This is the first test of a rocket igniter (i.e. small rocket engine) running on compressed air and propane. Using compressed air as the oxidizer is notorious for causing difficulty in keeping a rocket engine lit, as you can see here where the rocket only ignites a few times (by the spark plug) and never stays lit for any length of time. Basically, it appears like the spark just ignites what's in the ignition chamber before it all goes out. I tried some nozzles slightly smaller and slightly larger, and one just very slightly smaller seems to work best. (I also tried increasing the spark frequency from 31Hz to 122Hz, which helped slightly.) All of them seem to work best after the chamber becomes warmer after more testing. I may try running it for longer to warm it up and see if it ever stays lit for an appreciable length of time. I'm just concerned about melting the plastic tubing which brings the propellants into the chamber if I get the chamber any warmer (yeah, I probably should've tried metal plumbing there...). I may try seeing if I can water-cool it a little.
http://youtube.com/7alGMFgz52s
I originally had a lot more trouble with the spark generator producing a whole bunch of EMF which caused the microcontroller to go crazy. The relay which I was using for isolation earlier wasn't helping anymore for some reason, so I switched to an LED/photoreceiver setup which worked pretty well then had its own problems. I separated the power supplies to separate AC circuits and then used a metal mesh to shield the non-sparker side of the test stand, and that seemed to do the trick, 95% of the time, at least (good enough for me, though I can do better). I think I'm slowly wearing out the microcontroller somehow.
If I'm ever able to get it working smoothly, I will make a parts list. I need suggestions about how to keep it lit.
Here's a picture of the chamber with the nozzle unscrewed:
Have you looked at flame velocities vs the actual gas velocities in your igniter? Some googling suggests flame velocities for propane / air mixtures are on the order of 10 cm/s to 1 m/s depending on temperature, pressure and mixture ratio. If the actual velocity is higher than the flame velocity you couldn't expect to get continuous burning.
Those plastic hoses look dangerous. Maybe you should talk to an expert to make sure what you're doing is safe. Do you have flashback arrestors and burst valves on your propane tank?
I have not checked the actual vs flame velocity. That does sound like a good explanation, though. I had not considered it. If I had considered it, I may still not have done any testing, however, since if you don't truncate your analysis at some point, you'll never test anything.
As far as safety... I haven't done any more test firing because my hoses get too hot if I test too long, hot enough to be in danger of melting. I was originally planning to keep my tests very short, since there isn't really any kind of cooling mechanism. Before I do any more testing, I will add metal tubing. It isn't a safe setup, not by a long shot. My propane tank is just one of those disposable camping tanks. I don't need to talk to an expert to know this is nothing like safe!!! But if I do find an expert, there's no harm in me asking.
Thanks for the comment. I sort of doubt I'll be doing any more testing for a while. I have started grad school since I filmed this.
Another consideration is flammability limits. Apparently propane / air mixtures need to be within the 2-10% propane by volume range to be ignitable. So maybe you would need a separate pre-igniter with a mixture ratio that is easy to ignite and a main igniter to produce maximum heat once it's been lit.
As for safety, please talk to an expert before you decide an upgraded setup *is* safe. I know very, very little about this, but some googling suggests you may want at least flashback arresters (or explosion arresters, apparently a more powerful variant) both near the tank and near the engine and excess flow valves in case your hose / plumbing bursts. But you know what they say, I'm not a safety engineer, this is not safety advice, contact a safety professional if you need safety advice.
Thanks! I have considered flammability limits in the previous post about propane flow rate.
I want to stress that I am using just regular air, not pure oxygen like many other applications where flashback arrestors are used. They aren't used on gas stoves, are they?
Trying to keep temperatures and energies as low as possible to avoid needing complicated safety mechanisms (prone to failure) has been my design philosophy for this project. It has worked so far, but maybe TOO well, since I haven't got sustained combustion!
Heh, that's precisely how I found your blog! I was googling for propane / air rockets in the belief that that would be the safest step up from a compressed air cold gas thruster.
Have you looked at flame velocities vs the actual gas velocities in your igniter? Some googling suggests flame velocities for propane / air mixtures are on the order of 10 cm/s to 1 m/s depending on temperature, pressure and mixture ratio. If the actual velocity is higher than the flame velocity you couldn't expect to get continuous burning.
ReplyDeleteThose plastic hoses look dangerous. Maybe you should talk to an expert to make sure what you're doing is safe. Do you have flashback arrestors and burst valves on your propane tank?
ReplyDeleteI have not checked the actual vs flame velocity. That does sound like a good explanation, though. I had not considered it. If I had considered it, I may still not have done any testing, however, since if you don't truncate your analysis at some point, you'll never test anything.
ReplyDeleteAs far as safety... I haven't done any more test firing because my hoses get too hot if I test too long, hot enough to be in danger of melting. I was originally planning to keep my tests very short, since there isn't really any kind of cooling mechanism. Before I do any more testing, I will add metal tubing. It isn't a safe setup, not by a long shot. My propane tank is just one of those disposable camping tanks. I don't need to talk to an expert to know this is nothing like safe!!! But if I do find an expert, there's no harm in me asking.
Thanks for the comment. I sort of doubt I'll be doing any more testing for a while. I have started grad school since I filmed this.
Another consideration is flammability limits. Apparently propane / air mixtures need to be within the 2-10% propane by volume range to be ignitable. So maybe you would need a separate pre-igniter with a mixture ratio that is easy to ignite and a main igniter to produce maximum heat once it's been lit.
ReplyDeleteAs for safety, please talk to an expert before you decide an upgraded setup *is* safe. I know very, very little about this, but some googling suggests you may want at least flashback arresters (or explosion arresters, apparently a more powerful variant) both near the tank and near the engine and excess flow valves in case your hose / plumbing bursts. But you know what they say, I'm not a safety engineer, this is not safety advice, contact a safety professional if you need safety advice.
Good luck with grad school!
Thanks!
ReplyDeleteI have considered flammability limits in the previous post about propane flow rate.
I want to stress that I am using just regular air, not pure oxygen like many other applications where flashback arrestors are used. They aren't used on gas stoves, are they?
Trying to keep temperatures and energies as low as possible to avoid needing complicated safety mechanisms (prone to failure) has been my design philosophy for this project. It has worked so far, but maybe TOO well, since I haven't got sustained combustion!
Heh, that's precisely how I found your blog! I was googling for propane / air rockets in the belief that that would be the safest step up from a compressed air cold gas thruster.
ReplyDelete