SSTO HELICOPTER From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) Newsgroups: sci.space.tech Subject: Making Orbit 94: SSTO helicopters Date: 19 Jan 94 15:44:01 GMT I recently attended the Making Orbit 94 conference in the San Fransisco area (San Mateo to be exact). One of the more interesting presentations was by Bevin McKinney (vice president of AMROC) and Gary Hudson (general purpose launcher guru, who at various times has been interested in just about every conceivable non-traditional launch strategy). They jointly described some parallel work that they have been doing on SSTO helicopters (no, I am not kidding!). The concept has been around for some time, but only recently has been subject to serious examination, computer modeling, and wind tunnel testing. Patent applications are in progress for certain aspects of the recent work, so details could not be given for some aspects of the proposals. It was stated however that the presentation at Making Orbit 94 was the first public announcement of the work in progress. The concept goes under the general name of "Roton". The name originated with a child's toy of that name, which was some sort of helicopter-like device. During the early days of the work on the concept, the people involved jokingly called the launcher after the toy - the name then stuck. In its simplest form, imagine: 1) A barrel shaped propellant tank with a smoothly rounded bottom, and a bullet shaped nose. The propellant tank carries jet fuel and LOX. 2) Helicopter blades originating from a hub located below the nose. 3) High pressure, high expansion ratio rocket engines at the ends of the helicopter blades, fed by propellants which come down through the helicopter blades. To operate: 1) Rotate the blades to zero angle of attack, and spin the blades up (monopropellant thrusters or whatever; the startup sequence wasn't described) 2) Admit low pressure jet fuel and LOX to the propellant passages in the whirling blades (dual concentric hollow shafts and rotating seals are needed, but are said to cause no problems). 3) The propellants reach the rocket engines in the tips of the blades under very high pressure (3000 psi) due to centrifugal force. 4) Ignite the rocket engines, spinning the blades even faster. Note that except for bearing friction, there is no torque on the body of the launcher. 5) Crank in a substantial angle of attack on the blades. The launcher takes off vertically like a helicopter. Body twist from bearing friction is countered by one of several schemes under active investigation. 6) Ascend through the lower atmosphere using "air augmentation" of the rockets (the helicopter blades). The augmentation is said to be about 10 to 1 (a rocket generating 1 unit of thrust can generate 10 units of lift, due to the favorable lift to drag ratio of the blades). This gives an effective Isp through the lower atmosphere of roughly 3000 for LOX and jet fuel. 7) Modulate the angle of attack of the blades as necessary to maintain lift. Lift from the blades is said to be ok even through the transonic region, up to about Mach 1.2. 8) As the air gets thin, the blades are twisted to a higher and higher angle of attack until they are nearly vertical with the rockets pointing downwards. The vehicle is now operating under rocket power, and more propellant is poured into the engines to keep up the thrust as the air augmentation disappears. About half a degree of angle from the vertical keeps the blades rotating to keep up the propellant pressure and to keep the blades in tension as they drag the vehicle up. 9) Enter orbit, and shut everything down. 10) To reenter, start the rotor again and use as an autogyro for lift during descent. Computer modeling suggests that heat loads are manageable, particularly as the "hot spot" on the blades keeps shifting as a given edge of a blade is alternately a leading and a trailing surface. Land where you want, with quite a bit of cross range. Final hover and landing is done with power on, and can be quite lengthy due to the effective Isp of 3000. Some points: Many configurations are possible. McKinney showed a single rotor version, while Hudson said he was working on a dual rotor version. McKinney's vehicle had a collapsing propellant tank, as apparently on descent it is aerodynamically better not to have a bulky tank below the rotor (Hudson's vehicle would have the tank slung between two rotors where presumably it would not get in the way of the airflow on descent). Jet fuel and LOX are used as jet fuel is available everywhere, and LOX can easily be made everywhere. They are thinking of operation from normal airports (although there are noise issues which may prevent this). Peroxide has some advantages as an oxidizer (non-cryogenic, dense), and some disadvantages (availability) - a peroxide and jet fuel vehicle is a distinct possibility. Liquid hydrogen is out (its density is so low that sufficient pressure can't be generated by the centrifugal force). The vehicles don't scale well to large sizes. Typical upper limits for payloads seem to be a couple of tons or so. This is seen as a potential advantage, as the vehicles may provide "personal" access to space. The vehicles would do well for sub-orbital transport, with somewhat higher payloads. There was some discussion about their potential utility as sort of a super "executive jet" for transport of executives to Japan or whatever in a couple of hours. Costs of a complete vehicle might be on the order of the cost of the more expensive executive jets. The specific impulse is very high in air (air augmentation) and high in space (high chamber pressure, high expansion ratio engines). There are no turbopumps, nor are there any high pressure tanks. The whole device is something like a flying self activated lawn sprinkler with its own water supply, and the rotor control is by standard helicopter linkages. Initial development costs were estimated to be in the 10s of millions of dollars. All in all, a fascinating concept with no apparent show stoppers at the moment. -- Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca