# Some jet engine idea.

Gold Member
What do you think of this?. I was a brainwave when I sat on the toilet :rofl: .

A typical jet engine compresses the air when it comes into the compressor, burns it with fuel, and expands it in the turbine. Approximating there is no difference of static pressure between the entrance and the nozzle, and neglecting the mass flow of fuel, the integral momentum equation yields:

$$F=\frac{dm_a}{dt}(U_2-U_1)$$

is the force applied to the engine structure.

2=flow conditions behind of the engine
1=flow conditions in front of the engine.

Watching the Turbomachinery equations, one can guess that a compressor accelerates the absolute flow: the blades acts on compressing the relative flow, increasing the static and total pressure, decreasing relative velocity to the blade, and increasing the absolute velocity respect to the stator. We can say that behind the compressor the velocity is the largest (but it will have a lot of swirl). By contrast, the turbine deccelerates the flow, but it provides the energy of rotation to the compressor. So that, we are paying a lost of velocity to gain efficiency on the consumption of energy.

The question is: remove only the turbine and put around the compressor fairing a larger fan coaxially (I mean, like the turbofan engines). In flight conditions, the external fan will provide energy to the compressor. Additionally, the combustion chamber will provide with more enthalpy to the flow. So that we have a maximum $$U_2$$, without losts of velocity in the turbine.

What do you think?.

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russ_watters
Mentor
Clausius2 said:
What do you think?.
How do you takeoff if the engine requires you to be moving to power itself?

Gold Member
russ_watters said:
How do you takeoff if the engine requires you to be moving to power itself?
Good reply russ, but now the air turbine engine needs a complementary booster engine, don't it?. When the plane is stopped, no air flows inside the engine, so theorically no power is supplied by the turbine to the compressor. But in fact it takes off. I don't know very much about jet engine mechanics, but some complementary booster system is neccesary in all jet engines, also in my "dreamt" engine.

An extra source of power is required to start a jet engine, but once it is turning it can suck in air through the front, into the compressor and out through the turbine which keeps the engine running (even when stationary). If the turbine was effectively external, that is not in the accelerated airflow, then it could not sustain itself on the ground without quite significant forward movement

LURCH
There are jet engines that cannot function without forward motion (ramjets and SCRAMjets), and these work by being built into other, more conventional engines. The conventional jet engine pushes the vehicle up to the required speed, and then switches over to a different operating mode.

i would like to ask the question, do you think it's possible for someone to make a model- working jet engine rocket? id put in my idea but i dont know how to put in pix

Gold Member
I was not intending to init a discussion about engine boosting. Take it as in steady and cruise flight.

LURCH said:
There are jet engines that cannot function without forward motion (ramjets and SCRAMjets), and these work by being built into other, more conventional engines. The conventional jet engine pushes the vehicle up to the required speed, and then switches over to a different operating mode.
True, Thr SR-71 Blackbird is a good example of this.

russ_watters
Mentor
i would like to ask the question, do you think it's possible for someone to make a model- working jet engine rocket? id put in my idea but i dont know how to put in pix

A few years back, there was an article in RC modeler about a guy who built a working turbo-jet engine for an RC plane. So yes, it is possible (though not easy).
True, Thr SR-71 Blackbird is a good example of this.
The SR-71 engine is actually a turbo-ramjet hybrid that works just fine at low speed. The main thing that differentiated it from an ordinary turbojet is an inlet cone that moved to focus the shock-wave on the inside of the inlet. It didn't so much "switch modes" as continually adjust for its current flying conditions.
I was not intending to init a discussion about engine boosting. Take it as in steady and cruise flight.
It would probably work, but I think the drawbacks are too big.

my idea was more trying to actually make a miniture jet engine prototype into an actual rocket, i think ive figured it out and im gonna try it