Bill,
The second stage grain design, on this rocket, is "Moonburner". It has an inherent large mass offset during the burn.
The second stage grain design, on this rocket, is "Moonburner". It has an inherent large mass offset during the burn.
Bill,
The second stage grain design, on this rocket, is "Moonburner". It has an inherent large mass offset during the burn.
I completely agree. 75k is amazing, and you desiged and built everything yourself, including the motors and electronics, even sewed your own 'chutes. I enjoyed following this thread.Congratulations! 75K isn't anything to sneeze at, now you can set your sights on that 100K goal for next time.
I think the important thing to keep in mind is that this is a system and compromises have to be made. I'm not particularly interested in optimizing any single component but I'm VERY interested in optimizing the system. For example, it seems like a lot of Ex-ers chase maximum Isp, with effects on altitude due to issues like volume loading and burn times be damned.I did notice that; “…assuming the rest of the vehicle is axisymmetric in mass.”
Look, this thread has been hijacked long enough: I understand why moonburner grains seem the right answer for sustainers on high performance vehicles. But that choice will *always* lead to coning, that’s just physics. A BATES (or any other axisymmetric grain design) with a slower burning propellant will avoid coning and get the longer burn time desired for an upper stage.
Bill
Yes, I'm a little surprised and quite pleased that so much worked. Thanks a bunch for all the documentation you've posted over the years, your threads were goldmines for me.Rob, you got it off the ground, got a decent altitude and got it back. Not bad at all.
For what it's worth, I've flown moonburners in second stages over 100K multiple times without coning.
Jim
One last post here on @robopup's thread ...I did notice that; “…assuming the rest of the vehicle is axisymmetric in mass.”
Look, this thread has been hijacked long enough: I understand why moonburner grains seem the right answer for sustainers on high performance vehicles. But that choice will *always* lead to coning, that’s just physics. A BATES (or any other axisymmetric) grain design with a slower burning propellant will avoid coning and get the longer burn time desired for an upper stage.
Bill
I also went from one-DoF in 1998, to 3-DoF, to 6-DoF, now at 9-DoF (High & Low accelerometers + gyros).-- kjh( a 1990's one-DoF Rip Van Winkle suddenly awakening in the new 6-DoF world of rockets )
I think the important thing to keep in mind is that this is a system and compromises have to be made. I'm not particularly interested in optimizing any single component but I'm VERY interested in optimizing the system. For example, it seems like a lot of Ex-ers chase maximum Isp, with effects on altitude due to issues like volume loading and burn times be damned.
It's not that I didn't consider bates grains in the second stage. I even flew (successfully) 27" of orange sunset (as far as I know, about the slowest propellant there is) as an L-800 last year in a mockup of my second stage. It's that it seemed likely it wasn't the correct answer for the system: sims suggested that the choice of that same motor in a second stage would result in 15k ft less altitude than the second stage moonburner I used (even with 4" more propellant!), while also raising the maximum speed by an additional mach number (something I was and am very concerned about).
I accepted that there could be some penalty due to coning because it seemed likely the overall system would still perform better. And I still think that there were likely other factors which caused the majority of my problem - looking at data and video I have two theories (one being that I simply didn't get a fin on straight enough). If you look at Kip's or Curt's big flights, both used moonburners in their second stages and, while there was some spin+coning, it didn't dramatically effect the final altitudes compared to expected from straight flight sims.
And many thanks all for the kind words, it means a lot!!
Looking at your circuit you really want the resistor to the left of the shunt switch. That way the shunt switch wins in the fight against the altimeter output.Yes, I'm a little surprised and quite pleased that so much worked. Thanks a bunch for all the documentation you've posted over the years, your threads were goldmines for me.
Speaking of, I have a question I've been meaning to ask you: what is the specific usecase you're trying to handle by having a shunt which can take the full output of the battery without firing the igniter? My best guess is to last longer than the altimeter FET in case it's locked?
My ignition circuit was as below, but I ended up omitting the low ohms resistor in series with the igniter for the flight - accordingly, my igniter would fire with if both the inline and shunt switches were closed. I did bench test this circuit with the resistor in place and had things working, but decided to leave it out in favor of simplicity for the flight. The reasons being:
- Open/closed switch meant to handle any bad altimeter altimeter output on powerup
- Shunt meant to handle any weird static stuff etc, just like I always twist my igniter and ematch leads together until immediately before use.
- Checked before mounting electronics that all FET outputs were behaving as expected with light bulbs, then didn't fire any more ematches until the actual launch
- Could test my full setup without disassembly (but outside motor) - 1) turn on altimeter, 2) close open switch, 3) open shunt, 4) wait for 30 seconds, 5) if nothing lights, reverse the process then install closure in motor.
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For my drawn circuit diagram, that's not correct. You need to keep the circuit from altimeter output to shunt as low resistance as possible so that it sucks as much current from the battery as possible. And then use the resistor on the ematch side of the shunt to reduce current through the ematch below no fire. I tested my calculated resistance value for my battery configuration on this circuit and it worked - 1) top left switch and shunt were closed 2) battery connected to left side of circuit directly (rather than through altimeter fet) 3) ematch doesn't fire in this configuration 4) open shunt a few seconds later (before battery lets the smoke out) and ematch fires.Looking at your circuit you really want the resistor to the left of the shunt switch. That way the shunt switch wins in the fight against the altimeter output.
My preference is to put the resistor close to the altimeter in the wire to the shunt. The logic of this is that the resistor will control the maximum current from the altimeter, and then the relative resistances of the shunt and the ematch will control how much current the igniter sees. Using the resistor downstream can also work, but the difference is that you have a situation where high current is going to the circuit and bad things could happen as a result. For example, if the shunt wire or switch burned out, then the igniter fires. There is no problem with two resistors either.For my drawn circuit diagram, that's not correct. You need to keep the circuit from altimeter output to shunt as low resistance as possible so that it sucks as much current from the battery as possible. And then use the resistor on the ematch side of the shunt to reduce current through the ematch below no fire. I tested my calculated resistance value for my battery configuration on this circuit and it worked - 1) top left switch and shunt were closed 2) battery connected to left side of circuit directly (rather than through altimeter fet) 3) ematch doesn't fire in this configuration 4) open shunt a few seconds later (before battery lets the smoke out) and ematch fires.
It just wasn't clear to me what the safety usecase was for the shunt having this behavior (other than some corner case like an altimeter fet which latched, but this can be tested beforehand other ways) so I abandoned the resistor altogether. Still curious on Jim's thoughts though.
Other than one manufacturing snafu on one of the booster casings which caused it to be half an inch shorter the rockets were identical. That and the "A" rocket got all the parts which I felt were meaningfully better. Both were flown at XPRS, somewhat ironically "B" was the one that worked. Long story short the second stage motor on "A" cato'd shortly after ignition. Moderately disappointing as I'd made a point of testing this motor and its ignition beforehand, but high L/D motors and acceleration are... a challenge. Still trying to understand what exactly happened - second stage of "A" was recovered with no damage (motor sheared at aft snap ring grove).Great flight,
At the start of this build, it looked like the 2 sustainers had slightly different fin shapes. Were they? Did they stay different? Was the second one launched? Or was that at ARLISS?
Haven't caught the bug yet to really dig through data to understand the under-performance, so no update yet. I did put this little diddy together though; it doesn't hold a candle to Kip's stuff but I enjoyed editing it.
@eggplant Thanks! And I'm probably going to be reaching out to you shortly on details about your closures on your composite case; I think that's where I'm taking this next.
I need to go through the data to verify, but I suspect the primary cause for under-performance WAS the coning. I'm just not at all convinced that the dominant cause of the coning was the off axis mass of the moonburner. I've flown several moonburners in min diameter rockets with very little coning, as have many others on this forum. My bet right now is that "B's" particular fincan simply had at least one fin which wasn't on quite straight enough. I think Kip's two flights (with two different fincans) also support this guess - if the cause was dominated by off axis mass, why wouldn't those two flights be similar?
There's definitely dynamics here that I don't really understand right now, I just think there are too many clean flights floating around the forums at least for me to write this configuration off and be okay with hammering this thing on a long bates motor.
I use a line laser, v-block, and ruler mounted to the wall. Based on how high off the end the tube the laser is and the measured wobble as you spin the tube you can calculate the perpendicularity. I got this from Jim Jarvis and it’s how I square all my tubes. But now it’s how I check my cases that I have machined too.Interesting on the snap ring grove - that particular problem hadn't occurred to me, but I'll go back and double check. How are you measuring that precisely?
I don't know what I'm looking at, but I'm interested ! Do you have more details you can share ?@Kip_Daugirdas I've squared tubes that way but hadn't done casings before, makes sense. Copy that on Utah!
Speaking of squaring (but not measuring) tubes, I meant to post these here a while ago. A buddy turned me on to these things - I think he got the idea from an article in Sport Rocketry. They're... amazing.
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They are sanding jigs like I made for my Thunderbird 3 build. They were 3D printed in my LPR case. They work really well for squaring up the tubes. See this post for what the jigs look like:I don't know what I'm looking at, but I'm interested ! Do you have more details you can share ?
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