Monday, October 21, 2013

It's Always Interesting

One of the things I love about model rocketry is that even when you're in familiar territory, the unexpected can happen and there are always new things to learn and deal with.

I recently launched my Estes Mini Comanche 3, configured with all three stages. It's kind of obvious from this picture that it didn't go quite as planned.

The problem is that the second stage failed to ignite. The first booster ignited, the rocket flew about 40 or 50 feet up, then gracefully arced straight down for a perfect lawn dart landing.

One of the only good things about this is that I was left with a situation that allowed for full investigation. Nothing bugs me more than a failed launch that results in a lost rocket (or bits of it) that make it hard to piece together what happened.

Not that I can fully explain it in this case. I of course verified that  the correct motors were installed, in the correct order, and not backwards. It just looks to me like the first booster (an A4-0T) burned through and failed to ignite the second stage (another A403T). What seems really odd is that there are no burn markings at all on stage two. It's like the first booster never burned all the way up - though it did.

Of course I will try again. The other good thing was the the only damage was to the long skinny body tube of the main rocket. This was easily cut off and I've replaced it with a new section, so I'm ready to go.

Tuesday, August 6, 2013

Swing Test - oops!

Here's one thing that authors never tell you when they suggest a swing test to check your rocket's stability: if your string is wrapped around a section of plain body tube on a longish rocket, don't swing too fast.

I'm currently finishing up work on an Estes Super Neon XL. It strikes me as bottom-heavy, so I was more concerned than usual about the stability. I probably shouldn't worry because it's a proven Estes design, built to spec, with plenty of successful launches listed on (I actually did one small modification - I added an ejection baffle. But this should push the CG forward, if anything.) But I obsess.

I looked online at two posted Rocksim files. I don't have Rocksim (I use OpenRocket, but it doesn't support tube fins), but the pages show the calculated CG/CP values: 18.25"/25.25" and 19.5"/28.8". Quite different, though the former doesn't appear to have included the balsa fins. What then concerned me was that my measured CG value was more like 25" with no engine, and 28.25" with an E9-6, and that seemed a bit too close for comfort even if the CP were truly 28.8. Hence the swing test.

Sure enough, I couldn't get a stable swing, so I began taping lead weight onto the nose. Playing with the Rocksim design in OpenRocket, I figured that I'd need at least a couple ounces at the tip of the nose to get very comfortably stable. Sure enough, once I'd added 2.25 oz I got my first clearly stable swing test. But of course I couldn't stop there, and I had to back off and try again with a little less weight. Again, I had trouble getting it to go stable, so I thought "maybe if I swing just a little faster…" and that's when - FOLD!

Fortunately I had the couplers and spare tube on hand for a quick splice.

But what to do about the stability? I've decided to add just 0.25 oz at the tip of the nose and go for it. Here are my reasons:
  • I've read that the swing test is a very conservative measure, and difficult with larger/longer rockets. 
  • It is a proven design, As stated, I built it to spec, and my finished rocket (minus primer/paint) is 6.25 oz - less than the Estes estimated weight of 7 oz - so it's not like I incorrectly piled weight on the back end.
  • None of the reviews I've read suggest any instability.
  • While the rocket is well under the max lift weight for a D12-5 or E9-6, the idea of piling on a couple of ounces also concerns me. As is now, OpenRocket simulations show a speed of just under 25 mph off the rod with an E9-6 (better with the higher max thrust of a D12-5), which again seems to be cutting things close.

So I'm going to prime it, paint it, and cross my fingers. I'll update this post once it's flown.

Update: Back in September the Super Neon flew beautifully - then landed at the top of a very tall tree, where the bright parachute and nose cone still flutter in the wind five months later. (Oh, and I lost my Little Bucky Jones on its first flight the previous month. Losing streak.

Friday, July 12, 2013

Where and When?

There are always questions about where and when to launch. The circumstances vary wildly depending on where you live: city or rural, rainy or none-dry. And of course, what are the local laws and regulations? This is another topic that is interesting to revisit now that I'm a "responsible adult" rocketeer, as opposed to my days launching as a kid, without adult supervision. And I'm not lucky enough to live near friendly Farmer Brown and his gazillion acre fields. So I thought it would be interesting to document what I've learned in case anyone else can benefit from it - particularly any rocketeers in the Denver area.

I'll start by stating two assumptions: (1) All laws and regulations should be followed: those of the city, state, and the NAR. (2) You have a car to get you someplace (not really an option when I was a kid).

The first and most obvious answer is to launch with a club. Here in the Denver area there are CRASH and NCR, or farther south, COSROCS. I've launched with CRASH once and plan to do it again - especially when I'm launching high or with mid-power engines. But I also like to launch on my own schedule, with family and friends, sometimes making a picnic of it. So that is more the direction of this post: where can I do that?

There I several things to consider, so I'll discuss them in sections:

Finding a Site That is Physically Appropriate

NAR guidelines specify minimum site dimensions. Maybe there is an obvious spot near you, maybe not. I've used Google Maps to search for areas that look big and open, are mostly devoid of trees, and are public. Car accessibility is also a factor. But you can't tell everything from a map, so you need to scout it out to see what it really looks like. I'd rather launch in the soft, well-watered grass of a public park than in a field of tinder-dry brush or in the middle of a bunch of dirt and prairie dog holes.
Also, who else uses the park? A big open field is no good for a launch if it is full of soccer games. You need to be aware of the safety of others who won't be looking out for rockets. 
Finally, what borders the area? We all know that rockets can and do go out of the "minimum site dimensions." Could it land on private property? Power lines? In a lake? On the roof of the nearby rec center? On a highway?

Is It Legal to Launch There?

So you've got a site that you think will work. Is it legal? My initial assumption was to check fire codes, again turing to Google. In Denver (and every other municipality in the area that I've checked), the fire code does make a distinction between fireworks and model rockets. In all cases, the wording is something like this: 

The storage, handling and use of model 
and high-power rockets shall comply with the requirements 
of NFPA 1122, NFPA 1125, and NFPA 1127

Inconveniently, the NFPA rules are not immediately available online. But you can read (not download) them for free if you create a logon to the NFPA site. PITA, plus they then send you weekly (or more) emails about buying their products. But here it is in a nutshell:

NFPA 1122 -- Code for Model Rocketry - This is the NAR code, almost word for word. I don't know the history, but I think the NAR helped develop it. Hooray for the NAR!
NFPA 1125 -- Code for the Manufacture of Model Rocket and High Power Rocket Motors - A big N/A for me.
NFPA 1127 -- Code for High Power Rocketry - Maybe this will apply to me someday, but in that case I'd be off in the desert launching with NCR.

It is interesting to note that while the fire code makes a distinction between fireworks and model rockets, any fire fighters you talk to may not. Consider their perspective. They're not exactly in the business of encouraging people to find new ways to burn things fast and hot. If they aren't familiar with model rocketry, they may just assume that rocket = fireworks if you ask. I was lucky enough to speak with a very nice guy from the Denver FD who, despite a lack of familiarity, took my name and number, looked into it, and called me back a couple hours later to confirm that yes, I'm "good to go" in Denver.

But then there is another question. Recently I was launching in a public open space in Northglenn, Colorado. An officer drove up and approached - very friendly. I immediately told him that, though I'd never launched there before, I'd checked fire codes for the town. His answer was that while it may be OK according to fire codes, it may be against municipal codes. Though he said it in an odd sort of way, like he didn't know or care all that much. He checked out my gear, asked a few general question, took my name and number, and left with a friendly "be safe" and that was that.

(I'll also point out that, while he said that he saw a launch from the road and came to check it out, the area was also bordered with houses on one side. While I never got anything near them, that is something else to consider: Are nearby residents going to freak when they see missiles going up from the field near their house, and call the police? It may be legal, but it may also be a hassle for you and the police.)

I've honestly never known of a place where it was actually, all-around illegal to launch model rockets, but I'm sure some places exist. Bottom line: Ask the Fire Department. Ask the Police. CYA.

(You might also ask your local hobby shop or other rocketeers that you meet. I imagine the local hobby shop may have liability concerns about recommending places. As far as any other advice, I'll say it again: CYA.)

Really? Is It Legal to Launch There *Now*?

As I write this, at least half of Colorado's counties have fire restrictions in place due to wildfire danger. The rules on what constitutes a fire restriction vary by county, and even within counties there may be different stages or levels of restrictions - and they may split the county, with different restrictions in different areas. It gets complicated. And on top of that, there may also be municipal or state-wide bans. And not every county or city clearly posts when there is or is not a ban in place. Colorado just lifted its statewide ban a week ago, and the only way I know is from a Denver Post article that came up in a Google search.

One handy place to check for county fire restrictions is the Colorado Office of Emergency Management.
The Denver Fire Department can be contacted at 720-913-3473 Monday through Friday, and they've always been helpful when I've called to check on things.

There are a couple big parks in the area that I like for launching. One is in Golden, but Jefferson County is always among the first to enact fire restrictions, so this year and last, this has been out from late spring through summer. I've launched many times at Ruby Hill Park in Denver. It's not perfect - it's surrounded by big trees, and beyond them are power lines. But while I've had a couple of severe weathercocking launches, I've never gone near the power lines or surrounding roads (though I did have to wade into a pond once) and I've never lost a rocket in a tree. The Denver FD has confirmed for me that I'm OK to launch there. The center of the park is a big basin (the name Ruby Hill strikes me as the opposite of what it should be) that is never overly dry, and that section has no soccer fields or anything. I never see more than the occasional person stroll through, though the nearby playground gets a lot of action, and I have had a landing there. It's a bit of a walk from the parking area to the center of the field, but it is a pleasant setting, and feels surprisingly isolated given its location.

Update - 12/2016: It gets even more complicated. I was with a small group of friends and their kids, many launching their first rockets, at Ruby Hill Park a couple months ago. We were approached by a ranger from Denver Parks (who even know that never parks had rangers?) who told us that although it is OK with municipal and fire codes, launching is a violation of parks codes. Jeez, how can you ever know? He was super friendly, and lest us do a last launch or two, but then we shut it down. Sadly no more launches at Ruby Hill.

Since I first wrote this post over three years ago, I've attended many launches with CRASH, NCR, SCORE, and even one with Tripoli Colorado. Unfailingly, they are fun and friendly. So while I'll miss the family-only spur-of-the-moment option, it looks like it will only be club launches for us from here on in. (Unless we make a new friend with loads of empty farmland!)

DISCLAIMER NOTE:    This information  presented  only as my personal experience, and is not intended as specific or authoritative advice..  The  author  is  not   responsible  for  any  liability  or  loss  related  to  the  launches of others, and all individuals are responsible for selecting and verifying the viability of their own launch areas.. 

Tuesday, July 9, 2013

The Secret to Perfect Fins

It's shaping up to be another skimpy summer for rocket launching in Colorado. Fire bans have been declared almost everywhere in the state, and even in places where there are none (including Denver) you can't help but feel guilty just thinking about launching with all the wildfire destruction going on.

But there is always building. I've just finished two kits: Pemberton's Little Bucky Jones and FlisKit's Buck Shot.

The Buck Shot is my first MicroMaxx kit, so I'm excited to venture into that territory. I know my limitations, and my build skills aren't up to the challenge of making a high quality small model, so I just put it together and painted it as best I could without sinking in time going for perfection.

The LBJ, on the other hand, is such a unique and gorgeous kit that I found myself obsessing over it for weeks. Which brings me, sort of, to the topic of this post.

For several months I'd been vaguely planning on writing my tract on the ultimate solution to perfect fin finishing. Of course, most of the delay involved figuring out how to perfectly finish fins. There is no shortage of information out there on blogs, vendor videos, and message boards, and everyone has their favorite techniques. But something must be the best, right?

I have, I'm proud to say, discovered the true, absolute best solution. Are you ready?


There's sanding sealer, Elmer's Fill 'n' Finish, microballoons, CA glue, Kilz, urethane, on and on. But ultimately, no matter what materials you use, it comes down to patience. Lots of layers, lots of sanding, until you have it just right.

Sanding sealer stinks, FnF can be a little delicate, watered down Elmers can warp fins. But for me, the patience thing is harder to manage than any of those. Though if I stick to a willingness to always do another layer of fill/sand, or primer/sand, the imperfections that I see (and there are always some) get smaller and smaller.

That said, here is my current process when I'm tying to do a really nice job on a rocket. I'm not going to go into extreme detail on any particular technique; there is so much info on the web, and really you just have to try things out and experiment. But here are my favorite methods. And I couldn't help myself - I've gone beyond fins to describe all of my "best practices."

Construction Adhesive

I've barely dabbled in mid power rockets at this point, so for me the go-to glue is wood glue. I've used a lot of Elmer's, but lately I've been using Titebond. Used properly on balsa and cardboard/paper, this will make a bond stronger than the materials themselves. I do, however, sometimes use 5 minute epoxy in two cases. First, because it doesn't grab as quickly as wood glue, it is handy for doing things with a tight fit, like engine mounts, centering rings, and couplers. Second, it is nice to use if I have to fill a nose cone with some lead shot to weigh it down. Stinky stuff though. I avoid it when I can, and wear nitrile gloves when I do use it (but even here, the web has lots of conflicting information on what gloves are best protection from epoxy).


Light plywood and basswood are great because the grain is typically a lot tighter than balsa. I've gotten some terrific, smooth finishes on these with just one thin layer of FnF, sand, and then a couple primer/sand cycles.

I find balsa tricker. I've gotten decent results with one or two FnF/sand cycles, followed by a couple of layers of sanding sealer (sanding in between), then primer/sand/primer/sand. But lately, I've become a fan of this:

  1. Before mounting, laminate the fins with ordinary copier paper, glued on with a thin layer of wood glue. It is very important to make sure you smear the entire fin surface with the glue before sticking on the paper. I smooth out the paper as best I can - I use a small section of PVC pipe. The fins may warp like crazy with the wet glue on them. Put them between wax paper sheets and press them between two very flat surfaces, weighted down amply, for a day or so to dry.
  2. Smear the fins with a thin layer of CA glue (crazy glue) and let them dry. This will turn the white paper covering translucent as the CA glue penetrates and hardens the paper and wood fibers.
  3. Sand sand sand until smooth. The CA glue will leave a bit of texture, but with a lot of sanding you can get them glassy smooth.
  4. Mount the fins on the rocket.

This process adds a ton of strength, and you don't have to worry about grain showing. I recently did a test where I weighed fins before and after this process, and also compared the weight to lite ply fins of the same size. The laminated fins were still a lot lighter (but I lost my data!).

Body Tube Spirals

This is something that I never considered early in my rocket career, but now I can't stop myself from doing it - I'm compulsively bothered by visible spirals on a painted rocket. I use some slightly watered down Elmer's FnF (now just called Wood Filler) and paint it along the spirals. Then after it dries, I sand the whole tube. It's not a lot of effort, so it is totally worth it (though I might not say the same thing if I built an Estes Mean Machine). Some people claim to get rid of spirals with several coats of primer and sanding, and that might work with a thick primer like Kilz, but I think this is a good approach. Sanding the tube also helps with fin and paint adhesion.


I have two favorites here. For small, light rockets, I like Titebond Molding and Trim glue. It doesn't run, and shrinks and bubbles less than other wood glues. (I had a hard time finding it. Regular hardware stores don't seem to carry it. I had to go to a specialty woodworking store.)

For bigger and higher power rockets, I like Fixit epoxy clay. You can find some good directions for its use on the Apogee Components website. It's not quite as easy as it looks. When mixed, it's just a little too firm to just smooth out with a swipe of your finger. You have to work at it. In fact, you need to work at it. When dry, it sands nicely, but of course is much harder than the body tube and fins, so if you need to do a lot of sanding to get the fillets nice and smooth, you might end up digging in to the other components. It takes practice - my first rocket using this had OK-looking fillets. My second one had a few that were perfect, and a couple with minor imperfections. But in the end, you can get super solid joints, with fins that look like they grew right out of the body tube.

Nose Cones

For balsa, I've gotten amazing looking results with a layer or two of Fnf/sanding, then the same with sanding sealer, then a few coats of primer and sanding, finishing with wet sanding with a really fine grit. Totally worth the effort, though it still results in a nose cone that will get dinged the first time you chuck it in the back of the car with a bunch of other stuff. I've seen recommendations to add a couple layers of CA glue to harden it up and give it a sort of shell, so I'm still experimenting with that.
For plastic nose cones, like BT spirals, I'd never considered anything to do in my early days. Now, I always sand the seams *completely* down so they cannot be seen or felt at all. Then I need to smooth it out with successively fine layers of sandpaper. Ten more minutes of effort, but a way way nicer looking nose cone.


Lots and lots of people swear by Kilz because it goes on so thick. And it is great for hiding all the little blemishes - just like the "orange peel" texture applied to drywall in all modern residential work. So there is typically a lot of sanding needed to get back to a really smooth finish. If you're willing to do that, Kilz is great. However, I prefer not to use it on small models, or ones with lots of detailed pieces and angles that are hard to sand. In that case, I'll use a "regular" sandable primer (I use Rustoleum).


I don't have experience with enough brands to give a strong recommendation here. I can only say that Valspar is awful, and that I use Rustoleum Ultra Cover 2x glossy pretty exclusively and am happy with it. It goes on pretty well and there is a good color selection.

Clear Coat?

Question mark here. Do you need a clear coat? It is often recommended over decals (though I'm not a huge decal user), but many say that it will eventually dry and crack/peel the decals. I found this process on the web, using Pledge and Simple Green. It works, but it is not magically transforming. I use it occasionally and am still undecided if it is worth it.

So there we are - that's the state of my technique as of mid-2013. Perhaps, even hopefully, I'll want to totally revise this in years to come. I like to try new materials and methods, and I'm sure I'll get new favorites.

But it all comes back down to the P-word. Without lots of patience, nothing is going to look good.

Of course, despite all this, I look at every rocket I build, especially the ones I really really tried on, and just see the little flaws. And the parts, that were perfect? Well, they get bashed up in flight anyway. So at some point in a build, I always end up saying "enough is enough" and I just call it done so I can get the bird in the air. Patience has its limits, at least with me.

Saturday, June 22, 2013

The largest rocket I'll ever build

Non-operational, unfortunately.

The body tube is a 6 foot length of 30" diameter concrete column form from a construction supply place (Whitecap). The fins are two layers of corrugated cardboard, laminated with corrugation in perpendicular directions, lined and attached with black duct tape. The nose cone has a frame built of 2-by-2's, then is made with cardboard and covered in a mylar. It was painted with Rustoleum indoor/outdoor latex using a roller. It is kept from tipping by lashings to the playset behind with long, thick zip-ties.

Perhaps the most impressive part is that I had only three days to acquire parts, assemble, and paint it. So most of the work was done late at night, in unusually high winds. No time to smooth out body tube spirals on this beast.

The very top was meant to look like the escape tower of the Saturn V, but most of it was broken off by a rambunctious young guest at the birthday party the day before. Given more time, my idea was to make this part into an actual rocket that could launch off of the top, just as the actual escape tower was meant to do. Alas, the whole thing has found a permanent position elsewhere in our yard, under a big tree, so that won't happen.

As always, I learned something about materials and techniques during the build. My big lesson on this one? Duck brand tape is far superior to the Scotch brand duct tape.

Tuesday, June 11, 2013

How High?

I'm horrible at judging distances. I like to blame this on the fact that I've got great vision in one eye, and am very nearsighted in the other, effectively leaving my brain with no parallax to work with. After a rocket lands, I'm the guy searching for it in bushes twice as far from the pad as it actually landed.

I figure that I'm probably the same way about height, but I have another excuse there. I've run 1000 feet, or 2000 or 3000, but I've never jumped that high. But on a recent vacation to Arizona, it occurred to me that it might be interesting to compare the heights of some familiar things to the heights of some familiar model rockets. So here we go:

   14 ft - Robert Goddard's first liquid fuel rocket
  154 ft - Space Shuttle external tank
  305 ft - Statue of Liberty
  363 ft - Saturn V rocket
  455 ft - Great Pyramid at Giza
  500 ft - Estes Big Bertha (C6-5)
  564 ft - Highest baseball pop fly (Ted Williams, 1941)
  570 ft - Depth of Meteor Crater in Flagstaff, AZ
  630 ft - St. Louis Arch
  698 ft - Denver "Cash Register" building
  900 ft - Estes Mean Machine max height (E9-6)
 1100 ft - Estes Alpha III max height (C6-7)
 1454 ft - Empire State Building
 1463 ft - Eiffel Tower
 1600 ft - Estes Mongoose (C6-0 + C6-7)
 1771 ft - My personal height record - Estes Magician (E9-6)
 2722 ft - Burj Khalifa (Dubai)
 2887 ft - NAR altitude record for a D engine flight
 4030 dt - Snowmass ski resort "True-Up Vertical Descent"
 6000 ft - Grand Canyon (depth)
 8664 ft - NAR altitude record for a G engine flight
14,509 ft - Kite AGL record
20,000 ft - NCR standing FAA waiver ACL height
6 miles - Homer Hickam's final BCMA launch, June 1960 - Auk XXX1

There you have it. Your "typical" BP model rocket will go around as high a big skyscraper, but you need a G engine and a good design to fly out of the Grand Canyon.

As a rocketeer, I find the NAR records and Homer Hickam's launches (detailed in the very readable Rocket Boys) pretty darn impressive.

Looking for record heights on more advanced amateur rockets becomes a pretty divergent (but interesting) task because you start to consider clusters, multiple stages, and even experimental rockets. So I didn't include anything from that realm. The amateur record height is 72 miles - technically in outer space - so we'll just leave it at that.

Monday, May 6, 2013

Rocket Snobbery vs Continuous Improvement

I came across an interesting post on the Rocketry Forum. It touched on a lot of things that have been going through my head lately, so I wanted to put down my thoughts. It's an older thread, and much of what I have to say overlaps with what others have already said, so I haven't posted there (I still may).

A few of the responses dealt with the idea of a rocket's build quality being somewhat indicative of the builder's character. I agree with that to some extent, but with the very big qualification that it is only a small part of the equation. You have to consider the builder's experience level and motivations to a large extent. Several posters brought up examples such as kids doing their first build, or folks with disabilities. You can't ignore those contexts.

Plus there are contexts that you can't know. Someone who is intelligent and skilled in other areas may just dabble in rocketry, but devote a larger portion of their energy to other endeavors. Or I might do a far less than perfect build because I'm working on it with my five year old son and don't want him to lose interest over days of iterative finishing steps. And just because you see a forty year old with a rocket doesn't mean he's been building and flying for 25 years.

One poster brought up some ludicrous examples such as how he'd never go to a doctor who did a half-assed job on a rocket, stating (and I'm paraphrasing here) that there is a definite correlation between the hypothetical doctor's demonstrated rocket building effort/skill and his professional effort/skill. Extending this, would he avoid doctors that are poor golfers, or doctors that don't dress impeccably? Crazy. One could even argue for the opposite: a doctor that spends so much time on rocket perfection may do it to the exclusion of honing and updating his medical skills.

And several people wrote about how different people have different foci. The rocketry field is so vast, you may be interested in performance, or appearance, or scale models, or avionics. Or you may just like chasing down rockets. You don't have to be perfect in every aspect of rocketry, let alone life. You can't be.

Then there were digressions about the whole issue of events where "everybody wins." I agree with the tenet that there are definitely cases where "one is better than the other," and of course the design of certain events requires a winner. But there are also contexts where, I think, just showing up and participating is deserving of recognition, regardless of the relative quality of your work. That takes effort that a lot of people don't put in. I recently attended a friend's "rocket party", where the hosts generously supplied small, already built Estes E2X rockets for the kids (5-ish) to decorate and fly. Awesome - you've never seen so many excited kids. But a couple families who didn't know which way to stick in an engine bought, built, and brought their first kits. Build quality? Totally beside the point. Those folks are beyond cool.

I think the core of the issue, and I'm not sure anyone stated this explicitly, is that it's hard to divorce your experience and background when making judgement, silent or otherwise. Consider the forum audience: people who obsess over details like fin finishing techniques and baffle construction. Much to the credit of the group, a majority of people seemed to come down on the side of "hooray for anyone building rockets and enjoying themselves," though there was frequent and honest acknowledgment of silently judging and comparing craftsmanship. But that is normal and natural. Just keep your outward expressions appropriate. As one poster put it: "You don't have to be a d-bag."

There are a *lot* of people building model rockets. The odds are overwhelmingly in favor of the fact that many people will build rockets that look worse than yours, and many people will build rockets that are better.

So ultimately I think that you have to just focus on your own self, and absorb the experience of others in the pursuit of your own goals (and vice-versa). I like to try and live by the notion of continuous improvement. Each day, each rocket, is a chance to do a little better. I like to experiment with new techniques. I've gotten very interested in build and finish quality, but I didn't used to be. I've explored other areas:  using simple avionics, and building my own ground support equipment. But bottom line is that I like to build 'em and fly 'em. I'll admit that I never have, and probably never will, complete a rocket and look at it thinking "That is perfect!" But at the same time, I still have some of my older builds that are objectively and unquestionably awful looking. And I still like to take them out and fly them. They're old friends.

Would I love it if someone came up to me and complimented the craftsmanship on my rocket? Hell yes. But I'd also love it if an experienced builder approached and shared a good new technique with me, even if the implication was that my method was not-so-good. Not to be cliche, but it's all about the journey, because there really is no final destination. And there's no benefit in not being nice to each other along the ride.

Tuesday, April 30, 2013

My Take on a Mid-Power Launch Pad

Having recently completed my first mid-power build (a Madcow Mozzie) I realized that I probably needed to upgrade my ground support equipment with a beefier pad. The two most readily available options looked like these:

Estes Porta-Pad E  While this is quite inexpensive and does come with a 1/4" rod, several reviews I read described it being crushed under the force of an F or G engine, so that ruled it out for me. (And also put to rest once and for all any thoughts of trying to use my standard Estes LPR pad!)

Aerotech Mantis  Reviews on this seem to be mixed. Some people say it is great, some people report issues. Plus it's huge and doesn't even break down well for storage. For $100+, I'd want something that was absolutely ideal. (Some people seem to like this because the rocket sits so high off the ground, making it easier to set up. Really? You can't get on your knees every now and then? Do you train flying monkeys to chase down your rockets?)

There are other (harder to find and equally expensive) options out there, but the general consensus on discussion boards seems to be to build your own out of PVC. Inexpensive, accessible, reliable, and - of course - fun.

I found plans/descriptions of a few things that came close to what I wanted, but they all have, to my mind, some shortcomings (see the appendix below). Ultimately I used these ideas and came up with my own design.

My criteria:
  • Low cost.
  • Easily obtainable parts.
  • Easy to build.
  • Compact for storage/transportation.
  • Adjustable launch rod angle.
  • Easy way to change launch rod diameters.

Parts List
1" PVC, 10' (*)

1.25" PVC, 5'

1" PVC side outlet
(Be sure all joints are slip-fit. Not everyone has this - Lowe's does)
1" PVC end cap (2)

1.25" PVC end cap

1" PVC couplers (2)

1.5" 1/4-20 thumb screws (2)

1/4-20 wing nuts (2)

2.5" 3/8-24 hex bolt

drill chuck
(Salvaged from an old cordless drill, but probably obtainable cheap at a thrift store)
stainless steel bowl or pot lid for blast plate
(from thrift store)

Total: $19.02 (Plus launch rod(s) - I went for 4 foot,  1/4" diameter stainless from a local supply house - less than $10)
Low cost - check!
Easily obtainable parts - check!

(*) Aside from strength, the choice of 1" PVC for the legs was deliberate. Since one leg is made adjustable with a telescoping tube arrangement, I needed two PVC sizes, one sliding neatly inside the other the other. 3/4" PVC does not fit into 1" PVC.

Tools needed: drill, drill bits, saw, PVC cement or other suitable glue. Optional but nice: drill press, mitre box

figure 1
  1. Cut the 1" PVC into three lengths. I did 16", 16", and 14". (Longer if you so desire)
  2. Cut off a 14" section of 1.25" PVC.
  3. Glue one 1" PVC end cap onto one (only one!) of the 16" PVC legs.
  4. Slide the 14" 1" PVC section into the 14" 1.25" section, with an inch or so sticking out. Drill 10 1/4" holes straight through both pieces at one inch intervals. A drill press would be ideal here because the idea is that you want the holes to line up no matter how far you slide in/out the 1" section, giving you an adjustable length leg that can be secured in place with the thumb screws and wing nuts. But even with my lousy drilling skills I was able to get this to work after widening the holes a bit. (See figure 3 below to see how this looks on the assembled pad.)
  5. Glue the 1.25" PVC end cap on to one end of 1.25" PVC section.
  6. Drill a 3/8" hole straight through the top of the PVC side outlet, all the way through to the bottom. Slide the 3/8-24 bolt through from the bottom and screw on the chuck (see figure 1)
  7. Drill a 1/4" hole in the center of your blast plate.

Easy to build - check! The hardest part for me was getting the chuck off the old drill.

Putting your launch pad to good use
figure 2
For storage/transport, Slide one holey leg all the way into the other and put in the thumbscrews/wing nuts to hold them together. Add the other two 16" legs onto this with the two 1" PVC couplers, storing your launch rods inside. This protects your rods during transport, and it all stores easily in the rafters of a garage or the corner of a closet. (OK, I know, the side-outlet-chuck, one PVC end cap, and the blast plate float around loose. Not perfect. I'm sure I'll forget to bring them to a launch one day.)
Compact for storage/transportation - check!

figure 3
To set up, separate the legs, remove the couplers, insert the legs into the tripod top, add the extra end cap on the third leg, put the launch rod in the chuck, and slide on your blast plate. Adjust the angle of your pad by removing the thumb screws and changing the length of the telescoping leg.
Adjustable launch rod angle - check!
Easy way to change launch rod diameters - check!

I just tested mine out last weekend, launching B through F engines with varying rod diameters, and it worked great.

I admit that the adjustable leg doesn't allow for a huge range of angular adjustment - maybe 6 degrees is all. But rarely if ever do I launch at a higher angle, and I especially wouldn't do it with a mid-power rocket.

One modification that I've made since this is to widen the blast plate hole to 3/8" and put in a 2" threaded wiring post from an old lamp. This will prevent the blast plate from tipping and wobbling so much at launch. I'm also considering creating an easy way to stake in the ends of the tripod legs to hold the pad solidly down.

Appendix - Inspiration

Four Legged Launch Pad by John DeMar
Judging from the number of references I've seen to these instructions, this seems like a very popular design. What I don't like about it however is the lack of angle adjustment, the less compact head assembly, and the fact that four legs could be inherently unstable on uneven ground.
This is an interesting idea for angular adjustment, but it's complex and bulky.
I think these two references show a great idea to allow for angular adjustment, and I think a drill chuck is ideal for holding/changing the rod. However, as noted in the discussion thread, a typical drill chuck has 3/8-24 mounting threads. Good luck finding a suitable eye bolt. I called all over town with no luck. I found them online for around $6 - but with $14 shipping. Ha! Still, if I ever feel like I really need this, it could easily be added to my rig as an upgrade.

NOTE:    These  instructions  are  presented  without  warranty  of  any  kind.  The  author  is  not   responsible  for  any  liability  or  loss  related  to  the  construction  and  operation  of  the  launch  pad. 

Wednesday, April 24, 2013

Rear Ejection

The latest build I'm working on is Pemberton's Little Bucky Jones, and I think it is going to be a slick looking model. Last week I was silently lamenting the fact that it will have a seam where the nose cone detaches, and I thought about the idea of making it, or really any other model, a rear-ejection system. It wouldn't be too difficult: seal on the nose cone and construct an engine mount that is blocked in the front by some sort of engine block, but can slide out the back and drag the parachute with it when the ejection charge fires. Should work.

But if it is so obvious, why isn't it done? The best reason I could think of concerns what would happen if your parachute failed to open for any reason. Since the node cone would still be on, you'd have an aerodynamically stable rocket in the sky, making a very ballistic return to earth. 

Ironically, I was just talking about this very scenario to my father-in-law on Saturday as I prepped my US Rockets Dual 18mm Rear Eject  (also linking to Apogee since they have a much nicer presentation and pics of the rocket) for a flight with two C6-5 engines. The rocket took off, flew high, arced downward, and as soon as the ejection charges fired it was clear things were not good. Two separate pieces were visible, and the big one was headed downward fast.

We recovered the rocket with the nose cone and crumpled front of the airframe buried six inches in the soft ground. At that point it broke off, and the rest of the rocket was intact, except of course for the missing parachute, which had detached as a result of a broken shock cord.

(I should point out that on a previous flight the Kevlar shock cord that came with the kit actually burned all the way through. I replaced it with a cloth-elastic cord mounted in the end of the tube with a paper mount, Estes-style.)

So from what I can tell, one, or possibly two things went wrong:

First, the delay was too long. From what I observed, the rocket had fully arced over and was starting to head down with some speed when the ejection charge blew. I'm sure this put a lot of stress on the chute as it popped out, and it likely led to the snapped cord.

Second, I had set up a camera to get a movie of the take off from the pad. Watching this frame-by-frame clarified what seemed like a not-quite-right launch. Despite the parallel hookup of the two engines, the video clearly shows that they ignited at different times, maybe 1/15th of a second apart. Not a big difference, but I wonder if this led to the first ejection charge pushing out the wadding and chute, but the second charge burning the shock cord on its way out and weakening it. Though I didn't see much evidence of this on what we recovered (we never did find the chute and the end plug to which it was attached).

  1. Verify the proper ejection delay with flight simulation software whenever possible.
  2. I'm done with the idea of rear-ejection. I've had my share of chute failures in other models, but they've always had enough drag after popping the nose that they are recovered with little or no damage.

And here's a semi-related epilogue:
About fifteen to twenty minutes after that launch, I was at the launch base preparing another rocket while my father-in-law was out still looking for the parachute. A police officer drove up and approached, saying that he saw the launch from the road and came to check things out. I told him that although this was my first time launching there (at the Northwest Open Space in Northglenn, CO) I had checked the Northglenn fire code (which is basically the IFC) and verified that model rocketry was permitted. He responded that "it may be allowed by fire code but prohibited by municipal code," and added that there was currently no fire ban in effect - a big issue at the end of last summer around here.

The municipal code issue is something I'd never thought of. But he said that in a very shrugging-it-off sort of matter, then just asked a few general questions about my rockets (how high, do I ever use payloads like an altimeter) and casually looked over my gear. His last question was whether I planned to launch any more, so I figured I was in the clear. He checked my ID and wrote down the info, then left with a not-too-stern "be safe." Another police car rolled up as he was leaving, and my father-in-law overheard him say to the other officer "just someone launching model rockets." 

I've since looked up Northglenn municipal code and can't find anything about rockets. I've really never heard of low power model rockets being illegal to launch anywhere. They're certainly sold all over the metro area.

I still wonder if what he saw from the road was a scary ballistic return - though he never mentioned that.

Update - 06-May-2013
I attended the CRASH launch yesterday (my first time - it was pretty great actually) and talked to one guy briefly as he was preparing to launch his Estes Gemini DC, sharing my experiences. Guess what happened next? Lawn dart.

Monday, April 8, 2013

Then and Now

Here's where I'm coming from:

As a kid, I launched rockets, but never to any degree of sophistication. My neighborhood friends and I all did. We built Estes models, shot them with spray paint, launched them, and had a lot of fun. Did we meticulously seal the balsa parts? No. Did we join the NAR? No. Did we even use a launcher or even a launch pad? Well, no - we had speaker wire with a 9V and paper clips, and a piece of music wire stuck in the ground. And no, we never bought flame-proof wadding either. I probably reached farther than anyone in my group when I checked out Harry Stine's book from the library. But I certainly didn't read it cover to cover or follow all his advice.

I don't exactly remember when it stopped. Certainly by the end of high school. I think I may have launched one or two rockets as a young adult.

Jump forward 20+ years, to the summer of 2012. I'm a father of a two year old girl and a five year old boy who is intensely curious and physical science-minded. And friends of his/ours announce plans for a rocket party. My son, Jack, and I get psyched and in advance of the party I pull an old Estes starter set out of the garage - it's got the launcher, the pad, and a rocket - all we need. So we do the build together and go out for a launch, and we're hooked. The day of the rocket party arrives and it is insanely fun.

So now here I am as an adult. I don't exactly have lots of spare time, but I do spend almost every minute outside of my job with my kids, and I need a hobby. Jack is still interested in rockets, and we work on some together, but my interest can't remain at the pace of a five year old. And I'm older, more thorough. And I have more money. Plus, this rocketry business - it's a whole different deal now. Sure, the Estes-level stuff is still at the local hobby store, almost identical to 1978. But now there is the Internet, with loads of information, loads of online vendors for parts and models. And microelectronics - cameras, altimeters! So, so much to explore.

I won't say I've become obsessed, but sometimes it feels like it. I've built about a dozen models since last summer, and have a half dozen kits in my closet waiting to be built. Plus an impressive stock of pieces and parts, with ideas for at least three or four custom models. With every new build, I'm exploring new techniques - adhesives, finishes. I've got my first composite motors that I'm itching to use. I just joined the NAR, and am hoping to go to my first club launch this spring. 

 So this is my blog, to document and reflect on what I do. I may not have anything original to say, but I will try to distill what I learn.