THRP-1

[G_T]

I've been having a bit of fun with the body tube. It has a slight internal taper. When I go to slide it on, it essentially runs out of travel at least 5" short of being all the way to where I need it to be. It's snug for the whole way, just really snug towards the end. I think this is an artifact of my tank's OD likely being slightly greater than the OD of a typical 3" motor. It is DOM tubing, but raw on the outside.

With a running start, it's quite consistent where it stops.

That's a problem I didn't anticipate. There's always room for another problem, right?

The easiest solution might just be to cut the tube about 6" shorter and live with it being possibly a 2 person job to put on and align with the bolt holes.

Gerald

 

[StreuB1]

I would strongly disagree with that.

TP

I guess context on my part would be important and not just sling statements around willy nilly.

Metals in a small hybrid for the effort of performance in a flight weight rocket, are generally worthless without a catalyst.

Where "small" means less than 1000lbf.

It has been my experience that any gains that are possibly realized by any metals being added are rarely realized and it is easier to yield gains through grain and injector design. Casting a straight HTPB grain, without having to add literally anything, and achieve a smooth running system which yields over 220 seconds [measured]. In my opinion, is superior to adding metals and achieving lower impulse. There is also no density impulse gain, from my experience. Where in HRP and smaller rockets, density impulse reigns supreme.

 

[StreuB1]

Caveat, ULTRA fine metals which border on pyrophoric are a bit of a different animal.

 

[G_T]

The Ti I am using is, shall we say, more than a wee bit on the fine side. I handle with care... I only came across a small quantity of it once, and snagged while the snagging was good. It has a lot of similarity to the 16 micron Mg I've played with a fair bit. Not sure of the actual particle size of the Ti but it could easily be smaller. Avoidance of moisture is a good thing.

On the tube fit problem, turns out the limiting piece is the upper fiberglass ring at the top of the electronics bay. With that end taken off, the tube will go down full length - snugly, but it will do so.

Gerald

PS - Sanded on that piece of fiberglass tubing for a bit. Now the limit is about an inch farther. Progress...

 

[RalfB]

Hello Gerald, Thank you for the extensive information. I've already taken the right direction in my development. I had studied many master theses on hybrid engines over the last two lockdowns and realized everything that was plausible and technically feasible. Also the afterburner chamber. On the test stand we were able to prove that a grain with an afterburner burns much more evenly and achieves a higher thrust level. In contrast, the grain with 20% titanium had a significantly lower performance. However, the grain was also relatively coarse. I will take up your suggestion with the manganese oxide and do a comparison measurement at one of the next test stand campaigns. My biggest concern at the moment is to implement ventilation through the combustion chamber in order to stabilize the tank pressure during combustion. Many thanks and greetings Ralph

 

[G_T]

The MnO wasn't my suggestion but another poster's. I did mention MnO2. There are some other catalysts worth looking at. You can find them looking for decomposition catalysts for N2O and check what temperature they need and if you can find any other info. If your catalyst of choice is good with N2O, good with your metal of choice if you use any, and bonus if it is also an opacifier, give it a shot.

Also, you may not need a catalyst.

 

[RalfB]

Das mit dem MnO und MnO2 war mein Fehler mit der Übersetzung, sorry. MnO2, in Deutschland Braunstein genannt, wurde in Deutschland bereits zu einer unziemlichen Zeit als Katalysator in Walter-Motoren verwendet. Aber ich stimme dir zu, dass das Getreidedesign wichtiger ist, besonders bei den kleinen Hobbyhybriden.

 

[G_T]

Moving forward; working on the nosecone.

The thread for the screw-on tip was a little off center. I bent it over to center the metal tip, then filled with RocketPoxy to stabilize the new position. Now the tip is centered. Particularly since this rocket will go over mach, I wanted the tip centered.

There is going to be a small electronics bay in the nosecone to hold the tracker. The picture should make it pretty obvious how this will work. More updates later.

And yep, still sanding that coupler section at the top of the cagebay. It almost slides far enough into the body tube!

Gerald

 

[G_T]

Epoxy is curing for installing the tube in the nosecone. I mixed up extra, and dribbled it down over the back of the bolt at the nosecone tip. I figure I want more weight anyway... Then I put blobs and rings of epoxy thickly on the inner tube above each area that is getting glued. I quickly put the assembly into the nosecone, and let gravity do its thing. Then I twisted it around a little to work epoxy into the joints.

Temporarily I've put the rear ring over the threaded studs to maintain centering. It's unnecessary; just a precaution. It isn't going anywhere.

Now it needs to sit point down for a while.

Yes, this nosecone is being way overbuilt. That's the point actually. I want more weight in this rocket to lower the altitude, within reason. A heavy nosecone helps do that. I am making sure to use materials that shouldn't interfere with GPS reception or RF transmission from the tracker.

After dinner, it will be back to the final sanding of that annoying oversized coupler section!

Gerald

PS - Coupler sanding complete! It fits!

 

[G_T]

It's a flying toothpick. I know the next time I make an EX hybrid, it will not have as high a length to diameter ratio. The original design years ago used coupler tubing for the body tube above the motor, a trimmed down nosecone, different fins, and was perhaps 8" shorter. It was a more compact, cooler design. But with the reduction in the waivers in the east, the design became unflyable. So this is the porky version. More drag and a lot more weight.

Tomorrow will be some combination of foaming the nosecone, casting the fuel grain, and testing the GSE. Hopefully 2 out of 3. The launch countdown is running.

Gerald

 

[G_T]

Working on the nosecone...

I made the first pour of expanding 2 part 2 pound density polyurethane foam with the bulkhead not installed. That makes it much easier to pour in the foam slurry quickly. I had rasped out three sections of the little forward centering ring before epoxying in center tube. That left channels for the foam to get up to the front of the 38mm tracker bay tube.

I used 10ml each of part A and part B. That turned out to be about perfect, bringing the foam level up near the bulkhead location but still safely below. It did have me a bit nervous for a few minutes though! If one goes a little over, it is a running battle scooping excess foam out before it hardens. Better to go short. One can do more than one pour.

Immediately upon pouring the foam. I smacked the nosecone tip on my carpet several times to settle the sludge down around the base of the 38mm tube.

Once the foam had hardened, I went ahead and epoxied the bulkhead in place. I put rings of epoxy around the 38mm tube and inside the nosecone, then pushed the bulkhead in place. I kept a witness mark on the underside of the bulkhead to indicate orientation. Otherwise it could go on four different ways. The way I gang-drilled it fits the best.

I used the bay cover as a press to squish it in place, then removed the cover. I did carefully wipe up epoxy left behind before putting on the cover, of course!

The nosecone is sitting point-up so the excess epoxy on the inside flows back down onto the bulkhead forming an internal fillet.

That extra hole in the bulkhead is for finishing the foam filling job.

I added a couple pictures of the cover.

Gerald

 

[G_T]

Fuel grain is cast and is curing. I forgot to take some pictures. Sorry!

Foaming the nosecone is complete.

I took a look at the volume remaining to be filled, and compared it to the volume achieved in the first fill, paying some attention to the conical volume equations. It looked to me that 6ml each A and B should yield about the righ amount of foam.

I didn't trim it. It filled exactly. Sometimes it is better to be lucky than to be good!

This foam completes anchoring the eyebolts being used as threaded studs.

Next issue is the coupler fit to the airframe. It is wobbly loose. I'm thinking about adding a layer of light glass over the shoulder before installing it into the nosecone.

Then there is the nosecone fit to the airframe. The nosecone has a smaller OD than the tubing. I'll have to bevel the upper edge of the tubing to get rid of the ledge.

Mix and mismatch... I think it is doubling the work of this build.

Gerald

 

[G_T]

Enlarging the nosecone coupler... via adding a layer of fiberglass.

I apologize for the length of this post. It might be worth reading since I put some technique info here, sort of a tutorial.

Unless you are a composites worker you might not know all this stuff, and it is useful.

I've taken the coupler and using some relatively coarse sandpaper have block sanded the ends of the coupler fairly square. The cuts weren't square to start with.

I also scuffed the surface with some sandpaper to knock down the semi-gloss for better adhesion.

The fiberglass I'm using is 1.1oz or 1.2oz (I forget which) E-glass. It is a plain weave fabric like the readily available 3/4oz glass, same thickness, but a tighter weave. Once you've used this stuff you'll never use the 3/4oz stuff again. The layup weight ends up being nearly identical, just with more glass in it and less epoxy, therefore stronger. It used to be occasionally available. I have no idea now. I don't have much left. At least I don't think so. I'd have to go through my roll remainders to find out. Not doing it!

Note, E glass is electrical insulation grade. S glass is structural grade. S is quite a bit superior to E for layup work, but costs a fair bit more and is much harder to get. For most usage, E glass is fine. I'm using it here as a filler rather than for structural reinforcing. Hence E glass.

I'm not covering the entire coupler. The coupler inserts into the base of the nosecone until it wedges in place, right where it meets the bulkhead. But it does have a slight gap at the base of the nosecone. So I'm starting the fabric where it will have about a quarter inch of insertion, to act as a centering aid when gluing in the coupler.

I'm using MGS L285 epoxy with the fast hardener. Fast is a relative term. This is a laminating resin, and I find plenty of working time for most things using the fast hardener. Only for larger jobs do I blend fast and slow hardeners.

Weigh the two parts accurately! It matters. Mix well, about twice as long as you think you need to. For best work, then transfer the epoxy to a fresh mixing vessel and use a fresh mixing implement, and mix again. I was lazy here and only swapped to a fresh mixing implement.

I painted the appropriate area of the coupler with a thin layer of epoxy using an acid brush. Operative word here is thin. Wetted, but no more than that. Close to the minimum you can get on there while still providing coverage. Essentially, spread the epoxy as far as you can with the acid brush before picking up another drop of epoxy on the brush.

Note, if the epoxy doesn't wet the surface out, then insufficient prep was done. You might have mold release on there, or oils from hands or contamination, or might not have scuffed up the surface enough with sandpaper beforehand. If you ever hit an issue with wetout, it is better to clean off the epoxy and fix the prep work rather than just continuing. Not wetting the surface out looks like the epoxy wants to bead up in little droplets rather than going on like a thin layer of paint.

Don't use any "rubbing" alcohol in surface prep. It has oils added to prevent drying out the skin. Oil is the enemy of epoxy.

In some stubborn cases you might be trying to apply epoxy over a surface that is just not compatible with epoxy. If you hit that, a trick you might try is to take a torch and reasonably quickly pass the reactive part of the flame over the whole surface. You are not trying to burn or melt the part, or even heat it up. The chemically reactive part of the flame will break up the outer molecular surface leaving it chemically reactive in many cases. That can give you about 15 minutes to get epoxy on the surface.

Of course I don't need to do that here - just passing on something you can try if you hit certain classes of problems.

So here's the deal with laying up fabric. You want to be putting layer(s) of dry fabric one at a time on top of a damp surface. That way the fabric gets wet from the bottom not the top, and therefore CANNOT trap any air bubbles. Putting down dry fabric and applying epoxy from the top GUARANTEES air bubbles. There is a big difference in the quality of the result. For that matter, there is a big difference in the effort involved. Putting dry fabric on a damp surface is easy. It sticks. And you are handling dry fabric with your gloves. It isn't messy work.

Once the fabric is down, take your fairly dry brush or roller or scraper and sweep along the fiber direction, center to edge, to seat the fabric down everywhere. In the process the fabric will be taking up epoxy from the bottom. If it is glass fabric, it should end up fairly transparent.

Then put a layer of paper towel over the surface and brush, pat, roll, ... method of choice ... to pull up any excess epoxy from the fabric. Throw the paper towel away. If the paper towel came out wet rather than lightly damp, you need to do it again, and you started out using too much epoxy!

If the fabric is a little too dry, then congratulations, you are doing it right! NOW you add back just a little epoxy. But only if you need to.

Now the layup should look uniform.

If you need to add another layer, then put the dry fabric over the wet surface and repeat the procedures again. I don't need another layer for this application.

Don't trim off excess fabric yet. Let the epoxy get to the "green" state, where it is sort of rubbery but not wet. It will cut easily in that state with a clean new blade. Angle the blade such that it pushes the fabric edge down, NOT up, as you sweep the blade around the edge of the coupler. You should get a nice clean trim without moving or lifting the fabric. If the fabric moves or lifts, your blade was dull, your blade was sticky, your technique was poor, or you did it too soon.

I'm leaving the coupler under a halogen bulb now for a little bit of temperature elevation, perhaps 120F. This speeds the cure and promotes a slightly harder cure. I guarantee you don't want a soft cure when you test fit the result. If it has any ability to tack, with that much surface area, it can easily become a permanent fixture!

With MGS fast I can easily handle the part in about four hours.

Note how little epoxy I used. 0.9g. That includes losses due to what stayed in the acid brush I was using, and what I took up in the towels I used. Most people use way way more epoxy for layup than they should. You are not trying to get epoxy with some fabric in it. You are trying to get fabric with just enough epoxy there to fill the pores. No more than that. It is very little volume of epoxy.

I have test fit the coupler, risking glue-in. It was close... I had it stuck for a moment. I expected that, but I wanted to know. If it had been loose, adding a second layer while it is still a bit green doesn't require surface prep. You'll still get a chemical bond.

This coupler has a taper to it. It will go into the tube all but the last inch. So I'll end up lightly sanding a counter-taper into the end once it is fully cured.

I didn't care about surface finish, or ultimate compaction, so no plastic film or vacuum or etc. I didn't even try to keep dust away from it, since I knew it was going to get lightly sanded.

Don't even think about trying this sort of thing with an epoxy glue. You need to use a laminating resin. You can't effectively do thin layup work without it. That's why it was invented. But even more import is you need the hard surface produced with laminating resin. Otherwise your coupler will probably glue itself in place. You really don't want that.

And don't just thin some random epoxy with alcohol as a substitute. Even just 5% added alcohol costs half the strength of the epoxy and also acts a bit like a plasticiser, resulting in a rubbery layup.

Gerald

 

[G_T]

I prefer deploying everything out the nosecone, since the rocket body is stronger. I also consider it more reliable. If the drogue and main parachutes are sized such that the nosecone descent rate is slightly slower than the rocket body descent rate, it is extremely reliable.

I've included a quick picture of the recovery concept, for those who have not used deployment bags and deployed everything out the top. This is the simplest version.

Note, I'm using CO2 and a tether, so I don't have to worry about black powder blowing holes in chutes or damaging lines.

The nosecone is popped off via CO2 system, and the drogue is mostly stuffed into the nosecone coupler, in a way that it falls out effortlessly. The 2' drogue keeps the rocket strung out vertically during descent, or at least keeps the nosecone above the rocket body.

The nosecone is stuck right under the drogue, and the 1/4" Kevlar drogue line goes right beside the nosecone down to the tether's releasable eyepin.

The drogue line has an attachment point at the top of the deployment bag. This keeps the dbag pulled down into the top of the rocket body tube until the main is to be deployed. The deployment bag is taking none of the drogue loading. That is important.

The 5/8" tubular nylon main line is neatly zig-zag coiled in a layer right on top the upper electronics bay bulkhead, in that cup-like area on this rocket. The dbag is squished right on top of that, held down by the bottom section of the drogue line.

When the tether is released, the drogue line is now pulling only on the top of the deployment bag. The rocket falls away under the bag, initially pulling out the main line and then the main's shroud lines, and then the canopy itself. Because of the rocket falling away underneath, it is all pulled out in a vertical string, ideally oriented. The main then opens.

This is why it is important to have the drogue sized such that the nosecone descent rate under drogue is lower than the free-fall descent rate of the rest of the rocket. Otherwise there won't be any pull. Of course this is easily achieved, unless one has a very heavy nosecone with a small drogue, and a lightweight rocket body.

Ideally you string the rocket fairly vertical under the drogue, but still bring it down pretty quickly. On a larger rocket I did this with a descent rate of over 100fps routinely. Bringing it down fast helps to minimize drift on higher flights. This one won't be that fast. My drogue is perhaps slightly larger than necessary.

Once the tether has been released, the drogue chute is the nosecone's main. And the main for the rocket body is the bigger main. They come down separately. In practice they tend to drift apart a bit. That can make finding both parts a bit more... fun. So there is an optional very light line, more like a thread, that can connect the two descending units together.

I have not yet decided whether I'll thread the two descent units together. I've tended not to do so in the past. But it is an option.

If you want to see something more complex using a drogue and then separate main chutes for the nosecone and the rocket body, check out: https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/

Gerald

 

[G_T]

It turns out there is enough taper in that coupler section that I'll have to add a strip of glass away from the edge to even it out to be more cylindrical, less conical. Otherwise I have what amounts to a snug fitting wobbly nosecone. I don't want wobble on something which can go over mach.

I'm not pleased with this set of tubes. At least I know how to fix it and can do the work. Not everyone is set up to do it or has the experience. I did acquire the tubes at different times. Perhaps it would not have been an issue otherwise.

I suppose I could have taken the lazy way and used tape to make the tubes fit.

 

[G_T]

About 10 minutes later...

I've applied a strip of the light glass to the coupler. I marked with a pen where the coupler inserts into the nosecone and put the strip just below that mark.

I put the fiberglass on as before, just mixed up a lot smaller batch of MGS. But since this epoxy is about 10 years old, and the hardener has not been in its original can for most of that time, I added one extra drop of hardener compared to the normal 100:40 ratio. One drop of this hardener is 0.03g approximately, in this case in a 7g batch (5g epoxy + 2g hardener, or now, 2.03g). So only a small compensation. MGS lasts a long time; it's one of the numerous things I like about it.

For convenience I keep a quantity of MGS L285 epoxy and some L286 and L287 fast and slow hardeners in squeeze bottles. You can blend the fast and slow hardeners to set the working time you want.

Ideally you don't want air exposure, and ideally you don't want UV exposure. I admit the bottles have been sitting out on a bench for much of a decade now. Though I think this is the second set of squeeze bottles, the first having gotten pretty grungy. So perhaps these are only out 6 years. Don't try this with most epoxy. Oh, and don't handle the bottles without gloves. Just saying...

When MGS is new, the pale blue color is more blue than blue with a hint of green, and is less prevalent. That's the oxidization of the hardener making the color stronger. You can see that in the pictures posted earlier. In a layup MGS is crystal clear.

Anyway, when an epoxy system such as this ages, it is the hardener that goes gradually bad. The epoxy can last a very long time, but it may crystallize. If it crystallizes, warm it up for a while to around 140F and the crystals will dissolve back into solution. Then you can use it. No big deal; you can do this many times. I haven't had to here, but we had to do it a lot at a friends shop. Don't even think of using crystallized epoxy without dissolving the crystals back into solution!

I'm doing something unnecessary here, just to show off a technique. Here i have taken some vinyl tape and wrapped it around with some stretch, sticky side out. I'm using close to a 50% overlap. The ends are secured by strips of tape, sticky side down. This provides some compression of the layup.

Old magnetic tape works super well for this by the way. That's with the recording side down, surprisingly. You can re-use that tape.

You can also use shrink tape, perforated release tape, lots of things. I recommend avoiding peel-ply, but I guess technically you could use that as well. Just peel-ply damages the epoxy matrix when you peel it off. It makes sub-standard layups.

If you actually used more epoxy than you should have, this wrapping will squeeze a lot of the excess out. I know I didn't use excess, so no oozage.

 

[G_T]

I force cured the MGS epoxy with my little halogen lamp. At ambient temperatures it would normally take about 4 hours to be reasonably cured to the touch. I ran it probably 125F and it was glassy hard in two hours. I don't normally recommend doing this, and you shouldn't do this with any random epoxy.

Each 10C temperature increase roughly doubles the rate of a chemical reaction. So I sped up the reaction by more than a factor of 4.

I attached a picture showing the sort of surface you should get with a good layup and the spiral wrap compression. You'll always have those ridges. You can minimize them by very carefully having each wrap butt against the next one perfectly, then throwing a second wrap also as carefully done on top, but with the seam offset.

I knew I was going to be lightly block sanding this thing anyway, so I didn't care. For that matter, for my usage the wrap didn't serve any purpose. I just wanted to show the surface obtained. Compare that to the typical tube you see where peel-ply or other methods were used. Here you block sand the ridges off and you are on to finishing work with essentially no filling required. Composite layup shouldn't be adding to the work you have to do; it should be reducing the work.

The ridges are not very high, basically the thickness of the ribbon you used for the wrap. Thinner ribbon gives thinner ridges. I used vinyl electric tape but thinner stuff is better. Anyway they sanded off easily enough.

Be very careful not to sand epoxy that is not cured. If you want to help yourself develop an epoxy allergy, that would be a good way to go about it. Sanding dust gets everywhere and there is no avoiding getting some of it on you. I'm sanding outdoors in the breeze. That helps. I also know this epoxy has reached the glass hard stage. So I could go ahead and sand it. Note, if it gums good sandpaper, it isn't cured. I also clean up the dust with running water. Dust is not your friend. We all get exposed to too much of too many different types.

 

[G_T]

Now the nosecone coupler properly fits the nosecone end of the body tube. It has to be aligned right for it to start in without jamming. But once started it slides in smoothly with no appreciable effort and no wobble. Mission accomplished.

So now it is time to check that the nosecone aligns properly with the center of the body tube. The faster the rocket is going to go, IMHO the more that matters.

I started by checking the end of the body tube to make sure it was fairly square. I did this by wrapping a sheet of paper around the tube. It looked quite square. That means it is at least in the ballpark. It does not mean it is perfect. It isn't a sensitive enough test.

Of course I can't use that test on the nosecone!

Caveman testing...

I used vinyl electric tape (had it on hand) to tape the nosecone in place. Then I rolled the body across the floor, looking for any wobble in the tip of the nosecone. You can see a fairly small wobble that way. I didn't see any wobble. So any error in alignment is fairly small.

This isn't a definitive test by any means, but it is good enough for the majority of rockets, at least most subsonic ones.

If the alignment shows wobble, try rotating the nosecone to a different position. See if you can get lucky and come up with an orientation that doesn't wobble.

Regardless of how you get there, including block sanding the tube or the nosecone shoulder if necessary, put a witness mark on the nosecone and the tube when you are done. Forever after that is the alignment to use.

 

[G_T]

Of course a more accurate way to check is to block the tube in some V-blocks or on a large surface plate or some other stable structure, and measure to the tip at different rotation positions. If you are going high speed I recommend doing this sort of checking.

 

[G_T]

Smarter Caveman method employed.

I wanted a more accurate check than the roll method. I have a magnetic base dial indicator, and some steel channel that I could rig up for accurate measurements. Nah... Caveman time!

Every Caveman has a doorway!

Use the doorway as an alignment guide, and a knee to hold the tube in place. Measure from the edge to the tip of the nosecone. Rotate and repeat. Ideally there is no variation.

In my case, of course I was not that lucky. There was no rotational alignment of the nosecone that could get the wobble below about 3/32". So the actual misalignment was about 3/64" at best (half the wobble).

So I resorted to a bit of block sanding on the nosecone shoulder to fix the alignment. Now the wobble is on the order of 1/64". And yep, put witness marks on the nosecone, the body tube, and the nosecone shoulder.

I've epoxied the shoulder into the nosecone with RocketPoxy. I used an acid brush to smooth a thin coat on the inside of the nosecone and the outside of the coupler. Then put a bit thicker ring at the edge of the coupler and edge of the nosecone. Screwed together to nearly all the way in, then cleaned up most of the excess with paper towels and an epoxy brush to reach inside the nosecone. Then screwed it in the rest of the way, aligned with the witness mark, and cleaned up all the excess.

Cleaning up inside the nosecone was done with a fresh acid brush. Swipe up some epoxy, clean off brush with paper towel, repeat...

Once it was clean of external epoxy I put the nosecone back on the body tube and checked alignment one more time. It was good. Of course I didn't leave the nosecone on! That would be tempting fate a bit too much!

 

[G_T]

I'm being tempted to use three 2-56 shear pins with no vent in the deployment bay.

Being lazy, pretend the launch site is at sea level, so 14.7 psi. Expected max altitude is a hair over 15Kft (various reasons it should fall short of that in real life) so dropping to 8.2 psi. Airframe is 3", so close to 7.1in^2 surface area. So the pressure drop times the area equals just over 46 pounds. That would be with no venting at all. Now I'm trying to make the bay have a good fit at the top and bottom coupolers, within reason. But it isn't going to be perfect. There will be gas escapage during the roughly 30 seconds to apogee. The bottom side will vent into the electronics bay, which is itself vented, and the top will vent around the nosecone shoulder. The bottom coupler is a pretty good piston fit. The top, not nearly so.

A single 2-56 shear pin can probably take somewhere around 25-35 pounds to shear, give or take. I've seen a range of numbers, the lowest being 21.4 pounds. So two shear pins might be sufficient but it could be borderline. It depends on the rate of air escape on the way up.

I did a quick and dirty test, pushing the nosecone on abruptly and checking by feel how long it took to equalize pressure. About 2 seconds. Roughly 2" compression on a 15" bay (tentative) so 17" to 15" x 14.7psi, equalized pretty well in 2 seconds -> gas can easily escape at the rate of pressure drop on the way up at the expected speed. That would be about 6.5psi over about 29 seconds.

It will lag a little on pressure drop of course. But the differential cannot get large given that rate of equalization.

It also implies perhaps I should tighten up that nosecone shoulder a hair more!

So two 2-56 pins does look pretty safe. Perhaps I'll just use two.

 

[OverTheTop]

Nice project Gerald. I wish I had access to the research threads but not being in the USA is a problem.

I will just make one comment on using stainless allthread with stainless nuts. This combination can result in the binding together of the parts. It seems to be more problematic in smaller threads so I am not sure if you will see the problem.
https://en.m.wikipedia.org/wiki/Galling
If things start to bind up apply some oil and back out straight away. Then chase the threads with a tap and die nut. That has been successful for me.

Troy may have comments on this as he has worked extensively with SS. I have seen problems with this at work where all SS materials were specified.

 

[G_T]

Thanks for the link! I do have some familiarity with galling but the metal choice was a tradeoff vs corrosion. I've used non-stainless before. The fasteners on this allthread are not tight by most standards so the surface pressure is quite low while being moved. So I'm not really concerned about it. Worst case I can always cut out and replace allthread. If I start to see problems I can switch to the much more expensive brass nylon lock nuts.

There are only 4 nuts on this rocket where I need to pay attention - the two retaining the nosecone bulkhead, and the two retaining the upper electronics bay bulkhead. Those will see two removals per flight; perhaps three for the nosecone when I count recharging. If all the others freeze in place, that's actually an advantage. The three nuts on top the camera and battery bay might get a little blue Loctite to encourage them not to move under shock and vibe.

 

[G_T]

Tracker... Whacking the nosecone does result in the board touching the 38mm tube. I'll pad it a little with something, possibly some scrap fabric. Or perhaps I just need to bend the brass a bit to center the board better.

PS - Just a little bending was all that was needed.

 

[waltr]

I would not trust the brass strip to ensure the module doesn't hit the nosecone.
How about getting a centering ring that fit inside the nose just above where the battery is. Then cut groove to slide it over the GPS wood frame.

 

[G_T]

Perhaps. But it is reasonably stiff in one direction (took a fair bit of force and a long wrench to re-bend), very stiff in the other, and the stuff on it is fairly low mass with not all that much leverage. It takes a lot of lateral G's to bend it over to contact the tube. If a contact were to occur, it would be the basswood that makes the contact. The electronics cannot contact short of destruction.

Over the entire flight cycle there is no reasonable mechanism for generating high lateral G loading. It would take a catastrophic failure (folding airframe rather unlikely - no center joint. Fin failure unlikely, metal and good for about double the max speed, and CATO on a low pressure chilled hybrid is difficult to achieve. But if achieved, it would be tank detonation. The electronics won't survive that), and this is a very stout rocket. At drogue deployment, the drogue comes out the base of the nosecone while it is flying away from the booster. With the drogue at the nosecone and the rest of the rocket having decent mass, the two will not contact. Even on the landing, the nosecone is coming in point down.

So I feel pretty safe with not altering it. I'd considered the stiffness issue before taking this approach. It isn't the first time I've done it. One advantage of having the tracker essentially stiffly floated in the tube without contact is that even high G loads, and moderate impacts, get absorbed in flexure so the peak lateral G loading (shock) experienced by the battery and electronics is lower. Of course, a small electronics board can take a lot of G's intrinsically, and the battery can take a decent G loading. That said, your suggestion is generally a good one. I appreciate your observation and suggestion!

 

[waltr]

Ok, was just my feeling. I may be concerned of vibration, hybrid pulsing, causing to mass on the brass strip to oscillate.
I've been following this build and wish best of luck on the launch.

 

[G_T]

Thanks! Any chance you'll be at URRF? I'd love to show this thing off and chat shop! The warts are more visible up close! One annoying wart I'm working on is the tube being a fair bit larger diameter than the nosecone. More fiberglass sanding... It's going to show, but it's going to be aero. I probably should have just purchased replacment fiberglass parts and not had to deal with all this fitting. But, what I had was already paid for!

In the static test, this hybrid did not produce any pulsing at all. It is designed not to... but I'll believe it when it actually flies rather than just having a static test! G loading and the effects of vibration on the oxidizer mass are fairly well neutralized in a static test, but not at all in an actual flight.

Any pulsing with a hybrid will be axial. The tracker mounting is extremely stiff and strong axially. The transfer to lateral oscillation would be pretty small given the geometry with the mass fairly close to centered unless a resonance frequency is encountered. The board will have two primary resonance frequencies, and both are well above what a hybrid of this size should be capable of matching. However, the average hybrid motor is certainly a source of lots of vibration!

 

[waltr]

Thought about URRF but its a 5 hour drive (I'm near Philadelphia) and I don't have my L1 yet. Plan was this past weekend but launch got rained out. Next launch is this Saturday so hoping weather and winds are good.

I may make an MDRA launch (2.5 hours away) when they move back to Higgs farm this autum to fly some HP, or in the spring, for Red Glare.

Would love to see this rocket and see a hybrid fly.

 

[G_T]

Working on the nosecone fit...

Two issues to solve here.

(1)

First, the nosecone shoulder doesn't test as air resistant as I'd like. The reason is there are two bands of good fit, one near the bottom edge, and the other where I added that second strip of E-glass. In between these areas there is a depression.

I probably have some quarter ounce scrim glass around somewhere that I could have used to infill the shallow depression region, but I didn't feel like looking for it.

Besides, there is another way. What I needed was a gel epoxy paint. So I made some. This is the same MGS laminating resin and fast hardener I've been using. Note, fast hardener would be what you'd consider very slow hardener if it weren't a laminating resin. Plenty of working time and then some.

Here I added quite a lot of West Systems 423 graphite powder. I progressively stirred in graphite until I had achieved a gel paint. Stir in; don't whip it in. Otherwise you'll get graphite dust everywhere!

I painted a layer of this concoction in the depressed area, using an acid brush. Then I took that brass strip shown in the picture and used it as a scraper. I made sure to keep the scraper parallel with the axis of the nosecone. Not doing so would result in a hyperbolic section not a cylinder (or conic, depending). Try to rotate the item smoothly without stopping, holding the scraper blade fixed.

After the first pass, clean up the scraper. You've removed nearly all the excess. Then make a final pass just to make sure it is level and even.

That's it. Leave it to cure. It's not going to flow.

It will probably need a light pass of a sanding block. It will pick up some dust, and the infill won't have been perfectly level. Anyway I don't plan to really sand it, just knock off the dust.

This gel paint looks quite black, but in thin enough layers it is translucent. So the fabric weave shows through somewhat making the job look more textured than it really is.

Using graphite as the source of pigment and gelling agent results in a lower friction surface than would be the case for most other things you could use. I thought that was appropriate in this case. It would NOT be appropriate if you had an RF transmitter or receiver such as GPS in that band area!

(2)

Second, the nosecone has a smaller OD than the body tube, by a fair bit actually. I put a masking tape ring about an inch from the edge of the tube, and sanded the tube down. The masking tape acts as a stop for sanding the taper.

To sand smoothly for something like this, use a sanding block and rotate the tube with little motion of the sanding block. Resist the temptation to saw at the sanding with the sandpaper. The latter is faster, but you'll have flat spots and ripples. That's harder to remove than to put in! So use the slower method; getting good results is a lot easier!

I actually used a carbide sanding strip to start the job, there was so much fiberglass to remove. From there, to medium coarse emery paper, followed by medium emery paper. I didn't go very fine. I figure I might get some paint on the tube, so it is fine enough.

Hopefully the tapering doesn't show much just sitting there. It does show with the nosecone on, inevitably, because of the angle change at the shoulder. It makes it obvious. Not much I can do about that. I could have gone the other direction and filled the nosecone. That's a lot more work and more likely to chip out on hard landings.

-----

PS - No sanding of the nosecone coupler gel paint filling was needed.

PPS - The fit was noticably better. But decided on a second coat. I probably should have put it on slightly thicker, even the second coat. It wasn't bad after the first coat; I just wanted it to be good! It did hold pressure slightly better after the first coat. BTW, I had to sand away at least a third of the thickness of the body tube at the nosecone end.