Built from Scratch - A Tale of Two Berthas - Part 1

I recently finished building two Big Bertha rockets - from scratch, rather than from kits. As a guide, I used the original plans, published by Estes in Model Rocket News in 1963, when Bertha was a free plan, before it was sold as a kit (the Big Bertha is now the longest continually produced model rocket kit in history).

I meant to build just one scratch Bertha, but ended up building two of them, almost by accident. I do this a lot.

I started this project for a few reasons. My original Big Bertha kit was one of my earliest builds, detailed a few years ago on this blog. I mention the Bertha a lot here, not because it's always been my favorite model rocket (it hasn't), but because it's so iconic, and simple, and with its large parts, it makes a good demo rocket.

But last season, I flew my now beat up Bertha for my final flight of the fall on a C6-3 motor. Our Launch Control Officer, Kenn, commented "There's no better combination of rocket and motor, folks!" and I thought, you know what, he's right. That is a great rocket!

The Bertha is such a pleasure to see fly. I always brought it to a launch as a kind of afterthought, thinking I should take something that doesn't fly too high, just in case. But each time I flew it, it was just... fun! It goes up relatively high, but you can still keep your eye on it, and it floats gently back down on its 18 inch parachute, and... Well, I guess I just have a soft spot for the Bertha. So, I wanted another one.

That was reason 1. Reason 2 is that, while it does fly really well on C motors, I've always wanted to put a D12 in it, but I built the Bertha kit before I knew you could upgrade these things. The Berthas I've seen fly on D12-5 black powder motors are really fun. With its large fins, the Bertha is plenty stable, and can handle extra weight at the back. So, I decided to build a Bertha with a 24mm diameter motor mount, for those Estes D12 motors.

Reason number 3 is that I always wanted to build one of the plans from the old Estes Model Rocket News. Back in the 1960s, many of these early newsletters featured rockets either designed by Vern Estes himself, or by readers of the Model Rocket News, who'd send in their own designs, which used Estes stock parts. In those days, it was pretty common for model rocketeers to have a fleet which was at least in part built from scratch, rather than from kits.

I figured the Bertha would be a good place to start. No odd, out-of-production tube sizes or nose cones to buy. Just a BT-60 tube, some 1/8 inch fin stock, and a stubby elliptical nose cone. Speaking of which...

Reason 4: I had always wanted to try turning my own nose cone from a block of balsa.

 The plan was to see how hard this could be, using a hand drill as a kind of lathe. From what I'd read in both the Model Rocket News and the Handbook of Model Rocketry, this was supposedly not only doable, but not all that difficult. I wasn't sure that was true, but I wanted to try my hand at it.

It's been a while since I've done this kind of Rocket N00b stuff where I try to figure out how something is supposed to be done and then publish it here. I thought this would be a good place to start, and it would save me having to buy a cone. I wanted to keep this Bertha cheap, if I could.

Spoiler alert - it turned out pretty nice!

So, despite my promise to myself to finish building everything I'd been working on for a year and a half before starting any new rocket builds, I began working on the Bertha. Then I ended up making a second one. More on that when we get to it.

The next few posts will detail my scratch build of the two Berthas.

Click here for Part 2 - The Plans

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Stability - or - What Happened to Homer's Rocket? (Part 4) - Finding the CP: Method 1 Continued

This is a continuation of a series on model rocket stability for beginners. Click here to go go the beginning of the series. Click here to read the last post.

Last time, we discussed the earliest method of finding the Center of Pressure (CP) on a model rocket - the cutout method. This simple method ensured stable flights on every model in the early days of model rocketry. Finding the CP is a crucial problem to solve, because in order for stable rocket flight, the CP must be behind the Center of Gravity (CG).

But, of course, the cutout method had drawbacks. Rocketeers had to be reasonably skilled at drawing an accurate representation of the rocket on stiff paper or cardboard, with all the parts in correct proportion. In other words, in order for the cutout method to work the drawing had to look just like the real thing.

Since I'm not a skilled draftsman, I cheated a little. I illustrated the cutout method with a design I'd created in OpenRocket - Sounder IB - which I printed on heavy card stock, cut out, and balanced on a piece of aluminum angle.

This showed us another drawback of the cutout method - accuracy. While balancing the two-dimensional cutout of Sounder IB did find the center of area for the rocket, that point was far forward of the red CP mark on the drawing itself. In other words, OpenRocket told me that the CP was in one spot, but the cutout method indicated that the CP was a good two inches further forward. So far, in fact, that the CP as determined by the cutout method was in front of the CG, as calculated by OpenRocket.

So, while the OpenRocket design showed the rocket to be perfectly stable, the cutout method showed me a dangerously unstable rocket - one which would flip violently around if it were launched!

So, does the cutout method represent the Center of Pressure at all? Or were rocketeers merely fooling themselves? And how do we - how does OpenRocket - know where the CP actually is? Who's right, who's wrong, and why?

The answer is that they're both right - kind of.

In the cutout method, we're balancing a 2D representation of the rocket - on its side. The cutout is resting on its balance point, so as the force of gravity pulls on it, the force is equally distributed in front of and behind the aluminum angle. This force - gravity - is acting a substitute for another force - air pressure - in the real rocket. So, for the cutout method to represent reality, the air pressure would have to be hitting the rocket directly from the side. The cutout method shows you were the CP would be if the rocket were flying sideways!

In this case, that means that all the air is hitting the rocket from the side - at an angle of 90 degrees. The angle the wind is hitting a rocket is known as angle of attack.

Alpha represents the angle of attack. Image from Centuri TIR-30, by James Barrowman.

In The Handbook of Model Rocketry, a 90-degree angle of attack is described as "the worst possible flying condition." In fact, it's an imaginary flying condition, because rockets do not fly sideways. They fly pointy end first!

Under normal flying conditions, with the proper motor (providing enough thrust for the weight of the rocket), model rockets fly at or near zero degrees angle of attack. While the ambient wind tends to blow horizontally along the ground, the rocket flies fast enough upward that the effect of the wind is minimized. If the wind on launch day is, say, 8 miles per hour, and the rocket is flying upward at, say, 200 miles per hour, the rocket will barely notice the wind coming from the side.

Under those conditions, the determination of the Center of Pressure is dominated much more by the fins and nose cone than by the surface area of the body of the rocket. As the rocket wobbles during flight - totally normal for a model rocket - the angle of attack will swing back and forth between zero and a few degrees. As this happens, the fins, which stick out from the body of the rocket, use the oncoming air pressure to correct the rocket's path, causing the back end to rotate away from the wind.

The pressure on the body tube at or near zero degrees angle of attach is much lower, and has much less effect on the CP.

But if the angle of attack were to suddenly increase significantly, then the air pressure on the nose cone and body tube becomes much more significant. The effect is that, at high angle of attack, the Center of Pressure moves forward. If, due to some (imaginary) catastrophic event in flight, the rocket were to fly sideways, then the CP would move forward enough that it would be where we see it when we do the cutout method.

As angle of attack increases, the influence of the nose cone and body tube increase -
the CP moves forward! Image from Centuri TIR-30, by James Barrowman

There are only two situations I know of when a normal rocket experiences these conditions. The first is when the rocket is sitting on the launch pad, and the breeze is blowing across the field. But when the rocket is sitting still on the pad, it's not flying, so this doesn't count.

The other is a rare, pretty strange event, which I've seen twice - recovery.

Once in a while, a rocket will launch, fly to apogee, and then due either to an ejection charge failure or a nose cone which is stuck on too tight, the nose cone doesn't eject. The rocket stays intact, the parachute does not come out, and the rocket begins to fall back to Earth.

Normally, when this happens, it's pretty frightening. Because the rocket is stable, with its CG in front of its CP, it will tend to fly nose first. So a rocket which has an ejection failure usually comes in ballistic - taking a nose dive straight at the ground with increasing speed. This usually destroys the rocket.

Sometimes, very rarely, an odd thing will happen. The rocket will go up, tip over at apogee, and begin falling back down. In rare instances, the CP at 90 degrees angle of attack will be the same spot as the rocket's CG. The rocket is then neutrally stable. The forces of gravity and air pressure are both centered on the same spot. The rocket descends sideways. Since the Center of Gravity is the point of rotation, and the Center of Pressure is the balance point of the force of the air of the rocket, the whole thing is in balance - just like a balanced scale.

If she weighs the same as a duck...

Both times this happened, the rocket fell very slowly, and came to a soft landing. Both times, I was filming, but both times, I was so stunned, I missed getting the slowly descending sideways rocket in frame. But it was pretty cool - and certainly a relief not to have the rocket come in ballistic.

I should mention that you shouldn't try to replicate this, by gluing on a nose cone or something. It's a chance event when it happens, and the same rocket might not do it twice - a slight difference in Center of Gravity could change everything, and the rocket would come in ballistic. But if you do see it, it's kind of amazing.

* * *

The fact that the CP can shift forward is really important. It means that the CG and CP could be too close together for the rocket to remain stable. If the angle of attack suddenly increases, due to a gust of wind, or off-center thrust of the motor, or any number of things, having the CG too close to the CP means that under certain circumstances, the CP could move forward of the CG! If these two switch position, you now have a dangerous, unstable rocket.

This brings us to the question How far forward of the CP chould the CG be? I was going to save this for a later part of this series, but I think it makes sense to mention it here.

In general, the rule of thumb is that the CG should be at least one body tube diameter ahead of the CP. That means that if the rocket is, say, 1 inch in diameter, the CG must be at least 1 inch forward of the CP. This margin is known as caliber, and refers to the diameter of the rocket.

Sounder 1B is 0.976 inches or 24.8 millimeters in diameter. If the CG is exactly 0.976 inches or 24.8 mm ahead of the CP, we say the rocket has a stability margin (sometimes called the static margin) of 1 caliber. If the CG and CP are 1.952 inches or 49.6mm apart - twice the diameter of the body tube - the margin is 2 caliber.

As you see, Sounder 1B has a static margin of 1.63 caliber. The CG is 40 millimeters forward of the CP. Since the minimum static margin is 1 caliber stability, this is fine. The ideal, especially if you want to fly as high as you can, is a static margin between 1 and 2 caliber. More is usually OK, up to a point. Less is generally not enough for safety, except in the case of some short, stubby rockets.

For most model rockets, however, the minimum safe static margin is 1 caliber. Having a static margin of 1 caliber or more ensures that, even if the rocket encounters a high-degree angle of attack for a moment, the CP isn't likely to shift forward of the CG. The rocket should remain stable.

* * *

To be clear, the cutout method does work to make stable rockets. But it's what we could call overly conservative with its CP location. A rocket designed using the cutout method would certainly be stable and safe, but it errs so far on the safe side, that you may end up building rockets which are far heavier in the nose cone than they need to be, or with more fins or larger fins than you need. That means you may rob yourself of performance, or you may shy away from building a rocket which is perfectly safe and stable, because you worry it might not be.

A better, more accurate method of finding the Center of Pressure was called for. We'll discuss that method in the next post.

Click here for Part 5.

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Build It Yourself - Rocket Cradle

Recently, on The Rocketry Show Podcast Facebook discussion group, someone asked about a photo. CG, one of the hosts of the show, was presenting at NARCON 2016, and needed pictures of me and of Gheem, the other host, recording the podcast. CG posted our pictures on the discussion group.

Someone in the group asked, "In your pic showing you in your rocketry den, there is an apparatus that holds rockets horizontally and looks like it is made with PVC tubing. Where can I buy one of those?"

That object is a rocket cradle, and you don't buy them. You make them.

A rocket cradle holds a rocket on its side while you work on it, and keeps the rocket from resting on its fins on your work table. It can be very useful for when you're making fin fillets or applying decals, or even in the field when you're prepping larger rockets for flight. It's one of the easiest and cheapest pieces of equipment to make.

Rocket cradles are even used in real rocketry, during construction, storage and transportation of rockets. Here are two images of the Iris sounding rocket in cradles.

I'm going to show you how to make three versions of the rocket cradle.

Version 1 - For Small Model Rockets

 

This simple rocket cradle is described in The Handbook of Model Rocketry. It's made from balsa wood, but can also be made from basswood, cardboard, foam board or whatever you have on hand that's flat and can be easily glued together. Balsa is probably the easiest material to use, and if you build your own rockets, you should have some on hand anyway.

I used two pieces of 1/8 inch thick balsa stock, 3 inches wide. It's important that the ends be square, so if you're using scrap pieces with unevenly-cut ends, you'll want to square them up. One piece will serve as the base of the cradle, and the other will be the upright supports on the ends.

I drew a dividing line down the center of the shorter piece I'd use as the upright supports. The upright support board is 7 inches long, which will give me two 3 1/2 inch tall uprights.

I found the center of that dividing line, then marked spots 1 inch from either side of that center. This would give me a cradle with a 2 inch wide span.

I then marked the board so I'd have lines stretching from the outer marks on the support center line to the center of the board, at a 45 degree angle. You can use a compass to do this, but I found it easiest to use the drafting triangle from a Crayola drafting toy I had as a kid. I always figured I'd have a use for that thing if I saved it, and it has come in handy since I started building rockets.

You will end up with a square in the center of the board.

With a hobby knife, carefully cut the centering line, so you have two even pieces. As with cutting all balsa, use a new, sharp hobby knife blade, and don't try to cut in one pass - use several, light strokes to cut the wood.

You can lightly sand the cut ends so they're square if you need to. This balsa was very soft, so I got a jagged cut.

The square has now become two triangles, hard to see in the above photo, but on the right side of the upright support pieces.

Carefully cut the two triangles out with a hobby knife and a metal ruler.

You can stack the two triangles together and sand the edges even, just as you would do with a set of fins when building a rocket. These will become support struts for the uprights.

You'll be gluing the uprights onto the ends of the base board, and gluing the support struts to both pieces. I found the easiest way to do this was to glue the support struts onto the upright pieces first.

You want to glue them flush with the bottom of the upright boards, and as straight as you can. Drawing a centering line from the bottom of the cut-out triangles on the uprights helps to align them.

A piece of aluminum angle will help you get the support struts glued on straight and even with the bottom of the upright boards.

When gluing on the struts, a double glue joint will make building easier. Apply a thin layer of wood glue to one edge of the support strut, attach it to the upright board where you want it, and then remove the support strut. Allow the glue to dry for a few minutes.

Once the glue has dried a little, apply a little more wood glue to the same edge of the support strut. Again, you want a thin layer. Then, using your guide line and aluminum angle, attach it to the upright board.

The double glue joint will grab much more quickly, and you won't have to stand there holding the piece in place as it dries. It will take only a moment of your holding it in place for it to stand up on its own.

Repeat this for the second upright and support strut.

Let the glue dry for a little while, and once it's relatively set, apply fillets to the joints, just as you would do with fins on a rocket. This will strengthen the joint, so the uprights don't fall apart when you glue them to the base.

After the glue has dried for a little while - 30 minutes to an hour - you can glue the uprights to the base. The glue doesn't have to be completely dry on the struts, as long as you're careful not to break them off.

Apply a thin layer of wood glue to the bottom edges of the uprights and the struts, and make another double glue joint. Again, a piece of aluminum angle or other square object will help you get the uprights on straight and even with the edges of the base board.

The rocket cradle is complete. Allow it to dry thoroughly.

Note: I probably made the base of this cradle a little too long. I built it just for this blog post. It's a 9-inch long board. This cradle is good for smaller rockets, and probably shouldn't be longer than 6 or 7 inches. See below photo.

The Estes Viking is too short to be supported by the body tube.

Version 2 - For Medium to Large Rockets

This is the cradle seen in the background of the Rocketry Show picture. It's made of PVC, and is pretty versatile. It can hold medium and larger rockets, and you can add to it or change its configuration to fit your needs.

You see a lot of PVC cradles in online forums and at club launches. PVC is relatively cheap, useful for a number of rocketry accessories, and is pretty easy to work with. The tricky part is cutting it. You will need either some kind of saw or PVC pipe cutter.

I have cut PVC with a hack saw before, but it's hard to cut it straight. When I realized I'd probably end up building a number of things from PVC, I went ahead and got myself a pipe cutter which can handle PVC pipe up to about an inch and a half in diameter. It's a large, ratcheting clipper which looks a bit like an angry parrot. Whereas a hand saw leaves the edges of the PVC jagged, requiring you to sand the cut ends, the pipe cutter makes a clean, straight cut.

It cost me about $12 at a local hardware store.

The cradle in the above picture is actually two parts, which can be moved relative to one another, and which don't take up much space when not in use. I'll show you how I made it, and some variations you may consider.

You will need the following PVC parts, available at a hardware store:

• A length of 1/2 inch diameter PVC - at least 20 inches (it's usually sold in 5-10 foot lengths)
• Two 1/2 inch PVC cross fixtures (non-threaded)
• Four 1/2 inch PVC t-fixtures (non-threaded)

You will also need one foam pool noodle, which you can usually buy for less than a dollar from spring through summer, and sometimes into the fall.

It is important that the PVC fixtures be non-threaded. Some fixtures have threads inside them, to enable you to screw parts together. We want to be able to simply slide the parts together.

The first step is to measure out eight 2 1/2 inch lengths of PVC pipe and cut them.

Assembly is simple. Start by making the cross pieces. Put one 2 1/2 inch length of PVC pipe into each opening in the PVC cross fixtures.

Attach T fixtures to the ends of two adjacent pipes on each cross piece, with the open ends of the T fixtures perpendicular to the cross piece.

The cross pieces should now be able to stand on their own.

With a hobby knife, cut four 2-inch lengths from the pool noodle.

The lengths of pool noodle should fit right over the ends of the PVC pipe on the cross pieces.

Now you're basically done. You can move the two cradle pieces closer together or farther apart depending on the size of your rocket.

You might think you need longer lengths of PVC for the tops of the cross pieces. So did I, when I first started building this. But, unless you have a really fat rocket, it's going to nestle right in the corners of the cross pieces, and anything more on the upper arms is really just extra length.

Now, these two parts will stand perfectly well on their own. But if you want to take this cradle out to a windy field, and need some more stability, you can add feet. Simply cut two lengths of PVC pipe for each T fixture foot, put them into the openings, and add a PVC end cap fixture. Now you have a sturdy pair of cradles which are still small and portable.

If you want something even sturdier, or you simply want something that's one piece, you can join the two cross pieces with lengths of PVC pipe. In that case, you can use the T fixtures, as seen above, or you can substitute elbow fixtures, as seen here below.

You could even add a third cross piece for really long or heavy rockets which need extra support. Simply use a T fixture to attach the middle cross piece to the longer pipe, and T's or elbows for the end cross pieces.

You'll find this cradle to be a very useful tool.

Version 3 - Fast, Cheap, and Easy

Serious rocketeers never throw anything away before considering whether they might be able to use it for rocketry first.

This box once held a 12-pack of ramen noodles.

Get yourself a small cardboard box. Find the mid point of two opposite sides of the box, and mark it with a pencil.. As with the balsa cradle, measure out an inch or so on either side of that mark. Then, as with the balsa cradle, draw two 45 degree lines inward from those outer marks until they connect.

Cut out the triangles you've created with scissors.

Finished!

Now you have not only a quick and easy rocket cradle, but a box to keep parts in while you're working on the rocket.

You can cut notches on the two far sides for longer rockets, and the two nearer sides for smaller ones. You can even cut multiple sets of notches, and have a cradle that can hold two or more rockets at a time.

It's very satisfying to make a useful tool yourself, especially so inexpensively. And once you start using a rocket cradle, you'll wonder what you did without one.

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Bill Stine Himself on Quest Igniters

I think something incredible just happened.

If you're new to rockets, this might not mean anything to you, but stick with me. I'll explain.

So, Estes is not the only rocket company in the rocket biz. There are lots. One of them, Quest Aerospace (perhaps Estes' biggest rival in the low power rocket arena) is another. I have a few of their rockets, including the Magnum Sport Loader, a two-motor cluster rocket. Here's the video:

Quest has a line of motors and igniters. The igniters are called Q2G2, and they're sought after because they're very reliable - especially if you do cluster launches of low power, black powder motors (that's all I'm up to at this point). They require very low amperage to get them to fire, which means it takes less juice to get them to work quickly and properly - very important, since you want all your motors to ignite at the same time in a cluster rocket.

Problem is, Quest motors and starters have been out of stock for months - since long before I got involved in clusters. I kept hearing they'd be back in stock "in November," but, they're just not. Apogee Components has lately been saying "July 15."

I posted a thread on The Rocketry Forum to ask what the deal was. Were they ever coming back? Some insisted that, no, they are gone for good. Something to do with a recent merger between Quest and AeroTech (a company that specializes more in mid and high power rockets and motors). This Quest Q2G2 issue has been a hot topic in rocketry circles lately, and the thread has generated a lot of replies.

OK, that's Part One.

There's a guy named Bill Stine. He's basically been around hobby rocketry since the beginning, and he's the author - along with his father, the late G. Harry Stine (one of the founding members of model rocketry) - of The Handbook of Model Rocketry. If you're serious about pursuing rocketry as a hobby, and want to understand more about it, this book is required reading. I bought a copy for my Kindle and devoured it a few months back, and I turn back to it frequently. Seriously, get this book - it has a lot of information important to understanding rocketry basics. Point is, Bill Stine is one of the biggest names in hobby rocketry. He's like the Richard Leakey of model rockets.

Well, it appears as though Bill Stine himself responded to my thread.

Here's what he had to say:

Originally Posted by OldRocketeer"II"

OK, here's an official response from Quest:

First, NO, Q2G2 igniters are NOT gone forever and you will be able to purchase them again soon (both shorts and longs). The version that comes packed with the motors will not have any difference at all from the "originals" you guys love! But due to new pending BATF regulations, the ones sold as accessories are going to have a slightly different pyrogen on them - but you likely won't notice any difference.

And yes, we have been out of stock on A's, and then B's and then C6-5's. New production was completed in September and was expected to arrive via ocean freight then. Some regulations on shipping HazMat materials have recently changed in China - and we had to test new shipping cartons. Unexpected after unexpected delays just keep happening, and therefore I won't announce a "solid" arrival date now. but it will be soon...

We appreciate your patience and loyalty.

As to quality, the Quest China motors are very bit as reliable and safe as Estes BP motors (NAR MESS reports certainly support this). They are not made in a fireworks factory and the AQL process and testing is every bit as tough as it was in our own US factory years ago. In fact they are made from military grade BP - it just has a slightly slower burn rate than the US BP does. Most folks love our slightly slower burns, especially the C's. Teachers especially like the slower burn times because the kids get a bigger thrill out of slower boost that they can see...

The merger with AeroTech has been great, but of course there have been some "bumps". Running out of motors certainly was the biggest one...

We hope to have some new product announcements soon that I think the rocketry community will get a big buzz out of.

Bill Stine

Wow, I thought, Bill Stine saw my TRF thread? He REPLIED to my thread? THE Bill Stine?? For real?? Am I reading this right???

I went to his profile page. Looks like he's been on TRF for almost six years, and though I may simply not be understanding what I'm seeing on the website... it looks like my post on TRF is the only one he's ever responded to.

And if that's actually true... it kind of blows my mind.

Anyway, whether or not Bill Stine wrote this post himself, or it's someone quoting Bill, it's good to know Quest will eventually put out new starters, because I want to build bigger clusters, and I want them to launch correctly.

And I choose to believe Bill Stine thought my post worthy of comment. Because... wow...