THRP-1
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Coupler and injector assembly separated...
The first and second pictures show the custom little alignment and retainer pin I machined. This keeps the injector assembly from being able to be blown back into the flight tank.
The coupler was a bit greasy from all the O-rings, and had carbonation on the parts exposed to flame. I cleaned them up and even used a rather fine file and some 800 grit sandpaper on the carbonized end. It cleaned up somewhat. If I cared to have it truly clean, I'd probably re-face it on a lathe. It is purely cosmetic so not worth the bother.
The injector assembly surprised me. I literally had to hammer it out with a wooden dowel and a 2# rawhide mallet. It moved only slowly. I think I need to use a little more grease on it next time! The O-rings were in excellent condition except for the first one, which has a small chunk missing. Whether that happened on original installation, or removal, is hard to say. In any event there is no evidence of gas getting past that first O-ring. The second O-ring is slightly damaged as well.
The reason for the 4 O-rings is the alignment bolt. Having it there means the area in the middle is essentially slightly vented to atmospheric pressure. So 2 O-rings on each side, for redundancy. Overkill, but effective.
Like the coupler and the forward bulkhead, there is no evidence that the injector assembly needed to be removed. It could have lasted many more firings.
The injectors themselves of course are toast. They are definitely single use!
Gerald
The first and second pictures show the custom little alignment and retainer pin I machined. This keeps the injector assembly from being able to be blown back into the flight tank.
The coupler was a bit greasy from all the O-rings, and had carbonation on the parts exposed to flame. I cleaned them up and even used a rather fine file and some 800 grit sandpaper on the carbonized end. It cleaned up somewhat. If I cared to have it truly clean, I'd probably re-face it on a lathe. It is purely cosmetic so not worth the bother.
The injector assembly surprised me. I literally had to hammer it out with a wooden dowel and a 2# rawhide mallet. It moved only slowly. I think I need to use a little more grease on it next time! The O-rings were in excellent condition except for the first one, which has a small chunk missing. Whether that happened on original installation, or removal, is hard to say. In any event there is no evidence of gas getting past that first O-ring. The second O-ring is slightly damaged as well.
The reason for the 4 O-rings is the alignment bolt. Having it there means the area in the middle is essentially slightly vented to atmospheric pressure. So 2 O-rings on each side, for redundancy. Overkill, but effective.
Like the coupler and the forward bulkhead, there is no evidence that the injector assembly needed to be removed. It could have lasted many more firings.
The injectors themselves of course are toast. They are definitely single use!
Gerald
Attachments
Not much time available tonight, unfortunately. I got all the O-rings removed and the grease cleaned up. Got the old injectors removed from the injector assembly and initial cleaning completed, so it is ready for final cleaning and new injectors. One injector was less snug than I would have liked. Perhaps that was the source of the slight high pressure leak. The others were more properly torqued.
It took me longer than I would have thought to get the teflon tape off the right angle fitting that goes on top the cryovalve.
Speaking of the cryovalve, a new problem - So far it is stuck in place. I clamped the wrench flats in a 6" vice and couldn't torque the upper bulkhead off. I think replacement of the cryovalve might have to wait until I reassemble the flight tank. Then I can get a couple people grabbing the tube and put a wrench on the valve.
Gerald
It took me longer than I would have thought to get the teflon tape off the right angle fitting that goes on top the cryovalve.
Speaking of the cryovalve, a new problem - So far it is stuck in place. I clamped the wrench flats in a 6" vice and couldn't torque the upper bulkhead off. I think replacement of the cryovalve might have to wait until I reassemble the flight tank. Then I can get a couple people grabbing the tube and put a wrench on the valve.
Gerald
Attachments
Whatever the contamination giving the faint film coating inside the flight tank, whether it was sucked in wax vapor as I speculate, or compressor lube or grease from a contaminated batch of nitrous, it is gone. It cleaned out super easily with acetone. Then I used denatured alcohol to remove any traces of the acetone. It's now a nice shiny cylinder on the inside.
Had I the capability, and the paranoia, the next level of cleaning would involve up to 30% hydrogen peroxide rinse. That tends to consume any remaining organics! Don't have it; not doing it. I'm not set up for working with something like that anyway.
Had I 50% hydrogen peroxide, I'd be thinking oxidizer.
Gerald
Had I the capability, and the paranoia, the next level of cleaning would involve up to 30% hydrogen peroxide rinse. That tends to consume any remaining organics! Don't have it; not doing it. I'm not set up for working with something like that anyway.
Had I 50% hydrogen peroxide, I'd be thinking oxidizer.
Gerald
Forward bulkhead is reattached. I managed to get the pyrovalve to move but have left it in place for now. I'll replace it once the lower assembly is reattached. Anyway it is nice and shiny on the inside now that it has been cleaned.
I picked up some injectors from Tom at Contrail. Turns out these are much higher quality than the ones I've been using from Fastenal. It also turns out the threading is a little different. The threads have better depth and notably greater length of engagement. To use the new injectors I'll have to deepen the 1/8NPT threading a bit on the injector block. These new injectors won't quite start to thread. I had carefully set the depth of the tapered thread for the other injectors when I machined the injector assembly, so they would bottom on a shoulder at appropriate torque.
Tough call which way to go at this point. Leave alone, and use the Fastenal injectors as before, or slightly increase the threading and go with the better injectors from Contrail? I don't have anything like the machining setup I used to have, so this would be hand work this time around. I'd have to count turns of the tap handle to be consistent for the three holes. Picture from two posts ago shows the holes.
Gerald
I picked up some injectors from Tom at Contrail. Turns out these are much higher quality than the ones I've been using from Fastenal. It also turns out the threading is a little different. The threads have better depth and notably greater length of engagement. To use the new injectors I'll have to deepen the 1/8NPT threading a bit on the injector block. These new injectors won't quite start to thread. I had carefully set the depth of the tapered thread for the other injectors when I machined the injector assembly, so they would bottom on a shoulder at appropriate torque.
Tough call which way to go at this point. Leave alone, and use the Fastenal injectors as before, or slightly increase the threading and go with the better injectors from Contrail? I don't have anything like the machining setup I used to have, so this would be hand work this time around. I'd have to count turns of the tap handle to be consistent for the three holes. Picture from two posts ago shows the holes.
Gerald
Attachments
It's hard to tell visually, but it looks like one of the threads could be NPT and the other is BSP? The pitch is close 27 TPI vs 28. Also, generally when there's a gasket, the thread is typically parallel whereas it's always taper on the male thread (at least) for a thread seal. So the top pictured fitting looks as though it needs a nice generous taper to seat against ie. something flat & smooth?
TP
TP
They are both the same thread, 1/8 NPT. Just the threading is sloppier on the Fastenal unit, and there are only a few threads to engage before the seal. I think one of these was the high pressure leak I had on last year's flight. When I disassembled, one wasn't as tight as I thought it should be.
I decided to change brand of injectors. I don't like the low thread engagement of the Fastenal units. I don't think these things are rated for the pressure of the flight tank anyway. I also don't know if that seal ring is compatible with nitrous. The Fastenal units only engage about 3 threads and the threads aren't stellar.
If I had the time, I'd make a new injector assembly to swap in. But instead I'm just deepening the tapered thread. Tonight I just deepened the thread of each hole 1 3/4 turns of the tap. An injector will now thread in a turn before resistance starts to build. After work tomorrow I'll deepen them 2 more turns and then start to torque on an injector to see if that's about right. So I'll sacrifice one for fit check and thread conditioning.
I wish I had the appropriate forming tap to use for the last tiny bit of threading but I have very few forming tap sizes on hand.
I'll end up with one spare injector.
Gerald
I decided to change brand of injectors. I don't like the low thread engagement of the Fastenal units. I don't think these things are rated for the pressure of the flight tank anyway. I also don't know if that seal ring is compatible with nitrous. The Fastenal units only engage about 3 threads and the threads aren't stellar.
If I had the time, I'd make a new injector assembly to swap in. But instead I'm just deepening the tapered thread. Tonight I just deepened the thread of each hole 1 3/4 turns of the tap. An injector will now thread in a turn before resistance starts to build. After work tomorrow I'll deepen them 2 more turns and then start to torque on an injector to see if that's about right. So I'll sacrifice one for fit check and thread conditioning.
I wish I had the appropriate forming tap to use for the last tiny bit of threading but I have very few forming tap sizes on hand.
I'll end up with one spare injector.
Gerald
If you need a bottoming tap to get right to the bottom of the hole and have a spare taper tap, grind the front tapered bit off.Sorry for sounding obvious, but buy one. Considering the amount of money you are spending it is insignificant.
Great project by the way
No... A forming tap is a different sort of tap. It does not cut threads. It forges threads into the metal, giving very smooth threads. They take a lot of torque to use. For critical applications I like to use a forming tap as the last stage of finishing threads.
I designed the injector assembly with relief on the inside for passage of taps. They are tilted thru holes. Too long a tap would be a problem, but not 1/8 NPT taps.
The threads are enlarged now. I just finished cleaning the injector block with detergent to get rid of cutting oil. I'll test fit the sacrificial injector with some torque to see if the holes are now tapped to the right diameter and are matching each other. After that I'll clean with acetone then denatured alcohol, before installing injectors and O-rings, and assembling into the coupler.
My matching method was pretty crude - sharpie marks on a pair of taps. I used a pair since I have two on hand with microscopically different taper rates. Swapping between the two made it easier to tap the length of full diameter threads. I definitely didn't want to break a tap! That would have ended chance at a launch in two weeks.
The approaching launch date is also why I couldn't accept the delay for getting the specialized tap.
BTW, Rustoleum Professional rattle-can takes about 6 weeks to fully cure at room temperature. Then it is pretty hard. Glad I did the painting a couple months ago!
Gerald
PS - For everyone paying attention to this project, thanks for the interest! And if you get to Potter, stop by! Hopefully I won't be as brain dead tired as I was last time around.
I designed the injector assembly with relief on the inside for passage of taps. They are tilted thru holes. Too long a tap would be a problem, but not 1/8 NPT taps.
The threads are enlarged now. I just finished cleaning the injector block with detergent to get rid of cutting oil. I'll test fit the sacrificial injector with some torque to see if the holes are now tapped to the right diameter and are matching each other. After that I'll clean with acetone then denatured alcohol, before installing injectors and O-rings, and assembling into the coupler.
My matching method was pretty crude - sharpie marks on a pair of taps. I used a pair since I have two on hand with microscopically different taper rates. Swapping between the two made it easier to tap the length of full diameter threads. I definitely didn't want to break a tap! That would have ended chance at a launch in two weeks.
The approaching launch date is also why I couldn't accept the delay for getting the specialized tap.
BTW, Rustoleum Professional rattle-can takes about 6 weeks to fully cure at room temperature. Then it is pretty hard. Glad I did the painting a couple months ago!
Gerald
PS - For everyone paying attention to this project, thanks for the interest! And if you get to Potter, stop by! Hopefully I won't be as brain dead tired as I was last time around.
Since lots of people won't even know they exist or what they are, here is a bit of info on thread forming taps: https://www.mscdirect.com/basicsof/thread-forming-taps
Gerald
Gerald
Why the blazes you'd need a thread forming tap for this application is beyond me and I've built my fair share of injector heads. Besides, thread forming taps are specifically designed to be tapped automatically by either a tapping head, but more ideally by a CNC with rigid tapping capability. I have hundreds of regular HSS taps, forming taps and thread mills and it's the forming taps that would by far get the least work of all by quite some margin.
TP
TP
I like them for providing an improved surface finish on a thread as the last operation, in situations where it is expected the thread will get a bolt installed and removed a number of times, with essemtially full depth threads and tight engagement. Sort of a combination of smoothing and surface work hardening. But it is rare that I use one.
I miss my Bridgeport! The holes were drilled and going by memory, tapped on my old Bridgeport when I had it. The rigid setup made for very nice threads.
These were redone by hand though. Sorry for the color balance of the last picture. My phone went crazy yellow on it and I corrected as best I could.
I won't get back to working on this project until Sunday. I'll finish cleaning the injector assembly, install the injectors, clean the coupler and install the injector assembly, and install the coupler into the bottom of the tank. Then replace the cryovalve and re-fit its exhaust diverter cap. Then I'll make the two injector plugs. The third gets the fill line. That will complete the tank reassembly and prep.
Gerald
I miss my Bridgeport! The holes were drilled and going by memory, tapped on my old Bridgeport when I had it. The rigid setup made for very nice threads.
These were redone by hand though. Sorry for the color balance of the last picture. My phone went crazy yellow on it and I corrected as best I could.
I won't get back to working on this project until Sunday. I'll finish cleaning the injector assembly, install the injectors, clean the coupler and install the injector assembly, and install the coupler into the bottom of the tank. Then replace the cryovalve and re-fit its exhaust diverter cap. Then I'll make the two injector plugs. The third gets the fill line. That will complete the tank reassembly and prep.
Gerald
Attachments
Thread forming taps are generally for use in sheet metal and pipes where there is a finite amount of metal to be moved. They initially push the metal through the pilot hole or push the wall to form the thread or an extended burr on the other side of the sheet to increase the possible thread length. They are not really suitable for a blind hole as the material has to be pushed forward out of the way to form the thread and has nowhere to go. Even a forward cutting tap has a fluted slot for the cut material to be removed to. Generally, full strength on a threaded connection is achieved after 3 threads of contact. Obviously, with a tapered thread you have to have 3 threads of good fit contact, not a rattling fit.
That said, I know nothing.... so I'd be interested to see photos of the thread before and after the thread forming was applied. I can see that it might be useful to prevent galling on a high-use thread. I assume you're hard anodising the part too.
That said, I know nothing.... so I'd be interested to see photos of the thread before and after the thread forming was applied. I can see that it might be useful to prevent galling on a high-use thread. I assume you're hard anodising the part too.
Ummm, this sort of tap pushes the metal radially not down the hole if the initial drill size is right. They are just fine for either through or blind holes. They wouldn't be my choice for thin sheet though I suppose using a smaller than standard hole it might well form a thickened thread section on the back of the sheet. I've never tried it. The initial drill size is larger than for conventional taps generally at least for most applications. The threads are formed radially, displacing metal both outwards and inwards.
Assembly progress... The flight tank is reassembled, except for replacing the valve. That's probably next.
One shouldn't be handling Krytox grease. It is not good for you and quite bad if you get it in your eyes IIRC. Wear appropriate gloves and take care.
Gerald
Assembly progress... The flight tank is reassembled, except for replacing the valve. That's probably next.
One shouldn't be handling Krytox grease. It is not good for you and quite bad if you get it in your eyes IIRC. Wear appropriate gloves and take care.
Gerald
Attachments
I didn't get much done today, just replaced the cryovalve. I call it a cryovalve, but it is really a calibrated pressure relief valve that has a teflon seal so it is compatible with nitrous or oxygen, and has been cleaned at the factory for oxygen service. I call it "cryo" because that is also one of the requirements. Nitrous that vents here causes the valve to chill down to quite negative temperatures. If the valve can't handle cryogenic temperatures, it is likely to freeze or stick, either of which would terminate a launch.
There aren't many sources of a good valve for this application, or weren't many when I designed this motor several years back (it took a while to get it made, then there was covid...). This one is made by Rego, and vents at 550psi.
You can read off this handy little chart to see what this valve accomplishes. The nitrous ends up being somewhere around 45F in the tank, at a density of about 0.87g/cm^3. At the exhaust side of the valve, venting from 550psi to atmospheric pressure, the temperature is MUCH colder! Instant frostbite temperature range. Stay away!
In a normal hybrid in the summer, the flight tank would be somewhere closer to 80F (guessing, which is the heart of the problem) with a resultant pressure of about 860psi and a resultant density of only .66g/cm^3. But you can see that even small changes in temperature cause considerable changes in density and pressure. With a conventional vented hybrid, you only know roughly how much nitrous is in the tank, and at what pressure and temperature. Hitting consistent performance is impossible under those conditions.
So I give up tank pressure which then gives up chamber pressure so ISP is definitely reduced. But the oxidizer density is increased by about 32%. The density x ISP ends up being a bit better. But on the flip side the oxidizer is quite a bit harder to ignite - or to get it to detonate. That temperature drop quite significantly affects the energy needed to trigger nitrous decomposition. So it is even safer.
And, rather harder to light
On the major plus side, I know fairly closely the density, pressure, temperature, and total oxidizer mass that is going to end up in the tank. That's a major plus for designing the motor, and for getting repeatable burns even when the outside temperature and the source tank temperature vary by quite a bit.
It isn't worth playing this tradeoff game until the motors are at least this large. In any event this motor is a test motor to see how all this works before scaling up. Except for the part about igniting it, it is working quite well.
Next version will not vent out the top of the tank. It puts annoying constraints on the rocket design! Next time around I'd like to put the valve internal to the tank, minus the sticker of course! The older valves didn't have that sticker.
The valves are available in roughly 50# set pressure increments, last I checked, with a max of 600psi. Unless repressurizing, I think only the 550 and the 600 make sense for our purposes. I suppose the 500 could be used if you are crazy!
Last advantage is you don't need a large margin of pressure safety on the flight tank. It will vent rather than overpressurize. The tank can use a much thinner wall than this one has (0.120" roughly). You could safely use a tank which would burst on a conventional hybrid filling on a hot summer day.
And for a bigger motor, that helps the mass fraction a lot! That's the long term game plan anyway. Densify nitrous by chilling, reduced hardware mass allowed by greatly reduced MEOP, thereby getting a large boost in propellant mass fraction.
Gerald
There aren't many sources of a good valve for this application, or weren't many when I designed this motor several years back (it took a while to get it made, then there was covid...). This one is made by Rego, and vents at 550psi.
You can read off this handy little chart to see what this valve accomplishes. The nitrous ends up being somewhere around 45F in the tank, at a density of about 0.87g/cm^3. At the exhaust side of the valve, venting from 550psi to atmospheric pressure, the temperature is MUCH colder! Instant frostbite temperature range. Stay away!
In a normal hybrid in the summer, the flight tank would be somewhere closer to 80F (guessing, which is the heart of the problem) with a resultant pressure of about 860psi and a resultant density of only .66g/cm^3. But you can see that even small changes in temperature cause considerable changes in density and pressure. With a conventional vented hybrid, you only know roughly how much nitrous is in the tank, and at what pressure and temperature. Hitting consistent performance is impossible under those conditions.
So I give up tank pressure which then gives up chamber pressure so ISP is definitely reduced. But the oxidizer density is increased by about 32%. The density x ISP ends up being a bit better. But on the flip side the oxidizer is quite a bit harder to ignite - or to get it to detonate. That temperature drop quite significantly affects the energy needed to trigger nitrous decomposition. So it is even safer.
And, rather harder to light
On the major plus side, I know fairly closely the density, pressure, temperature, and total oxidizer mass that is going to end up in the tank. That's a major plus for designing the motor, and for getting repeatable burns even when the outside temperature and the source tank temperature vary by quite a bit.
It isn't worth playing this tradeoff game until the motors are at least this large. In any event this motor is a test motor to see how all this works before scaling up. Except for the part about igniting it, it is working quite well.
Next version will not vent out the top of the tank. It puts annoying constraints on the rocket design! Next time around I'd like to put the valve internal to the tank, minus the sticker of course! The older valves didn't have that sticker.
The valves are available in roughly 50# set pressure increments, last I checked, with a max of 600psi. Unless repressurizing, I think only the 550 and the 600 make sense for our purposes. I suppose the 500 could be used if you are crazy!
Last advantage is you don't need a large margin of pressure safety on the flight tank. It will vent rather than overpressurize. The tank can use a much thinner wall than this one has (0.120" roughly). You could safely use a tank which would burst on a conventional hybrid filling on a hot summer day.
And for a bigger motor, that helps the mass fraction a lot! That's the long term game plan anyway. Densify nitrous by chilling, reduced hardware mass allowed by greatly reduced MEOP, thereby getting a large boost in propellant mass fraction.
Gerald
Attachments
Even that's probably not as bad has it sounds. The change in pressure from tank depletion will be smaller with the chilled tank and the max loss of thrust coefficient will be @1atm from operating at the lower Pc, which will be gradually eroded away with reductions in Pa (altitude); so for high altitude flights, not only will the (tank) pressure loss gradients start to converge with depletion, the gains from higher Pc also erode with altitude.
That's assuming you can maintain the same level of combustion and chamber stability with the cooler N2O which is certainly no fait accompli.
What's also not mentioned is that chilling N2O isn't free. It costs N2O to achieve dT which is vented to the atmosphere. On hot days, that cost isn't negligible.
All that aside, I'm certainly interested in how you progress with your ignition. That's the one single challenge I'm unaware of anyone really getting on top of with chilled N2O, although you haven't quite gone to the heavy chilling degrees that others have.
TP
3.798v in the tracker LiPO. It's been sitting for a while. Needs recharging and retesting.
RunCam 2 4K still works. Has new battery.
Casting the fuel grain on Friday. Need to regenerate mix sheet. Somehow I misplaced the electronic copy! It got separated from the THRP-1 project files. I think I still have a paper copy from last year's mix, and I do have a paper copy from the static test. I know what the changes were so I can recreate it if I can't find it.
Have cut preheater casting tube. Working on formula. The big gamble... I know I need to hit it really hard, so I'm working on something that is a cross between thermite and a fast burning but low thrust solid propellant. Sort of a flaming thermite sprayer. I need a few more weeks so I can test some options but I don't have the time. I'm aiming for close to instant on.
I'll be using fiberglass sleeves over the lines where I don't want them to burn through.
Gerald
RunCam 2 4K still works. Has new battery.
Casting the fuel grain on Friday. Need to regenerate mix sheet. Somehow I misplaced the electronic copy! It got separated from the THRP-1 project files. I think I still have a paper copy from last year's mix, and I do have a paper copy from the static test. I know what the changes were so I can recreate it if I can't find it.
Have cut preheater casting tube. Working on formula. The big gamble... I know I need to hit it really hard, so I'm working on something that is a cross between thermite and a fast burning but low thrust solid propellant. Sort of a flaming thermite sprayer. I need a few more weeks so I can test some options but I don't have the time. I'm aiming for close to instant on.
I'll be using fiberglass sleeves over the lines where I don't want them to burn through.
Gerald
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