Launch tower diametral clearance?

[SolarYellow]

Question for experienced launch tower users.

I'm looking at building* a range of low-cost, non-adjustable towers for LPR rockets. BT-20 and BT-50 will be first, but I intend to cover a range from BT-5 to LT-125, including a lot of oddball sizes in between, and may go as large as BT-60 with the system. Yes, I really hate launch lugs.

How much diametral/radial clearance around the airframe are most people using? Are we talking 0.010 or more like 0.040 inch? A few wraps of masking tape? How many layers? Does this scale by airframe diameter?

*To be clear, this is for my own use, not contemplating a commercial product. In fact, it's quite the opposite. I'm looking at <<$5 of materials per size with quick, easy construction.

 

[robopup]

Obvious bare minimum for me is that the rocket cannot fall out of any of the gaps between the rails. Depending on how much effort I've put into the project, I'll adjust further. The perfect fit for me is one in which the rocket freely moves the length of the tower and has a small (but not too much) amount of rattle everywhere. I've never actually measured this (one of those know it when you see it types of things for me), but I'd guess it's something like 1/16 in between each rail and the rocket for a 54mm airframe. This scales some with diameter but not directly.

I definitely would oversize by more than .010 in - rails usually aren't that straight over 4+ feet.

 

[James Duffy]

I build a 2mm clearance (1mm on each side) into the towers I use for FAI rockets, ranging from 25mm to 100mm.

In other words, for a 40mm model the diameter of the imaginary circle is 42mm. The photo below shows a 71mm tower designed to accommodate a model with a 69mm OD. The uprights are .505" OD fiberglass kite spars.

 

[SolarYellow]

I build a 2mm clearance (1mm on each side) into the towers I use for FAI rockets, ranging from 25mm to 100mm.

In other words, for a 40mm model the diameter of the imaginary circle is 42mm. The photo below shows a 71mm tower designed to accommodate a model with a 69mm OD. The uprights are .505" OD fiberglass kite spars.

Exactly the kind of input I was hoping for. Thanks!

What I'm doing will be really close to this photo of Vern Richardson's setup:
https://www.nar.org/wp-content/uplo...ision.-all-hand-rolled-paper-copy-720x960.jpg
Anyone here have experience on small setups like that? I'm thinking maybe reduce the clearance to 1mm total for BT-5 to BT-20 ish rockets to keep them from bouncing around? Since I'm not supporting the upper ends, I don't need to worry as much about binding due to the rods not being perfectly straight.

On the other hand, any free clearance is scope for the rocket to yaw and/or bounce back and forth against the rods, which could induce vibration in the rods with potential for one to come back and kick the tail of the rocket just before it clears. Thinking I'll start with 1/4-inch rods, three feet long as a default setting for things smaller than BT-50, probably jump to 5/16 for BT-50 up to LT-125. I'm thinking that should be rigid enough to avoid rod whip slapping the rocket around before it leaves.

The setup will be cheap, quick and easy enough to make that I can always chop or build another one with shorter rods to get the length only as long as it needs to be per the sim & motor combination. I can also use the clothes pin trick to elevate the starting point, although that increases the flexibility of the rods at that point and increases the potential for rod whip. Certainly, I'll have the ability to experiment and see what works optimally. I'm just trying to save a little time and see if someone who has already done that can tell me, "I tried a bunch of different ways and ended up with this..."

 

[mh9162013]

Exactly the kind of input I was hoping for. Thanks!

What I'm doing will be really close to this photo of Vern Richardson's setup:
https://www.nar.org/wp-content/uplo...ision.-all-hand-rolled-paper-copy-720x960.jpg
Anyone here have experience on small setups like that? I'm thinking maybe reduce the clearance to 1mm total for BT-5 to BT-20 ish rockets to keep them from bouncing around?

Thinking I'll start with 1/4-inch rods, three feet long as a default setting for things smaller than BT-50, probably jump to 5/16 for BT-50 and up. I'm thinking that should be rigid enough to avoid rod whip slapping the rocket around before it leaves.

The setup will be cheap, quick and easy enough to make that I can always chop or build another one with shorter rods to get the length only as long as it needs to be per the sim & motor combination. Certainly, I'll have the ability to experiment and see what works optimally. I'm just trying to save a little time and see if someone who has already done that can tell me, "I tried a bunch of different ways and ended up with this..."

I have nothing to add here, but I am always looking for a different way to launch LPRs without using permanently attached launch lugs. So I'll be on the lookout for your final results.

 

[BEC]

I have one of the lovely adjustable Apogee towers and I typically adjust it about as James described. I'm mainly looking for a nice smooth slide down the full length when loading the tower. I haven't actually measured clearances.

Vern and his young proteges were all using towers like that in your picture at NARAM-64 (for which Vern was the contest director).

 

[shockie]

For my FAI piston tower, the ID of my tower is 41mm, with the body tube 40mm OD.

It's 3D printed brackets with 13mm 1m carbon fiber rods.

It has rail buttons molded into the brackets for attachment to either a Rockwell or Triton jawstand.

 

[Buckeye]

For my FAI piston tower, the ID of my tower is 41mm, with the body tube 40mm OD.

It's 3D printed brackets with 13mm 1m carbon fiber rods.

It has rail buttons molded into the brackets for attachment to either a Rockwell or Triton jawstand.

Nice! How are the carbon fiber rods attached to the top and middle brackets?

 

[shockie]

The CF rods plug into the brackets. These are 3D printed from PETG-CF.

I use slide-in t-nuts(3204) from 8020.net at the brackets(top,bottom and middle) to hold everything in place.

Creates a very rigid tower.