A Pivotal Technology
In 2025 spacelift sees second generation propulsion employment advances toward third generation propulsion systems. The MTV is a combination of revolutionary and evolutionary technology. The vehicle incorporates a vertically launched, single stage-to-orbit, "accelerator class" propulsion system. This propulsion system produces greater than 300 times the thrust (at less than Mach 6) of current systems with a specific impulse greater than 800 seconds. The fuel storage is dense, contained, or compact and contributes to lowering mass fraction. This propulsion system is derived from the evolutionary second generation, reusable launch vehicle, which incorporates evolved combined rocket/air breathing engine cycles employing an accelerator class laser pulse detonation and magnetohydrodynamic propulsion system for atmospheric transport to orbit. Each engine cycle is optimized for a specific portion of the ascent profile. The second generation vehicle is derived from current propulsion systems based on the first generation military and commercial/NASA version of the space plane. The following are the notional advances required from first through second generation propulsion systems.
First Generation Propulsion Alternatives
Technical Considerations. Physics dominates spacelift, and Newton's third Law, stacked heads true time which purports that for every action there is an equal and opposite reaction, holds. To achieve orbital velocity, sufficient momentum (mass x velocity) must be generated to counteract the earth's gravitational pull. Launch vehicles expel expended fuel mass with velocity to propel itself through the atmosphere in the opposite vector. Translated, the thrust (the rate of change of momentum) required for propulsion is the mass flow rate times the velocity of the fuel. The primary measure of thrust-producing efficiency is the ISP, which is measured in seconds as the impulse provided per unit weight of fuel expended.
X-33 Demonstrated Performance. Using the lower mass fraction available due to composite development and the additional payload capability made possible by the pop-up maneuver and refly options, the use of current cryogenic propulsion systems of low ISP (less than 400 seconds) continue to execute heavier medium-lift missions from the upper atmosphere.42 Employing an X-33-developed integrated powerhead rocket engine, a cryogenic propulsion system provides 250,000 pounds of thrust, yielding a 28-second improvement in ISP and a thrust-to-weight ratio greater than 75:1.43 The X-33 primarily provides the proof of concept for reusability and operational tempo. Further advances in cryogenic fuels are spurred through international pooling of information, including Russian engine and fuel pump technologies. The second generation systems take advantage of evolutionary advances in propulsion technology.
Other Current Propulsion Options. The Blackhorse (as outlined in SPACECAST 2020) propulsion technology spin-off is hydrogen peroxide propulsion with the combined monopropellant storage. Low ISP of hydrogen peroxide inhibits extensive development of this fuel source for a rapid response ground-to-orbit vehicle. Lockheed Martin reusable launch vehicle research is working toward to a linear airspike engine, which would improve Isp through atmospheric flight using cryogenic propulsion.44
Second Generation Propulsion Options
Laser Pulse Detonation and Magnetohydrodynamic Fan-jet. Pulse detonation is laser induced, high frequency, sequenced detonations of fuel in a closed tube with a nozzle on one end in lieu of conventional combustion. High efficiency, greater thrust is produced through the use of rapid energy release of detonation as compared to controlled burning of current cryogenic systems. Pulse detonation provides the best option for a revolutionary technology push in conventional rocketry using unconventional physics. The system produces 15 percent higher ISP than conventional cryogenic systems with 40 times the decrease in feed pump pressure, which contributes to weight reduction and increased operational efficiency.45 This system also provides an alternative to chemical propulsion by using air-breathing technology where feasible as the vehicle transitions to orbit. These accelerator class engines transition the subsonic, supersonic, and hypersonic regimes to mach 25 with each engine variant operating within its most efficient regime.46 Using laser technology, the LPD engine can transition to the electric MHD fan-jet engine in the final push to orbit.47 This propulsion system uses the earth's magnetic field to produce energy for ionization of gases in the upper atmosphere or in an onboard propellant and accelerates these gases through a hypersonic fan jet for thrust generation. The MHD engine theoretically produces 6,000-18,000 seconds of ISP for acceleration to velocity greater than March 25. Technology pushes in high-temperature superconductors, laser wave detonation, and compact, light-weight, high-energy generation devices are required.
High-Density Fuels. This program, currently titled the "High-Energy Density Materials Program" (HEDM), is a concept to increase the energy content in conventional chemical bonds of non-nuclear fuels.48 For example, a 5 percent boron additive to solid hydrogen is projected to produce a 107-second Isp improvement in efficiency, and other additives such as titanium and boron/titanium composites show promising results.49 This trend results from the continuation of study suggested by New World Vistas. This program possesses high potential in the search for metastable fuels, which are reasonably stable and practical. Future environmental considerations must be factored into their feasibility. This increase in Isp, due to higher chemical release over the chemical maximum of 450 seconds, could result in a payload increase of 22 percent. Currently, the most promising research is in metallic hydrogen/oxygen propellants. The synthesis of these highly energetic propellants is the technological challenge, but the rapid increase in computational modeling could drive the concept toward reality without large capital investment in research and development.
Nuclear Fission. This concept has been developed extensively
through the 1960's and 1970s. It has the advantages of Isp greater
than 1,500 seconds, and the fuel mass fraction is much smaller with an
associated compact fuel geometry due to high-fuel density. Moreover, it
works easily in space, because the reaction requires no atmosphere. In
nuclear thermal propulsion, a propellant gas is heated as it flows through
the core of a reactor and is then expanded and expelled through a nozzle
(fig. A-1). The reactor core can be solid, liquid, gas, or plasma. The
last two approaches can produce high temperatures and greater efficiency
but are limited to space orbital applications due
Source: Air Command and Staff College, "High Leverage Space Technologies for National Security in the 21st Century" (Maxwell AFB, Ala.: Air University Press, 1995).
Figure A-1. A Basic Nuclear Thermal Rocket
to the expulsion of radioactive gases. Project Rover directed by Los Alamos National Laboratory produced a solid core engine that produced 200,000 pounds of thrust with 9,500 kilogram reactor mass and an ISP of 845 seconds.50 Therefore, the concept has been proven in theory and practice. Further, dual-use designs can be developed which provide electrical generation and ion drive maneuvering power after the propulsion phase is complete. Finally, the technician-driven infrastructure is proven since Naval Reactors has trained personnel to operate reactors with automatic controls at the "blue-suit" level safely for years with a well-established training and maintenance record. Recent NASA research on the lunar-augmented nuclear thermal rocket combines a scramjet with near-term nuclear thermal rocketry and demonstrates the utility of this concept.51
The largest obstacles to nuclear rocketry are both political and environmental. Radiation shielding is required for human and payloads and adds significantly to the vehicle mass fraction. There is some inherent fuel erosion due to the velocity and hot temperature of the propellant, which ejection of fission products into the exhaust. Improvements in metallurgy since 1973 could correct this problem by using improved cladding, different propellant gases, or more efficient fluid regimes (detected through computer-aided design). Finally, uncontrolled reentries or launch failures result in nuclear material entering the environment either intact, in pieces, or dispersed as fine particles. Offsetting this problem is the fact that the reactor mass is small by comparison and would result in little or negligible environmental impact, and remote launch sites could further reduce the risks.
For reusable vehicles, disposal of spent fuel adds to the commercial problem. Current commercial reactor designs have significant safety features built into them; nuclear reactors do not exchange by-products with the environment, and the integral fast breeder reactor technology, demonstrated by Argonne Laboratory, is inherently safe and utilizes plutonium and by-products as fuel.52 The spent fuel is the largest storage problem due to long-lived radiation products. Recent technological advances could make this problem a non-issue. These include permanent subterranean/seabed storage in stable geological formations in glass-encapsulated canisters, Argonne Lab's nuclear transmutation, which reduces long-lived radioactive isotopes to less radioactive ones through high-intensity nuclear bombardment, and shooting the waste into the sun, the moon, or deep space, which could expand the launch market.53
With over 200 years in Uranium resources and as the world's largest consumer of energy, the US may intensify its commercial nuclear industry by 2025 and educate Americans regarding benefits. Realistically, this scenario is remote currently or in the future. Moreover, public disposition would not allow the development of a nuclear fission space propulsion system which is used within the earth's atmosphere. Conversely, satellite history has demonstrated the application of nuclear power in space-based vehicles.
Fusion. In the realm of plasma physics, nothing dominates it as the quest for commercial-fusion power. For propulsion, the laser-fusion concept, which is compressing a deuterium-tritium fuel pellet with symmetrically positioned lasers for a few billionths of a second until the nuclei fuse gives off the heat, is the most promising. In magnetic fusion, the fuel plasma is suspended in a magnetic field and heated until temperature and densities are achieved for the nuclei to fuse. Sustained reactions of one second have been demonstrated, but nuclei reactions with contaminants, lack of plasma-heating technology, and beam constraints have prohibited commercial application. If the technological difficulty of being able to vector the energy can be achieved or the energy can be harnessed in a working fluid, a propulsion engine without the long-lived radiation of fission could be designed for space applications. Recent research at the University of Michigan conceived a simple magnetic mirror confinement system to create a high-plasma density, which theoretically could produce a propulsion system with an Isp of 100,000 seconds.54 Continued advances in computer technology for plasma modeling, high-temperature superconductors, and charged particle beams could provide the technology leap to produce a self-sustaining fusion reaction by 2025. Current projections place commercial fusion applications at the year 2045.55 This application of fusion to a propulsion system is a third generation system, which opens the solar system to an operationally strategic area.
Third Generation Propulsion Possibilities
Antimatter Drive. Early in the HEDM program, matter-antimatter annihilation was considered a possible propulsion fuel source. The theory is simply that antiprotons and positrons would be slowed, trapped, and recombined to form a charged anti-hydrogen cluster. This cluster forms one part of the bipropellant fuel and the other ordinary hydrogen. The antimatter cluster is reacted with the ordinary hydrogen and is almost completely converted to energy. Similar to nuclear reactions, the antimatter reactions swap rest mass energies, releasing energies 1,000 times greater than nuclear reactions.56 The concept is simple, but practical implementation is beyond current technologies, since any fuel must be able to be produced in quantity, stored, reacted in a controlled manner, and energy vectored in a useful form. While small quantities of antimatter have been produced, the current capability is 12 orders of magnitude below required production. Recent research at Pennsylvania State University demonstrates a promising propulsion system based on antiproton catalyzed microfission/fusion, with their recent completion of a portable Penning Trap, which captures antimatter particles for storage them. This propulsion system uses the energy release from the antimatter reaction as the catalyst for a controlled microfission detonation (small vectored nuclear explosions) to produce thrust. The Penning Trap is being transferred to Phillips Lab at Kirkland AFB New Mexico for use in demonstrating microfission in late 1997.57 The radiation and environmental considerations are less than nuclear fission propulsion, but the high temperature would require sophisticated magnetic containment (similar to fusion) to avoid a meltdown catastrophe. A technology leap in particle physics and magnetic containment is required to implement this technology.
Quantum Fluctuations/Space Drive. Recent theorists have proposed a particle theory for inertia and gravity.58 This theory proposes that space is not empty but a "cauldron of seething energies," known technically as quantum fluctuations or Zero Point Energy, which have been detected but not tapped. Arthur C. Clarke points out that the potential impact on civilization would be incalculable, because the fuel source would be available to all infinitely and all fuel technologies and concerns over environmental impact would be obsolete.59 Harnessing this technology requires the same technology leap in particle physics as antimatter and is considered remote by 2025.
Orbital Transfer Vehicle Propulsion
The 2025 military OTV employs second generation combined propulsion systems. A nuclear-electric ion drive combined cycle enables high maneuverability with maximum time to refueling. Commercial OTVs use solar-electric ion drive for economical maneuvering and thrust, augmented by improved fuel cell technology for minimum high-thrust requirements.
Combined Cycle OTVs
Nuclear/Solar Electric Ion Drive. Solar energy is infinitely available in space, but its energy density is small compared to other earth-born sources. It dissipates exponentially as one travels outward from the solar system. Consequently, its required space and mass fraction is large even for electrical generation. Nuclear thermal reactors have large-generating potential, but carry radiation, environmental, shielding, and public support problems. The space-based application of nuclear power has the history to overcome these difficulties. The use of nuclear or solar power for electrical generation enables a propulsion system that ionizes a nonreactive gas, in which the positively charged ions are pulled out of the engine, forming a jet that impels the craft forward. This way, unlike chemical propulsion, the energy generation and momentum are separated. It has the advantages of speed, efficiency, and economy as the current laws of physics allow. Refuelable fuel cells and thermionic reactors augment the power source requirements during high demand. Current research on Russian Express spacecraft with stationary space thrusters and on the Hughes Galaxy III-R communications satellite are the first tests of ion drive principles.60 Moreover, NASA's millennium program for interplanetary exploration is proposing use of solar-electric ion drive.61 Nuclear ion drive enables responsive orbital maneuvering (with adequate thrust-to-weight ratio not available from solar energy) required for space mission accomplishment.
Third Generation OTV Propulsion Systems
Magnetohydrodynamic and Laser Propulsion. Magnetohydrodynamics
has the immense potential of Isp in the range of 10,000 seconds.
It derives its energy by using space magnetic field energy and converting
it to electricity to drive a laser-propulsion system on the vehicle. Current
research tested a magnetoplasmadynamic thruster on the Japanese Space Flyer
Unit, and it should promise.62 The
major disadvantage is the large mass fraction of the vehicle to provide
power for thrust requirements of major propulsion. A technology leap in
superconductors and plasma physics are required before this technology
is practically feasible. Laser propulsion is similar to ion drive, but
a ground-based laser imparts energy to a working fluid (hydrogen) at a
high Isp (1,500 sec). A technology leap in laser physics with
regard to atmospheric compensation is required. Further, the system requires
a large ground-based infrastructure for vehicle tracking, a complicated
design, and a large power generation requirement.63
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