Valved Pulsejet Design Thoughts

[StorkTorque]

I’m working on a pulse jet that is supposed to be attached to a UAV i’m making for senior design.

Our advisor put a battery limit on the design, it has to carry and drop the payload, has to fly 120mph, and need retractable landing gear. As you can see the design requirements make things pretty tight. I talked to my advisor though and he was ok with me getting around the battery limit with a pulsejet.

So far the design I used the excel calculator for a 7.7 lbf design, because that seems around before where they suggest to use a grid design and it is a little above the calculated drag at top speed. I got 26 gauge stainless steal from a thread somewhere here that mentioned that was around the thinnest they suggested. I also got a Hago spray nozzle and a fuel pump but I havnt had the chance to wire them up yet. Valves came out beautifully.
I had some issues finding a cheap lightweight fuel pump that can get to the right pressures to flow the calculated fuel consumption but I managed to get one that was on but still heavier than I’d like, but I like the idea that the pump can defy gravity a bit. I have no Idea how i’m going to be able to implement a gas tank into this thing. I know I want it between the wings but the payload is also there so it a little tight.

I think I figured out how to hook up the spark plug I just got a taser transformer.

I really wanted to use gasoline as it’s a lot more lighter and efficient than using propane. I also plan on injecting it into the main chamber as I saw it allows some throttle. Any thoughts or concerns before I commit any more to this? I’m honestly pretty worried I’m not going to be able to pull off a gasoline powered design. Or there’s some fatal flaw i’m missing.

[tufty]

Rather than a pump and the various ancilliaries, a pressurised bladder tank might be a decent option. You absolutely need to avoid bubbles in fuel supply, they'll kill it stone dead.

[StorkTorque]

Do you know where I can get a good cheap bladder? Because I really do like this idea especially if I’m able to fit them in the wings to keep the fuel over the center of mass.

I have heard of people making them but I have not heard of one I would be particularly confident in.

Or do you think there’s possibility to combine both pump and bladder? Because I also like the ability to change throttle, atomize the fuel and fight gravity during flight.

[tufty]

there's a few ways of thinking about this, and there's a good amount of information buried in these forums. I suspect the motor you're using is too large to take a simple "bladder tank" as was used for similar (Dynajet-sized) pulsejets and control line speed models (and piston driven combat models in my case), so although the principle remains more or less the same, there's a few "wrinkles"

I really wouldn't bother with fuel pump and high pressure fuelling, throttling varies from hard to impossible - go for a dynajet style "all or nothing" naturally aspirated setup. Run it balls out until it runs out of fuel. Far simpler, less to go wrong. Simple is good. In the case of flat out running, the only control you need is an emergency shutoff valve on the fuel line.

It's worth mentioning, if you're not already aware, that once it's running you should not need spark at all. All that spark ancilliary stuff doesn't need to be carried in the vehicle unless you're wanting to cut off and restart in flight (that would be *very* ambitious)

You need to know how long you want it to fly, and how much fuel is going to be required. Start the jet, measure how much fuel it uses at full chat.
That sizes the fuel tank for you. You may well be surprised how much these drink.

For bladder material, I wouldn't worry too much. Treat your bladders as expendable, that's what we did back in the day (2.5cc motors fuelled on methanol / castor oil / nitromethane, the fuel will kill pretty much anything), we used cheap balloons. You don't have to (and shouldn't) rely on the elasticity of the material for pressurising the fuel, instead have the bladder in a sealed container (which could be the wing) which is pressurised off the exhaust in our case, or a tap on the combustion chamber in yours, and a simple one way valve. pre-pressurise the chamber before starting with a bicycle pump. There's various threads on here on using bladder tanks.

[StorkTorque]

I like this method a lot but my flight time is estimated to be 3-5 minutes. I had concerns because I heard people saying making a system that uses gasoline is very hard so I also wanted to be able to adjust how much fuel was going into the system to make sure it’s the right air fuel mixture.

From the math I did, propane would be pretty hard to fit and get the wanted flight time. This is mostly based of the paper linked in this site that estimates fuel consumption. I did want to have a enclosed ignition system hoping to be able to reignite it in the air but that might be a bit to ambitious as I don’t really have any experience with this kind of thing.

So do you think a UAV could siphon the fuel just from bladder pressure and the engines vacuum reliably during flight? Because that’s what I’m mainly concerned about

[tufty]

So do you think a UAV could siphon the fuel just from bladder pressure and the engines vacuum reliably during flight? Because that’s what I’m mainly concerned about

Worked pretty well for the Germans in the mid '40s.

The Argus pulse jet which powered the V1 flying bomb had a simple pressurised fuel feed. They were capable of about an hour of flight.