(Yet Another) 100k Attempt

[Finicky]

@Finicky Exactly like @OverTheTop 's thread. I 3d printed mine also - flat plate with sandpaper on it. Then a cylindrical piece screws on top, which holds the airframe tubing you're trying to square straight. Makes short work of a rockets worth of tubing.

I will have full set made by this weekend !!!!!!!

 

[kjhambrick]

There is a short column on the "Tube End Dressing Tool" gadget in the July/August 2023 Issue of "Sport Rocketry" on page 36.

So if I don't have a lathe nor a 3D printer, is there a simple way to dress the ends of the cylinder to make them absolutely square ?

Thanks.

-- kjh

 

[robopup]

@kjhambrick You could always get creative with leftover rocket parts - I would start with some G10, coupler tubing and a coupler bulkhead. There's always also Jim's method, shown by Kip on the previous page, which doesn't require any fancy tools; just patience.

 

[kjhambrick]

@kjhambrick You could always get creative with leftover rocket parts - I would start with some G10, coupler tubing and a coupler bulkhead. There's always also Jim's method, shown by Kip on the previous page, which doesn't require any fancy tools; just patience.

Funny about squaring tube ends.

It's come up several times this week ...

There was a thread on the local AARG mailing list about Jeff Wagstaff's Level 3 Cert at Airfest and Jim's method of squaring tube ends was a topic in Jeff's Level 3 Documentation.

Thanks Rob !

-- kjh

 

[Adrian A]

There is a short column on the "Tube End Dressing Tool" gadget in the July/August 2023 Issue of "Sport Rocketry" on page 36.

So if I don't have a lathe nor a 3D printer, is there a simple way to dress the ends of the cylinder to make them absolutely square ?

Thanks.

-- kjh

I have used a combination square standing up on a flat granite floor tile and then turn the tube around to see where it's not parallel to the square. Then drag the tube along a taped-down piece of sandpaper with more pressure on the high side.

More recently I have dialed in the support on my bench-mounted belt/disk sander so that I can quickly make the tube end square on the disk.

 

[robopup]

Flight 1 Summary:

I'm not going to get through all the analysis I want to do in a reasonable amount of time, so I figured I'd at least get some data up.

I spent Monday - Sunday on the playa, for both ARLISS and XPRS. ARLISS is amazing - Becky, John, Tony and all the other volunteers don't mess around; it's a great crowd. Flying dozens of nearly identical rockets, prepped in similar ways and seeing the different flight results (most successful, but a few failures) was interesting, and also a foreshadow of what was to come for me.

I also appreciated having the extra time to get my tower dialed and do final button up on the rockets - because of work, in past years I was traveling, then rushing onto the playa Thursday afternoon for a Friday morning flight and it always felt like I couldn't quite catch my breath. I think the slower pace really helped make for cleaner, accurate final prep.

The weather for the full week was also unbelievably good. No wind (Windy was forecasting something like < 15knts at 50k the days I flew), good temps and a new moon which led to some killer starlink and ISS passes.

Flight 1 ('A' airframe):

After a couple friends got their big two stagers off, I was in the tower late morning on Friday.This flight was a little bit of a CF - I had two separate failures:

1. My booster came in hot. No charges fired and my suspicion is that one of the power wires solder directly to the altimeter (didn't have the height for terminal blocks) broke. One of the wires was very thin stranded and was kinda weak, the other was thicker solid, both of which I'd had trouble with previously. On flight two, I potted everything in silicone just in case. While the rocket gods didn't exactly smile on me, it sure felt like it when I drove up on the crater on my way back from retrieving the second stage - I'd spent a couple days at each of the previous XPRS events driving grids without success. My uncle and I got it all dug out (using his patented post hole technique), and I found most of the altimeter so we'll see what else I can learn here...
2. My second stage motor cato'd shortly after ignition. The aft snap ring grove failed, the propellant and nozzle were dumped and the rest of the rocket continued to apogee at 24.5k and successful recovery. Due to the cato, I'm having some trouble getting the full data set off my altimeters but I think there's still a chance. My suspicions at this point are that it's one or more of:
   • Long (for me anyway) motor, likely errosive burning which I'd also seen in tests, tight cores and nozzle, not a lot of margin
   • aggressive igniter (one full pyrodex pellet + ematch with ~.25g magnalite in a 3/8 x 23.5in core) - this had worked successfully twice, but potentially not this time.
   • grain cracking due to acceleration - I've figured my group has been curing propellant too hard for a long time, but we've also had a lot of success with the version of OS we used so why change it? Maybe this is why...
   • burn rate changes due to acceleration - I don't know much about this at this point, only that burn rates seem to go up.

It's too bad the second stage motor let go on this flight - the booster burn was much straighter than 'B's, and I had made some minor tweaks on how I made the 2nd stage fincan - I believe the fins on this one were on straight (and it's recovered so I'll be able to double check).

Flight 2 ('B' airframe):

My first reaction after 'A's failure was to bail on the second flight. After some long talks with the DAWG Pack, Becky and some final convincing by Tony (much appreciated), I was prepped and in the tower again Saturday morning.

Other than some hard coning, previously discussed, and a torn seam on my 2nd stage main, this one worked. Some altitude/velocity/acceleration data:





A few comments on this:

• I'm still looking into what exactly happened at ~28 s; a few seconds after second stage burnout. You can also see in the video that something... significant happened which put a big aoa of the rocket
• Not surprising, but the speed on descent from apogee to about 10,000 m is fast, not a lot of air up there
• You can see at lower altitudes that there is a fair amount of change in descent speeds. From the video, this is the rocket transitioning between the fincan tumbling/spinning, about 90deg from the flight path and it becoming ballistic, dragging the nosecone behind it. Unfortunately, it was in the later state when my main fired which tore a seam. I'd used a very small streamer (sorta the shape of a nomex pad), because I was concerned about the descent rate from my June test flight. Lotsa work to do on drogue descents I'm realizing...
• Looks like instant on of the second stage again which I'm really happy about. Instant on, or at least repeatable time-delay-on makes altimeter programming so much easier

Spinning and conning - I posted the video here already. Just adding the rotation data. The rocket spends 20 seconds pegging my 2000deg/s sensor, lovely... Like the acceleration plot and video, this plot also shows something intense happened at ~27 s, about 5 s after second stage burnout.

Because I had altimeters in the nosecone with vent holes that were still in the curvy part, as well as an altimeter with vent holes several inches behind the shoulder of the NC, I figured I'd make this plot comparing baro pressures. No major surprises - you can see that the pressure in the NC becomes increasing higher the faster the rocket goes, and also some difference as the rocket passes through mach 1.


It's not in the same ballpark as some of the flights on the forums, but the Interlux boat paint and G10 fin leading edges held up well. The rocket spent 36 seconds > M1 (including the brief dip below mach 1 before staging) and 10 seconds > M2. Max was mach 2.8. Albeit this was at altitude so much less intense than these crazy cats.

One spot on the fillet where it looks like the primer melted (and maybe a clue to something else going on like crooked fin or flutter...?). All others are pretty clean.


This is the one leading edge with a little noticeable damage of the fin material - the rest just have small paint bubbles like towards the tip on this one.


The nosecone just has some minor bubbling. Much improved from the BBQ paint I used to use (shown here flown on a K250 to ~M2.1:


I did not get drag separation, it occurred 3 seconds after booster burnout at the programmed separation charge firing. Speed was M1.4. The charge was passed through a small hole in the side of the ISC 54mm tubing, sealed with rope caulk. You can see what a terrible job I did sealing things from the pressure plot below - nothing a good kalman filter can't handle though! Booster continued on it's way to apogee at 19k ft.

 

[robopup]

Part 2:

I'm a big fan of "what would you do the same and what would you do differently" in other threads. Accordingly, here's mine:

Do differently:

• More margin on second stage motor design. Maximizing sims is addictive, but sometimes not doable in the real world.
• I'll be moving away from my fin attachment guides shown on page 1. They work great for most project I do, but there is very small amount of slop so they didn't cut the mustard on this one. For a rebuild, likely I'll 3d print a large single part guide custom for my fincans.
• My second stage igniter still needs some work - they're just too hard hitting. For my flight 'B' I used half a pyrodex pellet rather than the full thing, along with the same .25g of magnalite.
• Correct ignition circuit per Jim and @OverTheTop 's descriptions above
• Settings and mounting for camera - other than the window, the camera settings and installation were basically afterthoughts. In particular, I need to play with camera settings like (I think) turning off auto-exposure, to get good shots at apogee.
• Potentially simpler traditionally mounted booster ebay - saving length is nice, especially in the ISC, but this one was just a little too tight...
• Need better guides for initial tower setup - I made a couple but there was still a ton of fiddle factor due to multiple (rather than my usual 1) squares (rather than my usual triangle).
• No break in second stage airframe - I used that this year because I wasn't totally sure how I would make my camera window. This will be one piece next year so that the only airframe break is at the nosecone.
• Flight test and qualify all fincans - other than primer and paint, these are pretty quick for me to build now. I think I'm going to try to go into next season with at least 4 fincans and test them on small motors to qualify for larger flights.
• No more fancy EA-60HP for fillets. It's too brittle for this use. Back to my laminating resin + filler.
• Drogue descent - it's rare I actually see my rockets fall under drogue, but I did several times this year with the smaller motor tests, as well as with video. I need a serious redesign from my current one - a lazy bit of streamer - I could have used a gentler descent.

Do the same:

• No tip to tip, G10 fins
• Interlux boat paint - 404 primer + perfection
• 1 or more second stage motor static fires with exact igniter setup. I believe this was a large part of the reason I lit both sustainers succesfully - I had several wiffs on static test motors where I wasn't getting the "on" I was looking for
• 3D printed ISC with carbon tube inserts
• Tony's high altitude charges
• 4 fins
• Moonburner second stage
• 2 rockets - it's not that much more time or cost to build two vs one, I only get 0 to 1 opportunities to fly something like this per year, that choice last winter definitely made my year...
• Headend ignition with the 1/4-20 bolt and a very thin, compliant burst disk (I used 1/32 in TPU for the XPRS flights).
• Full up flight firmware + programmed settings testing is a requirement for me. It's crazy to me that this isn't a standard feature for most commercial altimeters (granted I'm mostly several years out of date on the commercial stuff, Blue Raven has a nice implementation though), and that you basically have to iterate on settings by actually flying.
• Full CAD for big projects - my final pad weights and Cg's were very close to those predicted by CAD in the very early design stage. It's kind of a pain, but it can be done far in advance and saves a ton of time during flight season. At some point I'll post my predicted CAD numbers vs my measured numbers before flight.

Other items I'm going to look into for the next attempt:

• Pourable (but hopefully still slow) propellant. My motor buddies and I made (don't have our notes in front of me) ~45 kg of orange sunset this season. Not a lot for some... But packing that much propellant is kind of a pain.
• Composite motor case for second stage - mass here makes a huge difference and I have access to a bunch of fancy uni prepreg which is sitting in a freezer, slowly going bad.
• Bolted booster case - snap rings are kind of a pain on big (big at least for me) casings.
• Phenolic + graphite second stage nozzle - integrated nozzle and tailcone.

And also attaching the final altimeter settings as a text file.

 

[James Keller]

Hey Robopup, something that I have been doing is measuring my fins with a digital level. I level the airframe then check to see if the fins are parallel with the airframe. I aim for +-0.1 degree. Every rocket that I have done with this has very minimal roll and flies very straight. All that being said I haven't flow a rocket nearly this large and aggressive. Just food for thought.
-James

 

[Adrian A]

Spinning and conning - I posted the video here already. Just adding the rotation data. The rocket spends 20 seconds pegging my 2000deg/s sensor, lovely... Like the acceleration plot and video, this plot also shows something intense happened at ~27 s, about 5 s after second stage burnout.
View attachment 606546


One spot on the fillet where it looks like the primer melted (and maybe a clue to something else going on like crooked fin or flutter...?). All others are pretty clean.
View attachment 606548

This is the one leading edge with a little noticeable damage of the fin material - the rest just have small paint bubbles like towards the tip on this one.

That plot looks an awful lot like my confirmed fin flutter experience, and the photo lends support to that idea IMO.

 

[robopup]

That plot looks an awful lot like my confirmed fin flutter experience, and the photo lends support to that idea IMO.

Thanks for the reminder! Certainly was possible on mine...

 

[Spacedog49Krell]

That plot looks an awful lot like my confirmed fin flutter experience, and the photo lends support to that idea IMO.

I agree with Adrian, the severe roll, pitch, and yaw oscillations at 27-28 seconds does fit a fin flutter pattern. Your fin fillet wrinkles would indicate high stress levels on the joint.

I did some preliminary frame by frame analysis of the on-board video for roll data. The video corroborates the roll gyro data. Your rocket was spinning at 6 rps - 8.5 rps during the pegged time with a possible peak of 10 rps.

 

[kjhambrick]

I agree with Adrian, the severe roll, pitch, and yaw oscillations at 27-28 seconds does fit a fin flutter pattern. Your fin fillet wrinkles would indicate high stress levels on the joint.

I did some preliminary frame by frame analysis of the on-board video for roll data. The video corroborates the roll gyro data. Your rocket was spinning at 6 rps - 8.5 rps during the pegged time with a possible peak of 10 rps.

I agree with Adrian and Krell.

And the raw pressure measurements during that 27-28 sec window also suggest that something unexpected but very real was going on with the rocket's attitude:

The fact that pressure anomolies were measured in two different AV-Bays ( ports at 8.5 in and 17.3 in ) might hold some clues too.

Both ports would have been forward of the CG so they would have been 'scooping' air on the same side of the rocket but at different positions.

I've never flown two barometers at the same time, let alone at different positions along the rocket but that's something I'll keep for my do-list.

EDIT: oops, I must be color blind. Mach number is green and is always positive !! One thing I don't understand in the plot are the negative Mach Number Units ?

I couldn't make the t-axes align in my post for the pressure plot and the gyro plot but these are the raw gyro readings for the flight:

The roll gyro was pegged, but I wonder what are the frequencies of the pitch/yaw oscillations during that same time window ?

And even the z-axis accelerometer saw something happening then:

I'll bet the pitch and yaw accelerometers saw it too ...

Whee !

I love high-frequency, raw, unfiltered, real-time rocket data !

-- kjh

 

[robopup]

@Spacedog49Krell - Wow, nice, thanks!

@kjhambrick - that acceleration data was filtered. Here's unfiltered 3 axis (nothing particularly interesting happens after 35 s so I trimmed it):

 

[Spacedog49Krell]

@Spacedog49Krell - Wow, nice, thanks!

@kjhambrick - that acceleration data was filtered. Here's unfiltered 3 axis (nothing particularly interesting happens after 35 s so I trimmed it):View attachment 606667

Thanks for posting this data. It gives me a new perspective of the 27-28 second event and what to look for in your on-board video. I see your pitch and yaw accelerometers are railing at 16 g's. What is your longitudinal accelerometer range?

 

[robopup]

Thanks for posting this data. It gives me a new perspective of the 27-28 second event and what to look for in your on-board video. I see your pitch and yaw accelerometers are railing at 16 g's. What is your longitudinal accelerometer range?

Yes, this is something I need to look into. I have two 3 axis accelerometers - an MPU6050 which is +/-16g's and and H3LIS331DL which I have set to +/-200g's. The idea is that when the mpu rails, the h3lis data is pulled instead - this works fine in the z axis and it makes that switch on basically every flight I do. You can see this in action from ~16-18 seconds where the z axis acceleration goes > 16g's and becomes much more noisy.

I thought I had implemented something more general which worked in x and y as well, but from the data, apparently not - and I don't think I'd actually hit 16gs in these axes before to notice this particular problem. Unfortunately I also had my full record data debug mode turned off so I was not recording all 6 raw 2 byte values, only x,y,z in engineering units with the intended logic above.

 

[robopup]

I should add - this certainly could be caused by the fins + rotation, but I do have another theory which could have played a part: On the video I posted of the static fire of the 2nd stage motor here, you can see the tail off/after burnout has some detached flow(?) which seems to throw some off axis thrust around, abnormally more than I'm used to. I didn't have a sea level nozzle on hand so I used one designed for ~25k, probably this is the cause. But it does give me... pause. It's an absolute swag right now and haven't looked into it yet beyond this.

 

[eggplant]

I should add - this certainly could be caused by the fins + rotation, but I do have another theory which could have played a part: On the video I posted of the static fire of the 2nd stage motor here, you can see the tail off/after burnout has some detached flow(?) which seems to throw some off axis thrust around, abnormally more than I'm used to. I didn't have a sea level nozzle on hand so I used one designed for ~25k, probably this is the cause. But it does give me... pause. It's an absolute swag right now and haven't looked into it yet beyond this.

The long tail-off on a moonburner results in a small but considerable amount of propellant getting burned at very low pressure, which both hurts Isp and causes flow separation as shown in the video. The chamber pressure gradually drops to 0, so even an optimally-expanded* nozzle will show this effect for some amount of time. That said, I'm guessing the thrust is so low and the momentum of the rocket is so high by that point that any impact on the flight would be really minor? That is another reason why I'm not a big fan of moonburners, though. Between the insulation challenges (especially in a composite case where you don't have the nice big aluminum heatsink), potential Cg/balance weirdness, and lower Isp, I'm much more tempted to just develop a slow-burning propellant and run it in an aft- or forward-finocyl configuration. Even if slower burning propellant also typically delivers a lower Isp and won't be quite as long of a burn as a moonburner, at least it will shut down cleanly due to the geometry and not waste impulse there.

* Optimal as in, exit diameter set to minimize exit pressure mismatch over the entire burn.

 

[robopup]

@eggplant - excellent post, thanks for the explanation. I'm still an ex moonburner noob... Even long orange sunset bates motors were too scary speed wise for me on this project this year. I've flown several long 54's as single stagers and even being a slow propellant, they're still kinda intense pushes, but this has given me more to think about. The ultimate solution like Kip and others have alluded to is an end burner but that has its own significant challenges.

On a related note, one of the things I'd like to look into for the next flight is a higher impulse booster so that I'm lighting the second stage in thinner air.

I guess this is why two stagers are fun!

 

[kjhambrick]

Rob / Propellant Engineers --

What is detached flow after burnout ?

Are you referring to the 'puffs' in the motor exhaust at the tail of the staic test burn video ?

Do the 'puffs' in the static test toward the end of the burn correspond to the spikes in the latest unfiltered green z-axis acceleration trace between 20 and 23 secs ?

EDIT: I guess I had better break out my old 'Sutton' book ...

On another note ...

It is interesting that the roll rate ( green gyro line ) suddenly slows when the rocket decelerates thru the transonic zone starting about 13 secs or so.

And it shows in the video too ...

Thanks for letting me play along !

Krell: Thanks for pointing me at the video !

I had to learn the YouTube Keyboard Shortcuts to play frame-by-frame but there is a lot of info in the video frame-by-frame playback !

-- kjh

EDIT: I am sold on the idea that there was fin flutter. I wonder how much, if any of the acceleration / gyro noise is due vibration carried by the airframe from the fins to the sensors in the AV-Bay, in addition to real changes in the flight of the rocket ?

I imagine that would be hard to put a finger on ...

 

[Spacedog49Krell]

Yes, this is something I need to look into. I have two 3 axis accelerometers - an MPU6050 which is +/-16g's and and H3LIS331DL which I have set to +/-200g's. The idea is that when the mpu rails, the h3lis data is pulled instead - this works fine in the z axis and it makes that switch on basically every flight I do. You can see this in action from ~16-18 seconds where the z axis acceleration goes > 16g's and becomes much more noisy.

I thought I had implemented something more general which worked in x and y as well, but from the data, apparently not - and I don't think I'd actually hit 16gs in these axes before to notice this particular problem. Unfortunately I also had my full record data debug mode turned off so I was not recording all 6 raw 2 byte values, only x,y,z in engineering units with the intended logic above.

For my experimental motors I also use the MPU-6050 with a H3LIS331 set to +/-200g's, recording all 9 channels with no switch over. This combination was fine at 400Hz, but I started testing at 1000Hz and the H3LIS331 output became too noisy. I'm searching for a filter combination that will work at 1000Hz for the H3, but I'm also testing the ADXL375 as a replacement. Preliminary testing of the ADXL375 exhibits reduced noise at 1000Hz and I can push it to 2000Hz in the future.
My commercial motor flight computers used the LIS331 with the 6050. After consistently railing the 24g accelerometers. I switched to the integrated ICM-20649 for increased range and simplicity. After studying your gyro data, I may switch everything to the ICM-20649 for the higher 4000°/sec gyro range. I've also flown an optical Yawsonde system which can handle spin rates over 11 rps as a simple backup system.

 

[Spacedog49Krell]

Do the 'puffs' in the static test toward the end of the burn correspond to the spikes in the latest unfiltered green z-axis acceleration trace between 20 and 23 secs ?

The three green acceleration oscillations between 20-23 seconds can result from two events that I've seen in the past.
1. The propellant sliver breaking up, increasing the burning surface area and pressure.
2. Unburned propellant or liner insulation passing through the nozzle.
Both can produce a similar acceleration change. Higher resolution data of just the events would help in determination.
This is also new territory for me. I have extensive experience with Moonburners, but I've never subjected a Moonburner to nearly 600 RPM during a test or flight.

 

[kjhambrick]

The three green acceleration oscillations between 20-23 seconds can result from two events that I've seen in the past.
1. The propellant sliver breaking up, increasing the burning surface area and pressure.
2. Unburned propellant or liner insulation passing through the nozzle.
Both can produce a similar acceleration change. Higher resolution data of just the events would help in determination.
This is also new territory for me. I have extensive experience with Moonburners, but I've never subjected a Moonburner to nearly 600 RPM during a test or flight.

Thanks Krell.

When I viewed Rob's Static Test Video at the end of Post #67 above there were some 'puffs' toward the end of the burn.

I am not familiar with the terms 'detached flow' that @robopup used in Post #166 above nor 'flow separation' wrt propulsion that @eggplant mentioned in Post #167 above.

I understand the terms detached flow and flow separation but I am not a propulsion guy so I looked in the index of "Rocket Propulsion Elements' by Sutton and neither term is listed there ...

Could the puffs in the static test video and the acceleration blips at the end of the sustainer motor burn between 20 and 23 sec be the same phenomena on the ground and in the air ?

-- kjh

 

[Spacedog49Krell]

Thanks Krell.

When I viewed Rob's Static Test Video at the end of Post #67 above there were some 'puffs' toward the end of the burn.

I am not familiar with the terms 'detached flow' that @robopup used in Post #166 above nor 'flow separation' wrt propulsion that @eggplant mentioned in Post #167 above.

I understand the terms detached flow and flow separation but I am not a propulsion guy so I looked in the index of "Rocket Propulsion Elements' by Sutton and neither term is listed there ...

Could the puffs in the static test video and the acceleration blips at the end of the sustainer motor burn between 20 and 23 sec be the same phenomena on the ground and in the air ?

-- kjh

The simple answer is, No. The static test flow oscillations at the end of the burn are a low-pressure nozzle flow phenomena. The flight accelerometer readings are a high-pressure thrust chuffing. I'm being vague because this is not the (restricted) Research thread.

George Sutton was a guest speaker for my rocket propulsion class at UCLA. Sutton doesn't discuss these flow problem areas. "Rocket Propulsion Elements" is a technical teaching book. A Fluid Dynamics class would cover flow separation issues.

Edit: The full title of the book is "Rocket Propulsion Elements ---- An Introduction to the Engineering of Rockets"

 

[Alan15578]

The simple answer is, No. The static test flow oscillations at the end of the burn are a low-pressure nozzle flow phenomena. The flight accelerometer readings are a high-pressure thrust chuffing. I'm being vague because this is not the (restricted) Research thread.

George Sutton was a guest speaker for my rocket propulsion class at UCLA. Sutton doesn't discuss these flow problem areas. "Rocket Propulsion Elements" is a technical teaching book. A Fluid Dynamics class would cover flow separation issues.

Edit: The full title of the book is "Rocket Propulsion Elements ---- An Introduction to the Engineering of Rockets"

I am not a propulsion engineer, and I have not read Sutton. However, I thought what was meant was over expanded flow where the exhaust separates from the nozzle before it reaches the exit, a condition that can often lead to unintended thrust vectoring.

 

[Spacedog49Krell]

I am not a propulsion engineer, and I have not read Sutton. However, I thought what was meant was over expanded flow where the exhaust separates from the nozzle before it reaches the exit, a condition that can often lead to unintended thrust vectoring.

If you over expand the exhaust flow while maintaining a normal shock at the throat, the flow separation can lead to uncontrollable thrust vectoring and destruction of the motor. At burnout, propellant slivers do not produce sufficient gas flow to maintain supersonic flow at the throat. The resultant off axis flow thrust is very small, insufficient to produce the 150 m/sec^2 - 70 m/sec^2 acceleration spikes presented in the flight data.

 

[Spacedog49Krell]

A summary of bullet point concerns noted during the Rocket B second stage phase of the flight.

• The accelerometer readings for the second stage motor indicate a hard start ignition. I see the same pressure spike in the static test data and test flight of the second stage. The CATO of the Rocket A second stage motor is another data point that must be considered.
• The erratic Moonburner thrust tail-off of Rocket B at 21-23 seconds into the flight appears to indicate propellant sliver breakup or a reduction in nozzle throat area from ejected solid debris. I see fluctuations in thrust during your second stage test flight at the 3 second and 7+ second marks in the data. You showed the motor liner from the static test. What was left of the liner from the test flight and Rocket B flight? Most of my Moonburner work used case bonded EPDM.
• The pitch-roll coupled event at ~27 seconds produced a 27°-29°coning flight profile for approximately 2 seconds. The accelerometer and gyro response look similar to a fin flutter event occurring after motor burnout. The high spin rate of 6rps – 9rps together with a change in wind AoA could account for the fin flutter and coning. Northern Nevada has high velocity upper level wind patterns. I searched for 150mb – 200mb wind charts for Sept 16. I found only 500mb wind charts freely available. There are indications at the 500mb level that there could be high velocity winds at the 150mb altitude level.
• The fin can design produces high spin rates. The single stage test of the second stage spun up to 5rps from the video data.

Keep up the great work.

 

[robopup]

A summary of bullet point concerns noted during the Rocket B second stage phase of the flight.

• The accelerometer readings for the second stage motor indicate a hard start ignition. I see the same pressure spike in the static test data and test flight of the second stage. The CATO of the Rocket A second stage motor is another data point that must be considered.
• The erratic Moonburner thrust tail-off of Rocket B at 21-23 seconds into the flight appears to indicate propellant sliver breakup or a reduction in nozzle throat area from ejected solid debris. I see fluctuations in thrust during your second stage test flight at the 3 second and 7+ second marks in the data. You showed the motor liner from the static test. What was left of the liner from the test flight and Rocket B flight? Most of my Moonburner work used case bonded EPDM.
• The pitch-roll coupled event at ~27 seconds produced a 27°-29°coning flight profile for approximately 2 seconds. The accelerometer and gyro response look similar to a fin flutter event occurring after motor burnout. The high spin rate of 6rps – 9rps together with a change in wind AoA could account for the fin flutter and coning. Northern Nevada has high velocity upper level wind patterns. I searched for 150mb – 200mb wind charts for Sept 16. I found only 500mb wind charts freely available. There are indications at the 500mb level that there could be high velocity winds at the 150mb altitude level.
• The fin can design produces high spin rates. The single stage test of the second stage spun up to 5rps from the video data.

Keep up the great work.

Thanks @Spacedog49Krell. I've been too busy catching up on life to think much more about this flight, but your summary hits major points of concern for me. Responding in order:

• I spent a full day testing igniters with inert grains, and then several static tests in 38mm land before the second stage static test. Priority #1 was consistent and repeatable on, not so much instant on. In my experience, usually these two things go somewhat hand in hand though. Unfortunately I was having some trouble with my pressure transducer so no hard pressure data on the inert tests, just observations discussed previously. I started off with @jsdemar 's calcs at 300 psi and was generally disappointed with results - 1 38mm failed to light, 1 chuffed for ~5 seconds. Finally, with 2 38mm motors left and the 54mm and the weekend running out I decided to see how things would do with a full pyrodex pellet, obviously very aggressive. These ignitions were instant on, albeit with the pressure spike at the beginning of the burn, but all three held, as well as 2 of three flight motors. I decided I would rather fail with an over-pressurization than failure to light and after experiencing the former, I stand by that. For the 'B' flight, I backed off to half a pyrodex pellet - still aggressive, but this probably is the path I'll continue to investigate. I'd have liked to do more static tests but unfortunately only have so much time for rockets.
• The liner from the B flight looked horrible, it came out in little chips. Granted I only got it out yesterday. Any actionable suggestions to reduce moonburner tailoff if it's possible? I need to start looking into this - I subbed a long bates configuration back into my flight and the profile still just looks too scary (for me). One thought off the top of my head is to cast a small amount of faster propellant on the edge which burns last.
• My suspicion is the event at 27s is some combination of:
  • wind shear - balloon from Reno about 4 hrs before flight doesn't show anything particularly nasty, but winds do go from ~5-20 knts over 2000m at about the altitude the event took place
  • moonburner shutdown - I need to do some moment of inertia and pressure force calcs to see if this guess holds water
  • fin flutter, although I don't think this is a root cause. I took a closer look at the fin fillet with the damage I showed and it is only the paint - no sign of anything going on in the fillet itself. I usually get a little stripped paint along my fillets on fast flights but this looks different, still not convinced either way whether the paint did this as a result of flutter or melting.
• I'm not convinced that the design itself produces the high spin rates; my bet right now is still that I simply got a fin a little off. I built three of these fincans this year, the two others flew with very little spin on I-200 test flights - obviously the off axis Cg was not the same but the axial Cg was very similar to the full flight profile. The 'B' two stage fincan was the same one which had the spin in the single stage test. There was a reason it was 'B' and not 'A'; unfortunately some other stuff came up this summer and I didn't have time to build a replacement. It's too bad it happened to be this fincan, for the most part I've been getting pretty straight flights the last few years. I have ideas on how to better measure and build these, we'll see how that works out.

Definite points of emphasis for me on the next flight will be flight motor testing and flight qualification of fincans. In general, I feel pretty good about the rest of the configuration and its performance.

 

[jsdemar]

I started off with @jsdemar 's calcs at 300 psi and was generally disappointed with results - 1 38mm failed to light, 1 chuffed for ~5 seconds.

What did you use for KNO3? And how thoroughly did you mix the pyrogen? I've never had an issue lighting 38mm motors with BKNO3-V. Just need a lightly dipped ematch.

 

[robopup]

What did you use for KNO3? And how thoroughly did you mix the pyrogen? I've never had an issue lighting 38mm motors with BKNO3-V. Just need a lightly dipped ematch.

I used your Quickburst design, followed the instructions and mixed thoroughly. Both inside and outside motors, they were disappointing to the point I wonder if I somehow messed up making it, which I'd mentioned previously. I used a .01 g scale to weigh out the amount calculated using 300 psi - obviously this is kind of ridiculous for small igniters but I wanted good igniter weights to start; it took me a few tries for each and I saved the mis-weighed dips to burn outside motors and for sport flights.

I have a big matrix of ~50 igniter tests, weights calculated from you "impletus" thread, I'm going to redo when I have a properly working pressure transducer. Many thanks for all you've written on TRF on igniters (EDIT: among other things, been reading about Bi2O3 and bolted cases lately), I've learned a lot.

 

[eggplant]

A long tail-off is pretty much guaranteed with a moonburner or D grain, because of the way that the propellant burns away to a sliver. If you want to see why, you can download my openMotor software and use the "grain" tab to see the shape of the propellant grain as the burn progresses. The only way I can think of to get a clean, BATES-style shutoff is by casting something inert into the space where the slivers would occupy, so the motor runs out of propellant when it is still at a reasonable Kn. That would be a bandaid solution that reduces impulse and mass fraction, though if that sliver of propellant isn't helping you maybe you don't want it.