Monday, November 18, 2019

Sounding Rockets; Cutting-Edge Science, 15 Minutes at a Time


At 2pm today, NASA live-streamed an hourlong program on sounding rockets - research rockets which carry scientific payloads on short, suborbital trajectories.

Here is the video:


If the player does not work for you on this blog, CLICK HERE to go directly to the video on YouTube.

I like sounding rockets, because of all launch vehicles, they're the ones which most closely resemble what we do in hobby rocketry. They're usually fin stabilized, they usually use solid propellants, and rather than go into orbit, they go up and come back down.

Sounding rockets look much like sport models, and doing a scale model of one is often much simpler than doing an accurate scale model of, say, a Saturn V or Mercury Redstone or Falcon Heavy, or Atlas Something.

In fact, one of the earliest model rocket kits, the Aerobee Hi, was a scale model of a sounding rocket which looked so much like a sport model, that when you see a scale model of it, you'd be forgiven for thinking it was just another model rocket.

The Aerobee Hi sounding rocket

I've been wanting to see more on YouTube about sounding rockets from NASA, but until today, I didn't find a lot that was terribly informative. Now that I have time to sit down and enjoy it, I'm hopeful it will have some interesting stuff in it.

Enjoy!

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Sunday, November 17, 2019

The Semroc Bandit - Part 2


The ducted ejection motor mount system for the Semroc Bandit can be seen above. This is apparently a re-creation of the original Estes baffle system in the 1971 Bandit. The parts fit together well, and it's pretty neat - although it does add significant weight. If you went with a simple motor mount and just used recovery wadding as per usual, you'd probably get the Bandit to fly a lot higher.

But part of the pleasure of flying the Bandit is watching the whole flight. It's a pretty bird.

The end of the motor tube is plugged with a balsa bulkhead or nose block. Into that bulkhead, you attach one of the supplied screw eyes to anchor the shock cord.


The shock cord which comes with the kit is a thin piece of sewing elastic. I decided against using it here for a couple reasons.

First, it's a little short for my taste. While I do use sewing elastic in my low power model rockets - a lot - I like to make my shock cords very long (more on this in a future post).

Second, despite the ejection baffle, I worry that the elastic will wear out over time after repeated ejections. Not from the heat, but from the chemistry. The caustic vapors from black powder charges have a negative effect on the latex in the elastic, making it brittle and prone to snapping. If I mounted an elastic shock cord that far down into the rocket, when it broke, I wouldn't be able to replace it.

I prefer to attach my elastic cords to something less prone to wearing out - Kevlar kite string.

While Kevlar will burn through after so many charges if it's right by the ejection site, with the baffle system, it will probably be just fine here.

I replaced the elastic cord with a Kevlar string, which I attached to the bulkhead on one end. On the other, I tied a loop. The Kevlar cord is just long enough that you can see it at the end of the body tube.


This will allow me to attach an elastic cord, which I can cut off and replace when it becomes brittle.


With the elastic attaching to the Kevlar inside the body tube, the shock cord is less likely to damage the tube should the parachute come out while the rocket is moving at high velocity. If the Kevlar extended beyond the end of the tube, there's a chance that the cord could cut into the end of the tube, possibly causing a jagged tear down the side of the rocket, known as a zipper.

I sanded the fins into airfoil shapes, which I often do. I like the finished look it gives to a model rocket.


While these aren't perfect, they turned out better than this picture implies. Sometimes a closeup can distort things in a photo. They look pretty even in real life, and once on the rocket, you won't be able to see the imperfections without close examination. I could have taken a second crack at them and probably gotten better airfoils, since I have a spare set of fins (see Part 1), but I decided I was happy enough with these.

I sealed the balsa nose cone and even the fins with thin CA - cyanoacrylate or Super Glue - and sanded them smooth. On the fins, I left the bottom 1/4 inch unsealed. If you get CA on the root edge, wood glue won't bond to it as well, and a fin could easily pop off.

I used the Guillotine Fin Jig to glue the fins on. I made a couple layers of glue fin fillets and then treated the root edges of the fins with CA, then sanded again.


The whole kit looks pretty nice.


You'll notice that I did not fill in the spiral grooves on the main body tube, but I did on the white upper payload tube. That's because the lower tube didn't really have grooves as such. The spirals were more of a tight overlap, so there was nothing to fill in. My hope here is that a good few coats of filler primer will hide those overlaps.

You can fill in spiral grooves with heavy coats of filler primer, too, but I prefer carpenter's wood filler. Filling grooves in with primer alone takes a lot more primer, which is more expensive than wood filler. I like hiding the grooves, and I like getting more than one rocket out of a can of primer, so I usually use the wood filler method. You don't have to fill spirals if you don't want to, but I like doing it, and I have a pretty hassle-free (i.e. not much sanding involved) method.

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Monday, November 11, 2019

The Semroc Bandit - Part 1


I've realized I made a mistake with #NaRoBloMo. That is, I promised to follow up the first N00b T00b post with another N00b T00b post, addressing specific topics. Then, I kept feeling obligated to write that before writing anything else, making this blog feel like work. This is part of my hobby - and supposed to be fun. So, I'm going to jump around as I see fit.

Semroc is one of those model rocket companies which nearly disappeared a few years ago, when its previous owner, Carl McLawhorn, unexpectedly passed away. Eventually, Randy Boadway of eRockets.biz purchased the company, and continues to produce its high quality model rocket kits and parts.

Despite the fact that eRockets is a sponsor of The Rocketry Show podcast, and that I've had several Semroc models waiting on my build pile for a long time, I'd never actually built one. My Estes back stock was just so out of control.

But I hadn't built a kit in a while. After finishing the N00b T00b, which was just a quickie scratch build to get me back into it, I decided it was time to grab something off the pile of kits and start. But what?

I haven't counted my pile. It's... well, it's enormous. If I tried to count it, I'd probably miss something, as I have kits stored here and there. So, what to build next seemed like a huge decision.

I decided to build something non-Estes to start with. I had some FlisKits, some AeroTech, a Squirrel Works Arapahoe E (which looks like a really great kit), plus a number of Rocketarium scale sounding rocket kits, plus some stuff from Quest. And maybe more, I'm not sure.

I thought this was all of my smaller non-Estes pile. I realized later it wasn't even close.

I made a small pile of what I thought were all of my non-Estes low and mid power model rockets (I decided not to start with high power). I realized later there were a few missing from the pile.

Nonetheless, after some thought, I decided it was high time I built a Semroc, since it was one of the first non-Estes companies I had heard of, but I'd never actually started a model. And I chose the Bandit - a "Retro Repro" model - a pretty close recreation of a classic, out-of-production kit (in this case, a 1971 Estes model). It was a nice, classic (I know I have used that word too frequently already, but it's what it is), great-looking model. Having seen a few online and one really nicely built one at a club launch, I felt I needed to have one.

One thing which impressed me about the Bandit was the quality of the parts. The tubes are all very nice, and the balsa nose cone and nose block fit very well. The other thing about the balsa nose cone I liked was how closely the base of it matched the outside diameter of the body tube.


A lot of long-time model rocketeers seem to prefer working with balsa nose cones, rather than plastic, if what I've seen in various online forums is true. Since I've only been practicing rocketry for about five years, I actually haven't worked with that many balsa nose cones. Estes has switched over to plastic, and the few kits they produced with balsa cones either are out or production, or soon will be.

The thing I've encountered when working with balsa cones - again, in my limited experience - is that the base of the nose cones often seem to have a wider diameter than the body tube, so that the cone overhangs the tube by a significant step.

A balsa nose cone which overhangs the tube a bit. It's not so easy to see on camera with the black paint.

A balsa cone with a significant overhang. I turned this one upside down into the light so you could see it better.

I always assumed that maybe it was harder to get balsa to a specific diameter on a larger scale of manufacture without taking off too much material, and so they'd err on the side of slightly too large. Something like that - it was a guess.

Anyway, I encounter that far less frequently with plastic nose cones, and I really don't like that step. If I were to sand the base of the cone down to match the size of the tube, I'd change its shape. So I don't want to bother with that.

When I made my hand-turned balsa cone for my scratch-built Big Bertha, I tried - and succeeded - to get the diameters to match pretty closely. But that was me spending a lot of time and effort on just one cone. Maybe they couldn't do that with a large batch, I figured.

But the Semroc nose cone is such a nice fit.

The Bandit comes with a ducted ejection charge system. Basically, the motor mount is an ejection baffle. Rather than allowing the ejection charge to blow straight up through the tube, the hot gasses pass through a series of holes in two tubes. The flame shoots straight up and hits a balsa bulkhead, but the air passes through the baffle system before pressurizing the rocket and ejecting the parachute. This means you should not need recovery wadding (though the instructions do recommend a sheet or two, just to be on the safe side).


The holes are all laser cut and quite neat.

The one issue with the parts was that the fin sheet was kind of beat up, and one fin had snapped in two along the grain.


This was unfortunate, but not reason enough to my mind to complain.

I can't remember if I bought the Bandit directly from eRockets.biz, or from JonRocket.com. In either case, if I had sent an email to complain, I'm sure would have done something to make it right. But both are - quite literally - "mom and pop" type operations, small family businesses. If there had been something badly wrong with the rocket (say, it had been crushed or something), I'd have sent an email. But in this case, it's not like there was any negligence or bad handling, and I didn't feel like making anybody jump through hoops for me for a bit of balsa.

The kit was otherwise in fine shape. This is one of those things that happens, and it's an easy fix. First, the fin split cleanly along the grain, and could easily be glued together. The bond would be strong and invisible.



Another option would be to trace a fin onto some balsa or basswood fin stock and make a replacement.

I often make an extra whole set of fins, so I can sand them to a particular shape if I want to. Say, I want to sand an airfoil. It's nice to have a set of spares, so if you make a mistake, you have extras. Frequently, they all turn out pretty good, so I'll build a second rocket - a copy of the kit. Then I have two.

I glued the broken fin back together, and after stack sanding the set, I traced them onto both some 3/32 inch thick hard balsa stock, as well as onto some basswood. I liked how the balsa came out better, so I used that. After stack sanding, I had two identical sets of balsa fins to play with.

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Tuesday, November 5, 2019

Technical Difficulties

I was trying to illustrate the next post on the N00b T00b last night, with a screenshot from OpenRocket, on the MacBook Air I use for The Rocketry Show podcast.

But the 3D image of designs in OpenRocket on this MacBook all display as really tiny.


This is as large as I can make the image.

Looking at it in preview, I guess in this case it's not terrible, but I wanted a larger image, and after an hour of messing with it, I got fed up and went to bed.

I might see if I can dig out the really old, slow, clunky laptop I used when I started The Rocket N00b blog, because at least I can load photos from my camera onto it, even if it's old junk.

We shall return to #NaRoBloMo shortly...

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Sunday, November 3, 2019

Junior Level 1 Rule Changes - Upcoming Segment on The Rocketry Show

You may have heard the news: Starting January 1, 2020, rocketeers under the age of 18 who wish to get a Junior Level 1 high power rocketry certification from the National Association of Rocketry will take a written test prior to conducting their certification flight.

The test will be twenty questions long, and there will be a study guide available. If you fail the test once, you may take it again - immediately, if you wish. The test can only be taken twice within a seven-day period, so if you fail twice, you do have to wait a week before trying again.

The purpose of this test is to increase safe flying for Junior Level 1 members, and to bring the NAR in line with some of the standards practiced by the Tripoli Rocketry Association. It's an improvement, and will cover things that any HPR flier should know.

Randy Boadway, of eRockets.biz, who is a sponsor of The Rocketry Show podcast, is also on the NAR board, and we've recorded a short segment on the upcoming changes. Look for it in the next episode of The Rocketry Show.

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Saturday, November 2, 2019

Remnants

Today was a launch day in Amesbury, MA. While I had anticipated pretty clear skies and low wind, by the time I arrived (late), clouds had rolled in, and the wind was blowing toward the power lines. Not enough to halt the launch, but enough to disallow high power rocket flights.

The winds were slight enough to fly, but chilly, and made me a bit skittish. There were a lot of CATO's today, too, and the whole thing just felt a little spooky.

I had brought a number of rockets which had never flown, including the Estes Saturn V which I finished building before the July 20 anniversary of the Apollo 11 landing, but which I've been unable to get off the pad for one reason or another.


My first attempt was thwarted by the extreme heat. The launch was called short before I got the chance to get my Saturn on the pad.

The second time I took my Saturn to a launch, the shoulder of the upper section had grown tight. I was worried it wouldn't come off, that there would be no parachutes, and that the whole beautiful thing would come in ballistic and smash to pieces on the ground.

I took it home and worked on the shoulder until it could slide in and out of the rocket without too much force.

Last month, I was finally going to fly the Saturn, but as I was setting it up on the launch pad, the string holding the upper portion's recovery system broke. It was too short to re-tie it on the field, so I had to come home without flying it once again.

Today, after all that, the wind had me worried, and though I want to see how this rocket flies, I wasn't willing to launch it if I was likely to lose the upper section. "Go fever" has caused me to lose a number of rockets I worked really hard on, so I've learned to say no if the weather isn't feeling right.

So, I didn't fly much today - not the Saturn V, nor a number of other unflown builds.


I started with the N00b T00b, its second flight. The first flight, in October, should have been low risk. I only put it up on an A8-3! But I had a PerfectFlite Firefly altimeter on board, and the nose cone wasn't tight enough, so at apogee, the nose and altimeter both popped out of the rocket and I didn't see where they fell! I found the nose cone later, as it had landed right at the base of Pad 7, which I was assigned for my to-be-aborted Saturn flight. The altimeter never turned up, despite my combing the area. Good thing they're pretty inexpensive.

Today's N00b T00b flight went better. No altimeter, but a high flight on a B6-4, and the nose stayed on.

I also flew my Estes Photon Probe on a B6-4, a nice flight with a nice motor. I used to put C's or nothing in my rockets, but the last few years I've learned to really appreciate the B motor.

I'd have flown more B stuff, but I didn't bring anything that small, and realized I'd left my motor adapters at home. There were one or two things I'd have been willing to fly on a D12-5, but I didn't have a spacer, and the motor tubes were too long.

Finally, the weather cleared, and the wind died down a bit, and I though I should have a bit more guts. There was only 45 minutes left in the launch, so I didn't want to do anything too complicated (like attaching the metric rail I'd brought for some of my rockets to the high power pad), but I felt pretty good about putting up my Estes Leviathan on an F26-6 composite motor.


Because I built the Leviathan for my high power level 1 certification flight, I of course way overbuilt it. It's heavy and wouldn't go too high on an F26. That, and the Jolly Logic Chute Release, and the fact that the Leviathan is just... big... made me feel it was unlikely I'd lose it.

I think this was only the third flight of the Leviathan since I'd built it. Maybe fourth.

I taped a camera to its side and filmed the launch from the ground. The F26 is a fun motor. The flight was great. Chute Release performed perfectly. Of course my Rocketry Show MacBook Air doesn't recognize the little camera I had taped to the side of the rocket, but once I figure out how to get the video off of it, it should be a fun little short video.

After the site was broken down, I combed the area again, wondering if maybe I'd stumble across that Firefly altimeter I'd lost over a month ago. You never know. You can find all sorts of things at regular launch sites, even years later. Someone today found an Estes Mosquito they'd lost in early October, in perfect shape, so anything's possible.


I didn't find the altimeter, but I did come across the remnants of an old CATO. A damp, spongy blue propellant grain from a 38mm composite motor which must have been here since at least last month. Probably the nozzle blew out and the fuel grain fell out, unburned. It had sat there unnoticed, blending in with the ground, till I saw the unusual shape and kicked at it with my boot to reveal the blue grain.


There's one more launch this year, in two weeks. This time, I'll prepare better, ahead of time, go early, and see if I can't get that Saturn off the ground.

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Friday, November 1, 2019

The N00b T00b - A Quick, Easy, Scratch Built Tube Fin Rocket - Part 1: The Basic Build


I have a bunch of random model rocket parts lying around the Rocket Room. Some are things I intentionally stocked up on, and some are leftovers from kits I modified, or from other projects.

I have a bunch of short, BT-20 sized tubes (0.736 inch in diameter). They're most certainly supposed to be motor tubes, though they're a little short, so I'm not sure where they came from. Most Estes engine tubes are about the same length as an 18mm (A, B, or C) motor, while these are shorter. If you used them for a motor mount tube, you'd have to have the motor hang out the back by at least a half inch, whereas most Estes kits have the engine stick out about 1/4 to 3/8 inch.


I think they're leftover stuff from Rocket Camp which ended up in my pile of parts, and I think they might be from Pitsco, which a previous Rocket Camp teacher seems to have used.

I also had one 8.75 inch long BT-20 body tube which I know is a leftover from Rocket Camp. When you teach model rocketry to kids, there will always be parts left over. Not from the kits the kids build, but from the extra kits you have on hand.

Kids at camp will lose parts, glue parts in the wrong spot, shove parts up their noses or get them stuck firmly on their fingers and need them cut off by the nurse (yes, this all happened). Consequently, some of them won't have a rocket to complete if you don't have spare parts, and a kid at Rocket Camp with no rocket to build is likely to get bored and become disruptive. This is really a digression, but if you ever teach model rocket camp, make sure you have extra kits, and be prepared to end up with a bunch of random parts left over.

So, having given away launch lugs, engine blocks, fins, etc., I had one body tube left over from an Estes Viking.

I hadn't started building a rocket in a long time. The N00b family had recently moved, and for a while I was trying to just finish building and painting all the stuff I'd started in the previous two years. My build pile of kits is so big, I didn't know where to start.

Then I thought: maybe I should build a quick tube fin rocket, just to get back into it.

When it comes to model rockets, my tastes are pretty traditional. I like rockety-looking rockets - a nose cone, long body tube, and three or four fins. I'm not as interested in odd-rocs, saucers, boost gliders, or tube fin rockets. That's not a criticism - I enjoy seeing them fly at launches. It's just for my own fleet, I like mainly sport models and the occasional scale model.

But tube fin rockets can be fun. They have a lot of devotees. They can be simple to construct, and unlike rockets with flat fins, they apparently do not weathercock.

N00b Note: "Weathercocking" is when a rocket arcs into the wind. Almost all model rockets do this to some extent. It's a kind of side effect of fin stabilization. As a rocket's fins correct its trajectory in flight, lift on the fins rotates the rocket body around its Center of Gravity. As it oscillates, the rocket will tend to correct more in the direction the wind is coming from. Rockets with larger fins or rockets which lift off slowly tend to weathercock more severely than faster models, which may only weathercock slightly closer to apogee. See The Handbook of Model Rocketry by G. Harry Stine and Bill Stine for more on this phenomenon.

I had lots of parts, so building a tube fin model would be easy. Tube fin rockets can come in different designs, and I'm no expert on them, but it seems that the easiest to build quickly are rockets which use the same diameter tube for fins as it does for the main body tube. You can fit six tube fins perfectly around the center tube, so getting them glued on straight is easy.

As far as "designing" the tube fin rocket would go, there wouldn't be much design. I had parts, so I'd glue them together. I wasn't going to have to cut anything or shape anything. I'd just see how it turned out.


The rocket used a BT-20 body tube, the diameter used for 18mm A, B, and C motor mounts. So this would be what's called "minimum diameter." No centering rings, no engine hook, no motor mount - the body tube is the motor mount. Minimum diameter rockets tend to fly very high. Some examples beginners might know are the Estes Viking, Wizard, Hi-Flier, etc. The motor would be kept in the rocket though friction fit - a wrap of masking tape around the engine until it's tight enough that it won't fall out of the rocket, or be ejected out by the ejection charge.

Construction was simple. First, I glued in a spare engine block I found in my spare parts. I pushed it n place with an old motor casing so that the motor would hang out the back by about 1/4 inch.

Next, I glued on the fins. This is the part that makes a tube fin like this a snap. To get the fins on straight, you start with one fin. Run a thin bead of glue down one side of the tube fin and attach it to the main body tube. Then, lie both tubes on the work table, side by side.


A straightedge helped me make sure the ends of the tubes were even with one another, and the fact that both tubes were lying on a flat surface as the glue dried ensured that they were parallel - that the tube fin was straight.

As the first fin dried, I ran a bead of glue down a second tube fin and attached that to the body tube by laying it down next to the first tube fin.


A third tube fin was glued to the body tube laid on the table opposite the first fin.

I needed to figure out where to put the launch lug, since I'd never built one of these before. I tried to see if I could hide it in a gap on the body tube between two tube fins, but it turns out that BT-20 tube fins are a little too small to hold a 1/8 inch launch lug between them. So I glued a launch lug inside the fourth fin and glued that in place in the remaining gap on the body tube.








I made sure all the tube ends were even using my sanding block, and then I let the glue dry a little bit. The first four fins went on in about 10-15 minutes.


I flipped the rocket over when I felt the glue would hold without things moving, and quickly glued the remaining two fins in place in about 1 minute's time.


That was it. I popped on the nose cone, and had a look. With the exception of putting in a shock cord, the rocket was built.


Coming up: Stability? Payload? Paint? How about a name for this rocket??

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