Old tractors, new tractors: Flying Pitchfork, Cluster F, TetraJet

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ReynoldsSlumber

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Inspiration​

In the days of yore, when reading G. Harry Stine's Handbook of Model Rocketry, an image toward the end of the chapter on staging struck me:
parallel staging Artis.png
Pat Artis of Ironton, Ohio, perfected parallel-stage model rockets and demonstrated them at NARAM-7 in 1965. By 1966, Artis had refined his designs to carry the booster pads up near the nose of the core model, thereby bringing the CG of the model well forward during the critical boost phase and eliminating the need for large fins to ensure stability during the boost phase. Artis showed that parallel-stage models had superior performance for payload carrying because of their high liftoff acceleration.
The wheels in my head started spinning and couldn't be stopped.

Pitchfork​

I built an Estes BT-50-based tractor parallel-staged model, stabilized by two T-fins like on Artis's design. It used Estes B6-0s for the boosters and a C6-7 with its longer burn duration for the core. Hooks were attached on either side of the main body tube, and the booster pods each had a pylon with a slot to go onto the corresponding hook (the reverse of a typical boost glider pop pod). Booster motor burnout was to act as the ejection charge to kick each pod off and deploy a streamer. I don't have a photo of the "Pitchfork," as I dubbed it for the obvious reason, but here's a CAD re-creation:
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When I went to fly it, my "that looks about right" friction fit on the booster attachment was too loose, so at liftoff things disassembled themselves and flew in two different directions. If memory serves, what happened was that the middle motor lit first and the core took off, while a split second later one of the now-orphaned booster pods cooked off and flew sideways. Lesson learned: forces on a model rocket were stronger than I thought. I also concluded that the igniter lead wires needed to be as long and as lightweight as feasible given the electrical current draw, in order to give the igniters as much of a chance to do their work as possible after the first motor to light started firing. Plus I rediscovered that the boosters' wires should be suspended on stanchions, to relieve their weight and to prevent tangles with the fins.

Flying Pitchfork​

Next I built a BT-60-based model, powered by D12-0s and an E15-8, again with streamer recovery on the booster pods. This time it had conventional fins, in order to reduce the risk that a booster streamer would cause a "Red Baron" scenario. Also, given the equivalency to flying on a F motor, getting rid of T-fins reduced the forces on the fin roots as well as the risk of fin flutter. I dubbed it the "Flying Pitchfork" 'cause I meant business this time (or something).
pf.jpgpf1.jpg

Funny enough, when I went to fly it, it garnered more skepticism from the high power RSO than the low power club folks had given the earlier Pitchfork. Anyways, with a tighter friction fit than before on the pod attachment, and with the aforementioned igniter wire arrangement, it flew great! The boosters' separations appeared to be simultaneous with each other. The booster exhaust heat did cause bubbling of the white paint near the fin roots, so if anyone does a similar model, the effort to make T-fins may be worth it after all. Or at least use paint that can handle higher temperatures.

Further parallel staging thoughts​

Early in the Cold War era, Atlas and R-7 ICBMs used parallel staging in order to avoid the difficulties of igniting a second stage mid-flight. For model rockets, is series staging harder than parallel? In the case of core-only black powder motor models with simple tumble recovery on the first stage, series is of course easier. That said, in windy conditions, parallel staging may actually be the better bet, by way of the high initial thrust preventing excessive weathercocking. That goes doubly for tractor parallel staged models, which with their forward CG don't need excessively-sized fins.

One more note: staging is allowed in NAR eggloft competition. Has anyone tried to compete using a parallel staged model? Could be advantageous; would be extremely oool.

Cluster F​

In thinking about clustered rockets, I hated the thought that a bunch of motors in a side-by-side bundle could cause excessive base drag. I don't know if there's any merit to the idea, but I guessed that by putting some of the motors in tractor pods, it could act as a sort of subsonic version of the "area rule" by spreading base drag out among the aft ends of individual motors. Such a tractor-pusher configuration would form in effect a "minimum diameter" cluster rocket, without the excess cross-sectional area of an enclosing main body tube.

What ensued was a BT-20-based balsa nose cone extravaganza. It took a lot of effort to build, most notably when it came to cutting chunks out of nose cones for the front of the pusher section and sanding them to fit snugly against the center tube, in good alignment and with a good seal for ejection pressure. No pop pods here—those tractor booster pods were permanently attached. The model packed a lot of power for its size with three C6-0s up front and three C6-7s in the tail, totaling to a mid-impulse F36, hence the (retroactive) name. In a bold move to minimize drag and weight, motors were friction fit and taped in place, without motor hooks. Again no photo was taken at the time, so here's a CAD representation:
cf16.pngcf25.png

cf6.pngcf5.png

When it came time to launch it, the winds were too strong to go all-out, so I elected to use just the pusher motors. The tractor motors stayed installed, sans ignitors, for proper CG. On ignition, it leapt off the pad and sped upwind, straightening its arc as it accelerated. Pretty darn cool cluster flight.

Though I had tried to reinforce the center body with a BT-5 stuffer tube, when I recovered the model, it was kinked just ahead of the aft section. I never repaired it. I'm guessing that all it would've taken not to bend would've been a thicker-walled tube, better internal reinforcement... and/or perhaps just going for it with all six motors! Although given how straight the flight path was, perhaps the kink happened when it landed under parachute rather than when under power. As with the Flying Pitchfork's fins, there was some scorching on the nose cones at the front of the pusher section.

TetraJet​

A drawback of tractor motor models with a full-length core body tube is that, due to the motor exhaust's heat, the tractor pods have their thrust lines spaced away from the CG. So all of the motors had better light, or else a smoke rainbow into the ground is almost assured. How would one instead configure a tractor-pusher rocket on which all of the thrust lines go through the CG?

Behold the TetraJet:
tj8.pngtj4.png

tj5.pngtj6.pngtj7.png

Now, this is a concept that I came up with upon reflection on these prior configurations, not a model that I've ever attempted to build. As-designed, it would use 18mm motors: booster motors in the pusher pods, and long-delay motors in the tractor pods. The tractor pods would each house a parachute. If any one motor didn't light, the model would stand a good chance of flying reasonably straight or at least of making it to ejection with sufficient altitude remaining. As shown, the pods are minimum diameter, and the structural connections are 1/4" dowels. The CG would be on centerline a bit ahead of the forward launch lug. The four fin tips sticking out at right angles might not be necessary, although I'd want to have some kind of assurance on stability given the airfoiled forward strut between the two tractor pods. Assembly would no doubt require some fancy jig work, given all the weird angles.

After construction, the other tricky part would be how to handle the igniter wires for the tractor pods, to assure that the igniter clips wouldn't catch on the fins. Perhaps a stanchion up the middle would work, or else one on each side pulling in tension, like umbilicals on a real rocket.

Has anything like this been attempted before? I'm not building and flying rockets currently, but if anyone were serious about bringing such a monstrosity into being, I'd be glad to partner up and supply plans and suggestions.

By the by, there's a lot of potential for fun alternative names for this thing:

Pat Artis's Fever Dream​
Robert Goddard's Nightmare​
Crossfire Hurricane​
I Crossed the Streams​
Hot Dog Cookout​

;)

Flare​

To close out our tractor rocket journey, here's a fun little 13mm motor model called the "Flare." It was an exercise in making a two-finned rocket. In flight it coned into a feverish spiral, so perhaps the fin sweep wasn't doing it any favors. Maybe unswept square fins would do the trick.
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Some may consider canted tractor motor oddrocs with widely spaced pods as ...unnatural. I have seen only a few use such Dark Arts techniques to fly the rockets they love. The great book I once looked at had a picture of a model rocket with widely spaced pods, the caption reading something like "an example of poor rocket design." Such abominations are possible if you successfully transmute the dominant 3-4FNC mindset. Things you will not learn from a Jedi.

It has been done before, but only with strict supervision insuring high levels of safety.

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Xenomorph Barbie Transport

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Crazy Train

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And many, many more silly airplane oddrocs, tapeworms and other flying critters/undead skeletons.

Canted tractor motors can solve many of your oddroc dilemmas faced when conjuring up very naughty designs. Don't just think outside the box, just toss the box aside and put all your doubts and fears in it. :)
 
Tractor motors with two big wings, I MEAN FINS, rule! Put on a silly kite tail to help out too! Watch all the town folk flee the launch site as Smaug is announced to fly.

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Luckily one of the guys had his hunting crossbow in the back of his truck and saved the day. All the folks were safe and sound. :)

P.S. I had to tape that good looking claw fin with some red tape to prevent clip catching. Good looks are paramount.
 
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On the Tetra Jet just evoke the patience of a Jedi, no fancy jig needed. Just good ole fashioned craftsmanship. Keep your mind on where you are at, what you are doing. Learn the ways of the Jedi and we will watch your career with much interest. :)
 
The RSO will point out tractor motors can burn things near them and on the back of your rocket. Reach out with your feelings to learn the flaming danger zone behind the motor. Shape your fins with fire in flight...a totally natural process. That argument never went well with my old RSO but it did work.
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Canting half way between the CP and CG with traditional motors at the back of the rocket, WHERE THEY SHOULD BE, along with fins at the back of the rocket, are long held hallmarks of good rocket design. Just build and fly a Duces Wild or TRES. You are now a Jedi Master!

An inverted canted tractor motor through the CG, being placed at the top of the rocket, WHETE IT SHOULD NOT BE, will, if one fails to light, just provide a tremendous amount of top mounted, off centerline thrust resulting in...I can't mention such horrors on this family friendly forum.

Outward canted tractor motors result in a sooty mess after flight. Inverting them on a frame rocket would be even worse, resulting in tedious clean up and dirty, smelly hands on recovery. Yucky! After the first flight you might call it Tetra Jet: A flying soot monster.

The T-motors have to light at the same time. My inner RSO doesn't want to hear "Oh it is just an ignition issue!" This is not a Hollywood fantasy. This is real life rocket launching! Even slight gaps in ignition and different burn rates inherent in mass produced black powder motors are frightening, especially up front! YIKES! Keep that centerline thrust centered! 3-4FNC beauties rule. Safe and sound.

Parallel staging is also frightening. There is a thread going on now about the old ARC PSR 1824 rocket kit.
 
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Motors up front and in the back of the Tetra Jet are cool. Maybe consider only big motors up front, like 24mm E and D, or even awesome 29 mm E or F! Weight up front, GOOD. Weight in back, BAD.

But Mr. RSO, as the tractor motor burns up front the rocket becomes less stable! Every thing is forcing me to think backwards.

The CG is in space! Where do I attach for a swing test?

Computer simulation is not working. Cardboard cut out is trash.

How many potential thrust centerlines going off at once?

Singing: "I remember when, I remember-I remember when I lost my mind..."

Beware of Dangerous mindsiming. Cogs turning in your mind can decive you. :)
 
Hi @Daddyisabar, wild collection of oddrocs there. I especially like the Crazy Train. If I'm looking at that dragon correctly, that's pretty bold to have outward-canted motors ahead of the CG. (It is the year of the dragon, after all.) Though I guess those who made fully functioning scale models of Mercury and Apollo pad escape test articles achieved a similar feat of daring... just with less flair.
 
Hi @Daddyisabar, wild collection of oddrocs there. I especially like the Crazy Train. If I'm looking at that dragon correctly, that's pretty bold to have outward-canted motors ahead of the CG. (It is the year of the dragon, after all.) Though I guess those who made fully functioning scale models of Mercury and Apollo pad escape test articles achieved a similar feat of daring... just with less flair.
Motors up front worked well for like 800 years and under the Pendulum Falicy until the late twenties. That ain't bad. Should still work today. Kite tails still work too. Flying those pad abort models is just a bit crazy. Flying Halloween decorations you find on sale at Party City is completely fine and dandy!
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Flying frame rockets like yours is totally cool. Yes Mr. RSO. It is just like flying a box kite, easy peasy lemon squeezy! This is as close as I ever came to flying a frame rocket. Very naughty.
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Tractors on a Seattle Space Needle rocket!

Old tractors rule. I always liked the Ford 9N.
 
Do you need the extra fins down low on the Tetra Jet? Ask yourself "Does it look good?" Looks are the most important aspect. Must be really really good looking on the assigned pad. Once this question is asked you are well established on the path to the Oddroc Dark Side. Strayed from the straight and narrow path have you. Orthodoxies no longer apply.

On the tape worm I told the RSO I put the hook fins up front because they looked cool and I didn't mind bringing that nasty.old CP way up. Not a good idea.
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I say Tractor motors and fins up front are for the bold and beautiful! CeeGee smeegee. Blast th CP! Mindsim it.

After launching the tapeworm back in the day an old timer came up to me and said that it was the most stable rocket launched here today. No need for everyone to be so scared. Boy was I relieved.

Staging tractor motors is fun too. Lots of tiny little motors raining down. One of the four booster motors Catos on take off, no problem. Just one stack is out of sequence and all the sin takes place high in the air!
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But only with Strict Level Two Certified RSO supervision! Safety First!

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Mr. Reynolds,
Your post, on the second reading, made me think of a twin pusher. I've included a picture of one. Maybe we can get Daddyisabar to build one as a rocket. I think there were twin tractors, too. I've included a picture of a tri-tractor. Put them together and maybe we have something like you've been thinking about. ;-)
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Mr. Reynolds,
Your post, on the second reading, made me think of a twin pusher. I've included a picture of one. Maybe we can get Daddyisabar to build one as a rocket. I think there were twin tractors, too. I've included a picture of a tri-tractor. Put them together and maybe we have something like you've been thinking about. ;-)
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I have built a Shinden and Blackbird Triplane, both pusher warbirds. All are here on the forum.
 
Tractor motors with two big wings, I MEAN FINS, rule! Put on a silly kite tail to help out too! Watch all the town folk flee the launch site as Smaug is announced to fly.

View attachment 635130
Luckily one of the guys had his hunting crossbow in the back of his truck and saved the day. All the folks were safe and sound. :)

P.S. I had to tape that good looking claw fin with some red tape to prevent clip catching. Good looks are paramount.
1711817256843.jpeg@burkefj, gonna bite on making this one glide?
 
I say Tractor motors and fins up front are for the bold and beautiful! CeeGee smeegee. Blast th CP! Mindsim it.

After launching the tapeworm back in the day an old timer came up to me and said that it was the most stable rocket launched here today. No need for everyone to be so scared. Boy was I relieved.

Staging tractor motors is fun too. Lots of tiny little motors raining down. One of the four booster motors Catos on take off, no problem. Just one stack is out of sequence and all the sin takes place high in the air!
View attachment 635177

But only with Strict Level Two Certified RSO supervision! Safety First!

View attachment 635178


That is sick. I've done rack rockets but not parallel.

I approve.
 
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