We didn't have a thread, only passing mentions here and there.
- https://www.secretprojects.co.uk/threads/late-1960s-space-tug.6178/#post-72943
- https://www.secretprojects.co.uk/threads/x-37b-orbital-test-vehicle-otv.5232/page-15#post-261150
en.wikipedia.org
PROFAC is closely related to RLVs and SSTOs, but among them it is unique. Think of it as a in-between HOTOL / Skylon air collection , and an orbital propellant depot. Put otherwise: a LOX orbital depot that fill itself sucking atmospheric air, splitting the nitrogen away to get the oxygen.
PROFAC was detailed in Demetriades papers and also by Aviation Week in 1963.
en.wikipedia.org
- https://www.secretprojects.co.uk/threads/late-1960s-space-tug.6178/#post-72943
- https://www.secretprojects.co.uk/threads/x-37b-orbital-test-vehicle-otv.5232/page-15#post-261150
Propulsive fluid accumulator - Wikipedia
PROFAC is closely related to RLVs and SSTOs, but among them it is unique. Think of it as a in-between HOTOL / Skylon air collection , and an orbital propellant depot. Put otherwise: a LOX orbital depot that fill itself sucking atmospheric air, splitting the nitrogen away to get the oxygen.
PROFAC was detailed in Demetriades papers and also by Aviation Week in 1963.
There are three basic types of PROFAC Systems:
(a) PROFAC-A; accelerating or suborbital PROFAC. An Aerospace Plane, with the LACE (Liquid Air Cycle Engine), an air-breathing rocket engine which manufactures its own oxidizer by liquefying atmospheric air and uses hydrogen as fuel engine, is a system of this type. Hydrogen or other chemical fuel reacts with the atmospheric gasses to furnish the power required to accelerate the vehicle, overcome drag and accumulate atmospheric gasses for further missions. Although, if sufficiently optimistic assumptions are made concerning wing structure, lift, etc., there is not much doubt concerning the feasibility of this vehicle, additional work is required to prove its economic advantages, if any. In particular, it remains to be proven that the mass of atmospheric gasses collected per unit mass of fuel expended and the collection rate are lucrative from the economic point of view. PROFAC-A is also called either LACE or ACES. A hydrogen-burning PROFAC-A using the liquid hydrogen fuel as a heat sink would collect approximately 4 kg of air per kg of hydrogen consumed and would require a collection and liquefaction rate of the order of 227 kg/s, making it necessary the use of a huge heat exchanger which severely decreases the payload. A 20 minutes ascent would require liquefaction of 272400 kg of oxygen !
(b) PROFAC-S; stationary PROFAC. This system is an automatic propellant or expellant accumulator on the surface of a satellite or planet.
(c) PROFAC-C; constant velocity or orbital PROFAC.
The essential feature of this scheme is to lift only the energy source into circular orbit at approximately 100 km and at that point to collect the propulsive fluid (air) for continuing the journey into space or for satellite/excraft maneuvers. This system consists of two vehicles. The Orbital Vehicle, which contains PROFAC apparatus, is one, and the Space Vehicle, which is the maneuverable satellite, lunar or interplanetary vehicle, is the other. The feasibility and economic advantages of PROFAC-C for certain missions are quite definite. Note that the PROFAC-C collection rate is of the order of only 0.0453 kg/s: 41 grams per second. Perhaps the problems encountered in a recoverable booster of the PROFAC-A type can be eased by refueling PROFAC-A from PROFAC-C on the way to orbit as well as in orbit. Thus the two systems are complementary rather than competitive.
Since all the accumulator gasses (which are collected and stored, as opposed to the propulsion gasses which are used for propulsion) have to be stopped with respect to the vehicle, there are two main variants of the propulsion cycle for PROFAC.
The first variant involves completely stopping all the propulsion air with respect to the vehicle, in addition to the accumulator air, and is known as the interrupted flow PROFAC engine. A hydrogen-burning PROFAC-A with an interrupted flow engine cannot possibly accumulate significant amounts of air at speeds in excess of 2042 m/s.
The second involves a partial stopping or slowing of either all or part of the propulsion air with respect to the vehicle, known as the uninterrupted flow PROFAC engine. The power requirements for these engines were given elsewhere.
A typical Orbital Vehicle power sources will require only about 0.26 MW per m2 of inlet area for the practical plasmatization of the stream and 0.24 MW for the actual acceleration of the stream, for operation at about 100 km altitude.
Assuming that the Orbital Vehicle inlet is 10 m2 this imposes a requirement of 5 MW, the attached PROFAC equipment with scoop area of 1 m2 will require 1 MW. Thus, a total of 6 MW will be required. Such a power source with auxiliary equipment would weight about 11 metric tons with the present state of the art and perhaps as low as 2 metric tons with the expected development of nuclear power sources in 10 years. Using the same dimensions, the PROFAC equipment will accumulate 430 kg of liquid air per day: 43 metric tons in 100 days.
One approach for collecting air involves aerodynamic flight at hypersonic speeds between 100,000 and 120,000 ft. altitude. The upper altitude limit for aerodynamic flight is about 150,000 ft. At these altitudes, the drag produced in maintaining lift and by the air scoop would require high engine thrust. Thus the air scooping cycle would have to be short because of limited fuel endurance. This flight profile would also require a high capacity internal power supply to operate the air reduction plant.
Because of the high thrust required, combined with the problems of heat dissipation from aerodynamic heating and operation of the power supply and reduction plant, this method has fewer adherents than the others.
A second approach envisions an elliptical orbit whose perigee would be about 250,000 ft. During flight in this region, say from 360,000 to 250,000 ft., there would be a concentrated period of air collection. The scoop would be a straight cylinder. As with the first concept of collection at 100,000 to 120.000 ft., the power requirements for the collection period would be so great that whatever power supply system was used, it would be prohibitively heavy. This could be batteries, recharged from solar collector cells, from fuel cells or from nuclear power units.
Another idea, which has survived more feasibility studies, is one which prescribes a circular orbit between 325,000 and 360,000 ft. This vehicle could operate from a nuclear power supply, simplifying the entire system by eliminating storage batteries and using heat sinks to capture heat generated by the concentrated scooping cycle. At these altitudes the design of the vehicle and the scoop are highly critical, because pressure interactions along the side walls would create undesirably high drag. For this reason, the scoop design would be more of a tapered shape.
These three schemes are predicated on continuous flow of air over vehicle and scoop surfaces. Above 360,000 ft., this continuum flow begins to change to molecular flow so that ram scooping is no longer possible.
There is another concept which would provide collection of air molecules by absorption in a carrier fluid or substance. This would operate in the region above 550,000 ft. Its shape would be dictated bv its solar collector power supply. It could be rectangular so the body could carry a solar collector curtain which would be unfurled after orbit is achieved.
Each of these concepts obviously requires a means of propulsion during the collection cycle, and all combustion engines proposed would burn liquid hydrogen and gaseous or liquid oxygen. During its development life, which is now more than 20 yr.. the turbojet has increased its capability from subsonic flight to the region of Mach 3. The ramjet in its present form can go to Mach 7, but must be accelerated to its starting speed, so it cannot be used for takeoff. A combination of the two, called the turboramjet, has the ramjet placed behind the turbojet engine. For takeoff and high power requirements to Mach 3, the two would operate as a turbojet with an afterburner. When the compressor blade speed limits are reached in the turbojet, it would be closed off and the ramjet would be able to function up to Mach 7.
For air scooping at 100,000 to 120.000 ft., the ramjet would provide the hypersonic flight needed. This would require use of a considerable amount of the stored liquid hydrogen and would reduce the payload for orbital flight or transfer to another vehicle. The jump from hypersonic flight speeds to the speeds needed to achieve orbit requires different propulsion techniques.
The most obvious method, and one which is available now, is the nozzle rocket system. This would be uneconomical, however, in that it would need liquid hydrogen and liquid oxygen for operation. Thus, takeoff with an appreciable load of liquid oxygen would be required, or a pause would be necessary along the way to scoop in sufficient air for the next cycle orbit.
A simpler powerplant is within the state of the art, however. The present ramjet reduces air flow into its chambers to subsonic velocity, no matter what the external speed is. A theoretical alternative on which there has been some research and a little development is the ramjet which uses supersonic internal flow. Considerable work would be necessary in the dynamics of supersonic flow-flame propagation. If it were provided a separate, extensive development program the supersonic flow ramjet is considered capable of success. Instead of being limited to Mach 7, it could operate fast enough to accelerate to the 25,000 fps needed for orbital flight. Once orbital flight is achieved at air scooping altitudes, it is considered uneconomical to burn the scooped air for propulsion rather than liquefying it.
The system closest to practical application for flight beyond orbital speeds is the magnetohydrodynamic accelerator. This device would require storage of liquid nitrogen in addition to liquid oxygen. However, air liquefaction systems can easily convert both from air. MHD would also require a nuclear power supply.
The magnetohydrodynamic accelerator uses the principle of superheating a gas to the point where it is ionized, and then accelerating it with a magneto coil. In ground test installations these gases can be heated to 16.000K. The gas could melt the container walls, but it is kept away from them by the magnetic field.
For a flight installation, a low temperature MHD accelerator, operating at about 4.000K would be practical. The nitrogen would be introduced past an electric arc which would heat it. An alkali metal vapor, such as calcium, sodium or potassium, 1% by volume, would be introduced to make the gas a better conductor of electricity. It would be choked in a nozzle to accelerate it to a low supersonic speed, then sent past the entrance to an acceleration chamber which can be cither a cylindrical or a rectangular cross section duct. Field coils then produce a magnetic field across the duct. A direct current is passed through the gas between electrodes, and in the presence of the magnetic field this current produces a force which accelerates the gas, thus producing a thrust. A theoretical system which might be applicable is the monatomic ramjet. Due to action of particles and rays from the sun, upper atmosphere molecules are dissociated, or ionized. This state represents stored energy. If the ionized air can be properly gathered and associated, energy in the form of heat will be released. In theory, a monatomic ramjet could operate without fuel, using onlv the energy stored in the upper atmosphere. If developed, it could readily be applied to a space plane.
Another possibility besides the MHD accelerator for reaching the moon in minimum amount of time, would be use of a variation of the ramjet. When ready to leave orbit and gain escape speed, the ramjet front end would be closed. Then it could be operated by introducing gaseous oxygen, which would burn with liquid hydrogen sprayed from the flameholder. One company claims this method is the answer to the complex plumbing of nozzle rockets.
The four types of air collection flight cycles arc tied to different time schemes for doubling their weight. The 100,000- to 120,000-ft. profile would have the mission accomplished in hours, but it would be hampered by its inherent dificulties. The elliptical orbit method would be on the order of hours also, but it is considered economically unsound.
Circular orbits, using nuclear power, offered the best method of collection from the standpoint of economics. The collection cycle would be in the order of days, and it could be repeated many times.
The vehicle in the upper reaches of the atmosphere where there is molecular flow would probably be unmanned and capable of operating for years.
Without considering development expenses, proponents claim that the cost of launching Aerospace Plane into orbit would be about $100 per pound, much less than the cost of rocket boosters available now.
One of the problems which will tax the skills of thermodynamics engineers will be the liquefaction of air and its separation into nitrogen and oxygen. Air entering any liquefaction svstem must be at a pressure of 10 cm. of mercury or more and a temperature below 600K. In the ordinary cryostat method of liquefaction, successive cycles of compression, cooling, recompression and cooling are used. This is a mechanical system, requiring moving parts and complex plumbing and cooling equipment. It also is heavy.
Other methods of compression are being developed, however. Various means of creating shock waves can be used for compression, among them the electric spark. To separate oxygen from nitrogen, if both are to be kept in liquid form, a centrifugal separator, similar to a cream separator, must be used. If oxygen alone is to be kept in liquid form, it alone can be liquefied since its boiling point of -183C is above that of nitrogen which is — 195.8C. Oxygen thus will liquefy first in the cooling cycle.
For short-term operations, fuel cells offer an attractive method of providing electrical power. In this method, the fuel is converted directly into electrical energy by electro-chemical means. For longer-term operations coupled with heavy power demands, a nuclear power source is required. Steady developments in this line, such as the SNAP series, promise lightweight space power.
Present conceptions of the entire space vehicle show aerodynamic surfaces with blunt trailing edges. Techniques for re-entering the atmosphere have yet to be tried. Too steep a re-entry path would burn the vehicle, too shallow a path would result in too little deceleration, allowing it to escape again, with the necessity of having to use power to return.
Magnetohydrodynamic drive - Wikipedia
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