I've heard pulsejet powered aircraft can fly at very fast speeds but are there any drawbacks to this type of engine? And are the disadvantages greater than the advantages or vice versa?
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5 Answers
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The pulse jet engine works because it uses the dynamic pressure of air for compression. Consequently, it needs speed to work well. If the pulse jet is on the ground, it is not easy to start - it needs to be fed with compressed air, and once it runs (it can run even when not moving because the oscillating pressure wave inside the tube will compress the inflowing air), it develops litte thrust.
Thrust will go up with airspeed, so you need additional thrust sources for initial acceleration and take-off. In case of the V-1, this was done by a steam catapult or by air-launching the device. Note that the V-1 was accelerated to 370 km/h on a 45 m long catapult which means more than 100 m/s² (10g) average acceleration. Most of the development work went into making the airframe withstand this acceleration. A modern design would need additional engines as well, and once those are included, the rationale for pulse jets becomes very weak. They make sense if you have an air-launched, disposable application, such as long-range air-to-air missiles. This is a niche in which their higher-speed cousins, the ramjets, will still be used.
Pros:
- Very easy to build, very light.
- Runs and produces some thrust when at rest (as opposed to ramjets).
Cons:
- Thrust grows with flight speed. This can be seen as a pro, but normally requires additional means of acceleration. Therefore listed as contra here.
- Low fuel efficiency. The fuel energy is converted to other forms of energy: Just listen to the videos of Colin Furze to get an idea.
- High levels of vibration due to the intermittent operation. When the Heinkel 280 was tried out with Argus pulsejets, the aircraft experienced unacceptable vibration.
- Need heat-resistant materials. The central part will get very hot in operation, so at least nickel-chrome stainless steel is needed if air cooling is insufficient.
- Cannot go supersonic. For this, there are ramjets.
EDIT:
NACA tested the As 014 pulse jet and wrote a report, from which the performance data depicted below is taken:
It seems that Bruce Simpson over in New Zealand has improved the pulse jet design a lot: He needs no compressed air for starting and makes clever use of surrounding cold air to increase mass flow and keep material temperatures down.
answered Mar 13, 2015 at 15:01
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First of all they are loud there is a reason that the V1 bomb is called the "buzz bomb".
They are also fuel hungry due to the low compression ratio.
The biggest advantage it that they are very simple. Once you have them ignited they will continue to burn until they run out of fuel or the airflow is disturbed.
answered Mar 13, 2015 at 11:11
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2$\begingroup$ not to mention that pulsejets are by far the easiest jet engines to build. there are plenty of good videos about pulsejets by colinfurze on youtube. $\endgroup$ Commented Mar 13, 2015 at 11:25
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Pulse jets were big during WWII but the have a few issues.
Advantages:
Aside from throttle flaps pulse jets have essentially no moving parts which makes them not only simple to build but simple to maintain. People build them in their garages at home all the time. From a war time standpoint this was great since you could make many of them in a short time.
Disadvantages:
Noise: Pulse jets are much louder than their spinning counterparts. This makes them some what less useful from a stealth/wartime standpoint and from a commercial standpoint in regards to general noise pollution concerns.
Low Impulse: From my understanding of them pulse jets have a low specific impulse due to their more piston like ignition pattern. They don't deliver the kind of thrust their fan like counterparts can.
Size: If memory serves they are also much bigger than a regular jet (for a given power output).
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Disadvantages:
Very low thermodynamic efficiency due to very low compression ratio. Therefore they are not very fuel efficient. They consume a lot of fuel.
They are very very noisy.
Reed valves fatigue quickly. Reed valves can also be subjet to heat and loose their temper. Bruce Simpson has done a great deal of work to improve the lift of reed valves.
High temperatures involved.
Valveless pulse jets tend to be less efficient than valved (but a lot simpler).
Most of the literature on valveless pulse jets is substantially incorrect as to what is happening with the gas flow within the engine.
Advantages:
- Simplicity
Pulse jets can be designed to develope maximum thrust at stand still or at any particular sub-sonic speed. If they are designed for maximum thrust at a reasonably high speed then their stand still thrust will be low. A low velocity pulse jet will suffer from the flame tending to be blown out the back of the engine as the forward velicity increases. There are ways of making pulse jets so they can develope maximum thrust over a wide speed range.
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One other advantage of pulse jets is: they are dirt cheap to build, as compared to other forms of aircraft engine. A primary reason they were used in the V1 was their very low cost.
One other disadvantage: they really have only one efficient power output: their maximum. While this does vary with airspeed, it also means that pulse jets can't really be throttled down without risking a flameout. That would make landing a pulse jet aircraft a challenging affair.
Since the V1 was never designed to make a controlled landing with a pilot on board, that wasn't an issue.
