by Dean Ing
There’s reason to suspect that simple air-breathing jet engines such as the Schmidt pulsejet can also operate as ramjets by clever modifications to pulse vanes and duct inlet geometry. In this way, sophisticated design may permit a small have-not nation to produce air-breathing power plants to challenge those of her richer neighbors, in overall utility if not in fuel consumption. A pulsejet develops thrust at rest, and could boost a vehicle to high subsonic velocity where ramjets become efficient. Supersonic ramjets need careful attention to the region just ahead of the duct inlet, where a spike-like cowl produces exactly the right disturbance in the incoming air to make the ramjet efficient at a given speed. A variable-geometry spike greatly improves the efficiency of a ramjet over a wide range of airspeeds, from sonic to Mach five or so. We might even see pulse-ram-rocket tribrids using relatively few moving parts, propelling vehicles from rest at sea level into space and back.
For a nation where cost-effectiveness or material shortages overshadow all else, then, the simplicity of the pulse-ram-rocket could make it popular. A turbine-rocket hybrid would yield better fuel economy, though. The choice might well depend on manufacturing capability; and before you can complain that rockets absolutely demand exacting tolerances in manufacturing, think about strap-on solid rockets.
MHD is another possible power source as we develop more lightweight MHD hardware and learn to use megawatt quantities of electrical energy directly in power plants. An initial jolt from fuel cells or even a short-duration chemical rocket may be needed to start the MHD generator. Once in operation, the MHD unit could use a combination of electron beams and jet fuel to heat incoming air in a duct, and at that point the system could reduce its expenditure of tanked oxidizer. We might suspect that the MHD system would need a trickle of chemical, such as a potassium salt, to boost plasma conductivity especially when the MHD is idling. By the year 2050, MHD design may be so well developed that no chemical seeding of the hot gas would be necessary at all. This development could arise from magnetic pinch effects, or from new materials capable of withstanding very high temperatures for long periods while retaining dielectric properties.
It almost seems that an MHD power plant would be a perpetual motion machine, emplaced in an atmosphere-breathing vehicle that could cruise endlessly. But MHD is an energy-conversion system, converting heat to electricity as the conductive plasma (i.e., the hot gas stream) passes stationary magnets. The vehicle would need its own compact heat generator, perhaps even a closed-loop gaseous uranium fission reactor for large craft. A long-range cruise vehicle could be managed this way, but eventually the reactor would need refueling. Still, it’d be risky to insist that we’ll never find new sources of energy which would provide MHD power plants capable of almost perpetual operation.
Whether or not MHD justifies the hopes of power plant people, other power sources may prove more compact, lighter, and-at least in operation-simpler. Take, for example, a kilogram of Californium 254, assuming an orbital manufacturing plant to produce it. This isotope decays fast enough that its heat output is halved after roughly two months; but initially the steady ravening heat output from one kilo of the stuff would be translatable to something like 10,000 horsepower! No matter that a kilo of Californium 254 is, at present, a stupefyingly immense quantity; ways can probably be found to produce it in quantity. Such a compact heat source would power ramjets without fuel tanks, or it could vaporize a working fluid such as water. In essence, the isotope would function as a simple reactor, but without damping rods or other methods of controlling its decay. Like it or not, the stuff would be cooking all the time. Perhaps its best use would be for small, extended-range, upper-atmosphere patrol craft. There’s certainly no percentage in letting it sit in storage.
For propulsion in space, several other power plants seem attractive. Early nuclear weapon tests revealed that graphite-covered steel spheres survived a twenty kiloton blast at a distance of ten meters. The Orion project grew from this datum, and involved nothing less in concept than a series of nukes detonated behind the baseplate of a large vehicle. As originally designed by Ted Taylor and Freeman Dyson, such a craft could be launched from the ground, but environmentalists quake at the very idea. The notion is not at all far-fetched from an engineering standpoint and might yet be used to power city-sized space dreadnoughts of the next century if we utterly fail to perfect more efficient methods of converting matter into energy. Incidentally, the intermittent explosion rocket drive was tested by Orion people, using conventional explosives in scale models. Wernher von Braun was evidently unimpressed with the project until he saw films of a model in flight.
This kind of experiment goes back at least as far as Goddard, who tested solid-propellant repeater rockets before turning to his beloved, persnickety, high-impulse liquid fuels. No engineer doubts there’ll be lots of glitches between a small model using conventional explosives, and a megaton-sized version cruising through space by means of nuke blasts. But it probably will work, and God knows it doesn’t have a whole slew of moving parts. Structurally, in fact, it may be a more robust solution for space dreadnoughts than are some other solutions. It seems more elegant to draw electrical power from the sun to move your space dreadnought, for instance-until we realize that the solar cell arrays would be many square kilometers in area. Any hefty acceleration with those gossamer elements in place would require quintupling the craft’s mass to keep the arrays from buckling during maneuvers. The added mass would be concentrated in the solar array structure and its interface with the rest of the craft.
On the other hand, there’s something to be said for any system that draws its power from an inexhaustible source-and the Orion system falls short in that department since it must carry its nukes with it. The mass driver is something else again. It can use a nearby star for power, though it must be supplied with some mass to drive. Lucky for dwellers of this particular star system: we can always filch a few megatons of mass from the asteroid belt.
The mass driver unit is fairly simple in principle. It uses magnetic coils to hurl small masses away at high speed, producing thrust against the coils. Gerard O’Neill has demonstrated working models of the mass driver. In space, a mass driver could be powered by a solar array or a closed-cycle reactor, and its power consumption would not be prohibitively high. The thrust of the device is modest-too low for planetary liftoff as currently described. Its use in an atmosphere would be limited, power source aside, by aerodynamic shock waves generated by the mass accelerated to hypersonic velocity within the acceleration coils.
For fuel mass, O’Neill suggests munching bits from a handy asteroid-though almost any available mass would do. The mass need not be magnetic since it can be accelerated in metal containers, then allowed to continue while the metal “buckets” are decelerated for re-use.
In case you’re not already ahead of me, notice that the mass driver offers a solution to the problem of “space junk” that already litters orbital pathways. The mass-driver craft can schlep around until it locates some hardware nobody values anymore, dice and compact it into slugs, feed it into the mass driver buckets, and hurl the compacted slugs away during its next maneuver. Of course, the craft’s computer will have to keep tabs on whatever is in line with the ejected masses, since the slugs will be potentially as destructive as meteorites as they flee the scene. Imagine being whacked by a ten-kilogram hunk of compacted aluminum garbage moving at escape velocity!
Solar plasma, the stream of ionized particles radiated by stars, has been suggested as a “solar wind” to be tapped by vast gossamer sails attached to a space vehicle-with the pressure of light radiation adding to the gentle “wind.” Carl Wiley, writing as “Russell Saunders,” outlined the space windjammer proposal in 1951. His sail was envisioned as a parachute-like arrangement of approximately hemispherical shape, made of lithium, many square kilometers in area. Wiley argued that, while such a craft could hardly survive any environment but space, it could be made to revolve with its sail as it circles a planetary ma
ss. By presenting a profile view of the sail as it swings toward the sun, and the full circular view as it swings away again, the craft could gradually build up enough velocity to escape the planet entirely. Even granting this scheme, a sail quickly deflated or rearranged into windsock proportions, it seems unlikely that a starsailer could move very effectively into a solar wind in the same way that a boat tacks upwind. The interstellar yachtsman has an advantage, though: he can predict the sources of his winds. He cannot be sure they won’t vary in intensity, though; which leads to scenarios of craft becalmed between several stars until one star burns out, or becomes a nova.
It takes a very broad brush to paint a military operation of such scale that solar sails and mass drivers would be popular as power plants. These prime movers are very cost-effective, but they need a lot of time to traverse a lot of space. By the time we have military missions beyond Pluto, we may also have devices which convert matter completely into photons, yielding a photon light drive. In the meantime, nuclear reactors can provide enough heat to vaporize fuel mass for high-thrust power plants in space. So far as we know, the ultimate space drive would use impinging streams of matter and antimatter in a thrust chamber. This is perhaps the most distant of far-out power plants, and presumes that we can learn to make antimatter do as we say. Until recently, there was grave doubt that any particle of antimatter could be stable within our continuum. That doubt seems to be fading quickly, according to reports from Geneva. Antiprotons have been maintained in circular paths for over eighty hours. The demonstration required a nearly perfect vacuum, since any contact between antimatter and normal matter means instant apocalypse for both particles. And as the particles are mutually annihilated, they are converted totally into energy. We aren’t talking about your workaday one or two percent conversion typical of nuclear weapons, understand: total means total. A vehicle using an antimatter drive would be able to squander energy in classic military fashion!
The power plants we’ve discussed so far all lend themselves to aircraft and spacecraft. Different performance standards apply to land- and water-based vehicles, which must operate quietly, without lethal effluents, and slowly at least during docking stages. Turbines can be quiet, but they produce strong infrared signatures and they use a lot of fuel, limiting their range somewhat. When you cannot be quick, you are wise to be inconspicuous. This suggests that electric motors might power wheeled transports in the near future, drawing power from lightweight storage batteries or fuel cells. The fuel cell oxidizes fuel to obtain current, but the process generates far less waste heat than a turbine does. The fuel cell also permits fast refueling-with a hydride, or perhaps hydrogen-which gives the fuel cell a strong advantage over conventional batteries. However, remember that the fuel cell “burns” fuel. No fair powering a moonrover or a submarine by fuel cells without an oxidizer supply on board.
When weight is not a crucial consideration, the designer can opt for heavier power plants that have special advantages. The flywheel is one method of storing energy without generating much heat as that energy is tapped. A flywheel can be linked to a turbine or other drive unit to provide a hybrid engine. For brief periods when a minimal infrared signature is crucial, the vehicle could operate entirely off the flywheel. Fuel cells and electric motors could replace the turbine in this hybrid system. Very large cargo vehicles might employ reactors; but the waste heat of a turbine, reactor, or other heat engine is always a disadvantage when heatseeking missiles are lurking near. It’s likely that military cargo vehicles will evolve toward sophisticated hybrid power plants that employ heat engines in low-vulnerability areas, switching to flywheel, beamed power, or other stored-energy systems producing little heat when danger is near. As weapons become more sophisticated, there may be literally almost no place far from danger-which implies development of hybrid power plants using low-emission fuel cells and flywheels for wheeled vehicles.
MATERIALS
Perhaps the most direct way to improve a vehicle’s overall performance is to increase its payload fraction, i.e., the proportion of the system’s gross weight that’s devoted to payload. If a given craft can be built with lighter materials, or using more energetic material for fuel, that craft can carry more cargo and/or can carry it farther, faster.
Many solids, including metals, are crystalline masses. Entire journals are devoted to the study of crystal growth because, among other things, the alignment and size of crystals in a material profoundly affect that material’s strength. Superalloys in turbine blades have complex crystalline structures, being composed of such combinations as cobalt, chromium, tungsten, tantalum, carbon, and refractory metal carbides. These materials may lead to hyperalloys capable of sustaining the thermal shock of a nuke at close range.
As we’ve already noted, graphite-coated steel objects have shown some capacity to survive a nuke at close quarters. There may be no alloy quite as good as the old standby, graphite, especially when we note that graphite is both far cheaper and lighter in weight. Superalloys aren’t the easiest things to machine, either. Anybody who’s paid to have superalloy parts machined risked cardiac arrest when he saw the bill. Graphite is a cinch to machine; hell, it even lubricates itself.
More conventional alloys of steel, aluminum, and titanium may be around for a long time, with tempering and alloying processes doubling the present tensile strengths. When we begin processing materials in space, it may be possible to grow endless crystals which can be spun into filament bundles. A metal or quartz cable of such stuff may have tensile strength in excess of a million pounds per square inch. For that matter, we might grow doped crystals in special shapes to exacting tolerances, which could lead to turbine blades and lenses vastly superior to anything we have today. Until fairly recently, quartz cable had a built-in limitation at the point where the cable was attached to other structural members. Steel cable terminals can simply be swaged-squeezed-over a steel cable, but quartz can’t take the shear forces; you can cut through quartz cable with a pocketknife. This problem is being solved by adhesive potting of the quartz cable end into specially formed metal terminals. Your correspondent was crushed to find himself a few months behind the guy who applied for the first patents in this area. The breakthrough takes on more importance when we consider the advantages of cheap dielectric cable with high flexibility and extremely high tensile strength at a fraction of the weight of comparable steel cable. Very large structures of the future are likely to employ quartz cable tension members with abrasion-resistant coatings.
Vehicles are bound to make more use of composite materials as processing gets more sophisticated. Fiberglass is a composite of glass fibers in a resin matrix; but sandwich materials are composites too. A wide variety of materials can be formed into honeycomb structures to gain great stiffness-to-weight characteristics. An air-breathing hypersonic craft might employ molybdenum honeycomb facing a hyperalloy inner skin forming an exhaust duct. The honeycomb could be cooled by ducting relatively cool gas through it. On the other side of the honeycomb might be the craft’s outer skin; say, a composite of graphite and high-temperature polymer. Advanced sandwich composites are already in use, and show dramatic savings in vehicle weight. The possible combinations in advanced sandwich composites are almost infinite, with various layers tailored to a given chemical, structural, or electrical characteristic. Seventeen years ago, an experimental car bumper used a composite of stainless steel meshes between layers of glass and polymer to combine lightness with high impact resistance. A racing car under test that year had a dry weight of just 540 lb., thanks to a chassis built up from sandwich composite with a paper honeycomb core. The writer can vouch for the superior impact and abrasion resistance of this superlight stuff, which was all that separated his rump from macadam when the little car’s rear suspension went gaga during a test drive. The vehicle skated out of a corner and spun for a hundred meters on its chassis pan before coming to rest. The polymer surface of the pan was scratched up a bit, yet there was no structural damage whatever. But we
considered installing a porta-potty for the next driver . . .
Today, some aircraft use aluminum mesh in skins of epoxy and graphite fiber. The next composite might be titanium mesh between layers of boron fiber in a silicone polymer matrix. The chief limitation of composites seems to be the adhesives that bond the various materials together. It may be a long time before we develop a glue that won’t char, peel, or embrittle when subjected to temperature variations of hypersonic aircraft. The problem partly explains the metallurgists’ interest in welding dissimilar metals. If we can find suitable combinations of inert atmosphere, alloying, and electrical welding techniques, we can simply (translation: not so simply) lay a metal honeycomb against dissimilar metal surfaces and zap them all into a single piece.
Several fibers are competing for primacy in the search for better composites; among them boron, graphite, acetal homopolymer, and aramid polymers. Boron may get the nod for structures that need to be superlight without a very high temperature requirement, but graphite looks like the best bet in elevated temperature regimes. Sandia Laboratories has ginned up a system to test graphite specimens for short-term high temperature phenomena including fatigue, creep, and stress-rupture. The specimens are tested at very high heating rates. It’s easy to use the report of this test rig as a springboard for guessing games. Will it test only graphite? Very high heating rates might mean they’re testing leading edges intended to survive vertical re-entry at orbital speeds. Then again, there’s a problem with the heat generated when an antitank projectile piles into a piece of Soviet armor. Do we have materials that can punch through before melting into vapor? And let’s not forget armor intended to stand up for a reasonable time against a power laser. For several reasons, and outstanding heat conductivity is only one of them, graphite looks good to this guesser. If the Sandia system isn’t looking into antilaser armor, something like it almost certainly will be-and soon.