Model turbojet tips and calculations

[shin]

Hi All,

New member here joined a little while back and been having a mosey through the site and resources available and decided I want to get the ball (slowly) rolling on a first project.

I have seen the designs of John, Andy and Johansson, they have been quite inspiring, particularly the attention to detail and high quality craftsmanship in each of the engines produced! I would like to design and manufacture a turbojet preferably the axial flow (with radial flow compressor/turbine) design by the likes of the folk mentioned above rather than the external combustion chamber design however this is something that has to be seen to later depending on my own fabrication abilities and cost. No plans to put it in a vehicle however. If I get addicted enough though I'll consider converting to turbo shaft or putting an afterburner on it.

Out of curiosity what are the typical differences in these two different designs of combustion chamber in terms of power, suitability as turbojet or turboshaft and cost/time?

I have had a read through some of the linked reading material such as the books by Kurt Schreckling and the jet specs designer program. I understand general thermodynamics and blade exit angle efficiency calculations however that is the extent of my experience.


I'm an engineer by trade so I'm not afraid of a few hard sums. First things first I will be running a few calculations and preparing a CAD model. I was wondering if anyone has any good resources that are practically applicable to DIY turbines, or any calculations or tips they are willing to share?

I have found that searching for information that is any more detailed than just a general overview of turbojet/shaft operation quite difficult to find outside of this board.

I am particularly interested in finding out:

1. What you look for or avoid in turb/comp wheels and scrolls
2. how the combustion chambers are designed (dimensioning/sizing, efficient placement and sizing of holes/louvres in flame tube
3. fuel delivery design (direct injection or evaporation) and the effects and efficiencies of each type
4. Any surprises or unusual behaviors one could expect and general tips
5. Useful calculations you may use to design the components

I have already compiled a short list of best practices in design I have picked up from browsing other users projects (thank you racket/john) but I would like to greatly expand on this and ideally prepare an excel spreadsheet to calculate the required dimensions and flow properties of both an axial flow with radial comp/turb design (not sure what you actually call this design) and external combustor design from intake through to exhaust as I am not comfortable with being unable to see the calculations behind the JetSpecs designer and the fact that it does not address everything. I would like to check as I wish to have everything that I can feasibly and reliably check with calculations alone squared away properly.

I understand I'm probably asking a lot for only my second post here and in return I'll be willing to share my calculation sheet.

~Shin

[racket]

Hi Shin

Have you looked at the rough info here jetandturbineowners.proboards.com/thread/680/diy-turbines

The difference been a "sore thumb" DIY turbo engine vs an "micro" type axial one aren't huge, the sore thumb is easiest because we only need to make a combustor and stick it on a turbo , but most turbos will have lower component efficiencies , both comp and turb , compared to a micro with a vaned comp diffuser and NGV stator on the turb , this does reduce potential power output, a bit, and the engine will be heavier , but that can be traded off by using a ready made turbo which saves a huge amount of work .

1 ....But picking the right turbo with the right configuration isn't always easy, a lot will depend on the power output you're after.

2....Combustor sizing will depend on the size of the comp and its flow , again that will be determined once you have a rough idea of the engine .

3....Direct injection or evaps , it'll depend on a lotta things .

4 .....LOL, always surprises

5........ jetandturbineowners.proboards.com/thread/680/diy-turbines

Sorry about the brief reply , but it really needs thousands of words..................the best way is to find an engine amoungst the ones here and copy shamelessly :-)

Cheers
John

[shin]

Hi John,

I had a look at the thread you linked but images are not available, I followed the link to the yahoo group but you cant view them without being a member... but apparently I'm already a member. I sent another request to join the group regardless.

1. Would like to go for something on the larger side but readily available to keep costs down something like a Holset HX80 or wheels from KTS if doing an axial design, I have seen both of these used here with success, I haven't got a particular power output in mind. but I would like to design it to be as efficient as possible, still undecided on what design, maybe one of each...

2 I would ideally like to develop a calculation sheet that I can use to calculate the required dimensions based on the compressor, flow and other properties and dimension in addition to power output based on the calculated design as that may help in selecting the most suitable compressor. This is something I tend to do with all my engineering design and verification work.

3. Should have mentioned I will be using a liquid fuel, kerosene, diesel or jet-A1, What other things does this depend on and how does it affect combustor design and power output? I've never seen these used with the "micro"axial flow type, is there a reason for this?

4. I suppose I'll deal with them as I find them 

5. unfortunately images not available.

I was not particularly keen on copying a design that feels like cheating a bit but Johansson's JU-02 does seem to have the most documentation available, I'll have a further look around the forum and do some more reading.

Thank you for your quick reply

~Shin

[racket]

Hi Shin

Did you click on the icons ??

1 ........the larger ones will generally give better efficiency

2 ......thats what Jetspecs is nuclearprojects.com/jetspecs/ Jesse and I made it up a long time ago .

3.... all the liquids have been used successfully in both sore thumb and micro engines.

4 .....LOL, as we all do

5...........some people have problems but others don't , I just click on the pic icon and up she comes.

LOL.................never concern yourself with copying a design , the full sized aero companies do it , why shouldn't we :-)

Anders JU-02 is an update on his JU-01 which was a copy of my 10/98 engine which was an update on my 9/96 and FM-1 engines .............its a slow progression, but we're getting there .

Cheers
John

[shin]

Hi John

I didn't realise those broken image link icons actually linked to an image, apologies.

2. Didnt know you did JetSpecs, excellent stuff. Is the result that provides a more conservative design that has room for improvement? if so it might be worth doing some updated calculations. are the same calculations applicable to a micro engine combustion chamber?

3. is there a reason I haven't seen injectors used in the micro engine design?

And thank you for answering all my questions. hopefully I can help contribute to the development of the engine further down the line.

~Shin

[racket]

Hi Shin

Yep , we had problems with the pics last year and needed to change things.

Jesse did the Programing to accommodate my numbers from the Rules of Thumb , we we're entirely happy with the outcome as the huge number of variables with our engines made it impossible to accomodate them in a "simple" easy to use design .................I've looked at updating it a number of times but keep on reaching the stumbling blocks , its not only the actual component efficiencies that need to be considered but also the "human" component of doing a build ............its a crude device that should produce a working engine .......................for sophisticated builds , the GTBA www.gtba.co.uk/ is where to go for more finely tuned technical data.

Injectors for micro engines isn't easy , too many needed which means very small flows as well as costs ..................though Andy Ms new engine "injects" .............but we don't know how thats going to workout just yet.

Cheers
John

[shin]

Thank you very much for your help John, I've had a look through Finiteparts combustion design thread and it has some excellent information, I'll see what I come up with in my calculations...

Would you know if its possible to join GTBA without giving away surname and address?

Lastly one more request and I'll soon be out of your hair. Would you have any recommended reading that goes into more detail about the particulars of engine design and analysis beyond the kurt schreckling books? I have come across two interesting textbooks that may be of use, Gas turbine Engineering handbook, 4th edition by Meherwan Boyce and Blade design & analysis for steam turbines by Murari Singh (whilst neglecting compressor and combustion design and being geared towards steam turbines the principals are not too dissimilar)

~Shin

[racket]

Hi Shin

With the GTBA I think you can keep your name and address "private" from other Members , but maybe still need to provide it when joining , its a well run Site thats been around since the beginning of micro jet engines , virtually all the development work for them was done by Members before they started getting made commercially, firstly by some Members and then Companies , they take things "seriously" at the GTBA ...........LOL, so not as many laughs as on here :-)

As for books , it'll depend how deep you need to go , which will again depend on what you want to build , theres plenty of books on Ebay , Amazon etc ,

I love my old 1950s Cohen and Rogers www.ebay.com/itm/Gas-Turbine-Theory-H-Cohen-G-F-C-Rogers-1958-ID-56128/382870213333?hash=item5924d7d2d5:g:dvAAAOSwkp5cneWc

also www.ebay.com/itm/Aircraft-Gas-Turbine-Engine-Technology-Textbook-Irwin-Treager-1970/293076600003?hash=item443cba04c3:g:r90AAOSwFbJZ3QF8

and www.ebay.com/itm/Aircraft-Gas-Turbine-Powerplants-by-Charles-E-Otis/143131210508?epid=315622&hash=item215348f30c:i:143131210508


But , I think the GTBA is the best place for you, the amount of information is staggering , both historic and current , there are also some highly educated and knowledgable Members that are willing to help.

Cheers
John

[shin]

Thanks for the list of extra reading, I sure do love technical detail

I will definitely join GTBA but I don't intend to leave JATO, I will stick around and hopefully sometimes soon have some progress being made on a project that I can share.

~Shin

[racket]

Hi Shin

I might catch up with you over on GTBA , I've been a Member, on and off, for a long time ...............the two Groups compliment each other , between both we cover most guys needs .

Cheers
John

[britishrocket]

Hello Shin,

I just found your post and I thought my films might be of use to you.

youtu.be/5M_FPAULPJI?si=3KwNcaiLS9_H1Io0

youtu.be/USl_X_nOD_s?si=gPdRws0ymxFzj7Nf

Best wishes,

Carl.