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Work has kept me busy, and after work, doing maintenance on my microlathe. New variable speed motor, cleaning, and some mods. OT though!

I looked at some pictures that James Fields took at Potter. One of the pictures shows something very interesting. In the vent bay, I have the main exit hole for the cryovalve vent. That is jetting out a stream of vapor at a pretty good clip. Really that valve should have a flow restrictor on it, but oh well, something for the next design motor.

What is interesting is the vapor stream from a couple inches lower down. That's the drain hole for the ventbay. Little should be coming out there, but clearly that's not the case. That is leading me to think the leak I had is not the cryovalve misbehaving, but instead that I might have not had the cryovalve screwed into the upper bulkhead solidly enough - leading to leakage at the base of the valve - or the upper bulkhead O-rings were failing. With the age of the latter, and all the handling the cryovalve got while I was building the rocket, either is quite possible.

I think the venting was sufficient that when I pull it all apart, there will be evidence of where the gas was coming from.

As an aside, seeing venting in a chilled hybrid is super easy. There's no need for a sensor with this sort of system! You can also easily hear the vent cycle from a thousand feet or more away. The vent generates enough thrust to push the rocket sideways a couple inches on the rail. It could use a flow restrictor to reduce the vent rate when the valve is open.

However, watching it push the rocket sideways (a few pounds of thrust) makes me think a cold flow RCS could work for our scale rockets, at a weight penalty of course.

I've attached two pics from the flight, when the motor was up to full burn. That's an 8' plume... which is a bit bigger than it should have been. And a bit bigger than it was in the static test. I think I know why the motor went unstable halfway through the boost. One of the injectors burned out or otherwise lost the internal tube. That opened that injector up larger than design diameter.

The larger total injector area that resulted decreased the pressure drop between the flight tank and the combustion chamber. I didn't design with much margin there. If one has too little pressure drop, combustion will be unstable.

On my static test it was very stable. On this boost it was very stable, until suddenly it was not. At that same time, the acceleration suddenly jumped up roughly an additional 2G. That would be fully explained by the tube core letting go in one injector giving a sudden increase in oxidizer flow. The combustion chamber pressure would have gone up a little, and the pressure drop across the injector would have decreased a little.

The plume size in flight would have been slightly larger than from a static test anyway, but not a couple feet larger! At least not at that point in the boost. The increase would have been from acceleration-induced pressure increase on the supply side of the injectors, and aero effects at flight speeds.

Assuming a half full tank that would have been about four pounds of nitrous, under 12G acceleration, so roughly 50 pounds over a surface area of about 6 square inches. So it only adds about 8PSI to the effective tank pressure. Or, about 1.5%. As in, not much.
 

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Here's the normal expected flame size from this motor, about 6' long, perhaps a half second earlier in the flight.
 

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I've finally started back at cleaning up the rocket. Generally one shouldn't leave things like this, but a hybrid motor has less issues that way. No AP; no corrosive residue! Bonus points that it doesn't stink.

It also (in my experience) doesn't stick the same way an AP liner will stick after the burn. Admittedly I have to use masking tape layers to get the liner to fit the casing properly, and I grease the masking tape to prevent adhesive ooze causing a real mess. It works well as you can see.

I definitely recommend the Krytox coating on the outside of the grain. With APCP, gasses that get around the backside are quite hot, but generally reducing. With a hybrid, they may not be as hot, but they are very oxidizing. They can potentially burn the casing since the casing looks like fuel to hot oxidizer! That's something to be avoided.

Additional bonus points for not slagging the nozzle. The fuel here is a mix of wax and HTPB with some additives. The wax forms a cooling film on the nozzle. One simply wipes the expansion section off with a paper towel and it looks better than new.

The combustion temperature is of course well above the melting point of the fuel, so the grain glues itself to the nozzle forming a pretty effective bottom end seal. It also takes a little force to break the nozzle free of the spent grain after the burn.

I just used a razor blade to shave the excess off the flat top of the nozzle. No point in removing the wax from the convergent section. It'll go pretty quickly on the next light, and provide a lot of initial evaporative cooling.

The top end did get a little gas this time, but the char residue wiped off with a paper towel. I have a semi-O-ring molded into the fuel to help seal the top end, but it doesn't work perfectly and doesn't work towards the end of the burn since it has burned away by then.

Gerald
 

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I plan to cut the spent grain lengthwise and post some pictures.

This burn, like the previous, did not start up immediately but had cold flow for a couple seconds. That dumped a fair bit of nitrous costing a lot of altitude. That caused it to burn less of the fuel than intended. Best I can figure, the regression rate is about right for a full burn to use up nearly all the fuel.

I plan to fly at URRF again next year. Hopefully, solving the things which did not go completely correctly in the last flight! I'd like to have a fully nominal flight out of this rocket.

I have ordered new cryogenic pressure relief valves to replace the one on the flight tank. One of them is the same as used in the previous two burns, and one is calibrated at 50psi higher pressure - in case I tweak the design for a higher tank pressure and temperature.

I haven't decided whether to try another approach for startup with the existing injector assembly, or replace that assembly with something else that might be intrinsically easier to start. In this motor it's a replaceable module.

Gerald
 
To-do items before the next flight, based on lessons learned:

Replace the oldest altimeter with a newer one. Not sure if I'll go with the same thing or something different.

Replace the screw switches. IMHO the ones I used are not good. I had a lot of trouble with them. It's even slightly possible a bogus electrical connection under flight shock and vibe might have triggered main release at apogee. With a tether release system the tether can fire early and nothing visible happens outside the rocket. But when the drogue releases the main comes out with it.

I couldn't get the switches I wanted. If I can't find some good ones I'll make something a lot better. I might just do that anyway since it is pretty easy to do. I'd use silver strips, a real screw, and a nylon lock nut. It would have better electrical contact and not be subject to shock and vibe issues.

Anybody selling REAL 1/4" wide tubular Kevlar? Most of what is being sold as 1/4" isn't. It is larger. What I have is the larger, and it is a tight fit. I'd like the real 1/4" since that would let me run more line. More line is an easier to reach rocket if it gets stuck in a tree.

This time around I'm not going to freebag the main. I got lucky last time. The issue is having the tracker in the nosecone, and everything of importance in the rest of the rocket! A freebag separates them. This summer's flight was the first time ever I had main at apogee. The bag line gently snagged on the main line so they stayed together. If that hadn't happened, I might not have the rocket today. So I'll run some heavy thread between the two recovery units to keep them from drifting apart. The descent rates are essentially identical so there is no load to be concerned about. The thread in this case need only be a few feet long. The drogue line itself will make up most of the link.

Perhaps I'll even consider a little paint. Or not. It certainly isn't on my priority list. But it would make the rocket more visible.

I'll be replacing all the O-rings in the flight tank. It was first assembled some years back, and I have suspicions the upper O-rings might have been leaking a little this last time out. In any event they aren't rated for long term nitrous exposure, though are quite good for short term exposure such as they get on a flight or a test burn. Anyway, time to inspect and replace. I'll probably have to buy more O-rings though. I think I'm out or nearly out.

I'm open to suggestions of others as well! I might not follow advice, but I'm happy to hear it! Or if there are any questions, ask away! I'd like to get more interest in hybrids going. They are different than solids, and pretty cool. I do EX with both.

Gerald
 
To-do items before the next flight, based on lessons learned:

Replace the oldest altimeter with a newer one. Not sure if I'll go with the same thing or something different.

Replace the screw switches. IMHO the ones I used are not good. I had a lot of trouble with them. It's even slightly possible a bogus electrical connection under flight shock and vibe might have triggered main release at apogee. With a tether release system the tether can fire early and nothing visible happens outside the rocket. But when the drogue releases the main comes out with it.

I couldn't get the switches I wanted. If I can't find some good ones I'll make something a lot better. I might just do that anyway since it is pretty easy to do. I'd use silver strips, a real screw, and a nylon lock nut. It would have better electrical contact and not be subject to shock and vibe issues.

Anybody selling REAL 1/4" wide tubular Kevlar? Most of what is being sold as 1/4" isn't. It is larger. What I have is the larger, and it is a tight fit. I'd like the real 1/4" since that would let me run more line. More line is an easier to reach rocket if it gets stuck in a tree.

This time around I'm not going to freebag the main. I got lucky last time. The issue is having the tracker in the nosecone, and everything of importance in the rest of the rocket! A freebag separates them. This summer's flight was the first time ever I had main at apogee. The bag line gently snagged on the main line so they stayed together. If that hadn't happened, I might not have the rocket today. So I'll run some heavy thread between the two recovery units to keep them from drifting apart. The descent rates are essentially identical so there is no load to be concerned about. The thread in this case need only be a few feet long. The drogue line itself will make up most of the link.

Perhaps I'll even consider a little paint. Or not. It certainly isn't on my priority list. But it would make the rocket more visible.

I'll be replacing all the O-rings in the flight tank. It was first assembled some years back, and I have suspicions the upper O-rings might have been leaking a little this last time out. In any event they aren't rated for long term nitrous exposure, though are quite good for short term exposure such as they get on a flight or a test burn. Anyway, time to inspect and replace. I'll probably have to buy more O-rings though. I think I'm out or nearly out.

I'm open to suggestions of others as well! I might not follow advice, but I'm happy to hear it! Or if there are any questions, ask away! I'd like to get more interest in hybrids going. They are different than solids, and pretty cool. I do EX with both.

Gerald
nickle strip is readily available for making battery packs and less prone to oxidation.
 
Ok, good thought. I have fine silver on hand and it isn't too prone to oxidation either. That's what I figured to use, but nickel is fine. I might order some strips, if I can find as thick as I want.
 
I split the fuel grain to show the burning surface. I tried to cut it as smoothly as possible, but it wasn't easy with the medium phenolic liner. The phenolic edges are ragged.

These pictures show the high turbulence surface and the internal structure of the grain.

The injector assembly is to the left; the nozzle to the right. Ruler for scale.

The light colored balls you can see in the walls are paraffin beads. The rest of the material is polyurethane with metal and nitrous decomposition catalyst. The paraffin boils out faster than the polyurethane, leaving a cratered surface. During the burn it would actually be a little rougher than in the pics. The surface layer will be above the melting point of the polyurethane at the end of the burn so it smooths out a little due to surface tension post-burn. But not much.

The pits in the grain wall are due to pullout from cutting, and some are probably casting bubbles. This goop is pourable, slowly. It's a sticky mess. I didn't bother vacuum processing it like I would have for a solid propellant. I may do so next time around, or not. It isn't the same sort of problem with a hybrid grain that it is with a solid propellant grain. Having longer to pour is probably worth more to me than having a slightly higher density. If I change curative at some point to something that is a little slower, that could change.

I'll measure the remaining fuel thickness and compute a regression rate. The burn was not complete due to cold start. Had it been a full burn, very little fuel would have been left. My eyeball guesstimator says the O:F ratio was right around where I designed it to be. The fuel does not appear to need any significant change and may not need any change at all.

Gerald
 

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I could only give you theoretical numbers which might not be worth much. Every burn of this motor has had slow ignition. That results in dumping lots of oxidizer out without generating useful thrust (ie, not enough to get it up the rail). Because of that I do not have anything other than guesstimates for how much oxidizer is actually getting burned. Mass flow rate is higher with cold flow since the chamber is way under pressure under those conditions. So two seconds of cold flow is probably worth three or four seconds of hot, if not more.

But net results is I don't know how much oxidizer burned. So I can't give a useful number for ISP. There would be too much WAG in there. Sorry!

In any event the ISP won't be super impressive no matter what. One cost of densifying nitrous by chilling is the chamber pressure having to be lower than other hybrids. That costs ISP. The tradeoff is more N2O mass fits in the same volume tank. So the total delivered ISP is greater for that size motor. It's a tradeoff. Density ISP should be better.

Gerald
 
Just thought I'd mention that I plan to fly this rocket again at URRF in June. I think I know what to do about each of the issues that showed up last time. I'll continue this thread and try to post details and pictures, as before.

I have a new cylinder on order, and will see about getting it filled once it arrives. I have a new solenoid valve setup, and am trying to get a wireless launch controller. So the whole GSE will be new.

I have some new switches arriving this week. I'm never again using the particular screw switches I used last time. Best I can tell, one of them unscrewed and turned off its altimeter. At least, only one altimeter was still running when the rocket was on the ground. Not good. That switch might be the root cause of the main deploying right after the drogue. It was possibly making jittery contact at apogee.

So, this is the new switch candidate: https://palmbeachbots.com/products/fingertech-mini-power-switch I'll post pics when they get here, and give you all my initial assessment.

Gerald
 
I have been using that switch (and many others here have too) for many years now. It won't unscrew during flight.
 
I picked up some new batteries (look like old stock actually) for the RunCam 4K. Charged them up, then went to charge up the old battery... The connecting terminals of the old battery are looser than the new batteries. The charger wouldn't charge it unless I pushed on it. Hmmm... Not cool. Don't think that battery is going to fly!

Picture of the old switches, with the replacements to show size. I'll make a new switch board, or more likely reuse the existing one. But not tonight.

Gerald
 

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It's no longer green. No final sanding; no clear coat. I'm not interested in making it a showpiece. Just less ugly and a little easier to see. Rattle-can Rustolium Pro light grey primer and white paint. It needs to be sprayed heavier than I did for it to flow out well. I'd not used this paint before. It is ok just a little different. It is what it is. I did do a little fine scuff sanding and degreasing before painting. That way the paint should at least stick!

The motor will stay it's natural aluminum color, and I doubt I'll get around to painting the fins. Not a high enough priority. This is a research rocket, not a showpiece. So it will be white and silver and a bit boring.

Gerald
 

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Time to get serious about prepping this rocket for flight at Potter, NY, URRF. Lots of little things to do to get it ready. I have a cylinder and a dolly for it, have the valve assembly, and some hope that I'll get the wireless GSE in time.

The tank part of the motor has been assembled for a few years now, through two firings. I figure it is time to disassemble and inspect. I plan to replace all the O-rings. They actually don't get heat exposure; they get chilled. But nitrous is a pretty good organic solvent. I used krytox grease for assembly (required) but even so the O-rings on the liquid side might have suffered some swelling or other degradation. That's what I want to inspect for. I have both silicone and viton rings on hand for replacement.

I've removed the bolt-pins and the alignment pin, but at the moment the coupler and injector assembly seems pretty solidly frozen. This might end up being a two person job to get it apart.

I also had trouble with the four little bolts that are retainers for the preheater grain. These are the round head screws on the end of the coupler. Two came off, and two have not. Four is overkill; one might be sufficient and two certainly are sufficient. I'll tackle the two reluctant screws once the coupler is out. It will be easier that way.

Gerald
 

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I managed to get the coupler and injector assembly out of the tank. I ended up having to clamp the coupler in a big vice. Once I got enough torque on the tank tube it broke free and came off without issue.

It's interesting that the inside of the tank is coated with a fine white film... It is more visible in person than in the picture. I'm thinking that is wax that was vaporized in the combustion chamber and condensed on the inside of the cylinder which would be quite cold at that point when the combustion has completed. The cylinder was very thoroughly cleaned before assembly originally but has now had two burns.

This is actually a dangerous situation. Nitrous can probably dissolve the wax since it is a good solvent. The nitrous probably contributed heavily to getting the wax in there in the first place, since the remaining vapor at atmospheric pressure would be N2O and it would cycle in and out of the combustion chamber a few times when combustion is complete, simply due to the large temperature difference causing some low amplitude pressure oscillations. There would have been remaining vaporized wax in the combustion chamber due to that end being hot.

The result would be sensitized nitrous; not a good thing. I'll have to dissolve out the wax and thoroughly clean the tank before reassembly. It's a good thing I took it apart for inspection. The next burn might not have ended well.

Chalk it up as something to be aware of when using wax as part or all of the fuel grain. Strip it down and clean everything each burn.

I think the residue is probably wax, but it could be Krytox grease or a combination of the two. That's if Nitrous can dissolve Krytox which I didn't think was the case? Seems like there is a fair bit more residue than the available Krytox could have provided.

Second picture is the inside of the coupler looking down at the back side of the injectors. It actually looks cleaner than the inside of the cylinder.

Gerald
 

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I managed to get the coupler and injector assembly out of the tank. I ended up having to clamp the coupler in a big vice. Once I got enough torque on the tank tube it broke free and came off without issue.

It's interesting that the inside of the tank is coated with a fine white film... It is more visible in person than in the picture. I'm thinking that is wax that was vaporized in the combustion chamber and condensed on the inside of the cylinder which would be quite cold at that point when the combustion has completed. The cylinder was very thoroughly cleaned before assembly originally but has now had two burns.

This is actually a dangerous situation. Nitrous can probably dissolve the wax since it is a good solvent. The nitrous probably contributed heavily to getting the wax in there in the first place, since the remaining vapor at atmospheric pressure would be N2O and it would cycle in and out of the combustion chamber a few times when combustion is complete, simply due to the large temperature difference causing some low amplitude pressure oscillations. There would have been remaining vaporized wax in the combustion chamber due to that end being hot.

The result would be sensitized nitrous; not a good thing. I'll have to dissolve out the wax and thoroughly clean the tank before reassembly. It's a good thing I took it apart for inspection. The next burn might not have ended well.

Chalk it up as something to be aware of when using wax as part or all of the fuel grain. Strip it down and clean everything each burn.

I think the residue is probably wax, but it could be Krytox grease or a combination of the two. That's if Nitrous can dissolve Krytox which I didn't think was the case? Seems like there is a fair bit more residue than the available Krytox could have provided.

Second picture is the inside of the coupler looking down at the back side of the injectors. It actually looks cleaner than the inside of the cylinder.

Gerald
Are you venting the tank to atmosphere or into the combustion chamber? If to atmosphere you could put a non return valve on the feed to the injectors.
Norm
 
Is it possible the white coating is from an inert filler/additive in the Nitrous? Kind of like compressor lube in your HVAC system?
Was it coated thicker on the nozzle end of the tank, or uniform throughout?
 
Norm,

It is vented to the atmosphere through a cryovalve that pops open at a set pressure. One of the injectors has to be bi-directional for flow since it is also the fill point.

Handeman,

I haven't had a chance to investigate further. It was a rather long day at work today. I'll be pulling the upper bulkhead when I get the chance and then I can get a good look at both ends. For the upper end I need to label or track each of the 16 screw pins. Their heads are contoured to fit the curve of the tube, so the motor slides into the fiberglass airframe tube that encloses recovery and electronics. That tube only goes over the upper half of the tank.

It's sort of funny, but if the available waiver were higher, I could replace the fiberglass tube with PML tube or similar. The cagebay and the compression packing of the recovery put the tube in tension before the motor is fired. So it never sees as much compression as would typically be the case. Since this rocket only pulls about 12G, has very little unsupported length, and can only go a little supersonic, even fairly light tubing would be fine. The fiberglass tube is overkill. But I needed the extra weight to keep the altitude down.

The original design was way cooler. It used coupler tubing for just a stub tube above the motor, and much more was housed in the trimmed back nosecone. With a smaller OD, several inches reduced length, and a few pounds less weight, it would have gone somewhat faster and a fair bit higher. On that one, the cryovalve would not have been sticking out the top of the upper bulkhead. But when the URRF waiver was reduced by several Kft, I needed to alter the design to keep it under the new waiver. :( So I did what I had to do.

Gerald
 
As I think I mentioned, two of the four little button head cap screws that retained the preheater liner were frozen in place. The allen head wouldn't take the torque required to break them free. Tonight I used a jeweler's saw to cut off the sides in a parallel fashion so that I could get a good grip with vice grips. Even then it took a fair bit of torque to get them out. But they are out.

After all that, the preheater liner came out smoothly. The grease on the protected side was still fine.

BTW, the O-rings were perfect. No evidence of any issues at all from years of assembly and two exposures to nitrous. Like new.

Gerald
 
I've received word that the GSE I have had on order has been shipped. I also checked with my nitrous supplier and he has plenty on hand. I just have to get the new cylinder there for filling.

Now if I can just get some time to work on the rocket!

Gerald
 
I've received word that the GSE I have had on order has been shipped. I also checked with my nitrous supplier and he has plenty on hand. I just have to get the new cylinder there for filling.

Now if I can just get some time to work on the rocket!

Gerald
Are you pumping your nitrous? Or just using solenoids? What pressure does your cryovalve vent at?
 
As I think I mentioned, two of the four little button head cap screws that retained the preheater liner were frozen in place. The allen head wouldn't take the torque required to break them free. Tonight I used a jeweler's saw to cut off the sides in a parallel fashion so that I could get a good grip with vice grips. Even then it took a fair bit of torque to get them out. But they are out.

After all that, the preheater liner came out smoothly. The grease on the protected side was still fine.

BTW, the O-rings were perfect. No evidence of any issues at all from years of assembly and two exposures to nitrous. Like new.

Gerald
With button heads you've got so little head depth you need a good quality allen key so it's a really good fit for the hex hole. I sometimes regrind the end of the allen key flush so there is only a good hex part of the key rather than the slightly rounded forged end. This gives you maximum key in the short depth of the button. Also, some button heads will be forged better than others, so pick the best ones if it's a critical application.
Torx head button would be better. Grinding the end of the torx bit to fit it similarly.
 
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Torx would absolutely be better. Heck, for this application, straight bladed screwdriver or square head would also be much better. These screws get torched! Thanks for reminding me I need to get replacements.

I have two cryovalves on hand; one is calibrated to vent at 600psi and the other at 650. When I designed the motor a 650 wasn't available. I've done all the burns with a 600. I could change to a 650 but would need to make a new injector assembly, or remachine the nozzle. I'd lose some total impulse with the change if I did it, but gain a little thrust and/or a little more pressure drop across the injectors.

When I redesign for the next motor, it's hard to say what I'll choose.

I decided to stick with the existing injector assembly this flight. As it is, I get 10-12G during the first few seconds which is right where I want it. The motor is doing what I designed it to do.

The real problem is igniting the nitrous. I haven't tested it yet, but plan to use a modified thermite this time around (a version where one of the major reactants is a nitrous decomposition catalyst, and there will be a light binder that generates a little gas). I'm through messing around. I'm going to hit it very hard this time.

I think the GSE will arrive Friday, and I might get nitrous this weekend. If so, I'll get to perform a test pretty soon. I'll post a few photos. That's a test of the GSE; not the motor!

I might post a short vid of the new preheater testing when I get to it. That's going to be the first real use of the wireless GSE - to get well away from that test! This won't be some wimpy little igniter. 12g (or however much it was) of BKNO3 augmenting some aggressive solid propellant wasn't enough last time. I still had two seconds of cold flow. That cost near half the expected altitude and kept it subsonic.

When the injectors open, think of it as a dry ice maker with a flow rate of perhaps 3 pounds per second of ice production! I have to raise the output temp to up around 600C for quick ignition. I need well over a half megawatt to do that if one discounts the catalytic component of the preheater (so I don't necessarily have to get it quite as hot). Once nitrous decomposition really gets going it generates enough heat to keep the reaction going.

Gerald
 
I might post a short vid of the new preheater testing when I get to it. That's going to be the first real use of the wireless GSE - to get well away from that test! This won't be some wimpy little igniter. 12g (or however much it was) of BKNO3 augmenting some aggressive solid propellant wasn't enough last time. I still had two seconds of cold flow. That cost near half the expected altitude and kept it subsonic.

Gerald
Yeah, BKNO3 bound with Viton is a good igniter composition and very easy to ignite, but I certainly wouldn't put it in the extremely thermally energetic category.

TP
 
B50 cylinder on wheels, with most of a full load of nitrous. It's a 2 person job to get it in and out of the back of a pickup truck! On the plus side, with those wheels it rolls nicely over grass.

GSE is here (Wilson Fx, HLS-w-Pel) and looks quite nice. I expect to test it sometime in the next week, and will post pictures then.

Flight is now registered as a BFR for URRF.

Upper bulkhead removal...

Each bolt-pin is specific to the hole, due to my having had to contour file the heads so the upper end of the motor would slip into the fiberglass tube. So I used a rather tiny ball bur in a micromotor to mark the number of each bolt on its head, and numbered the first few holes on the case. The sharpie numbers will end up coming off when I clean the case. Just about everything removes sharpie.

Like the lower bulkhead, the upper was frozen solidly. I expected that. So I used a wooden dowel for leverage and got it rotating. Then I used the same dowel inside the tube to pound then push the bulkhead out. It came out pretty easily once it was sliding.

The O-rings are perfect, as I expected from the condition of the coupler O-rings. They don't need to be replaced but I'll do it anyway.

Looking inside the cylinder, there is really hardly any coating on the inside. None towards the upper bulkhead. I expect the total mass of coating is on the order of milligrams - a barely visible whitish film. If nitrous dissolves it, the total mass fraction is small enough not to be a concern. If nitrous doesn't dissolve it, then it isn't a concern anyway.

I'll clean it this time, but I'm not expecting to do a full disassembly again. I think the cylinder can stay assembled indefinitely. Perhaps I'll remove just one of the bulkheads after three or four more burns for an inspection.

Cleaning will be interior scrubbing with acetone, then denatured alcohol.

I might put the tube on a lathe and give it a shine on the lower half (visible when rocket is assembled) with WD-40 and 600 grit wet/dry sandpaper. It would make it prettier. I'm not sure I care enough one way or the other to do it. We'll see.

Anyway, current theory on the high pressure leak I had at the pad is the cryovalve was sticking not quite closed. It will get replaced.

Though, I still haven't pulled the injector assembly from the coupler. I'm not expecting any issues there with the 4 O-rings.

Gerald
 

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