THRP-1

[G_T]

Waltr,

Heck, do your L1 at URRF! You'll have no problem getting witnesses. And you'll get to see a whole lot of really cool projects fly, from the simple to the absurd. If you aren't really hooked yet, you will be!

I think my drive exceeds 5 hours, coming from Virginia.

Our group is planning to get our projects in the air Friday morning. Then after recovering them we'll hang out with everyone.

Hopefully it won't get rained out. Typing as a thunderstorm rages outside...

 

[G_T]

Taking a brief break, and posting some more pics of the electronics cagebay. The board I've added holds an RRC3 and a Raven. The board is held on by three zip ties, forming a hinge. The board swings out for working on the boards. Two zip ties on the opposite side will anchor the board in place for flight.

The thread teeth of the allthread hold onto the zip ties rather well. There is no ability for the board to move vertically.

With this arrangement it is easy to swap electronics. Unzip one board, zip in another, and hook the wires back up.

The third picture shows how much additional room is available for more electronics and the switches.

Doing this sort of thing is why I went with a cagebay (and three rods). That of course is not an option with a break in the middle rocket, but it is feasible when everything deploys out the top.

BTW, this rocket is also a prototype potential arrangement for a Balls project for next year.

 

[G_T]

Nosecone ends up weighing 1pound 10.2oz unpainted. After the second round of epoxy gel graphite paint, the fit is snug. Finally! So it will get just a touch of sandpaper if it doesn't loosen up a tiny bit before the launch.

I had to unseal the bottom of the coupler. I'd left it sealed after cleaning and installing new injectors, a few years back. Yes, that is cleaned!

I had forgotten how long the preheater was to be. I'll be casting it tonight. It's sort of a single grain fast burn high Mg solid propellant. First picture is the precombustion chamber with the impinging injectors.

Actually, that's not the whole preheater. There will be Pyrodex, and BKNO3 also involved. It's amazing how much heat it takes to get chilled nitrous burning quickly.

Here I'm marking a phenolic 54mm liner for the cut length. Preheater is cast into that segment. It is held in place in the motor by four button head cap bolts, and the fuel grain butts up against it. When the preheater is burned out that forms the precombustion chamber.

 

[G_T]

The fuel grain, round 2. This is a mix of Paraffin and HTPB, with some metal and a catalyst. Why round 2? Well, round 1 cured rather soft. I could push my thumb into it. I tried to remove the fuel from the liner so I could use the liner again - ever tried to tear out chunks of half inch thick rubber bands? That's what it felt like. I gave up after getting a third of it out.

I've made hundreds of pounds of APCP, and for that matter cast this grain before with a very similar formula. No problems on all of those.

So on round 2 I was super careful, and also upped the cure point. It is slightly harder - probably from upping the cure point. It is still soft.

So I think either the curative or the R45 variant is contaminated. Either the R45 with moisture perhaps, or something diluted the curative, or the curative is suddenly showing age issues with loss of equivalent weight. I don't bother vacuum processing hybrid grains. Vacuum processing would remove water from a mix.

It did experience some grain swelling during cure, so I suspect the R45 variant is contaminated with moisture.

The grain might be flyable. In APCP I wouldn't do it - it would CATO. With a hybrid the softness will simply increase the regression rate. It is still more than strong enough mechanically for the G loads.

The risk is running out of fuel before running out of nitrous. The lack of hardness is not a problem per-se, just that I didn't design for it. Key point being with an N2O hybrid, the fuel is only a fraction of the reactants being burned (depending on fuel choice, ideal O:F can range from around 7:1 down to about 3.5:1, with an unenriched fuel), and one cannot create a sustained significant overpressurization with a pressure fed system. So CATO is not an issue. Burn-thru is an issue, since the fuel is also part of the insulation.

N2O hybrids are somewhat insensitive to small shifts in O:F ratio so a somewhat greater regression rate really doesn't hurt the performance - as long as it doesn't run out of fuel! The ISP might drop slightly, but the effective propellant mass fraction would increase slightly. These affect rocket performance in opposite directions.

Note, with polyurethane (HTPB+curative) and wax blends, one generally controls the regression rate via the ratio. More wax has a higher regression rate. This grain will behave as if it were higher in wax content.

@$@#$%!

 

[OzHybrid]

The fuel grain, round 2. This is a mix of Paraffin and HTPB, with some metal and a catalyst. Why round 2? Well, round 1 cured rather soft. I could push my thumb into it. I tried to remove the fuel from the liner so I could use the liner again - ever tried to tear out chunks of half inch thick rubber bands? That's what it felt like. I gave up after getting a third of it out.

I've made hundreds of pounds of APCP, and for that matter cast this grain before with a very similar formula. No problems on all of those.

So on round 2 I was super careful, and also upped the cure point. It is slightly harder - probably from upping the cure point. It is still soft.

So I think either the curative or the R45 variant is contaminated. Either the R45 with moisture perhaps, or something diluted the curative, or the curative is suddenly showing age issues with loss of equivalent weight. I don't bother vacuum processing hybrid grains. Vacuum processing would remove water from a mix.

It did experience some grain swelling during cure, so I suspect the R45 variant is contaminated with moisture.

The grain might be flyable. In APCP I wouldn't do it - it would CATO. With a hybrid the softness will simply increase the regression rate. It is still more than strong enough mechanically for the G loads.

The risk is running out of fuel before running out of nitrous. The lack of hardness is not a problem per-se, just that I didn't design for it. Key point being with an N2O hybrid, the fuel is only a fraction of the reactants being burned (depending on fuel choice, ideal O:F can range from around 7:1 down to about 3.5:1, with an unenriched fuel), and one cannot create a sustained significant overpressurization with a pressure fed system. So CATO is not an issue. Burn-thru is an issue, since the fuel is also part of the insulation.

N2O hybrids are somewhat insensitive to small shifts in O:F ratio so a somewhat greater regression rate really doesn't hurt the performance - as long as it doesn't run out of fuel! The ISP might drop slightly, but the effective propellant mass fraction would increase slightly. These effect rocket performance in opposite directions.

Note, with polyurethane (HTPB+curative) and wax blends, one generally controls the regression rate via the ratio. More wax has a higher regression rate. This grain will behave as if it were higher in wax content.

@$@#$%!

I did a test with a PVC grain years ago. 400N Thrust 30 sec burn. At 28 sec it started using the case as fuel, That was expensive.

 

[G_T]

I'll be casting another fuel grain tomorrow, different batch of R45 and curative. The preheater also came out soft. I don't consider that a problem for the preheater. It's a thin web fairly high exponent ineffficient fairly high temp APCP. It'll be gone in the first second of burn. It will probably add a little bit of kick off the pad from briefly upping the Density ISP and mass flow.

All this having to repeat work, and having to rework fiberglass tubes, has put me behind reasonable schedule. I need to get a lot done, quickly. It may look rocket-like, but there are many details left to be done.

I've painted the nosecone fixed bulkhead and removable bulkhead with water-based polyurethane to add some water protection.

 

[G_T]

A box of GSE needing testing, and some screw switches added to the rocket.

I was going to put another board there, but realized I don't have the time to deal with it. So it became a good place to stick the switches. The holes to access the screw switches will be fairly close to the hole needed to access the video controls. I'll climb the left side of the tower to access them. The video camera will point right towards the crowd. The nitrous vent is on the other side of the launch rail, pointed slightly towards the rear. It will all work out.

I soldered some 0.2" pitch PCB mount screw terminal blocks onto the screw switch boards.

 

[G_T]

Added USB interface board for RRC3. It's above the screw switches. I figured it would get use, so might as well just mount it in the electronics bay. I'll not connnect the ribbon cable to the RRC3 unless I'm using the USB board. The board is just there for convenience.

 

[G_T]

The string of bad luck continues. My old Raven is reporting incorrect battery voltage now. I no longer trust it. Raven 3 on the way...

Making replacement nosecone bulkhead to replace super sticky bulkhead from water based polyurethane that just this once decided not to cure...

And I can't find my 1/4" Kevlar!

PS - The reason I want a Raven in there is (1) has high rate accelerometer data that I need to analyze the hybrid motor, and (2) compact. The altimeters will be set for baro deployment as a precaution in case the hybrid goes into pogostick mode. Of course the RRC3 is baro-only. Unstable combustion will throw off acceleration based apogee detection since most accelerometers do not have a sample rate high enough to follow the vibrations accurately. This motor shouldn't produce those vibrations, but of course this is Rocketry... If it CAN go wrong, it WILL! If not now, eventually!

 

[G_T]

Fuel grain cast, round 3. This time, different batch of R45 and curative. The difference was visible, and via smell. Processing was notably different; much improved. This one should be good. Seems most likely the old batch of R45 was contaminated with some water.

I've replaced the tracker bay bulkhead, this time finished with superglue. It's fast, and reinforces the plywood. Beware the fumes!

Here's the preheater. There's more to it to come, but think of it as a very hot burning APCP 54mm grain in a phenolic liner. Previously the liner was doubled, but it was overkill. With a single liner there is more room for preheater propellant. That's likely a good thing. Anyway it will all be gone by the end of the first second of combustion pressurization. Afterwards this becomes the liner for the precombustion chamber.

 

[G_T]

I've not had luck getting 1/4" Kevlar from any of the locals. So what I have is 3/8", that actually measures a little smaller than that. Here is roughly 18' mock packed into the cup intended to hold the main's line. It's just taped bundles of lines squished on top of each other. It keeps the arrangement neat so the line cannot snag on itself, assuring an orderly deployment.

I had wanted to use 1/4" and use a fair bit more line.

For those concerned about the masking tape - this is pretty pathetic masking tape. It has enough stickyness to stay put, but the paper itself is quite flimsy. I have to be gentle removing it from the spool and even then sometimes it tears out diagonally. It seems to be what passes for masking tape now. It is garbage compared to the old stuff. However, for this sort of application it is superior. That tape can't resist the weight of the booster section. It will tear and deploy line when the main starts taking up the load.

18' is actually much more line than needed for this sort of deployment. Exception being if the rocket lands in a tree. Then I'd rather have hundreds of feet of line! Unfortunately a tree landing when flying in the east is always a distinct possibility.

This type of stowage does not offer very much shock absorption though it does offer a little. With the type of freebag everything out the top deployment I'm using, there is no possibility of the main being deployed at exceptional velocity. It is the drogue which pulls the main out, and only once the tether has released. The tether can support much more force than the drogue could produce before it shreds. If the drogue shreds, the result may still produce enough drag - along with the nosecone - to pull the dbag off the main. But it won't be as rapid or clean a deployment. The rocket would be coming down in drogueless fashion. But it would take a substantial horizontal velocity at drogue deployment to end up in this scenario. Since there is no break in the middle, the main isn't coming out until main deployment altitude and can't come out if the drogue hasn't already been deployed. So in a nutshell, that's why I'm not concerned about stowing for shock loading.

The drogue of course could easily generate shock loads if the rocket deploys at the wrong time or ends up with notable horizontal velocity at altitude. I'll be using a technique for stowing the drogue line for shock absorption. I'll show that later.

I'm using this 3/8" line because that's what I have. It is severe overkill. I may elect to re-rig the deployment at the field with 1/4" line if I can find some! The possibility of trees is a powerful motivator!

I do have some 1/8" Amsteel Blue line that I may consider using for the drogue. Amsteel Blue is much stronger than Kevlar for the same diameter, and is light enough it floats on water. It isn't affected by moisture. It has two issues to consider. First doesn't apply to me - max temp is about 300F. It melts around 350F going by memory, so a lot like nylon line in that regard. It would have to be protected from pyro events. But this rocket is pyroless. Second issue is although it is very strong, it doesn't do as well with shock loading which is what deployment can generate. However it is so much stronger the resulting size and weight would still be smaller than the equivalent using Kevlar.

I know the rocketry crowd doesn't seem to be familiar with it, but it is an alternative to tubular nylon and tubular Kevlar that is worth considering. Unfortunately I have only one 13' piece and one 7' piece. Usually one doesn't tie knots in it, and one certainly doesn't stitch up a loop! One example of making a fixed loop on one end of Amsteel Blue and similar round tubular lines:

 

[G_T]

Showing packing the drogue into the base of the nosecone.

An important point is the length of the short line from the drogue to the nosecone. It needs to have sufficient length that a pull on the drogue line gets the chute out. Otherwise there is a small chance it may not come out. I'm using Amsteel with some figure-8 knots for that line. It is much stronger than it looks.

When packing the drogue - or any chute for that matter - start by detangling all the lines. Then flake out the shrouds.

Here I have half on one side and half on the other. The lines are gathered into two bundles (or three for a toroidal chute). Then the canopy is folded to bring all the lines into a single gathering. Because of the width of the chute, it is folded in half again. Now it is narrow enough to zig-zag fold, and stuff into the nosecone shoulder.

Neatness, forethought, and consistency are needed for reliable recovery. It helps to have no free space, so things stay exactly the way you packed them. If stuff can move, it will, and you have to plan for that. Or plan so that it can't happen. I prefer the latter!

 

[G_T]

Packing the main in its deployment bag.

This deployment bag has a nylon flat line inside that is intended to attach to the top of the chute. It is optional, an option I'm not using. So I cut that line out.

Start out by detangling the lines, then flaking the gores. On a toroidal chute that will result in two bundles of lines plus a central line.

Fold the left and right side panels in half so all the lines now run parallel in the middle.

The chute is wider than the deployment bag. So, fold the left and right panels in again so they meet in the middle. If it were still too wide, one would continue doing this until it would fit in the bag opening. The panels are never crossed over each other. We are creating a compact arrangement, but one where there is little holding it together so the chute comes open easily.

Now grab the top end of the chute and stuff it all the way into the bag. Grab just outside the bag, and stuff that part in, shoving it all to the top of the bag. Repeat until all the canopy is in the bag.

This packing is a bit compressed, so left to its own devices, the canopy will start coming out of the bag. Hold the bag up and it would fall out! You don't want it jammed in, just stuffed in.

The lines are stowed under the elastic bands.

Lastly, the flap is folded over the lines to cover and protect them - and to help keep the chute in the bag.

Chances are the bag isn't quite round at this point. Grab it in your hands and squish it into a round shape so it will slide easily in the tube.

 

[G_T]

Back to the cup that is stowing the main line. I've cut off the excess line, and terminated it in a figure-8 knot to form a loop.

That loop is connected to the quick link of the main chute. The dbag sits right on top of the cup, with the opening of the bag (covered by the flap) oriented downward. When the main is deployed, the bag is pulled up and out of the rocket. The shroud lines deploy first, then the canopy comes out. The chute is strung out ready to open. That's part of the beauty of a deployment bag. It helps the chute open in a predictable repeatable fashion.

The Kevlar drogue line is terminated by a Boline or Bowline knot. It is attached to the tether release eyebolt. The top strap of the deployment bag (where it is pulled from) is tied down over the flap by running a Figure-8 knot through the loop at the end of the strap. This pulls the bag down solidly on the top of the main line cup. When the tether bolt is released the drogue will pull the dbag up and out of the rocket via the drogue line, starting the main deployment process.

To make stowage neater and provide some shock absorption I've braided the line into a more compact form. It releases smoothly with a steady tug or a jerk. But the overall length is perhaps a fifth of the stretched out length, so it packs much more easily. When I pack it for real I'll leave a little more unbraided most likely, to increase the nosecone separation before the braid starts slowing it down.

All this of course slides into the rocket body tube, and is squished down. I will organize the drogue lines neatly in the process.

In practice, the drogue connection to the drogue line won't be hooked up until the body tube is slid over everything. Then the drogue is attached and the nosecone pinned.

Hopefully I've made the reasons and methods clear for this type of deployment. If I haven't, feel free to ask!

 

[G_T]

Disconnect the drogue, slide the tube over the top of everything, reconnect the drogue, then pin the nosecone.

Next major step for me is to drill all the holes in the fiberglass body tube. There will be the tank vent hole and drain hole, the video lens hole, the hole to access the video controls, the two screw switch holes for arming/disarming the electronics, the nosecone shear pin holes, and the holes for the bolts which retain the body tube. I'm not looking forward to all this fiberglass drilling.

 

[G_T]

Final length is going to be right around 101.4".

It's nice one can see the innards. That will make placing the first drilled holes much easier. I just have to make sure not to place any holes in the direction that will be towards the launch rail. That would not be good!

It looks cool enough it almost makes me want to skip any paint. But the current color scheme is a bit too close to woods camo.

I don't think I mentioned it in this thread before, but I'm considering a modest 2 stage flight for Balls next year. The electronics bays and recovery organization in this rocket are a prototype test, though the rendering won't be in fiberglass and the sustainer overall will be a bit larger. Custom nosecone is on the way, and I have the tubing. I've yet to decide between a modest 2 stage and a larger but not all that big single stage. EX motors no matter what, of course. Anyway, it's a subject for another thread!

 

[G_T]

Deployability Testing

When I assembled everything I left the tether pin in place but not locked down. In this state a fairly light pull is sufficient to pull the pin. I also left everything compressed in the rocket overnight.

I removed the nosecone, and tugged on the drogue line, to simulate drogue forces for deploying the main.

Nothing happened. So I pulled harder. Nothing happened. Even pulling with a fair bit of force it would not deploy.

I pulled harder than the force that would be applied by the weight of the rocket, and the pin popped free. But even then the dbag took too much force to slide up the tube. The drogue might not get the main deployed in a short time.

So this packing job is too tight. Or more accurately, these deployment bags are slightly oversize for a 3" airframe so one needs to avoid packing them tight. The chute deploys easily from the 6" bag once it is out of the airframe.

I'll need to move from the 6" deployment bag up to the 9", the next longer one. I have it on hand, since I didn't know for sure that the 6" would work. The packing job will then be fairly loose so the bag won't be under the same pressure. Presumably that will make it slide out the airframe tubing much more easily.

And that is what testing is for!

It's also why I haven't yet drilled any of the holes in the airframe tubing.

If it turns out the tether pin won't release on a slightly off-axis pull, then I may have a design problem to solve. Or I might just need to use a little grease. Probably the latter.

 

[G_T]

I spent a while fiddling with and studying the tether. It wasn't originally designed for bulkhead mounting the way I'm doing it. It looks to me that with off-axis pull, it has a pretty fair probability of jamming. With the deployment bag in the tube, the pull will always be off-axis. I'll not fly it like it is without compensating for it or fixing it for my application.

I've fiddled with the former, and decided to do the latter. I'm about a third through machining an extension to go on top of the body on the pin side to extend the cylinder the pin rides in. It doesn't have to be extended very far before jamming should become impossible.

Used as originally designed, off-axis pull is fairly improbable. It's likely only an issue for how I'm mis-using the unit.

The Raven to replace the one that no longer works isn't here yet. Overnight shipping; should have been here yesterday. Now it is scheduled for tomorrow by 9pm. Another delay.

Paint is no longer an option.

 

[G_T]

Here's the fuel grain for this flight. I should have upped the curative ratio a couple points due to age of the curative, but otherwise this is a normal grain. It isn't sticky super soft like the previous two attempts. It's flyable.

I've machined a press fit cap to go on the tether that solves the off-axis jamming issue. That 5C Taig manual microlathe keeps coming in handy for one-off odd jobs! I have a nice variable speed motor and control box for it, but haven't gotten around to putting it on. The various chucks are mounted on spindles, in this case 17mm, that fits nicely into a 17mm 5C collet.

I had to keep putting the part back on the lathe to take off a little bit at a time, until the tether functioned normally.

The box that holds my replacement Raven arrived.

I'm moving up to the 9" deployment bag so the fit isn't too tight in the tube. The rocket will be a little longer.

Problems are getting solved. Now it's just a race with time. I don't think I have any remaining showstoppers that are out of my control.

 

[G_T]

The electronics bay is prepped, except I need to charge the video battery and stick in the memory card. I'll post pictures tomorrow when I have some light to take them.

I've been doing this backwards. I haven't drilled the holes in the airframe tubing yet.

This design has some unusual features. The airframe tubing takes very little load at any point in the flight, compared to normal rocket designs. It holds the nosecone forward under acceleration and drag loads, however there is going to be a little compression when it is shear pinned. So the tubing doesn't take all the load. Plus the nosecone only weights ROM 2# and the max acceleration is perhaps 9g. So, 18# from acceleration and of course somewhat more added to that from drag. But it isn't a large number.

The tubing itself doesn't weigh all that much. It is a 4' piece of 3" filament wound fiberglass.

So from acceleration and drag, probably not even 50# force on the retention.

For deployment, there will be 2x 2-56 shear pins on the nosecone. Ballpark ROM 50# to break them.

Since all the heavy loads are transmitted from the motor's case edge and forward bulkhead through the ventbay and allthread rods to the eyebolts, the tubing really is mostly just holding things in a straight line.

So in a nutshell, very little retention is required for the airframe tubing, even after a healthy margin is applied. I'm very tempted at this point to just use some push pins.

They are Nylon I believe... Nylon has only fair compatibility with nitrous oxide exposure. Exposure in this case will be short term, and the pins can be considered single use. Hmmmm. That would save some time and effort! I was going to use bolts.

https://www.calpaclab.com/nylon-chemical-compatibility-chart/

 

[G_T]

Electronics bay.

Since the Peregrine CO2 system uses a smidgen of black powder, I decided to put some nomex around where the wires exit. If it exhausts out the back, which it shouldn't but could, this will catch sparks and at least some of the smoke.

Having the boards able to hinge out then be tied down was convenient.

This isn't wired for high-G flights. For high G, there should be essentially no lengths of unsupported wires. Here the wires are mostly supported and it will take much more of a G loading than this rocket should be capable of producing short of a CATO or cruise missile deployment.

BTW, new final length is 104.4", due to the longer deployment bag. I've marked up the tube; time for drilling!

PS - Turns out I have the wrong length of push pin rivets. I figured that out AFTER I'd already drilled the pilot holes for the rivets! Of course... Well, a few extra little holes in the ventbay is probably a good thing. So it's getting three 10-24 stainless button head screws into round base T nuts. Prep for those - scuffing up with diamond wheel, and rough sanding the fiberglass. Rocketpoxy + milled fiberglass, used liberally. I waxed the threads with Partall #2 mold release wax. They're curing now. After dinner its back to more drilling.

PPS - Still holes left to drill. I had to wash out all the fiberglass dust from inside the tube so I can check the video lens hole position before I make it full size, and verify locations for screw switches. Tube is drying...

 

[G_T]

For hole drilling, I just have the nosecone shear pin holes left, and enlarging/centering the video port, round 3. Grinding out the fiberglass makes so much dust that I have to wash the tube out each test. And then of course it needs to dry.

For the holes that are larger than tiny, I've beveled the bottom edge to prevent the hole from becoming a ram air scoop. It might not matter as much with the average rocket with only little holes and not many of them, but this thing is swiss cheeze with large holes for the vent and the video port. So those two holes, and the lesser holes for screw switches and video control access got the NASA standard bottom bevel.

---

I've coated the core of the preheater with a thin layer of BKNO3, and will be putting some BKNO3 pellets by the forward bulkhead. This is computed to pressurize the combustion chamber to 300psi. I expect it is only a ROM estimate and I have no good way to test that right now. The proof will be when the button is pushed.

The intent is to greatly reduce the pressure drop across the injectors when the fill line or other injector plugs burns through. If I get that 300psi, the initial oxidizer surge will be cut down by close to 70%. That reduces the refrigeration from the expansion of the initial oxidizer stream by much more than that, since there is less of a stream and it is expanding into 300psi gas rather than into roughly sea level atmospheric pressure. So instead of needing a half megawatt roughly to reliably start this thing instant-on, it becomes a much more manageable fraction of that power.

Computation is in the Research forum THRP-1 thread for those who are interested. Chilled nitrous systems are no joke to get lit reliably. Even only moderate cooling such as this one utilizes creates a requirement for much more agressive startup methods. It turns out to be the most difficult part of this project, a difficulty I didn't recognize until the first test smacked me in the face with that little detail I'd overlooked! Rocketry has a way of doing that!

Short version, that's the reason for the reformulated preheater with a bit more propellant than the previous burn, and moving to BKN03 to get it all going.

I ran out of BKN03; need a few more grams to make the pellet portion. It's in process.

Video battery is charged, and firmware updated. Lots more electronics stuff to do though. Have put a little blue loctite on the threads of the screws for the screw switches. They were way too loose for my liking. I'll re-install the screws and test when the loctite has dried.

Now back to holes in airframe tube...

Must remember to mount the conformal rail guides!

 

[G_T]

Airframe tube is done. Rail guide on it is curing. Tracker battery charging. About to install the conformal rail guide on the fincan.

I don't have aluminum prep on hand for making the aluminum surface strongly bondable. Rough sanding and cleaning aluminum just results in a mediocre mechanical bond. Aluminum forms an essentially impervious coating of aluminum oxide in milliseconds of actual aluminum exposure to air.

Slightly better than sanding and cleaning is to clean, then sand with the aluminum coated in the epoxy you intend to use. Keep the surface wet. Clean the sanding gunk mostly by diluting with fresh epoxy. Don't clamp hard! Epoxy needs at least a tiny glue bond thickness or you get what is called a dry joint and a poor bond. I'm not clamping at all. I just pushed the part around a little to force the glue layer to be thin but not very thin, then cleaned up the excess.

Again, the right way is to use an aluminum prep solution. Or at least one of the right ways. But this wet epoxy sanding method works better than nothing.

https://www.aircraftspruce.com/menus/me/metalprep.html

 

[rocket_troy]

Chilled nitrous systems are no joke to get lit reliably.

As I've mentioned in Arocket previously, chilled N2O is like bashing your head against multiple brick walls: your injector can likely go from mutiphase to single (condensed) making both ignition and steady combustion significantly more challenging and energy intensive. What's more, unless you're providing a 2nd pressurant, your tank pressure & consequently Pc will be much lower (with the lower vapour pressure) also providing ignition and steady combustion challenges with the N2O molecule.
In many respects, it's defeating the primary virtues of N2O hybrids.

TP

 

[G_T]

On the flip side, by establishing a known temperature and pressure in the flight tank, one can have more consistent performance from a hybrid, and greater total impulse in the same volume. Yep, the tradeoffs are extensive. Anyway this is just a flight to gather data from this motor, for designing round 2. I'm considering oxygen repressurization to a second higher set pressure. The boiloff in the second stage process will be primarily oxygen. I just need a higher pressure calibrated oxygen certified cryogenic pressure relief valve, and a servo or solinoid controlled ball valve. Still just in the early stages of considering it.

I think hybrids are a fun deviation from solids. However I do wish we were allowed to fly liquids. And I do know that the resulting potential CATOs can be more extreme.

 

[rocket_troy]

On the flip side, by establishing a known temperature and pressure in the flight tank, one can have more consistent performance from a hybrid, and greater total impulse in the same volume.

You can also achieve a pretty precise temperature and pressure in a flight tank by implementing either a PRV vent or an actively controlled (throttled or bang-banged) vent. I would argue - based on the often encountered issues with chilled N2O these options are easier, but then again, that's a biased opinion as I have industrial scale subtractive capability at home.
Yes, you certainly win with *potential* impulse-density by chilling. The question is though, is it easier just making a larger tank and perhaps rocket to *only* deal with the other myriad of potential hiccups with flying N2O hybrids without having to also deal with ignition and combustion challenges. I guess everyone has different goals and pet challenges they want to conquer - which is a good thing!

I think hybrids are a fun deviation from solids. However I do wish we were allowed to fly liquids. And I do know that the resulting potential CATOs can be more extreme.

So, we (as in Tripoli) used to be allowed to fly liquids subject to being a member of good standing & appropriate level of certification and having the system being subject to board approval. If that's changed, I'm curious to know when that happened?

TP

 

[G_T]

Interesting. I thought we were limited to 1127, which allows methyl and ethyl alcohol tribrid rockets but that's it. I thought liquids essentially were class 3 no matter what, get permission to launch somewhere, and get your own waiver, but not under TRA.

 

[G_T]

Catching up on some pictures here.

Fins on combustion chamber, with rail guide curing. Rookie move here; I should have placed the rail guide perhaps an inch higher to avoid heat soak from the nozzle. However if this motor works correctly there really shouldn't be that much heat soak. It is employing wax film cooling for the nozzle. A decent bit of heat should be carried off from boiling off the wax. This occurs because of the high wax content of the fuel, and the lack of a post-combustion chamber. In the static test I approached the motor right after the burn and the nozzle end was only about the temperature one would find after a solid had finished a burn 10 minutes ago. Qualitative, not quantitative judgement based on experience. So I think the cooling is working pretty well.

The second picture shows the preheater which is essentially a Mg rich AP grain with a fair bit of KP subbed against AP. The inner surface is painted with a layer of BKNO3, and three mounds of BKNO3 are placed at the injector end to sit between the injectors.

This motor uses three impinging injectors. For the static test I did the usual Contrail-type setup with a tube plugged into one injector looped out the motor and plugged into another, with the remaining injector getting the actual fill line. Instead, I am plugging two of the injectors with sealed end tubing sections. It's a much less cluttered arrangement. I have other reasons for doing it this way, but they are somewhat speculative. Mostly if a loop burns through, the bottom part is still fed from the fligh chamber and sprays cold nitrous under pressure on the preheater inhibiting it's burn. Here there's no loop. I'm hoping it improves startup.

These plugged tube sections are formed by heating an end to near melting point and squishing it together to close. Then raising the end to just above melting point and forming it into a thicker wall smooth end. Smooth is a relative term here... this is not easy to do. In the process it will bubble some so one has to let it cool some then surface flash to pop and flow over the bubbles. Inspect with loupe for any pinholes and close them off as well. 4 attempts, 2 successes. Beware fumes. And, dripping hot plastic!

As the ends of those plugged tubes cool, the outside cools first. It produces a crinkle finish and a stressed part. So as the crinkles form, it is helpful to briefly flash heat the surface to flow them back out. It's a timing and practice thing. Note I made these parts with a micro butane torch as the heat source. That worked a lot better than a heat gun. Expect to light the end a few times. When it is actually burning like a candle, one can rotate it around and flow plastic to where it is needed.

The resultant ends are of course stressed. That's bad since this tubing is rated for only about 300psi and we're going to hit it with up to around 650psi along with thermal stresses. So the ends are annealed in boiling water to make them less brittle.

I've wrapped the ends with layers of masking tape, to try to make it so the end next to the injector is where it will burn through. I'm showing two of the three injectors plugged with these tubes, and then the preheater installed and locked in with four little bolts with washers. That finishes the precombustion chamber end of the coupler.

 

[G_T]

THRP-1 flew Friday afternoon at URRF. I want to thank the crew there for being very helpful with this unusual project. It was also nice to chat with some of the people on this forum - John Derimiggio, Paul Gagnon, Don Livingston, and many others whose names I unfortunately don't recall offhand. Also a big thanks to friends Al Anderson, Jerry O'Sullivan, Mitch Guess, Ivan Galysh, Ben Russell and Bill Schworer for their help all along the way. Thanks guys!

I was mentally fried before I got there and didn't take the pictures I should have taken. Others took pictures and video, perhaps some of that can be posted in this thread.

I have not disassembled the rocket beyond what was necessary for transport, and have not grabbed any of the data yet. Some things worked well, and some did not. Quick look report in my next post. But it has flown and been recovered. That much is good!

 

[jderimig]

THRP-1 flew Friday afternoon at URRF. I want to thank the crew there for being very helpful with this unusual project. It was also nice to chat with some of the people on this forum - John Deremigio, Paul Gagnon, Don Livingston, and many others whose names I unfortunately don't recall offhand. Also a big thanks to friends Al Anderson, Jerry O'Sullivan, Mitch Guess, Ivan Galysh, Ben Russell and Bill Schworer for their help all along the way. Thanks guys!

I was mentally fried before I got there and didn't take the pictures I should have taken. Others took pictures and video, perhaps some of that can be posted in this thread.

I have not disassembled the rocket beyond what was necessary for transport, and have not grabbed any of the data yet. Some things worked well, and some did not. Quick look report in my next post. But it has flown and been recovered. That much is good!

Gerald, that was a beautiful flight and very nice burn. I was very impressed!