So, maybe I'll try a three-stager

[jsdemar]

Okay, that makes sense. Is there a way to prevent such a sudden change in pressure, such as perhaps a thermoplastic restrictor in the nozzle that burns out over the same time period the motor comes up to pressure?

Yes, it's an engineering problem... design it based on requirements, implement it with a limited model and materials, test, and reiterate. I'm at phase 1 with the process.

The burst disk (or plug) at the nozzle must hold until the critical pressure is exceeded. That's the pressure where the propellant has reached a self-sustaining combustion (the chuff-free zone). This pressure depends on the propellant formula. Next requirement is that the igniter pyrogen amount and its configuration need to provide the heat flux level and spreading to transition to full ignition. One can size the igniter to give the pressurization needed, but if the pyrogen doesn't provide sufficient heat production, the motor won't light. Also, if the heat production is good but it's constrained and directed toward the burst disk, the disk will pop too early. (This is likely what happened with Jim's second stage in October). The head-end "basket" should be smaller than the grain ID and directed evenly.

One brute-force approach is to design the disk/plug to hold statically beyond the motor's operating pressure! The hope is it will melt before the motor overpressurizes.

Another way to "wing it" is to make the igniter essentially it's own rocket motor which will provide choked flow at the nozzle before the main propellant gets to that point! It also has to provide heat flux which is spread out enough to ignite the main propellant. A burst disk is optional in this case. The risk is that the shock of this transient could damage the propellant.

For both of the oversized igniter approaches, things get scarier for long motors. It can cause a shock wave or acoustic resonance. This will increase the propellant burn rate along the shockwave, and also could damage the grains. Even with a correctly sized igniter at the head end, this can happen with weak/erosive propellant.

Summary: 1) Design the burst disk to hold between the critical pressure and the operating pressure, and hold long enough to allow for self-sustained combustion. 2) Size the igniter pyrogen to produce that pressure. 3) Choose a pyrogen with high heat flux per gram. 4) Physical design of the igniter "basket" must hold the pyrogen to complete combustion and allow unrestricted flow to a wide area of the grains near the head end. 5) Oversizing the igniter has risks, especially with long motors.

 

[FredA]

that burns out over the same time period the motor comes up to pressure?

That's the rub - gotta match the burst disc [or whatever] to hold pressure for just the perfect amount of time.........

 

[TonyL]

You guys make this so complicated. Ultimately it is all about heat transfer [as are many things]. I would claim that if you get enough surface lit, your problem is solved. Creating a brief pressurized period of time to emulate sea level or better conditions is one way to get there, but not the only way.

While I am not a particular advocate of copper thermite, it does illustrate the principle: Makes little gas, not pressure sensitive, and transfers a lot of heat. A big enough charge gets you there in 30ms or so. For a less nerve-wracking design, a large enough charge of BKNO3 with a short burn time also should get you there. Why try to do it with less when you can do it with more

 

[0011001100]

Would painting the grain surfaces with a pyrogen (like BKNO3) and then having a smaller igniter work better?

 

[TonyL]

Yes, but it really becomes an 'enormous igniter' at that point, and not separate from the motor.

 

[0011001100]

But the idea is to get heat into the grains is it not? In that case you would be able to get the entire surface area burning and not need a burst disk. I've never attempted anything like this, just spit-balling ideas out.

 

[jsdemar]

You guys make this so complicated. Ultimately it is all about heat transfer [as are many things]. I would claim that if you get enough surface lit, your problem is solved. Creating a brief pressurized period of time to emulate sea level or better conditions is one way to get there, but not the only way.

While I am not a particular advocate of copper thermite, it does illustrate the principle: Makes little gas, not pressure sensitive, and transfers a lot of heat. A big enough charge gets you there in 30ms or so. For a less nerve-wracking design, a large enough charge of BKNO3 with a short burn time also should get you there. Why try to do it with less when you can do it with more

Copper thermite does not transfer "heat" as defined by radiant energy. It blasts your propellant with molten, abrasive, hot liquid. The grain gets stressed, the surface gets eroded, and a portion of your propellant becomes the igniter. The propellant surface at the top grain is sacrificed to produce pressurization gas instead of contributing to the total impulse of the motor (albeit small). As many have found out the hard way, some propellants (due to formulation or processing) will crack and CATO. If the thermite is oversized, this may happen to even the best propellant.

A "proper" igniter provides pressurization gas and radiant heat without abusing the propellant. It's not complicated!

 

[TonyL]

Agree to disagree, I am only saying that there is more than one way, not who is "better".

I would note that the Space Shuttle SRMs are on an especially short timeline and they do it differently

br/

Tony

 

[FredA]

Don't the SRB's use NASA Standard Initiators? BKNO3 in a basket as I recall......
And they are lit at sea level.

 

[jsdemar]

Tony,
What I've written above are correct facts on how an igniter should work and how thermite does it differently. Which of those facts do you disagree with?

You seem to imply that the Shuttle SRBs used thermite. They did not. A NASA standard initiator lit a BPN basket which lit a propellant load as a flame thrower and gas producer. This was the size of a Q motor and shot down the length of core with about a 100 msec start-up time.

To find a military or commercial rocket motor reference to thermite as an igniter, you have to search quite far back. What you easily find is thermite being used as a means of "decommissioning" weapons.

Here's the correct way to use copper thermite in a "rocket"...

 

[TonyL]

Just to clarify: In this thread I did not say or suggest that one "should use thermite". I did not say or suggest that the Shuttle SRMs used thermite. I did not say or suggest that any commercial system used thermite.

I would argue that a mass of material residing at a temperature around 4000F [3000F if you prefer] can radiate heat effectively enough to initiate combustion of nearby surfaces [who said thermite?]. If there is additional heat transfer by conduction, so be it. I draw this conclusion from personal experience, not measured data, so sadly it is merely my opinion. The appropriate counter argument would be to show that the mass required to radiatively deliver sufficient joules/cm^2 to initiate combustion would require a prohibitively large mass or too long a duration to be practical.

I will suggest that I would be fast friends with anyone who takes the time to do the math.

Whether the impingement of liquids and or solids causes the effects that you state would be the most interesting as a documented result. I would love to see measured results that show these effects.

I will suggest that reading the words as written is best, and that any "implication" resides in the reader

br/

Tony

 

[boatgeek]

I'm sort of curious to go back to Steve's suggestion of a thermoplastic ring with a small exit diameter that fits in the contraction section of the nozzle. In theory at least, you could use a 38mm or 54mm grain as the igniter in a well similar to a smoke grain. Yes, you have to light the initiating grain, but that seems somewhat easier because the diameter is quite a bit smaller. If you, say, 3D printed the ring out of PLA, then you'd have the second or so of initiator grain burn time to melt all of that out, which seems pretty reasonable. The main motor coming up to pressure would also rip it out pretty quickly. That would let you pressurize a small volume in a relatively predictable way with a gradual release of pressure to reduce the chances of chuffing.

Obviously, it needs thought, calculation, and testing, but would it work in theory?

 

[Reinhard]

A NASA standard initiator lit a BPN basket which lit a propellant load as a flame thrower and gas producer. This was the size of a Q motor and shot down the length of core with about a 100 msec start-up time.

There is also the igniter initiator - what appears to be a roughly L1 sized rocket motor - between the BKNO3 booster and the igniter in the SRBs lovely 4 stage ignition scheme.

Reinhard

 

[OverTheTop]

I have successfully used a D motor to light an M2020.

 

[jsdemar]

Whether the impingement of liquids and or solids causes the effects that you state would be the most interesting as a documented result. I would love to see measured results that show these effects.

From my July 2021 TRATECH presentation:



Back in 2005, I also tested igniters on 75mm inert grains (replacing AP with TiO2 and SiO2). This was back when there was the "1 gram per 1000 N-sec" rule of thumb. The thermite sand blasted the surface of the grain and cracked an end of the grain.

After that, I modeled the heat flux of a "doobie" of thermite in Matlab. The following plot shows the results of the model for different lengths of motors based on core volume. Many people have spot-checked this graph with successful ignition without oversizing the thermite load.



Based on that, I would need 10+ grams of thermite to light my Q motor. I used 6 grams of BKNO3-V.

 

[TonyL]

Hi John,
Thanks for re-posting your data. I can say that back in the day I would use your thermite sizing chart and about triple it with satisfactory results.

While one could talk about interpretation, ultimately I [still] wasn't talking about thermite. BKNO3 has a similar reaction product temperature, and the fundamental question was whether one could just 'go big' with a conventional igniter, rather than have to develop a burst disk system.

Nothing against burst disks other than I am lazy and I don't like having to work that hard. Seems like one could take your 150 cal/cm^2/sec threshold [or maybe 500 cal/cm^2/sec] and just size it.

br/

Tony

 

[jsdemar]

Hi John,
Thanks for re-posting your data. I can say that back in the day I would use your thermite sizing chart and about triple it with satisfactory results.

While one could talk about interpretation, ultimately I [still] wasn't talking about thermite. BKNO3 has a similar reaction product temperature, and the fundamental question was whether one could just 'go big' with a conventional igniter, rather than have to develop a burst disk system.

Nothing against burst disks other than I am lazy and I don't like having to work that hard. Seems like one could take your 150 cal/cm^2/sec threshold [or maybe 500 cal/cm^2/sec] and just size it.

br/

Tony

Hi Tony,
In the professional literature (Sutton, NASA, and others), the recommended heat flux for an igniter is 100 cal/cm^2/sec. My graph was based on a 50% increase over the recommended output. The risk for most igniter pyrogens at 500% (as you suggest), is over-pressurization; for thermite at 500%, the risk is physical damage to the grain. Something to consider.

Ignition time for most SRM systems is more important to be repeatable and reliable than to be instant on. This is also true for upper stages. Simply designing (or guessing) for instant-on comes with added risks.

That's all I have. We should turn this thread back to Jim's flight musings.

 

[JimJarvis50]

That's all I have. We should turn this thread back to Jim's flight musings.

I'm pretty much out of musings. Have we adequately scoped out your homework assignment?

Jim

 

[Kip_Daugirdas]

I’ll muse about thermite lol.

I once poo-pooed thermite but decided to revisit it after some wasted test burns of EX motors last year using BKNO3 igniters. Think awful starts with a characteristic pop…nothing….and then the slow pressure build up. I’m clearly doing something wrong but I have not been able to sort it out and get good consistent startups. I plan to revisit BKNO3 in the future as it is considered the gold-standard.

In the meantime, thermite has been good to me, it is simple to make and size for a given motor once you understand it. Next year, I will try it on a sustainer at high altitude (ignition >30k ft). I haven’t had any of the characteristic problems that some see using thermite. I did have one pop-nothing-full pressure startup but that was fixed by dropping the aluminum size in the thermite mix and it never happened again.

My propellant doesn’t seem to mind thermite and start-ups are insanely fast to the point that they look professional. I need to remember to stop pressing the launch button at T-1 because the motor is at full pressure and the rocket is already in the air.

A good overview of thermite can be found on AIAA. Darren Wright and David Reese published a paper on it, it’s worth the $20: https://arc.aiaa.org/doi/abs/10.2514/1.B34771?journalCode=jpp

NACA/NASA did use thermite way back in the 50s. They called these igniters powder cans. Some information on powder cans can be gleaned on page 36: https://ntrs.nasa.gov/api/citations/19710020870/downloads/19710020870.pdf

Food for thought…

 

[Steve Shannon]

I’ll give my usual reminder that when using copper thermite, be very careful because copper thermite can explode and, in very small particle size, can be static or impact sensitive. Kip is right, you must know what you’re doing before messing with it.

Google “copper thermite explosion”

 

[Kip_Daugirdas]

I’ll give my usual reminder that when using copper thermite, be very careful because copper thermite can explode and, in very small particle size, can be static or impact sensitive. Kip is right, you must know what you’re doing before messing with it.

Google “copper thermite explosion”

Yeah that YouTube of the bottle rocket that John posted is definitely sub-micron (German Black or equivalent). It’s a detonation and that is definitely not what my igniters look or sound like. And I’d avoid it for safety reasons. The danger, like with everything, increases with the smaller mesh sizes. Start large and work your way down until you get the reaction/motor ignition that you like. Some good safety information in the AIAA paper. And safety will vary widely on particle size.

 

[jsdemar]

Yeah that YouTube of the bottle rocket that John posted is definitely sub-micron (German Black or equivalent). It’s a detonation and that is definitely not what my igniters look or sound like. And I’d avoid it for safety reasons. The danger, like with everything, increases with the smaller mesh sizes. Start large and work your way down until you get the reaction/motor ignition that you like. Some good safety information in the AIAA paper. And safety will vary widely on particle size.

David's paper is available for free on his old website. He wrote that as a student and may have different input now that he's a professor at USC. I shared with him the models and references I had gathered at the time.
The particle size issue has been the major safety risk with thermite. Inexperienced rocket flyers mix whatever they find to make a thermite igniter. One experienced guy needed his thumb reconstructed from a skin graph. Powdered pryogens without a binder bring a higher level of risk.
Without moving the discussion to the Research section, we shouldn't discuss specifics. Keep in mind that particle sizes are also critical in BKNO3 and all pyrogens, powdered or with binder. Thorough blending makes a significant difference, too.

 

[JimJarvis50]

Looks like I haven't posted to this thread for a while. It's where I typically document high-altitude two or three stage projects. There is a Balls project this year, spearheaded primarily by a guy who doesn't really want to be involved (but he is). The project will be a 6" barely Q to a 4.5" mid O. It will go somewhere between 0 and 325K feet.

Typically at this time of year, I post the electronics programming and request some QC. I really do need the QC, as I have mucked up the programming many times in the past. Please take a look!

This year, the sustainer electronics will include Kate and an EasyMega. The program screen for the EasyMega is attached for starters. Channel A would be for deployment of the main at 1800 feet. The question I have about this approach is whether I should instead use the "main" channel on the EasyMega. The concern I have about this is that I don't know exactly how the "apogee" channel is programmed (it's not specified), and I don't know if there would be any issues using it coming from high altitude. This might be a better approach, though.

Channels B and C would share a common ematch. Channel B is intended as a straight timer set for 150 seconds. Channel C is intended to be barometric apogee if the altitude is less than 90,000 feet (i.e., the sustainer motor doesn't light).

Again, comments on the plan would be appreciated!

Jim

 

[ksaves2]

I'm too stupid Jim to comment but I suspect you mastered augmenting low voltage ematches to ignite upper stage motors. I spent a summer making some but I never flew multistage. All my igniters worked with a AA battery when I ground tested though! They really flared up when the ematch popped! I never used them for single stage flights as all of the club panels might have lit them off with a continuity test. I never went multi-stage but it was fun working on the ignition profile. Nice thing about igniters of any kind is it just takes a very small amount of the "fixin's" to make 'em. That includes ematches too but we don't have to worry about that anymore. Kurt

 

[JimJarvis50]

I'm too stupid Jim to comment but I suspect you mastered augmenting low voltage ematches to ignite upper stage motors. I spent a summer making some but I never flew multistage. All my igniters worked with a AA battery when I ground tested though! They really flared up when the ematch popped! I never used them for single stage flights as all of the club panels might have lit them off with a continuity test. I never went multi-stage but it was fun working on the ignition profile. Nice thing about igniters of any kind is it just takes a very small amount of the "fixin's" to make 'em. That includes ematches too but we don't have to worry about that anymore. Kurt

I'm pretty good at dipping ematches. I like to use the PML magnalite for the dipping. I'm not sure if this will still be offered in the future with the LOC acquisition? Anyone know.

Anyway, our motor last year didn't light using a fair number of KBNO3 pellets. I strongly suspect that won't happen again this year!

Jim

 

[heada]

I'm pretty good at dipping ematches. I like to use the PML magnalite for the dipping. I'm not sure if this will still be offered in the future with the LOC acquisition? Anyone know.

Anyway, our motor last year didn't light using a fair number of KBNO3 pellets. I strongly suspect that won't happen again this year!

Jim

PML was a reseller for Rocketflite who makes Magnelite. If PML via LOC discontinues it you can go to Rocketflite directly.

https://www.rocketflite.com/

 

[JimJarvis50]

PML was a reseller for Rocketflite who makes Magnelite. If PML via LOC discontinues it you can go to Rocketflite directly.

https://www.rocketflite.com/

Thanks Heada. I didn't know where to look for this. I just noticed it wasn't on the LOC website (Oh, oh). I like that stuff.

Jim