PISTONLESS DUAL CHAMBER ROCKET FUEL PUMP

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1 39 th AIAA/ASE/SAE/ASEE Joint Propulsion Conferene and Exhibit AIAA July 2003, Huntsville Alabama PISTONLESS DUAL CHABER ROCKET FUEL PUP Steve Harrington, Ph.D. Flometris, In. Solana Beah, CA Abstrat A positive displaement pistonless roket fuel pump uses two pumping hambers alternately filled and pressurized in sequene to maintain a steady flow of pressurized propellant to a roket engine. This pump fills the gap between pressure fed and turbopump rokets by making a lower ost roket feasible without the weight of a pressure fed design or the high ost and omplexity of a turbopump. The pump, ombined with a lower pressure tank, saves up to 90% of the tank weight in a omparable pressure fed system. Thrust to weight ratios are alulated for the pump using typial fuel ombinations. For a 2219 aluminum LOX/RP-1 pump at 4 Pa the thrust/weight ratio of the pump is ~700. Design and test data for a prototype whih pumps water at 3.5 Pa and 1.2 kg/s is presented. The simple onstrution of the pump allows for low ost, reliable propulsion systems. This pump has been tested with liquid nitrogen and kerosene. It has also been used to pump kerosene with a roket engine. Introdution With the advent of low ost ablative liquid fueled roket engines and omposite tanks, the problem of propellant pressurization beomes the last stumbling blok to affordable launhers. Turbopumps are urrently used in the majority of launh vehiles, although piston pumps have been designed and flown 1 and pneumati diaphragm pumps have been proposed by Godwin 2 and Sobey. 3 The pump onsidered herein is muh simpler and less expensive than a turbopump. The pump onept is simple: instead of having the whole fuel tank pressurized to 2-7 Pa, the main tank is at pressurized to kpa and it is drained into a pump hamber, and then the pump hamber is pressurized to deliver fuel to the engine. An auxiliary hamber supplies fuel while the main pump hamber is being refilled. This type of pump has benign failure modes, an be installed in the fuel tank to minimize vehile size and uses inexpensive materials and proesses in its onstrution. With the right hoie of materials, the pump will be ompatible with all ommon roket fuels. The pump an be started instantly, with no spool up 1 time required. It an be run until the tank is dry with no onerns about avitation or overspeeding. The simpliity and low ost of the pump allows for systems with engine out apability or allows for the use of tri-propellant systems. This pump lends itself to mass prodution tehniques for low ost systems with multiple engines and tanks. The pump an be easily saled up or down with no loss of performane. The pump an be stored for a long time with no degradation. Basi Pump Design The basi pump design is shown in Figure 1. In this design, two pumping hambers are used, eah one being alternately refilled and pressurized. The pump is powered by pressurized gas whih ats diretly on the fluid. The pump is designed so that the time required to vent, refill and pressurize one pumping hamber is less than the time to dispense a given quantity of fuel from the other. Figure 1 Basi Pump design The pump ontrols are set up so that when the level in one side gets low, the other side is pressurized; and then after flow is established Copyright 2003 Steve Harrington. Published by the Amerian Institute of Aeronautis and Astronautis, In., with permission.

2 from both sides, the low side is vented and refilled. This results in steady flow and pressure. A model of this pump was designed and built out of lear plasti, and it performed as expeted delivering steady flow and pressure 4. The pressures and flow rates were measured and the data was analyzed to determine how to improve the pump performane. Pump design Considerations Although the pump design is simple, the optimization proess is not. aking the pump yle as fast as possible would make it lightweight, but higher flow veloities ause problems. A pump with a small hamber must be filled and vented quikly, with minimal head loss through the gas and liquid valves and plumbing. The maximum inflow rate is limited by the main tank pressure (usually about 300 kpa) and the area of the inlet valves. Also, if the inflow veloity is too high, the propellant will be aerated, whih may ause problems with the engine. The ullage volume in the pump hamber should be small to minimize gas usage, but if it is too small, there will be a loss of propellant through the vent. Furthermore, the pump yle frequeny must not exite any ombustion instabilities in the roket motor. The seond generation pump design proess started with the realization that by plaing the pump hamber inside the main tank and inreasing the size of the hek valves, the pump an be filled very quikly. One the pump is being filled muh faster than it is emptied, it beomes lear that the two hambers do not have to be symmetrial. The optimized pump design is shown in Figure 2. Instead of two similar pump hambers, it uses one main hamber whih supplies fuel for most of the time and an auxiliary hamber whih supplies fuel for the rest of the time. The main hamber is plaed inside the tank, and it is filled through a number of hek valves so that it an be filled quikly, thereby reduing the size of the auxiliary hamber, whih is typially one fourth the size of the main hamber. The optimized design offers a substantial weight savings over the basi design, in that it uses one primary pumping hamber and one auxiliary hamber instead of two pumping hambers. A prototype of this type of pump is shown in Figure 3. The tank is made of stainless steel, the valves are brass, and the seals are Teflon so that it an be used to pump LOX. Figure 3 Photo of pump assembly with flange for attahment to tank. The prototype inludes a flange to easily attah it to the bottom of a tank. The prototype uses ylindrial tanks instead of spherial for ease of manufature. It weighs approximately 6.8 kg exlusive of the air valves and the bottom flange. Figure 2 Prototype optimized pump design Optimized Pump Design Pump Weight One of the most important benefits of this pump is the low weight for a given propulsion system. The weight may be alulated by determining the weight of the pump hambers and the valves. For valves or hambers, the weight is found to be proportional to the flow rate and the pressure. The weight of the hambers an be easily figured as spherial or ylindrial pressure vessels. The weight of the fluid and pneumati valves may be estimated based on the weight of ommerially available hek valves and atuated butterfly valves. For example an aluminum 200 series 2

3 Cirle Seal hek valve flows up to 200 liter/minute at 20 Pa for a 25 mm size whih weighs 250 grams. Assuming that the weight is proportional to the design pressure times the flow rate, a valve for use at 4 Pa and 200 liter/minute would weigh 50 grams. If eah pump requires 5 valves as in Figure 3, then for a pump whih supplies 200 lpm at 4 Pa, the valves would weigh 250 grams. Note that not all the valves will be the same design, but we are assuming an average weight. If the pump supplies roket fuel to an engine at a given mass flow rate, the weight of the valves may be alulated as a funtion of the engine thrust. Sine the thrust is proportional to mass flow rate, a thrust to weight ratio for a set of valves may be determined. For example, if a set of roket pumps supply LOX and kerosene at 200 liters/minute to an engine with a speifi impulse of 285 s, the thrust would be about 8800 N. This gives a thrust to weight ratio for the fluid valves as The gas valves will be substantially lighter. Therefore the valves an be onsidered to be a small perentage of total pump weight. The weight of a pressure vessel an be found as a funtion of volume, pressure and the speifi strength of the material. The required volume for the main pump hamber V is: V = Q T yle (1) Where Q is the propellant flow rate and T yle is the yle time of the pump. We will use spherial pump hambers in the alulations that follow. Cylindrial vessels would be heavier. The diameter of a spherial pump hamber D is given by: V 3 6 D = (2) π The required thikness for a spherial vessel made from a material with a given maximum stress is given by Roark 5 : P f D t = (3) 4 σ Where P f is the fuel pressure and σ is the allowable stress. The mass of the hamber, an be omputed by using the thikness t, the area of the spherial hamber, and the density of the hamber material, ρ : 2 = t π ρ or D Pf = π D 4 σ 3 ρ (4) Beause the hamber size is a funtion of the flow rate, we an put the hamber mass in terms of the flow rate: Pf 6 Q Tyle = π ρ σ π 4 or Pf 1 Q Tyle ρ (5) σ =. 5 In order to alulate the thrust to weight ratio, we need to determine the pump required for a given propellant and thrust. The thrust T is given by the momentum equation for the ase of ideal expansion: T = Q ρ f g I sp (6) where ρ f is the average density of the propellants, g is the aeleration of gravity and I sp is the speifi impulse of the propellants at the fuel pressure. The optimized pump mass is the mass of one full size hamber, one ¼ size hamber, 5 hek valves and three or four air valves. Therefore, the total mass of both pump hambers is 125% of the mass of one hamber. If we assume that the valves and the ullage add another 25% to the pump mass, the total pump mass is or 1.56 times the hamber mass. Now we an alulate the thrust to weight W ratio for the pump: T ρ f g I sp =. 43 (7) W Tyle P ρ f σ This equation applies to a single pump in the ase of a monopropellant, or to a number of pumps for bipropellant systems. Beause the pump mass sales linearly with flow rate, the flow an be divided among a number of pumps. If the pumping hamber is made ylindrial instead of spherial the weight will be twie as muh, but it may be easier to integrate into a fuel tank. The thrust-to-weight ratio an be alulated for a number of propellant ombinations: Using the equation (7) above with a yle time of 5 seonds, and density and speifi impulse data from Huzel and Huang 6 for engines running at 4 Pa at sea level, pump thrust-to-weight ratios were omputed for typial roket fuels Aluminum with a design stress of 350 Pa and a density of 2.8 gm/ was assumed to be the pump material. The pressure drop through the 3

4 injetors was not inluded. Higher T/W an be ahieved by using titanium or FRP pump hambers. Propellant Average Density (kg/m^3) ixture ratio I sp (se) Pump Thrust/ Weight LOX/RP LOX/LH H2O2/RP N2O4/N2H Table 1. Pump Thrust-to-weight ratios. Reent testing indiates that the yle time may be redued to 1 seond or less. Commerial diaphragm pumps operate with a 1 seond (60 RP) yle time. Therefore the numbers above are onservative, even as safety fators are added in. As far as the salability of the pump is onerned, for spherial pumps with a similar time to fill, the flow veloities need to sale linearly with the pump size. The veloity through the filling hek valves is a funtion of main tank pressure, so larger pumps will require more or larger inlet hek valves. However, if the pump hamber is made larger in diameter, but not taller, the pump will sale with minimal hanges. Pressure Fed Weight Savings Pressure fed systems inlude the weight of a high pressure tank, whereas the pump fed system inludes the weight of a low pressure tank and the pump. Assume both systems use a similar high pressure gas supply. The weight savings of the pump fed system an be alulated based on the fuel pressure and the burn time. Note that the pump onsists of hambers whih hold the same pressure as the tank in a pressure fed system, but the pump is muh smaller, and the tanks in a pump fed system are muh lighter than in a pressure fed system beause they need to hold a muh lower pressure but are of the same volume. The weight of any pressure vessel is proportional to the volume of the vessel and to the pressure inside the tank. Higher pressures require thiker, heavier walls. Given: The volume of the pump hamber is equal to the flow rate times the yle time The volume of the tank is equal to the flow rate times the burn time. Therefore the ratio of the pump hamber volume (or mass) to the tank volume (or mass) is equal to the ratio of the yle time to the burn time. This allows us to alulate the pump mass for a given fuel volume and pressure as a funtion of the tank mass for an equivalent pressure fed system. The ratio of the pump hamber mass to the pressure fed tank mass is the ratio of the yle time to the burn time. The mass of the pump is 1.56 times the mass of one pump hamber. (See eq. 7).. Furthermore the tank weight is proportional to the fuel pressure, so when we replae a pressure fed system with a pump fed system, the mass of the pump fed tank is equal to to the fuel pressure divided by the tank pressure times the mass of the pressure fed tank. Therefore we an alulate the ratio of the weight of a pump fed system to the weight of a ass Savings over pressure fed system ass Savings 100% 80% 60% 40% 20% Delivery Pressure 2.4 pa 4.2 pa 6.2 pa 0% Burn Time (seonds) Figure 4: ass savings as a funtion of burn time and delivery pressure. 4

5 pressure fed system as follows: (the first term is the mass of the pump and the seond is the mass of the tank) pumpsys Tyle Ptan k = (8) pressure _ fed Tburn Pfuel Where pumpsys is the mass of a pump fed system inluding the pump and the tank and pressure_fed is the mass of the tanks in a pressure fed system. T yle is the yle time of the pump, usually 1 to 5 seonds and T burn is the burn time of the roket, usually 120 to 300 seonds. P tank is the pressure in the tank, usually 300 kpa and P fuel is the fuel pressure delivered to the roket engine, usually 2.4 to 8 Pa. The pump fed system is 5 to 10 times lighter than the pressure fed system for burn times longer than a minute. The graph in Figure 4 shows the mass savings for a system with a 5 seond yle time and 300 kpa tank pressure. odel Design and Test Results A model has been designed that performs as expeted pumping water at 1.2 kg/s and 3 Pa. See Figure 4. A onservative yle time of 6 seonds was used for these preliminary tests. The main hamber supplies fluid for 5 seonds and the auxiliary hamber supplies fluid for 1.5 seonds, allowing approximately 250 ms of overlap during eah valve swithover. The model is onstruted of off-the-shelf industrial and onsumer valves, level sensors and fittings. The sequening is ontrolled by the same omputer that aquires data on the pump operation. Note that there are 20 mse wide pressure spikes as the seond hamber is pressurized. The minor pressure spikes at swithover may be due to the mass of fluid in one pumping hambers slowing down as the other pumping hamber starts to flow during the overlap time when both hambers are pressurized. Pistonless Pump Output Pressure Pumping water at 1.2 kg/s with Compressed Air Pressure Out (pa) Time (seonds) Figure 5 Prototype pump output. Integration Into Roket Vehile To use the pump in a vehile, a pump would be plaed in both the oxidizer and fuel tanks and a third pump or a set of pumps would be used to supply pressurant to a gas generator. The pressurant pump(s) would run on pressurized helium or air. The pump(s) would either supply propellant(s) to a gas generator or liquefied gas to an engine-mounted heat exhanger. Heavy pressurant tanks would not be required. Beause the pump weight sales linearly with flow rate and pressure there is no penalty assoiated with size. Therefore the roket vehile an use a number of independent tanks, engines and pumps to ensure redundany. For example, there ould be 6 or more propulsion modules arranged in a ring, If one system failed, the one on the opposite side ould be shut down, the remaining engines ould be throttled up and the vehile ould ontinue. Also, beause the pump hamber is relatively small, the fuel pressure an be easily ontrolled to vary the thrust without having to vent pressurant. Analysis of the system and mission will determine the optimum hamber pressure for the vehile, whih will 5

6 probably be higher than for a pressure fed vehile and lower than for a vehile whih uses a turbopump. Testing with Liquid Nitrogen The pump has been tested with liquid nitrogen and it works well. Figure 6 shows the test in progress, the pump is inside the tank. Figure 6 Liquid nitrogen test. When pumping liquids near the boiling point, the liquid must be prevented from boiling exessively during the vent yle. This may be ahieved by venting through a bak pressure regulator or by shutting the vent when the pump hamber pressure falls below a preset level. This is partiularly important when the pump is used at altitude or in spae. Testing with Atlas Vernier Engine The pump has been tested with an Atlas vernier engine as a proof of onept test. The pump worked well maintaining pressure even as the engine suffered an o-ring failure whih aused exessive fuel flow. The pump delivered kerosene at 2.8 pa and 1.1 kg/s. The LOX was pressure fed for this test. Figure 7 shows the engine running with kerosene supplied by the pump. Conlusions and Further Work The pump has been shown to be a viable alternative to turbopumps, with a omparable thrust to weight ratio. It is lear that the pistonless pump will ost at least 10 times less than a turbopump. It is also lear that the pistonless pump will be more inherently reliable than a turbopump, with no issues related to bearings, seals or vibration. Further optimization of the pump design will result in redued yle times and better pump thrust to weight ratios.. One potential use for the pump would be to pump liquid Nitrogen or liquid Helium through a ombustion hamber mounted heat exhanger to provide the gas to operate other versions of the pump whih would be pumping propellant. 1 Whitehead, J.C., Pittenger, L.C.,Colella, N.J. Design and Flight Testing of a Reiproating Pump Fed Roket, AIAA , Godwin, Felix Exploring the Solar System Plenum press Sobey, Albert J. US patent 3,213, Warren C. Young, Roark's formulas for stress and strain Graw-Hill, Dieter K. Huzel, David H. Huang, odern Engineering for Design of Liquid- Propellant Roket Engines (Progress in Astronautis and Aeronautis, Vol 147); Figure 7 Atlas vernier engine running with fuel pumped and LOX pressure-fed. 6