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Orbital SkyDart Project

 

I've been wanting to build a big sport model using the "original concept" for the Space Shuttle. A winged flyback booster, plus winged orbiter of course. No ET, no SRB's. Both gliding back, both R/C.

I decided for practical reasons to not build it all from scratch, since it would mean building two new separate R/C models. So, the SkyDart 2X model from 1999 was selected to be used as the "orbiter". There is more info about the SkyDart 2X model at this link.

SkyDart 2X at an "undisclosed location"
First plan, with 4.6" dia winged booster
Revised plan, 3" dia winged booster

With the SkyDart chosen as the orbiter, that freed up the flyback booster design to be of a style of other Estes/Centuri pseudo-scale/fantasy kits of the SkyDart's era (Also by borrowing a certain old Estes kit's markings). The flyback booster wing design was a swept double-delta similar to a model I had made in the 1970's. A canard was considered, but ultimately dropped. Twin rudders at the wingtips were chosen to keep the rudders clear of the SkyDart and to improve yaw stability while hopefully avoiding yaw/roll coupling problems.

The winged booster originally was going to use 4.6" tubing, old NCR Shuttle ET tubing. And a vac-formed ET nose. But as the raw parts were being gathered up and weighed, it looked like the model might be unlikely to make it under the 1500 gram target weight (being under 1500 grams would allow it to be entered in the "Imagination Celebration" event at NARAM-46). As well, the vac-formed ET nose was going to be pretty fragile, a poor landing would have crushed it.

So, the winged booster's nose and tubing was changed to 3". An Estes "Big Daddy" nose and LOC 3" tubing. Unfortunately the LOC tubing was more like 3.0" inside diameter, not OD, so a 3/8" lengthwise gap was cut out of it then spliced back together to make it 3" OD to match the nose cone. Before that was done, the very heavy tubing was peeled to reduce weight. One 34" tube was peeled to cut the weight from 240 grams to about 120 grams. Another 34" tube was peeled even more, down to about 80 grams. The 80 gram tube was used for most of the main body, with 12" of the 120 gram tube used for the front section since the front section ahead of the wing is what has a lot of stress on landing.

Before the project got any further, boilerplate testing was needed. A 1/3 scale boilerplate of the winged booster and Skydart were built (Technically a 2/3 scale of the original SkyDart, but 1/3 scale relative to the 2X Skydart). A series of hand throws worked out the glide CG location for the winged booster. After that, attachments were added to the booster to mount the SkyDart to it. The SkyDart was located so its glide CG was dead even with the glide CG for the booster. This was done so that if the SkyDart 2X didn't come off (didn't "stage"), the combination of the winged booster and SkyDart 2X could glide back down under control. The winged booster used an internal pop-pod system to add noseweight and hold down the elevator, like the original SkyDart used. The boilerplate SkyDart did not glide, its elevator was taped flat and it was rigged not to separate for the flight tests.

      

The boilerplate flew well, confirming the boost stability. The combo also glided back down well. The boilerplate has since been modified so that the SkyDart will separate at ejection and glide down by itself.

With the boilerplate testing done, main construction continued for building the full sized winged booster. 3/4" thick blue foam was used for the wings. Since the model has to be shipped, the outer wing panels were cut off and a wing joiner system added to them. Since the blue foam leading edges would be pretty badly dinged up on landing for a model of this size and weight, pre-shaped balsa leading edge stock was used. The rudders are 3/16" thick medium-light balsa. When the rudders are glued on, some fiberglass cloth will be added to reinforce the joints.

Disassembly, with yellow yardstick
Rough assembly, tube & rudders taped on

The servos for the booster will be Hobbico CS-35MG servos. They have a lot of torque but are not as big and heavy as full size servos.

Engine power for the booster, two G12 clustered reloads. If one does not ignite, the model will definitely be in trouble. It may be launched at 45 degrees so that if only one lights, there will be a chance of getting it into a safe horizontal attitude then make a shallow climb.

The SkyDart might use a D12. An E6 might also be possible, though ignition will not be as straightforward. The SkyDart's engine ignition will be under control of the person flying the winged booster, using the Transmitter's trainer switch (spring return toggle).


July 5th update: The booster was completed enough to test fly - lacking decals, some markings, and some of the in-flight staging details. The model came in light enough to add a water ballast system to the nose cone to counteract the propellant burnoff, as with the X-1 model. The first test flight was all-up with the SkyDart 2X set up for a captive flight, to test the boost with two G12 reloads.

The first attempt was half misfire, half ignition. Only one G12 lit. Fortunately the model stayed in the tower. For the second attempt, two ignitors per engine were used. Both ignited, and the model took off well. It was easily controllable, near the end of the boost I wished I could have ignited an engine in the SkyDart, but that will have to wait for NARAM (for a 2nd pilot). After transition, the glide elevator trim was adjusted so the combined model could glide well. Some left aileron trim was also added, as the model had tried to roll to the right a bit on boost. It was easily controlled to a safe landing. The glide CG seemed to be just about perfect, validating the 1/3 scale boilerplate's CG.

Another flight was made, using the booster only, without the SkyDart, to check out the glide trim and handling of the booster by itself. First attempt was with a single F16 reload. It didn't light too well, finally getting enough thrust to leave the tower. The combination of too little airspeed and being extra nose heavy (since the water takes 12 seconds to drain out and a burned out G12 was in the second mount so the model would have the correct glide CG) allowed the model to get horizontal and hit the weeds. It was pretty close though to almost catching itself into pure horizontal flight at 2 feet off the ground, where after building up a little more speed it could have then been climbed upwards. Fortunately there was no damage, thanks to the very very shallow angle (so shallow the nose cone did not dig into the ground) and flying into the weeds.

After checking it over to confirm it was OK, another flight was made using a G25. That took off great - plenty of speed but not too much speed (A G40 would be too much.). After transition, it went into a nice glide, much better than when gliding with the SkyDart 2X on its back.

So, the flight testing is done. Time to add the decals, markings, and button up other little details to make it ready for the 2-stage flights at NARAM-46. And to get on to making contest models......

 

The obligatory
"before the first flight,
just in case" photo


July 10 update: All finished up. Including the Orbital Transport style decals and markings. Last pics before the model gets disassembled and boxed up.

The booster gets a name.

Barely visible under the door area is a spruce fairing that protects the water dump fill /drain and vent/overflow fittings from landing damage.

 

 

The two forward-pointing dowels on the short "rudders" on Sky Booster's body tube plug into two 3/16" launch lugs glued on the SkyDart. The front end of the Skydart has an old-style dowel mount that plugs into a 3/16" lug on the booster.


The SkyDart's engine will be ignited by R/C using a servo and switch onboard the Sky Booster. The servo at right (Hitec HS-50) has a mini lever switch CA'ed to it, arranged for the switch to be pressed when the Transmitter trainer switch (spring loaded) is engaged. When the switch closes, power from a second battery is sent both to an electric match and to a small Piezo beeper (wired in parallel). The Piezo beeper sounds to indicate that the system is "firing", which acts as both a warning and for early system checking before SkyDart's ignitor is plugged in. Clear heat shrink covers the solder joints and gives extra support to the Piezo beeper housing.

To save mass, a very low current electric match is used, which only needs two cells from an 8 cell 12 volt "N" sized battery. The two cells plus connector only weighs 2 grams. The reason for not simply using the onboard R/C battery pack is that in case of a continuous short during ignition, it would shut down the R/C system, probably making the model crash.

In the photo at right, the wiring coming out to the left are the servo leads to the receiver. The wiring at bottom is to the 2nd battery (2-cell ignition battery). The wiring to the right runs along the inside of the body to an externally mounted SIP socket for the ignitor leads to plug into.

 

 NARAM-46 flights

The photos below are by Chris Taylor, from his NARAMlive.com website, unless otherwise noted.

The 1/3 scale boilerplate was flown Saturday. The SkyDart coming off at ejection to glide back as well as the Sky Booster gliding back.

The first all-up staged flight of the full sized Orbital SkyDart was planned for Saturday at NARAM, but it was way too windy. So it was pushed back to Sunday afternoon, for the Imagination Celebration qualification flights. The SkyDart made three trim/practice flights by itself, the last one flown by Bob Parks. But before the whole Orbital SkyDart model could be prepped, the rest of the day was rained out. Monday was too contest-intensive (with three duration events), and Tuesday had no flying. So, the first flight was Wednesday morning as soon as it could be prepped.

The pilot for the SkyDart was Bob Parks, who commutes frequently from California to Virginia to work for John Langford's Aurora Flight Sciences company. It was really great to be able to have Bob involved in flying this project, given that he is one of the early R/C RBG pioneers from the MIT Rocket Society, the first R/C Scale model at an FAI World Spacemodeling Championships (X-2 in 1978), and his full-scale work thru the years ranging from the F-117 project, man-powered aircraft, and more recently at Aurora, designing test models and full sized prototypes for winged aircraft to fly in the Martian atmosphere, the Mars Flyer.

George Gassaway & Bob Parks, before flight one.
Sky Booster & SkyDart together in the tower. The orange flags are on "remove before flight" plugs that keep the R/C battery power off.
Loading up the water ballast in the nose, as George Rachor (background) looks on. The water counteracts the weight of the propellant in the two G12 reloads, for proper CG for boost and glide.
An umbilical plugs into the nose at about 30 degress for the water ballast fittings to interface. At liftoff the umbilical unplugs itself, allowing the water to drain out in flight.

The first flight used an E6 in the SkyDart. The combo took off well, and at liftoff Bob counted down from six so that the SkyDart's E6 could be ignited 1-2 seconds before the two G12's burned out. The sky was hazy and at first it seemed like the SkyDart had not staged, then a separate smoke trail could be lightly seen (E6's usually do not leave a visible trail, but it was humid). I didn't see what was going on with the SkyDart, needing to fly the Sky Booster. Turned out Bob Parks had not seen it for a few seconds either, a combination of the sky conditions (white on white is not so visible) and distance. It turned out the Sky Booster was a bit tail heavy, and got into a stall which made things a lot worse when the removable engine mount slid half-way out to make it even more tail-heavy. It ended up with a very hard landing, no major damage but putting a slight spiral crimp into the 3" body tube. Bob had no problem flying the SkyDart once he got it in sight, flying it very well and doing a 360 degree roll before landing. The Sky Booster had a bit of noseweight added to it, as well as adding some masking tape to improve the friction fit (the 3" tube had swelled a bit in the humidity)

The Imagination Celebration finals was flown a bit after 5:30 PM Wednesday. It started drizzling, then lightly raining for awhile, some of the fliers flew in it. Fortunately the rain stopped (for awhile) by the time the Orbital SkyDart flew. Again, Bob Parks was the pilot for the SkyDart. The liftoff was good, but got a little squirrely in roll. Later review of photos and video showed why, the SkyDart was not attached properly to both of the aft mounts, it was tilting. The boost was easily controllable anyway.

Bob had suggested igniting the SkyDart's E6 at four or five seconds into the boost, so it would not be quite as high as before. At first it seemed like it had misfired, as the switch was thrown but the SkyDart didn't stage. Then as the G12's burned out, the SkyDart separated, and burned for about 4 seconds or so. So, the SkyDart's E6 had ignited on time, but it didn't have the "oomph" to push forward off of the Sky Booster. At any rate, the Sky Booster was flown back under good control thanks to the added noseweight, and Bob Parks flew the SkyDart near the booster for a bit. When the Sky booster landed, Bob flew the SkyDart past it doing a 360 degree roll, then landed it near the Sky Booster.

Installation of the two G12's
Loading the water ballast
Launch (Tom Pastrick pic)
Sky Booster glide (Tom Pastrick pic)

It seems like the improper attachment of the rear fuselage of the SkyDart added some extra friction and drag that the E6's thrust spike was unable to overcome on that flight. After evaluating the situation later, it made sense, the sustainer thrust of an E6 is not enough to make the SkyDart fly faster than the Sky Booster on two G12's. So for future "staged" flights, the SkyDart will need more thrust, such as a D11-P or an E9-P. But E6's are still a good option for air-started flights, where the whole model would be put into a stable glide before the SkyDart's engine would be ignited (See the R/C RBG Safety Code regarding air-start procedures from level glide).

The Orbital SkyDart didn't take either of the top two places in the Imagination Celebration event. The wimpy "staging" didn't help things.

Friday at NARAM-46:

I had planned for the Orbital SkyDart to fly a lot more than twice at NARAM, but the weather sure didn't cooperate. Friday, when Scale & PMC were flown, it was pretty windy, but there were occasional moments of "not too much wind", due to thermals being generated. So, after charging up batteries and prepping engines, it was taken to the sport range and readied to fly. Chris Taylor was tabbed for the SkyDart pilot. The engine combo for the SkyDart was changed to two CHAD-staged C6's, and a flight profile where the SkyDart would air-start from level glide, not stage.

Nearly ready for flight, the SkyDart's R/C system would not come on. The 3/32" jack used as an on-off switch failed to close after the "remove before flight" power plug was removed. So, back to the prep area to so some mods to bypass the switch, using the old fashioned but workable "plug the battery in last" method.

Finally ready for flight, and a reasonably low wind for the day, the countdown began. At zero, the model started moving up and then "BANG",  everything fell to the ground in several pieces. But there was also the noise of something thrusting up and away rapidly, end over end. It was one of the G12's, thrusting free from the model.

The SkyDart was not damaged, beyond losing its forward attachment dowel hook. The cato had made it separate completely from the Sky Booster, so there was no danger of the staged C6's igniting (not that any of the wiring or batteries in the Sky Booster were any shape to ignite anything either).

The Sky Booster shortly afterwards.
The R/C compartment (see hatch at upper right of torn tube) took a direct hit. Unknown how one of the two engines managed to burn the forward left wing area.
Forward portion of the 3" body tube.

This tube was donated for the Best Midwest Qualified Flight "trophy".

The Sky Booster though looked a mangled mess. The main wing had the 3" diameter body blown apart in the middle, starting at the R/C access hatch, with the front half of the tube completely knocked off. However the only significant damage to the wing was an area near the upper left, melted blue foam and some ripped/melted Econokote covering. That area can be easily cut out and a new piece of blue foam glued in, as that area of the wing is not structurally critical (If the wing had been damaged where the joiners are attached, that might have required replacing the whole wing). So, the model looks a lot worse off than it is. The only part of the model itself that was badly damaged was the 3" tubing, which all needs to be replaced. The vast amount of assembly time and effort went into the wing (including servos), the nose cone's water ballast tank and plumbing, plus all of the pre-build work from the first sketches, to the 1/3 scale boilerplate, to the custom-drawn and printed decals.

So, if I really had to do it in a short time (and had the 3" tubing on hand), the model could be repaired in about 3 nights. So, it will be fixed and flown at NARAM next year. Well, I need to get another 32mm reload casing, I only have one good one left now.

There were some onboard casualties though. The 300 mAh NiMH battery pack got hit by one of the two engine casings, tearing the shrink-wrap on it and leaving an impact mark. I'm already leery of small NiMH packs now, so after a hit like that I won't trust that pack for flying anymore. And the HS-50 servo used for pressing the lever switch for staging was MIA. We looked and looked for it, it was gone, though the lever switch and Piezo beeper formerly attached to it were found. The Hitec 555 receiver LOOKS like it survived, but it has not been tested out yet.

So now, what about the cato? It was actually my fault, I had had something like this happen before (1996) but not so badly. I had not cleaned the front end of the engine casing out like I should have from previous flights. So, a bit of crud had built up, enough to allow a little bit of an air gap (not much, but any is not good). When the G12 ignited, the endburning propellant was forced by the pressurization to move up into the top of the casing like a piston in a cylinder. This made the slight bit of air get intensely hot (Diesel effect), enough to ignite the forward end of the propellant grain. When the front end of the grain ignited, it instantly forced the propellant DOWN, something had to give at that point (in 1996, the molded nozzle had fractured and forced the extinguished propellant out the aft closure like Play-Doh - the casing survived and the impact to the model was not nearly as severe). What gave this time was the back end of the engine casing, where the threads begin for the aft closure to screw on. The casing broke right where the threads start, which is also a "safety" design of the engine so it will structurally fail up and down and not outwards. That shot the aft closure and propellant grain (which then self-extinguished) into the ground, and made the engine casing ram up into the body tube at great speed. The engine mount was never built to take that kind of abuse, it was built for maybe 2X of max flight loads, not 100X plus cato loads. Judging by the streaks of white paint on the casing, it seems like the casing ripped thru the body tube since the only paint on the model was on the tube.

But that was not the only cato. The second G12 was at first assumed to have just been knocked loose and flown free. But when it was found, very hot, on the ground, it had a nasty hole burned in the side of it, and the casing was bulged out near that hole from what seemed to be heat deformation. How did that happen? It's not really clear how. But it is assumed to be as a consequence of the first G12 cato, not an independent cato (I've never heard of an endburning reload to burn out of the side of the casing). The first G12 did not split apart, it only shot the aft closure and so forth right down into the ground, so that could not have affected the side of the second G12 casing. And the first G12's casing, while fractured at the back, had nothing sticking out of the side. Also the area with the hole burned in the casing was not directly next to the other G12. it was about 30-45 degrees away roll-wise. So something very odd happened to it. I'm starting to wonder if when it flew free it hit an HPR pad or some other hard object to damage the casing in the side like that to allow it to vent gases out the side.

Have you seen me?   

 

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