Dumb newbie question about fins

Of course fin increase stability. In the real world some (many) rockets have little or nothing in the way of fins (Atlas, etc.) Is this do to secret elf magic? Or gyros?

tomrcast said:

Of course fin increase stability. In the real world some (many) rockets have little or nothing in the way of fins (Atlas, etc.) Is this do to secret elf magic? Or gyros?

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Rockets use something called "thrust vector control" (TVC for short) to steer on most rockets...

Most large rockets use gimbaled engines for stability... what that means is that on rockets like Atlas, the shuttle, the Saturns, Titan, most rockets in fact, the engine is mounted on a swivel that allows the engine to swing back and forth in all directions, moved by hydraulic (or increasingly, electric) actuators that move the engine in the correct direction and amount under control of the guidance system. This swiveling of the nozzle directs the thrust opposite the direction of the unwanted movement, or forces the rocket to turn in the desired direction.

Now, there's a second type of system that's used in some systems, in particularly solid propellant missiles and some solid rocket motors like those used as boosters on the Titan III/IV. It's called
"fluid injection TVC". This system was originally developed for the Polaris missile, since it had a single nozzle and space for the missile aboard the submarine in its firing tube was at a minimum. Hydraulic swiveling rocket engines works great for large missiles, especially liquid fueled ones, but the technology for swiveling nozzles for solid rockets hadn't been developed yet when the first solid propellant missiles were fielded. Plus, they're rather heavy systems and take up a lot of room, so a different, more compact system that was lighter in weight was developed for Polaris-- and subsequently used on a few other different missiles and solid rocket motors as well. The system works by having a series of injectors radially aligned around the circumference of the nozzle, all the way around, like the numbers on a clock face. As the guidance system detects unwanted motions in the rocket or the need to steer the rocket in a certain direction, it commands the valve opposite the direction the rocket needs to steer to open, injecting a liquid "steering fluid" (commonly nitrogen tetroxide, since it's an oxidizer and actually will contribute to the propulsion of the vehicle) into the exhaust stream of the rocket engine coming down the nozzle. The fluid forces the exhaust stream toward the opposite side of the nozzle, in effect "steering" the rocket exhaust and thus creating the same effect as if the entire rocket nozzle had been swiveled in the desired direction, causing the rocket to move in the desired direction. The amount of steering fluid necessary for a normal flight, plus a certain reserve amount, is calculated beforehand and the rocket is equipped with a sufficient quantity of the steering fluid in special tanks. These tanks are very visible on the outside of the SRM's of the Titan III rockets... On Polaris and other such missiles, they are spherical tanks placed in the aft thrust structure of the rocket.

Now, if you go back even further, V-2 and Redstone used a similar system, in that the engine itself did not gimbal to provide stability. They had fins, but the fins were insufficient to stabilize the rockets by themselves, and at any rate, provided less and less force as the rocket ascended out of the sensible atmosphere. To augment the stabilizing forces created by the fins, those rockets used "thrust vanes" that were placed in the exhaust stream coming out of the rocket engine, directly below it, slaved to the servomotors that steered the fins per commands from the guidance system.

These vanes were typically made of carbon, since they had to hold up to the extreme heat of being immersed in the white-hot rocket exhaust which would melt steel or other uncooled metal parts. The problem is, while this system works, it's very inefficient and reduces the performance of the rocket. That's why it was phased out in favor of other systems as soon as they became available...

Later! OL JR

luke strawwalker said:

Now, if you go back even further, V-2 and Redstone used a similar system, in that the engine itself did not gimbal to provide stability. They had fins, but the fins were insufficient to stabilize the rockets by themselves, and at any rate, provided less and less force as the rocket ascended out of the sensible atmosphere. To augment the stabilizing forces created by the fins, those rockets used "thrust vanes" that were placed in the exhaust stream coming out of the rocket engine, directly below it, slaved to the servomotors that steered the fins per commands from the guidance system.

These vanes were typically made of carbon, since they had to hold up to the extreme heat of being immersed in the white-hot rocket exhaust which would melt steel or other uncooled metal parts. The problem is, while this system works, it's very inefficient and reduces the performance of the rocket. That's why it was phased out in favor of other systems as soon as they became available...

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Ok, I didn't know that. Hardcore! Hrm, heat transfer into the control mechanisms must have been a Problem...

I sorta see/guess, where your going with your question. For the most part we --as a hobby- use fins for staballizing our birds. You can make a longer rocket with smaller fins-or in some cases even no fins. Your talking about an active control system vs. a passive system. We lean towards the passive system--fins. If you start trading length or weight or both to get smaller fins you pay a price in performance---gravity hates us !

Zeroignite said:

Ok, I didn't know that. Hardcore! Hrm, heat transfer into the control mechanisms must have been a Problem...

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Well, if you go under the Mercury Redstone at Johnson Space Center Rocket Park, you can see the pads that the carbon vanes bolted to... they're just basically IIRC about half inch thick steel plates with four 3/4 bolt holes through them, about maybe 5-6 inches long and about 3-4 inches wide... not very big at all... the were on the inner end of the "axle" shaft that the outer square-looking wedge shaped outer fin bolted to, at the rear part of the longer, inner swept fin which was affixed to the aft end of the rocket... only the vane and outer part of the fin actually moved, controlled by a servomotor setup of some type receiving commands from the guidance system.

V-2 worked basically the same way but I think the aft trailing edges of the fins on V-2 would move along with the vanes in the exhaust...

There's a study summary in the scale section by Grand Central Rocket Co. that I did, detailing their plans for large solid rocket vehicles, which would have used vanes installed at the aft end of the nozzles themselves, for control purposes... this was before liquid injection TVC, let alone swiveling nozzles for solid rocket motors, had been developed.

Later! OL JR

I was gonna post a link to some ABSOLUTELY AWESOME video of the five J-2 engines on the S-II Saturn V second stage being fired and gimballing, with the native sound... absolutely AMAZING sound... but it seems to not be on youtube anymore... which sucks...

There's some video, but not of the engines gimballing. If you have "The Mighty Saturns" from Spacecraft Films, you'll have the footage and know what one I'm talking about. In one test, they programmed the system to gimbal the engines as they would in flight... since the S-II had five engines, in a "+" configuration, for pitch one pair of opposing engines would swing in unison parallel to the center engine (which was fixed and not gimbaled) either fore or aft... for yaw the other pair of engines 90 degrees from them on opposite sides of the center engine would swing together either right or left. To roll right, all the outer four engine nozzles would swing in one direction in unison (like dominoes tipping over in a circle). To roll left, all the outer four nozzles would swing the opposite way in unison. It was really cool to watch, and just amazing with the beautiful clear/blue hydrogen flames and mach diamonds in the exhaust streams...

Later! OL JR

Here's an excellent photo of the steering vanes on a V2 rocket:

Tremendous answers! Thanks to all.

So them am I to assume....Gimballing is obviously a much more complicated answer than fins. It must be more effective? Fins create too much drag?

Fins do create drag, but real big rockets leave the thick atmosphere very quickly and fins do no good at all when there is little or no air.

Air to air missiles steer with fins.

tomrcast said:

So them am I to assume....Gimballing is obviously a much more complicated answer than fins. It must be more effective? Fins create too much drag?

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One more way sometimes used in sounding rockets is spin stabilization. The old Super Loki Dart employed this to stop the dart from tumbling on its way to 200,000 ft+; it was launched from a rifled tower that spun the whole vehicle up to several revs/sec. Of course this does not provide any guidance, it just keeps the rocket pointed in the direction of travel. At high altitude the restoring moments on the fins are too small to stabilize the vehicle, but angular momentum works even in deep space...

tomrcast said:

So them am I to assume....Gimballing is obviously a much more complicated answer than fins. It must be more effective? Fins create too much drag?

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Gimballing more complicated.....I think it's probably a toss-up. Here's a good explanation of fin (and missile) design. Scroll down to ""Wing Design":

https://www.okieboat.com/Talos aerodynamics and control history.html

Heck, read the whole thing, it gives great insight. It's kinda hard to follow the entire missile story, it's not indexed in any way, and the links to individual pages seem somewhat random or hidden.

Once again, many, many thanks!