My first jet engine build

[nima0908]

Hi, im planning to build a jet but im very new in this topic so i dont know much about it. I want to start slow so i think a simple thermo jet is a good start. It does not have to produce a lot of thrust. Maybe 10N?. I dont know what tools i need for this project. For the fuel i want to use something simple like methane. One of my biggest problems i that i dont know where i can get the parts from.
Nima0908

[racket]

Hi

Could you provide a bit more info on your concept , there are different "thermojets " , also the level of your construction abilities so that we have an idea of any complexity limits .

Cheers
John

[nima0908]

Hi,
my concept is to power the compressor for the air with a electric motor from a seperate Lipo. The fuel then gets burned normaly in an combustion chamber and goes out without powering anything. My construction abilitys are very low so i thought this would be a simple design to start with
Nima0908

[racket]

Hi Nima

OK , so using an EDF for the "compressor" .

You'll need to be able to work with metal to produce the combustor section , which will need to be considerably larger in diameter to the EDF because you'll need to allow for the expansion of the air once its "burnt " , most EDFs don't produce any static pressure rise , so you won't be able to use a jet nozzle to produce your thrust but simply rely on the expansion from heating .

Most of the thermojets I've seen on Youtube make the mistake of maintaining the same diameter for the combustor as the fan , big mistake as they end up with thermal choking which eventually can cause the EDF fan to surge.

The combustor probably needs to have a cross sectional area increase roughly in line with the change in absolute temperature change , so say 300K out of the fan and 1200K in the combustor , 1 :4 ratio , fan annulus to combustor cross sectional area .

A combustor similar to our dump style afterburners should work , though I'm not certain how well any EDF plastic parts will cope with radiant heat from combustion

Cheers
John

[nima0908]

Hi John,
thanks for your help. I have still one question left. How do i inject the fuel right. I think you can do a lot wrong with how you inject the fuel and what you use for fuel so i want to ask before i do anything wrong.
Nima0908

[racket]

All you'll need is a ring of metal tubing, approximately the diameter of the fan hub , positioned downstream of the fan, with maybe 12 or more small holes on the outer side so that the fuel sprays radially into the airflow from the fan , same as we use in our afterburners

[nima0908]

Hi John,
if i can only rely on the expansion of the gas for thrust and not use a nozzle does that mean i should let it end as big as the diameter of the combustor without making it any smaler or is there something to look out for?
Nima0908

[racket]

Hi Nima

During the heat/burning expansion process there is a "loss "of energy due to having to increase velocity , an axial EDF fan produces minimal static pressure rise that can subsequently be used for expansion in a nozzle .

The EDF produces a total pressure rise due to the exit velocity , if that velocity was slowed down in a downstream diffuser there could be a slight static pressure rise , the fuel could then be burnt in that "higher?? " static pressure , and a nozzle downstream could be used for "expansion", but there are losses in both the diffuser and the nozzle which would reduce overall efficiency and thrust .

Axial compressor stages have rather narrow flow ranges as does an EDF fan , if you start putting backpressure on the fan by adding a jet nozzle downstream all that will happen is the flow will start to reduce and there could be a slight pressure increase , but overall thrust will probably be less.

As I mentioned in an earlier email . all the thermojets on Youtube make the same mistakes , .................LOL, I really should do a build and find out for myself :-)

Ideally we'd install a pitot tube at the EDF outflow and adjust the outflow area of the "afterburner" so as to maintain the same pitot reading as when the EDF was standard configuration .



Cheers
John