The Joy of High Tech

This being a collection of random thoughts on bits and pieces of technical information which should interest the technically oriented reader.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. And if you find a mistake, please let me know about it.

The Forerunner Aircraft

Forward

     The goal of the Forerunner project is to provide a safe, high-performance homebuilt plane with an endurance at cruise of at least 20 minutes, using available or near-term technology. Below are descriptions of the Forerunner II and III, and the history of their development. What, you might ask, became of the Forerunner I? I saw early in the design process that this first attempt would not be stable much above Mach 1. I therefore abandoned it. Strangely, in a NASA paper on the feasibility of a Mach 5 bomber, which I read later, the simplified airframe used in their evaluation was almost identical in shape to the Forerunner I!

     Back in the Summer of 1989, I mentioned to a technically oriented friend that Burt Rutan's Advanced Composites - or one of the other homebuilt aircraft companies - should be able to design and market a kit aircraft capable of exceeding Mach 3. Estimated component price would be under a quarter of a million dollars. My friend was skeptical. In essence, he said "put up or shut up." Well, I started doing some research, and quickly realized that I had greatly underestimated the technology available. Indeed, the upper limit for the engine I planned to use was Mach 6. If researchers are correct in their computer modeling, adding a cooling cycle at the inlet will allow a modified version of this engine to function at speeds up to Mach 8! That's faster than anything short of a space vehicle has ever gone!

     A few months after I started this project, I learned that a kit plane company was developing the BD-10, a Mach 1.4 homebuilt aircraft kit, for $140,000. The design was very conservative, using mostly steel for the structure, and an existing commercial aviation turbojet engine for propulsion. The company also planed to get the plane certified by the FAA, so they could sell completed aircraft as well as kits. I was also told that several advanced homebuilders are working on supersonic planes. However, in the years since I have not heard of any of these efforts being successful. Indeed, the BD-10 project seems dead. Both prototypes crashed, in each case killing the pilot. Attempts to offer a variant to the military as a drone have also been unsuccessful.

     Neither the BD-10 nor the proposed Forerunner aircraft require techniques which have not been used before by kitbuilders. Indeed, the only unique feature in the basic Forerunner II design is the engine, and those are usually bought as completed units anyway.

     The project is by no means finished. Among other work which remains to be done, I need to locate a bonding agent which can withstand temperatures of up to 600 Kelvins. (Some polyimides look promising.) Also, I have not even begun to determine the exact cost of the finished kits. As mentioned above, I am aiming for a completion price of under a quarter of a million US dollars for the Forerunner II.

     The first production (non-research) aircraft to cruise at or beyond Mach 3 for long periods were members of the Blackbird family. The first of the line, the A-12, made its initial flight in 1962, with the YF-12 and the SR-71 following later. A Blackbird exceeded Mach 3 for the first time in 1963. These planes were built with late fifties technology, and the fact that they still hold speed records is testimony more to the fact that most military planes don't need to fly that fast than to how inherently difficult it is to do so. The XB-70 Valkyrie was not quite as fast as the Blackbirds, but the two test articles both cruised in excess of Mach 3 for over an hour at a time. Except for using compression lift (see below) the Valkyrie was even more conventional than the Blackbird!

     The infamous Mig-25 Foxbat doesn't really count here as a operational Mach 3 aircraft, since only the reconnaissance versions can make that speed, and those for only short bursts - and afterwards need to have the engines replaced!

     The Airturboramjet (ATR) was first invented back in the fifties, but has not been successfully built yet. (Though by 1994 at least one effort was underway to do so.) This isn't surprising when you consider the operating parameters. Given the advances made in materials technology over the past 10 years, though, it should be possible to construct one now. In fact, this engine works better on a small scale, such as for use in a cruise missile or a homebuilt aircraft, than in a full-sized military plane. A civilian variant built around an airliner Auxiliary Power Unit (which is already flight rated, reducing development costs and time) would be safe, economical and reliable. Though the particular development I'm thinking of would technically be an afterburning ultra-high bypass turbofan and not an airturboramjet, I'm lumping it in with the true ATR for purposes of this article.

     In essence, the ATR is a ramjet which creates its own head wind. It can start from static conditions, and continue to Mach 5 or possibly Mach 6. The concept is deceptively simple: a small turbine burns fuel and oxygen (or air, for my variation) to drive a one- or two-stage compressor, with the fuel-rich exhaust then passing into a combustion chamber where it burns in the air driven backwards by the compressor section. Additional fuel can be injected into this airstream and ignited for more thrust, creating a true afterburner. Above Mach 0.8 the ATR begins to act more and more like a ramjet, and above Mach 2 behaves almost exactly like one. Thrust and efficiency increase until above Mach 3, then slowly decline at higher speeds.

     Unfortunately, the operating temperatures and stresses involved require the compression blades to be made from something like ceramic whisker/metal matrix composite and they will probably have to be cooled by air or fuel pumped through their hollow centers. Such materials and techniques have only recently become practical. The interior of the engine would have to be lined with a ceramic or some other material which not only withstands high temperatures for long periods, but which is resistant to thermal shock. If the engine flames out at high altitude, or even if the pilot simply throttles back quickly, the temperature drop will be extreme.

     The engine inlet is critical, and the design I have used is apparently unique in its details. Normally, any interruptions in the surface of an aircraft - such as engine inlets - are stepped away from the skin, to avoid disrupting the boundary layer flow. This is a thin film of air which tends to hug the surface of the plane's body. High-performance aircraft from the P-51 Mustang to the YF-23 use this stepping technique, which is why the F-14 looks as if its engines are about to fall off. However, one of the greatest performance aircraft of all time, the F-104, uses a different approach. It has long, tapered half-cones attached to the fuselage ahead of the inlets. These gently redirect the boundary layer stream, turning some away from the body and into the engines, increasing the mass flow and the oxygen available for combustion. (Note that the actual inlet ducts are indeed stepped away from the sides of the aircraft, so that much of the boundary layer flow passes between the inlets and the fuselage.) This is one of the reasons that the Starfighter, designed in the mid-fifties, can still show its heels to most first-line combat aircraft of today.

     Most designs for hypersonic aircraft have the inlet or inlets underneath the plane, using the underside forward of the engine to help gather and compress the air. The inlet of the Forerunner is similar, using an angled ramp (think of a corner cut on a slant from a cube) projecting down and back from the belly of the plane. This ramp helps create the complicated shock pattern needed to slow the inlet air to below the speed of sound. If necessary, strategically placed holes in the ramp can direct boundary layer air away from the turbine face.

     Supersonic flow is counter-intuitive; compressing a stream moving faster than sound slows it; expanding it increases the speed of flow. Since the ATR is a subsonic flow engine, like any turbojet or conventional ramjet, slowing the inlet air to below Mach 1 is necessary. This is done by creating a controlled standing shock wave in the inlet. Supersonic jets of the Fifties and Sixties mostly used fixed ramps, optimized for one or two airspeeds. Most more modern craft use adjustable ramps, but on a small plane the weight and complexity this requires might be prohibitive. A two- or three-shock (that is, shaped to work optimally at two or three different speeds and supersonic shock wave conditions) fixed ramp should be sufficient.

     The exhaust nozzle for a jet engine is equally important. The ratio of the convergent/divergent section must be adjusted to match the current conditions of flight, including engine power. I am hoping that a simple "blow open" nozzle will be sufficient, but a more sophisticated system may be required, using pressure sensors and electrical or hydraulic actuators. Here we can't simply used a fixed ratio; the change in requirements for changing flight conditions is too great. While fixed-nozzle ramjet engines have been used, these were in vehicles - usually missiles - which were boosted to near speed before the ramjet took over. The engine was therefore operating only in a narrow set of parameters, and didn't need to adjust the nozzle.

     The Forerunner aircraft depend on an ATR being available to achieve top performance. With conventional high-performance afterburning turbojet engines the top speed would be between Mach 2 and Mach 3, and the cost would be greater, although cruise endurance would also increase. Other engine options are also available, at least in theory, but the ATR is the best solution to the goals of the design.

     The basic form of the Forerunner family of aircraft consists of a twin-tailed, swept delta wing with blended wing chines and canard foreplanes, so that it strongly resembles several past and current homebuilt aircraft. While these features were originally adapted due to this source of inspiration, there are legitimate technical reasons for using them.

     High-speed flight is, of necessity, also high-altitude flight. In order to reduce the drag and heating from air friction a Mach 3+ plane needs to cruise above 60,000 feet. Altitudes as high as 250,000 feet would be necessary for Mach 6+ speeds. However, at such altitudes the thin air reduces the effectiveness of the controls. This is mitigated somewhat by the fact that the high speed gives the controls a little more bite. Still, the Blackbird aircraft sometimes find themselves in a situation where the stall speed is only a few knots below the cruise speed. In order to gain as much control authority as possible in the Forerunner aircraft, the control surfaces need to be placed far from the center of gravity, so they have a longer moment arm to work through. This means using a canard design, and twin vertical tails placed at the wing tips.

     The shape of the wing/chine combination is reminiscent of that used on such planes as the Blackbird, the F-16XL and the F/A-18. The chines add pitch stability as well as lift, and help reduce the movement of the center of lift as the Mach number increases. Underwing fences, essentially extra fins, are used in the Forerunner to increase the lateral stability. Adding ventral fins to the rear of the engine may also be necessary.

     Great care was taken to make sure the form followed area rule, which dictates that there be no sudden changes in cross section in supersonic aircraft. However, remember that the designs as shown are strictly back-of-the-envelope, "If it looks good it will perform good." work. While the final version will be close to this and have the same main features, a great deal of computer simulation and wind tunnel testing will be required to optimize the design.

     The Forerunner is intended as a sport craft, not a military or commercial plane. It can therefore be built more simply and lightly than otherwise, while still retaining an adequate safety factor. Other features of the design also help reduce complexity and cost. For instance, flaps are not needed, since the delta wing has good low-speed stall and handling characteristics, especially when in ground effect during takeoff and on final approach.

     For the Forerunner II, a Mach 3+ cruise time of 20 minutes is the target for the basic design. This does not sound like much, but with a ground speed of over 2000 mph you can travel about 700 miles in that time. Since the materials used in the skin and structure (see below) conduct heat poorly, this means that internal temperatures will stay low for such a short flight. There is no need for elaborate air conditioning to keep the pilot cool, or for exotic, high-temperature materials inside the plane, and the engine can burn ordinary JP, these factors greatly reducing costs.

     The fuel tanks will be bladders supported by the structure, with one in the fuselage and two in the wing, one on each side. I considered using the "wet wing" approach but decided that this was too complicated for the average builder. It could be offered as an option for those more skilled and/or more ambitious.

     The main limiting factor on subsonic flying time for a true ATR is due to the fact that the engine will only run below Mach .8 if bottled oxygen is supplied. Onboard O2 would probably still be needed to run the turbine of an ATR below Mach 1.5 to maintain full thrust. Other versions of the engine may use any of several strategies to increase this duration. If nothing else, bolt-on external tanks, similar to the conformal tanks used for the F-14 and F-15, may be available as an option. A more streamlined option is to use an airbreathing power turbine to drive the compressor section, instead of a pure ATR, as mentioned above.

     As it stands now, the general flight profile calls for a steep climb to high altitude, to keep the speed below Mach 1 until the plane is high enough to avoid offending anyone with its sonic boom, followed by rapid acceleration in a dive to get into ramjet mode as quickly as possible and a final climb to cruising altitude.

     The cabin will be pressurized to 10,000 feet by a pump, driven by an electric motor or bleed air, and oxygen will be supplied through a mask or cannula (nose tube). By eliminating whole-cabin air-conditioning I avoided a great deal of extra weight and cost. Military surplus masks and helmets are cheap and, if rebuilt by someone proficient in the task, very reliable. Equivalent commercial gear is also available.

     As stated above, a major reason that a plane like this can be built now and not before is due to recent developments in materials technology. Ceramic fibre fabric composite is already being used to replace metal in homebuilt aircraft firewalls. While the ceramic cloth is heavier and more expensive for the same strength than graphite or Kevlar, it is still vastly cheaper than titanium or the more exotic steels and other high-temperature alloys which have been used or proposed in the past. The skin and main internal structure of the Forerunner should be made of the ceramic composite. As mentioned above, I am still looking for a bonding agent which is very temperature tolerant. It wouldn't do for your plane to come unglued in flight. Since the metals mentioned above are becoming cheaper - especially the titanium - a final design could make use of those.

     The cockpit canopy is simple in concept, though a good design and proper construction are critical. Corning has the ceramic glass used in their Visions cookware available in large sheets for industrial use. The outer layer of the canopy would be made of this, the inner of an aviation-rated acrylic or something similar, perhaps Polycarbonate. Low pressure dry nitrogen between the layers would serve as insulation. The ceramic is quite tough enough to handle the aerodynamic loads at maximum speed, even when heat-soaked, and you don't need to worry much about bird strikes at 60,000+ feet. At lower speeds and altitudes, an accident which shatters the outer layer would still leave the inner intact and capable of holding out until an emergency landing can be made.

     As I proceeded with the design process, I discovered serendipity at work. Elements which I had included for one reason turned out to have additional benefits. For instance, I had not only swept the canards back, but down as well, so that the shock waves trailing off the tips would not interfere with the wing or rudders. Then I noticed that these same shockwaves were going between the engine nacelle and the wing fence on each side of it. By giving the nacelle flat sides, I enabled the Forerunner to trap this shock wave to produce compression lift in certain parts of the flight envelope. As far as I know, only one model of aircraft has done this before, the magnificent XB-70 Valkyrie. Additionally, spreading the lift over two sets of surfaces - the wing and the canards - reduces the intensity of the sonic boom and lowers the frequency to a range where it will be less damaging. This makes getting permission for supersonic overflights much easier. Similarly, placing the vertical tails at the tips of the wings helps recover some of the energy from the tip vortices, reducing drag.

The Planes

     This small craft is intended to sell for under $250,000. It will come in a kit form, with all the components needed to build a flyable plane. The top speed will be Mach 3.5, with a cruise of Mach 3.2. Endurance will be around 20 minutes at cruise speed, provided cruise-optimized climb and acceleration are used. Careful attention was paid to area rule and streamlining. The seat is reclined, both to ease the g loads on the pilot during maneuvering and to lower the height of the canopy and thereby reduce drag. The fuel is ordinary JP, as mentioned above.

     This is the base model of the Forerunner II, but there is plenty of room for improvement. The estimated top speed of operation for an airturboramjet engine is about Mach 6. This plane would only be able to achieve a little over half that. How, then, to make better use of the engine's potential? The main problem in flying faster is heat from air friction. At speeds above Mach 3.5 the temperature at the hottest locations on the plane - the nose and the leading edges of the wings, fins and engine inlet - will be high enough to weaken the bonding agent, causing the composite to delaminate. The aircraft would literally come apart in the air. Going to a higher altitude only helps to a point. Above about 100,000 feet the increase in benefits with increasing altitude declines rapidly, so that you would have to rise above 150,000 feet for any real improvement, and maybe as high as 200,000. It is more practical to add foamed ceramic overlays to the nose and leading edges. There are three benefits to this: First, the ceramic is a very high-temperature tolerant material. Second, it is a good insulator, reducing the transfer of heat to the rest of the plane. Third, and this will require some explanation, the blunt surface of these added pieces would create a stagnation layer.

     Look at winged re-entry vehicles, such as the X-24 and the Space Shuttle Orbiter. The nose and leading edges are very blunt, just the opposite of what you would expect for high speed flight. What happens is that this blunt surface traps a layer of air, which prevents direct contact between the surface and the slipstream, reducing heat transfer. In the thin air at altitudes where high supersonic and hypersonic flight takes place, the added drag from these blunt surfaces is minimal.

     Okay, foamed ceramic, plus larger control surfaces, will get us up to Mach 4 or even Mach 4.5. Which is about the limit for conventional jet fuel, anyway. (Though some research shows that Mach 5+ can be achieved with such hydrocarbon fuels as JP-8+100.) Not bad. We have a basic, bare bones aircraft which will reach Mach 3.5 and a deluxe version which might go as fast as Mach 4.5. Still, there is that extra margin of performance in the engine...

     The Forerunner II gave me a homebuilt which could fly formation with an SR-71, but to fully realize the potential of the ATR would require a new design. The estimated cost is just about double that of the previous plane, being somewhere around $500,000. There are enough people who will pay this - and more - for a sports car which will only do 200 mph to keep several production lines running, so that high cost shouldn't be too much of a problem in creating a marketable aircraft.

     To fly faster the new plane must also fly higher. Which means it needs more control authority and a finer shape. I made the Forerunner III longer, gave it a somewhat larger wingspan, and made the control surfaces even larger in proportion. The added control authority these changes provide allows the Forerunner III to fly safely at higher altitudes, perhaps as high as 250,000 feet, though a more likely cruise altitude is 100,000 to 150,000 feet. This means less heat and drag, so the cruise and top speeds are higher, Mach 5 and Mach 6, respectively. The finer shape creates a smaller angle between lines drawn from the nose to the wingtips. This angle is better than that of the standard swept delta for high supersonic and hypersonic flight.

     The two best choices for fuel in this higher-performing version of the Forerunner are liquid hydrogen and liquid methane/LNG. Liquid hydrogen is expensive, hard to store and requires special materials in the fuel tanks and delivery systems. LNG has a higher storage temperature, has been in industrial use for decades, and has a much higher energy density, so you need smaller tanks on the plane. Given the size of the Forerunner III, this fuel compactness is important.

     Unfortunately, liquefied natural gas contains carbon and nitrogen, both of which form combustion products which can be harmful to the ozone layer. Liquid methane lacks the nitrogen, but is more expensive and less commonly available. I chose LNG, but worried about the pollution this might cause. As it turned out, serendipity struck again, as outlined below, greatly reducing this concern.

     Okay, I now had a plane that could reach Mach 6, which is as fast as the basic ATR can go. It could fly formation with whatever craft the Aurora Project has produced, and possibly even go faster. I was happy. Then I read a brief about some computer simulations run by a Japanese researcher. If he is right (and others have since produced similar results) you can raise the top speed of an ATR to Mach 8, simply by adding a cooling cycle at the inlet. The challenge was on again.

     There are several ways to cool the air coming into an engine. I could have used regenerative cooling, the method NASA planned for the ill-fated NASP and X-33. That involves running the fuel through tubes around the inside of the leading edges, inlet and ramp, then passing it to the engine. This not only removes heat from the hottest spots, but uses this heat to improve performance in the engine. However, this complexity was one of the reasons the X-33 was cancelled. My intent was still to keep things as economical and simple as possible, and the simplest and most economical method of cooling the air was water/methanol injection. A ring of misting nozzles around the inlet and along the crest of the ramp would do the job. As an added benefit, this would also add mass to the flow, increasing both thrust and efficiency. Not bad. The engine could now do Mach 8. But how could I keep the plane cool at this higher speed?

     As I pondered this, I recalled something else. Back in the early seventies, experiments were performed on using transpiration cooling to protect ballistic missile warheads during reentry. Water was ejected through a porous ceramic cap on the nose; as the water evaporated, it carried the heat away, at the same time forming a protective film which augmented the stagnation layer. I already had foamed ceramic on the hot spots. I already had water injection for the engine. All I had to do was provide a larger coolant tank, some extra plumbing and a bigger pump, and I was in business.

     Now I had something! This gave me an engine possibly capable of Mach 8 and an airframe theoretically capable of surviving re-entry! Moreover, the engine exhaust would be surrounded by a sheath of water vapor, streaming back from the nose and leading edges, as well as already containing significant water from the inlet injection. This would combine with the combustion products, either taking them into solution or causing simple physical capture, in both cases creating large molecules which would settle quickly into the lower atmosphere. There went my main objection to Methane!

     So, want to go for a quick ride to the Bahamas?

     Both aircraft would have available a number of options at additional cost. The B model of each would have two seats, the second included in a stretch section which could be built in from the start or added to the A model later. Bolt-on conformal tanks over the wings would extend the range, at a slight increase in drag. A supercruise version of the engine would use regenerative heating of the fuel to reduce the amount needed, which requires a bit of explanation.

     As mentioned above, the regenerative cycle passes fuel around hot spots in the engine (and possibly the airframe), then on to the power turbine. Since the expansion of the vaporized fuel could drive the power turbine without combustion, less onboard oxygen is needed for engines which are true airturboramjets. Also, for both a true ATR and my version this would increase fuel efficiency by using waste heat to help drive the turbine. This would give improved subsonic and low supersonic cruise economy. The supercruise version of the engine would also have bypass doors for full ramjet operation, which would reduce wear on the compressor blades and power turbine at high speeds.

     Okay, let's get a bit crazy with technology, here. The Forerunner III is theoretically capable of surviving re-entry. So, as a promotional stunt, we strap on some solid rocket motors and do a suborbital hop from Baja, California to Hawaii. (An unofficial land speed record was set several years ago by adding a Sidewinder solid rocket motor to the Blue Flame rocket car, so there is precedent.) The trip would take about thirty minutes, not counting landing.

     Okay, you say, playing devil's advocate. You can get enough thrust to make the hop, and the plane will survive re-entry with some modifications - but only if it is lined up nose first! (Unlike orbital and extra-orbital vehicles, which enter at a much higher speed, and therefore use a large, blunt portion of their exteriors to slow more quickly and dissipate heat over a larger surface.) What do you use for attitude control? Simple. A parachute.

     A small ceramic fiber cloth parachute on a long tether attached to the rear of the plane would provide enough force to keep the nose pointed in the direction of travel. It would be deployed near the top of the trajectory, where the extremely thin air would produce just enough drag to inflate it and swing the plane's nose around. Once the spaceplane was deep enough into the atmosphere for the control surfaces to work, the chute would be jettisoned.

     A scramjet (Supersonic Combustion Ramjet, which maintains supersonic flow all the way through) module mounted on each side of the engine nacelle could raise the top speed of the Forerunner III to above Mach 20. This probably isn't practical in such a small plane, but see below for another application.

     NASA has recently developed an engine which combines the functions of the ramjet and scramjet, and does this without internal changes of configuration! They just change the location of fuel injection depending on the flight conditions. This ramscram could make the NASP practical, if the concept is ever revived.

     Other technologies also hold promise for higher performance. The ATR can be modified into an ATR-R, or airturboramjet-rocket. This allows one engine to provide propulsion from a standing start all the way to orbit. The configuration is much more complicated than a simple ATR, but could be developed later, using data gathered from the early Forerunner aircraft. Also, several small, Russian-made liquid fuel rockets - including some burning LNG and LOX - have become available in recent years. Put one of those in the tail of a Forerunner III for suborbital hops.

     If the ATR can't be used - for reasons of expense or technical difficulty - either design could make it to Mach 2.5 with a standard high-performance turbojet. That is fast enough for a ramjet to function. And we know that ramjets work. There are also a number of exotic engine configurations which have been studied - and some of them tested - for hypersonic flight. These tend to be much more complicated than the ATR, however, and much more expensive.

     One reason for going the homebuilt route is avoiding the hassles of FAA certification. Can you imagine what would happen to a request to certify a private plane which can reach Mach 8? The company would be lucky to get a reply! After Forerunner planes have been flying for a while, and the data obtained from them studied and used to make refinements and improvements, the basic technology can be applied to other areas.

     The Forerunner IV is a proposed Mach 6 reconnaissance plane. It would have about the same performance as the fabled and officially nonexistent Aurora (actually the name of a research and development program rather than an aircraft), but with a shorter range and a much smaller payload. It would also cost much less, perhaps as little as a million dollars per item (omitting the money needed for the dedicated military and surveillance equipment).

     The Forerunner V would be a four-place business jet. The pilot and three passengers could be anywhere in the world in an hour or less. Scramjet modules under the wing would allow a cruise speed of Mach 15. If hydrogen is carried for short dashes, it might reach Mach 20! Cost would be under $5 million - less than many conventional business jets. But then, it only carries four people.

     And there you have a brief overview of the Forerunner concept. The technology is available; if the applications are practical and if we can keep the safety nazis and bureaucratic bean counters out of the picture, there is great potential not only for fun, but for the development of a technology base which could result in an economical aerospaceplane. A Model A for the 21st Century. Spread the word.

     Most of the reference materials mentioned in the first section below are reports given at conferences. The rest are articles from technical magazines, except for the NASA Technical Note on propulsion systems for an advanced Space Shuttle. I obtained copies of these documents from the NASA since-closed liaison office at the University of Kentucky. They are now available through NASA's Internet site. The NASA catalog numbers are included. The references in the second section are textbooks.

     "Turbojet-Ramjet Propulsion System for All-Body Hypersonic Aircraft," Mark H. Waters, Office of Advanced Research and Technology, Mission Analysis Division, Moffett Field, California.

     Published January, 1971. (Thorough analysis of high-speed propulsion, although the ATR is not included.) N71-15380.

     "Performance Estimates for Space Shuttle Vehicles Using a Hydrogen- or Methane-Fuelled Turbojet-Powered First Stage," Gerald Knip, Jr. and Joseph D. Eisenberg, Lewis Research Center, Cleveland, Ohio, January, 1972. (Moderately good general reference.) NASA TN D-6634.

     "Mach 5 Cruise Aircraft Research," NASA Langley Research Center, Hampton, Virginia. Published February, 1984. (Although this is a fairly recent report, it uses some very conservative materials technology. Ceramics aren't even mentioned, with the authors recommending the use of superalloys for high-temperature areas. It does have a good discussion of the merits of LNG or Methane over hydrogen as a fuel.) X86-10217.

     "A Parametric Study of a Gas-Generator Airturbo Ramjet (ATR)," Christopher A. Snyder, Lewis Research Center, Cleveland, Ohio. Presented at the 22nd Joint Propulsion Conference, cosponsored by the AIAA, ASME, SAE, and ASEE at Huntsville, Alabama, June 16-18, 1986. (Short overview of the operational characteristics of an ATR.) N86- 31586.

     "Airbreathing Propulsion Concepts for High Speed Tactical Missiles," Fred Zarlingo, Naval Weapons Center, presented at the AIAA/ASME/SAE/ASEE 24th Joint Propulsion Conference, July 11-13, 1988. (Good comparison of various engines for vehicles of the same general size class as the Forerunner planes.) A88-44749.

     "Advanced Airbreathing Powerplant for Hypersonic Vehicles," F. A. Hewitt and B. D. Ward, Rolls-Royce. Presented at the International Symposium on Air Breathing Engines, Athens, Greece, Sept. 3-8, 1989. (Review of general issues in Mach 5+ propulsion systems.) A90-12607.