Evaporator needle question.

[madrocketscientist]

Hi everyone,

First post.

I am designing and building a couple of 25kg thrust turbines and while I have read the Kamps book more than a few times I cannot seem to find any info on the length of the fuel injector needles. From looking around the net there seems to be a lot of designs with short needles into the evaporator tubes. However some of the model size turbines have longer needles either through the front face of the combustion chamber, or coiled around the distribution ring at the rear of the chamber.

I am wondering if the length of the needles make any difference and why would some designs have longer coiled needles?

Thanks in advance.

[racket]

Hi

I think the longer needles are so that larger bore ones can be used to prevent blockages whilst still maintaining enough "backpressure" when flows are minimal .

As long as the flows are even amongst all the needles, the length isn't important , its more to do with being able to use a decent sized bore , fine bored needles are rather fragile as well, and very fiddly to work with .

Cheers
John

[madrocketscientist]

Thanks John,

Are you able to elaborate on the effect of insufficient back pressure at low fuel flows? Does this make it hard to control the turbine speed at low speeds?

I have some 0.7mm internal bore SS tube so hopefully that will be suitable.

Shannon.

[racket]

Hi Shannon

0.7 mm ID should be fine .

What you want is a good spread of fuel pressures between idle and fuel power , this should make throttling less touchy.

Are you a Member of the GTBA www.gtba.co.uk/ ?? theres a wealth of info on there

Cheers
John

[madrocketscientist]

No I'm not a member... yet at least.

[racket]

Hi Shannon

I just had a look in my Kamps ( 1995 edition) and he used "Size 2 ,0.8 dia, 40 mm, from pharmacist" , though there is only 12 mm projecting from the manifold ( page 75) , but on a previous page 73 he says the injectors are 12 mm long length of syringe silver soldered into the fuel manifold.

The GTBA membership cost is money well spent if you intend building that type of engine .

Cheers
John

[madrocketscientist]

I will consider joining the GTBA, but the site terms of not allowing sharing anything do seem quite outdated? Not that I personally know of anyone else building a turbine!

I am loosely basing my design on the GR180 and UT160 turbines. I just need slightly more thrust than those particular designs put out. So Far I have a 84mm Compressor wheel and most likely will use the Jetmax 84mm turbine wheel and NGV. I have my own CNC router so things like the compressor diffuser etc shouldn't be a problem. I have a couple of shafts turned up but not yet usable, the first one I didn't machine in the correct order and the bearing surfaces aren't concentric,, the second I turned a bit short but everything is concentric to approx .005mm

[racket]

Hi Shannon

The GTBA is a bit fussy about its "intellectual property", I think it stems from earlier times when the first of the commercial RC turbines were coming onto the market and the manufacturers were undoubtedly using the free R and D supplied by all the experimenter Members of the GTBA .

Times have changed, and with the wealth of knowledge available via the Internet , which didn't exist when the GTBA was founded, their "restrictions" are a bit obsolete , especially for amateur builders ..............and is probably one of the reasons why the GTBA is a fairly quiet Site these days .

Its still good value for a home builder though :-)

Cheers
John