Subject: Space-tech Digest #151 Contents: Bruce Dunn's P2 booster design: P2 1/5 Introduction P2 2/5 Design Description ------------------------------------------------------------ Date: Mon, 14 Jun 93 22:46 PDT To: space-tech@cs.cmu.edu Subject: P2 1/5 Introduction From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) It's time for everyone to take another crack at my evolving P2 booster. This is the third version of the launcher, and I have made a number of changes. Many of these were the result of finally getting software to calculate specific impulse for arbitrary propellant combinations, chamber pressures, and expansion ratios (provided generously by Mitchell Burnside-Clapp), and my development of a spreadsheet to calculate rocket nozzle expansion and thrust in relation to chamber pressure and physical dimensions. These tools have allowed me to treat expansion ratio and chamber pressure as variables to be optimized in the design of new engines for the booster system. I have split the posting up into 5 parts: Introduction (what you are currently reading) Design Description (what the P2 booster looks like) Design Choices (why the P2 is designed the way it is) Calculations (assumptions and methods for performance calculations) Economics (how the P2 can compete) I would like feedback of any type on the material. I have found the suggestions and comments of members of the space-tech mailing list invaluable in the evolution of the design and my thinking about it. I would particularly like to acknowledge Paul Dietz and George Herbert for their thoughts. The following changes have been made from the previous version of the proposal: The fuel has been changed from propane to RP-1. This change was made after detailed specific impulse calculations indicated that RP-1 was the superior propellant, having only slightly lower specific impulse than propane but a substantially better density. Chamber pressure is now 3 MPa compared with a previous 6 MPa. Tank mass and helium mass is down significantly, at the cost of lower expansion ratio and Isp. However trade studies indicated that for a given liftoff mass, the lower pressure vehicle can deliver the same payload, while having a lower dry mass. The upper stage has been changed to from a pump fed LOX/kerosene system to a pressure fed peroxide/RP-1 system. This gives a common storable propellant system for the upper and lower stages, at the expense of requiring a new upper stage engine. The use of storable propellants in the upper stage has several advantages for missions delivering cargo to a space station. Principally, it allows a system in which propellants can be positively expelled from tanks in free fall. This allows the main upper stage propellants to be used for orbital maneuvering, and allows their transfer to the space station. The turbopumps for liquid helium are eliminated. Liquid helium is now stored in a spherical pressure tank and pressurized with hot gaseous helium. This approach is made feasible by the lower chamber and tank pressure in the current version of the design. The P2 design now has no rotating machinery at all - the only moving parts are valves. The use of the SRB derived nozzle for the lower stage engine has been abandoned in favor of a newly designed ablatively cooled non-steerable nozzle. Thrust vector control is by peroxide injection into the nozzle. By assuming a new nozzle design, the throat and expansion ratio can be optimized for different chamber pressures (which was not possible with the SRB nozzle whose throat is fixed by the existing hardware). The vehicle has been resized to a diameter of 6.1 meters, and a point design consisting of an 800 ton lower stage and a 200 ton upper stage has been investigated. At these dimensions, the lower stage fuel tank is identical in construction to the upper stage peroxide tank, and both are perfect spheres. To keep the same name for the booster, the "P2" part of the name, which formerly meant propane/peroxide, now means "pressurized propellant". In the interests of simplicity, I have dropped the "E" part of the former P2-E name. It meant "expendable" and was meant to distinguish the second generation design from the original design which assumed considerable recoverability. -- Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca ------------------------------ Date: Mon, 14 Jun 93 22:51 PDT To: space-tech@cs.cmu.edu Subject: P2 2/5 Design Description From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) This posting describes the P2 family of low cost, pressure fed liquid fueled boosters. At this point, the design is at the "conceptual" stage, and no part of the vehicle has been subjected to a detailed design procedure. Component masses and performance are estimated from existing hardware and engineering practices. GENERAL DESCRIPTION The basic P2 booster is composed of one lower stage of 800 metric tons mass and one upper stage of 200 tons mass. Both stages burn RP-1 (refined kerosene) and hydrogen peroxide from pressurized tanks (P2 stands for "pressurized propellant"). Heavy lift variants use two or more lower stages as boosters, a lower stage as a "core", and an upper stage. P2 ENGINES The P2 booster uses pressure fed engines burning RP-1 and peroxide. The lower stage engine has a sea level thrust of 13,500 kN, while the upper stage engine has a vacuum thrust of 1000 kN. The engines are rigidly bolted to the bottom propellant tank in each stage, and propellants are supplied through rigid high pressure lines. The top of the combustion chamber contains an injector system which is fed propellants via throttling valves. Propellant flow and mixture ratio is adjusted on a feedback basis. If necessary, the engines can be throttled partway through the flight to lower aerodynamic forces or G loadings. The combustion chamber, throat and nozzle are ablatively cooled. Part way down the nozzle there is a ring of peroxide injection ports in the nozzle wall which are controlled by quick reacting flow control valves. In order to vector the thrust of the engine, peroxide is injected into one side of the nozzle and flashes into superheated steam and oxygen. This results in deflection of the main exhaust stream without requiring the complications of a movable nozzle. A further series of ports allow tangential injection of peroxide into the gas flow for roll control. The lower stage engine has a chamber pressure at full thrust of 3 MPa (approximately 450 psi), supplied by propellants pressurized to 4 MPa. The engine has a throat diameter of 1.98 meters, and a nozzle exit diameter of 5 meters. Expansion ratio is 6.4, which is slightly overexpanded at sea level. This chamber pressure was selected as the result of a trade studies; higher pressures allow higher expansion ratios and specific impulse, but add tank weight. The engine has a sea level thrust of 13,500 kN and a vacuum thrust of 15,500 kN (approximately 3.5 million pounds force). For the purposes of calculations, the engine is estimated to have a mass of 15 metric tons. The upper stage engine has the same chamber pressure, a throat diameter of 0.58 meters, a nozzle exit diameter of 3 meters, and an expansion ratio of 44. Vacuum thrust is 1000 kN. For the purposes of calculations, it is estimated to have a mass of 1.25 metric tons. Isp values are taken as being 95% of theoretical values obtained by direct calculations (which take into account propellant chemistry and mixture ratio, chamber pressure, expansion ratio, and ambient pressure). The lower stage engine is estimated to have a real-world specific impulse of 231 at sea level and 276 in a vacuum, while the upper stage engine is estimated to have a vacuum specific impulse of 314. DESIGN OF THE P2 LOWER STAGE The basic lower stage has a mass of 800 metric tons at liftoff. Larger versions of the stage can be built by stretching the propellant tanks. All stages are 6.1 meters in diameter and have a single engine. Propellants are contained in high strength welded steel tanks and are pressurized by room temperature helium gas. Separate fuel and oxidizer tanks with no common bulkhead are used. The fuel tank is a 6.1 meter diameter sphere, while the oxidizer tank is a cylinder with hemispherical ends with a total length of 18 meters. Tank wall thicknesses range from approximately 5 mm thick (fuel tank and end domes of oxidizer tank) to approximately 10 millimeters thick (barrel section of the oxidizer tank). The RP-1 tank is on top of the peroxide tank, and an internal fuel line descends through the oxidizer tank from the RP-1 tank. The space between the fuel and oxidizer tank is enclosed by a light non-load bearing fairing. Propellants are pressurized with warm helium gas. Liquid helium is stored in an insulated 3.5 meter aluminum sphere, designed for an operating pressure of 4.5 MPa. Liquid helium is expelled from the sphere with helium gas, provided by helium from a smaller high pressure storage sphere. The liquid helium is vaporized and warmed by a gas generator which burns pressurized peroxide and fuel from auxiliary propellant tanks, and mixes the hot combustion gases with the helium stream. The resultant helium (carrying a small amount of CO2 and water) is conducted to the propellant tanks. To maintain the temperature of gaseous helium from the high pressure storage sphere as it is expanded from its storage pressure, a controlled portion of the gas is run through a heat exchanger in the gas generator before being used to pressurize the liquid helium and the gas generator reactants. All components of the tank pressurization system (liquid and gaseous helium tanks, gas generator and gas generator reactants) are located in the interstage area, on top of the fuel tank and below the nozzle of the upper stage. This space is surrounded by a cylindrical interstage structure which has the same diameter as the upper and lower stages. The interstage joins the upper and lower stages, and also acts as a bay for the tank pressurization equipment, avionics etc. Because the main engine nozzle exit cone diameter (5 meters) is substantially less than the booster diameter (6.1 meters) and the nozzle does not swivel during thrust vector control, the vehicle can be directly supported on the pad, with the nozzles protruding through holes in a platform. There is no need for the "skirt" assembly of the Shuttle solid rocket boosters, which protrudes in order to provide pad support points for the booster which is narrower than its nozzle. MASS BUDGET (METRIC TONS) FOR LOWER STAGE The following dry masses are independent of the size of a P2E stage - all masses are in metric tons. Masses are estimates, subject to revision upon execution of more detailed designs. The figures below refer to a "core" lower stage, bearing an upper stage on top. Boosters would have similar masses, but substitute a nose cone for the interstage adapter and are provided with a solid rocket stage separation system derived from SRB hardware. Engine 15 Gas generator and plumbing 1 Inter-tank structure (non load bearing) 1 Avionics, range safety 1 Inter-stage structure 5 Miscellaneous, including thermal protection 4 Growth Margin 3 Fixed mass total 30 Actual stage mass includes the 30 tons of fixed mass, plus propellants, plus tank mass, plus helium and helium storage tank masses. The masses involved are listed below for a 800 metric ton stage; overall mass fraction (usable propellant fraction) is 0.913 Main propellants 734 Helium 2.5 Fuel tank 5.3 Oxidizer tank 25.7 Liquid helium tank 0.9 Gaseous helium tank 0.9 DESIGN OF THE UPPER STAGE: For baseline performance calculations for launchers using P2 boosters, an upper stage of 200 metric tons mass has been assumed. This size is near optimum for a two stage vehicle using an 800 ton lower stage, and is easily stretched for a larger vehicle. General layout and construction of the stage is the same as that of the lower stage. The oxidizer tank is a 6.1 meter sphere (identical to the lower stage fuel tank for economy of production), surmounted by a 3.9 meter spherical fuel tank and a 2 meter liquid helium tank. Tank pressure is 4 MPa, and wall thickness is approximately 5 mm for the oxidizer tank and 3 mm for the fuel tank. The overall mass fraction of the upper stage is 0.927. As with the lower stage, the space between the fuel and oxidizer tank is surrounded by a fairing, and tank pressurization equipment is all situated at the top of the fuel tank. The arrangement of a spherical fuel tank on top of a larger spherical oxidizer tank gives a semi-conical shape to the upper stage, and results in the top of the upper stage being easily adaptable to payload support structures and payload shrouds of approximately 4 to 5 meters in diameter, to accommodate Shuttle or Titan class payloads. LAUNCH SEQUENCE: A launch begins by filling the tanks with storable propellants from tanker trucks. Several hours before launch time, the liquid helium reservoirs in the upper and lower stages are filled. The helium reservoirs are heavily insulated, so long holding times are possible with little helium boiloff, and continued top-up of the tanks immediately prior to launch is be needed. Just before launch, the tank pressurization system is activated. As the propellants reach full pressure, the engine throttling valves are opened to allow propellant into the combustion chamber, and a pyrotechnic igniter starts the engine. Once full thrust is reached, the vehicle is launched. The launch sequence may be aborted by closing the throttling valves while simultaneously shutting down the helium pressurization system. Sufficient helium is carried to pressurize the propellant tanks to the full rated pressure with warm helium during the expulsion of two thirds of the propellant load. The tanks are then allowed to blow down during the remainder of the burn. By the end of the burn, the helium pressure (and thus thrust) drops to approximately half the initial value as a result of the cooling and volume increase during adiabatic expansion. The thrust decrease in the last third of the burn acts to limit the acceleration of the vehicle. PERFORMANCE: Configuration 1: Two stage P2 This is the basic launcher in the P2 family. It is sized to give a payload somewhat in excess of common commercial launchers, and employs only two stages to put a payload into low earth orbit. Lower stage: 800 metric tons, Isp 231 SL, 276 vac, MF = 0.913 Upper stage: 200 metric tons, Isp=314 vac, MF=0.927 Payload to LEO: 14.6 metric tons gross (including shrouds) Lower stage acceleration: 1.35 g to 2.78 g, staging at 3160 m/sec Upper stage acceleration: 0.48 g to 1.75 g Configuration 2: Three stage P2 This is a heavy lift variant, using largely using stock components from the basic 2 stage launcher. A stretched core stage (lengthened tanks), the upper stage, and the payload are stacked vertically, as for configuration 1. The core stage is flanked by two boosters. At takeoff, both of the boosters and the core stage ignite. No cross-feeding of propellants is used during the parallel burn. Booster stages: 2 x 800 metric tons, Isp 231 SL, 276 vac, MF=0.913 Core stage: 1204 metric tons, Isp 231 SL, 276 vac, MF=0.923 Upper stage: 200 metric tons, Isp=314, MF=.927 Payload to LEO: 50.6 metric tons gross (including shrouds) Booster stage acceleration: 1.35 g to 2.75 g, staging at 3140 m/sec Core stage acceleration: 2.18 g to 2.30 g, staging at 5160 m/sec Upper stage acceleration: 0.41 g to 0.77 g final These performance estimates indicate that heavy payloads can be orbited with an extremely unsophisticated, albeit very large vehicle. The payload of the nominal 1000 ton vehicle considerably exceeds that of the commercial Delta, Atlas and Ariane 4 launchers, although it is less than that of Titan launchers. The payload of the vehicle with two boosters exceeds that of any current US launcher and approaches that of various proposed shuttle derived launchers. Even higher payloads would be possible by using four boosters rather than two, using a hydrogen/oxygen upper stage, or both. With four boosters, a core, and a stretched peroxide/RP-1 upper stage, approximately 90 metric tons can be delivered to low earth orbit. If an upper stage powered by a single SSME is used, the vehicle can deliver 158 metric tons to low earth orbit, which exceeds the payload capability of the Saturn V. PAYLOAD DELIVERY MISSIONS: For the boosting of communication satellites, the P2 employs a third stage to complete the delta V required to place the satellite into a geosynchronous transfer orbit. In the near term, this third stage is one of several solid upper stages available commercially. Because the P2 does not need to establish a low earth orbit before igniting the third stage (as does the Shuttle and proposed SSTO vehicles), the staging velocity for the third stage is a free parameter and the distribution of velocity increments between the stages can be optimized. To some extent this will allow placing higher payloads in a geosynchronous transfer orbit than would be allowed by using the same stage from the Shuttle, or a hypothetical SSTO vehicle in low earth orbit. For delivery of cargo to a space station, a modified P2 upper stage is used. Both the peroxide and RP-1 tanks of this version are provided with a flexible membranes separating the pressurizing gas and the propellant, and allowing positive expulsion of the tank contents in free fall. The modified upper stage has an attitude control system using hydrogen peroxide thrusters, and carries a small restartable bipropellant orbital maneuvering engine for circularization, phasing, and rendezvous burns. After payloads have been delivered to the station, residual propellants (left over from unused performance margins) can be salvaged and transferred to the station. This is much easier with storable propellants from diaphragm equipped propellant tanks than it is with cryogenic propellants from a pump fed upper stage. Just enough propellant would be left on the P2 upper stage for a de-orbiting burn. The salvaging of unused propellants from the upper stage will act as a built-in form of load balancing, making sure that the payload capability of the P2 is always fully utilized. The delivered propellant can be stored indefinitely on orbit, and used when needed for attitude control (using hydrogen peroxide thrusters) or station re-boost (using a small bipropellant engine). Interestingly, nearly 90% of the mass of the propellant from unused flight margins is hydrogen peroxide. This dense storable material is simultaneously a good oxidizer for bipropellant engines, a good monopropellant for attitude control, and (after decomposition) a potential source of both water and oxygen for crew resupply purposes. CONSTRUCTION AND OPERATIONAL ASPECTS: The P2 boosters are very simple. The tanks are heavy walled and welded, and need neither the aircraft-style light weight construction of conventional boosters, or the carefully shaped, elaborately jointed heavy segments needed to construct a large segmented solid rocket. It would probably be easiest to weld together the tank sets near the launch pad, using shaped steel segments shipped from a primary supplier. The tanks would be hydrostatically tested, then used to assemble booster elements. The development of the P2 family would start by building the minimum 2 stage launcher, which even conservative calculations suggest has the capability to lift most current payloads with considerable spare capacity. The main items requiring development are the two new engines and two helium gas generator systems. Engine testing should be easy, as no turbo-pumps or complicated startup sequences are involved. By design, the gas generator systems are independent of the engines, and can be developed and tested without a rocket test stand. Once the basic P2 vehicle was operational, it would go through an initial flight test program designed to demonstrate its performance, and establish a preliminary reliability record. Once several successful flights have been achieved, it would be able to compete with existing launchers. Until a more extensive reliability record has been built up, the launcher would probably have to compete on the basis of superior price or payload. Because of its inherent simplicity however, it is likely that in the long term the launcher will be very reliable, and become attractive on that basis as well. With the establishment of ongoing production of P2 lower and upper stages for the minimum launcher, boosted P2 variants would become feasible with little development cost. -- Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca Date: Mon, 14 Jun 93 22:54 PDT ------------------------------ End of Space-tech Digest #151 *******************