Re: Motor Jets and thermojets.

Eunometic wrote:
The motorjet/thermojet.

The motorjet or thermojet are early forms of jet propulsion that were
seen as a way of achieving near transonic flight well in excess of
what was possible with propeller driven aircraft.

Basically they consisted of a piston engine driving a compressor
thereafter which the airflow was further accelerated by heating to
achieve reactive thrust. They could be described as a ducted fan with
after burning. In some cases they were 'cold jets'. Sir Frank
Whittles papaper on the topic of flight at 100,000ft was actualy built
around a motorjet, his proposals involved a diesel with compressors
built around the involved double reverse flow combustion chambers
latter seen in the Welland. There is an outline of his early
proposals here:
http://www.angelfire.com/art/jetengine/

Although decried as inefficient or heavy I think this is unfair and
based on a premature evaluation of very early experimental engines.

There were actually quite a few built and even flown.

Henri Coandă's 1910 Biplane had a 50hp engine with a burner system
that produced a respectable 220kg thrust.

The Italian Caproni Campini N.1 had a 750hp or 900hp engine and could
produce some 750kg (kilopond for purists) thrust ie 1650lbs. One
unusual feature of this engine was that the burners heated the air
indirectly via the wall of the 'afterburner', presumably to avoid the
complication of burners opperating in the high speed airstream. (Both
problems solved by Whittle and von Ohains partner Max Han )

The N1 had a speed of 329km/h without the 'afterburner' and 375km/h
with. This 14% increase in thrust suggests atleast a 30% increase in
thrust. After burning is said to increase thrust by between 30% to
100% over the cold thrust.

There was a great deal of work in Germany at Junkers, Heinkel and
BMW. Almost to detailed to go into.

Among these was the Heinkel HeS 60 which was a cold thrust engine
looking not unlike a modern high bypass ratio podded turbofan nacelle.
It had a 32 cylinder 2000hp engine and produced some 1250kp (2750lbs)
thrust. It was of the cold thrust type with no burners.

In the USA the NACA under Eastman Jacobs (inventor of the Mustang
laminar flow Wing) a 1942 Research proposal known as Jakes Jeep based
around an R-1530 engine (typically worth about 1000hp). It was
expected to be capable of at least Mach 0.75 (500mph) which is good
for an engine not over boosted and likely to burn out.
http://history.nasa.gov/SP-4305/ch8.htm (pic half way down)

In the Soviet Union hybrid motor jets driven by an auxiliary of a
piston fighters main engine entered service. The Mikoyan-Gurevich
I-250 could tap power of its 1650hp engine to generate an additional
600kp thrust. This fighter could achieve a very respectable 510mph.

In Japan thermo jets were also under development, The Nakajima Keika,
which ended up be powered by BMW 003 clones, also had a thermo jet
version.

The other was the TSU-11 of 200 kg thrust of which the compressor was
driven by a conventional engine Hatsukaze of 110 hp (principle of
Campini ). The engine was started in the ground and then paused during
the flight under and restarted during the attack of the Ohka suicide
bomb.

Now consider an engine such as a DB605DCM of 1850hp or say a Rolls
Royce Merlin two stage with 1690hp both for a weight of 730kg or so.
Scaling up the thrust results for the Campini engine of about 830kp/
1000hp we would expect a thrust of about 1406kp or 3000lbs. The
main weight would be the ducting and engine with the compressor
perhaps not to heavy. For comparison the Jumo 004B had an uncowled
weight of about 710kg for 900kp thrust. On top of this the Merlin
would be producing about 300lbs of jet thrust from its exhaust. (due
to its high supercharger pressures and low compression ratio). A
PW2800 of some 2000hp would have produced about 1660kp/4000lbs more or
less enough for a P80.

Therefore I conclude that the motor jet could have made an effective
propulsion system able to operate at somewhere from Mach 0.75 (500mph)
and beyond probably to the speed of sound. It simply seems that
advances in the pure jet engine and materials came along in time to
nip in the bud development of the motor jet.
How would a diesel run at 100,000 feet? Carry it's own oxygen?

Dean
.

Relevant Pages

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