Return to contents.
Back To The Future: The Rotary Rocket Roton
by Jack Hagerty, LUNAR #002
When the 20th Century opened, the spaceship was just a dream, albeit one that some scientists and visionaries were beginning to realize was possible. At mid-century it was a reality, although an immensely complicated and inefficient one requiring armies of specialists and providing high drama with each launch. At the century's close the spaceship is on the verge of becoming a commodity, a workaday service requiring no more support and providing no more drama than your average air freight shipment.
The theoreticians of the century's first two decades gave way to the experimenters of the next two, who in turn supported the hardware developers of the next two. With a push of cold war paranoia, one more decade saw human footprints on the moon, an event that still ranks as the supreme accomplishment of our age, but one that more and more is looking like a costly detour away from the development of the true spaceship.
Once the experimenters were flying hardware, authors and movie producers provided us with images of the spaceship in its true form; a utilitarian machine used for leaving the Earth. It takes off, it does it's job and it lands again. It can take off from anywhere, land anywhere, and doesn't require an army of specialists working for months to get it ready to fly again. This did not agree at all with what we saw flying off the pads of the world's space launch centers; huge behemoths that flew only once while constantly shedding parts of themselves until only the payload was left.
Well, just as technology historians consider the vacuum tube as being a costly detour that delayed the development of true electronics by 50 years, so do the Saturns, Titans and other descendants of wartime missiles appear to have stood in the way of developing the real spaceship. As the century draws to a close, it looks as if the version of space flight depicted in books and movies of the '30s, '40s and 50's is finally coming to pass. As of this writing, there are no fewer than five Reusable Launch Vehicle (RLV) development programs dedicated to creating a spaceship that is not only 100% reusable, but also vastly simpler and cheaper to fly
Of these five projects, though, only the Roton by the Rotary Rocket Company fits the classic archetypal image of the spaceship: a single vehicle which ascends vertically into space, performs its mission then descends back to Earth and lands the way it took off, namely vertically and intact needing only to be refueled and reloaded before flying again.
The crew at Rotary, lead by DC-X alumnus Gary Hudson, get the lateral thinking award for attacking the problem of getting payloads into Low Earth Orbit (LEO). They first asked themselves what are we actually trying to accomplish here? The mission, they decided, was to take a reasonable payload all the way to LEO in a single vehicle that was both complete and completely reusable.
VEHICLE - To accomplish this goal they followed the old auto racers' maxim "simplificate and add lightness." Lots of lightness. The key to making this work is to cut the structural and fuel mass fractions to absolute minimums. As they quite proudly point out, this is a stripped down "Volkswagen" of a space vehicle; the least possible amount of spacecraft necessary to do the job. The automotive analogy is quite appropriate because to reduce weight they took advantage of advances in materials that started in the auto racing field.
While composite materials are used extensively in aerospace for subassemblies, most airframes and major structures are still built of metals such as aluminum, titanium and steel. However, in auto racing, especially Formula 1, they have been using carbon fiber/epoxy composites for decades to build the main structures of the cars. One aircraft designer who embraced composites early and completely is Burt Rutan, who is probably best known as the designer of the Voyager which circled the globe without refueling in 1986. Rutan's company, Scaled Composites, is acting as vehicle integrator and is lending his expertise to the design. The main "aeroshell" structure and both propellant tanks are made of carbon fiber/epoxy composites.
The fuel for the Roton is kerosene, not the more efficient liquid hydrogen. Why should it use the less efficient fuel when you have to carry more of it (from a weigh standpoint) for the same amount of energy? The answer, ironically, is to save weight. While the weight of the kerosene is more than an energy-equivalent amount of liquid hydrogen, kerosene is so much denser than liquid hydrogen (some seven times denser, in fact) that you only need a tank about one fifth as big. "It's easier to make a small tank light than it is to make a big tank light" notes Geoffrey Hughes, Rotary's business director. And a smaller tank means a smaller amount of airframe to cover it leading to a greater overall savings in vehicle weight. While the tank is light, it still has to be strong since it is the Roton's main load bearing member. Sitting in the base of the craft, it has to absorb the engines thrust loads and transmit them up into the aeroshell structure and ultimately into the LOX tank in the nose.
The engines using this fuel are another innovation. The cryogenic pumps and plumbing for a hydrogen engine are much heavier than those used by the ambient temperature kerosene fueled one (the LOX part is the same in both cases). But Rotary did away with the heavy turbopumps all together showing that the "rotary" concept goes beyond helicopter recovery. Rather than two or three relatively large rocket engines, the Roton has 72 small thrust chambers arranged in a ring. This means that each chamber need produce only 7,000 lb. (31,000 Nt.) of thrust to achieve the half million pounds (2.2 million Nt.) needed for liftoff. This is a thrust level that even amateur rocket designs are achieving, which means a much simpler development and testing program plus a much higher degree of reliability. To feed such an array (half again as many as von Braun had envisioned for his most massively clustered design, the Collier's Ferry), the thruster wheel is set in bearings allowing it to spin. The centrifugal force of the spinning ring provides the pumping action. In addition to the 72 main combustors, there are four more smaller ones in the wheel for use on-orbit. Their main job is to stop the engine wheel from spinning after the main boost phase, but they're also used for orbit circularization and the deorbit burn. Attitude control is done by small monopropellant (peroxide) thrusters.
One area in which they decided to give up some precious payload weight was in the addition of a crew since there are times when having people aboard can save a mission. The Roton has provisions for a crew of two (a pilot and a payload specialist) placed amidships next to the cargo bay and in between the two propellant tanks. Having a payload specialist on board allows them to not only deploy satellites, but retrieve them as well.
FLIGHT PROFILE - The Roton's flight profile is extraordinarily simple. It lifts off from its small concrete pad and drives itself directly into orbit using most of the propellants in the main tanks. Once on orbit, the Roton can stay as long as 72 hours, but most missions will probably be much shorter.
Up until the time of the deorbit burn, the rotor blades are folded down along the sides of the vehicle. Just before reentry they are unfolded up to their horizontal recovery position (this helps stabilize the craft as it comes back base-first). Despite being fully deployed, Rotary assures us that they are within the plasma sheath during reentry. After it drops below Mach 1, which happens just below 30,000 ft, the blades begin to autorotate. Once under rotor descent, the glide slope is about 1:1 so the pilot has a five mile (8 Km) radius circle in which to find his landing pad. For the terminal landing maneuver, small rotor tip rocket motors are started to allow a powered landing. The tip motors use the same monopropellant as the attitude control system. Since it lands so gently, the landing gear is likewise very small and (again) light weight.
One down side to the vehicle being so light is high accelerations. F=ma and when the you have high thrust with low weight it's going to accelerate quickly. Going up isn't too bad with a maximum four gees just prior to burnout. Coming back, though, the crew experiences a maximum eight gee deceleration during reentry. As Geoffrey Hughes notes, "it's just a big, empty plastic tub" so it slows down in a hurry. Of course that's the key to the whole thing. The accelerations may be high, but they're over with quickly.
The project is still on schedule and the first craft, the Atmospheric Test Vehicle (ATV) was rolled out on March 1 to the cheers of aerospace professionals, space flight enthusiasts and just plain groupies. The ATV is full sized and weighted like an operational Roton would be after reentry. It is used to test the helicopter approach and landing systems. The nice thing about the design is that they don't have to haul it up on the back of a 747 like the Shuttle tests. They just use a bigger monopropellant tank and drive it up to 30,000 ft or so using the rotor tip motors.
There are two more prototypes to be completed. First is the Structural Test Vehicle (STV). While the STV isn't intended to fly, it allows them to subject a flight equivalent frame to the same kinds of loads it will receive in flight to check their design calculations. Finally comes the Propulsion Test Vehicle (PTV). This one will actually fly to orbit as a final all-up systems check, but won't carry payload. Once they're assured that everything works, they go into production.
The first approach and landing tests using the ATV are scheduled for this summer and rocket powered flights should start early next year. The countdown to future has entered its final phase!
Quickspec: Rotary Roton
Vehicle Morphology.................Standard Year ...............................Current Manufacturer.............Rotary Rockets Co. Length.........................63 ft (19 m) Max Diameter .................22 ft (6.7 m)
Modelers' Note: All of you scale modelers who always wanted to enter Helicopter Duration events, here's your chance. Rotary Rockets did, in fact, sponsor the HD events at NARAM 39 and 40.
Copyright © 1999 by LUNAR, All rights reserved.
Information date: Mar. 16, 1999 lk