Could the rocket helicopter be the space equivalent of the personal computer?
__ Gary Hudson wants to build an inexpensive helicopter powered with rocket engines that will lift tourists into space. "it's such a wacko idea," he says, "that I have a great deal of faith that no competitor will think it'll work." That sort of clears out the field. Hudson has spent 25 years as a maverick in the commercial space industry, including a long stint in the 1980s as founder and president of Pacific American Launch Systems Inc. For most of that time, Hudson tried to buck NASA and develop reusable single-stage commercial rockets, which look pretty normal. Then a few years ago, he came across this helicopter idea:__ About three years ago, my friend Bevin McKinney sat in a conference room at American Rocket Company, a business he co-founded, and told me about an idea he had. He wanted to build a space helicopter - a rocket ship powered by a huge propeller. My first reaction was to patiently say, "Bevin, that's insane."
My second reaction was to keep listening. The difference between "insane" and "insanely great" is often only a matter of shifting perceptions, something Bevin excelled in. Some of his earlier insane ideas - such as a hybrid liquid- and solid-fuel rocket that couldn't blow up - turned out to be insanely great.
For years both Bevin and I had been frustrated that space was the exclusive province of government space agencies and hero astronauts. We both wanted to go to space for the sheer fun of it and, in the finest capitalist tradition, make a few bucks along the way. Our careers had taken us, independently, in virtually the same direction. When we first met, we were competitors working on expendable commercial launch vehicles - throwaway rocketry. But we both believed that these expendable rockets were just the first step to achieving our real goals of opening the space frontier to the common man - in other words, ourselves.
Bevin arguably had been more successful than I. He'd founded two rocket companies and had built and flown the Dolphin, a prototype of an expendable commercial satellite launcher. While I had gotten the first private launch vehicle in the US to the pad a few years before him, his actually worked. While I was working on ideas for inexpensive reusable rockets during the 1980s, Bevin was racking up successful firings of large hybrid rocket engines more often than anyone else. But, sadly, when we talked that spring of 1993, both our companies were being driven out of business by government-funded competitors. We needed an edge, and this time we needed to cooperate, not compete.
Contrary to its image, the aerospace industry is hidebound, with scant reward for innovation. Little has changed in fundamental rocket technology since the V-2 rockets were flown 50 years ago. Much of that is due to the politics played within NASA, within the private sector industry, and within Congress. It's in the interests of many of these players to keep expensive, multistaged rockets as the norm.
That political environment changed somewhat after the Challenger exploded, but commercial rocketry still has had a hard time getting off the ground. In my view, it's because the roots of rocketry were not in air transport but in artillery. Today's American commercial launchers are all derived from military ICBM technology. Throwaway rockets became the only way to go to space. But imagine throwing away an airliner after a single flight: the price of a ticket would be - pardon the pun - astronomical.
As far back as 30 years ago, a few brave souls began offering an alternative idea: the single-stage reusable rocket or spaceship. They weren't talking about what became the US space shuttle because that vehicle uses multistage boosters and a throwaway external tank to get into orbit. These ideas, including a few of my own, eventually led to the government's successful DC-X program, now segueing into the X-33 near-orbital reusable launch vehicle. Developed in only 18 months for 10 percent of the cost of a single space shuttle flight, the DC-X went a long way toward demonstrating the promise of a reusable, affordable, single-stage spaceship that eventually could carry humans.
As I sat in that American Rocket conference room absorbing Bevin's wild notions about a rocket with a propeller, I began thinking that maybe his idea held the most promise yet. This idea just might make the transition from "insane" to "insanely great." He'd even come up with a cool name for his spaceship, full of sound and maybe a little fury. The Roton.
__ Spinning into space__
Reusable rockets need both high-performance engines and very lightweight structures. Bevin proposed to cut weight by putting a rocket engine at the tip of each of the four rotor blades, using the rockets to shoot horizontally and spin the blades. The spinning rotor blades would create a downward thrust that would provide lift. The rotor would maximize the efficiency of the rocket thrust, which normally just exhausts downward.
This increased performance would - in rocket engineer terminology - "pay for the weight of the rotor." The Roton also promised to reduce takeoff noise substantially, because the vehicle would require only a fraction of the rocket thrust at liftoff, and the rotor would be generating thrust more efficiently than a conventional rocket at lower altitudes.
A key function of the spinning rotor would be to siphon propellant into the engines at very high pressure. (This takes advantage of a simple principle of hydrodynamics that you can prove by standing on the roof of a two story building and dropping a garden hose into a 55-gallon drum of water on the ground. Swing the other end of the hose over your head like a lariat and you'll drain the drum dry.)
These high pressures were previously achieved only by using very expensive, very heavy pumps driven by hot engine gases. Eliminating engine pumps, in rocketry terms, is heavenly. Any weight saved in building an engine is a compounded saving. A significant portion of the propellant a spaceship carries is used just to lift the engine, so that the less engine weight, the less fuel it needs to carry; thus the less engine it needs, the less fuel it needs to carry - and so on.
Once it runs out of atmosphere, the rotor could no longer provide thrust to push the vehicle along the trajectory to orbit. At this point, the rockets at the rotor tips would swivel to point their exhaust thrust backward. Of course, the rotor would have to continue to spin even without air, otherwise there would be no pumping power to feed the engines. A tiny fraction of that thrust would be deflected to the side to spin the propellers. Still, overall, you'd save on propellant because the high performance of the rotors in the atmosphere would more than compensate for the need to spin the rotor in space.
Bevin was not the first to propose putting rockets at the tips of helicopter blades. Others had kicked around the idea and a few experimental helicopters had been built. But nobody had ever suggested building a vehicle capable of powering itself all the way into space. Likewise, Bevin borrowed some ideas about using a rotor during reentry. Engineers at Bell Helicopter and French aviation company Giravions Dorand had proposed using rotor blades as a "drag brake" to slow down reentering space capsules. NASA engineers had confirmed the concept with wind tunnel tests at the Ames Research Center in Mountain View, California, as early as the late 1960s.
Bevin saw that the rotor would solve the biggest problem for any true spaceship: landing. The standard solution - retro-rockets for touchdown - does work, as demonstrated by the DC-X landing on rocket thrust at White Sands Missile Range in New Mexico in 1994. But retro-rockets have many problems: they need more propellant; they're very noisy; and, most important, you have to worry about whether they will fire up at precisely the right time. Waiting for that relighting enhances what test pilots quaintly call "the pucker factor."
On the other hand, a low-speed rotor landing would be much less risky, far more quiet, and would consume less fuel. The spaceship would weigh less since the extra landing propellant needed in the final seconds of a flight wouldn't have to be carried all the way to orbit and back again.
That leaves the most frequently asked question about the Roton: wouldn't the rotor blades burn off in the atmosphere? The remarkable - and counterintuitive - answer is No. During the long climb into orbit, the atmosphere steadily decreases in density. The Roton starts out at very low speeds in the high-density atmosphere. As it picks up speed and climbs higher, the atmosphere thins out. The "dynamic pressure" (think wind) would actually be lower for the Roton than for many high-performance aircraft, including fighters.
During reentry, the Roton would encounter a pretty benign environment as well. The Roton would start out at high speeds, but the atmosphere would be very thin. As the atmosphere becomes more dense at lower altitudes, the rotor would slow the vehicle down. Also, the load on the blades would be rather small because most of the propellant would have been consumed - meaning more than 90 percent of the overall weight would be gone. Wind tunnel tests have shown the heating would be no worse than that experienced by the space shuttle or other reentry vehicles.
__ Stumbling blocks__
OK, so the Roton is a nice concept, but could a bunch of engineers on a budget really go out and build it?
Yes. The key to Roton development is to use inexpensive technologies already created by the homebuilt-aircraft community, known in the industry as "homebuilders." Right now thousands of homebuilders are producing sophisticated flying machines in their garages by using graphite-epoxy composite materials, modern electronics for both design and onboard avionics, and an abundance of innovation. These craft range from copies of World War I fighters to personal jet aircraft.
Indeed, a whole industry has grown up in the shadow of the military industrial aerospace complex. It's led by people like Burt Rutan, whose Scaled Composites Inc. has produced everything from the body of the GM Ultralite automobile to the aeroshell of the DC-X experimental rocket. Today sophisticated amateurs and interdisciplinary professionals are leapfrogging the tottering space establishment.
The Experimental Aircraft Association, representing these homebuilders, reports that more than half a million aviation and space enthusiasts show up each year in Oshkosh, Wisconsin - the Woodstock of homebuilders. For 8 days the little airport at Oshkosh becomes the busiest in the world. Almost certainly someone in that crowd is already thinking about building a personal rocket.
In this environment, developing a working Roton becomes easier. A Roton could use the high-tech materials already developed by the homebuilders market. It could use low-cost aviation kerosene and cryogenic oxygen liquefied from air. It would require no launchpad, since no rocket thrust would ever touch the ground. No longer would vast, overpriced, government-owned launch sites be required. Any small county airport should do.
The early Rotons may well be flight-tested with a human crew onboard or possibly be tele-operated from the ground. The vagaries of flight testing generally require the intuitive response of a human pilot, whether it's one sitting in a cockpit or controlling the vehicle from a virtual reality terminal on the ground. This human involvement in flight testing will speed development, as it allows incremental testing: first flying the vehicle in hover, then up through Mach 1 and, finally, after many test flights, into orbit. This is how airplanes are tested, but it's dramatically different from missile flight tests. Since there is no way to land an expendable missile after liftoff, it must be tested to orbit on its one and only flight. Because of the cost of these missiles, they rarely fly more than one test flight before carrying a paying cargo. In contrast, aircraft routinely take dozens if not hundreds of test flights.
A prototype Roton could be developed for tens of millions of dollars instead of the tens of billions of dollars it took to develop the space shuttle. Within 10 years, an off-the-shelf Roton might cost no more than a light private jet - between US$5-10 million.
Safety? A reusable Roton should be as safe to operate as a small business jet - mainly because it will have redundant systems comparable to aircraft. This is crucial to the Roton's development success and operational safety. Without multiple rocket engines and rotor blades, and redundant avionics, the Roton would likely have the same abysmal failure rate as other boosters - about one in twenty of those never makes it into orbit.
What are the downsides? The Roton seems to have some size limitations. We'd probably not want to build a Roton with a rotor more than about 150 feet in diameter because of manufacturing and handling complexities. So the Roton seems destined to fly mostly light cargo. But this certainly could include ferrying people to space and back again. It is perfectly suited to the emerging opportunity of space tourism.
As with any technology, one can imagine other concerns. A spectacular increase in spacecraft traffic might also increase atmospheric pollution or contribute to space debris. There are some who worry about exploitation of near-Earth orbit by terrorists or military powers. And, of course, like any transportation system, Rotons will crash, collide, and otherwise fail, leading to loss of life.
But the genie is out of the bottle. From an engineering point of view, the problems are essentially solved. The technology is in place, and someone is going to do it. If Rotons or their equivalents are not built and flown in the United States, then we can expect that they will be developed elsewhere. The only issue is whether or not the development will happen soon or be delayed by financial and bureaucratic impediments.
Could the Roton be the space equivalent of the personal computer, challenging the mainframe-like expendable missiles of today? It could certainly go a long way toward making space accessible to many of us. And continuing the metaphor, it might make its inventor, and a bunch of third party vendors, a little money on the side.
Sitting in the American Rocket office three years ago, I thought Bevin was insane. Today there's still no doubt in my mind: this is an insane idea. But it's a great one - and it will work.
