Rocket with RC Recovery...

Hi guys... I'm new to this forum, and model rocketry, and I wanted to run my latest "experimental" project by you to get comments/suggestions...

I'm planning to build a rocket with moveable control surfaces on the fins, so that the rocket can be guided back to the launch area while it descends. It won't be a rocket boosted glider, really, since there will be no wing, just moveable fins. The idea is to come in with a relatively fast, steep descent and try to flare up to a horizontal position for a belly landing.

I plan to use a Hitec Feather micro receiver, two HS-55 micro servos, and a small 4 cell 2/3AAA batt pack. I'm going to start with an Estes Patriot kit (BT-60 body tubes) and modify it. I'll custom make 3 large balsa fins, two of which will have hinged control surfaces on them (these will act as "elevons" during descent). I also plan to install a "firewall" and make a vented section in the body tube to vent the heat and gases from the "burn through" on unplugged engines. I'll build the Patriot with a D sized engine mount, since this rocket will weigh close to 8oz. I plan to fly D11-Ps in it, which should get me up to about 600ft, but for the first flight I'm planning on a C11-0, which should only take it up to about 300ft.

My primary concerns so far are:

1) Will the hinged control surfaces hold up to the 90-140mph top speeds? CA hinges and hinge tape are the common methods of hinging very lightweight aircraft control surfaces, but those aircraft typicall don't reach these high speeds!

2) How will the stability be affected by only having fins at 0, 90, & 270 degrees around the body tube (see attached diagram... no fin at 180 degrees so I can belly land it). I'm guessing that the drag caused by the 0 degree fin will cause the rocket to arc slightly in that direction, since there is no fin at 180 degrees to offset the drag.

3) How to calculate the CP when the projected area with the 90 & 270 degree fins will give a CP further back than the projected area with just the 0 degree fin. Right now, I figure I'll just calculated the CP with just the 0 degree fin in the projected area to get the worst case stability factor.

Thanks for reading all of this... if you made it this far! Any comments and suggestions are welcome!

Welcome aboard!

That is a very interesting concept.

It's sort of a RC rocket boosted glider...but not really...sort of an X-15 recovery.

I would do this to be safer.

Add a bottom fin for launch stability at the 180 degree location or two smaller fins at 135 and 225 degree location.

The problem will be on the "glide" it won't be easy to control unless it stablizes "up and down".

In order to get it to come in "somewhat" stable on the "glide" and to keep it from spinning you may have to add some stubby wings with dihedral right at the CG. Maybe something that looks like X-15 or F 104 wings.

Without at least stubby wings, I don't think the fins will have enough control surface to bring the nose up.

That is gonna be a bear to trim!

Thanks for the welcome and the advice, sandman!

You're probably right about having some stubby fins at the CG to have any chance of pulling the nose up to horizontal... I'll probably have to do that.

I'm assuming that as long as I keep it in a pretty steep nose dive (say within 45 degrees of vertical) I should have enough airflow over the control surfaces to keep it from rolling and to guide it back towards the launch site. If that assumption is wrong, then all bets are off anyway!

I wasn't planning to do any pre-launch trimming, since I can eyeball the control surfaces as straight as just standard balsa fins. I'd need a lot of airflow over the fins to get any control authority for trimming, and just giving it a toss is unlikely to get it moving fast enough to get much control.

My plan is to launch it, let it go into a nose dive, trim the aileron control to stop any spinning, and then use the elevator control to get it to arch back towards me. If all else fails, hopefully I can just give it hard right (or left) aileron and get it to spin for a helicopter-like recovery.

I plan to have my low rates setup for very small control surface movement for launch, and then high rates setup for max throws (with a good amount of exponential dialled in) so that I can get as much tail authority as possible with minimal airflow. I'm not planning to use the controls while launching, but if it starts arching badly, with low rates I might be able to pull it back or give a hard right (or left) aileron input to spin stabilize it a little.

With no wing to generate lift near the CG, I'm not really sure what this thing is going to do, but as long as I can recover my radio gear without busting it up, I'll have fun trying it out!

If this idea is a flop, I figure my next project will be to put some RC control into a scissor wing kit, or that new Estes Cosmos Mariner... those are rocket boosted gliders, so I should have a much easier time controlling those!

Some comments.

You have to be sure that the elevons are dead flat perfect for launch. If they are off a little, the rocket might roll, or might pitch. Pitch would be bad. If it pitched you might want to try to correct the boost by giving an elevator response to correct it....but if it happened to roll some you would have no dead if you needed t give up elevator or down elevator to correct the pitch. That’s among the problems with trying to do an R/C rocket, it’s very difficult to impossible to see the roll orientation of it. Whereas a glider type tend not to roll as fast (higher roll moment of inertia) and the wings help show you the orientation.

As to the 3-fin design, a 4-finned rocket missing one fin. Well, it might have a yaw-roll coupling problem. Imagine it starts to roll to one side, and the one single fin is the only fin in that axis to try to correct it. Well, it will start to correct in yaw, but since there would be no opposing fin then the rocket also would roll due to the corrective force of that one fin. Now some configurations will have that problem, and some will not, so I can’t say for sure that this would or would not.

You could consider attaching the two horizontal fins as though they were one small “wing” to the underside of the body tube (thinking of this in the glide-mode orientation). Then also have two small fins glued to the tips of those fins, like winglets. Those would add to the yaw stability and would not add to the yaw-roll problem. So, you could make the single fin on the “top” have less area, since the smaller that single fin on top would be, the less likely it would have the yaw-roll coupling problem. Another thing to try to counter yaw-roll coupling would be to attach the tip plate fins bowed inwards, tips closer together, as that would produce an anti-roll effect when they were yawed (Imagine the SR-71 rudders which are angled in towards each others, of course in this case they would be at the tips.

Or, don’t give it a big single fin on “top” at all, use two normal sized fins attached to the tips of the horizontal fins.

If it was a model with wings, the key thing would be to work it out so the wings were located at the right CG location, or else flip the problem around and make sure the CG was located at the right pace on the wings (If I was designing something like this, and I've thought about something that boosted like a normal rocket but had some small wings in the mid-body, I’d do some pre-assembly of the main components without wings and get a ballpark idea before choosing where to locate the wings on the body, planning to achieve the final CG location be locating various R/C components more fore or aft in hopes of not needing to add extra weight for balancing purposes). Since you are not talking about adding any wings (yet?), you have a very tricky problem to work out since the body tube itself would be acting a bit as a lifting body, plus the main part of the fins at the back. The most practical way to find out would probably be to just glue together a scale version out of scrap parts, glue the elevon tabs at a high angle, and keep adjusting the CG with clay till it started to glide. Then if you get it to glide you’d scale it up.

Another problem with this, if it’s not to eject anything, is that for the model’s CG to be far enough aft to glide after burnout, the CG would be more rearward for liftoff due to the propellant mass. This is why most rear-engine gliders eject something between boost and glide, whether it’s ejecting an internal pod like he old SkyDart, or dumping water out the nose as I’ve done with my X-1 and Orbital SkyDart SkyBooster models. Here’s a link to some pics and info on some R/C models:

https://members.aol.com/GCGassaway/rcgliders.htm

The “SkyBooster” has twin rudders glued to the wingtips. They are of the proper total area to stabilize that model in yaw even if it did not have wings. But imagine if that model didn't have wings, it had normal sized horizontal fins like you described, then those twin rudders would be huge compared to the fins they were glued to.

BTW - what I had considered doing years ago was from an idea suggested by a person who did demos at schools. They wanted to fly something bigger than normal but didn’t want to risk losing it. I thought up a BT-80 (2.6” model) of around 5-6 foot length that would have normal fins at the back, and some stubby wings in the middle (just big enough to help maneuvering to glide down, not big enough for safe landing), and color-code (one side black, one side fluorescent orange for example) it to help see the roll orientation. Though for higher flights color coding is not as obvious so it would still be tricky (probably would have had to figure out the orientation by seeing which way it responded in glide to a roll command). The idea being to boost up then glide back towards the school field. But since it would be tricky to land it safely on a small field, it would use R/C to eject a parachute when it was 100’ or so above the ground, over a clear safe area. Since that time I’ve given the same idea to converting an Estes BT-101 (4”) diameter V-2 kit into an “A4-b”, a V-2 with wings that was actually tested by Germany, to do the same parachute eject during glide trick. Haven’t done anything like that yet though.

- George Gassaway

George,

Thanks for all of your suggestions!

I agree with the yaw-roll coupling issue you brought up, and I'll consider the suggestions you had for alternative fin positions.

I realize this thing is not going to "glide" w/o a wing. My intent is that this will be a guided "ballistic" aircraft on the descent. Because of this, I believe that it is best to look at the aerodynamics of it from a ballistic point of view (as a rocket) and not from an airplane/glider point of view. Since it will still be a stable rocket geometry after burnout (CP aft of the CG by 1-2 calibers) I believe that it will go nose down after apogee and build up a good amount of speed, at which point I can control the fin "elevons" to control roll and pitch to get the rocket to arch back towards the launch site.

The problems I see now are...

1) I know I can control the roll after it noses down and builds up speed (i.e. I can make it roll right or left) , but how stable will the roll control be (i.e. can I stop the roll and hold it in the position I want it), especially when I start to pull some "elevator" and start the arching. As soon as I start arching the rocket, it wll slow down... if it slows too much, airflow over the rear fins may not be enough to stabilize the roll axis or control it. I'm thinking maybe sandman's suggestion of stubby wings with some dihedral may help give a little "right side up" roll stabilization.

2) Will I be able to get this thing horizontal at all before it lands (or before impact, whichever the case may be)?!

This will be very much like a radio controlled JDAM. For those unfamiliar with that... it's just a dumb bomb (no propulsion) with some aerodynamic strakes (body fins near the CG) for stability and some moveable tail fins for guidance. I'll have to do some research on JDAMs to see what their flight characteristics and performance is like. I might have to use those "strakes" on my model!

Here's a a JDAM photo/drawing for those interested...

Here's a JDAM "exploded" view (no pun intended)!

Gentlemen... paint your targets carefully... LOL...!!!

Just a thought, but you could use the ejection charge to actuate two small winglets near the CG to provide lift - scissoring out of the body tube. Sure, it makes it a bit more like a boost glider. Less like a bomb.

I would recommend against this (or at least tread VERY carefully), for the following reasons.

Surface flutter will be a potentially very large issue. I used to fly RC planes, and watched the guys who flew racing planes. You have to be very careful about construction of the trailing edge of the stabilizer, and the leading edge of the movable control surface. Typical nylon aircraft hinges (which are glued into the wood) start to get squirrely around 130mph unless built right. Flutter is the result beyond those speeds. Makes a great "PPpppbbbbbbtttt" rasberries sound, followed immediately by balsa confetti, and a large crash. For sure hinge tape won't work at all for this application, I guarantee it. Loss of project will result.

The other issue is the RC systems themselves. They aren't NAR compliant for a good reason. Even a good FM system is going to be prone to outside signals (cell phones, walkie talkies, Nextel style two way radios, etc.) This intereference can cause spontaneous control surface movement even with the transmitter on and in range. Not a huge deal to have a 1/3 second 20 degree deflection in a trainer plane traveling 60 mph. Could be 'interesting to have it happen to a rocket coming off the pad, or worse at max velocity.

Plus, the servo adjustments on the transmitters are never 100% precise. Everytime you turn on the receiver and the transmitter they reset, but if you've bumped the trim gimble you have to reset the control surface and hope you got it straight. Plus theirs the possiblity of bumping the stick as it ascends, which would fall under the 'unexpected result' category. You can't keep the transmitter off until ejection either, that leaves the receiver prone to reacting to every stray signal bordering on its frequency, it will really dance then. Only viable option would be to have the receiver turned on by an ejection charge based switch (with surfaces preset to 'level' before turning it off).

Not a huge idea that I would pursue.

I hesitate to weigh in on this thread, but I've got a thought. Could you design the body so that it was a glider by itself? The reason I say this is because my son built a CC express and the booster glides because of the orientation of the fins and the balance (of course). At that point it would seem like you're only adding corrections to the natural glide with the control surfaces rather than trying to heave the body around with elevons.

I know there has been some study of "backsliding" rockets (Peter Alway comes to mind and I believe it was something like a Mean Machine). Maybe this would be a place to jump off from.

At any rate, please ignore if this is gibersish. And I'd be very interested in what you come up with for the Cosmos Mariner. That would be a cool RC glider!

MetMan

I really don't know much about RC so this is kid of an outside opinion, but couldn't you use the ejection charge to unlock your control surfaces? Your relying on it to safely recover you rocket anyway.

If this "JDAM-like" rocket recovery idea doesn't work, I'll certainly be moving on to trying a convential glider with RC control... either scratchbuilt or a kit like the Cosmos Mariner. I wanted to give this a try because I thought it would be pretty cool if it works, and I've never seen it done before (I may soon find out why ).

I have a Futaba 9C computer radio, so trimming the control surfaces and keeping them there (on the ground) is not a problem. I'll have it in low rate mode for launch, which will be set up for very small control surface movement... bumping the stick during boost should only cause it to veer off slightly.

In the air, during the boost phase, flutter is one of my main concerns (as stated in the original post). One way to help eliminate flutter is to seal the hinge gap with tape or covering. My current plan is to use CA hinges and seal the gap with hinge tape on both sides. This will provide ~3X the hinging strength normally used on lightweight aircraft (usually just CA hinges or hinge tape on only one side is used).

True, a bad radio glitch could make things ugly, but thats true for any RC aircraft that you can create. Even the Estes RC Centurian Boost Glider will have this as a potential problem. I've been flying on the same frequency at my club for over a year now, and have had no glitching problems, so the liklihood of me having a bad glitch during my two seconds of boost is pretty small, IMHO...

Thanks for all of the input... you guys know much more than I do about rockets, so I'm considering everything said here very carefully!

Originally posted by stevecarr
I really don't know much about RC so this is kid of an outside opinion, but couldn't you use the ejection charge to unlock your control surfaces? Your relying on it to safely recover you rocket anyway.

Click to expand...

Steve,

I'm an RC junkie... I like to fly RC planes & helicopters, and have a few RC cars too (no boats yet)! I really want something I can control via RC after launch. The Estes Centurian boost glider looked pretty good to me, but that's $100 and looks like it's being discontinued (some online stores still have it, though).

After seeing some rockets come down, nose first (because the nose didn't pop to deploy the chute), and after reading about the JDAM bombs being guided by moveable fins, I figured I might give this idea a try.

It's an extremely simple design... not nearly as complex as a boost glider. If it works, it will be pretty cool... if not, it will be just another one of my dumb ideas!

What? No boats? Boats are the coolest of all RC (see avatar)
On a side note, JDAM bombs come in at >45 degree angle. The fins guide them, but wouldn't do squat for anything but a very minor change (enough for homing, not enough for pulling up). A lifting body or stubby winglets at the CG would be the only options short of a full RC BG, imo.

Is it a boat? Is it a plane? Watch.

Another R/C rocket glider is Edmonds Aerospace's Arcie II:
https://members.aol.com/RobEdmonds/RC.html

The T-25 Centurian is still on Estes' website:
https://www.estesrockets.com/cgi-bin/products.cgi?view,406

More R/C weirdness: a lawnmower.

seen those two MANY times before. Both have been over rc forums about a million and a half times. The company that makes the lawnmower also makes a doghouse, a racecar, and a tank.

My concern would be that without some kind of wing, you're not going to slow this down much even if you get it to flare up some.

I would hate to think that the radio gear will be "single use", just like the motor.

Originally posted by BobH48
My concern would be that without some kind of wing, you're not going to slow this down much even if you get it to flare up some.

I would hate to think that the radio gear will be "single use", just like the motor.

Click to expand...

If all else fails, I figure I can at least get it to spiral or spin for a helicopter-like descent. The radio gear should survive... I've piled in planes several times when I was first learning to fly (well, OK, I still pile one in on occasion ) ... I've yet to total a receiver or battery pack... just a servo or two if they took a direct hit.

Originally posted by pkh
If all else fails, I figure I can at least get it to spiral or spin for a helicopter-like descent. The radio gear should survive... I've piled in planes several times when I was first learning to fly (well, OK, I still pile one in on occasion ) ... I've yet to total a receiver or battery pack... just a servo or two if they took a direct hit.

Click to expand...

I hear you on that!

When I was learning to fly, I had my share of crashes without too much radio damage, but a few years back I was doing a few high g turns with a .40 size model and the wing bolts ripped right out and the fuselage came in ballistic from about 300ft. The battery pack and one servo survived. The receiver case just blew apart and two servos were junk. One servo was still usable and the aileron servo was fine as it fluttered down with the wing.