The Finless Rocket Flies Again

(Copy & pasted from my blog)

I redesigned the Finless Rocket. The launch lugs have been replaced with rail buttons for a more streamlined look. This version uses a 100+ gram plastic ogive nose cone that has 200 grams of lead in the very tip. Loaded with a motor and parachute, the entire rocket weighs about 825 grams. The concentrated weight in the nose shifts the center of gravity of this rocket well in front of its center or pressure, even without fins. The first version of this rocket included a long section of body tube behind the motor nozzle. When it flew the first time it appeared very underpowered. On a rocketry forum I've recently learned about the "Krushnic Effect" which basically boils down to rocket motors lose significant power if the nozzle is blasting through a tube. I cut the tail tube down from about 9 inches to about 2 inches. The 70 Newton, 1 second burn motor lifted the rocket very well. It flew very straight for a few hundred feet where it wobbled before apogee. It was windy and gusty today. Based on parachute decent time and ground distance covered I calculated that one rocket was pushed along by over 20 mph winds. I think the wobble may have been a gust induced wind cocking. Even if the finless rocket naturally wobbles while coasting up, I'm very happy with its performance. It proves the point that rocket stability really does come down to the primary rule of placing the center of pressure behind the center of gravity. The finless rocket proves that some of the most trusted rocket simulation programs can be wrong. It also proves that the old-school string test has a lot of validity. Check out the flight video and "wind tunnel" test.

Finless Rocket Movie https://www.hallbuzz.com/movies/2014/finless_rocket.wmv
(47 seconds, 37MB, 1080p)



My thumb shows the CG with a spent motor. The CG with a live motor is about 2 inches back.

That's pretty cool. I think I'll have to build one. Can you provide some specs: length, diameter, CG, overall weight, exact motor used.

Thanks.

Ditto Dick!
Super Kewl! Looks like another Micro Downscale Odd-roc in the works LOL! Original dimensions and mass would certainly helps

I have to admit that I pretty much gave up on swing tests when the rockets got too big. I also have had little ones that never swung right but fly just fine. I will definitely confirm any finless rockets with a swing test.

My virtual world is now completely shattered. I thought swing tests were old school and of little to no value. That true faith could be had in computer simulation. That more power and nose weight was just a band aid for poor rocket design. That no finned or two finned rockets were dynamically unstable, especially if those two fins were way up high on the rocket. I heeded the warnings of not launching my Interceptor E on an E9-4 off a short rod in the wind. I felt such youthful joy watching the simmed rocket fly virtually on the computer screen that a real world launch seemed of little value. Say it ain't so! The computer must be right! Science must prevail! What am I going to tell all the kids huddled around the lap top? Those old farts with their balsa nose cones and sanding sealer can't be right. It must have a gimbaled motor and active stabilization!

If this is true then someone could make silly airplane rockets with canted motors and claim the exhaust produces flame fin action. Weighted and canted motors in the nose cone, outboard motors located in places they should not be. Silly ducted designs in an attempt to move the motor farther forward. Oh the oddroc horror! People using their minds and not the machine? That puts us right back to wearing togas and sitting around just Philosophizing all day. Now I am so upset I need to get back to my Play Station or at least watch the Iron Man movie again.:grin:

I'm not sure I like your attitude. I think you ought to mail me ALL your airplane rockets for immediate disposal on the fields of MDRA.

Daddyisabar said:

My virtual world is now completely shattered. I thought swing tests were old school and of little to no value. That true faith could be had in computer simulation.

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Meet Green Cheese:


Depending on which method you use, Rocksim says it's either unstable or marginally stable. A swing test says it's stable. Let's see which one is telling the truth:


Eat Cheese, Rocksim!

Looks like it is close to being around a 10:1 L/D ratio and needs some base drag correction in Rocksim.

I'll remain a heretic and stick to Rocksim. Unless the rocket has no fins.

rstaff3 said:

I'm not sure I like your attitude. I think you ought to mail me ALL your airplane rockets for immediate disposal on the fields of MDRA.

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With the fire season around the corner and given half of my latest airplane launches have crashed, they might just tell me to pack up and head East of the Mississippi. Maybe I can finally install ROCSIM on the way.

If you ever get a rocket that your RSO rejects, MDRA is your place.

Nice project! It shows that experimenting can be good. I tend to think that Rocsim is conservative, which reports on this thread show. Although, one time I had a very short stubby rocket with lots of nose weight and fins that extended way past the aft end. Rocsim said it was stable and it was not. Like some say Rocsim may be used as a guide line, but for models that are out of the norm, the flight is the true test. In such cases it may be good to have light proto-types and some stand-off distance to the launch pad.

Too bad Gerald Bull isn't around these days. :kill: He could take out the wobble at the end of the flight.

rstaff3 said:

That's pretty cool. I think I'll have to build one. Can you provide some specs: length, diameter, CG, overall weight, exact motor used.

Thanks.

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Loaded CG is 15.75" from the tip of the nose, 21.25" from the tail. When the motor is spent the CG moves forward about an inch.
825 grams fully loaded. LOC 3" nose cone and body tube, 29mm motor mount
Motor: Cessaroni Pro-29 / F59 White Thunder (70 Newton, 1 second burn)



There's really nothing special about this exact design, however. Except that, all efforts have been made to shift the CG as far forward as possible. The motor is mounted forward a couple inches and the motor mount tube is extra long to force the chute into the nose cone. Then there's the 200 grams in the tip of the nose. I think this could be done at almost any scale or sized motor. I think a fat body helps though.

rstaff3 said:

I have to admit that I pretty much gave up on swing tests when the rockets got too big. I also have had little ones that never swung right but fly just fine. I will definitely confirm any finless rockets with a swing test.

Click to expand...

If its too big to swing, just hang it from a clothes line in the wind. The swing test has a disadvantage of the fact that the aero-stability of the rocket has to be overstable enough to overcome inducing a 360 spin for every lap the rocket must make. Heavier/dense rockets are harder to get to fly straight in a swing test. Borderline stable rockets also are harder to test. I tried to swing test an arrow once, it tumbled. Clean wind is the ideal test in my opinion.

Thanks.

On short fat rockets, base drag does help a lot. The references in the Apogee newsletter cite a 10:1 ratio. However, it's clear that the effect doesn't abruptly turn on.

That's really cool!

Daddyisabar said:

My virtual world is now completely shattered. I thought swing tests were old school and of little to no value. That true faith could be had in computer simulation. That more power and nose weight was just a band aid for poor rocket design. That no finned or two finned rockets were dynamically unstable, especially if those two fins were way up high on the rocket. I heeded the warnings of not launching my Interceptor E on an E9-4 off a short rod in the wind. I felt such youthful joy watching the simmed rocket fly virtually on the computer screen that a real world launch seemed of little value. Say it ain't so! The computer must be right! Science must prevail! What am I going to tell all the kids huddled around the lap top? Those old farts with their balsa nose cones and sanding sealer can't be right. It must have a gimbaled motor and active stabilization!

If this is true then someone could make silly airplane rockets with canted motors and claim the exhaust produces flame fin action. Weighted and canted motors in the nose cone, outboard motors located in places they should not be. Silly ducted designs in an attempt to move the motor farther forward. Oh the oddroc horror! People using their minds and not the machine? That puts us right back to wearing togas and sitting around just Philosophizing all day. Now I am so upset I need to get back to my Play Station or at least watch the Iron Man movie again.:grin:

Click to expand...

When I was a kid I built this Superman model. When I grew tired of it I taped an Estes D12 onto him. My friends and I experienced 1.7 seconds of bliss and terror as that SOB circled us in an incomprehensible blur. It was like he was everywhere all at once.



My next bad-idea rocket was when I took an Estes V2, carved out the nose cone to make it lighter and faster and then, based on a photo in a book, made scale wings so that it would fly.
The pic in the book looked like this:

aerostadt said:

Nice project! It shows that experimenting can be good. I tend to think that Rocsim is conservative, which reports on this thread show. Although, one time I had a very short stubby rocket with lots of nose weight and fins that extended way past the aft end. Rocsim said it was stable and it was not. Like some say Rocsim may be used as a guide line, but for models that are out of the norm, the flight is the true test. In such cases it may be good to have light proto-types and some stand-off distance to the launch pad.

Click to expand...

I built this rocket in Rocksim and Open Rocket. The thing that bothered me was that the CP was modeled in the nose cone no matter how much body tube there was. The cardboard cutout mode got it right, but that mode certainly has it's flaws.

David Hall said:

If its too big to swing, just hang it from a clothes line in the wind. The swing test has a disadvantage of the fact that the aero-stability of the rocket has to be overstable enough to overcome inducing a 360 spin for every lap the rocket must make. Heavier/dense rockets are harder to get to fly straight in a swing test. Borderline stable rockets also are harder to test. I tried to swing test an arrow once, it tumbled. Clean wind is the ideal test in my opinion.

Click to expand...

Since many rockets need a good head of steam, I guess you need a good period of high sustained winds (up to ~30mph) that are nominally going in one direction. I'll stick with my non scientific software.

I haven't seen a clothes line in over 30 years. LOL

David Hall said:

I built this rocket in Rocksim and Open Rocket. The thing that bothered me was that the CP was modeled in the nose cone no matter how much body tube there was. The cardboard cutout mode got it right, but that mode certainly has it's flaws.

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RockSim basically ignores the body tubes as it assumes a zero angle of attack...ie 'static stability'. I forget what the RockSim proprietary mode does vs. the Barrowman, but I have found it more accurate. In some cases, these simplifications can hurt ya (what appears stable can be unstable)...and you found a case where it shows the opposite. I still believe that the finless rockets may not react consistently to gusty winds....but time will tell.

I just tried changing a body tube in OR. It did show a change at angles of attack other than 0, more at higher angle. It's possible this finless rocket wobbled some because it's not particularly stable at extremely low angles of attack, but at higher angles it is.

I've done wind testing, and wind strength isn't near as big an issue as consistency. With a well-designed normal rocket, stable, string and wind tests can be tricky to perform in way that clearly demonstrate it, because they are only stable over such of a small angle of attack (due to relying on fins to counter a CG near the rear). Not a problem here. However, aerodynamic forces are low, so its ability to overcome "errors" such as misalignment of thrust and mass is low, so it needs a high velocity off the rail.

The body tube is not without effect at low angles of attack, but compared to fins it is very small. Base drag stability is HUGE in a finless rocket, and must be compensated for, again because other forces are small. Perhaps sims should always include base drag, even though can be of small effect. However, drag stabilization is different and perhaps unpredictable regarding angle of attack. Fins help limit angle of attack by changing actual direction of movement, providing a sense of forward. Some saucers become unstable after burnout, because the motor helps it keep moving, defining forward.

The swing test has a disadvantage of the fact that the aero-stability of the rocket has to be overstable enough to overcome inducing a 360 spin for every lap the rocket must make.

Click to expand...

It is my belief that the swing test actually has an artificial advantage to appear "stable": if the rocket is going perfectly sideways, all parts are moving in the same radius, but otherwise, some parts are following a wider radius. The CG/Center of Mass isn't, but the different masses don't experience the same force. This is orbital mechanics, and in a vacuum the rocket would eventually end up going perfectly forward ... or backwards.

rstaff3 said:

Since many rockets need a good head of steam, I guess you need a good period of high sustained winds (up to ~30mph) that are nominally going in one direction. I'll stick with my non scientific software.

I haven't seen a clothes line in over 30 years. LOL

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There is an old dude at the club who has become so cantankerous with the computer that he has worked out a wind tunnel design using his 50's Dodge pickup with his wife driving while he observes the flight characteristics of his big 50's SI-FI rockets. Extreme boat tails with tiny curved fins are almost as bad as no fins at all.

David Hall said:

When I was a kid I built this Superman model. When I grew tired of it I taped an Estes D12 onto him. My friends and I experienced 1.7 seconds of bliss and terror as that SOB circled us in an incomprehensible blur. It was like he was everywhere all at once.

My next bad-idea rocket was when I took an Estes V2, carved out the nose cone to make it lighter and faster and then, based on a photo in a book, made scale wings so that it would fly.
The pic in the book looked like this:

Click to expand...

The Pad Fuhrer would want to see an accurate computer simulation on Super Man before he would give out the pad assignment.

Daddyisabar said:

The Pad Fuhrer would want to see an accurate computer simulation on Super Man before he would give out the pad assignment.

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Back when I flew totally dumb stuff there was no pad fuhrer, no club for that matter. Only lizards, snakes and jack rabbits.

Now I only fly 75% dumb stuff.

This got me LOLing at work. :^)

I once built a rocket using one of those Designer Special kits. It was short, fat, and had a box section tail like a bomb. It flew great on a C6. My Dad thought it would be more fun to try it as a two stage and taped another C motor onto the end of it (chad staged?). Anyway that little bastard scared the bejesus out of me. It made it about 15' off the rod, spun around like a pinwheel, staged into cruise missile mode and then cratered in about 100' away. By far the most exciting model rocket flight of my childhood.

David Hall said:

When I was a kid I built this Superman model. When I grew tired of it I taped an Estes D12 onto him. My friends and I experienced 1.7 seconds of bliss and terror as that SOB circled us in an incomprehensible blur. It was like he was everywhere all at once.

Click to expand...

rstaff3 said:

Back when I flew totally dumb stuff there was no pad fuhrer, no club for that matter. Only lizards, snakes and jack rabbits.

Now I only fly 75% dumb stuff.

Click to expand...

Oh the good old days of country living. Now it is the big city and a litigator around every corner. I have also got to cut down on the dumb stuff.

Looks like this finless rocket was tested on the open snow covered prairie.

I bet that motor could have more tube behind it with a little fire proofing. What would be better for further rescission? A cooler long burning motor with a bigger flame or a hotter short burning one with a narrow flame? How big and fast can the motor go? Where is the Rocket Scientist when you really need one?

Daddyisabar said:

I bet that motor could have more tube behind it with a little fire proofing. What would be better for further rescission? A cooler long burning motor with a bigger flame or a hotter short burning one with a narrow flame? How big and fast can the motor go? Where is the Rocket Scientist when you really need one?

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I was wondering if a longer or shorter body is preferable? Shorter L/D means base drag will help more. Will longer help in some other way?

The 107mm has to be the worlds most impressive Finless Rocket. We used to get attacked by these frequently in Camp Junction City.

TopRamen said:

The 107mm has to be the worlds most impressive Finless Rocket. We used to get attacked by these frequently in Camp Junction City.

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Complete with break-apart recovery.

rstaff3 said:

Complete with break-apart recovery.

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I don't think you could cert on it though... probably not able to be prepped for another flight.