These cycles are thermally integrated combined cycle propulsion analogous to the LACE rocket-ram-scramjet and the deeply cooled rocket-ram-scramjet except the thermally processed air is separated into nearly pure liquefied oxygen (Liquid Enriched Air, LEA; LACE stands for Liquid Air Cycle Engine; and ACES for Air Collection Enrichment System) and gaseous nitrogen (Oxygen-Poor Air, OPA). This is possible because the boiling point of liquid oxygen is 90.03 K and the boiling point for liquid nitrogen is 77.2 K. Just as in a fractionating tower for hydrocarbons, where hydrocarbons of different boiling points can be separated, the oxygen can be liquefied while the nitrogen remains gaseous. This means that most of the oxidizer carried on-board the launcher was not loaded at takeoff but loaded during the flight to orbit. The result is that the carried oxidizer-to-fuel ratio at takeoff is less than for a non-ACES system. Thus the takeoff gross weight and engine size are reduced. Whether also the volume (size) of the launcher is reduced depends on the volume of the ACES system [Bond and Yi, 1993]. The maximum weight of the launcher is then near the ascent climb to orbital speed and altitude, rather than at takeoff. The process is executed in steps, through temperature gradients where a fraction of the oxygen is liquefied at each step. As in all chemical processes, the difficulty increases as the oxygen purity increases, and for a flight weight system there is a practical limit. The liquid-enriched air has purity in the 85% to 90% oxygen range and is stored for use in the rocket engine during the rocket ascent portion of the ascent trajectory. The oxygen-poor air contains 2% to 5% oxygen and is introduced into the ramjet, creating the equivalent of a mixed flow by-pass turbofan. That is, the mass-averaged exhaust velocity is reduced but the specific impulse, engine mass flow and thrust are increased. Thermal integration means that the fuel passes through both rocket and scramjet to scavenge rejected heat and convert it into useful work before entering the combustion chambers, increasing the specific impulse at the same time oxidizer is being stored for the ascent to space. Just as for the LACE and deeply cooled rocket, both rocket and scramjet must operate as an acceleration system until efficient ramjet operation is reached. So the Mach number for air separation and collection is usually in the Mach 3 to 5 region. This is a very good cycle for launchers that require a launch offset to reach an optimum launch latitude and time window, for instance, when the vehicle must cruise some distance to the ascent to orbit point. The approach is applicable to SSTO vehicles. ACES has more significant payoffs for TSTO launchers that must fly an offset, because the air separation plant is in the first stage, not in the stage that flies to orbit. A good example of this is reaching the ISS 55-degree orbital inclination from Cape Canaveral, at 28.5 degrees latitude. The Space Shuttle loses a significant fraction of its payload because of the propellant required to move the orbital plane during a rocket ascent. To rotate the orbital plane 26.5 degrees requires a significant weight ratio increase to achieve low earth orbit (this will be discussed in Chapter 5). However, a first stage flying in the atmosphere can achieve this with a small fraction of the propellant required to do the plane change by rocket thrust, because the first stage accomplishes the turn simply using aerodynamics. The rocket in its acceleration-turning flight has thrust at least twice its weight with an effective Isp of perhaps 400 s, while the aircraft has the thrust of one-sixth its weight with a specific impulse about 10 times greater (Figure 4.12). This expands the launch window because the launcher can fly to intercept the ascending node of the desired orbit and not be confined to when the ascending node and launch site latitude coincide. The figure of merit for these systems is the weight of LEA collected per weight of hydrogen. A practical value is 6 kg of LEA per kg of hydrogen; for more details see [Czysz and Vandenkerckhove, 2000]. Examples of the thermally integrated enriched-air combined cycle propulsion are:
10. ACES-LACE ejector ram-scramjet-rocket. Figure 4.22 is an air collection and enrichment system [Ogawara and Nishiwaki, 1989] added to Propulsion System 6. The liquid air is not pumped to the rocket immediately, but passed through a liquid fractionating system to separate the oxygen component as liquid-enriched air (LEA contains 80% to 90% oxygen) and nitrogen component as liquid oxygen poor air (OPA contains from 2% to 5% oxygen) [Balepin, 1996]. The oxygen component is then stored for use in the rocket ascent portion of the flight. The oxygen-poor nitrogen component is injected into the ramjet, to create a hypersonic by-pass engine that increases engine mass flow, thrust and reduce the mass-averaged exhaust velocity. The hardware development in the 1960s was undertaken by the Linde Corporation under Air Force contract. Sufficient hardware was fabricated to design the operational system and confirm performance. ACES most significant
penalty was the volume required for the fractionating separator. For hydrogen-fueled hypersonic and space launchers, volume is a critical parameter, and increasing it comes at a significant size and weight penalty. At takeoff this propulsion strategy can significantly reduce the takeoff perceived noise. It is done for the same reasons a conventional mixed flow by-pass gas turbine was invented. ACES was originally proposed by the Air Force Aero-Propulsion Laboratory for the space plane of the late 1950s [Leingang, 1988; Maurice et al., 1992]. and was the subject of intense investigation in the 1960 to 1967 time period [Leingang et al., 1992]. Most of the original Air Force work was for a TSTO vehicle, although application to SSTO was investigated. For airbreather operation to the 12,000 to 14,000 ft/s range, its cycle can achieve weight ratios less than 3 with oxygen-to-fuel ratios approaching one-half.
11. ACES-deeply cooled ejector ram-scramjet-rocket. Figure 4.22. is an ACES option added to Propulsion System 7. Even in the 1950s, the paramagnetic properties of liquid oxygen were noted by the LACE and ACES investigators [Leingang, 1991]. Patrick Hendrick was a graduate student under the late Jean Vandenkerc-khove in 1988 who observed that Siemens sold an exhaust gas analyzer measuring gaseous oxygen based on the magnetic properties of oxygen. The magnetic susceptibility of oxygen at its boiling point (90.03 K) is 7699 x 10~6 in cgs units, that is, as large as some chromium and nickel compounds. During a visit to Jean Vandenkerc-khove at his Brussels residence, Patrick Hendrick [Hendrick, 1996] discussed his concept of gaseous air separation using the magnetic properties of oxygen. Collaboration with Vladimir V. Baliepin resulted in the addition of a vortex tube pre-separator based on the small temperature difference in the liquid temperature of nitrogen and oxygen. The result was a new approach to the ACES concept with much lower total volume requirements than the liquid fractionating equipment. The deeply cooled gaseous air is not pumped to the rocket immediately, but passed first through a vortex tube initial separator (at this stage the LEA contains about 50% oxygen) [Lee et al., 2003], and then into a cryogenic magnetic oxygen separator. The oxygen component is then liquefied as LEA (LEA contains 80% to 90% oxygen) and stored for use in the rocket ascent portion of the flight. The gaseous nitrogen component of oxygen- poor air (OPA) contains from 2% to 5% oxygen. The oxygen-poor nitrogen component is injected into the ramjet, to create a hypersonic by-pass engine that increases engine mass flow, thrust and reduce the mass-averaged exhaust velocity. At takeoff this can significantly reduce takeoff noise, for the same reasons a conventional mixed flow by-pass gas turbine was invented. This system is in laboratory testing and studies but has not as yet been developed as propulsion hardware. At this point in time it has potential to significantly reduce the volume and weight required for an ACES system, but is not yet proven. For airbreather operation to the 12,000 to 14,000 ft/s range, this cycle can achieve weight ratios less than 3 with oxygen to fuel ratios approaching one-half.
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