Min-Diameter Launch Lug Options

[LandonC]

For some context, I'm building a 98mm min-diameter carbon fiber rocket that's outlined in my build thread. I'm not 100% what launch lug options are available for these types of rockets. I've seen people build their own launch towers that don't require launch lugs for smaller-sized rockets. Considering our rocket will be upwards of 9ft tall, is building our own launch tower the only viable, but unrealistic option?

 

[Lord Rory Gin]

On my BT-80 sized rockets I use both 1/4" LL and rail buttons for flexibility. For any HP motors I would use 1010 rail buttons because that's what is required where I launch. I'm not sure at what mass/dia. when to upgrade to 1515 rail buttons; >15 lbs maybe?

 

[JimJarvis50]

I have used the PML rail guides.

https://locprecision.com/products/linear-launch-rail-lugs-2-lugs-per-kg?_pos=2&_sid=48309265c&_ss=r
The advantage is a lot of surface area to glue to the air frame. I sand them to conform to the air frame. I also sand out the openings a little so they are not so tight on the rail. They are brittle and not ideal, but they can be used for minimum diameter.

Jim

 

[MClark]

Use a tower.

 

[Sandy H.]

I've heard about pop-off rail guides, but have never used them. As they would be non-invasive (i.e. if you have or build a tower, use that, but if you want to fly at a club with only a conventional rail, just use the pop-off guides). Any other glue on (like the PML) or other solutions would add to the drag even if you chose to use a different launch method.

Having said that, I re-iterate that I've never used them and maybe there are downsides. Hopefully someone who has flown them can comment, as I'd like to learn too.

Sandy.

 

[JackC]

You can also purchase Acme conformal rail guides from Giant Leap Rocketry. Scuff the tube where you want to apply them with some 80 grit sandpaper and use JB Weld since the guides are aluminum. These actually have less drag than normal rail buttons. The best performance is a tower launch, though. But Acme convenience is a nice feature.

 

[Neutronium95]

For a 98mm minimum diameter rocket, you really want to use a tower. There are a few designs floating around for towers of various size, adjustability, and prices.

I think that flyaway rail guides just aren't stiff enough to ensure that the rocket goes in the correct direction. If you zoom into the attached picture, you can see that the rocket is not actually pointing in the same direction as the rail. Admittedly it was flying on an O5280, but I'm still not a fan of them. If a tower is not an option, go with conformal rail guides, but I'd still really prefer to use a tower.

 

[REK]

You can also purchase Acme conformal rail guides from Giant Leap Rocketry. Sand the tube where you want to apply them and use JB Weld since they are aluminum. These actually have less drag than normal rail buttons. The best performance is a tower launch, though. But Acme convenience is a nice feature.

I recommend these as well. Here is the link

https://giantleaprocketry.com/collections/launch-systems/products/acme-rail-guide-3-00in

 

[watheyak]

If you're putting rail guides on a 98mm minimum diameter rocket you're missing the point.

There's some serious, hard questions that need to be asked about your motives.

I second Mark Clark. Use a tower.

 

[Steve Shannon]

Considering our rocket will be upwards of 9ft tall, is building our own launch tower the only viable, but unrealistic option?

No, you may be able to borrow a tower. They’re out there. Go to some launches and walk around and see how others solve the problems you encounter. None are unique. In your build thread you mention Mach 3 and that you’re making your own CF tubes, but wondering what fin shape and nosecone shape to use. Take some time to see what others have done. Questions are great, but going to some of the larger launches and seeing the wide variety that people fly will give you some ideas.

 

[watheyak]

Also, when I bring home the club pads for storage I occasionally find conformal rail guides still in the rail from when they were torn off a rocket at launch.

I feel like they're not so great in practice.

 

[Steve Shannon]

Also, when I bring home the club pads for storage I occasionally find conformal rail guides still in the rail from when they were torn off a rocket at launch.

I feel like they're not so great in practice.

On larger or longer rockets I’ve seen the lowermost rail guide twisted right off while loading the rocket on a rail. I recommend using a button on the bottom. A rail guide works okay on the top because it is in alignment when loading the rocket, but that bottom one can really be torqued while trying to line up the upper one. Buttons aren’t bothered by side to side turning.

 

[LandonC]

No, you may be able to borrow a tower. They’re out there. Go to some launches and walk around and see how others solve the problems you encounter. None are unique. In your build thread you mention Mach 3 and that you’re making your own CF tubes, but wondering what fin shape and nosecone shape to use. Take some time to see what others have done. Questions are great, but going to some of the larger launches and seeing the wide variety that people fly will give you some ideas.

We've progressed enough to know what fin shape and NC profile to use. Currently building prototypes of the NC, but still deciding on what material to use for the fins. Will likely be CNCed aluminum.

I'll look into borrowing a tower. We would like to avoid building one, but if it comes down to it, it won't be an issue.

 

[manixFan]

I have flown both 75 and 98mm CF MD rockets, mostly using a tower. I have also used the aluminum conformal rail guides. As Steve says, they can be torqued off the rocket if things go 'sideways' during loading, which I've seen happen. But I've found that if you properly prep the surface of the CF (degrease and sanded to a water break) as well as the aluminum guide, and use JB Weld, they can form a very strong bond. I've done some destructive testing on a crashed rocket and was surprised at the strength of the guide. And since you know in advance if using a guide, you can take extra care when loading the rocket on the rail to prevent issues.

For what it's worth, in my experience, most of the failures of the conformal guides has been when the nose of the rocket was lifted too high during loading and pulled the bottom guide off the rocket. You can help that by having some thin pieces of wood or similar material that you can put in between the rocket and the rail and then basically keep the rocket against them as you slide it down. That makes it a lot easier to keep the correct spacing. Even a folded rag works well. Also, you should have plenty of helpers loading the rocket and one person who is eye level with the rail to guide everyone. And it sounds dumb, but use simple commands like 'nose towards my side', or 'nose closer to the rail', so there is no confusion as what to do. Talk over the loading procedure with all involved.

As an aside, one downside to a tower I've experienced is 'tower rash', or long scratches along the length of the rocket where it is in contact with the a tower rail. For that reason I use the Jim Jarvis method (at least that's who I learned it from) of coating the CF with a very thin layer of laminating epoxy, or, I'll just leave it naked. Either way, a little wet sanding after the flight and it looks pretty good.

Good luck,


Tony

EDIT: We also have a dedicated 'fin holder' - they keep hold of the fins as it slides down the rail to keep it vertical, and then are responsible for keeping it stable while it's being raised.

 

[Rocketjunkie]

I used a tower. This rocket was 4.5" diameter using a 4-5/16" O3000 motor in 1992. John Cato, me inserting the igniter, and Earl Cagle (Point 39 Productions) with the camera. Behind Earl is Pius Morazumi the launch director (LDRS XI).

 

[LandonC]

I used a tower. This rocket was 4.5" diameter using a 4-5/16" O3000 motor in 1992. John Cato, me inserting the igniter, and Earl Cagle (Point 39 Productions) with the camera. Behind Earl is Pius Morazumi the launch director (LDRS XI).

That's a great picture! Scratch build? How how did it end up going?