Wednesday, April 26, 2017

Upcoming Featured Vendor Posts and a Personal Note

A couple quick things...

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For a long time, I've had in the back of my mind that I should write a "Where To Buy Rocket Stuff" post, but have never gotten around to it.

Well, there are a lot of places to get stuff. Tons of great online vendors, some brick-and-mortar chain stores which tend to carry rocketry supplies, etc. And a post like that would tend to be 1) very long, and 2) incomplete.

But I think I'll have an occasionally-recurring "Featured Vendor" series. As this blog was, at the beginning, a venue for me to share what I was learning about rocketry as I went along, so that other beginners could pick up some tips, I think as I come across good suppliers, it might be helpful for rocket n00bs if I share that information.

Of course, I love JonRocket, and have talked about them multiple times here. And I turn to them often before looking elsewhere. But every vendor has certain things they really do well, and I've tried some new places when looking for certain things lately, so I think I'm going to start giving shout outs to good suppliers.

Now, this will certainly not be comprehensive. I'll only write about vendors I've had experience with. And I'm not in the business of flaming people online, so I may only mention those I've had positive experiences with.

Actually, apart from a couple eBay sellers and iffy third-party Amazon stores, all the rocketry suppliers I've dealt with so far have been great. It's a small community, and word gets around, so you tend to get really great service.

That said, I want this blog to remain positive, so unless I have such a bad experience I feel I must report on it, I'll only talk about the sellers I've gotten good service from.

Now if you don't see a certain supplier here, it doesn't mean they're bad. Either I haven't purchased from them, or haven't gotten to writing the post, or maybe haven't even heard of them before. I'm pretty sure I had a few stores bookmarked on my old computer, and lost them when I got a new one. I'm always looking for new sources for rocket stuff - if you have a tip for me, shoot me an email!

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It's been a busy winter, and Mrs. N00b and I have had a lot on our plates. Consequently, I haven't had as much time to devote to the blog as I used to. While writing this blog isn't my job, and is something I do for fun, I do feel a sense of responsibility to my regular readers. My daily page views have dropped, and it's not surprising - people only show up if you have something to offer.

If you're a longtime reader who checks back often to see if there's anything new here, well, thank you for your continued interest, and I think things will pick up this spring.

The beginners' series on stability will continue for a few more posts, and in between, I'll have other stuff. Once I'm done with stability, I plan to move on to multistage rockets, clusters (multiple motors side by side - a fun challenge), basic rocket design and building from scratch, and lots of other stuff. But I really need to get through stability before I can move on to that stuff.

I can't wait for staging, though. That's going to be a fun one.

Flechette, a small, high-flying two-stage rocket I designed and built last week
The finished prototype of Flechette. I can't wait to test this one out - and talk about it here.
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Tuesday, April 18, 2017

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.

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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.

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Wednesday, March 29, 2017

Rock-It Girls!

Rocketry is a great activity for schools, as it promotes interest in STEM fields (Science, Technology, Engineering and Math).

This group of middle school girls build and fly - from scratch - a huge, awesome Level 2 High Power rocket!

Their teacher, Dan Feller, has picked a great project for them. Building and flying rockets is not only a great learning experience, but a huge confidence booster. These girls are certainly more advanced than this rocket n00b, that's for sure!

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Monday, March 27, 2017

JonRocket Back in Business - For Real This Time

A few weeks ago, I mistakenly reported that was back in business, after their recent move to a larger facility. That news was premature at the time.

Well, it looks like now they really are back!

The message on their home page says:
We are completing our move to a new location. Please be patient with us as we settle in. We will be adding items to our online catalog daily and it may take us a few days longer than normal to fill your order. Thanks!
Well, that is certainly good news!

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Saturday, March 18, 2017

Stability - or - What Happened to Homer's Rocket? (Part 3) - Finding the CP: Method 1

This is the continuation of an older series of posts on model rocket stability for beginners - rocket n00bs. Click here to return to Part 1, and here for Part 2.

In the previous posts on model rocket stability, we talked about Center of Gravity (CG) and Center of Pressure (CP) on a rocket, and where the two should be in relation to one another (CG ahead of CP). We learned that the purpose of fins is twofold: to move the CP aftward - behind the CG - and to correct a rocket's trajectory and dampen the back-and-forth oscillation you naturally get in rocket flight through the air.

But how do we know where the Center of Pressure is? How far behind the Center of Gravity should it be - can the CP be too close or too far from the CG? And what can we do to fix an unstable or understable rocket?

We're going to devote the next few posts to different ways of finding the Center of Pressure, then move on to other questions on stability. This is exciting stuff, because once you understand the basics of model rocket stability, you can do some interesting things. You can design and build your own rockets, knowing they'll fly safely. Even if you mainly prefer to build kits, understanding stability will enable you to modify those kits - such as building them to fly with larger, more powerful motors, or converting single-stage rockets to high-flying multistage rockets by adding a booster section.

An upcoming project - an Estes Photon Probe* kit with a booster - now a two-stage rocket!

* * *

First, finding the Center of Gravity - also known as the Center of Mass - is simple enough. All you have to do is balance the rocket on its side. You can do this on a finger, the back of a chair, or the edge of a ruler (if you can get it to stay still). I like to use a piece of string with a loop tied in the end. Balancing a model rocket on a chair back, which I have done, you run the risk of it falling off and breaking. With a string, I don't worry about the rocket falling to the floor.

Finding the CG on an Apogee Avion rocket
When locating the CG for checking stability, it's important to have the rocket prepped to fly. In other words, you need to install a motor, recovery wadding, and the parachute.

Once you've located the point where the rocket balances without tipping one way or another, you've found the Center of Gravity. The CG is the rocket's balance point, and as it flies, the rocket will rotate back and forth around this point as the fins keep the rocket pointed upward.

Well, the Center of Pressure is another kind of balance point, but rather than being a balance point of all the mass or weight of the parts of the rocket, it's an aerodynamic balance point. It's the theoretical point on the rocket where the sum of all the aerodynamic pressure is in balance. It has to do with surface area rather than the relative weight of the rocket's parts.

So, while a heavier nose cone might change the CG, its weight has no bearing on the CP. It has to do with the shapes and sizes of all the external parts of the rocket. How on earth do you figure out where that point is?

That question plagued rocketeers in the early days of model rocketry. They knew what the CP was, and knew it had to be behind the CG, but how were you supposed to know where the CP would be on a given rocket design?

There are three basic methods. Today, we'll look at the earliest and most basic one.

The Cutout Method

From Estes' The Classic Collection

In the early days of model rocketry, people knew that the CP had to do with surface area, and needed to find a simple way of locating the center of the surface area of their rockets. Specifically, the center of lateral area - the center of the area of the rocket as viewed from the side. It would be easy to find the center of area looking at the rocket from straight on - it's the tip of the nose cone.

How do you find the center of lateral area of a 3-dimensional rocket-shaped object? The answer is to simplify things a bit.

It's actually simple to find the center of area of an oddly-shaped two-dimensional object. You balance it. If we had a two-dimensional representation of our rocket, we could find its geometric center or centroid.

Using the plumb line method to
find the centroid of an odd shape

The cutout method involved making an accurate drawing of a rocket on a stiff material such as card stock or cardboard, cutting the drawing out, and balancing it. Since the card stock is of uniform thickness and density throughout, its Center of Gravity and Center of Area are the same thing!

Here's a cutout of a simple model rocket - Sounder IB - balanced on a piece of aluminum angle.

Once we've found the center of lateral area for our rocket design, we know that as wind hits that object, it should be balanced at the geometric center. Because the air pressure would be equal on all sides of that point, that's our CP. If you were to balance the rocket at that point and hold it in the fast moving air of a fan, you could point the rocket sideways, but it wouldn't pivot - the air pressure would be equal in front of and behind the CP.

As long as when we build the rocket, we make sure that the CG is ahead of that point, we should have a stable rocket.

From Centuri technical report TIR-30, by James Barrowman

Of course, the cutout method has some drawbacks, a couple of which can be deduced from the photo above.

The first is that it requires that you be able to draw an accurate representation of your rocket design, with all parts correctly proportional and in exactly the place they will be on the finished model. Not everyone is terribly gifted at drawing these days, so you'd have to be able to draft an accurate design with tools - rulers, curves, maybe a compass, etc. (Since am not skilled at drafting, I used a printout from an OpenRocket design just to show you the cutout method above. And since I have OpenRocket, I really don't need to use the cutout method - but I wanted to show it, and since I can't draw, I cheated here.)

Another drawback is this: Drawing a two-dimensional representation of your three-dimensional rocket may not tell the whole story. A rocket seen only in silhouette is not the same as the real, 3D thing.

As an illustration, here are three very similar - but significantly different - model rocket designs.

Sounder IB is a four-finned rocket, so it's simple to create a symmetrical, reasonably accurate drawing of it.

The three rockets above - which I haven't named - are all the same design. They have 18-inch long body tubes, a 4-inch plastic nose cone, and trapezoidal fins. The only difference is the number of fins - three, four, and eight.

Let's start with the four-finned rocket, since that's symmetrical in multiple directions. Here's what the drawing we would make of it on cardboard would be shaped like.

Pretty simple.

For the moment, ignore the blue and white CG marking and the red and white CP marking. We'll get to those in a bit. Also, ignore that I've done this drawing using model rocket design software. Let's pretend - just for now - that we're looking at a good drawing done by hand.

If we cut out along all the lines of our drawing, we can see that the end of our two-dimensional cardboard cutout with the fins on it will be heavier, and that the CP of the rocket will be closer to that end than to the nose cone end. As we look at the design, we see two of the fins directly from the side - in other words, we can see their full outline straight on.

Of course, if we turn the rocket 45 degrees, then our two-dimensional drawing looks a little bit different.

The fins of the rocket are the same size as they were before, but in our two-dimensional representation, they look smaller. That means that, if we were to use this drawing to find our CP, it would seem like it was further forward than if we used the first drawing. But, of course, the actual CP on the rocket isn't dependent on which way you look at the rocket.

Of course, most likely nobody would have drawn their rocket like this to find the CP, so this might seem a bit silly. But it does begin to give an idea of the limitations of the cutout method.

So, let's look at a three-finned model.

Now, with this drawing, we're looking at two of three fins, which would be 120 degrees apart on a rocket. Since we're seeing one fin directly face on, we're seeing another one at an angle, and so in this drawing, the fins are lopsided. That's OK, of course, because we're not trying to balance our cutout along the rocket's vertical axis - from nose tip through the motor nozzle. And, of course, we could rotate the view of the rocket by 30 degrees and see it like this:

Now we're seeing two fins at an angle, so they're smaller than they would look face on. The third fin is on the opposite side of the rocket, pointed directly away from us.

Because the fins in this drawing look smaller, a cutout of this balanced on a ruler would indicate that the CP is further forward than on the four-finned rocket - which would be correct. If you add more fins, there is more surface area on the back of the rocket, and the CP moves aftward.

So, which way should you draw your rocket if it has three fins? Well, it might not matter. Perhaps you'd find that both drawings have the same area, and the balance point of the cutout would be the same. But, in fact, I've found no explicit instructions about what to do for a three-finned rocket when using the cutout method. Again, either one will work, and if you build your rocket with the CG forward where the balance point is on the two-dimensional cutout, the rocket will be stable.

Since we've established that adding more fins moves the CP toward the rear of the rocket, let's go in the opposite direction. Instead of three or four fins, let's build a rocket with eight.

If you draw a two-dimensional outline of the four-finned rocket seen above, you get this:

Because you're creating a two-dimensional outline of this rocket, the four-finned version and the eight-finned version look exactly the same, which means that the cutout method suggests that these two rockets have the same Center of Pressure, despite one having twice the fins of the other!

Now, of course, you may well already know that, in OpenRocket or RockSim, the Center of Pressure is indicated by the red circle with the red dot in the middle, seen above in all the designs. And the CP on the cutout of Sounder is far behind the spot where it is balancing on the aluminum angle.

You probably also know that the CG is indicated with the blue and white checkerboard circle. The CG in these designs is an estimate, calculated based on what I've told the software each of the component parts weighs. You'll notice that, regardless of what the rocket looks like in silhouette, as I add more fins, the CP moves aftward.

You can see that all of these rocket designs have the CG well ahead of the CP, and are stable. That includes Sounder - even though the blue CG mark is behind the spot where my cutout balances on the aluminum angle! In order to make Sounder stable, according to the cutout method, it looks like I'd have to make the front end of the rocket much heavier, to move the CG further forward.

So, what gives? Can the cutout method be said to represent the CP of a rocket at all?

Well, the answer is yes - but only in certain circumstances. We'll talk about that in the next post.

Click here for Part 4.

*Original OpenRocket file by Jim Parsons - a.k.a. K'Tesh. His OpenRocket work can be found on The Rocketry Forum in this thread. It's helpful to have these, because you can take them and tweak them, which is a lot of fun.

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Sunday, March 12, 2017

NARCON 2017 Coverage

While I was on vacation this weekend, the NARCON episode of The Rocketry Show hit the web. It turned out nice, and there's a little extra bit at the end that I'm glad we captured. Click here to listen online, or search for "The Rocketry Show Podcast" and click subscribe on iTunes.

We also shot video, which was originally only available as a sneak peak to our patrons on Patreon, but now you can see it too. It's got some good stuff in it, and apart from the interview with James Barrowman (which was too wonderful not to share twice), is completely different material from the audio podcast. There's a fun bit at the end of this one too.

Subscribe to our YouTube channel to see video content as we put it out - not as often as the audio podcast, but from time to time.

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Wednesday, March 1, 2017

JonRocket Back In Business [EDIT]

[Edit: It appears I spoke too soon. Looks like JonRocket is still in the process of getting set up. I'll try to update you a soon as they're really back in business.]

JonRocket, one of my favorite suppliers of low and mid power model rocket kits, parts, and accessories, is back in business after more than a two-month hiatus while they moved into bigger and better facilities!

This is great news, as I have a growing shopping list.

A lot of items are still listed as "out of stock," but as this appears to be the first day they are back up and running, that is probably due to them being in the process of unpacking. I imagine they'll be fully-stocked very soon.

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Tuesday, February 28, 2017

NARCON 2017 Review

A Black Brant V sounding rocket, from Wallops Flight Facility, parked all weekend outside NARCON
I'm finally back from NARCON, and what a weekend it was! Three days of presentations on all aspects of rocketry, plus museum tours, dinner, and a speech by one of model rocketry's beloved company founders, all took place at the Crown Plaza Dulles Airport hotel in Herndon, Virginia, just outside Washington, D.C. The hotel was conveniently located a short shuttle ride from the airport, and not far from the National Air and Space Museum's Udvar-Hazy Center.

Plus, there was a real NASA sounding rocket - the Canadian-built Black Brant V, seen above - on display in the parking lot outside all weekend. This in fact caused a bit of a stir, when an F.B.I. agent driving by noticed the hulking rocket sitting in a hotel parking lot, got a little suspicious about this, and called the police. NAR officials quickly cleared this up, and the police apparently had a bit of a laugh about the whole thing.

This year's convention was hosted by NOVAAR - the Northern Virginia Association of Rocketry - and they put together a great event.

Orange and black are the official colors of NOVAAR.
The event director was Trip Barber, a longtime rocketeer who did some important early research in model rocket staging, and the founder of TARC - the Team America Rocketry Challenge contest for student teams.

I arrived at the hotel around 11:30 in the morning on Friday, and met CG, co-host and creator of The Rocketry Show podcast, for the first time. Despite the fact that we're on the same show, we'd never met in person before.

He gave me my official Rocketry Show shirt to wear at the weekend's events. It's pretty nice!

We went to the Udvar-Hazy Center to look around and record some video and audio for the podcast, including the teaser video I posted yesterday.

Friday night began with a town hall meeting where NAR president John Hochheimer discussed the state of the NAR and the board's pre-NARCON decisions about the organization. Membership continues to reach record highs, and the organization is in good financial shape.

This was followed by Research and Development presentations. This is a competition event for those wishing to present projects on technical development in the hobby, and presenters were vying for $1,000 in cash prizes.

International competitor Stoil Avramov showed techniques he uses for building incredibly light, perfectly airfoiled wings for competition rocket gliders. Building up wings from multiple materials - a foam core and various materials for skins and hinges, he has perfected wing building.

My camera had a hard time with some of the lighting this weekend, so some of these photos are a little blurry.

Stoil Avramov shows off one of his competition rocket gliders.
Matt Steele, of North Coast Rocketry, presented an analysis of S1 performance in the 2016 World Championships for Spacemodeling. The S1 competition is a two-stage altitude competition, where juniors fly 1/2A to 1/2A staged motors, while seniors fly A to A stages.

These are very lightweight rockets. Matt made a couple of interesting points. For best altitude, it is best to have the booster (first stage) as light as possible. If it were possible to have a massless booster, that would be best.

But a sustainer (upper stage) performs best between about 9 and 11 grams. That is the optimal mass.

Another interesting result of the analysis has to do with the timing of the staging. Multistage model rockets typically use direct staging, in which the lower, booster stage rocket motor ignites the upper stage motor, when the propellant burns its way through the top of the motor. There is no delay grain or ejection charge - just a propellant grain which is exposed at the top of the motor.

Image from

As such, there is only about 0.001 second between burnout of the lower motor and ignition of the upper motor (this 0.001-second delay was actually described by Trip Barber during his college days - the work I mentioned above - and is thus known as the "Barber delay"). In other words, staging is nearly instantaneous.

By doing this, the upper stage model is "launched" in midair, but already traveling upward very fast. Therefore, the velocity of the boosted rocket is added to the sustainer or main stage, giving the model a much higher performance than if it were launched standing still.

High power rockets and those that use composite propellants, however, often have a delay between the lower stage burnout and upper stage ignition. The booster will burn out, the stages will separate due to the drag on the lower stage, and the upper stage is then ignited by an electronic system carried on the rocket itself.

What Steele said is that, according to his analysis, there would actually be an altitude advantage on an FAI competition model if there were such a delay between booster burnout and sustainer ignition. This surprising is in contrast to what most people assume, because of the way staging is described in The Handbook of Model Rocketry. The reason is that most people forget that when describing the altitude gains in direct staging, G. Harry Stine was assuming that there was no aerodynamic drag. By increasing velocity of the rocket, you increase drag dramatically. Therefore, there is an advantage gained by allowing the rocket to coast a bit between booster burnout and sustainer ignition - provided the rocket doesn't begin to arc into the wind, of course!

In reality, delayed staging like that is impractical in a contest rocket, because there is a weight penalty. The electronics needed to ignite the upper stage would add mass to the rocket.

Tim Van Milligan of Apogee Components presented a computer analysis of drag on launch lugs, launch rail buttons, and launch rail guides. I was really interested in this one. I have seen in online forums that a lot of people assume that rail buttons create less drag than launch lugs. A lot is made of the drag of launch lugs, including in The Handbook of Model Rocketry. But to my knowledge, the drag of lugs and rail buttons had never been put to the test, and it seemed that everyone was assuming that buttons are of lower drag.

I won't go into too much detail here, because this subject will almost certainly be the topic of an upcoming Apogee Components newsletter. But I'll just say that, according to the airflow computer simulation software he used, Van Milligan found that launch lugs have the lowest drag, while rail buttons have the highest. Launch rail guides are in the middle.

Some other interesting findings - airfoiled rail buttons (such as the one seen above) actually do have lower drag than standard buttons, and the drag can theoretically be lowered further by rounding the sharp edges on the tops of them, and launch lug drag can be lowered further by shaping as well. Also surprising, long launch lugs appear to create less drag than short lugs.

Don't be fooled by these numbers. Simulations were run on an
extra large simulated model to get more clear information.
It is important to note that this is just a computer analysis using an airflow simulator. To get the real story, wind tunnel testing would need to be conducted, and flight testing would confirm whether the effects seen in this study would be significant enough to affect model rocket flight in a noticeable way.

Dan Wolf presented his project, creating a digital pressure sensor emulator, which he hopes to use in altimeter testing. This is to verify the accuracy - and consistency - of various commercially-available altimeters.

And Chris Flanigan, another contest flyer, presented comparisons of predicted and flight data for rockets flying from an 18mm piston launcher (a piston launcher is used in contest rocketry instead of a traditional launch pad and rod. It uses the motor's gasses to impart more velocity to the rocket at liftoff in an attempt to reach higher altitudes).

First prize went to Chris Flanigan, second to Stoil Avramov, third to Matt Steele, and honorable mention to both Tim Van Milligan and Dan Wolf.

The Breakout Sessions

On Saturday, the breakout sessions took place. There were four "tracks" you could choose from: TARC Rocketry, Professional Rocketry & Spaceflight, Model Rocketry, and High Power Rocketry.

Honestly, it was sometimes hard to choose what to see. There were seven scheduled hour-long sessions with one session in each track, so you could see up to seven presentations. I made it to five in total. I wish I had seen others, but CG and I needed to record some stuff for the podcast.

We chatted with the vendors in the Vendors Display Room. Those conversations will be on the forthcoming podcast. I saw presentations on NASA's sounding rocket operations, Tim Van Milligan's demo of doing a lightweight fiberglass layup for FAI contest rockets, and a presentation on painting and finishing which was really aimed at TARC rocketeers (this year's contest rules include the requirement that all rockets must be colored somehow or another - be it paint, marker, colored tape, Monokote covering, etc).

Carl Curling describes how this TARC rocket was finished and painted.

Later in the afternoon, I went to hear Jim Barrowman speak in a TARC Rocketry session. Barrowman created simplified mathematical equations for finding the center of pressure on a model rocket, which enabled rocketeers to create designs and know they would be stable in flight. His work is used in all rocket simulation software today, such as RockSim and OpenRocket. He based these equations - what became known as "the Barrowman equations" - on his work with sounding rockets.

The room was packed. He said at the outset that the session would cover the basics in Centuri TIR-30, and would not be about the Barrowman equations, and that he would understand if anybody felt bored or left. Of course, nobody did!

After the session, Jim came to the Vendor's Room where CG and I had set up a table to record, and he was gracious enough to grant us an interview. I turned to him as we were setting up and said "I have to admit that I'm pretty nervous."

But he was so easy to talk to. Jim Barrowman is a really approachable person, and we ended up having a great conversation. Once we ended the interview, we continued chatting, and had a conversation that I can only describe as delightful. CG said "I should have been recording this!"
Me, Jim Barrowman, and CG. Meeting this man was worth the whole trip.

But the interview was terrific, and I really think you'll enjoy it.

The Manufacturer's Forum

Some of the most exciting news came from the Manufacturer's Forum at 5p.m. There were nine vendors there to discuss new products, and each had a limit of five minutes to present. Here are the ones I think readers of this blog will find most interesting.

Jolly Logic

Jolly Logic has a number of exciting things in the works. First, the next iteration of the Chute Release will include more ergonomic, easier-to-use buttons. New bands and chute deployment bags for larger chutes for high power are being developed.

Second, a smaller version of the Chute Release is in the works! This was hinted at on Twitter a while back, and John Beans is currently working on it. He has to build a whole new servo in order to do it. The current Chute Release uses the smallest servo he can find. But the good news is that the new Chute Release should fit into a much smaller tube, and due to being smaller, should be less expensive than the current Chute Release. Both of these are great selling points, especially for model rocketeers with a fleetfull of smaller sized rockets. The new Chute Release will probably not be out this year, but likely in 2018.

What should be released sooner, however, is the Altimeter Four. Before Chute Release, Jolly Logic was primarily known as a maker of versatile, easy-to-use altimeters. Here is a photo of my Altimeter Two, which weighs about 10 grams.

It's a great altimeter which gives a lot of interesting flight information. But at 10 grams, it can be a little heavy for smaller low power birds.

Well, here is a 3D printed "size model" of what Altimeter Four will look like.

Altimeter Four's projected weight - one gram! Not only that, it will connect to a computer or phone and give all the flight analysis information you'd expect from a Jolly Logic altimeter, including a flight profile in graphic form like the Altimeter Three.

[EDIT] I nearly forgot to mention that John is working on a GPS location solution for rockets. Knowing how easy Jolly Logic stuff is to use, I can't wait for this to come out! It may take some time, but I'll definitely put it in the shopping cart.


AeroTech has a number of exciting things coming out - both new kits and new motors.

The four-inch diameter Monstra will be able to fly Level 1 and Level 2 HPR flights with its 38mm diameter motor tube, and four-inch airframe. It features a recovery harness from One Bad Hawk.

And the beautiful "fantasy scale" Arreauxbee-Hi is a cross between the AeroTech Arreaux and an Aerobee-Hi scale model. It flies on 29mm motors, and I wanted to take it home with me.

The new kits include screw-on motor retainers instead of motor hooks, and also have both launch lugs and rail guides, so the rocketeer has the option of either one without having to purchase additional hardware separately.

A new single-use F motor is debuting soon, the F67 Economax. Why an F? Gary Rosenfield, the owner of AeroTech, explained it was their attempt to get the most power they could from 30 grams of propellant. 30 grams is the most that can be legally shipped via the US Postal Service, rather than via UPS with a HAZMAT fee.

 The 14-second delay won't be featured. Delays will be 4, 6, and 9 seconds.

Speaking of US Mail shippable motors, another exciting development by AeroTech for HPR fliers is a non-HAZMAT J motor! This is pretty unusual, and it's accomplished by dividing the motor into 13 individual 30 gram propellant grains.

This, says owner Gary Rosenfield, will be about as far as they'll go with that, so don't expect a mailable M motor any time soon!

As for the Quest Q-Jet composite model rocket motors, they only had on display the A3 motors, which have passed certification, but the others are still waiting to finish the process. They'll all be released at the same time, once all motors have been certified. According to Gary, this is one of the hardest motors AeroTech has made, but it will be exciting for us when they are finally available.

Apogee Components

Tim Van Milligan announce Apogee's intent to release ten new kits this year. Apogee is hiring a marketing person and a web developer.

Aerospace Specialty Products

ASP showed off some of their new 29mm powered mid powered scale kits. These weren't new at NARCON, but it was nice to see them in person, particularly the Sandia Sandhawk, the D Region Tomahawk, and the WAC Corporal models. These are pretty simple kits that a relative beginner can assemble without much trouble, but with accurately-sized parts so that an experienced builder can add details and have a very faithful scale model.

Due out mid to late summer are three more scale kits, details of which will be announced later.

eRockets (including Semroc)

There are now 125 Semroc kits through eRockets, and the number continues to grow.

The Blue Jay is a new delta-wing glider with an extra wide keel and a beefed-up front end.

The Maple Seed is a sort of odd-rock with maple seed-shaped fins. At apogee, the seed-shaped fins detach and helicopter down, just like the real thing.

Also coming out later this year, provided Estes doesn't change their mind and re-release it, will be a Semroc version of the Scissor Wing Transport, a boost glider which always had a bit of trouble flying just right. Randy Boadway of eRockets says he's solved the problems with the Scissor Wing, and it should be flyable more than once or twice.

Chad had an Estes Scissor Wing Transport, and I can tell you that it was tough to get it to fly right.

North Coast Rocketry

 Matt Steele presented a new kit, an upscale of the Estes Goblin, called the Hobgoblin. It will fly on 29mm motors. It's actually a much smaller version of a North Coast Hobgoblin from years ago, which was 8 inches in diameter. This one is pretty fun and still nice and fat at 2.6 inches in diameter.

Also coming soon from NCR are 29mm screw-on motor retainers similar to those sold by Estes, but with rocket nozzle details attached, so your mid power model rocket can look more like a space launch vehicle.

Dr. Zooch

Wes Oleszewski of Dr. Zooch presented at the Manufacturer's Forum. While they won't have any new kits, Wes does have a series of books on the history of spaceflight, Growing Up with Space Flight (click here for an example).

You'll hear more from Wes on the upcoming podcast.

* * *

At the banquet, Lee Piester of Centuri Engineering, told the story of his time running a much-loved model rocket company. It was an inspiring story with some surprising details, and even made me nostalgic for Centuri rockets, even though I was far too young for rocketry when Centuri did most of its business, being nine years old when the Centuri line was finally discontinued.

We hope to have Lee Piester on the podcast soon.

Door prizes were handed out. John Beans had donated ten Jolly Logic Chute Releases, and while I didn't win one to replace my lost one, as I'd been hoping, I did in fact get a pretty great prize for me - a copy of Tim Van Milligan's book Model Rocket Design and Construction, something to add to the library.

* * *

After the proceedings on Saturday, I sat in the hotel restaurant with Bill Cooke of The Rocketeer's Corner blog and had a great chat. Bill's a really nice guy with a fun blog (check out his stuff about "Geezer TARC"), and we shared modeling tips - well, mostly I asked him how he builds such nice looking rockets.

Sunday I returned to the museum with those who stuck around, and stayed there for five hours, looking at planes, models and space ships. It was a perfect weekend, and I managed to only spend slightly too much money.

Thanks to NOVAAR and the NAR for such a fun conference!

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