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N93- 1G911 //7"o's//' Transport: Introduction William Lewis and Sanders D. Rosenberg The propulsion workshop addressed these scenarios. Descriptions of the current status and future these technologies can be found in requirements for space propulsion the individual contributions that by considering the demand for follow this introduction. transportation in the three scenarios defined by workshop 1. The low- It appears that current oxygen- growth scenario assumes no hydrogen propulsion technology utilization of nonterrestrial resources; is capable of meeting the the two more aggressive scenarios transportation requirements of all include the use of nonterrestrial scenarios. But, if this technology is resources, particularly propellants. used in conjunction with advanced The scenarios using nonterrestrial propulsion technology, a much resources demand that tens of more efficient space transportation thousands of tons of rockets, system can be developed. Oxygen propellants, and payloads be from the Moon promises to shipped through cislunar space by significantly reduce the yearly 2010. Propellant oxygen derived tonnage on the transport leg from from the Moon is provided in the the Earth to low Earth orbit (LEO). second scenario, and propellants Hydrogen from Earth-crossing from asteroids or the Mars system asteroids or from lunar volatiles are provided in the third. The (in cold-trapped ices or the lunar scenario using resources derived regolith) would offer further only from the Earth demands much improvement and reduce less shipping of hardware but much propulsion technology challenges. more shipping of propellants. Mars missions are supportable by propellants derived in the We included in our examination a Mars system, probably from range of technologies that could Phobos. Unfortunately, these be developed to meet the opportunities cannot be taken transportation requirements of at current funding levels. 82 PRECEDING P/IIGi; _LA_K NOT FILMED The NASA baseline scenario is The nonterrestrial resource shown in figure 1. This scenario scenarios, figures 2 and 3, initially assumes the development of a follow almost the same path but, space transportation network without after the space station is established, utilization of nonterrestrial resources. move less toward GEO and more The space station is developed first toward the Moon. In addition, these and used to support development in scenarios consider selective mining geosynchronous Earth orbit (GEO), of asteroids that cross the Earth's manned exploration of the Moon, and orbit. Nonterrestrial resources are unmanned exploration of the solar used to reduce transportation and system. Beyond the timeframe construction costs for projects in considered, the space station can cislunar space. Eventually, the serve as a base for lunar settlement space station and lunar base serve and manned Mars exploration. as production and staging areas for manned Mars exploration. Date 1990 1995 2000 2005 2010 2015 2020 2025 2030 "--" I, I,,,,I ,I , , ,I, I, ,I 1, 1991 1996 1999 2008 2012 2016 2022 2031 1997 2014 2O24 1998 Sitesurvey Manned Camp Figure 1 Mars Mars sam•ple return (rover) Baseline Scenario If NASA continues its business as usual Mapper _ Base without a major increase in its budget and (orbit) : Explorer Camp without using nonterrestrial resources Moon -" w° as it expands into space, this is the • ° development that might be expected in :::- the next 25 to 50 years. The plan shows : : Experimental an orderly progression in manned missions :-" platform Outpost from the initial space station in low Earth GEO Manned station orbit (LEO) expected in the 1990s, through :- !$ an outpost and an eventual space station = £- in geosynchronous Earth orbit (GEO) Space : u_ (from 2004 to 2012), to a small lunar base in station 1 .= 2016, and eventually to a Mars landing in LEO 2024. Unmanned precursor missions ,_ Growth would include an experiment platform in : space station : GEO, lunar mapping and exploration by robot, a Mars sample return, and an ,'Ill R Earth ,-"1 I I automated site survey on Mars. This plan Shuttle-based Manned orbital Shuttle-derived can be used as a baseline scenario orbital transfer transfer vehicle launch vehicle against which other, more ambitious plans vehicle can be compared. 83 Figure 2 Date 1990 1995 2000 2005 2010 2015 2020 2025 2030 t I I I -I I I I 1 Scenario for Space Resource Mars Utilization sample Site survey Manned Camp Space resource utilization, a feature t'eturn (rover) landings Mars lacking in the baseline plan, is emphasized in this plan for space activities in the same 1990-2035 timeframe. As in the basefine scenario, a space station in low Earth orbit Mine Explorer (LEO) is established in the early 1990s. Near-Earth This space station plays a major role in asteroids staging advanced missions to the Moon, beginning about 2005, and in exploring near-Earth asteroids, beginning about the Mapper • same time. These exploration activities (orbit) : Explorer Camp Moon lead to the establishment of a lunar camp and base which produce oxygen and possibly hydrogen for rocket propellant. Automated missions to near-Earth Experimental platform iOutpost asteroids begin mining these bodies by GEO about 2015, producing water and metals which are returned to geosynchronous i e_ Earth orbit (GEO), LEO, lunar orbit, and Space the lunar surface. Oxygen, hydrogen, and station :on Growth space station metals derived from the Moon and the LEO / near-Earth asteroids are then used to fuel Shuttle-based orbital transfer vehicle space operations in Earth-Moon space Manned orbital transfer vehicle and to build additional space platforms Shuttle-derived launch vehicle and stations and lunar base facilities. III I I I i These space resources are also used as Earth fuel and materials for manned Mars missions beginning in 2021. This scenario might initially cost more than the baseline scenario because it takes large investments to put together the facilities necessary to extract and refine space resources. However, thiS plan has the potential to significantly lower the cost of space operations in the long run by providing from space much of the mass needed for space operations. 84 Transportation System (aerobraking) can be used to Requirements offset requirements for inbound propulsion. Because of differences Table 1 lists the principal routes in mission duration and in the between nodal points in the Earth- accelerations achievable using Moon-asteroid-Mars system and various techniques, some identifies technologies for each of transportation modes are more the legs. The principal distinctions relevant to manned flights and between categories of space others to cargo flights• Manned propulsion are related to whether flights require fast and safe significant gravitational fields are transportation to minimize life involved. Leaving a gravitational support requirements and radiation field requires a high-thrust propulsive exposure. Cargo flights can be system• Orbit-to-orbit trips can be Slower, less reliable, and thus made with fairly low thrust, though cheaper. We also discussed to a such trips take longer and are less limited extent transportation on efficient because gravity reduces the surface of the Moon, which effective thrust. If a planet has an will require quite different atmosphere, atmospheric drag technologies. Date t990 1995 2000 2005 2010 2015 2020 -" I I t I I 1 I Mars sample Rover return Camp Mars • • Figure 3 $ 1 Scenario forBalanced Infrastructure Multiple Multiple Buildup Near-EaCh surveys rendezvous Automated materialreturn asteroids Inthis scenario, each locationinspace receives attentioninabalanced approach andnone is emphasizedtothe Lunar geochemical exclusionof others. Thescenario begins orbiter Rover Experimental station Base withtheestablishmentoftheinitialspace Moon _: : ,_=Li: station about 1992. Thisisfollowedby A_ theestablishment ofamannedoutpostin ::i Experimant;l_ geosynchronous Earthorbit(GEO)in _ platform. Outpost 2001, anexperimentalstation onthe GEO : : _;: .. Moonin2006, andamannedMars camp _ A_ "" in 2010. Inparallel withthese manned • • . • activities,manyautomatedmissionsare • • . • flown, including alunargeochemical Space : ::, : orbiterandalunarrover, multiple surveys LEO ofnear-Earthasteroidsandrendezvous withthem, andamartian rover anda Orbital Mars sample return. Automatedminingof transfer vehicle near-Earthasteroidsbeginning in2010 is I _ II I 1 , I I alsopart ofthis scenario. Earth Shuttle-derived launchvehicle 85 TABLE 1. Principal Routes Between Transportation Nodes (a) Nodes and their locations Node Location 1. Earth Kennedy Space Center 2. Low Earth orbit (LEO) Space station 3. Geosynchronous Earth orbit (GEO) Shack 4. Lunar orbit Shack 5. Moon Advanced base 6. Earth-cr,ossing Mining base carbonaceous chondrite asteroid 7. Mars orbit Shack 8. Mars Advanced base (b) Routes and modes of transportation for them Leg Transportation mode options Earth to low Earth orbit Chemical rockets LEO to LEO (plane changes) Chemical rockets Low-thrust orbital maneuvering vehicles (OMVs) Tethers LEO to GEO, lunar orbit, Chemical-rocket-propelled orbital asteroids, Mars orbit transfer vehicles (OTVs) Low-thrust propulsion OEO, lunar orbit, asteroids, Aerobraked chemical rockets Mars orbit to LEO Low-thrust propulsion Lunar orbit to Moon Chemical rockets Tethers Moon to lunar orbit Chemical rockets Electromagnetic launch Tethers Mars orbit to Mars Aerobraked vehicles Mars to Mars orbit Chemical rockets 86 The baseline scenario could be system. (See Salkeld and Beichel implemented with the Space 1973, Eldred 1982 and 1984, and Shuttle, Shuttle-derived launch Davis 1983.) These systems gain vehicles (SDLVs), and orbital efficiency by eliminating man-rated transfer vehicles (OTVs). The elements and reducing system nonterrestrial resource scenarios weight, rather than by improving require the development of the rocket engine (although some additional systems. While it is improvements in rocket engines technically possible to establish the are still attainable). It may be transportation network for these worthwhile to develop such scenarios with oxygen-hydrogen vehicles for cargo transport in the (OH) rockets alone, the expense baseline scenario over the next of operating the transportation 20 years. And the scenarios using network, even for the baseline nonterrestrial materials require scenario, could be reduced by the such vehicles for cost-effectiveness. introduction of non-OH rocket technologies. Let us consider Transportation from the lunar briefly the technologies that could surface to orbit could be be used for three categories of accomplished using OH rockets. transportation: surface-to-orbit, The advantages of choosing OH orbit-to-orbit, and surface. rockets are summarized in table 2 by Sandy Rosenberg, who points out that oxygen-hydrogen propulsion Surface-to-Orbit Transportation is likely to persist simply because (Earth to Orbit, Moon to Lunar the large amount of effort that has Orbit, Mars to Mars Orbit) gone into its development has led to Transportation from the Earth's a level of understanding which surface to orbit is conventionally surpasses that of any alternative accomplished using chemical propulsion system. In a separate rockets. There seems no readily paper, Mike Simon considers the available substitute for such rockets use of OH rockets in a systems on this leg. Shuttle-derived launch sense, showing how the introduction vehicles or, if traffic becomes heavy of nonterrestrial propellants can enough, heavy lift launch vehicles affect the overall system (HLLVs) could provide Earth-to-orbit performance and, eventually, transportation at a lower cost than reduce the cost. does the current Space Shuttle 87 TABLE 2. Selection Basis for Oxygen-Hydrogen Propulsion Factor Rationale 1. Common use of The exploration and exploitation of space is based water to support on a water economy because of the presence of human activity in humans. Water and oxygen are required for life space support. Therefore, use of oxygen and hydrogen in propulsion systems will benefit from synergism with other parts of the space system. APlant.Growing Module ata Lunar Base Plantswillrequire aconsiderable stock of water, butnearly allthe water can be recycled inaproperly designed controlled ecological fife support system(CELSS). 2. High performance The bipropellant combination of liquid oxygen (LO2) and liquid hydrogen (LH2), operating at a mixture ratio of 6:1, offers a vacuum specific impulse of 460 to 485 sec, with an environmentally acceptable exhaust. The LO2/LH2 bipropellant propulsion system offers a high thrust-to-weight ratio, an acceptable fraction of propellant mass to propulsion system mass, a short trip time (an important factor for all manned missions), and afirmly established technology base. 88 TABLE2 (concluded). Factor Rationale 3, Technological The technology for the long-term storage and transfer feasibility of cryogenic fluids in a low-gravity environment, which will enhance the efficient management of LO2/LH2propellant, is being actively pursued by NASA's Office of Aeronautics and Space Technology (OAST). Aerobraking is also being actively studied and appears promising. 4. Benefit from LO2/LH2 propulsion benefits directly from the nonterrestrial utilization of nonterrestrial resources; e.g., the resources manufacture of 02 on the Moon and 02 and H2 on Mars. Earth-crossing carbonaceous asteroids may be asource of 02 and H2. Oxygen Manufacturing Plant on the Moon Thisplant uses afluidized bed to reduce lunarilmenite withhydrogenand produce water. Thewateriselectrolyzed,the oxygen is collected, cooled, and cryogenically storedin thespherical tanks, and thehydrogen is recycled into the reactor. Theplant is powered by electrici_/ from thelarge solar ceil arrays, each of which can generate 56 kilowatts. Artist: MarkDowman 5. Programmatic LO2/LH2 propulsion gets more than 90 percent of support the investment that NASA's OAST is currently making in its research program. No change inthe current NASA program is required when LO2/LH2 propulsion is selected. 89 Other rocket propellants derived launch; the results of this program from nonterrestrial materials could might be fairly cheaply adapted to also find use in the future. Andy the space environment. This Cutler considers an oxygen- concept is considered in a paper hydrogen-aluminum engine as a by Bill Snow. possibility. Such an engine could use oxygen and hydrogen derived Several other technologies may from lunar or asteroidal materials be of value in surface-to-orbit and could also provide a second transportation. Tethers, in use for the Space Shuttle's particular, can permit an orbiting aluminum external tanks, which station to acquire momentum from are currently thrown away. a high Isp propulsion device over long periods of time and quickly Among the alternative technologies transfer it to a vehicle that needs that may be useful are the momentum to gain orbital electromagnetic launchers capable velocity on launch from the Moon of launch from the Moon to low (Carroll 1984 and 1986, Carroll and Specific Impulse (Isp) lunar orbit and of propelling Cutler 1984). In effect, high Isp is Specific impulse (Isp)is ameasure of the vehicles in space. The Department combined with high thrust, performance of arocket engine. Itis of Defense is funding a program of although only briefly. Andy Cutler equal tothethrust generated Fdivided by significant size in electromagnetic discusses this idea. theweightflow rate wof thepropellant used: Isp= F/ Itsunitsturn outto be seconds. Inthe JEnglishsystem,pounds of force (mass times acceleration orIb ft/sec 2)divided bypounds of weight (mass timesgravity or Ibft/sec2)per second equal seconds. Inthemetric system, newtons(kgm/sec2) divided bykilograms (kg)times gravity (m/sec2)per second equal seconds. Specific impulseis also equivalent to the effective exhaustvelocity divided by the gravitationalacceleration. This relationship can also be derived from a consideration of theunits. Force, or mass timesacceleration, can be seen as massper second times velocity,. Weight flowrate,ormass times gravityper second, can be takenasmass per second timesgravity. Thus,specific impulse equals velocity (m/sec) divided bygravib/(m/sec2), orseconds again. 9O Orbit-to-Orbit Transportation (LEO Figure 4 to GEO, Lunar Orbit, Asteroids, or Mars Orbit and Back) Orbital Transfer Maneuver A spacecraft orbiting the Earth can raise Orbit-to-orbit transfers withincislunar the altitude of its orbit by firing its engines space can be handled by OH to increase its velocity in a series of two maneuvers. In the figure, the spacecraft rockets. See figure 4. A series of in a low circular orbit fires its engines at space-based orbital maneuvering point 1. Its new velocity causes an vehicles (OMVs) and orbital transfer increase in orbital altitude on the opposite vehicles (OTVs) is now being side of the orbit. When the spacecraft considered by NASA. reaches the high point of this new elliptical orbit, at point 2, the engines are fired again to increase its velocity. This Aerobraking, which uses increase in velocity raises the low point of aerodynamic effects to lower orbit, the elliptical orbit and in this case results may be significant in cislunar space in a circular orbit at a higher altitude than transportation. This technology will but has not been tested inthe the original orbit. An orbit can be lowered be used primarily with high-energy context of GEO, lunar, asteroid, or by following this procedure in reverse. Taken from AC Electronics Division, systems, such as OH rockets, to Mars missions. No paper on General Motors Corp., 1969, Introduction slow spacecraft returning to the aerobraking was produced during to Orbital Mechanics and Rendezvous Earth (or entering the Mars the workshop, but the principles Techniques, Text 2, prepared under NASA atmosphere), reducing their need and prospects of aerobraking have contract NAS 9-497, Nov. for propellant. See figure 5. This been discussed by Scott and technology is under development others (1985) and Roberts (1985). Figure 5 Aerobrake Used To Slow Down Unmanned Spacecraft Returning From Mars Aerobrakes can reduce oreliminate the need for retrorockets because they use aerodynamic forces in the upper atmosphere of the Earth to slow down spacecraft for orbital insertion or for reentry. AerobrakJng could also be used on the Mars end of a voyage to slow down spacecraft. Artist Pat Rawlings 91