Jet Build Part 1 Jet Build Part 2

2nd September 2008

Kart is almost complete

Since the engine is running and appears to push out at least some thrust, the next logical step was to put together a basic gokart to sit it in.

Considering the size of the engine and the gas tank it requires, a regular gokart chassis would probably be too small for my needs, so I set about building my own chassis. As another project, I've been thinking about putting together a motorbike engine powered gokart, mainly inspired by the Haynes book by Ron Champion: Build your own off-road buggy (and yes, I realise the cover is somewhat questionable!). This book gives good advice on purchasing and building techniques.

With a little research, I picked up some box steel from my local DIY place, and some free-spinning pneumatic wheels and hubs from a marine store (think they're designed to be used on trailers, etc). The wheels were quite tricky to find, and cost a suprising amount (maybe £80 for all four and the hubs), but I think they'll do the job.
First up I welded two 2m sections of box steel to three 1m lengths to form the basic box shape of the frame, then I welded two hubs onto the rear. I also cut out the damaged section of the engine. Re-forming the section was a tough job: the original section wasn't perfect, and forming the new section by hand to fit it was tricky. I think we spent over four hours getting it as close as possible. It's not perfect, but hopefully the engine will still run fine and the non-perfect fit won't sap too much power.



I first created a basic braking system mounted on the very rear of the frame, unfortunately it really wasn't up to scratch. The metal simply wasn't thick enough, and even a reasonable about of force resulted in the steel simply flexing. I doubt this thing is going to that fast, but brakes are always important!

Next up, I needed to create some hubs for the steering. These were a little tricky. I'll create an exploded diagram showing how they went together. The front corners of the frame were reinforced to strengthen it ready for the steering. Word of advice: always go for titanium drill bits over steel. They last much, much longer and will save you endless frustration trying to drill with a blunt bit. They don't even cost that much more.



The whole frame needed a reinforcement structure; we figured out a basic triangular 'space' frame, then also mounted some footrests:



As part of preparing for the aforementioned petrol powered kart, I had sourced some second hand parts for about £80 off the internet. This included a complete rear axle, steering parts and brakes. I pinched the steering wheel and mounts for this kart, but had to fabricate some steering rods to fit my hubs. These were simply made from an M8 nut welded to a length of tube, which in turn was welded to a piece of angled plate. The steering works a treat, even under load (the leverage of the steering column mount is basically half that of the mount on the hub, so you have to move the wheel twice as much to get the same movement in the hubs, effectively doubling the force you can apply on them.
Like just about everything on this project, the welds are messy and need cleaning up!



Next up the brakes needed work. I cut off the old crappy brakes and started from scratch. Kudos to my buddy Chris who figured a really elegant solution to routing two cables to the rear which allows for some redundancy, and offers great strength. I used much thicker steel tubes that have no give in them at all. The brakes feel firm and have easily enough strength to lock the rear wheels up. Everything is lightly bolted in place on the images, cables will be doubly secured and probably welded, just in case.



So, not too much to do now. The engine needs some mounts and I need to mount the plumbing for the throttle, then it'll be ready for a test firing.
Having just watched the video where the engine imploded again, I think I'm going to put some cooling fins on the 180 degree elbow, as it appeared to be running white hot. I'll also have to be careful not to run the engine at max power for more than a few seconds.
Essentially, I need to pull my finger out and start thinking about building a proper stainless steel engine: this first prototype engine has been a success, but it's simply not made out of the right stuff for an engine that runs so hot. I've reinforced it as best I can, and I hope it provides at least a few minutes of running about before it packs up again!

25th September 2008

Kart complete, and a test run done!

We built the mounts for the engine, then stripped the whole thing down and gave it a lick of paint - I've got a feeling the kart may be left outside (covered) for quite a lot of time over winter, so I needed to give the metal some decent protection. Basic black Hammermite metal paint was used, and a decent coat was achieved with maybe three cans of the stuff. Another addition I forgot to mention was the mount for the gas tank. I used some off-cuts to create a cube into which the top of the tank fits quite snugly. On the cross member of the top of the fram we mounted some loops, and used the strap from a backpack to secure the tank in place. It really works a treat and there's no movement in it as you can tighten the strap as you please.



Once the paint was dry, we fashioned a firewall from a piece of MDF wood. It fits snugly into the frame. We've left the bolts long as at some stage I hope to find a decent piece of steel that can me mounted perhams 3cm from the wood which will take most of the heat thus preventing the wood from becoming scorched.

Next, it was a case of greasing up all moving parts, fitting them again and making sure everything was very tight and secure. It's suprising how 'half hour' jobs can run into several hours when you really want to put a lot of care into it (you don't want a wheel to come off!).

With the wheels, brakes and steering fitted along with the seat, we then set about plubing in the gas system. I did have pipe left from the previous test, but some areas had been bent and appeared to have some fatigue. I really didn't want the gas system to fail (remember there's no regulator, so the system has to cope with the full pressure of the tank) so I splashed out on some new copper. man has the price risen! £24 for 8m! ouch. But better to pay that than have freezing, flammable liquid propane squirting all over me.



The system comprises of a number of valves to give control - the gas from the tank is split on a T-joint. One goes to a wade valve that allows for precise control to set the idle fuel flow, and the second goes to a ball valve which can be opened and closed fully very quickly to provide a sort of throttle. These then join back via another T-joint which passes through a second ball valve that is left open, but can be used to shut the engine off entirely. For good measure, I mounted a secont cut-off valve towards the back of the kart in case I had to jump out, or someone else had to shut the engine down in an emergency (safety is paramount!).

Effectively, the kart was now built. We mounted the kart onto a roofrack and took it down to the local model aircraft runway to give it a try.



At last! the engine works and provided real thrust. The kart moved off very well, and I believe it would have reached a really decent speed, but unfortunately, the mild steel gave way again and the same section folded. Looking at the video again, it would appear that the fins I mounted had worked - the section was running far cooler - however, the u-bend again got too hot and gave way, which in turn made the rest fold.
It is a shame considering the amount of time spent repairing the section, but was pretty much expected. The engine is two years old and was literally the first thing I ever built once I had learnt to weld. Time to say good-bye to the old girl. Now that they've seen the kart in motion, people are enthused and keen to help out with the bigger, more bad-ass engine. Reading online, it would seem that the valved engine provides significatly more thrust for the size (at a cost of having fragile reeds) and seeing as I was able to flood the engine with too much fuel, the current kart and fuelling system can handle a more fuel hungry engine. Exciting times lie ahead!

I'd like to mention at this point my most sincere thanks to all those who have helped me along the way in making this engine, including: Joe Raymond, Peter Laurens (and Pete's Parents!), Fabrice Le Luyer, Chris Coates, Dave my welding instructor, Bruce Simpson, Eric and all the the others at pulse-jets.com. Without your help I'd not have been able to put this thing together - Cheers!



Jet Build Part 1 Jet Build Part 2