Several different types of fully reusable space transports are technically feasible, depending on the level of technological sophistication which a designer employs and economic requirements by which he or she is constrained. Three general classes of vehicles are being considered today by the U.S. aerospace industry. First is the VTVL (While present industry concepts focus on nose-entry VTVLs, we have concluded that base-entry is superior for operational vehicles, but the discussion of the trades relating to this issue are beyond the scope of this paper.) Second, the VTHL has certain potential for intermediate service as an earth-to-orbit transport. Finally and generally more difficult to build, are HTHL vehicles. Because of economic reasons, we evaluated these vehicles only in their single-stage-to-orbit ( SSTO) versions, though other forms of the vehicles might be considered, such as in-flight propellant transfer HTHLs or air-launched HTHLs. We have selected the VTVL as the basis for the ROTON as described below.
Vertical Takeoff, Vertical Landing Space Transports

VTVL space transports have certain characteristics which recommend them as a nearly universal form of launch system. Many thoughtful observers agree that of all the classes of reusable SSTOs, the VTVL may be the simplest to build (Ref 1). The basic concept requires a chemical bipropellant liquid propulsion system integrated into the vehicle, employing high energy, inexpensive propellants. The vehicle ascends vertically and flies a throttling trajectory to low earth orbit ( LEO). Once a mission is completed, the VTVL re-enters the atmosphere and lands using rocket thrust to cancel residual terminal velocity. Landing of the VTVL is at the launch site.

One important attribute of the VTVL which is generally not possible with the VTHL or HTHL is the ability to ascend to higher earth orbits following refuelling.

Finally, the VTVL does not require earth atmosphere for flight and hence can operate from planetary bodies such as the moon or Mars. This is true even for the ROTON concept which we discuss in the next section, in spite of the use of air-augmentation of the ROTON during flight from earth surface to orbit. Such an operational feature will be an important advantage for space resources development.

WHAT IS A ROTON?

We will describe the ROTON in terms of an orbital version (thought other types are feasible, for example, a sub-orbital or boost-glide transport vehicle). The configuration described is one of several which has been considered for our reference mission, and serves to illustrate the basic features and principles of the ROTON. We caution the reader that the final configuration of the prototype ROTON has not been determined as yet, and may well change from that described in this paper. An exemplar ROTON is shown in Figure 1.

The ROTON may be considered a ”space helicopter•. It has three primary elements. The first is a cabin containing crew or cargo and other miscellaneous equipment needed to support the flight. The next is the propellant tankage. The third element of a ROTON, and its defining one, is a multi-bladed propeller/rotor similar to a helicopter rotor. This rotor has one or more small high-pressure rocket engines mounted at the tip.

Figure 1. An exemplar small ROTON. Rocket engines are at the rotor tips, crew cabin is at the top of the vehicle. A number of alternative configurations are under active consideration.

ROTON combines previously developed technologies into a unique new concept for a space transport vehicle. As early as the late 1940s, rocket engines were built which used the centrifugal pumping power of a rotary arrangement of nozzles to provide the energy to force propellants into the combustion chamber. The ”cyclo-rocket• developed at Aerojet in 1946 (Ref. 2) was the first to employ this approach, though its designers failed to take the next logical step and use the actual centrifugal forces produced by spinning the rocket around to provide direct pumping power; rather it converted the rotary engine motion into shaft power to run a centrifugal pump. By positioning rocket engines at the tip of a rotating blade or propeller, the energy produced by the engine thrust can be used to spin the blade and to provide the desired pumping power. (Hero‘s steam engine is a simple expression of one element of this concept.) Propellants are pumped to the rotor blade tips by the centrifugal forces resulting from the spinning rotor, eliminating the need for turbopumps or high pressure propellant tanks.

While in the sensible atmosphere, the rotor can also generate thrust, often improving the rocket engine thrust by a factor of five to ten. This has the result of reducing the propellant consumption by the same factor. By combining this technique with the very high pressure being produced by the rotary action, engine efficiency can reach the near ultimate possible for liquid fuel rocket systems. This permits a vehicle to be built using inexpensive propellants (liquid oxygen and propane or kerosene) and with a loaded to empty weight ratio of about 14 to 1. We know this to be within the state of the art to manufacture, since boosters built in the late 1950s and early 1960s achieved propellant mass fractions of 0.95 or greater. Such a vehicle will be capable of flight to orbit.

A ROTON performs much like an air-launched rocket. Air-launching a single-stage rocket will reduce the required mass ratio for orbital flight (using cryogenic oxygen and hydrocarbon propellants) from over 18:1 to as low as 12:1. Advantages which accrue from air-launching are (1) reduction of drag and gravity losses, (2) increased engine performance from near-vacuum nozzle exit conditions and (3) reduction in aerodynamic loading and heating. The ROTON achieves roughly equivalent performance to an air-launched single-stage rocket by using the highly efficient air-augmented ”helicopter• mode to reach the altitude and velocity at which a typical single-stage rocket would be released for flight. It consumes only a small percentage of its propellant performing this feat, since the augmented rocket specific impulse (Isp) will be over 1500 seconds. (Pure rocket Isp would be around 300 seconds. This means the propellant mass consumed during the aerodynamic portion of the flight will be only 5-10% of the total propellant load. A comparable rocket might burn 50% of its propellant to achieve the same flight conditions of altitude, velocity and flight path angle.)

During ascent, the rotor is quite heavily loaded, and might be more properly termed a propeller. Typical disk loading will be in the 50-100 psf range. During descent, the loading reduces by a factor of about 14, or the same as the mass ratio. The rotor is now more like a helicopter rotor in terms of loading and control. The rotor has both collective and cyclic pitch control. Engines will also have the ability to alter their thrust vector slightly out of plane of the rotor pitch angle. While the baseline has four blades, a six blade version is under consideration.

The rotor provides an additional important function during re-entry. By acting as a combination rotary wing and large drag brake (depending on orientation of the blades), the rotor can either slow the vehicle high in the atmosphere (substantially reducing frictional heating) or extend the crossrange and downrange glide capability of the vehicle, improving landing performance and decreasing the decelerative loads on cargo and crew. In engineering terms, this means the rotary wing can produce L/D ratios around 1.3:1, yielding re-entry crossrange performance superior than the winged U.S. Space Shuttle (Ref 3, 4, 5).

On take-off, the blade is accelerated to operating speed, and collective pitch is added to the rotor blades. The vehicle begins to rise, propelled almost entirely by the aerodynamic forces generated by the rotor. This is a very efficient operating regime for the ROTON since the aerodynamic augmentation of thrust delivered by the rotor generates about 5-10 times the thrust of the rockets which drive the rotor. As the vehicle climbs and accelerates, more blade pitch is added, and because of the simultaneous pitch change of the rockets at the blade tips, the rockets begin to generate a larger share of the overall axial thrust.

At about 35,000 ft, the vertical speed has risen to approximately 700 fps and the ROTON transitions to pure rocket flight since the atmosphere has become too thin to provide useful assistance. At this point the blades are pitched to nearly vertical, much like a feathered propeller, and the rockets are throttled up, firing axially to the flight path with just enough pitch to maintain rotor rotation for centrifugal pumping. The vehicle continues in this manner until burn-out.

The vehicle remains in the same attitude throughout flight. As the vehicle reenters the atmosphere and aerodynamic forces increase, the rotor autorotates much like a helicopter or autogyro during vertical descent. Drag may be modulated by rotor coning, and can be controlled by a simple mechanical governor which adjusts blade pitch angle relative to coning angle. This mechanism has been tested in operational systems and has shown itself capable of smooth rotor drag modulation from speeds of Mach 3.5 to zero with no tendency for rotor overspeed.

As the vehicle approaches the ground, the pilot or flight computer performs a collective flare to reduce vertical speed to near zero before settling to earth. It is possible that the tip rockets will be restarted at low thrust so the ROTON can be hovered or otherwise maneuvered before final touchdown.

Some Features Of The ROTON

The performance of the rocket chambers is quite high by conventional engine standards. This is largely due to the high combustion chamber pressures generated by the centrifugal pumping of propellants by the spinning motion of the rotor. A limitation of this approach to propellant pumping is the concept favors high density propellants, essentially eliminating the opportunity to use hydrogen. Given the point of view that density impulse is more significant than performance specific impulse in determining the feasibility of fabricating a lightweight airframe for an SSTO (Ref. 6) this may be a ”blessing in disguise.• An important consequence of the use of high density propellants is the elimination of the need to store, ship or handle liquid hydrogen at conventional airports.

• Although it shares some of the mechanical and operating characteristics of a helicopter, the rotor dynamics are considerably simpler since there is no high speed horizontal motion required in a ballistic ROTON flight. High speed horizontal flight adds considerably to the complexity of helicopter rotor dynamics.

• On reentry, the ROTON is self-stabilizing. Even if the vehicle has not been stabilized before entry begins, it will rapidly rotate itself into proper entry attitude, purely through aerodynamic forces. (Failure of the attitude reference system to properly point the X-15 during entry caused the only loss of aircraft and pilot in the X-15 program.)

• The crew or cargo remains in the same attitude throughout flight. Acceleration and deceleration can be taken directly through the long axis of the vehicle in all flight regimes. Maximum g during lifting entry can be modulated down to 1.2. During a ballistic entry maximum g is about 7.5.

• ROTONs would be able to takeoff from any level ground - for example the taxiway of a small airport. No rocket blast touches the surface, eliminating the need for any sort of launch support equipment. Landing and takeoff can safely be performed at zero speed from very compact sites. This is most important during an abort. The ROTON can safely land at unprepared sites, which lack runways. In an emergency, it can be landed in any farmer‘s field. While noise would be substantially greater than a small aircraft, the noise footprint is quite compact and the high speed departure from the field would minimize the impact on the surrounding area. On landing, the much lighter vehicle would be significantly quieter, comparable to a conventional helicopter.

• Only kerosene and liquid oxygen, both common industrial chemicals, need be supplied to the vehicle. A medium-sized ROTON, capable of carrying one ton of cargo, will require less than $50,000 worth of propellant.

To permit the use of aviation kerosene in place of highly refined and expensive rocket grade kerosene (RP-1), the ROTON will likely employ LOX cooling* in the rocket chambers. This has been demonstrated by NASA-sponsored tests.

Vehicle avionics for a ROTON is virtually off-the-shelf. The principal means of flight control will be through fly-by-wire or fly-by-light technology. Because there is little probability of mechanical backup, the electronic control system must be highly reliable. Fortunately, this problem has been solved many times in recent aviation history: most new military aircraft use such control systems, and some (such as the X-29, F-16 and F-117) rely upon them for complete flight stability. Adopting the existing rules for flight safety for commercial aircraft should be sufficient for ROTON development to proceed.

Differential GPS will certainly be used to provide precise knowledge of the vehicle during terminal phase operations; conventional GPS will be adequate for ascent and on-orbit navigation.

Extensive health monitoring and built-in test (BIT) are probably not required, except where it clearly contributes to dispatch reliability and overall cost-effectiveness. Civil aircraft operated very profitably for decades without either.

The ROTON will have several advantages over commercial aircraft with regard to fire and explosion threats. The most important of these is that the vehicle uses up the vast bulk of hazardous propellants quickly and for most of the mission is largely inert, except for a small quantity of landing propellants. Another advantage is that the cryogenic LOX is rapidly dispersed in the event of an in-flight accident, and does not pool or coat vehicle components or payloads.

Examination of the Space Shuttle Challenger debris demonstrates this fact. Post-accident investigation showed virtually no blast or fire damage from destruction of the external tank. All of the damage to the orbiter came from aerodynamic forces and the exhaust plume of the right solid rocket booster. By one calculation, the explosive yield of the entire external tank‘s remaining propellants (about 60% full) was equivalent to a few tons of TNT.

It would be desirable for the ROTON to be able to launch from most large airports. This will require a ROTON to fit into the traffic patterns which have be established for present civil aviation aircraft. Both the climb-out and landing of a ROTON will be largely vertical, meaning that the airspace corridor required for routine operation is both compact and non-intrusive into the normal terminal airspace layout. The high maneuverability of the ROTON at landing suggests that there will no problem with moderate to heavy crosswinds at the landing field.

Although the vehicle will not interfere with normal airport traffic, there are other considerations we must address.

Hangers which can accommodate typical large aircraft will also be suitable for small to mid-sized ROTONs. A ROTON with a payload capability of 5,000 pounds will be shorter than the vertical tail fin of the Boeing 747, and even with rotors deployed in launch position will have a footprint smaller than a DC-9. ROTONs will also have landing gear which will permit them to be towed by conventional ground tractors. Crews can board the vehicle by use of mobile stairs or jetways, neither of which will require modification to be used with the ROTON. Light cranes can be used to load cargo, such as small satellites.

In the event the ROTON must be ferried from one point to another, there are two possible delivery modes. The most obvious method would be simply to ferry by means of ballistic flight from one site to another. If this is not possible for some unforeseen reason, then small jet engines might be fitted in place of the rocket chambers at the rotor tips. Using kerosene stored in the fuel tanks of the ROTON, it could be slowly ferried over distances of a few hundred miles at speeds of perhaps 50-100 mph. We expect that self-ferry by means of ballistic flight will prove most practical for most transfers.

ROTONs will require the delivery of cryogenic liquid oxygen to the vehicle launch site. If this is a commercial airport, issues of safety may need to be addressed relating to handling of the LOX. But we should note that LOX is routinely used in industry and deliveries do take place every day, without incident, at hospitals all over the world for use in medical treatment. A typical small ROTON flight might require three or four LOX tanker trucks to off-load their cargo either into the ROTON directly or into a storage tank. Storage vessels of adequate volume are commercially available. For early operations, it will be best to off-load propellant from tanker trucks rather than to build dedicated storage tanks*. Alternatively, it is possible to produce the required LOX on-site as was done with some early missiles such as the U.S. Army Redstone. The equipment needed to produce enough LOX to fuel a small ROTON over a 24 hour period can be purchased commercially LOX could be delivered from such equipment directly into the ROTON since evaporation of LOX from the insulated ROTON tank is relatively minor over the time period of interest. Using such truck-mounted LOX extraction equipment would mean any site which can provide a supply of kerosene will be able to easily support ROTON flights. Since LOX cooling is used in the chambers no specialized grade of kerosene is needed thus aviation kerosene can be used directly from existing tanker trucks or tank farms.

On landing and take-off, ROTON rocket exhausts are parallel to the ground, but hazards will remain from jet entrainment of ground debris. Whether or not this hazard is greater than the entrainment risks of conventional high-thrust jet engines remains to be evaluated.

There is also the question of whether it is necessary to hold down the vehicle prior to liftoff for a few seconds for full-thrust engine checkout. Though some believe this is necessary, it was not done on many liquid propellant rocket programs nor with any solid engines. Furthermore, the ROTON can lose one or more engines immediately and still save the vehicle.

We expect by resolving such issues as these, the integration of the ROTON vehicle into a routine airport operations can be managed.

Once the issue of vehicle safety is settled by adopting rules for ROTONs similar to those for aircraft, the biggest launch constraint will be the sound level. Rockets have higher velocity exhausts than jet engines and higher jet velocity means more noise. Very little work has been done on quieting rocket engines and sound level may be the constraining factor in using existing airports or building launch terminals convenient to cities. One possibility is water injection into the shear layer between the high velocity jet and the stagnant surrounding atmosphere. It is not clear how much can be accomplished, but ROTON development should pursue an aggressive program to reduce rocket noise. Another significant noise source will be the rotor aerodynamic noise. Since the rotor tip speed is close to the speed of sound at sea level at liftoff, the aero acoustic environment is expected to be a potentially serious issue. Analysis into this problem is underway, but a solution is simply to operate the ROTON at some appropriate distance from unsheltered humans. The distance is likely to be about the same as a large transport aircraft is removed from the terminal area when it begins its takeoff roll.

Atmospheric pollution should be no greater from a lox-hydrocarbon ROTON than for a commercial jet aircraft of comparable fuel load. Planned ROTONs would have fuel loads equivalent to mid-sized transport aircraft such as the Boeing 737.

We expect ROTONs will be used in a variety of roles and missions almost unfathomable by present-day planners. The launch of discrete satellites will be a small part of the business of a mature ROTON fleet. With a dramatic reduction in launch costs based on an understanding of the technology in the ROTON, we can expect several major markets to emerge.

Beyond the relatively small market for satellite launches, three larger potential markets to consider are: space power, space tourism and extraterrestrial resources. Interestingly enough, the market which is most probable to emerge first is tourism, as it requires a smaller initial investment (i.e., expenditures only for the vehicle, and not for solar-powered satellites, space platforms or lunar bases).

Such a market is important to vehicle design and certification, because the standard of safety for the ROTON is likely to be equivalent to commercial air transports. Further, there will be tremendous pressure to reduce the vehicle operating costs in order to expand the market, and this pressure will produce a very low cost per pound of payload delivered in orbit: a key criterion if satellite solar power or extraterrestrial resources are to be developed next. Finally, a future ROTON might enter the market for high speed transpacific travel in competition with hypersonic transports.

It is widely believed that the development cost of an unmanned launch vehicle will be less than that of a manned version. We regard this as a classic example of ”conventional wisdom• which is in error. When one adopts the ROTON philosophy of development, there can be no difference between the cost of the manned or unmanned version (except for incidentals such as crew accommodations, which are a trivial part of the vehicle cost). By analogy to aircraft development we can ask the questions: when do we find it necessary to build unmanned aircraft? The answer is: only when we intend to use them in an expendable role - as ammunition.

Therefore we assume without further comment that the ROTON will always be flown with a crew, except when it is uneconomic to do so. The manned versus unmanned debate will be determined by operational requirements, not by perceived risks or costs.

It should be apparent that the operation of ROTONs will be dramatically different than present ELVs. The attention which ROTON designers will pay to flight safety (in order to get the vehicle back) will clearly have a positive effect on range and third-party (i.e., public) safety. This in turn will have a major effect on insurance. It will be as inappropriate to require multi-billion dollar insurance policies for ROTONs as it is to require them for commercial aircraft (which operate under far less burdensome regulatory insurance rules).

Operation from conventional airfields will further affirm the inherent safety of the ROTONs. With such routine operation it is obviously impractical to require launch licenses for each flight, any more than a license is required for each aircraft departure (we accept the necessity of filing a flight plan).

This important issue of regulation of operations must be addressed. Present U.S. policy is to regulate vehicle safety for public safety. In addition, a Congressionally-mandated ”mission review• is conducted to determine the impact of the payload on national security and foreign policy. While this has not excited much concern to date, it will be utterly unworkable in a world with ROTONs. By analogy, when a private business jet departs San Francisco International, it does not require the approval of the Departments of State, Defense, Commerce, the White House and NASA. Neglecting troublesome issues relating to the Fourth Amendment involving unlawful search and seizure, it will simply not be tolerable to burden space commerce with such bureaucratic impediments to progress.

We believe the ROTON offers one means by which inexpensive commercial space access may be achieved within the next ten years. ROTONs will be able to be flown from almost any airport or heliport at low cost, using existing ground handling and servicing equipment, and without any greater risks than operating jet transports.

Yet, if we are to create a climate for the commercial development of the space frontier analogous to the tremendous success of American commercial aviation, we must abandon the desire to have the federal government fund and thus dictate the technical approaches to solving the problems of space access.

Fortunately, the ROTON technology may given us the a new method to access space at low cost and risk. It is up to us to grasp the potential and the opportunity.

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