Scott's Homage To the Motorjet (a.k.a. Afterburning Ducted Fan)
Info covering historical motorjets and my own homemade versions
Links to other motorjet pages
A motorjet is not a turbojet, pulsejet, nor ramjet--it’s yet another form of jet engine first thought up by Rene Lorin. Figure A, above, shows his unique 1908 motorjet in which piston compression ignition in a reciprocating engine was directly used to form a thrust reaction jet. No power was tapped off the crankshaft at all in his very unusual first motorjet design: he intended such devices to be attached directly the the wings of airplanes. By late 1910, Henri Coanda flew and and crashed a biplane powered by a motorjet. Although it is debated, this seems to have been the very first piloted airplane flight of utilizing jet thrust in history. Coanda's motorjet incorporated a 50 hp internal combustion engine driving a compressor through a gear box at 4000 rpm. A variable iris ahead of the compressor regulated the flow of air. Airflow proceeded through a ring of combustion chambers to finally be ejected with a force of 220 kg. In all, Coanda's motorjet produced half the thrust of Heinkel's HE 178 which flew decades later.
The motorjet was popular with the Axis powers before and during WWII. It is similar to the turbojet, but in the case of the motorjet, cold intake air is accelerated by the force of a compressor driven by a separate motor. This separateness has led many people to call these engines "hybrid jets" because they combine a reciprocating engine with a thermal jet (a ramjet--an afterburner). The easiest way to get your head around the structure of a motorjet is to simply think of it as an "afterburning ducted fan." You might already own a weak but in fact quite complete motorjet and just don't realize it: your handheld electric hairdryer.
The BMW company built motorjets in the early 1940s and experimented with these alongside their well known turbojets--at one point, some German designers deduced that the proposed long range Amerika Bomber would be best powered by motorjets. The Junkers company created their own large motorjet called the "jet reaction plant" which employed eight reciprocating cylinders driving a compressor section which fed accelerated air into a rearward combustion section (figure B, above). Heinkel made a series of motorjets that were similar to the Junkers and ran on up to 32 cylinders! The Italian Campini-Caproni jet aircraft flew several times using a motorjet in 1940 and, in 1941, it flew using motorjet propulsion from Milan to Rome (figure F, above, shows Campini's initial 1932 design in which a centrally mounted radial engine turns compressors which feed accelerated air back into an afterburning tail section which concludes with an adjustable venturi). I feel that there were two big problems with the Italian design: the aircraft was too heavy relative to piston engine employed and the design of the combustion chamber was inefficient (hard to ignite and keep ignited while being fed with strong air compression). One source online indicates that perhaps the real intention of this Italian design was: to use the piston engine to get up to speed and then turn over entirely to afterburner (i.e. ramjet) only for operation at high speed. That is still a pretty interesting prospect here in the 21st century.
In the last days of WWII, some of the Japanese Ohka kamikaze aircraft used the Tsu-11 motorjet instead of rockets to provide forward thrust; one of these actually resides completely intact today in the collection of the National Air and Space Museum, Washington, DC. Also, Japan’s proposed Me-262 clone, the Kikka, was originally slated to have a pair of Tsu-11s slung under its swept wings. The Tsu-11 motorjet consisted of an inline four-cylinder engine driving a compressor which fed a large afterburner. During and after WWII, the Soviets built and flew several hybrid motorjet airplanes which appear to have overcome the problems of the earlier Campini-Caproni design. American research into the motorjet concept of propulsion was pursued in depth at Langley in 1942 by Eastman Jacobs and others in a project called the NACA Jeep. This aircraft project seems to have been a research reaction to the Campini-Caproni motorjet flights in Italy.
These neat old jets--often called “primitive” and sometimes treated by conceptually-challenged historians as "not really jets at all"--are not widely remembered today. This is due to their shortlived and transitional form--they were definitely jets, but they tended to be less powerful than turbojets at the same rate of fuel consumption and they blended old and new technologies. Therefore, they quickly became obsolete. As they say: history is written by the victors.
Unlike the turbojet, pulsejet, and ramjet, a single concise name for these hybrid jet engines was never really universally accepted. They have been called motorjets, hybrid jets, piston-jets, compound engines, ducted fan engines, reaction motors, and thermojets. I think motorjet is the best term because no matter what, these things always include a motor and they do indeed produce a jet of thrust--sometimes hot thrust, sometimes cold thrust. Some people like the word thermojet--I definitely don't like that term at all because it makes no reference to the motor and it also assumes that the engine has a combustion chamber--this is not always the case as you can see in the quite awesome Heinkel-Hirth S 60 pictured below. Finally, the term thermojet is an even more serious problem because it is most commonly used today to refer to a certain form of valveless pulsejet.
No matter what the name, there are three attributes that tend to define motorjets: 1) air is mechanically compressed at the intake by a separate power source; 2) combustion of this cold, compressed air occurs in a combustion chamber that is downstream from the intake compressor***; and 3) these heated gases are then accelerated on through the exhaust section adding additional thrust to that which was initially provided by
the intake compressor. It has been reported that the thermal combustion section of a motorjet adds at least one third more thrust to the initial thrust created by the compressor. Another source claims that initial compressor thrust is doubled by combustion.
***An important note about afterburner combustion: I have this great book called The History of German Aviation: The First Jet Aircraft, by Wolfgang Wagner. The above He S 60 picture comes from that book. In it, I discovered that item number 2, mentioned above, is not necessarily required in motorjets. Certain early motorjets were created with a total absence of the combustion section. In these jets, the only heat addition to propulsion was caused by the friction of fast moving molecules of air bumping together during their rapid transit and also latent heat was input by exhaust trickling from the compression engine itself. Cold thrust motorjets similar to this are still common today: they're called ducted fans. They compress air within a tubular housing and they thereby create actual jet propulsion. This has led some people to call all engines of this class "pressurejets" and that is indeed a perfectly good term. As stated best by Larry Cotrill, a jet engine at minimum must possess "...a fluid flow accelerated in a particular direction by a nozzle..." Builders of light kitplanes often employ a pressure jet (i.e. ducted fan) rather than the plain old unshrouded propeller for forward thrust. Despite lack of combustion, these little pressure-pushed airplanes are, in fact, jets.
In most of the early 20th century versions of the motorjet, a piston engine was used to drive the compressor section and liquid fuel was employed for afterburner combustion (although, as stated above, some versions simply compressed
air to create a cold pressure jet and employed no combustion fuel at all). The simplicity of this brute force jet reaction scheme meant that designs could vary widely. In figure C, above, we see a motorjet designed by the famous Frank Whittle in 1936 that largely anticipated similar work produced some time later by Dr. Gustav Eichelberg in Zurich, Switzerland. In Whittle's motorjet design, a central diesel engine drove a couple of compressors--one ahead and one behind. He called this jet his "dual thermal cycle" engine. In figure D, above, we see a 1917 patented design by H.S. Harris of Esher, Surrey, England, in which a two cylinder engine drives a compressor fan in front which sucks in fresh air and rams it into two side mounted afterburners in which fuel is sprayed and combusted for added thrust. This was essentially the same design as the Morize ejector scheme also created circa 1917 in which a reciprocating engine drove a compressor supplying air to a liquid fueled combustion chamber which discharged into a convergent-divergent tube and ultimately out into the atmosphere. Figure E shows a two stroke "free flying piston" motorjet patented some time after 1920 by Melot in France. It is a motorjet that simply has its exhaust ported off to be fully burned and exploited for thrust via an expanding horn-shaped afterburner section.
ALVIN LOWI’S CONTEMPORARY MOTORJET DEVELOPMENT FOR NASA
In the 1990s, Alvin Lowi, Jr., P.E., worked on propulsion systems concepts for application in a NASA very high altitude subsonic unmanned airplane for use in atmospheric research. Lowi’s primary interest was (and still is) the application of internal combustion engines with concentration on axial cylinder (barrel) types. This engine type packages nicely into a compact nacelle or into a duct; this realization led Lowi to pursue what he then termed a hybrid system which would maintain some advantage over ordinary free-stream propellers.
NASA was interested in obtaining propulsion for the very-high-altitude of 90,000 feet. At the height, there would be little air available for internal combustion or cooling--but possibly there would be enough ram air (about 0.3M for a pair of ducted axial fans which would would be afterburning decomposed nitromethane).
Lowi did a design study for a 150 hp facultative adiabatic engine in a duct. The piston engine was built and ran on compression-ignited diesel and/or nitromethane, the latter with or without air. The engine was loaded by a shaft-driven fan and the engine exhaust powered a turbine-driven fan for a second stage of compression. Downstream was an ejector feeding an afterburner discharging through a variable plug nozzle. At the time in 1997, the only background Lowi had was his study of the Caproni and MiG motorjet research projects; information which he acquired from the Smithsonian.
CONTEMPORARY MOTORJETS AS PROPULSION FOR LOWER COST AIR TAXIS (contributed by Alvin Lowi, Jr., P.E.)
It may interest your readers to know that most of the historical motorjets or thermojets utilized the Brayton cycle, as did the turbojets that followed. Recall that the Brayton is the steady-flow external combustion thermodynamic cycle effected in the gas turbine engine. The two most famous motorjet powered aircraft are the Italian Caproni Campini N.1 and the Mikoyan-Gurevich I-250 (N). The Caproni was the first jet-powered aircraft to fly (1939-40) and MiG was the first (and only) motor-jet to see military service as the MiG-13 from 1946 to 1950. The MiG was actually a hybrid in that it also retained the traction propeller on the nose of the airplane.
It is true that the compressors in these historical motorjet engines were powered with internal combustion engines that operated on an intermittent-internal-combustion Otto cycle. However, the compressed air delivered for propulsion was heated in a continuously fired burner in order to boost the energy of the air before expansion in the nozzle to create jet reaction.
Note the occurrence of two separate fuel burning processes in the original motorjets. One is intermittent internal combustion at constant volume taking place in the reciprocating engine that drives the air compressor. The other is a continuous, constant-pressure burning in the air after compression prior to expansion in the jet nozzle. The latter combustion process is comparable to afterburning in a conventional turbojet. The result is large amounts of fuel consumed per unit of thrust (high thrust specific fuel consumption).
Afterburning turbojets are like that at subsonic airspeeds. Also like the turbojet, the propulsion efficiency of the motor-jet suffers at low airspeeds where the jet velocity may be excessive for efficient conversion to thrust. Unlike the turbojet, however, the thermal efficiency of the Otto-cycle compressor drive engine in the motorjet achieves much higher fuel efficiency (especially at part-load) than the Brayton-cycle gas turbine compressor drive engine in the turbofan. Thus, the motorjet obtains better performance at reduced thrust than the turbojet. Consequently, the motor-jet would have better “loiter” characteristics than the turbojet for aircraft propulsion.
In the quest for improved propulsion and fuel efficiency at subsonic airspeeds, the turbofan engine has become the preferred method of air-breathing jet propulsion. The turbofan evolved from the turbojet. It combines the turbojet with a concentric ducted fan or propeller driven by an extension of the turbine shaft or by a free turbine. It resembles a turboprop in that the turbine engine delivers some shaft power to a separate fan at the expense of residual jet power in its exhaust gas. The fan or propeller improves the overall propulsive efficiency at lower airspeeds by producing an additional parallel jet of high mass flow at low velocity that can be more readily matched with airspeed for low-speed thrust production. At higher airspeeds, the propulsion efficiency of the turbine exhaust jet improves, which offsets the loss of propeller efficiency and/or fan duct losses at high airspeeds. The large-diameter high mass flow fan jet is concentric with the high velocity, low mass flow turbine exhaust jet. The ratio of fan flow to turbine engine flow is called the “bypass ratio.” As the bypass ratio increases, so does the propulsion efficiency of the engine at subsonic airspeeds.
Without an afterburner, the motorjet is simply a ducted fan driven coaxially by an internal combustion engine. Such an arrangement constitutes a fan-jet propulsion engine capable of the highest possible bypass ratio (fan flow to engine exhaust flow). It could also achieve the widest possible economic operating range (altitude and airspeed) with a minimum of noise. At low airspeeds, its propulsion efficiency can rival that of a variable pitch free-stream propeller driven by an internal combustion engine. And since its prime mover is an internal combustion engine, its part-load performance would be far superior to any jet engine driven by a gas turbine. Such performance would enable loiter characteristics highly desirable for general aviation applications.
The principal virtues of turbojet propulsion are service ceiling and airspeed. The turbofan trades some speed for range. But in general aviation applications, speed, altitude and range are secondary to propulsion efficiency at low airspeeds and altitudes. This is where turbojet propulsion is at a disadvantage.
Good general aviation airplanes do not cost much more to acquire and operate than utility motor vehicles, and they must obtain good loiter performance under all weather conditions. They also require operational simplicity and flexibility appropriate to average operator skill, preparation and concentration. Since short hop flying is preponderant in general aviation, high-altitude climb-outs are unnecessary and cabin pressurization is superfluous. Thus, good (but expensive) turbojet propulsion is wasted on the short route, low speed, low altitude flying characteristic of most general aviation activity.
Good general aviation propulsion calls for low cost, low fuel consumption operation, which requires high propulsion efficiency independent of airspeed and high thermal efficiency independent of thrust level. Such propulsion is always best for fuel economy, short field and confined airspace performance and loiter endurance. Such attributes are also important for safety, navigation and air traffic control flexibility. The more fuel, thrust and lift reserve the better. Service ceiling over and above a safe margin of ground clearance only incurs extra hazard and cost.
The modern business jet has certainly carved out an aviation niche for itself, namely non-stop hops of 1000 miles for a couple of people in a couple of hours reaching altitudes exceeding 30,000 feet. Such transportation is a luxury beyond the reach or purpose of most prospective general aviation participants. Likewise, the helicopter fulfills a transportation niche that is practical or economic only under special circumstances. However, these heroic classes of airplane designs fail to satisfy the bulk of general aviation demands. The greater demand comprising the larger market for general aviation services, which is virtually unfilled, is for short hops point-to-point on short notice at low cost. What is missing for the lack of suitable equipment is jitney service (scheduled or for-hire) competitive with bus service to underserved towns and cities with underdeveloped airports. Given GPS and radar navigation as well as satellite, cellular and internet communication, autonomous route and traffic control is now as feasible as highway/roadway transit even for amateur operators.
Imagine an air taxi service that could deliver up to 10 people a trip anywhere and anytime in reasonable safety, convenience and comfort for less than $200 a flight hour all up life-cycle cost? An STOL airplane powered by a “motor-fan-jet” comprising a compact, light-weight, jet-A-burning, compression-ignition internal combustion engine of about 500 horsepower might be just the ticket. That’s about 500 lb. thrust at 250 mph airspeed for a 12,000 pound GTOW airplane.
SCOTT'S SIMPLE MOTORJET PROJECTS
I acquired the motorjet building bug back in 1998 and soon thereafter, I created this webpage. I made numerous motorjets using surplus electric compressors rather than gasoline piston engines--that was a completely new idea at the time that you often see people on YouTube repeating these days in the form of model airplane EDFs with afterburner sections. I used solid fuel in my motorjets initially--an idea I took from solid fuel ramjet cruise missiles built back in the 1950s. Later, I used gel fuels which cling nicely. Recently, I've been using liquid fuel delivered to the combustor in an unbelievably simple manner: just lying in a pool via the force of gravity at the bottom of the separate combustion chamber section. Once heated momentarily with a torch, no sparking mechanism is needed at all to maintain afterburning despite the full force of compressed air entering from the compressor section.
Once to take a break from the danger of pyrotechics, I made a cold thrust motorjet. I got some of its parts from a retired Eureka Whirlwind Lite vacuum cleaner: a really fantastic Whittle style electric rotary compressor, its big rubber mounting ring, and its long black power cord. I simply press fit this stuff into a suitably narrowing black plastic Blitz funnel and cut off some of the funnel tip in an attempt to get optimal jet output. I succeeded--when this motorjet started, it immediately moved out fast across the floor despite a total lack of wheels.
In November 2008, I built my latest motorjet pictured here--the most efficient one I've ever made. It is is able to overcome its own weight and drive itself forward and it is extremely loud. It is very simply constructed out of gallon and quart metal paint cans with a roughly 1.5 inch hole at the intake, at the middle joint, and with a 1 inch hole at the outlet. The electric compressor from a vacuum cleaner is mounted inside the gallon can (liberally glued in place within the front face of the can with silicone caulk), and a rubber plunger sits via suction on the intake end (the stick on the plunger is absent). The plunger is used to regulate air intake--any flat paddle of wood or metal would work alternately. At first, this obstruction reduces air intake. Later, I remove the obstruction and let the full blast of air inside to get maximum power output. This motorjet burns charcoal lighter fluid that is simply laying in a pool in the bottom of the smaller container. There is no source of spark for ignition--it maintains constant burning without it until all of the fueled is consumed. Once again, this is a very efficient motorjet: this is evidenced by the bright blue flame, the loud volume noise it produces, and the impressive amount of force it generates once turning and burning.
THE EFFICIENCY OF MOTORJETS ISSUE
I see debates online about the efficiency of motorjets: I'll add my two cents here based on over a decade of experience. Many people seem to think motorjets are too inefficient to bother with. This is shortsighted: why not first consider motorjets via your own experimentation? If you open up your mind and freely experiment, you may well invent something new.
I look forward to hearing about your own experiments with motorjets--feel free to send me email.
DISCLAIMER: IF YOU READ THIS PAGE AND DECIDE TO BUILD ANY MOTORJETS YOURSELF, YOU DO SO AT YOUR OWN RISK. This page is intended to document some rare jet engine history and to reveal some of my own creative work in the area of motorjets; this page is in not intended as a "how to build jets" instruction sheet.