THRP-1

G_T

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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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G_T

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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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G_T

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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
 

G_T

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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
 
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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.
 

G_T

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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.
 

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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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G_T

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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
 
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