Simple comparison between turbojet and pulsejet engines

[Alex Eng]

Well, everybody knows an answer — of course, turbojet is much better! But we would like to specify — which turbojet do you mean???

And, really, what is it — better? Everyone is well aware of the high fuel consumption of a pulse et engine, which is much higher than that of turbojet. And this is the most interesting question.

Let's start with something simple. What exactly determines fuel consumption? This is engine theory. To eliminate the influence of engine size, we'll consider the so-called specific fuel consumption per 1 kg of thrust (TSFC). Specific fuel consumption is inversely proportional to specific thrust, which depends primarily on the pressure in the combustion chamber.

Have you thought about it yet? So, a good pulse jet engine (and only a valved engine can be good — no valveless noise generators!) has a maximum combustion chamber pressure of approximately 2.5 bar. Accordingly, the pressure ratio is also approximately 2.5. This allows for a specific thrust of approximately 700-750 m/s and a specific fuel consumption of approximately 3.2-3.5 kg/kg of thrust per hour. Our Pulsejet-Sim program can calculate all this very simply.

And what about a turbojet engine? It's complicated. We accidentally discovered that for 75 years dozens and hundreds of smart scientists had been comparing pulse jet engine with... large turbojet engines! These engines now boast pressure ratios of 50-60! Like GE, PW or RR on Boing and Airbus aircrafts! This is 20 times higher than a pulse jet engine has! Naturally, all these scientists unanimously wrote about the complete inefficiency of pulse jet engines. But we had a question to them...

Why do we need to compare the large engines? Ours are small! So, let's compare a small turbojet engine with a small pulse jet engine, assuming they have the same thrust, say, 5 kgf. And then, surprisingly, we discover that their compression ratios are practically identical — 2.5!

Why is this so bad? Because in small sizes, the so-called scale factor comes into play, which prevents the manufacture of a small multi-stage compressor for turbojet engine. And a single-stage centrifugal compressor doesn't offer more than a pulse jet engine has! As a result, a small turbojet engine doesn't have a significant advantage in fuel consumption — it's only slightly lower, and not by a significant margin compared to a larger turbojet engine.

Why is it bad again? Because the so-called scale factor comes into play at small sizes, which prevents the production of a small multistage compressor. But a single-stage centrifugal compressor doesn't offer more than a pulsejet engine. As a result, a small turbojet engine doesn't offer a significant advantage in fuel consumption — it's only slightly lower, and not by a significant margin.

And we encountered also another problem. When 1 or 10 engines are needed, there are no problems at all — a turbojet engine is clearly much better. But what would be, if 10,000 are needed? Then it won't be possible to produce that many turbojet engines. Even in a year. But pulsejet engines can produce even 30,000, easily and very quickly — a small ventilation system factory would be sufficient. This was confirmed in Germany 80 years ago.

By the way, Germans considered V-1 missile with Porsche turbojet engine, which had pressure ration... 2,5! Same as their Argus 014! Argus won — and now we understand why!

From this, we came to the following conclusions:
If we're talking about engine thrust of up to 20-50-100 kgf, but this requires the production of a large number of engines, then all the advantages of a turbojet engine are lost, and a pulsejet engine gains clear advantages. Additionally, and especially in financing — you can save more than 10.000.000 USD from 10.000 pulsejets! However, if only one engine is needed, there's no point in pursuing a pulsejet engine — any turbojet engine would be more powerful, more interesting, better and cool.

Same with noise and high temperature of engine walls: if you need 10.000 engines, all this is real advantage of pulsejet! But if you have 1 engine only, all this is very bad, and you need turbojet exactly.

Perhaps this is the reason why the world has lost interest in pulsejet engines...

[Mike Everman]

I've seen the 2.5 pressure ratio before. Thanks for that.
Valvers are certainly less drag and shorter, but I wouldn't place valveless in the "only a noisemaker" category.
I have been won over by valvers a great deal, though. I've watched as valve life went from low to almost indefinite with superior engineering.
There were some nice ones in the Boonta Eve Classic pod races.