Being the occasionally interesting ramblings of a major-league technophile.
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.
Supersonic transports are in the news again, as they seem to be every few years. Aviation prognosticators are stating that in order to be worth the research money required to create them, new airliners must fly faster than Mach 3. The consensus is that this sort of sharp improvement in performance is required to make them desirable over improved versions of existing planes. Well, the prognosticators are wrong. Not about the economics of the situation, but about going from current speeds and altitudes for civilian transport aircraft to Mach 3+ at 25,000 meters or more being a sharp improvement in performance. That is merely an extrapolation of current conditions. Indeed, such developments are actually behind predictions of the sixties, which said we would already be flying Mach 5 airliners by this time.
The real "quantum jump" (following the popular usage rather than physics terminology) would be a suborbital transport. Take off at any one place on Earth and land at another in under an hour. To do this the vehicle has to boost out of the atmosphere and take a suborbital ballistic path.
Purely ballistic manned suborbital flight has occurred, but rarely. In fact, the only examples I can think of offhand are the early Mercury flights, which used a smaller booster than the one which carried John Glenn into orbit in the same type of capsule. This sort of flight is not very efficient, at least in terms of getting the most out of the energy input. Still, it is the fastest way we know to get from one place to another on Earth, barring trains running in evacuated tubes at similar velocities. With a rocket launch, a ballistic suborbital flight, atmospheric re-entry and a rocket landing (perhaps aided by parachute) a flight form New York to Tokyo would take a bit over an hour, ground to ground. Adding a wing to convert some of that reentry velocity to lift would mean the same trip could be made nearly as fast but with a much smaller energy input.
This is not a new concept. During the Second World War, German researchers evaluating ways of improving V-2 (model number A-4) altitude and range performance noticed something interesting; put wings on it and the range could be greatly extend. This led to the A-9/A-10 proposal, which would have essentially put a winged V-2 on top of a booster, resulting in a maximum range for the standard payload of 5000 kilometers, in contrast to a bit over 300 km for the V-2. This projected range would have allowed targets on the eastern seaboard of the US to be hit from Europe. Some tests were made and some plans drawn, but nothing practical came of the idea. Still, the payload was limited to the same 1000 kilograms as for the V-2. Without nuclear warheads or some devastating biological or radiological weapon, what was the point? And the V-2 itself was never quite reliable enough to make the launch of such a deadly warhead worthwhile, often blowing up on the launch pad or breaking apart in the air while still over German-held lands.
Later in the war, Austrian engineer Dr. Eugen Sanger and collaborator Dr. Irene Bredt authored the "Sanger-Bredt Report." This outlined the idea for a craft which could take off from Germany, bomb a site in the US, and land on the other side of the world two and a half hours later. This "antipodal bomber" would have been huge vehicle carrying a relatively small bomb load, and bomb aiming accuracy would have been a major problem, but it was technically feasible. There were other problems, though; for instance, the antipodal locations for Germany cover Australia and New Zealand, Allied territory.
In spite of these problems the Antipodal Bomber (the term was soon capitalized) made a lot of converts. By taking off and accelerating to 6000 meters per second it would reach an altitude of 162 kilometers, then drop down to 40 km some 3500 km downrange. Whereupon it would bounce off the denser lower atmosphere (well, dense in comparison to the "air" at the peak of the flight) and rebound to an altitude of 125 km. Another 2500 km and it again would hit bottom and bounce back up. On the third bounce the bomber would have been over New York. After the ninth bounce the craft would go into an extended hypersonic glide for perhaps six thousand kilometers, then descended for a conventional landing.
Increase the maximum velocity to 7000 mps and the craft would go all the way around, landing back where it started... just 3 hours and 40 minutes later.
Now, this is a simple concept but a bit inelegant. All that bouncing... So, in 1949, Dr. Tsien Hsue-shen of the California Institute of Technology developed an alternative plan, using a flight from Los Angeles to New York as his example. (Hsue-shen was one of the Allied rocketry experts sent into Germany as that nation collapsed in defeat in the Second World War. He came to the Massachusetts Institute of Technology on a Boxer Rebellion Scholarship in 1935, becoming a protégé of the legendary Theodor von Karman. Already arguably the leading theoretician in rocket and high-speed flight theory in the United States and instrumental in the founding of the Jet Propulsion Laboratory in California, he took full advantage of the German research recovered. Unfortunately, in the mid-Fifties he fell afoul of Cold War McCarthyism, eventually returning to China to work for the Communists on their rocket program.) The craft would boost in a steep climb for 150 seconds, coast to a peak of over 450 kilometers, re-enter the atmosphere some 15 minutes later and some 2000 kilometers downrange, level off at 43 kilometers and glide for another 2800 km. Total time, ground to ground, a little over an hour. And the trip works about as well the other way.
The difference here is that gliding without the bounce is less efficient, since more of the flight is spent actually flying; that is, in the upper atmosphere, subject to air drag. Of course, if the craft is optimized for high-speed, high-altitude flight the difference is reduced. It's still a nifty concept.
These flight plans work for two reasons. You see, there is very little drag at 43 kilometers. Also, when calculating the glide range for supersonic vehicles, to take into account the kinetic energy as well as the potential, you multiply the lift-to-drag (L/D) ratio by the Mach number to estimate the range. Since suborbital flights re-enter at about Mach 12...
There have been many proposals for suborbital shuttles, ranging from military sortie vehicles to cargo and passenger craft. These concepts also range from purely ballistic, all-rocket vehicles to hypersonic aircraft with rocket boosters. The level of development ranges from initial construction plans to back-of-napkin exercises.
A few years ago I designed (strictly as one of those back-of-napkin exercises, so take my numbers with a very large grain of salt) the Forerunner series of supersonic and hypersonic aircraft, two of which were designed as homebuilts. That is, they could be built in a home workshop, from kits. One of those, the Forerunner III, had a top speed of over Mach 6. Given a design cruise of thirty minutes, it was subject to considerable aerodynamic heating. I therefore included an active cooling system, using water transpiration through porous ceramic at the leading edges, engine inlet and other hot spots. I got the idea for this from an old report on tests of this idea for use in intercontinental ballistic missile warhead protection. Which meant that I now had an aircraft that - with some modifications - could survive re-entry.
The Forerunner III used liquid-methane (or liquefied natural gas, aka LNG) for fuel, and included liquid oxygen for low-speed operation of the airturboramjet (ATR) engine. LOX/liquid methane (or LOX/LNG, pronounced "lox-ling") has only slightly less performance than LOX/LH2 as a rocket propellant combination. Which meant that a small rocket engine burning the same propellants as the airbreathing engine could be added for the boost out of the atmosphere. For mounting the rocket and carrying the extra LOX/LNG needed I replaced the standard single-engine underbody nacelle with a larger one containing tanks for the extra fuel and oxidizer, mounting the rocket engine in the center rear of the nacelle. The single ATR engine was replaced by two, one on each side of the nacelle, to provide the increased thrust needed to compensate for the extra weight and drag of the larger nacelle.
The basic aircraft could (very theoretically) reach Mach 7 at 30,000 meters. That gives an initial velocity of about 2000 meters per second. Adding another 3000 meters from the rocket gives a suborbital hop of 3100 kilometers. The plane can then level off at 43 kilometers and glide for another 7000 kilometers. Then it descends, the ATR are restarted, and the plane flies to a standard airport. The craft would be about the size of a business jet and carry two people. Anywhere in the world. In a couple of hours. Assuming you don't have to wait in the landing pattern at your destination airport.
Aside from the joyride aspects, is there sufficient financial justification for building a ballistic transport? Oh, yes... Enough that several groups have developed serious business proposals around such vehicles.
There are many instances of companies keeping multi-million dollar helicopters on hand to ferry an important person or piece of equipment. An all-rocket suborbital craft has similar vertical takeoff and landing characteristics, but with many times the range. Yes, it also costs many times as much as a helicopter. But what if a broken part has to be shipped from Los Angeles to Sidney (over 12,400 kilometers)? And it's needed in a big hurry? Shipping that part by standard air express services would take two days, and for each hour of that time your machines aren't earning the $7000 they are supposed to, while expenses mount and contracts fall further behind. What sort of premium would you pay a shipper who can get what you need where you need it in one hour instead of 48?*
I'm not saying that we shouldn't develop supersonic - or even hypersonic - transports. For distances under 5000 kilometers the time difference between traveling at Mach 5 and going suborbital isn't all that great. For longer distances, though, there's a great deal of money out there to be made in fast transport of people and packages. And how many folks have ridden the Concord just to say they have? All we need to do is find someone willing to pay the development costs.
*This is taken from an example given in Halfway to Anywhere by G. Harry Stine, a book on the technology and economics of spaceflight which I very much recommend.
This document is Copyright 2002 Rodford Edmiston Smith. Anyone wishing to reproduce it must have permission from the author, who can be reached at: email@example.com