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BLG - Ballistically Launched Glider: A new project/build thread

RoyBro

Senior Member
Mentor
#21
Last time I used Swiss Post the company that sent to me was out of business long before the merchandise arrived some 3 months later! :(

Thurmond
I've used registered mail once and it took almost the full 45 day window. I've shipped via Swiss Post several times since, and they've been very consistent. Always within the 14 day window they estimate.
 

earthsciteach

Moderator
Moderator
#23
earthsciteach, I am seriously trying to do this also and I hope to start a build log thread on my progress. Although, I'm wondering if a glider is good enough or if I should put a folding prop on it. I also don't have a potato gun or compressed air cannon. So would building a glider with a 2 foot (not even) wing span that folds small enough to fit into a tube with pop out wings and rudder be enough proof of concept?
That's good enough for proof of concept. I'm planning on a 1 meter wingspan. I have a heavy duty cardboard tube that I am going to try as the shell of the sabot. Its about a meter long. I figure the glider's length will be in the neighborhood of 750-800 mm.

If you don't have a potato gun you can build one for a few dollars!
 

earthsciteach

Moderator
Moderator
#24
I put a bit of thought into this over the weekend and made a "full scale" mock up of the thing. I have my wing pivots worked out. They are sleeved CF and brass rods. I sort of like the profile idea. May make a prettier version of the fuse and stiffen with a cf rod.

Surprisingly, this thing flies pretty well for not having an airfoil shape whatsoever. But, I'm looking for decent performance, so I need to decide what materials to build my wings from. At this point, I'm thinking of going with a traditional built up, balsa wing. I can make the horz and vert stabs from $tree foam covered with fiberglass cloth. The original plan was to use a fishing rod blank for the fuse boom. May still go that route. But, the foam board would be lighter. Eh, either would work just fine.

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#25
I have used the spud-gun approach to launch "rockets" as a means to test a flight computer I was developing a while back. I can give you a heads up on some of the issues you are going to run in to:

Unless you are building exceptionally small and light, you are probably going to need a larger spud gun than you may be anticipating.

If you have the option of going with compressed air, take it.

If you are going to go combustion, don't waste your time with hair spray. Direct propane injection really isn't all that difficult and is far more reliable.

Maintaining a good seal of high pressure gasses is critical and challenging to implement.

You will have to deal with some serious G forces, as you are going to be accelerating to maximum speed in the length of your barrel in a very short fraction of a second.

As to the development of the gliders themselves, you should be aware that rocket-launched gliders are a "thing" and the rocketry community has been working on deployable wings for some 40-odd years now. That is going to be a great source to mine for design ideas.

Looking forward to see what you come up with!
 
#27
As violent as a rocket launch is for a RLG, it's fairly gentle compared to cannon shot.
Yes, this is absolutely correct. I do 100% scratch-built rockets (this includes motors and electronics) and have developed my own techniques for rolling strong and light convolute paper airframes for MPR (E-G class) motors. These happen to quite well suited for this launch method. A few other people have attempted the spud launcher approach with the typical commercial spiral-wound tubes without the best of results -- they usually wind up more as confetti cannons than anything else.

A successful launch and recovery with this method will be a true test of your design and build skills indeed!
 
#29
Mortar shells -- being round, relatively dense, and not terribly effected by relatively high G-forces -- are well suited to this lift method. This is generally not the case for ballistically launched gliders.

After thinking about this a bit, a fairing such as is used to cover the payload of a real rocket may be the way to go here.

In my ballistically-launched rockets, I was placing the airframe tube outside of the launch/pressure tube as is the standard method with stomp rockets. One thing I did notice is that with the weight of battery+electronics all the way forward, the fins could be remarkably small. I never attempted a launch with the fins removed entirely, but it is conceivable that this could work.

If this is the case, you could use something that is a combination of payload fairing and sabot, keeping everything inside of the launch tube. The fairing/sabot would keep things streamlined during launch, and then fall away at or shortly before apogee and wing deployment.
 

earthsciteach

Moderator
Moderator
#30
Thanks for the input, R4K. Its the difficulty in accomplishing this that has me excited about doing this.

I know it will take a large cannon. The barrel will have to be 3-1/2" pipe, which is just 1/2" short of the combustion chamber diameter on my current potato gun. I do utilize direct propane injection with a metering chamber for my spud launcher. The chamber is sized to provide approx 4.4% fuel to air ratio to the combustion chamber. All I have to do is pressurize the chamber to 20 psi and it will toss a tuber well over 200 yards downrange.

The glider will be contained in a sabot, which will seal the barrel and, if necessary, contain padding to absorb the shock. A tapered carbon fiber rod will form the tail boom. The electronics pod is made of xps foam. I'm quite confident that the glider will handle the launch.
 

xuzme720

Dedicated foam bender
Mentor
#32
If you use something like this, you can make a very form fitting sabot that should help support the airframe during the highest G's at launch. Wrap the glider in plastic while it's in it's folded configuration, place it in a tube of the barrel diameter, and pour in some mixed foam. Mythbusters used a similar method for their chicken gun.
 

xuzme720

Dedicated foam bender
Mentor
#35
Its vanilla pudding!

No, its a muffin!

Wait... Its a giant mushroom!


This is what went through my head as I watch the video.

Cool stuff!
Funny thing is I almost said that you could use it for mushrooms in a stop motion Super Mario Brothers video if it didn't work for your sabot...
 
#36
Ha! it's high speed Gorilla Glue!
Gorilla Glue cures much more slowly because the "curing agent" is the moisture in the air. This is why you can mix GG with water and get it to cure much faster. Even with this technique it is still going to be a fair bit slower than the two-part stuff, though.
 
#37
How strong is this mixed foam? Has anyone built a wing mold and tried using it to form a wing? It would be simple enough to add a CF rod during the production. I'm seeing some very interesting possibilities with this stuff.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#38
How strong is this mixed foam? Has anyone built a wing mold and tried using it to form a wing? It would be simple enough to add a CF rod during the production. I'm seeing some very interesting possibilities with this stuff.
The canned foam used to seal window joints (like Dow "Great Stuff") is also polyurethane foam. It's pretty sturdy stuff, once cured, and I've seen builders use it to fill wing cavities. If you did a paper-stripped versa/krakken and it came out floppy, this would be a perfect fix.

If you've ever had to cut back or sand excess Gorilla glue, you'll have a good feel for how durable the stuff is -- very, but kinda hard to work with once cured.
 
#39
With the two-part expanding foams you can adjust the density (and thus strength) of the final product. Different products achieve this in different ways. You need to get the volume/quantity dialed in just right, otherwise you will either have voids, a blown form, or a real mess.

I really wouldn't recommend it for model aircraft use. Not only is it a real PITA to use, but the curing agents often use isocyanates which are pretty toxic.

I'm always reminded of this video:

http://www.youtube.com/watch?v=zAIY0I5GGw4
 

con244

Senior Member
#40
I've heard mixed reviews about polyurethane. I've heard that it continues to expand for some time after it's cured, so for things requiring tight tolerances it may not be the right thing to use. I personally have no experience with the stuff except in building (house) repairs so maybe a test is in order.
Con.