Scott F. Hall's Homage To the Motorjet (a.k.a. Afterburning Ducted Fan)
Scott F. Hall's Homage To the Motorjet (a.k.a. Afterburning Ducted Fan)
CONTENTS
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 motorjet and 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 flight of a jet 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 very 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 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.
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 and 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 awesome Heinkel-Hirth S 60 pictured below. Finally, the term thermojet is a real problem because it is most commonly used 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 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.
I acquired the motorjet building bug way back in 1998 due to that race scene in Star Wars Episode 1. I made several motorjets using surplus electric compressors rather than gasoline piston engines--that was a new idea at the time that you often see people on YouTube repeating these days. I used solid fuel initially--an idea I appropriated from solid fuel ramjet cruise missiles that were built back in the 1950s. Later, I used gel fuels which cling nicely just like solids do. Lately, I've been using liquid fuel delivered to the combustor in an unbelievably simple manner.
Once to take a break from the 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 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.
HALL'S FIRE CANNON
In July 2007, I created a weak motorjet which I ended up calling a "fire cannon". Pictured below is the mark 5 version--the best one. The forward section which includes the compressor fan is driven by a separate power source--an electric motor. This creates cold thrust. Half of this cold thrust is bypassed and half makes its way through the duct to the smaller rear combustion section. Within the combustor, liquid fuel simply burns as it lays there like water in a tub and hot thrust is expelled out along with the bypassed cold thrust of the motorjet. The sound of this device is unique combining all in one the whir of a ducted fan along with the rumble of a ramjet and some rhythmic drumming akin to pulsejet sound. Ultimately, I decided it was a motorjet in principle but one put to best use as a sort of flamethrowing artillery piece. See the link below to watch my fire cannon destroying milk jugs in a video.
HALL'S LATEST MOTORJET
In November 2008, I built my latest motorjet pictured here--the most efficient and the simplest one I've ever made. It is is able to overcome its own weight and drive itself forward and it's extremely loud. It is very simply constructed out of two paint cans with roughly 1.5 inch holes made at both ends and in the middle joint region. The electric compressor from a vacuum cleaner is mounted inside the bigger can (glued in place inside 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. At first, it will burn with only a little bit of air intake--hence, the need for the plunger nose on this jet. Later, I can remove the plunger and let the full blast of air in to get maximum power. An alternative to this system would be to put a rheostat in the electric line to adjust the speed of the compressor. Like the fire cannon above, it burns charcoal lighter fluid 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 the fueled is consumed. See the link below to watch it operating in video at YouTube.
THE EFFICIENCY OF MOTORJETS ISSUE
I see debates online about the efficiency of motorjets so I'll add my two cents here. Many people seem to think motorjets are too inefficient to bother with. This is so shortsighted: would you likewise discard all mopeds just because motorcycles are available? Why not first consider motorjets via your own experimentation? Have you taken a look at the recent supersonic motorjet U.S. patent which was filed post-year 2000? If you open up your mind you just might 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 ENTIRELY AT YOUR OWN RISK. This page is posted with free speech in mind to historically document some rare jet engine history and to reveal some of my own creative work in the area of motorjets. This page is in no way intended as a "how to build jets" instruction sheet.
LINKS
(WinPC instructions: click the right mouse button and choose "open in new window" for each of these links below)
