vmax PSA

Exactimator said:

I wonder if the additional exposed surface area (or rough drilled vs. smooth un-drilled) helps it ignite and burn.

If that is the case, Maybe it's possible you just need a small nick in the grain to get it to light more reliably.

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FWIW I flew a vmax motor (I800) with an undrilled delay this past Sunday. The rocket had electronic ejection, but the motor ejection occurred about 1-2 seconds after the electronic deploy- like I had planned it.

I flew 2 CTI H400 V-Max motors. Both flights went perfect in my Mini Magg using only motor ejection. Maybe its the bigger V-Max motors where the problems shows up?

crossfire said:

Maybe its the bigger V-Max motors where the problems shows up?

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If someone wants to donate me a larger vmax motor (like a J1520 or a K2045), I'm willing to test this theory out. You know- for science!

djs said:

FWIW I flew a vmax motor (I800) with an undrilled delay this past Sunday. The rocket had electronic ejection, but the motor ejection occurred about 1-2 seconds after the electronic deploy- like I had planned it.

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I have no data to back this up, unfortunately. But I would assume the un-drilled delays would burn more often than not. But maybe the drilled delays burn every time or almost every time?

I wish we could access the MESS reports.

Is there a pyro pellet at the head end of those?
If not I would add one.
Is puzzling why it would not light. I guess the head end lights so fast and moves down to burn, that the top end does not burn long enough to light the delay grain????

Dave A said:

Is there a pyro pellet at the head end of those?
If not I would add one.
Is puzzling why it would not light. I guess the head end lights so fast and moves down to burn, that the top end does not burn long enough to light the delay grain????

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No, many vmax motors pressurize too fast for that, and come with a big fat pyrogen igniter instead.

I have lit them with my boron dipped igniters but I usually use a longer burning igniter. I just made some epoxy based igniters that use boron and they light and burn a long time compared to the boron dipped.

I have never seen VMAX fail to light the delay, but then again I just may not have been told that was the reason for the failure to deploy.

Exactimator said:

It looks like the unburned delay wasn't drilled/adjusted. I'm sure it was left that way intentionally, but it makes me curious. I've launched many 38mm VMAX motors with no issues. I always adjust the delay, either to the minimum delay or the next shortest.

I wonder if the additional exposed surface area (or rough drilled vs. smooth un-drilled) helps it ignite and burn.

If that is the case, Maybe it's possible you just need a small nick in the grain to get it to light more reliably.

Click to expand...

Exactimator said:

I have no data to back this up, unfortunately. But I would assume the un-drilled delays would burn more often than not. But maybe the drilled delays burn every time or almost every time?

I wish we could access the MESS reports.

Click to expand...

I remember a L1 Cert launch at Thunderstruck 4(?) that the Vmax load delay didn't burn, and I remember him adjusting the delay. Net result was a lawn dart...

Adrian

I'm really curious about these failures and would like to see some documentation on which Vmax motors failed, and what size rockets they were in.

A question I have is whether the failures occur when the delay has not been adjusted, and/or the specified CTI igniter was not used, and/or the igniter was not in contact with the delay grain. Here is a photo of an unburned delay grain from post 18.




In the photo it does not appear that the delay grain ever lit, and that is was not drilled. One wonders if the igniter was not in contact with the delay grain, or if a non factory igniter or e-match was used to start the motor.

I'm also wondering if contamination of the delay grain surface coupled to the shortness of the burn might inhibit initiation of the delay grain combustion if/when the grain is not drill for delay reduction. The act of drilling the delay grain exposes fresh surface and creates a cavity that might not be snuffed out due to the rapid shut down.

I have not seen any information whether the motors were prepared and used according to the printed instruction sheet, and I wonder if that could be part or all of the problem.

1.) Vmax motors do not have a BP starter pellet. If you use an e-match instead of the supplied igniter to start the motor, there may not be energy delivered to the delay grain to ignite it before the motor burns out.
2.) Evenif the proper igniter is not in contact with the delay grain, there may not be enough energy delivery to the delay grain to ignite it before the motor burns out.
3.) When used in minimum diameter rockets, the delay may not need to be drilled to shorten the delay which could reduce the probability of delay grain ignition for delay grain Kn issues, and
4.) In minimum diameter rockets, the velocity at burnout is supersonic, and the base vacuum on the rocket can be far below ambient which could quench a marginally burning delay.
5.) In larger diameter rockets, delay grain drilling is required, and the velocity is likely not to be supersonic at motor burnout. Under those conditions, the delay Kn is higher and the base vacuum is not that far below ambient so it is less likely that the delay grain would be quenched by a rapid decrease in chamber pressure.

I've heard that an unexpectedly large number of Vmax motors with motor ejection did not function properly but I don't believe a documented database has been complied to definitively prove why the rockets with Vmax propellant did not have their ejection charge fire. I think we need real statistical data other than anecdotal reports of ejection charge failures in Vmax motors to determine why this is happening.

I would like to see is a table of the failed flights containing

1.) the rocket motor label into (e.g. Pro38 H400) and the motor delay shortening value (e.g -0, -3, -5, -7, -9).
2.) Was the supplied igniter used. If not why not and what was used in its' place.
3.) Rocket diameter, mass, and anticipated maximum velocity. (If the velocity data is not available we can come close in a sim with the rocket diameter and mass.)

I speculate we are going to find that the problems occurs:

1.) In minimum diameter rocket attaining supersonic speed at burnout.
2.) In motors where the delay was not shortened.
3.) In motors where the supplied igniter was not used and/or not installed touching the delay grain.
4.) In the longer (higher grain count) motors of a given diameter where the nozzle area is large providing the lowest Kn for the delay grain, especially if they are not shortened.

I would appreciate anyone who have experienced no ejection recoveries to post the details of those flights.

Bob

The reason for the large number of delay failures is the sudden pressure drop at burnout causing the delay grain to extinguish. It doesn't happen every time but frequently enough This is why all Aerotech Warp9 motors are plugged only. I first came across this with Vulcan G500s. In this case, the shortest (4 sec) delays failed about 1/2 the time while longer delays worked every time (20 motors). In the picture in the previous post, it looks like this is whet happened - the surface is far too rough to be the original cut surface,

Gizmo on a K2045, no electronics leave the delay alone. Great flight, done it about a dozen times. Gotta use a tracker, though.

Rocketjunkie said:

The reason for the large number of delay failures is the sudden pressure drop at burnout causing the delay grain to extinguish. It doesn't happen every time but frequently enough This is why all Aerotech Warp9 motors are plugged only. I first came across this with Vulcan G500s. In this case, the shortest (4 sec) delays failed about 1/2 the time while longer delays worked every time (20 motors). In the picture in the previous post, it looks like this is whet happened - the surface is far too rough to be the original cut surface,

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In that photo of the delay grain it doesn't look like it's burned at all. It wouldn't have extinguished because it didn't ignite.

There is no way drilling or cutting into a delay should be required. Simply plug the motor if you cannot make the delay light reliably.

At least in my case this was a stock ignitor, all the way up, within reason.

Here's the details on my flight: Scratchbuilt 2.6" Phoenix, 29 mm MMT, 28 oz dry. H410, drilled to 9 sec (-5). Used the stock igniter, max velocity was 542 fps (Rocksim). Motor mfg date was Nov 2010, flight July 2014. Delay was at least partially burnt, and as I recall, actually saw some smoke after burnout. The ejection charge was completely intact.Best guess is delay snuffed out, although the smoke would suggest it was still lit after burnout, which is odd. It is possible that I'm wrong about the smoke, and it is also possible it burned fully but simply did not ignite the ejection charge. I don't recall if I saw the hole to the charge or not in the delay well. I took photos of the delay, but I can't seem to find them at the moment.

David

I wonder if painting the end of the grain with a pyrogen would help.

g zilla said:

I wonder if painting the end of the grain with a pyrogen would help.

Click to expand...

I like that idea, will give it a shot next time it comes up.

Or paint just 1/2 the grain and testwith electronic backup

bobkrech said:

I'm really curious about these failures and would like to see some documentation on which Vmax motors failed, and what size rockets they were in.

A question I have is whether the failures occur when the delay has not been adjusted, and/or the specified CTI igniter was not used, and/or the igniter was not in contact with the delay grain.

Click to expand...

This is from an I-800, flown in 2010 in a 3" 4FNC rocket with about 5,5lbs liftoff weight. I don't have performance data on hand, but I certainly wouldn't call that a high performance flight. The delay was not adjusted and the reload was simply transferred from the packaging to the casing. Two things to note: I got the motor second hand (unknown storage conditions) and the reload was about five years old when flown. Due to it's age and second hand nature, I just reported it to the dealer but didn't fill out a MESS form.
I used the CTI supplied igniter. The pyrogen on it had already become brittle and a small piece of it fell of, but the igniter worked flawlessly. I always put a kink in the igniter wire, about 1" away from the tip of the igniter, to ensure the pyrogen touches the grain but I don't bother to try to touch the delay element. On some AT hobby reloads, if assembled per the instructions, the igniter is both spatially and physically separated from the delay with a piece of masking tape between the igniter and the delay. Not touching the delay doesn't seem to be a problem.



Reinhard

Opened this email this morning:

To our members:

The National Association of Rocketry’s Standards & Testing and Tripoli Rocketry Association’s Tripoli Motor Testing Chairmen are hereby enacting temporary Safety restriction for all Cesaroni Technology, Inc. VMAX reload motors equipped with delay charges.

We have both seen a growing number of VMAX flights using delay only recovery coming in ballistic at our flying fields, due to delay snuffing. It is believed that delay snuffing is likely a result of any extremely high burn rate motor completing its burn with a significant drop in pressure and/or temperature, breaking the burn ‘chain’, extinguishing or ‘snuffing’ the delay slug.

In light of the unpredictable VMAX delay performance and the resulting significant safety issues, we will now require all VMAX motor flights flown at NAR and Tripoli launch sites to have at least one electronic system installed for primary recovery. These electronics need to be able to handle the short G period of these motors to insure “Flight in progress” triggers. The safety restriction will be lifted when the manufacturer has provided documentation to the three motor certification committee chairman which includes the root cause analysis, the corresponding design changes undertaken by the manufacture to address the issue, and implements the necessary changes..

Any questions about this restriction may be sent to your respective organization’s motor test chair

Regards,

Steve Lubliner
NAR Safety Committee Chairman

Paul Holmes
Tripoli Motor Testing Chairman
------------------------------------------------------------------------
You are receiving this message as you are a current member of the Tripoli
Rocketry Association.

Should you not wish to receive messages from Tripoli please reply to this message with "Unsubscribe" in the subject

Copyright 2002-2014 by Tripoli Rocketry Association Inc.

Very Interesting. At Red Glare this Spring I flew my Mega Der Red Max on a H400 Vmax and it came down ballistic apparently because the motor ejection charge failed (this was a MDRM clone built with BlueTube and plywood fins).

There was a Jolly Logic Chute Release in there which actually survived the crash. It was buried about 6" in the ground, in the remnants of the shattered nose cone.

tfish said:

For it to "snuffed out" it would have to lit in the first place. I see no signs of any burning of the delay grain.

Tony

Click to expand...

Any idea why it did not light in the first place?

Reinhard said:

This is from an I-800, flown in 2010 in a 3" 4FNC rocket with about 5,5lbs liftoff weight. I don't have performance data on hand, but I certainly wouldn't call that a high performance flight. The delay was not adjusted and the reload was simply transferred from the packaging to the casing. Two things to note: I got the motor second hand (unknown storage conditions) and the reload was about five years old when flown. Due to it's age and second hand nature, I just reported it to the dealer but didn't fill out a MESS form.
I used the CTI supplied igniter. The pyrogen on it had already become brittle and a small piece of it fell of, but the igniter worked flawlessly. I always put a kink in the igniter wire, about 1" away from the tip of the igniter, to ensure the pyrogen touches the grain but I don't bother to try to touch the delay element. On some AT hobby reloads, if assembled per the instructions, the igniter is both spatially and physically separated from the delay with a piece of masking tape between the igniter and the delay. Not touching the delay doesn't seem to be a problem.

View attachment 292932View attachment 292933

Reinhard

Click to expand...

The CTI instructions state the igniter should be inserted to touch the top of the grain. In most motors the top of the grain has a pressed BP starter pellet. In Vmax motors, there is no starter pellet so the top of the grain is the delay element.

The peak cut-off dP/dt of the I800 motor is high, probably ~-27 kpsi/second, a result of the high burn rate of Vmax and the large throat area of a Vmax nozzle. Contrast that to White Thunder which is ~ -10 kpsi/second, and the others which are slower. This is the leading speculation why the delay grain is extinguished on engine cut-off.

The "snuffing" of the delay grain may be analogous to the use of blow out panels on solid fuel ICBM motors, when the panels are blown off the sudden loss of pressure extinguishes the motor, in this case the sudden drop of pressure and a large nozzle throat diameter may do the same thing. However some of the pictures look to have delay grains that never even lit initially.

There is a difference between the ICBM motor and a CTI motor. The ICBM upper stage is in space when the panels are blown out, so the casing pressure basically goes down to full vacuum. This is not the case in a hobby rocket motor because the Vmax motors stays in the lower atmosphere, the motor will equilibrate at a sub-atmospheric pressure, and that's most likely is the root cause of the problem.

If you plot the propellant burn rate as a function of pressure as a log-log plot you nominally obtain a straight line so the burn rate = intercept x 10^slope. At some low pressure, there will be a negative deviation of the burnrate due to the lower energy feedback from the hot gases above the solid surface.

In a conventional SRM, if the Kn is too low, when you try to ignite the motor you get chuffing: an oscillation between slow burning and deflagration. I think the issue is similar with a delay grain on a Vmax motor. Once the chamber pressure drops below the stable burning point, the burn goes from deflagration to slow burning. Because the motor casing also acts a an organ pipe, there are acoustic resonances within motor and the pressure can oscillate around a mean value, in the case where the pressure is reduced, the pressure could oscillate below the level where sufficient feedback is lacking to sustain simple combustion and the delay grain combustion is quenched and will not continue. This would be different in each rocket and motor. At higher velocity, the base pressure is lowest, so the minimum oscillatory pressure could easily be below the critical quench pressure of the delay grain. The longer the motor casing the lower the acoustic frequency and the time below the minimum quench pressure lengthens, to the point where the surface temperature drops below the ignition temperature.

If I am correct, it would be difficult to simulate, and hard to predict.

Bob

bobkrech said:

The CTI instructions state the igniter should be inserted to touch the top of the grain. In most motors the top of the grain has a pressed BP starter pellet. In Vmax motors, there is no starter pellet so the top of the grain is the delay element.

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While instructions regularly state to insert the igniter to the top of the propellant grain(s), I don't think I've ever encountered language to the effect that the igniter needs to touch the delay grain. In motors with delay and/or grain spacers, this might put most of the igniter into a relatively inert area of the motor and AT advises to put a backstop on the forward surface of the propellant. In c-slot or moon burner geometries it would be exceedingly difficult to hit the delay grain after the motor has been assembled. IIRC, there have been motors from Ellis Mountain where it was advised to pull back the igniter about a third, after full insertion.

In a nutshell, it's my understanding that the delay gets reliably ignited by being exposed to the chamber conditions in slower propellants. Besides their short burn time, neither VMAX nor Warp9 appear to me to be motors with a high radiant output that might help to ignite the delay.

Because VMAX and Warp9 ignite easily, it might be interesting to put some effort into specifically targeting the delay for ignition.

Reinhard

Elllis is out of business and AT has many internal motor configurations. Regardless, their instructions are not relevant for this discussion since this thread is about CTI motors, with Vmax propellant in particular, and the CTI instructions are quite clear, the igniter is to be placed at the top of the grain, and on each instruction sheet there is a picture on how to bend the igniter so it will maintain contact with the top of the propellant grain. In most 38 mm motors, CTI has inserted a compressed BP pellet at the head end of the motor which permits the user to use a simple e-match to ignite the motor. Because of the high mass flow of Vmax, CTI does not use the BP starter pellet in Vmax motors to prevent overpressurization on ignition, so they supply a boosted igniter with Vmax propellant motors.

What is frustrating to everyone is that the delay grains on Vmax propellant function normally in test stand situations where there is no indication of -dP/dt delay grain quenching at motor cut-off. If this is not the problem, then what is? Two possibilities that I can envision is the failure to use the supplied boosted igniter supplied with the motor, and the second is a failure to place the igniter in contact with the delay grain at the top end of the motor, either which may prevent the delay grain from igniting fully before the propellant burns out. It would be really nice if either of these were the source of the problem since the solution is simply to follow the manufacturer's instruction on motor operation. Unfortunately I don't think it that simple.

CZTeacherman flew an I800 today at the Woosh launch. I asked to look at the delay+ejection charge afterwards (he flew it with electronic deployment). He'll probably post pictures in a day or two, but the delay grain was pretty much unburnt, and the ejection charge did not go off. Just before the flight, we were commenting on how many vmax motors we fly with no issues, but I guess this is an issue. I don't remember the datecode exactly, but it was Feb 2015, iirc.

Bumping an old thread - has there ever been a response from CTI about this issue? Want to fly a G250 on one of my rockets, but it doesn't have electronic deployment.

I was told that no-one officially approached CTI requesting that they "fix" anything. It was also suggested that there may be an easy "fix", which was to simply offset the top grain slightly, and that the impact on the performance would be negligible. This discussion was unofficial.

djs said:

Bumping an old thread - has there ever been a response from CTI about this issue? Want to fly a G250 on one of my rockets, but it doesn't have electronic deployment.

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The response is essentially that you do not fly vmax without electronic deployment. There is no fix forthcoming.