Rockets without Fins

[Alby]

Can you get away with building a rocket that doesn't have any fins?
The reason I ask this is because the US Navy launches rockets that
don't have any fins. So I'm wondering if that can be transferred into
model rockets?

 

[cjl]

Not easily.

The reason that they can get away with it is that those finless rockets all have active guidance. The motors can actually be steered. If that were not the case, none of them would fly straight.

 

[dmjung]

"Guided" missiles that don't have fins to do their guiding are typically vectoring the thrust from the motor. Other than imparting a spin to your rocket in some manner, I think you'll need fins...modeling rockets that don't have fins (or too small fins) can be done using clear fins.

 

[Alby]

Thanks. So without fins, you need thrust vectoring. Makes sense..

 

[ScrapDaddy]

Check this out: https://www.rocketreviews.com/reviews/all/scratch_no_fins.shtml

 

[Micromeister]

Thanks. So without fins, you need thrust vectoring. Makes sense..

Alby:
In addition to Thrust Vectoring it is also difficult to do with hobby motors because of their "Normally" very short burn times. Once the motor shuts off there is no further way of contolling or altering flight path direction or correcting upset do to crosswinds or distrubances.

Modeling Finless Scale Rockets & Missiles or Odd-rocs can much more easily be accomplished with the addition of Clean Polycarbonate (LEXAN) fins. From a short Distance away they are all but undetectable on display or in flight.
Hope this helps.

 

[The EGE]

There other ways of doing finlessness, but none of them produce a spectacularly aerodynamic rocket.

You've got drag-stabilized shapes like cones, pyramids, saucers, and spools.

You can use a stick fin, or many.

In some cases, even vaguely cone-shape rockets like the Russian space boosters can be stable with a small amount of nose weight.

 

[El Phantasmo]

What if you locate the rocket motor very close to the nose and have some sort of "fire proof" "engine tube or cone" extending all the way aft. This would require a fair amount of modification for motor loading and recovery systems. Kind of like a bottle rocket. Here's my very quick, very crude drawing.

 

[Micromeister]

What if you locate the rocket motor very close to the nose and have some sort of "fire proof" "engine tube or cone" extending all the way aft. This would require a fair amount of modification for motor loading and recovery systems. Kind of like a bottle rocket. Here's my very quick, very crude drawing.

For most of the Drag based configurations the exterior sides have to have some angle to cause air friction heavy enough to counter crosswind disturbances or other off path direction effects. In Cone in the photo below has the motor nozzle about 7" below the point of this 16" cone. Flys OK but does some tail wiggling after burnout.

the Corn stalk Rocket "U.S. Corn" uses the Root ball as drag fins but still needed a good bit of nose weight to be stable.

 

[cjl]

What if you locate the rocket motor very close to the nose and have some sort of "fire proof" "engine tube or cone" extending all the way aft. This would require a fair amount of modification for motor loading and recovery systems. Kind of like a bottle rocket. Here's my very quick, very crude drawing.

You'll get no thrust.


The long engine cone will act as a nozzle extension, grossly overexpanding the nozzle and killing the engine's efficiency. This is known as the Krushnic Effect.

 

[Alby]

I suppose, in theory, you could put some thrust vectoring fins behind the
motor's exhaust. But then the question becomes the speed an effectiveness
in the short time frame the rocket travels. Not to mention the amount of
electronics and computing that is likely required to make adjustments.

Although this rocket has traditional Fins, it also has fins to steer it, mounted behind the
motor.

 

[GregGleason]

The ballistic missile fleet of the USN have sophisticated guidance and control systems (like inertial navigation or perhaps even GPS, or perhaps a hybrid in case of a failure of one system) tied into to gimbaling rocket motors.

It is very difficult to miniaturize these kinds of complex systems without a significant weight penalty. The problem is exacerbated because of the typical short, burn times of hobby rocket motors.

That said, it can be done:

https://www.ukrocketman.com/rocketry/gimbal.shtml

https://www.accur8.com/Scale Projects/1-10Vanguard/NARAM_41_Vanguard.htm

Greg

 

[Alby]

The ballistic missile fleet of the USN have sophisticated guidance and control systems (like inertial navigation or perhaps even GPS, or perhaps a hybrid in case of a failure of one system) tied into to gimbaling rocket motors.

It is very difficult to miniaturize these kinds of complex systems without a significant weight penalty. The problem is exacerbated because of the typical short, burn times of hobby rocket motors.

That said, it can be done:

https://www.ukrocketman.com/rocketry/gimbal.shtml

https://www.accur8.com/Scale Projects/1-10Vanguard/NARAM_41_Vanguard.htm

Greg

Now that is what I'm talking about... w00t. lol

 

[gdiscenza]

There is also spin-stabilization.

I've seen pictures of a model that had 2 D engines and 2 AT engines canted in such a fashion as to cause the whole rocket to spin. The launch rod was threaded through the center of the rocket.

I haven't seen it fly, but ballistics imply that it should be stable.

G.D.

 

[luke strawwalker]

What if you locate the rocket motor very close to the nose and have some sort of "fire proof" "engine tube or cone" extending all the way aft. This would require a fair amount of modification for motor loading and recovery systems. Kind of like a bottle rocket. Here's my very quick, very crude drawing.

Doesn't work... Look up the "Krushnik Effect".

Basically the Krushnik Effect causes the engine to lose thrust to the point of uselessness. If the engine is recessed too far into the back of a rocket (or a body tube extends too far past it, the engine plume "over expands" into the tube and this over-expansion robs thrust. In short, the rocket won't lift off, or if it did it'd just stagger far enough into the air to crash.

You can recess the engine a LITTLE bit into the back of the rocket to help minimize noseweight requirements or move the CG forward, but not much... kits like the Estes Phoenix missile recessed the engine to help the CP/CG relationship. But it's only recessed maybe an inch or so at most.

Later! OL JR

 

[luke strawwalker]

I suppose, in theory, you could put some thrust vectoring fins behind the
motor's exhaust. But then the question becomes the speed an effectiveness
in the short time frame the rocket travels. Not to mention the amount of
electronics and computing that is likely required to make adjustments.

Although this rocket has traditional Fins, it also has fins to steer it, mounted behind the
motor.

Those carbon graphite fins in the exhaust stream were used on the V-2. Also the Redstone had the same fins in the exhaust for thrust vectoring. Only problem is, sticking fins into the exhaust stream robs engine efficiency. Aside from the problems of finding a material that could stand up to the heat/blast of the engine exhaust (carbon graphite filled the bill). Due to the inefficiency created by the fins in the exhaust, that method went away rather quickly. Interestingly enough Goddard also launched some rocket with 'thrust vanes' which acted like mini-deflectors to push the exhaust stream to one side or the other, kinda cup-like shaped, as well.

Larger rocket engines firing longer needed gimballed engines for steering. As rockets became more powerful and guidance became better, and hydraulics and control equipment (electronics, valves, lines, cylinders, etc.) got smaller, gimballing the engines rapidly became the technology of choice.

Interestingly enough, when the first submarine launched ballistic missiles (SLBM's) were coming about with the Navy's Polaris program, two requirements forced some novel innovations-- the new SLBM solid rocket motors made making a gimballing nozzle very difficult at that time. (that difficulty has since been overcome, as seen on the gimballing SRB nozzles on the Space Shuttle.) The other problem was the extremely limited space for the missile and missile tube inside the submarine. Nozzle gimbals and the equipment that moves and controls them take up significant space, and there was VERY little space in a submarine, so the keep the missile short, the gimballing nozzle and it's operating equipment were deleted, which allowed the missile to be shorter to fit inside the missile tube inside the submarine. The missile still needed active guidance, however, so a novel system was developed that consisted of a series of injectors placed around the wall of the nozzle radially all the way around it. These injectors were controlled by the guidance computer, which selectively opened and closed the injectors to inject an inert "steering fluid" into the nozzle exhaust stream, which acted to deflect the stream from one side to the other, in any direction, just as a gimballing nozzle would. This "steering fluid" system was MUCH more compact than a gimballing nozzle and it's support equipment would have been, requiring only the addition of steering fluid tanks to the missile structure, which theoretically could be placed nearly anywhere.

The success of the steering fluid system led to it's being used on the Titan III and IV solid rocket boosters, with the addition of two large-ish steering fluid tanks mounted on the sides of the SRM's about halfway up the side of the boosters, adjacent to the Titan core stage.

Later! OL JR

 

[luke strawwalker]

Now that is what I'm talking about... w00t. lol

Yep, that's John Pursley's Vanguard model... I've held that model and inspected it when we had a rocketeer's "guy's night out" at his place a year or two ago. It's a VERY cool model!

It uses a model airplane 'autopilot' type system (off the shelf) with 4 horizon sensors placed in the nose on a horizontally mounted ring, arranged at 90 degree angles around the rocket. These sensors look out to the horizon and sense it by "seeing" the horizon, and feed thier information into a central 'flight computer' electronics box housed in the rocket. The computer compares the information from each of the sensors-- ideally they should all be seeing about 50% "sky" and 50% "ground" if the rocket is flying straight up. If the rocket tilts to one side, the sensor on the "topmost" side sees much more "sky" than "ground", while the sensor on the other side sees much more "ground" than "sky", so the computer sends a signal to the servos to correct that and bring them back into balance. There are two servos mounted on baseplate above the motor mount, with the motor mount itself mounted on a U-joint type gimbal to allow full forward/backward and side/side freedom of movement, or any combination thereof. These servos then extend with pushrods to the motor mount itself, 90 degrees apart. The computer actuates the correct servo in the correct direction (you have to set it up right of course!) and swings the motor the "opposite" direction of the tilt, pushing the rocket back on course.

Of course, this only works so long as the motor is producing thrust, which for model rockets, is a rather short period of time considering the length of the flight overall. Once thrust drops to zero at burnout, if the rocket wanders off course, the computer will swing the gimbal the correct way, but the motor isn't producing any force to correct the flight path. Actually, even after burnout, the delay train of typical rocket motor does actually produce a very tiny amount of force which CAN HELP the correction, but for all intents and purposes you can say it's nil. SO, the rocket has to have a certain amount of 'dynamic stability' to ensure it continues flying 'stably' without tumbling. The rocket will continue to follow a ballistic trajectory even if it DOES tumble, because the engine has burned out and so it won't 'chase you around' like an unstable rocket under thrust will, but of course if it tumbles it won't fly very high or far! AS it happens, the Vanguard is an ideal rocket for this type of system... The upperstage is more slender, and the lower stage is bigger in diameter, with the transition very near the middle of the rocket. This in itself enhances stability (exactly the opposite of say Ares I, whom someone else recently inquired about making into a finless rocket using excess noseweight or a gimbal system-- the larger forward upperstage and smaller lowerstage SRB tube would tend to be very DESTABILIZING after burnout, wanting to send the rocket into an aerodynamically induced tumble). In fact, Mr. Pursley had also created a smaller model to test the concept, and flew it without active guidance stably, with sufficient noseweight, without fins.

Later, John also made a gimballed-motor Redstone, which he flew at NARAM. Despite the Redstone having fins, SCALE fins are pretty small on the Redstone, especially for large- G sized motor rockets, which have a lot of weight in the rear. The gimballed guidance kept the rocket stable and smooth until burnout, and the fins were more than adequate to maintain stability through the coast phase.

Hope this helps! OL JR

 

[Alby]

That Rocks... I want one..

 

[Science Explosions r nice]

What if you had one engine/motor (I dont know why people call them both) instead of having multiple angled ones but for this design you but 4 intersecting and fire proof fins below the motor some how and launched it using a flat surface and no launch rod.

 

[BABAR]

If I visualize you correctly you are thinking of using nozzle flow to replace aerodynamic flow over the surface of the rocket. Meaning High velocity gas traveling over your flameproof fins but because it is fixed it will have the same orientation relative to the rocket no matter which way the nose is pointing. Therefore it will not help.

Major NecroThread. Almost a decade old