic power supplies are to use the decomposed hot ases to directly power actuators or to power a hydraulic system [4]. Althouh a monopropellant driven hydraulic system is bulkier and less efficient than a system which directly uses the decomposed hot ases, it does provide several advantaes, which could make it more desirable for certain applications. Since hydraulic fluid is far less compressible than the hot as, hiher bandwidth actuation can be achieved. The hiher pressures that can be obtained in hydraulic fluid, when compared to compressed as, also allow the use of smaller actuators to achieve the same forces. A centralized hydraulic pump also contains the hot decomposition ases to a sinle location where they can be vented usin passive exhaust ports. Thus it does not require the development of control valves that can withstand the hih decomposition temperatures of the monopropellants. This paper investiates a hydraulic system which uses hydroen peroxide to drive a novel free piston hydraulic pump (FPHP). The FPHP combines two past areas of research: the use of monopropellants to power hydraulic systems with turbine driven pumps [2,3] and free piston hydraulic pumps driven by IC enines [7-12]. Althouh asoline and other hydrocarbon fuels have very hih enery densities, a breakthrouh in the development of a reliable IC free piston enine hasn t occurred, primarily resultin from several technical challenes that include maintainin a constant compression ratio with the absence of a crank shaft, properly timin the inition, and startin the enine. These challenes arise from the need to compress the air-fuel mixture in IC enines and to inite the mixture at a certain piston location. Since monopropellants systems do not require compression, these problems are eliminated. 2 DESCRIPTION OF FPHP The basic power source desin, illustrated in Fi. 1, consists of two Hot Gas Cylinders and a Hydraulic Cylinder. Left Catalyst Left Fuel Line Left Solenoid Fuel Left Hot Gas Cylinder Left Hot Gas Inlet Left Hot Gas Left Hydraulic Chamber Left Exhaust Port Connectin Rod Fluid Line to Accumulator Check Valve Hydraulic Cylinder Riht Hydraulic Chamber Hydraulic Piston Fluid Line from Reservoir Fiure 1. Monopropellant FPHP A cycle of the FPHP operation beins with the openin of the Left Solenoid Valve, allowin liquid monopropellant to flow into the Left Catalyst Bed. The Catalyst Bed, typically a metallic mesh, decomposes the liquid monopropellant into hih pressure decomposition ases, which enter the Left Hot Gas Cylinder throuh the Left Hot Gas Inlet (Fi. 2a). The expandin hot as performs work on the Left Hot Gas Piston, forcin it to the riht. Since the Hot Gas Pistons are riidly connected to the Hydraulic Piston by a Connectin Rod, formin a sinle free piston assembly (FPA), the Hydraulic Piston is also forced to the riht. This motion drives the hydraulic fluid in the Riht Hydraulic Chamber throuh a Check Valve and into an accumulator, and draws low pressure hydraulic fluid from a reservoir into the Left Hydraulic Chamber (Fi. 2b). When the piston reaches the end of its stroke, the ases are vented to the atmosphere throuh the Left Exhaust Port, which is machined into the cylinder (Fi. 2c). This marks the end of the first stroke of one cycle. Durin the second stroke, monopropellant is injected into the Riht Catalyst Bed, resultin in hot as expansion in the Riht Hot Gas Cylinder, which drives the piston to the left. This forces the hydraulic fluid in the Left Hydraulic Chamber into the hih pressure accumulator, and draws in more low pressure fluid into the Riht Hydraulic Chamber. This cycle is then repeated. Thus, the FPHP is able to produce power with each stroke, since the Check Valves ensure that the hydraulic fluid is drawn into each Hydraulic Chamber when the piston moves in one direction, and pumped out at hih pressure when the piston returns in the other direction. Since the area of the hydraulic 2 Copyriht 23 by ASME

3 piston is smaller than the hot as piston, pressure amplification is produced. This allows the FPHP to achieve hiher pressures in the hydraulic fluid. Hot Gas From Catalyst Bed Expandin Hot Gas (a) Hot as injected into cylinder Enterin Low Pressure Hydraulic Fluid FPA Velocity Exitin Hih Pressure Hydraulic Fluid (b) Hot as expands forcin FPA to the riht Althouh pure hydroen peroxide is desirable from an enery density standpoint, lower concentration 7% hydroen peroxide with 3% water and 9% hydroen peroxide with 1% water are less expensive and readily available for testin. The vaporization of the extra water in these lower concentration monopropellants further reduces the enery density, however. Table 1 outlines the monopropellant enery densities, and decomposition temperatures for various concentrations of hydroen peroxide: Concentration Enery Density Decomposition Temperature 1% 1.6 MJ/k 1269 K 9% 1.2 MJ/k 113 K 7%.4 MJ/k 56 K Exhaustin Hot Gas (c) Hot as vents throuh exhaust port Fiure 2. Operation of Free Piston Hydraulic Pump The desin of this enine is much simpler than existin IC enines. There are no cams, complex exhaust port routin, or fuel mixture requirements. There is only one basic movin part: the FPA. This simple desin results in a compact, reliable, and robust machine capable of a lon service life. Another important feature of this system is that as result of the simple radial eometry it can be manufactured fairly inexpensively. 3 H2O2 AS A MONOPROPELLANT FUEL Althouh the FPHP could make use of any monopropellant, hydroen peroxide was chosen for the prototype. Hydrazine, the most widespread monopropellant in the aerospace community because of its hih enery density, is carcinoenic and very costly to handle. Hydroen peroxide, on the other hand, has several characteristics makin it much safer to use. First, it has a very low vapor pressure allowin personnel to handle the monopropellant without respirator systems. Furthermore, by dilutin hih strenth peroxide with water, any immediate daners can be easily eliminated. Finally, the decomposition products of hydroen peroxide are hot steam and oxyen, which are nontoxic to humans. In addition to these benefits, since there is a relatively lare market for hih concentration hydroen peroxide in the textile and interated circuit industries, there is an infrastructure in place to commercially obtain the monopropellant. These advantaes make hydroen peroxide the best choice to study the FPHP in a laboratory environment. One hundred percent hydroen peroxide reacts accordin to the followin reaction: catalyticsurface H O ( l) 2H O( ) + O ( ) 1.6MJ / k (2) Table 1. Comparison of Various H 2 O 2 Concentrations 4 DYNAMIC ANALYSIS OF FPHP 4.1 Theoretical Modelin The dynamics of the FPHP are determined by the dynamics of the free piston assembly motion which are overned by: F = mx & = A ( PH PL ) A f ( PfH PfL ) F fric (3) where m denotes the mass of the FPA, & x& is its linear acceleration and ΣF is the sum of the forces actin on the FPA, which are illustrated in Fi. 3. No force is modeled on the back faces of the hot as pistons since both are well vented to atmosphere. A P H A fp fl F fric A fp fh A P L x F fric FrictionForce A Area of Hot Gas Piston A f Area of Hydraulic Fluid Piston P H Hot Gas Pressure on Hih Pressure Side P L Hot Gas Pressure on Low Pressure Side P fh Hydraulic Fluid Pressure on Hih Pressure Side P fl Hydraulic Fluid Pressure on Low Pressure Side Fiure 3. Free Body Diaram of FPA The hot as cylinder of the FPHP is modeled as a control volume with the hot ases enterin at the adiabatic decomposition temperature (T ad ) of the hydroen peroxide as illustrated in Fi. 4. Since each stroke occurs in a relatively 3 Copyriht 23 by ASME

4 short time, very little heat will be lost throuh the cylinder walls. The process is therefore assumed to be adiabatic. Decomposition Gases at T ad Catalyst Bed P H, x Control volume Fiure 4. Control Volume for Hot Gas Cylinder The enery balance for an adiabatic control volume with enterin as is: x d m & ihi W & = Esystem (4) dt where m& i is the mass flow rate of hot as into the control volume, h i is specific enthalpy of the as, W & is the rate of work done by the system on the surroundins, and E system is the total enery of the control volume system. The rate of work can be calculated from the FPA velocity, x&, the hot as pressure, P H, and the hot as piston area, A : W& = A P x& (5) Since the kinetic and ravitational potential eneries of the hot as are neliible, the total enery of the system is equal to the internal enery of the hot as. This internal enery, assumin an ideal as approximation, can be calculated from the as temperature, T, the total mass of the as, m, and the specific enery of the as, c v : E system H = U = m c T (6) The mass of the as can be also be expressed as the product of its density, ρ, the hot as piston area, and FPA displacement: m = ρa x (7) Assumin ideal as properties, the specific heat can be calculated from the as constant, R and the specific heat ratio k, which are known properties of the as: R c v = (8) Insertin Eq. 7 and 8 into Eq. 6 yields: A xρrt U = = E The ideal as law can be written as: P H RT Substitutin Eq. 1 into Eq. 9 yields: E v system (9) = ρ (1) A xph = system (11) Differentiatin Equation 12 with respect to time: d dt E system A = ( xp & + xp& ) H H (12) Since ideal as properties are assumed, the enthalpy of the incomin hot as can be determined from its temperature: kr hi = c pt = kcvt = Tad (13) Substitutin Eq. 5, 12, and 13 into Eq. 4 yields: P & x & = & H krt ad + kph x mi (14) A Althouh no detailed analyses of hydroen peroxide decomposition were found, past experimental results indicate a pure time delay of 37 msec between monopropellant injection and decomposition [6]. Thus, the mass flow rate of hot as into the hot as cylinder is approximated as the mass flow of monopropellant throuh the solenoid valve, m& mono, shifted by a delay time, τ, as illustrated in Fi. 5. m& i ( t) = m& mono ( t τ) (15) By combinin Eq. 14 and 15 and reorderin terms, an equation for the hot as dynamics is produced: 1 m ( t τ) krt P& & mono ad ( ) = kp ( t) x( t) H t x( t) H & (16) A Equation 4, and subsequently Eq. 16, assumes that the volume of the hot as cylinder is equal to zero when the FPA position, x, is equal to zero. Since the volume of the hot as cylinder is not zero when the FPHP beins a stroke, x can be defined as: x = x n + x clearance (17) where x n is the FPA position which is equal to zero at the beinnin of a stroke, and x clearance is the effective clearance lenth in the hot as cylinder: Vclearance x clearance = (18) A where V clearance is the volume of the hot as cylinder at the beinnin of each stroke. This extra volume includes any internal volume in the catalyst bed. 1 m ( t τ) krt P& & mono ad ( ) = kp ( t) x ( t) H t x ( t) + x H & (19) n n clearance A The dynamics of the mass flow of the monopropellant throuh the solenoid valve are estimated since there is no available data on specific valve dynamics other than the valve response time. The flow is modeled as a linear ramp to the steady state value over the valve response time as illustrated in Fi Copyriht 23 by ASME

5 solenoid valve response time decomposition time delay, τ t open t close time Monopropellant flow throuh solenoid valve Hot as flow into hot as cylinder Fiure 5. Model of Fuel Flow Throuh Valve For small injection times, which are less than the decomposition time delay, the pressure drop across the solenoid valve is constant, and the steady state monopropellant flow is calculated from the valve flow equation as: P m & mono, ss = ρcv γ (2) where ρ is the density of the monopropellant, C v is eometry dependent valve constant, P is the pressure drop across the valve and γ is the specific ravity of the monopropellant (ratio of density of monopropellant to density of water). Equation 19 is used to model the hih pressure as durin the initial portion of the expansion stroke. Once the hot as piston crosses the exhaust port, it is assumed that the exhaust ports are lare enouh to vent the hih as pressure to atmospheric pressure instantaneously. As the as in the hih pressure hot as cylinder expands, the as in the low pressure hot as cylinder is compressed. Durin the initial portion of the stroke, while the exhaust port is still uncovered, the low pressure hot as cylinder is still open to atmosphere so its pressure is assumed to be equal to atmospheric pressure. Once the low pressure hot as piston passes the exhaust port, the low pressure side behaves as an air sprin. Assumin the process is adiabatic, the pressure of the low pressure side is found from: k PL V = C (21) where V is the volume of the low pressure hot as cylinder, k is the specific heat ratio, and C is a constant determined from the pressure and volume when the low pressure hot as piston crosses the exhaust port. Since the pressure drops across the hydraulic check valves are small compared to the chanes in as pressures and the hih pressure hydraulic force, P fl and P fh are assumed to be constant with P fl set to the hydraulic reservoir pressure and P fh equal to the maximum load pressure of the fluid in the accumulator. Althouh the load pressure would vary in real applications, if the FPHP can pump aainst the maximum load, it can pump aainst all loads. Since there are no side loads on the FPA, the friction is not dependent on the location of the FPA, as with a piston connected to a crankshaft, so F fric is modeled as a constant. 4.2 Simulation Results The first FPHP prototype was desined for a taret power production of 2237 W (3 hp) at 6.9x1 6 Pa (1 psi) with an operatin frequency (f) of 1 Hz. The power output (P) of the FPHP is calculated from: P = 2P A L f (22) fh f stroke where P fh is the hydraulic pressure, A f is the area of the hydraulic piston, and L stroke is the stroke lenth of the FPA. Initial desin simulations were performed, assumin 9% hydroen peroxide and properties for off the shelf solenoid valves and catalyst beds, in order to determine a FPHP eometry which would provide the desired hydraulic power production. In order to maximize efficiency, the simulation varied the monopropellant injection time to find the minimum amount of injected monopropellant that would result in a successful stroke. The efficiency, ε, of the FPHP was then calculated as the ratio of work per stroke to the enery of the monopropellant injected (assumin an enery density, ED, of 1.2 MJ/k for 9% hydroen peroxide): ε = work extracted per stroke enery of fuel injected per stroke PfH A = m f mono L stroke ED (23) The simulation parameters, which represent the monopropellant properties, valve characteristics, and FPHP eometry of the taret prototype, are listed in Table 2. The monopropellant properties were taken from published data on hydroen peroxide [1]. The FPHP eometry, mass properties, hydraulic pumpin pressure, and reservoir pressure were taken from the desin parameters of the prototype FPHP [13]. The dry friction was estimated from the forces required to manually push the FPA while assemblin the pump. The steady state monopropellant flow throuh the solenoid valves was calculated from Eq. 2 usin the measured C v value of.15 (al/min/psi 1/2 ) and monopropellant tank pressure of 3.4x1 6 Pa (5 psi). The hot as cylinder dead volume and the catalyst bed volume were calculated from the FPHP prototype data, and the decomposition time delay was estimated from literature on past hydroen peroxide experiments [6]. The simulation, usin the parameters in Table 2, resulted in an estimated efficiency of 21% for the initial prototype with a monopropellant injection time of 19 ms. The simulation results showin the FPA displacement and velocity and hot as behavior over several cycles are shown in Fi. 6 and 7. Investiatin the time duration of each stroke, it can be seen that a FPHP with these parameters is able to execute a full cycle, consistin of a riht and left stroke, in approximately.12 seconds. Thus, accordin to the simulation, the FPHP can operate near the taret 1 Hz operatin frequency. 5 Copyriht 23 by ASME

6 H 2 O 2 Concentration 9% Gas Constant (R H22 ) 376 J/kK Specific Heat Ratio (k) 1.27 Adiabatic Decomposition Temperature (T ad ) 113 K Steady State Fuel Mass Flow ( m & mono, ss ).25 k/sec Decomposition Time Delay (τ) Hydraulic Pressure in Accumulator (P fh ) Hydraulic Reservoir Pressure (P fl ) Dry Friction (F fric ) FPA Mass (m) Hot Gas Cylinder Diameter.37 sec 6.9x1 6 Pa (1 psi) 2.8x1 5 Pa (4 psi) 44 N.544 k.465 m (1.83 in) Stroke Lenth.6 m (2.36 in) Gas Cylinder to Hydraulic Cylinder Area Ratio (A /A f ) 6.5 Clearance Volume (V clearance ) 7.5x1-5 m 3 (4.3 in 3 ) Table 2. Desin Simulation Parameters 5 EXPERIMENTAL FPHP In order to demonstrate the feasibility of the desin and the accuracy of the simulation, a prototype FPHP was desined and constructed followin the parameters listed in Table 2 [13]. 5.1 Hardware The peripheral mechanical components of the FPHP experimental system, shown in Fi. 8, can be rouped into two main systems: the monopropellant system and the hydraulic system. The monopropellant system controls the flow of monopropellant, which is pressurized to 3.4x1 6 Pa (5 psi), into the catalyst beds. In order to simulate a maximum hydraulic load of 6.9x1 6 Pa (1 psi), the FPHP pumps hydraulic fluid throuh a sprin loaded relief valve between the accumulator and reservoir with a relief pressure of 6.9x1 6 Pa (1 psi). H 2 O 2 tank Reservoir Accumulator 1 8 Piston Position (cm) Piston Velocity (m/s) Fiure 6. Simulation Results FPA Velocity and Position Hot Gas Pressure (psi) Fiure 7. Simulation Results for Hot Gas Pressure Enine/Pump Catalyst bed Fiure 8. Photo of the Experimental Power Source 5.2 Experimental Results The FPHP was tested usin a computer to control the time each solenoid valve was open. The best results were achieved by openin one solenoid valve for 5 ms and then waitin 45 ms before pulsin the opposite valve. Fiure 9 illustrates the recorded hot as pressures in both hot as cylinders over several cycles. Fiure 1 shows a more detailed view of the hot as pressures durin one stroke of the FPHP. Even thouh the FPHP successfully pumped hydraulic fluid, several undesirable phenomenon were observed durin testin, and the observed efficiency of 1.2% was sinificantly lower than expected. First, the FPA exhibited stiction-like behavior, with many of the strokes consistin of a series of small jerky motions instead of one smooth, continuous stroke. Second, most of the strokes resulted in a very slow exhaust of the hot as, which was observed by a hissin sound. Third, over the several cycles, the as pressures on both sides of the FPHP radually built up. Eventually, the excessive pressure on both sides prevented the FPHP from pumpin at all, and the FPA remained stationary. 6 Copyriht 23 by ASME

7 Gas Pressures, PH and PL (psi) Gas Pressures, PH and PL (psi) Left Fuel Valve Riht Fuel Valve Left Hot Gas Cylinder Riht Hot Gas Cylinder Fiure 9. Experimental Hot Gas Pressure with 5ms Injection Time Low Pressure Piston Hih Pressure Piston Fuel Valve Fiure 1. Experimental Hot Gas Pressure Over Sinle Stroke 6 DISCUSSION Durin the simulation, short injection times of 19 ms produced quick pressure pulses on the order of tens of milliseconds as shown in Fi. 7, while much larer injection times of 5 ms were required on the actual system, producin much more radual pressure rises and strokes lastin on the order of seconds as shown in Fi. 9. Upon careful evaluation of the experimental results and the prototype hardware this discrepancy may the result of poor delivery of the monopropellant from the solenoid valve to the catalyst bed. The required fittins to connect the solenoid valve and catalyst bed were rather lare, creatin a lare amount of empty volume between the solenoid valve and catalyst bed. Fiure 11 illustrates the interface between the solenoid valve and catalyst bed. Fuel Valve Empty Volume (1/4 Tubin) 3 Silver Catalyst Mesh Pressure Sensor Fiure 11. Diaram of Fuel Valve/Catalyst Interface The monopropellant leaves the solenoid valve as fine mist, which would ideally directly enter the catalyst mesh. Instead, it is believed that the mist strikes the walls of the tubin and collects in the tube. The monopropellant then slowly drains into the silver catalyst mesh. Thus it is hypothesized that the actual flow rate of monopropellant into the catalyst bed is much lower than the mass flow rate of monopropellant from the solenoid valve. In order to verify this hypothesis, several modifications were made to the oriinal simulation. First, the dead volume in the hot as cylinder was increased from 7.5x1-5 m 3 (4.3 in 3 ) to 1.15x1-4 m 3 (7. in 3 ) to account for the extra dead volume between the solenoid valve and the catalyst bed not accounted for in the initial desin. Second, the steady state mass flow rate was chaned from.25 k/sec to.1 k/sec to account for the hypothesis that the injected monopropellant collects in the space between the valve and catalyst, enterin the catalyst at a much lower rate. The results of the modified simulation, plotted aainst the experimental data in Fi. 12, support the hypothesis that the monopropellant is in fact poolin between the valve and catalyst. One important characteristic of both the experimental and simulated results in Fi. 12 is the smooth initial rise in pressure followed by oscillations in the pressure. This rise in pressure corresponds to the stae in the stroke when the FPA is stationary since the hot as pressure is not hih enouh to overcome the hih hydraulic force on the hydraulic piston. Lookin at Eq. 19, which overns the hot as pressure dynamics, the FPA velocity term, x&, is initially zero. Thus, the pressure chane is positive since the term overned by the monopropellant injected, m& mono, is always positive. Once the FPA beins to move, the velocity is no loner zero, and the neative FPA velocity term eventually dominates the positive monopropellant injection term since the actual rate of monopropellant enterin the catalyst bed is so small. This causes the time derivative of the pressure to become neative. As the hot as pressure drops, the hydraulic pressure slows the FPA. When the FPA is sinificantly slowed or sometimes stopped entirely, the positive monopropellant injection term of Eq. 19 aain dominates, causin the time derivative of the pressure to become positive. As the hot as pressure increases, it causes the FPA velocity to increase, resultin in a new drop in the hot as pressure. This cyclin of this process results in 7 Copyriht 23 by ASME

8 the pressure oscillations. These oscillations also account for the stiction-like behavior of the FPA, which was observed durin testin as the pressure oscillations caused FPA velocity to cyclically increase and then drop to zero. Hot Gas Pressure (psi) slow experimental exhaust instantaneous simulated exhaust Experimental Data Modified Simulation Fiure 12. Comparison of Experimental Data with Modified Simulation The collection of monopropellant between the valve and catalyst bed also accounts for the occasional increases in as pressure lon after the solenoid valves have been closed. A ood example of this is the rise in pressure in the riht as chamber in Fi. 1 at 14.2 seconds. This poolin of monopropellant also creates a steady eneration of hot as on both sides of the FPHP as the collected monopropellant slowly drains into both catalyst beds. This radual as eneration, alon with the slow ventin rates, accounts for the radual pressure rise in both hot as cylinders as seen in Fi. 9, which eventually prevented the FPA from movin. One difference that remains between the modified simulation and the experimental result is the ventin rate of the hot as at the end of the stroke. As a result of the poor delivery of monopropellant to the catalyst bed, the hot as pressures required to ive the FPA enouh momentum to fully uncover the exhaust port could not be achieved. Durin initial tests of the FPHP usin compressed air instead of decomposed hydroen peroxide to drive the piston, the exhaust port was uncovered and rapid exhaust was achieved, indicatin that the improvement of the delivery of monopropellant to the catalyst bed will also solve the ventin problem. 7 CONCLUSIONS The simple and compact desin of the FPHP allows inexpensive and robust power supply systems to be created. These systems, which offer a potential for improved enery and power density over electrical systems and the ability to produce intermittent power without idlin in oxyen free environments, could have applications in a variety of mobile robotics applications. Althouh the experimental prototype of the monopropellant driven free piston hydraulic pump was not able to produce the taret power output, it did demonstrate the feasibility of usin a monopropellant to drive a piston enine and pump hydraulic fluid. The analysis of the experimental results also revealed that the interation of the solenoid valve and catalyst bed is essential to improve the delivery of the monopropellant to the catalyst bed. This knowlede can be applied to future versions of this type of system to reatly improve performance. REFERENCES 1. McCormick, J.C., 1967, Hydroen Peroxide Rocket Manual, FMC Corporation. 2. Stokes, P.R., 1998, Hydroen Peroxide for Power and Propulsion, Presented at the London Science Museum, 3. Dismukes, K. (curator), 23, Auxiliary Power Units, NASA Human Spacefliht Website, spacefliht.nasa.ov/shuttle/reference/shutref/orbiter/apu/ 4. Johnsen, E.G. and Corliss, and W.R., 1967, Teleoperator and Human Aumentation, AEC-NASA Technoloy Survey, NASA Document SP Amendola, S.C. and Petillo, P.J., 21, Enine Cycle and Fuels for Same, United States Patent # 6,25,78 B1. 6. Barth, E.J., Goola, M.A., Wehrmeyer, J.A., and Goldfarb, M., 22, The Desin and Modelin of a Liquid-Propellant-Powered Actuator for Eneretically Autonomous Robots, 22 ASME International Mechanical Enineerin Conress and Exposition Paper IMECE22-DSC Beachley, N.H. and Fronczak, F.J., 1992, Desin of a Free-Piston Enine Pump, SAE paper Dalton, T.B., 1976, Dual Pressure Hydraulic Pump, United States Patent # 3,985, Heintz, R.P., 1978, Free-Piston Enine-Pump Unit, United States Patent # 4,87, Heintz, R.P., 1985, Theory of Operation of a Free Piston Enine-Pump, SAE paper Li, L.J. and Beachley, N.H., 1988, Desin Feasibility of a Free Piston Internal Combustion Enine/Hydraulic Pump, SAE paper Tikkanen, S. and Vilenius, M., 1998, On the Dynamic Characteristics of the Hydraulic Free Piston Enine, Second Tampere International Conference on Machine Automation. 13. Raade, Justin W., 23, Desin and Testin of a Monopropellant Powered Free Piston Hydraulic Pump, Masters Thesis, University of California, Berkeley, CA. 8 Copyriht 23 by ASME