# How to build your own Jet Engine

## Step 8: Making the flame tube

Well, for many builders, this is considered the hardest part. The flame tube is what lets the air enter into the center of the combustion chamber, but keeps the flame held in place so that it must exit to the turbine side only, and not the compresor side.

The picture below is what your every day flametube looks like. From left to right, the hole patterns have special names and functions. The small holes to the left are the primary holes, the middle larger holes are the secondary, and the largest to the right are the tertiary or dilution holes. (note that there are also some additional small holes in this design to help create a curtain of air to keep the flametube walls cooler)

The primary holes supply the air for fuel and air mixing, and this is where the burn process begins.

The secondary holes supply the air to complete the combustion process.

The tertiary or dilution holes provide the air for cooling of the gasses before they leave the combustor, so as to not overheat the turbine blades in the turbo.

The size and placement of the holes is a mathmatical equation at best and a logistical nightmare at worst. To make the process of calculating the holes easy, I have provided a program below that will do the work for you. It is a windows program, so if you are on a Mac or Linux box you will have to do the equations longhand. The program, Jet Spec Designer, is a great program, and can also be used to determine the thrust output of a particular turbo.

For the long hand calcualtions of the flametube holes and an in depth explanation of things, please go to our website at http://www.badbros.net/jetbike5.html

Before making any holes in the flametube, you will need to size it to fit into the combustor. As our combustor is 10 inches long as measured from the outside of the ring ends one side to the other, you will need to cut the flametube to that length (make sure you cut to fit your combustor length). Use the posterboard wrapped around the flametube to square up one end, then measure and cut the other. I would suggest making the flametube almost 3/16ths of an inch shorter to allow for expansion for the metal as it gets hot. It will still be able to be captured inside of the end rings, and will "float" inside of them.

Once cut to length, get going on those holes. There will be a lot of them, and a "unibit" or stepped drill bit is very handy to have here. The flametube can be made of stainless or regular mild steel. Stainless will of course last longer and hold up to the heat better than mild steel.
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hilalh1 month ago

hello. the jetspecs program is very useful but i need the long hand calculations of the flame tube holes to develop a better understanding. i have tried the link you have share above for those calculations but i can't find anything. can you please give me a current link and help me out regarding it?

rohitbansal1 year ago
will aluminum will work beter thn steel??
mhickl3 years ago
I am trying to use the jet spec program for our senior design project. We need to do some calculations with the mass flow. do you know the units that the mass flo rate is given in the jetspec program?
mech_manic7173 years ago
i am a little confused, is the flame tube supposed to touch both of the end caps? or is there a gap at one end?
TurbineJesse5 years ago
JetSpecs is an excellent program I had made several years ago for designing the flame tube/combustor. It will size the combustor/flame tube a bit larger than needed, but does provide a safe "buffer" to insure you don't make it too small. You can scale back the dimensions it gives by an inch or so. The flame tube holes it suggests is very accurate, and deviations from that are not suggested unless you understand what you're adjusting.
4 years ago
How does your program work. i cant add  custom dimensions. i can only put in the inducer radius. my flame tube is 4 inches in diameter and 18 inches long. so what would my hole pattern be? with the program it says my flame tube should be 9in. in diameter, so the holes would not be accurate for my project.
4 years ago
The program will give you very accurate flame tube holes and sizes, that is one thing the program does VERY well. The size calculations for the flame tube are separate from the flame tube calculations. Use whatever holes it tells you to use on your flame tube. 9in is certainly a bit big for a typical flame tube, I wonder what your inducer size is? 4-5" flame tube is pretty good. Like I mentioned before, the program tends to be a bit 'generous' on flame tube/combustor sizes, so you're at liberty to scale back those dimensions a bit. Hopefully one day well come up with a bit more accurate formula for sizing an appropriate combustor/flame tube! Visit www.DIYGasTurbines.com for any future updates to JetSpecs. An online version is currently being built... you heard it here first!
4 years ago
Thanks alot! i took the program calculations and scaled them down 2/3 becasue my flametube diameter is 2/3 of the size the cal. said it should be. i hope it works. i just drilled my holes last weekend. do you have any tips on making jets. this is my first one. its for a class project, we have a 7 person team and we are all sophmores. the teacher is not much help. we know probally more than he does on his subject.   we have some good material resources but we dont no anyone who has made one of these.