Water putty rocket nozzles

[yoyo]

Hello,

 

For anyone who uses Rockite, Durham's water putty, or some other variant for rocket nozzles, how do you make them properly symmetrical and straight? I'm using a conical cake mold, and too often the nozzles come out with a very slight tilt which is immediately noticeable during launches.

 

Any suggestions would be appreciated.

[eoneuk]

My experience is limited to 1/4" bottle rockets. I put the appropriate amount of mixed putty directly in the tube using a 1/4" rod as a gage. When cured I just drill it out in the center. Just a straight hole, nothing fancy. Works well in this application.

 

Ed

[stix]

Yoyo, you probably need to make some sort of tooling or jig to hold everything in place.

 

You use a "cake mold"!! What size motors are you making?

 

Cheers.

[nater]

When I use Durhams to make rocket nozzles, I just dampen it slightly and press a nozzle using a standard spindle and rammer just like when I use clay.

[stix]

When I use Durhams to make rocket nozzles, I just dampen it slightly and press a nozzle using a standard spindle and rammer just like when I use clay.

 

Nater, I don't think yoyo has any spindle and rammer tooling - I imagine he is making the putty, pressing it into the end casing, then trying to form the nozzle using a conical mandrel (ie. de Laval nozzle).

 

Durhams sounds like an ideal product mainly because it doesn't shrink. I looked into this a few years back when I was experimenting with my 'castable' nozzles due to a lack of tooling. I came up with a workable solution.

 

Yoyo, surely there is some way for you to make a simple jig or formework using a drill, wood, glue, sandpaper, dowels etc??

 

Cheers.

Edited December 20, 2014 by stix

[uncrichie]

Go here: http://www.nakka-rocketry.net/pvcmot12.html go down to pvc motors. Look at the plans for the F-70 motor. They show a way to make simple tools to form the nozzles after casting the putty/cement. This should give you plenty of ideas. Kurt

 

Actually that address puts you right onto the page you need to be on.

Edited December 20, 2014 by uncrichie

[yoyo]

Thanks for the info uncrichie.

 

Stix: Some basic pictures would be helpful because I've never made even a simple jig or stand. My motors are 1 inch ID, six or seven inches long. Right now I'm putting the tube directly on a conical cake mold, then pouring (3:1 ratio) the rockite into the tube until it looks like the throat will be between 3/8 and 1/2 inch in diameter, then leave it there to harden.

[stix]

Thanks for the info uncrichie.

 

Stix: Some basic pictures would be helpful because I've never made even a simple jig or stand. My motors are 1 inch ID, six or seven inches long. Right now I'm putting the tube directly on a conical cake mold, then pouring (3:1 ratio) the rockite into the tube until it looks like the throat will be between 3/8 and 1/2 inch in diameter, then leave it there to harden.

 

That's a pretty difficult ask yoyo.

 

Part of experimenting and making your own nozzles & motors is also making your own tooling if standard tooling is not available - which most of the time it isn't. This obviously varies greatly because of the materials and tools we may have available to us.

 

The only time I've ever done 'castable nozzles' was for my 'alpha size' 3/4 inch ID motors. I'll take some pics of the jig and post them as soon as I can. I don't think this jig will suit your larger motors though, but it might give you some ideas.

 

One method you could try is securing the conical mold on a piece of wood, then drill 6 holes centred around it to suit your motor OD, insert 6 pieces of dowel around 3 inches long. That way, you can insert your tube (it should be a nice snug fit) and secure it with tape. If made well, the tube will be vertical and the nozzle will be centred. Fill with putty and allow to dry.

 

Investing in or having access to a bench drill press will certainly save you a lot of frustration.

 

I assume you are making r-candy/sugar rockets?

 

Did you have a good look at the link that uncrichie posted? http://www.nakka-roc...t/pvcmot12.html

 

Cheers.

 

[EDIT] Also, if this is your first time experimenting with motors, I would suggest starting smaller, ie, 1/2 or 3/4 inch.

Edited December 22, 2014 by stix

[yoyo]

Great idea stix! I'll give it a whirl. I am indeed making sugar rockets, and the link uncrichie posted was very helpful. I'll try to make the nozzles like Nakka does, once I lookup most of the terminology, since I have zero experience with woodworking.

 

I've been working on these motors for about a year now, and quickly settled on 1" sizes because until about a month ago they were all nozzle-less, which consequently requires more power.

[stix]

Great idea stix! I'll give it a whirl. I am indeed making sugar rockets, and the link uncrichie posted was very helpful. I'll try to make the nozzles like Nakka does, once I lookup most of the terminology, since I have zero experience with woodworking.

 

I've been working on these motors for about a year now, and quickly settled on 1" sizes because until about a month ago they were all nozzle-less, which consequently requires more power.

 

Good to see that you've read some of the Richard Nakka stuff, In my view, he's "The Master" when it comes to sugar rockets. There is a lot to take in, especially all the graphs but just take what you need and develop your own methods. Also try to take heed of all the safety info provided.

 

As requested, here are the images of my jig (3/4 inch):

 

Image[1] The jig has two parts. The top section has a hole cut through to suit the motor casing OD. It also has two locating pins/screws. The bottom part shows the matching locating pin holes as well as the nozzle forming mold.

 

Image[2] This shows a close-up of the exit (divergent) nozzle cone. The 'cone' was formed around an inserted (metal) centre pin, then shaped by hand using a 2 part epoxy putty which sets over about 5hrs which allowed me to carefully shape it using my fingers, glass paper and a popsicle stick (from memory it was vanilla, coated with dark chocolate and crunchy little biscuit crumb thingies on the outside - but please don't quote me on that ).

 

Image[3] The two parts are mated together using the locating screws (I didn't bother screwing them in fully for this demo).

 

Image[4] The motor casing is inserted through the hole to sit flush with the base. From memory, I used baking paper with a hole in it to stop the clay/puttly sticking to the base. You could use silicon spray or some other release agent that doesn't interfere with the putty.

 

Image[5] The final result - well almost, as this nozzle was made from a different set of tooling but the same process. I don't have any of the originals left as they were used in model rockets which unfortunately I couldn't retrieve. Perhaps they went into orbit?

 

Important Note: This jig was used for casting nozzles for sugar rockets - I probably wouldn't use this nozzle throat diam. for bp versions as I think it would be too small and result in over-pressurization.

 

---

 

A simpler method as eoneuk suggested, is you could easily just form a solid piece, then drill it out. You could even shape the divergent section with your cake cone, with some glass paper wrapped around it.

 

I only made this jig as an exercise in jig-making. To be honest, unless you are into high powered model rocketry, I don't think you would see any major performance increase having a divergent section (de Laval) to your nozzle design. If you actually are into hp, then you should take a step back and re-evaluate where you are at. You at least need some decent tools like a bench drill. Tools, tools, tools, tools, tools...

 

My experience on this forum mainly relates to sugar rockets, "H" class being the biggest I've made (load cell test) but never flown. My "H" class nozzle jig was made on a lathe and the nozzles pressed with a "shop press" using fireclay and wax. Hopefully you can use some of the information provided by myself and others to achieve your goals.

 

Always happy to help in any way I can.

 

Also, other great resources are:

http://jamesyawn.net/candyrocket/

http://www.thefintels.com/aer/rocketindex.htm

 

Stay Safe.

Cheers.

Edited December 21, 2014 by stix

[MrB]

I only made this jig as an exercise in jig-making. To be honest, unless you are into high powered model rocketry, I don't think you would see any major performance increase having a divergent section (de Laval) to your nozzle design.

I'm not going to pretend to know much, at all. But rather ask a question. Speaking of our pyro applications, Convergence section seams more important then divergence. It lets us pack "stuff" in the fuel that finds it's way out without getting stuck, or crushed inside. But does it also add "motion" to the boat, so to speak, has it thrust increasing properties? I think so, and then, which is more important, convergence, or divergence zones?

 

"Everyone" has a divergence zone, since it's easily machined to the bottom of the spindle. But not everyone has the rammer machined to make a nozzle on the inside, which made me wonder what really is more important, if one starts pretending to care.

(I only have a few tools for rockets, drivers and fountains, all of which has both. Why a fountain needs the conical shaped nozzle on the outlet... Hey, i dunno.)

B!

[yoyo]

That is quite a nice jig, definitely on my things to do list. Could it be said that I'm into static-testing high-powered motors? In reality, high-powered rocketry is way above what I need or want to do, since it's so much more complex the bigger things get. Kind of like how ball shell construction becomes a different beast at 5" or more.

 

I do need a bunch more epoxies, tools, and jigs before my rockets get to uniform construction and performance.

 

MrB: I have wondered what could be done with melted sugar rockets using a theoretical paper or pvc tube with a taper on one end, so that one side was 1" ID, for instance, and the other was 1/2" ID. In fact I plan to experiment with small plug nozzles just to see if a performance difference could be detected. This (http://imgur.com/X5ecQtz) for instance uses rockite, with four holes drilled at an angle to converge at the bottom of the tube. It's untested as yet.

[schroedinger]

B! The relly intresting part is the covergent part (tapered rammer).

Some ti e ago there was a really god analyses about these here, but i can't find it at the moment, but in the resukt was, the covergent part can actually increase thrust a divergent is more likely to reduce it or make no difference.

The reason for this is, that the pressures in bp rockets are jus too small.

 

But there is a 2nd reason to use a tapered rammer, (specially with endburners). The cavity directs the flame towards the nozzle hole away from the tube. If you use a flat rammer the flame isn't directed away and always hits the wall and this can result in burn through. If this happens a end burner will lose all his thrust way to early. A core burner is of course less likely to have this problem as it burns up the thrust fuel within a second but still the delay is able to burn through the wall.

 

About the divergent part, it is also interesting to have, since it is an easy way to increase the touching surface between the nozzle and tube without creating a too long straight channel inside the nozzle.

This point is specially interesting if you are going for the highest power possible, as it reduces the risk of a blown out nozzle

 

In the end its again nice to have and the other one won't hurt to much.

[dagabu]

Yoyo,

 

Lots of good stuff here to read and learn from. Allow me to explain further some of the ins & outs of the simple black powder and sugar rocketry we often use here. Water putty is expensive when comparing it to regular clay, it can give you good results BUT if your tube walls are slick, the tube is really dry or you pound your comp above the nozzle, it can fail with rather spectacular results.

 

If you are in the USA, you have many sources for tooling at a minimal cost, Caleb and I both have metal lathes and can turn tooling for you. When making nozzles from water putty, I recommend you have a base, spindle and convergent rammer.

 

Convergent rammer ends of 45% give a smooth linear motion to the gasses flow, below is a picture of one of my nozzles, I made these from clay with graphite, the convergence is 45° and the divergence is 22.5°. In my early tests with the ACME test stand, the gain with BP rockets with the large divergence was a reduction of 4% (+- 2%) then with a flat divergence. The convergence did make a huge difference but NOT in a performance gain (BP is just too slow) but in tube life and burn through as well as lack of sputtering and inconsistent burns.

 

http://pyrobin.com/files/new%20nozzle%20mix_1.jpg

[stix]

I agree that the convergent section is absolutely very important to the performance of the nozzle in regard to the flow of hot gasses. However I'm not convinced that it's more important than the divergent section (in sugar rockets).

 

Black Powder and Sugar Rockets have a significant difference in specific impulse. ie, bp 80, KNSU 125-130 - this could account for variations in results whether a divergent section has a benefit or not. I haven't done any comparison tests myself, so I'm just taking it from what I believe I've read elsewhere. Sorry that I can't be specific.

 

My tooling as posted before (#10, Image 2) deliberately didn't show the "convergent rammer" for the simple reason that I didn't want to overwhelm yoyo with a piece of tooling that's even harder to make, given his apparent lack of tools.

 

Here it is:

On the left is a piece of dowel with a hole drilled in it with the convergent section made from epoxy putty using my standard popsicle stick method (this was used for "hand pressing" my experimental nozzle formula) - the two other pieces were lathed when I was fortunate enough to have access to one, used for ramming clay/wax nozzles.

 

This is a static firing test of one of my experimental nozzles:

There's no way that you would get a nice looking symmetrical flame like that with no divergent section - whether that equates to better performance, I don't know. But it does look nice - yum!

 

Here are the specs and data of that motor. 9 kilos (20 pounds) of max. thrust is not bad for a 3/4inch ID tube and 22grams of fuel.

 

This is one of Richard Nakka's beautiful lathed nozzles:

The divergent section is significantly longer than what you would want to ram inside a tube, and the extra weight, ie. to fill up the negative areas with clay, would make it prohibitive - or very inefficient at the least.

 

Black Powder Rockets and Sugar Rocket are not the same, they have their uses. Sugar Rockets are more of a day-time activity and more to do with raw power. BP rockets are a night-time "pretty" display. Sometimes I think that sugar rockets are viewed by some as just a 'play toy' that 'kewl dudes' muck around with.

 

Anyway, I'm on this forum to get away from sugar rockets, but I'm always happy to help others in that area if I can - I'm looking forward to making a simple small (3/4 inch / 1 pound) bp rocket that takes of gracefully into the night sky with a nice flowing tail, a simple 2" shell with a salute.

 

Surely that's not asking for too much?

 

Cheers.

[dagabu]

Stix,

 

Yes, you can easily lift a 2" shell with a 3.4" ID motor, even an end burner should lift if as long as your BP is hot enough. Using a core burner, you should be able to lift a LOT more, we have seen independent testing with special pyrotechnic propellants hit just shy of 100# of pressure on the ACME test stand. Theoretically should lift an 8" shell to height.

 

Back to KNSU motors: A de Laval styled nozzle does indeed make for a pretty flame but the lack of shock diamonds is a clear indicator that there is no significant increase in thrust since the entirety of the burn remains subsonic. The problem with both BP and KNSU (and other R-Candy motors) is that the gasses just don't have the velocity to take advantage of the nozzle configuration over time and even though the flow MAY hit a velocity that can take some advantage of the divergence, what happens most of the time is detrimental to the actual thrust and can actually choke the thrust.

 

The VERY basic drawing below shows what happens with various gas flow rates. The bottom line is that the bell shape and overall geometry has to be specifically tailored to the fuel/propellant, our attempts to do so are futile and the extra weight of clay or water putty actually hampers the overall performance of paper cased rockets (proven time after time in actual flights).

 

http://upload.wikimedia.org/wikipedia/commons/thumb/4/47/Rocket_nozzle_expansion.svg/200px-Rocket_nozzle_expansion.svg.png

 

If the pressure of the exhaust jet varies from atmospheric pressure, nozzles can be said to be (top to bottom):

Grossly overexpanded

Overexpanded

Ambient

Underexpanded

If under or overexpanded then loss of efficiency occurs. Grossly overexpanded nozzles lose less efficiency, but can cause mechanical problems with the nozzle. However, slightly overexpanded nozzles will produce more thrust than critically expanded nozzles if boundary layer separation does not occur. Rockets become progressively more underexpanded as they gain altitude. Note that almost all rocket engines will be momentarily grossly overexpanded during startup in an atmosphere.

Dexter K Huzel and David H. Huang (1971), NASA SP-125, Design of Liquid Propellant Rocket Engines Second edition of a technical report obtained from the website of the National Aeronautics and Space Administration (NASA).

Edited December 22, 2014 by dagabu

[MrB]

Some ti e ago there was a really god analyses about these here, but i can't find it at the moment, but in the resukt was, the covergent part can actually increase thrust a divergent is more likely to reduce it or make no difference.

The reason for this is, that the pressures in bp rockets are jus too small.

I sort of remember some of that, now that you mention it. It all passed right over my head, and, not being that much of a rocketman, i prefer mortars, i guess i didn't pay enough attention.

 

 

Surely that's not asking for too much?

Slapping a stick to a 3/4" engine, and lifting a 2" shell. U mad bro? No, this can't be allowed. You have to make some flash jokes first, threatening to blow your self, or the neighbors dogs to bits, and then we can get to the mundane rocketry.

 

 

 

If under or overexpanded then loss of efficiency occurs. Grossly overexpanded nozzles lose less efficiency, but can cause mechanical problems with the nozzle. However, slightly overexpanded nozzles will produce more thrust than critically expanded nozzles if boundary layer separation does not occur. Rockets become progressively more underexpanded as they gain altitude. Note that almost all rocket engines will be momentarily grossly overexpanded during startup in an atmosphere.

This relationship to altitude, is it due to atmospheric pressure? Does the area of the burn play in at all? I have a feeling the answer is yes, but i'd rather ask and look stupid, then not to ask, and remain stupid.Larger area of the burn means higher rate of gas production, well until it cant burn due to pressure. Not sure if we reach that point in our BP / sugar rocket engines. Do we?

B!

[schroedinger]

No we won't reach that point, the two main reasons for that are that we deliver the fuel and oxigen ourself, which means we don't depend on atmoshpheric oxigen and the pressure actually increases the reaction speed.

Due to these facts we rather blow up our casing then stop the reaction

[dagabu]

I sort of remember some of that, now that you mention it. It all passed right over my head, and, not being that much of a rocketman, i prefer mortars, i guess i didn't pay enough attention.

 

 

Slapping a stick to a 3/4" engine, and lifting a 2" shell. U mad bro? No, this can't be allowed. You have to make some flash jokes first, threatening to blow your self, or the neighbors dogs to bits, and then we can get to the mundane rocketry.

 

 

This relationship to altitude, is it due to atmospheric pressure? Does the area of the burn play in at all? I have a feeling the answer is yes, but i'd rather ask and look stupid, then not to ask, and remain stupid.Larger area of the burn means higher rate of gas production, well until it cant burn due to pressure. Not sure if we reach that point in our BP / sugar rocket engines. Do we?

B!

 

Schroedinger says it better than I but yes, there is a direct correlation between the area and the optimum thrust. With liquids, the thrust remains the same, with solids, you can change the shape of the bell in flight to make up for losses.

 

No we won't reach that point, the two main reasons for that are that we deliver the fuel and oxigen ourself, which means we don't depend on atmoshpheric oxigen and the pressure actually increases the reaction speed.

Due to these facts we rather blow up our casing then stop the reaction

 

Keep in mind that liquid fuels also deliver oxygen to the reaction as well and do not depend on atmospheric oxygen.

[FlaMtnBkr]

I was in Sears the other day looking at tools and saw some countersink bits in different outside diameters. Some also have a short section of drill bit so you can drill a countersink and thru hole in one shot.

 

Maybe this could be used somehow to drill a nice centered hole? The ones I saw had a 1/2" and 3/4" outside diameter. Slide the whole thing into the tube to center and then make your hole. You would probably need to do it while damp because that is some pretty tough stuff once set and could maybe slowly twist with your fingers to make the hole. A short section of tube can be aligned to the end of the rocket so the nozzle will be close to end of motor and use the removable section to center the bit.

 

Just a thought. I started with 3/8" ID tubes and tooling was pretty easy to make with wooden dowels and a nail or metal rod. At that small size the spindle didn't even need to be tapered and blew away commercial firework rockets. A drill press did make things a lot easier to get centered.

[stix]

 

Stix,

 

Yes, you can easily lift a 2" shell with a 3.4" ID motor, even an end burner should lift if as long as your BP is hot enough. Using a core burner, you should be able to lift a LOT more, we have seen independent testing with special pyrotechnic propellants hit just shy of 100# of pressure on the ACME test stand. Theoretically should lift an 8" shell to height.

 

Back to KNSU motors: A de Laval styled nozzle does indeed make for a pretty flame but the lack of shock diamonds is a clear indicator that there is no significant increase in thrust since the entirety of the burn remains subsonic. The problem with both BP and KNSU (and other R-Candy motors) is that the gasses just don't have the velocity to take advantage of the nozzle configuration over time and even though the flow MAY hit a velocity that can take some advantage of the divergence, what happens most of the time is detrimental to the actual thrust and can actually choke the thrust.

 

The VERY basic drawing below shows what happens with various gas flow rates. The bottom line is that the bell shape and overall geometry has to be specifically tailored to the fuel/propellant, our attempts to do so are futile and the extra weight of clay or water putty actually hampers the overall performance of paper cased rockets (proven time after time in actual flights).

 

http://upload.wikimedia.org/wikipedia/commons/thumb/4/47/Rocket_nozzle_expansion.svg/200px-Rocket_nozzle_expansion.svg.png

 

If the pressure of the exhaust jet varies from atmospheric pressure, nozzles can be said to be (top to bottom):

Grossly overexpanded

Overexpanded

Ambient

Underexpanded

If under or overexpanded then loss of efficiency occurs. Grossly overexpanded nozzles lose less efficiency, but can cause mechanical problems with the nozzle. However, slightly overexpanded nozzles will produce more thrust than critically expanded nozzles if boundary layer separation does not occur. Rockets become progressively more underexpanded as they gain altitude. Note that almost all rocket engines will be momentarily grossly overexpanded during startup in an atmosphere.

Dexter K Huzel and David H. Huang (1971), NASA SP-125, Design of Liquid Propellant Rocket Engines Second edition of a technical report obtained from the website of the National Aeronautics and Space Administration (NASA).

 

 

Thanks for the detailed explanation dagabu.

 

Is there a consensus that the divergent section is a least "useful" in directing the flow of gasses than just a blunt hole? I think it is.

 

 

. . . Slapping a stick to a 3/4" engine, and lifting a 2" shell. U mad bro? No, this can't be allowed. You have to make some flash jokes first, threatening to blow your self, or the neighbors dogs to bits, and then we can get to the mundane rocketry. . .

 

No probs B, what was I thinking?. I actually don't mind dogs but there are a couple of noisy cats (which I find are a complete waste of space) a few doors up that have really been pissing me off lately, as this could easily alert the authorities.

 

As a matter of fact, just the other night I was on my front patio with my 7yo son, doing one of my half-drunk "K motor short fuse" experiments. It was raining lightly at the time and the patio can become very slippery and quite dangerous.

 

As usual, my 10yo daughter was making flapjacks and flash powder in the kitchen. Fortunately the kitchen window is right next to the patio, so I can always give her a reassuring glance - although she does always look a bit worried, which I assume has something to do with those bloody cats!

 

Anyway, to cut a long story short, just as he was about to light the fuse, two of those freakin' cats started up - what a horrible screeching noise they made! This startled him and he slipped backward and banged his elbow on the patio. Bloody hell, he could have easily burnt his fingers on the match, or worse - fractured a bone or something. Very very annoying indeed!!

 

I still remember the incident vividly and three things immediately sprung to mind.

1. Get some anti-slip matting for the patio
2. Hang thicker curtains over the kitchen window (sometimes my daughters concerned looks can be very off-putting and would also help keep the flapjacks warm)
3. Do something about those bloody cats once and for all.

I'll certainly be putting a big effort into reading up on ballisitc trajectories etc... tee hee hee, those cats won't stand a chance!

 

Hopefully as a result, this will clearly prove my qualifications for discussing and handling flash.

 

Gotta go now, the flapjacks are ready.

 

Cheers.

Edited December 23, 2014 by stix

[dagabu]

I was in Sears the other day looking at tools and saw some countersink bits in different outside diameters. Some also have a short section of drill bit so you can drill a countersink and thru hole in one shot.

 

Maybe this could be used somehow to drill a nice centered hole? The ones I saw had a 1/2" and 3/4" outside diameter. Slide the whole thing into the tube to center and then make your hole. You would probably need to do it while damp because that is some pretty tough stuff once set and could maybe slowly twist with your fingers to make the hole. A short section of tube can be aligned to the end of the rocket so the nozzle will be close to end of motor and use the removable section to center the bit.

 

Just a thought. I started with 3/8" ID tubes and tooling was pretty easy to make with wooden dowels and a nail or metal rod. At that small size the spindle didn't even need to be tapered and blew away commercial firework rockets. A drill press did make things a lot easier to get centered.

 

I guess I don't understand the whole topic, why buy a drill bit from a store when the tooling made to form a nozzle is cheaper and overall better? One has to do side by side nozzle testing to see first hand how the extra work, time and money are wasted with water putty.

[yoyo]

I approached rocketry from the angle of melted sugar rockets, rather than black powder. The intelligent decision would have been to get BP tooling and go with 1/2" or 3/4" rockets instead, but by now I'm invested enough in sugar rockets to not want to switch quite yet.

 

Water putty isn't that expensive. A pound of rockite is 8 dollars, so quite affordable.

 

Edit: Also, black powder is very dusty and a bit more dangerous to work with than melted KNSU, which will stay within safety limits by simply underheating or over-caramelizing it (accepting a reduction in performance).

Edited December 23, 2014 by yoyo

[dagabu]

yoyo,

 

I made you a rammer (nozzle former) from Polyethylene, it has a 30° convergence, I will make you the matching base with divergence, what angle do you want the divergence and what throat diameter do you want? If you are looking for a de Laval, 15° would work well, just look at my clay nozzle above, that is what it would look like.

 

Dave

[yoyo]

Wow, thanks Dave! I'll go with your recommendation for 15° divergence. What diameter is the rammer? Could you please pick a throat diameter based on your knowledge? Otherwise I would say 3/8" throat diameter.