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Rocket Designs and other free flight fun

FAI-F1D

Free Flight Indoorist
#1
Following the many favorable comments, I'm succumbing to the demand for more details on swing wing rocket glider designs. Following completion of an article for publication in this year's Free Flight Symposium, I'll be converting the information into a format that is usable here, probably as a series of articles or something of that nature.

In the meantime, here is something to whet your appetite (and show how simple it is!): Rocket1.png Rocket2.png

Although it really doesn't compare to the amazing launch sequence footage that Flite Test captured, you can view more rockets and other assorted flying fun at my youtube channel: https://www.youtube.com/channel/UCCKDHzGUWhVXxdx0sZG7DCw

Here's a simple rubber powered model I built a while back:

And an unlimited class that produced flight times of over 5 minutes before I lost it, ending further development:
 

FAI-F1D

Free Flight Indoorist
#6
Skipo, Hip Pocket is definitely the go-to place for free flight stuff these days. Bill Gowen, who holds almost as many records as Jim Richmond, is quite an active contributor there. I make a few appearances as well.

Do you do competition F1D? I've seen a few people fly F1D at the AirVenture museum in Oshkosh, they really are amazingly light craft.
Well, I'll answer your question by sending you here: http://indoornewsandviews.com/2014/03/17/joshua-finns-ama-cat-i-record-f1d/

Since you've seen the flying at Family Flight Fest, you've witnessed the unbelievable skills of John Kagan and Brett Sanborn. Mike Kirda also flies there and is a skilled F1D flier as well.

Anyone interested in serious indoor competition should go look at the contest calendar at freeflight.org. We've got upcoming contests at West Baden (April 4--the best Cat III flying site on earth), Kibbie Dome, Colorado Springs (US Nationals--spectators are greatly encouraged to attend), Round Valley Dome, and Lakehurst (F1D team selection is Labor Day weekend--Hope and I and young Caleb will be there vying for spots on the team). We're hoping that the Mini Dome in Johnson City will become available again in 2016. There is also a crew that occasionally fly at Hershey Park Arena. Anyone in Georgia is encouraged to contact me for information on indoor contests in Dunwoody. Also, DC area folks get the privilege of occasional access to the National Building Museum through the DC Maxecuters.
 

FAI-F1D

Free Flight Indoorist
#7
Ok, here's the first installment of the fun. I need this stuff for another project, so it's a win-win to need it for this forum.

Let's start with piston launchers...

Floating head pistons have been around for years in competitive rocketry. They are virtually isolated to that discipline, and half the rocketry community doesn't even know about them. Half the rocketry community are scared to death of building a glider, too, but that's it's own sad story for another day. Hopefully we can take away some of the voodoo and make it more accessible to the masses. I've pulled in some of the still shots that FT captured for reference here since they did things with the camera that I'm not equipped to do. So let's get started.

Why would you want to use a piston? After all, it's complex, the alligator clips have to just barely be attached to the ignitor so they'll slip free with the slightest tug, and the tube is constantly filling up with soot!
Rocket3.png

Anyone who has launched model rockets is familiar with the hiss prior to the rocket whoshing away. That hiss is hot gas firing out of the nozzle, but at low enough speeds that there is no useful thrust. Propellant is being burned, but until the combustion chamber fully pressurizes, there is no useful thrust. A piston launcher confines that gas in a closed chamber, just like an engine cylinder, and puts its expansion to work. The tube pressurizes, and pushes down on the piston. Since the piston rests atop a stake that you've punched into the ground, it can't move.
Rocket4.png

Instead, the cylinder has to move upward to relieve the pressure, carrying the rocket with it. When the piston reaches the end of the tube, it hits a collar which prevents its escape. At this point, the tube fully pressurizes, and would rupture but for one thing: it's friction fitted to the base of the engine casing. The pressure separates the tube from the casing, propelling the casing backward and shoving the rocket forward.

Physics now comes into play. The pressure of the tube being shoved upward as it pressurizes develops significant speed--so much so that the entire rocket-piston assembly is catapulted into the air. When the tube fully pressurizes and separates, it is thrown backwards. Newton's third law now kicks it--the rocket is throw upwards--all of this having happened before the engine has produced any useful thrust! Now when the thrust kicks in, the rocket is already moving very, very fast, and in the case of a little A engine propelling a heavy glider, your altitude potential nearly doubles!

The result is displayed to an absolutely stunning extent by this sequence:
Rocket5.png
Rocket6.png

Notice that in the brief time that after separation, literally a fraction of a second, with the piston still flying through the air, the rocket has travelled a tremendous distance, now going well over 100 mph on its brief 0.3 second burn (yes, you read that correctly--the thrust lasts for less than 1/3 of a second!).

The acceleration is so much faster than a traditional rocket launch that you don't even need a launch rod like you used with your Estes rockets. The minimal guidance needed is provided by the stake that the piston rides on. Basically, wherever that rocket is pointed, that's where it's going--period. I've launched this model in winds exceeding 25 mph--more than double the strength at which most glider fliers call it quits, and it still tracks dead straight--no weathervaning into the wind.

In the next post, I'll show the guts of the piston system.
 
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FAI-F1D

Free Flight Indoorist
#8
Floating Head Piston Basics

So now that we've seen what makes pistons so awesome, here's how they work.

First, we see the total assembly. One rocket ready to go, one standard model rocket body tube, full length (18"), and a turned balsa plug--the piston. Nice that I've wrapped the piston in masking tape. I made this one a little too small and it almost managed to escape the tube one time--very, very bad things would happen if that took place. The bottom end of the piston is rounded so that it slides that much easier in the tube, and the upper end is flat. Both ends are hardened and protected with CA, although bagging it in fiberglass would have been a better idea (maybe next time). As it is, expect the piston and the tube to have a limited life expectancy. Also, metal and plastic are bad ideas for the tube--you want it to be as light as possible to achieve maximum acceleration.
DSC05162.JPG

Here the piston is slid into the front of the tube. It should slide freely in the tube. Were it to catch, the tube would overpressure prematurely, causing it separation to occur before all of the available energy has been captured.
DSC05163.JPG

You can see here that the piston is slid fully inside. Note that I've hardened the end of the tube with CA to reduce wear and tear.
DSC05164.JPG

Now it's friction fit onto the base of the engine casing. This should be a firm fit, but sand off the end of the engine casing as needed to keep it from being too tight. Even if it's loose, once the ignitor leads are routed in, it'll tighten up. The tube only needs to fit on about 1/4" for a good seal.
DSC05165.JPG


Look closely and you'll see the collar in the end of the tube. It's just made from another segment of cardboard tubing with a slit in it so it can be slid inside. It's glued permanently in place using CA.
DSC05166.JPG

That's all there is to it. Set the whole assembly upright onto a rod driven into the ground (I use a carbon arrow shaft at least when soft ground is available). Refer to the previous photos and the Flite Test video for the wiring setup. I keep the wires carefully routed and taped onto a mast so that they cannot entangle in the tail surfaces of the model (makes for a very bad day--I've had it happen several times).

It's worth noting that this system works for any model rocket that has the end of the engine casing freely exposed. I have noticed that it's unpopular for larger rockets, and that's mainly because the ignition sequence on composite engines is more sensitive. They also seem to waste less energy fizzing on the pad.
 
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jhitesma

Some guy in the desert
Mentor
#9
Great stuff. I've been intrigued by piston launching for some time, wasn't convinced at first that it could make as big of a difference as it does but I've never been one to let my concerns for whether an idea will work or not put me off from actually trying it to confirm :)

The white/orange rocket on the left here has a piston boosted second stage:
DSCN1579-L.jpg

The orange part is the second stage, and the upper payload section houses the piston assembly. Same idea as Josh's but implemented a little different so the tube is captive and retained afterwards. I'm still not convinced that this particular design is engineered well enough to actually be beneficial...and even though I built it almost 20 years ago I've yet to fly it for two reasons. 1) I didn't have the money or experience to complete the electronics to launch the second stage (due to the piston you can't just use a booster motor to ignite the upper stage, it has to be an electronic ignition.) and 2) While I was building it BATF changed the rules regarding rocket motors and the new restrictions weren't worth the effort to me. It also didn't help that shortly after building it I moved here to AZ and there's no local club flying - there is a group that flies about an hour and a half west of here in CA but that's a bit far for me to have to drive just to launch.

Oh yeah, I'm also worried that if I ever do fly it the second stage will more or less disappear despite the bright orange paint job :D

If anyone is interested I could dust it off and get some detailed photos of the piston assembly.
 

FAI-F1D

Free Flight Indoorist
#10
Do share. I wanna see it.

As for driving an hour and a half, alas, you're fortunate. For years I had to drive more than 3 hours to get to the nearest free flight contests. A number of folks in our club have drives that long or more, and it's their only opportunity to fly at all.
Field access has become the number one reason for the decline in free flight. It has also virtually destroyed rocket powered gliding.

Technology is your friend on retrieval of small, fast rockets. There are a multitude of telemetry systems available, although Walston remains the most respected (and most expensive) of the bunch. If you have a GPS setup for a quad, that's one way of getting full retrieval capability for your rocket. Most of those systems are now small and light enough to do the trick. Of course if you have the money on hand, the Walston is way easier--insert batteries, stuff it in its little compartment, and off you go.
 

jhitesma

Some guy in the desert
Mentor
#11
Do share. I wanna see it.
I'll try and get some this evening.

As for driving an hour and a half, alas, you're fortunate. For years I had to drive more than 3 hours to get to the nearest free flight contests. A number of folks in our club have drives that long or more, and it's their only opportunity to fly at all.
Field access has become the number one reason for the decline in free flight. It has also virtually destroyed rocket powered gliding.
Oh I know it could be far worse. And honestly it's closer to an hour than an hour and a half usually. In fact it's a shorter drive for me than it is for the club which is based in San Diego so I really shouldn't complain. But the big thing is the club is based in San Diego and just occasionally flies at Plaster City. So it's hard to feel like a member since their meetings and most of their launches are in San Diego (they fly smaller stuff on Fiesta Island) so coming from here in AZ it's tough to really feel like you belong. They're friendly and welcoming...but it's just hard to take part in a club under those circumstances. The closest AZ based club is up in PHX and that's about a 3 hour drive. When I started in HPR lone wolf flying was still an option and BATF LEU licensing wasn't that big of a burden. But in the mid 90's that all changed and now it's a lot more red tape and I hate red tape :D

Technology is your friend on retrieval of small, fast rockets. There are a multitude of telemetry systems available, although Walston remains the most respected (and most expensive) of the bunch. If you have a GPS setup for a quad, that's one way of getting full retrieval capability for your rocket. Most of those systems are now small and light enough to do the trick. Of course if you have the money on hand, the Walston is way easier--insert batteries, stuff it in its little compartment, and off you go.
Oh yeah, lots of options now. 20 years ago on a students budget the options were far more limited :) I had a small FM 2m transmitter I built with a little PIC chip programmed to beep out a beacon message which cost more than most GPS's do now :D I also built a primitive flight computer based on an 8051 all programmed in assembly - but never got it to a point that I was willing to trust it on a flight. Just the accelerometer chip for that cost more than most flight controllers do now...and that was just one axis! So many more options now and at far lower costs (both financially and time!)

I may dust it all off and start flying them again in the future if my daughter stays interested. She got a few packs of estes motors in her stocking from Santa this year to fly a few of my smaller rockets with and had a blast. In fact she's anxious to go out again but I need to finish a few repairs from some failed 'chutes (those little stickers don't hold on so well after a few decades!) Hope to get her building soon. And pushing the launch button herself - she loved watching them launch but was still scared to push the button :D

I really need to finish the launch pad for that Delta Thunder (You can see it in the back of the photo above) I bet she'd get a real kick out of that one. I even have a few packs of D's for it on hand still. Just need to trim it out and it will be ready to go.
 

jhitesma

Some guy in the desert
Mentor
#12
Ok, the piston stager is a bit dustier than the last time I looked at it...but not too dusty for some photos :D

Fully assembled it's 74" tall, but I don't have a motor on hand to couple it with and it's harder to get a decent photo of something that tall and skinny :D
10970354_10152567102381805_498865256_o.jpg

Both stages are minimal diamter. The top is sized for 24mm motors and uses a thin ultra light tube with thin ultra light ply fins. It has streamer recovery and is really designed to be as minimal as possible.
10951988_10152567102251805_500526481_o.jpg

At <2oz the sustainer is light but strong and designed to handle the most powerful 24mm motors that were available in the early 90's. (24mm E's were recommended IIRC, but none were available at the time I built it due to some issues with motor certifications from various manufacturers at the time.)
10964768_10152567104001805_886241630_o.jpg

At just over 10oz the booster is pretty light itself and is a 54mm minimal diameter design with plywood fins as well. It came with adapters to allow using 29mm motors or a cluster of 3-24mm motors as well - I still have them but they're stashed away in the box my NCR 1/72 shuttle is in and that's a bit more work to dig out :D
10958906_10152567103756805_1288392827_o.jpg

Here's the piston assembly taken apart. The top cone is meant to be fastened in place but I haven't done so yet since I was still debating just how to deal with the staging electronics. I really didn't like the idea of a timer but accelerometers weren't really reliable enough (at least not on my budget) for me to trust one at the time I built it....they weren't really small enough for the space available at the bottom of the piston either :D (They are now of course!)
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The piston is a loose fit through most of the nose that acts as a guide:
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But is a tight fit for the top few inches to keep it straight.
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The bottom of the piston is a good fit in the payload section - but I was considering adding some kind of seal around the outside to help it. Apparently the biggie that wasn't mentioned in the instructions but has since been figured out by other builders is adding a vent hole above the piston is necessary. I had suspected as much when building but was nervous about poking holes in it without knowing it would help and not make things worse ;)
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You can see the difference between the nose cone coupler on the left which isn't meant to come apart in flight and the payload coupler on the right which is. To help the payload coupler mate well but still come apart easily from the ejection charge I used to soak them in CA and then sand them down so they were a very smooth plastic kind of finish. Probably more work than it was worth but I never had a coupler jam in flight.
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And here's the piston fully extended.
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Apogee apparently has a really nice writeup on this kit including a video of one flying (Cool, I hadn't yet heard of anyone who completed one and flew it successfully the last time I searched!) They also have a few in stock apparently though the price has gone up considerably from what I paid when I got mine directly from the owner of US Rockets (long story there - he's a rather controversial figure in high power rocketry.) http://www.apogeerockets.com/Rocket_Kits/Skill_Level_5_Kits/Piston_Stager

I really like their solution to how to add the electronics. Though it's a little late to do it quite that way on mine....I do have a few ideas on how I'd do it if I ever get the itch to try and fly this one.

The main reason I was never convinced the piston in this setup would be very helpful is because of how large the chamber is below the piston. 54mm x 500mm is a lot of volume for a 24mm motor to fill with exhaust before you'd get any benefit it seemed to me. The bottom 100-150mm could be closed off to give a place for the electronics and I figured that would help since then there's be almost nothing to fill before the piston would start to help. But by the time I thought to do that I had already glued in the lower coupler making that option kind of tricky. All I really need is another 455mm long tube and coupler to rebuild that section...but USR used some funky tubes that were remarkably strong and light - but dimensionally just different enough from other manufacturers that it would look out of place using someone elses. And even the newer tubes from USR are different OD so I'd almost have to rebuild the bottom of the booster as well so it would all match (which is just a tube and 3 fins so not that big of a deal....but given my lack of flying abilities at the time I just kind of shelved it.)

I've been toying with just using the booster as a launch pad to test the piston from the ground...but have to admit I'm worried about loosing the sustainer given how small it is :) Maybe one of these days....
 

FAI-F1D

Free Flight Indoorist
#13
My word that stuff really is light. USR was known for making lightweight stuff if you could finish it as intended. Oh the controversies... I think one of the reasons for the bad rap they got was that they produced kits with the level of complexity and sparse instructions as what you'd find in a traditional stick and tissue kit, while a lot of folks in the rocketry community are used to these super detailed instructions. I grew up not even having instructions, so the whole series of complaints was a bit lost on me. Either way, you've done a fantastic job on yours. That's definitely one of the more complex rockets I've come across. I'd love to see footage of it being launched.
 

jhitesma

Some guy in the desert
Mentor
#14
My word that stuff really is light. USR was known for making lightweight stuff if you could finish it as intended. Oh the controversies... I think one of the reasons for the bad rap they got was that they produced kits with the level of complexity and sparse instructions as what you'd find in a traditional stick and tissue kit, while a lot of folks in the rocketry community are used to these super detailed instructions. I grew up not even having instructions, so the whole series of complaints was a bit lost on me. Either way, you've done a fantastic job on yours. That's definitely one of the more complex rockets I've come across. I'd love to see footage of it being launched.
Yeah the instructions that came with the PS were very lacking and outright wrong in some places. Though in his defense Jerry did warn me about that before I bought it :)

From what I understood...and experienced...the controversy wasn't so much about their kits as it was about Jerry's lack of diplomacy dealing with getting his motors certified with NAR and TRA. And his willingness to interpret the BATF regulations one way for his own dealings but another way for his competitors. Add in a willingness to get lawyers and courts involved and a reputation for slow shipping (and not shipping) but quick to cash checks and well...doesn't really matter how good his kits and motors were/are. I got the piston stager right when he tried to make one of his comebacks in the mid 90's and he tossed in a few of his motors (which he claimed were legally certified under his reading of the BATF/NAR rules and regs but which NAR insisted weren't.) I flew them out at a friends farm and have to admit they looked like they were made up in someones basement which didn't give me much confidence (so I only flew them in models I wouldn't shed a tear for if they left the pad in pieces instead of vertically). They were finicky to ignite and took some hot igniters to get going - but once they took...they were very impressive performers.

I did come in just a few grams over the official weight - but I wasn't building for light weight, I was more worried about making sure it would hold up to anything I threw at it. Though even then I never planned on using a 54mm booster, I was planning on a beefy 29mm booster and a mild 24mm sustainer for the first flight and then maybe something bigger on the sustainer once I was willing to risk it not coming back ;) Now I'd be willing to put a small tracking device on it and go big right from the start...if I ever finish it.
 

FAI-F1D

Free Flight Indoorist
#15
Interesting...I never knew about the rocket motor controversy. That's just plain messed up. I have zero respect for people who pull stunts like that. The only controversy I'd heard about regarded the kits...mainly people crying about the instructions.

I really hope you get that model flying...would be absolutely awesome to see. My few multistage launches have been breathtaking experiences.
 

FAI-F1D

Free Flight Indoorist
#16
I had a request for photos of the deployment sequence...this is the best I have on hand just now. Enjoy it for what it is and try not to hate it for what it isn't. ;)
Rocket7.png

Additionally, here's how it works. Bear in mind that this is the larger glider--it has a tensioner on the hold down line and floating hinges whereas the smaller ones need neither of those complications.
Rocket9.png

And when the ejection charge blows, it shoots the nose forward, releasing that line. There are a pair of wires on the nose cone with bend ends sliding in aluminum tubes such that the nose can only go so far forward. Once stopped, it is pulled back in place by that tensioner band so that aerodynamic cleanliness is retained. I'm still trying to decide whether the tensioner is necessary since there's really not that much of a gain to be had. But the cool factor does kinda win out... ;)
Rocket8.png
 

FAI-F1D

Free Flight Indoorist
#17
And there's more! This is on my desk at work. Yes, those are Solidworks drawings. One of my coworkers wants to 3d print the whole thing, but I'm still not so sure about that.
Rocket10.jpg