THRP-1
- Thread starter G_T
- Start date
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
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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
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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
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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
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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
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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
If you want really insane temps, you can consider Mg+fluoropolymer (with high F content) combo maybe augmented with a BKNO3+Viton coating for ignition or even a more classic thermalite combo throwing some thermal mass at it.
You've probably seen this, but just incase: https://ntrs.nasa.gov/citations/19710020870 (p26 for some classic compositions)
TP
You've probably seen this, but just incase: https://ntrs.nasa.gov/citations/19710020870 (p26 for some classic compositions)
TP
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Thanks for the link Troy. I'll read it over soon.
Tonight is occupied with charging batteries. Why does rocketry involve so many batteries?
Two new lithium batteries for the electronics bay - not yet replaced. I'm leaving the old ones in for testing; will swap afterwards.
Video battery topped off, along with spare video battery.
HAM handheld topped off.
GMRS handheld charging.
Tracker battery charging.
20Ah, 40Ah LiFePO4 batteries for GSE are charging.
The 40 is almost a perfect fit in an ammo can. Nice for transportation. Someday I'll come up with something for the 20Ah, but for now it fits well in a small cardboard box.
100Ah LiFePO4 battery not charged yet. I probably won't take the 100. Overkill.
Gerald
Tonight is occupied with charging batteries. Why does rocketry involve so many batteries?
Two new lithium batteries for the electronics bay - not yet replaced. I'm leaving the old ones in for testing; will swap afterwards.
Video battery topped off, along with spare video battery.
HAM handheld topped off.
GMRS handheld charging.
Tracker battery charging.
20Ah, 40Ah LiFePO4 batteries for GSE are charging.
The 40 is almost a perfect fit in an ammo can. Nice for transportation. Someday I'll come up with something for the 20Ah, but for now it fits well in a small cardboard box.
100Ah LiFePO4 battery not charged yet. I probably won't take the 100. Overkill.
Gerald
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Which is why I stick with 9V batteries for all my altimeters. They usually last 3 years. The only ones I need to charge is for my GPS tracker and receiver. I'm starting to run cameras now too, so that may up the charging needed.
Maybe I'm starting to get into the "battery charging" world too. I've been avoiding it, but it looks like it might be catching up!
Batteries are charged.
TAC-1 drogue z-folded and stuffed into the base of the nosecone. I just fiddled with packing arrangements until it fell out easily enough. There are pictures earlier in this thread showing the bulkhead of the nosecone is recessed to the forward side of the coupler. The nosecone contains the tracker, and the coupler has sufficient volume for the drogue. It's an arrangement I like to use.
There's a quick link hidden by camera angle. It's hanging under the swivel.
There are some fine threads hanging in some places. I'll pull it apart and burn them down soon. Anyway I have to remove the bulkhead to turn the tracker on or off. It's not hard to do but it is a minus of sticking the tracker up in the nosecone this way.
I also prefer simple switches to magnetic switches which I don't really trust, or wireless switches. The more complicated it is, the more there is that can go wrong. I turned the tracker on at the pad last time, right before we put the rocket on the rail. It slows operations down a bit, but this is a hybrid. A project like this just takes a bit more time at the pad!
Gerald
TAC-1 drogue z-folded and stuffed into the base of the nosecone. I just fiddled with packing arrangements until it fell out easily enough. There are pictures earlier in this thread showing the bulkhead of the nosecone is recessed to the forward side of the coupler. The nosecone contains the tracker, and the coupler has sufficient volume for the drogue. It's an arrangement I like to use.
There's a quick link hidden by camera angle. It's hanging under the swivel.
There are some fine threads hanging in some places. I'll pull it apart and burn them down soon. Anyway I have to remove the bulkhead to turn the tracker on or off. It's not hard to do but it is a minus of sticking the tracker up in the nosecone this way.
I also prefer simple switches to magnetic switches which I don't really trust, or wireless switches. The more complicated it is, the more there is that can go wrong. I turned the tracker on at the pad last time, right before we put the rocket on the rail. It slows operations down a bit, but this is a hybrid. A project like this just takes a bit more time at the pad!
Gerald
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Somehow I lost the files for the fuel burned last year, but did have notes on the back of a mix sheet which look correct. But since I had to regenerate a mix sheet, I decided to alter the formula. (In Darth Vader voice) Prey I don't alter it further!
This hybrid motor uses a very short combustion chamber. So getting combustion to complete within the chamber particularly at the start of the burn where port velocity is highest is a challenge.
Each burn has also dumped a good portion of the oxidizer before the burn became sustained in the nitrous. So I have been unable to adequately judge regression rates.
What I do know is the thrust produced is very very close to designed thrust - literally within a few pounds. So based on unsupportable assumptions, the regression rate also looks about perfect.
The problem is that about perfect means almost no fuel left at the end of a full burn - if I can get a full burn out of it. There's little safety margin. It was part of the original design, when I didn't anticipate I'd have such an ignition problem.
So there are going to be some alterations to address these concerns. These alterations are not based on PEP analysis as is my usual method, but instead based on tweaking a design based on what I've observed and measured in a couple of burns.
I haven't put much in the way of fuel details on this open forum, but it is pretty well known that I'm working with a blend of HTPB and Paraffin as the primary fuel constituents. Both are common for hybrids, though the blend is not as common.
Now with such a blend, one has control over the regression rate by altering the ratio of these two fuels. My original intent was to tweak the ratio based on tests to dial in the O:F ratio for max total impulse from the motor. Being able to do so was a good portion of the reason for my choosing a blend of two primary fuels.
My original ratio was slightly over half Paraffin. The new ratio is slightly over half HTPB. This will slow the regression slightly. Hopefully it will also let it pour a little faster into the liner! It was like cold molassas last two times.
Now about that short combustion chamber... When I used one metal fuel that I won't mention here, there was plenty of evidence that a lot of the fuel burned outside the motor. That's visually evident in the static test picture in the first post of this thread. So for the flight motor test I switched to a metal fuel that needs much less residence time and doesn't need as high a temp to get burning. That was a very notable improvement. I'm using the same metal but increasing the percentage slightly. It's mostly to get the burn a little hotter, a little quicker. I'm starting to run low on that metal and haven't found a new source for it. But that's a problem for another day.
What I am mostly changing is upping the catalyst by a large percentage, and going to a dual catalyst composition. These catalysts are nitrous oxides decomposition catalysts. They are not there to have anything to do with burning the metal. One is commonly used in solid propellants though not the most common, and the other is not used in that application. An advantage is I can easily get the additional catalyst in sub-micron particle size so it presents lots of surface area - exactly what a catalyst needs.
I might be easily increasing the total surface area of catalyst in the fuel grain by a factor of 10. This is to encourage the nitrous to decompose as fast as possible.
Mixing is tomorrow.
Gerald
This hybrid motor uses a very short combustion chamber. So getting combustion to complete within the chamber particularly at the start of the burn where port velocity is highest is a challenge.
Each burn has also dumped a good portion of the oxidizer before the burn became sustained in the nitrous. So I have been unable to adequately judge regression rates.
What I do know is the thrust produced is very very close to designed thrust - literally within a few pounds. So based on unsupportable assumptions, the regression rate also looks about perfect.
The problem is that about perfect means almost no fuel left at the end of a full burn - if I can get a full burn out of it. There's little safety margin. It was part of the original design, when I didn't anticipate I'd have such an ignition problem.
So there are going to be some alterations to address these concerns. These alterations are not based on PEP analysis as is my usual method, but instead based on tweaking a design based on what I've observed and measured in a couple of burns.
I haven't put much in the way of fuel details on this open forum, but it is pretty well known that I'm working with a blend of HTPB and Paraffin as the primary fuel constituents. Both are common for hybrids, though the blend is not as common.
Now with such a blend, one has control over the regression rate by altering the ratio of these two fuels. My original intent was to tweak the ratio based on tests to dial in the O:F ratio for max total impulse from the motor. Being able to do so was a good portion of the reason for my choosing a blend of two primary fuels.
My original ratio was slightly over half Paraffin. The new ratio is slightly over half HTPB. This will slow the regression slightly. Hopefully it will also let it pour a little faster into the liner! It was like cold molassas last two times.
Now about that short combustion chamber... When I used one metal fuel that I won't mention here, there was plenty of evidence that a lot of the fuel burned outside the motor. That's visually evident in the static test picture in the first post of this thread. So for the flight motor test I switched to a metal fuel that needs much less residence time and doesn't need as high a temp to get burning. That was a very notable improvement. I'm using the same metal but increasing the percentage slightly. It's mostly to get the burn a little hotter, a little quicker. I'm starting to run low on that metal and haven't found a new source for it. But that's a problem for another day.
What I am mostly changing is upping the catalyst by a large percentage, and going to a dual catalyst composition. These catalysts are nitrous oxides decomposition catalysts. They are not there to have anything to do with burning the metal. One is commonly used in solid propellants though not the most common, and the other is not used in that application. An advantage is I can easily get the additional catalyst in sub-micron particle size so it presents lots of surface area - exactly what a catalyst needs.
I might be easily increasing the total surface area of catalyst in the fuel grain by a factor of 10. This is to encourage the nitrous to decompose as fast as possible.
Mixing is tomorrow.
Gerald
If you want to maintain that O:F ratio (at least initially) with all else being equal, another option could be a dual/double casting whereas you maintain the existing ratios with the inner web section and perhaps use a higher HTPB ratio for the outer or use a plain vanilla engineering plastic for the outer eg. PVC conduit. So many solutions to that one.
TP
TP
Yep, lots of options. But they'll be for the next motor design.
Swapped some liner material with Jerry O'Sullivan. He had an old scrap which is a much better fit for the type of tubing I have for the combustion chamber. I don't think I'll need to add any masking tape this time. Or if I do it will be one layer instead of the three or four layers I've been having to use so far.
Mixed at Ben's. He was willing to help me out.
I did alter the formula some more. I thought about it, and decided to get rid of the metal. From the last burn I don't think close enough to all of it was burned in the combustion chamber to make up for the heat losses melting it.
Since this isn't a propellant, and has no oxidizer, can I talk about the formula here?
Batch size was 900g this time. That was essentially perfect.
I nearly screwed it up in classic fashion. Once cleanup was done, I went to move it to another location. So I grabbed the liner and lifted...
With solid propellant I run tape around the casting base to hold the liner. But in this case, no tape. Plus the base is perhaps 2" thick block of aluminum. I had it on hand when I was making this casting setup some years ago, so just chucked it in the lathe and turned it into a base. That helps make it quite stable, but also makes it a really bad idea to lift by the liner!
So I grabbed, lifted, and yes the liner came up...
I realized what had happened when I was perhaps a half inch above the base! I slammed it back down very quickly. Thankfully this is a fairly thick mix! I did lose a little out the bottom.
I also jammed the tape I'd used to fit the different sized liner to the base. That prevented the liner from going back on all the way and was trying to force it to tilt. I had to cut the bottom of the tape out with a knife before I could get the liner back down. It stopped a mm short but I can live with that.
There's a bit less in there then I had planned due to that event. It is close enough. I'll put the end with a touch more liner exposed as the nozzle end, unless some issue is revealed when I pull the mandrel - hopefully tomorrow.
That was a close one!
Gerald
Swapped some liner material with Jerry O'Sullivan. He had an old scrap which is a much better fit for the type of tubing I have for the combustion chamber. I don't think I'll need to add any masking tape this time. Or if I do it will be one layer instead of the three or four layers I've been having to use so far.
Mixed at Ben's. He was willing to help me out.
I did alter the formula some more. I thought about it, and decided to get rid of the metal. From the last burn I don't think close enough to all of it was burned in the combustion chamber to make up for the heat losses melting it.
Since this isn't a propellant, and has no oxidizer, can I talk about the formula here?
Batch size was 900g this time. That was essentially perfect.
I nearly screwed it up in classic fashion. Once cleanup was done, I went to move it to another location. So I grabbed the liner and lifted...
With solid propellant I run tape around the casting base to hold the liner. But in this case, no tape. Plus the base is perhaps 2" thick block of aluminum. I had it on hand when I was making this casting setup some years ago, so just chucked it in the lathe and turned it into a base. That helps make it quite stable, but also makes it a really bad idea to lift by the liner!
So I grabbed, lifted, and yes the liner came up...
I realized what had happened when I was perhaps a half inch above the base! I slammed it back down very quickly. Thankfully this is a fairly thick mix! I did lose a little out the bottom.
I also jammed the tape I'd used to fit the different sized liner to the base. That prevented the liner from going back on all the way and was trying to force it to tilt. I had to cut the bottom of the tape out with a knife before I could get the liner back down. It stopped a mm short but I can live with that.
There's a bit less in there then I had planned due to that event. It is close enough. I'll put the end with a touch more liner exposed as the nozzle end, unless some issue is revealed when I pull the mandrel - hopefully tomorrow.
That was a close one!
Gerald
Attachments
I did have to give the fuel some vacuuming to break up clumps of the half micron catalyst.
If the mandrel comes out tomorrow, then I'll go ahead and cast a test preheater. What I'm going to do hasn't been tried before, to my knowledge. I want to test it before sticking it in a motor. I't's designed to have a very hot very fast burn, but not to have enough gas generation to lift the rocket up the rail. Plus it has to light quickly. Hopefully it won't burn the injectors off rather than lighting the motor. The more energetic you go, the harder it can be to get right.
If the formula works it will also be useful for things like lighting sustainers during high altitude flights.
I do plan to take video...
Gerald
If the mandrel comes out tomorrow, then I'll go ahead and cast a test preheater. What I'm going to do hasn't been tried before, to my knowledge. I want to test it before sticking it in a motor. I't's designed to have a very hot very fast burn, but not to have enough gas generation to lift the rocket up the rail. Plus it has to light quickly. Hopefully it won't burn the injectors off rather than lighting the motor. The more energetic you go, the harder it can be to get right.
If the formula works it will also be useful for things like lighting sustainers during high altitude flights.
I do plan to take video...
Gerald
An 846 gram batch of fuel was essentially perfect. The waste was a layer about 1 wax ball thick around the bowl. It is hard to get that last layer out anyway. And the adjustment of the wax to rubber ratio helped with getting it into the liner. I used a cut off paper cup to scoop up a blob, then dumped it onto a sort of side funnel I made. It worked well, though slowly.
Turns out the tape was a very bad idea.
First, it just didn't work. It was still necessary to use a hydraulic press to get the mandrel out.
More importantly, the tape bonded to the rubber more strongly than the rubber bonded to itself. Removal of the tape was a pain, involving a long screwdriver, long forceps, and a second set of hands. The surface of the rubber came off with the tape. That shows quite clearly in the picture. Sorry; no pictures of the messy removal process! No spare hands.
I'm not sure enough of the regression rate to know whether this is a major problem or not (minor problem is not an option for this sort of issue). If fuel runs out before the nitrous runs out, the nitrous WILL eat the hardware. Aluminum is a fuel. So is graphite.
My recovery plan is to make a small batch of R45 with curative and some CuO as an opacifier and nitrous catalyst, and paint that in the core. Then put the fuel grain on a tube rotator. That will somewhat heal the surface.
Judging by how strongly the electric tape bonded, I'm tempted to recommend dissolved vinyl electric tape as a bonding agent for liners or for case bonded grains. Acetone + electric tape = cheap + easy. I might have to test that at some point on a srap of aluminum.
By the way, you can see the built-in O-ring at the injector end of the fuel grain. That seals against the preheater grain liner. It serves double duty. It is also the centering ring for the preheater liner when a preheater is cast. I rather like that feature.
There's nothing special at the nozzle end.
Gerald
Turns out the tape was a very bad idea.
First, it just didn't work. It was still necessary to use a hydraulic press to get the mandrel out.
More importantly, the tape bonded to the rubber more strongly than the rubber bonded to itself. Removal of the tape was a pain, involving a long screwdriver, long forceps, and a second set of hands. The surface of the rubber came off with the tape. That shows quite clearly in the picture. Sorry; no pictures of the messy removal process! No spare hands.
I'm not sure enough of the regression rate to know whether this is a major problem or not (minor problem is not an option for this sort of issue). If fuel runs out before the nitrous runs out, the nitrous WILL eat the hardware. Aluminum is a fuel. So is graphite.
My recovery plan is to make a small batch of R45 with curative and some CuO as an opacifier and nitrous catalyst, and paint that in the core. Then put the fuel grain on a tube rotator. That will somewhat heal the surface.
Judging by how strongly the electric tape bonded, I'm tempted to recommend dissolved vinyl electric tape as a bonding agent for liners or for case bonded grains. Acetone + electric tape = cheap + easy. I might have to test that at some point on a srap of aluminum.
By the way, you can see the built-in O-ring at the injector end of the fuel grain. That seals against the preheater grain liner. It serves double duty. It is also the centering ring for the preheater liner when a preheater is cast. I rather like that feature.
There's nothing special at the nozzle end.
Gerald
Attachments
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I'm not set up to do this sort of work at my place.
I mixed up three small batches of the fuel mixture minus the wax globules, and painted the inside of the core with a layer. A total of about 40g of new fuel was added into the core.
Then the grain was placed on a rotater to spin it. This helps get and keep the coating more uniform. No it doesn't do a perfect job, but it makes the job possible.
I have the rotator for coating insulation into tubes for cast in place solid propellant motors. I haven't done that experiment yet. But I'm ditching casting tubes whenever possible! Given the price of liners nowdays, I plan to be ditching them too when possible.
BTW, 40g was more mass than was originally lost. The mix without the wax has a higher density.
In any event, this should roughly restore the lost web thickness.
Somewhere in the process I dribbled some of the mix on my pants leg. The pants went into the trash, and I had to clean off that part of my leg with mineral spirits then denatured alcohol, followed by shower and detergent. Even without plasticizer it will readily go through fabric.
Like I said, I'm not set up to do this sort of work here.
My AC is not keeping up with the temperature. That's good for curing this coating, but otherwise...
Later this evening I hope to make a prototype preheater for static testing tomorrow (the preheater, not the motor). Happily enough it doesn't require any rocketry specific chemical$. Will video the test. I plan to use the new WilsonFX wireless GSE to fire it. I want some distance from this one! And I want to make sure the battery setup can handle three ematches in parallel.
Gerald
PS - Ignore the writing on the liner. It was a repurposed liner that I got from Jerry.
I mixed up three small batches of the fuel mixture minus the wax globules, and painted the inside of the core with a layer. A total of about 40g of new fuel was added into the core.
Then the grain was placed on a rotater to spin it. This helps get and keep the coating more uniform. No it doesn't do a perfect job, but it makes the job possible.
I have the rotator for coating insulation into tubes for cast in place solid propellant motors. I haven't done that experiment yet. But I'm ditching casting tubes whenever possible! Given the price of liners nowdays, I plan to be ditching them too when possible.
BTW, 40g was more mass than was originally lost. The mix without the wax has a higher density.
In any event, this should roughly restore the lost web thickness.
Somewhere in the process I dribbled some of the mix on my pants leg. The pants went into the trash, and I had to clean off that part of my leg with mineral spirits then denatured alcohol, followed by shower and detergent. Even without plasticizer it will readily go through fabric.
Like I said, I'm not set up to do this sort of work here.
My AC is not keeping up with the temperature. That's good for curing this coating, but otherwise...
Later this evening I hope to make a prototype preheater for static testing tomorrow (the preheater, not the motor). Happily enough it doesn't require any rocketry specific chemical$. Will video the test. I plan to use the new WilsonFX wireless GSE to fire it. I want some distance from this one! And I want to make sure the battery setup can handle three ematches in parallel.
Gerald
PS - Ignore the writing on the liner. It was a repurposed liner that I got from Jerry.
