...For those who understand German, please note the sexy comments from the cam lady. I wish I could meet her now and have a drink...what she says about the launch..is very sexy *g
Don't forget, you've got a lovely lady as your wife :wink:
...For those who understand German, please note the sexy comments from the cam lady. I wish I could meet her now and have a drink...what she says about the launch..is very sexy *g
Throttling a hybrid you definitely need some sensors on the motor to help. With your idea I don't think I'd pre-fill a homemade tube tank and transport it. A DOT rated tank would be much safer.
An engineered tank , like the one on my 75mm Contrail is no doubt strong enough and I would mount a high-pressure valve on the aft end.
I would not think that would be any less safe than with a DOT tank with a valve on it. I would fill it in the trailer as racers do and not transport it filled.
Isn't NO2 typically at most 900-1000psi unlike an oxygen bottle come 2200 psi, filled.
But then again, the Contrail tank may not meant to stay filled at too long of periods wiithout the o-rings leaking.
I guess I could forgo that and just fill it on the pad like normal.
The design I am currently developing is using the servo plug valve from Rocketmoonlighting. Plug Valve I have a pressure transducer connected to the chamber and one also connected to the nitrous tank. The inital program for the valve monitors a pressure differential between the chamber pressure and tank pressure. You don't want them to reverse and splatter your rocket over the field.
On the Contrail:
When the ends of the hoses burn back to the fittings on the tank, and the pressure in the tank is almost spent, there is little or no pressure in the chamber with such a large openning in the nozzle, I would not think there would be any blow-back in the tank.
The 2nd priority of the controller is to maintain a set chamber pressure during the burn. Chamber pressure relates to thrust and this is one way to throttle. The motor works at 100% open in case the valve gets stuck open. I didn't want full open to result in too high of an Ai/At ratio. Approximately 75% flow is the design parameter for the motor. It starts off at nearly full open for 2.5 seconds, then throttles back to the design thrust. Then, because the pressure in the tank is dropping the valve slowly starts opening and keeping the chamber pressure at design thrust. When the pressure in the nitrous tank reaches 100 PSI (blowdown phase) the valve closes and keeps approximately 50 PSI in the nitrous tank preventing any backflow of combustion products into the nitrous tank to keep it clean.
I would like to make some runs on the test stand and also graph the tank vs. chamber pressure during a burn. Then I can see the impulse change as it throttles back and find the "sweep spot" that the motor can ramp down to until burnout. I do not have the time or funding to invest in closed-loop control of the flow rate. I did design and implementation of HVAC controls systems but that is beyond of what I have time for.
I was thinking of running tests on a test stand as well with CO2 (that is a great idea, OBTW) to make test runs of progressive nitrous controller.
In the cold flow tests with CO2 and using a ball valve to simulate a nozzle we were able to keep the chamber pressure within 25 psi of design even when we drastically opened or closed off the outflow from the combustion chamber. The first hot fire we did we just opened the ball valve 100% and tested at full throttle. We haven't had time to test the throttle but I need to make a small injector change and then we will move on to throttling the hybrid.
Edward
@ Edward, pls keep posting about that beast, it is extremly sexy on the paper
@ dragon64: yeah I opened my 2015 hybrid rocketry year yesterday... 3 motors prepared, 2 beautifull flights and one non-starter.
We tried first to fly my beloved eagle-claw 4 with a K sparky at mid speed. Could not start a proper launch procedure as the fill and dump valves were immediately leaking. After several test we dismantled both fill and dump valves on the field. It was just dirt inside. Easy repair on the field...but the ignitors have been twice frozen and defrozen and were simply refusing to ignite anymore, too humid...Maybe I was swearing too much about the delivery delay of my black sales order 2014..so the eagle claw refused to pleased me ;-)
So we continued with another scratch bird in 4", a light weight glassed phenolic with a lovely contrail J...in WHITE SMOKE and slow nozzle. It went very long into the bavarian skies. WHITE SMOKE is the new fuel formula from contrail. As the name says...it has NO smoke *lol* but an incredible tense white flame....Went just perfect in the sky and cool recovery.
So we finished with the mighty KRAMER X4, now her flight #4, on her favorite fast nozzled K555 BLACK GOLD, a 1200Ns peak thrust beast to lift-off the 38 pound nazi gorilla :kill:
It went just perfect all the way thru.
Now I had a new friend onboard: the full kevlar tubular harness from ONEBADHAWK that did the work just bravely!
Film of said flight coming soon..
Now what I cannot really tell.... is that we share the field with an official student group of the german spatial state agency .. and a 30 men crew and 3 trucks... against me and my friend. They had tons of HW, flat screens under tents etc...and a comparable rocket to our phenolic in one in diameter and length. They went also hybrid, but homemade.
We went good. They had a professional setup.....but...no real high power experience. Result: fin flaterring and broken in flight, motor explosion just above and last but not least a destroyed harness and hard landing pieces without chute.
National Space Agency vs amateur rocketeers 0:2 :kill:
But I must say they were very nice guys to discuss and they even shared nitrous with us, as we went empty with this non starting wildman bird.
It was a great saturday!
Having combustion particles in the tank is something that typically happens with closed nitrous tank systems. The open vent style of monotube hybrids I have not seen this happen.
For the cost of two pressure transducers ($70 total) and an Arduino ($50) you have almost everything you need for a closed loop feedback system. The very first generation I did was on an Arduino but then a friend joined the project and we transitioned a PIC controller. I think I had maybe 150 lines of code for the first generation. Most was just making sure it knew where to go after it adjusted the plug valve. The PIC controller used a PID loop to adjust the chamber pressure.
A DOT 3AL tank that has a rating of 1800 psi will have a minimum test pressure of 3000 psi and minimum burst pressure of 4500 psi. Both of these are beyond the 1800 psi rupture disc pressure. I don't think the Contrail is rated for either of those and would be an unwise choice to pre-fill and transport, unless you like hugging untested pressure vessels.
Edward
I have been assisting forum member Stardust on flying his hybrids at our monthly club launches. Maybe he will get on the forum and chime in. He just sent a WAC Corporal up on an H90. All the folks at the launch are real interested, some club members think maybe we a bit crazy to fly such motors. So what if they are not as easy as AP, the sound of the H 90 and a longer burn are worth it. And up to the 3.3 pounds weight limit is they only thing the hybrid flyer needs to worry about, no heavy and inefficient black powder limited to 125 grams.
The issue we had was a short hiccup in power right after ignition; a little scary as it went up the rail. Maybe that chunk of blue thunder burned too fast before the plastic melted and came to full power? Maybe some slower burning white lightnin' will do better?
Just look at those guys way out at the far corner of the range. How silly are they! Look - one of them just wrapped a towel around the tank and poured water on it! They have their own launch system and I think one of them said something about taking a dump! How crude! Are they ever going to get it to work? Is it real rocket science? Are they going Hybrid Crazy? If the regulators have their way they may be the only one's flying.
Thanks for keeping this thread going guys. I am interested in trying hybrids one day but I want to learn a lot more first. I have not even seen a hybrid fly yet but I like the fiddling aspect of it.
I have tentative plans to fly a Rattworks K240 in October. Failing that it will have to wait until May 2016. But it is going to happen.
I received the longer 76mm motor to be the new combustion chamber for my EX M project, and have ordered the aluminum I need to machine for the rest of the parts. I'm figuring on a first static test in a few months if I can get some help with the GSE. I don't have my own yet and won't by then.
If this motor works out well enough, I hope to fly it as an intentionally overweight min diameter in NY presuming the club grants me permission, and as the motor in a large but reasonably light rocket perhaps at Red Glare or some other launch in the spring.
Gerald
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