Reducing rail button drag with fairings

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raptor22

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I am currently in the process of building a minimum diameter 38 mm rocket for launch later this summer. The goal of this launch is maximum altitude on the available power plant (approximately 750 Ns). Unfortunately, a launch tower is unavailable at this time. RASAero is predicting that the added drag from the rail buttons is reducing our ceiling from 13,000 feet to 8,500 feet, and our maximum speed from 1.74 to 1.32 mach.

So, I am considering ways to reduce the turbulence associated with the rail buttons. Does anyone have any thoughts on fairing them in as seen below? I was considering epoxying on small pieces of wood to accomplish this, would you consider this a safety hazard in terms of coming off and clogging the rail?

728m9j9.jpg


Thank you for your input.
 
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One word FlyAwayRailGuide...okay it is 4 words all strung together.

There is a guy with our club who has used those and they work fine. It helps to look at them as a rocket consumable. Plus the mass and the aerodynamic penalty is jettisoned after leaving the rail, where it counts the most.

Greg
 
There is a guy with our club who has used those and they work fine. It helps to look at them as a rocket consumable. Plus the mass and the aerodynamic penalty is jettisoned after leaving the rail, where it counts the most.

Greg

I haven't used them myself but I have watched quite a few videos of them in action and know they are gaining in popularity. I view them as the next best thing to a tower. As you pointed out the mass and aerodynamic drag is jettisoned quickly after it leaves the rail, but they are likely to be a consumable as well as subject to some drag from the rail. None the less they are a great option.
 
Interesting idea raptor. I personally think you should give it a try and see how it works. The fly aways are cool and everything but this hobby is all about trying new ideas and seeing what works best.
 
Thank you for the comments. I had never heard of fly away rail guides, but they seem like the perfect solution to my current dilemma.

I am still interested in pursuing the fairings, but I think the fly away is ultimately a lower drag option. I will only be able to launch once this summer, so want to make it count.
 
I think the fairings are likely to create more drag than the rail buttons. Also, you will be creating a mass imbalance in one side of the rocket that might create interesting flight characteristics.
 
The mini rail buttons are for a smaller rail, right? Most folks flying rockets like this are hitting them pretty hard off the pad. Are the mini rails and buttons up to this without significant deflection.
 
Is a Acme Conformal Rail Guide or a PML Linear Rail Guide any more aerodynamic than a button? I can see the ACRG being slightly more aero, and possibly the PML one with a bit of shaping.
 
The mini rail buttons are for a smaller rail, right? Most folks flying rockets like this are hitting them pretty hard off the pad. Are the mini rails and buttons up to this without significant deflection.
If you piggy back them to a 1010 or 1515 rail I think you would get the reduced drag of the smaller buttons but the strength and stiffness of the larger rail.
 
Fiy away guides are strictly DYI except for the 38mm Mayhem Rocketry offerings that are pretty nifty. I believe Bill Cook is working on larger ones to add to the lineup. Kurt
 
Wildman had a bag of super low profile 3d printed buttons at LDRS. sadly I didn't get any before he had to bail, (ironic word) but they were fantastic looking!
 
I am working on a CAD file for a 3D printed rail button fairing something like what you have.
 
I am very confident that this shape would constitute a significant drag decrease. From the way I can see it, we have two major contributors to the button drag at supersonic speeds: a large normal shockwave in front of the button and significant turbulence behind the button trailing continuously to the back of the rocket. The front of the fairing would replace the normal shock with an oblique one that does not slow local flow to subsonic speeds, which is major. In addition, the rear of the fairing would close that "wake turbulence" to some extent by filling in the low pressure area behind the button.

I think the offset mass of the fairing is probably minimal in comparison to the weight of the buttons themselves, which do not cause tracking problems. My primary concern is that a little epoxied-on wooden fairing could theoretically pop loose, clogging up the rails, and causing a launch malfunction. The rocket should be doing 170 feet/s before it clears the rail.

I have looked at the acme conformal rail guides, and my thought on those was that the reduced frontal area of those guides would not be significant because the boundary layer that close to the rocket would prevent much airflow between the little metal segments of the guides. In addition, the front and rear of the guide are still essentially blunt for our purposes, maintaining the problems with the normal shock and trailing turbulence.

Luckily, the Mayhem rocketry fly away guides should fit our rocket perfectly. Even the most aerodynamic rail guide is more drag than no guides at all.
 
Is there any reason why you didn't consider building a launch tower? you could do it for about $20 or free if you have some materials already on hand. A search of the forum or Google will bring up many different designs and ideas. rather than making them adjustable I have some that are set for different diameter rockets but its up to how much time you want to spend on it. This way you wont have any rail friction, rail button or fairing aerodynamic drag as well as a few less grams if you dont need it
 
Just an odd question... Has anyone ever tried using water flow to test drag characteristics? There's a tank at my local children's museum that would work perfectly to test out different shapes and watch "airflow"
They have different shapes that kids can move around in the water flow using magnets, in that way you get to see how turbulence forms around varying shapes. If you could replicate the concept, you could try ask kinds of shapes until you had one that works the best.
 
I was under the impression that a tower would be a lot more than $20 in material costs. If I truly can build one for that price it would be fantastic. I will look around.

As far as the water tunnel goes, yes that is a somewhat common technology and it is useful for simulating aerodynamics but only at subsonic speeds. The university I study at has one as part of our aerospace facilities.
 
How about about spring loaded buttons that sit pretty much flush with the body tube when retracted.
 
How about about spring loaded buttons that sit pretty much flush with the body tube when retracted.

May be difficult with MD. I'd say the flyaway rail guides, if not a tower. AMW sells them too.


Later!

--Coop
 
Just an odd question... Has anyone ever tried using water flow to test drag characteristics? There's a tank at my local children's museum that would work perfectly to test out different shapes and watch "airflow"
They have different shapes that kids can move around in the water flow using magnets, in that way you get to see how turbulence forms around varying shapes. If you could replicate the concept, you could try ask kinds of shapes until you had one that works the best.
Water is not a compressible fluid so simulating supersonic effects is difficult....

Building a tower launcher is not that difficult so if minimum drag is necessary, that's what you use.

Bob
 
Ok, I will bite. 3d printed PLA guides, right? What size screw, and what size airframes do you have them for?

Bingo! They are nominally 54mm but will happily sand to fit 29-76mm.

Center hole is for my standard plastic screw, M3, or 6-32.

Will be available from a major vendor in the very near future!
 
Your original post is exactly the same thing that happened to me. I simulated my design through RASAero and it said that I would lose ~5000 ft due to rail buttons. Yes, rail buttons on a 38mm min diameter add some significant frontal area as well as impart off-center drag/lift. But, 35% altitude loss??? I think RASAero may be way too conservative here. How good is the rail button drag model in the simulator? Anybody compare flights with/without rail buttons?

Anyway, the simulation put me in a panic that I would not reach my altitude objective with buttons. So, I started on a journey to build a tower. I went through 4 or 5 revisions of the design that ended up taking longer than building the damn rocket itself. All parts were obtained from the local Big Box Home Store. See my trials and tribulations here:

https://www.rocketryforum.com/showthread.php?69413-Jawstand-tower

If the fly-away rail guides from wfcook were available a few months earlier, I could have saved myself a lot of time and effort!

So, I think messing with rail button fairings is not worth the effort. Get rid of them completely with 1. tower or 2. fly away guides. Good luck.
 
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