38mm Space Shot tower

[Adrian A]

This is the build thread for the tower that is designed to launch this rocket. It's a 10 foot long, ~10 lb 3-stage rocket that could go past the boundary of space if it gets a straight boost.

I have made 2 previous towers, both for 3-finned rockets. The first one used three 6 foot 1.5" Al tubes and had mostly-wooden base and top:



My next tower is a big improvement. The build thread for it is here. Made of aluminum and fiberglass, this tower is easy to transport, and I can carry it across a field and set it up by myself in about 15 minutes. The internal clamps to join the two sets of uprights together work great. It's 12 feet tall, and I have gotten a lot of good straight boosts out of it.



But 12 feet is about the limit for this style of construction. It's not too wiggly when when the straps are cranked down, but there is some flexibility. The main uprights are 1" x 2" tubes, and they cover a lot of the rocket with three uprights. For a 4-finned rocket, there wouldn't be much exposed tube, and GPS lock would be very difficult. If I made the tubes larger for better rigidity with a longer height, that problem would get worse. I think a space shot tower needs a separate outer frame and some smaller guides that go against the rocket.

Here are the requirements and goals I'm working toward for the new tower

Requirements

• Accommodate 4 fins
• Straight and very rigid
• At least 12 feet tall
• Fit into my hatchback
• Assemble in 2 hours or less
• Ability to fine-tune fit
• Accommodate a 38mm minimum diameter rocket

Goals

• 20'-30' tall
• Keep GPS lock when the rocket is in the tower
• Be able to get GPS lock from a cold start while loaded in the tower
• No-frustration assembly in 30 minutes or less
• Less than $1000 materials
• Load rocket from the base
• Insta360 camera mounted high
• Long-range (1 mile across playa) remote camera activation
• Long-range (1 mile across playa) live wind direction and speed telemetry
• No excess weight, to make it easier to transport and assemble
• Accommodate a 54mm to 3" minimum-diameter rocket

I have been impressed with the rigidity of truss-based towers on the playa, but generally (with some exceptions) these are used for rails. They are also big and heavy, made of steel, and generally require their own trailers. This truss seems like it could work well for sending the rocket up the middle:

https://www.amazon.com/dp/B08D7MPNW5?psc=1&ref=product_details

4 sections of this truss would easily fit into my hatchback along with the rest of my stuff. Since it's not a lot larger in diameter than the rocket, the connecting hardware to the inner guide rails would be small and stiff. My first design plan had these truss sections and also 0.5" x 1" aluminum tube guide rails that would be put together similarly to my current 12' tower. But this was looking like about $1500 for 20 feet.

I got some advice on the discord server, and learned about low-priced towers that use these tube connecting nuts with conduit.

https://www.mcmaster.com/94290A510/

I considered making a tower wedding-cake style with 5 or 6 foot sections of inner and outer steel tubes clamped around 1' x 1' fiberglass rings. But I'm not convinced that if it goes up to 20 or more feet, that this would be rigid enough without having any diagonal cross bracing to take the shear. And I just like the idea of that 8" x 8" aluminum truss, so I bought two sections to take a closer look at. They are returnable if I don't like them. But the discord advice did help save money because my new plan is to use 5' conduit, probably trade size 3/4", for the inner guides. This should bring the price for 20' down below $1000. I'm thinking through how to connect the inner guides to the the outer truss, and I'm leaning toward designing some 3D printed clamping fittings that would work with 1010 rail every 5 feet and with some short metal tubes to connect to the diagonal bracing in between.

 

[Adrian A]

The first of two 8" truss pieces arrived today.


On the plus side:
The section feels stiff and strong, and reasonably light. I wondered if the diagonal truss bracing tubes would be stiff enough to mount the inner rails to, and they easily are. If I had to climb up the tower using the diagonal braces I could do that too.

On the downside:
I'm concerned about the flatness of the ends, because that will be critical for ensuring the tower is straight. The ends are clearly made from 4 aluminum pieces welded together rather than cut from one piece. I will need to wait for the 2nd section and bolt them together before I can tell for certain. If a base of a section is more obviously flawed I can probably tweak it with a belt sander, but I wish I had something like the benchtop disc sander I use to square up my tubes, but big enough to grind the whole end flat and square.



The corner-to-corner measurement of the opening on each end is right at 6.5". This is easily enough for a 38mm 4-fin rocket but it's questionable for a 54mm rocket. If I switch over to doing a 54mm-38mm 2-stage I think it would be enough because the booster fin span wouldn't need to be sized for high supersonic flights.

 

[pugachu]

The corner-to-corner measurement of the opening on each end is right at 6.5". This is easily enough for a 38mm 4-fin rocket but it's questionable for a 54mm rocket. If I switch over to doing a 54mm-38mm 2-stage I think it would be enough because the booster fin span wouldn't need to be sized for high supersonic flights.

Adrian,

For your potential 54mm booster, would you be considering a composite case design, or would it be a standard metal case with a min diameter CF airframe?

You've inspired me to work my way up to a composite case rocket for the Loki 54/4000 load. I've already gotten the mandrel for rolling the case/airframe, but there will be several steps before I get there.

 

[wsume99]

The ends of the vertical tubes are open. Could you put another tube inside to align the sections? Like a coupler fits inside a body tube. You could still join the sections using the bolt holes through the plates but the vertical alignment would be done by the couplers.

 

[Adrian A]

Adrian,

For your potential 54mm booster, would you be considering a composite case design, or would it be a standard metal case with a min diameter CF airframe?

You've inspired me to work my way up to a composite case rocket for the Loki 54/4000 load. I've already gotten the mandrel for rolling the case/airframe, but there will be several steps before I get there.

Good question. For now I'm focused on the 38mm 3-stage potential and part of that is because premade metric CF tubes are easy and a great fit for the Loki 38mm liner. I haven't really looked into the 54mm version of that, but I suspect that the diameter is larger than what is currently commonly made for drones, unlike CF tubes for 38mm reloads. I have made my own tubes before but it's not my favorite thing.

 

[Adrian A]

The ends of the vertical tubes are open. Could you put another tube inside to align the sections? Like a coupler fits inside a body tube. You could still join the sections using the bolt holes through the plates but the vertical alignment would be done by the couplers.

The vertical no longer have a precise ID because of the welding. And I think I need to join the sections with preloading to really make the tower rigid enough.

I'm starting to have ideas about making a fixture to use with my belt sander to get the ends straighter and flatter. Or maybe more of a larger belt sander would help.

Or this looks promising:

https://www.harborfreight.com/12-in...-WrnS1KmWZJ6kawhkOGo3iM3oogixbSBoC9JoQAvD_BwE

 

[Adrian A]

The tower is 10.5" on the diagonal, so if I could figure out a way to take the table off of that 12" disc sander and mount it with a big jig that centers a tower section square on the disc, then I could square up the ends at a reasonable cost.

 

[wsume99]

How much deviation from vertical are you looking to achieve? I'm asking because if you're a quarter degree off square that will put the opposite end of the 60" section 1/4" off vertical. One degree off would be an inch off. What kind of precision are you looking to achieve vs what kind of precision can you actually achieve attempting to hold a 5 ft long piece square to a sanding disc. You may be better off 3D printing some shim plates to fit between the segments.

 

[pugachu]

The tower is 10.5" on the diagonal, so if I could figure out a way to take the table off of that 12" disc sander and mount it with a big jig that centers a tower section square on the disc, then I could square up the ends at a reasonable cost.

Whatever solution that you come up with, make sure to take safety first. Power tools are generally safe when used as they are intended, while giving them the respect they deserve. When modifying how they are meant to be used, things can get ugly very quickly. I happen to like your products, and you seem like a good guy, I'd hate for you to get hurt.

 

[mrwalsh85]

Bolt together on the flanges. Utilize jack screws on the flanges to support getting the sections arrow straight as best as possible, pushing the flanges apart as needed to get perfect alignment.

 

[Adrian A]

Interesting to hear different perspectives and I’m glad to talk it through.

The 8” truss sections are very stiff at 5 feet long but 20 feet is a long way. The sections are rated for a 200 lb lateral load with 3” of displacement if 20’ worth of sections are bolted together. If I weren’t so into small rockets and keeping this portable snd saving some money, I would go for 12” wide truss for this height of tower.

For maximizing the stiffness I feel that it’s important to bolt the ends together with good preloading and with the whole surface in contact with each other, particularly the 1.5” tubes that are on the corners. So making sure the ends are flat is a high priority.

Making the surfaces square is the next priority, because it would be very convenient to have a consistent distance between the inner guide rails and the outer structure. But if I can’t achieve straightness in the outer structure, I can still adjust the guide rails that go against the rocket to be straight. In that case I would want to fine tune it at home and then carefully keep track of which corners get assembled together on the playa.

If I use a long table with an alignment fence and the sander bolted down, then the truss would be safely controlled. The 12” disc sander should be able to make the ends nice and flat, and how square they turn out would just depend on how squarely I could bolt down the sander relative to the spacers on the table and the fence.

The spacing between the inner guides needs to be controlled to around 1/8” tolerance over the whole length of the tower, and ideally their straightness would be about that good too. But the tower could flex more than that in the middle if it’s windy or the rocket is bouncing around much on its way up. Realistically, even with the top strapped down with beefy guy lines to big stakes, I might expect around an inch of motion at the top if there is a significant side load, which works out to about a quarter of a degree. That wouldn’t be a problem. But if the 9 foot long rocket has to deal with a kink in the guide rails inside the tower, that would be a bigger problem.

 

[mrwalsh85]

Forgive me, I do not know the design of your rocket. However, the 8x8 truss seems like it would be awfully small for a 54-3" diameter rocket with whatever fins you choose, given the fin span vs available area inside the trusses. Has that consideration been given thus far?

 

[SolarYellow]

If you're thinking of using those press-in tube nuts, do a search for "star nut installation tool." They are commonly sold for assembling bicycle forks. I have one I got from Bike Nashbar nearly 20 years ago. They make it easy to get the nut installed perfectly straight and evenly centered, a task that is nearly impossible without the proper tool. You'll need to machine or 3D print a sleeve to adapt the ID of the installer tool down to smaller tube sizes, and maybe make a different mandrel with the different thread spec for the nuts you use. But starting with a COTS tool will certainly be cheaper and easier than starting with bar stock.

Squaring the ends of the trusses might be best if you stop thinking of it as a DIY task. There might be a machine shop in your area with a horizontal mill that could get them perfect within 0.001 inch or so. It would increase the budget, but I would think it would be a simple task for a shop that had the right tool for it and would be well worth the expense.

 

[Adrian A]

Forgive me, I do not know the design of your rocket. However, the 8x8 truss seems like it would be awfully small for a 54-3" diameter rocket with whatever fins you choose, given the fin span vs available area inside the trusses. Has that consideration been given thus far?

It's a good question. Flying my 38mm space shot is the requirement. Fitting a possible future 54mm rocket is a goal, and fitting a 3" rocket would be nice to have but not necessary. The 6.5" span across the corners is fine for 38mm. For 54mm, it's probably still adequate for a 54mm booster in a 54mm to 38mm rocket. Unlike sustainer fins, I could design booster fins to fit with little/no performance penalty. Fitting a 3" rocket in there is out of the question, but I don't see a need for a 3" rocket for my foreseeable future, and I'd rather keep this as portable as it is.

If you're thinking of using those press-in tube nuts, do a search for "star nut installation tool." They are commonly sold for assembling bicycle forks. I have one I got from Bike Nashbar nearly 20 years ago. They make it easy to get the nut installed perfectly straight and evenly centered, a task that is nearly impossible without the proper tool. You'll need to machine or 3D print a sleeve to adapt the ID of the installer tool down to smaller tube sizes, and maybe make a different mandrel with the different thread spec for the nuts you use. But starting with a COTS tool will certainly be cheaper and easier than starting with bar stock.

Great tip. I am planning to use those nuts to join the conduit together for the inner guides. I see the tools go down to 1" or maybe 7/8" bike steerer tube diameters. Depending on how that compares with the conduit trade sizes, that may be small enough for my purposes already. I bet I could also keep them centered and aligned if I got 6 foot all-thread to pull them into place from the other end.

Squaring the ends of the trusses might be best if you stop thinking of it as a DIY task. There might be a machine shop in your area with a horizontal mill that could get them perfect within 0.001 inch or so. It would increase the budget, but I would think it would be a simple task for a shop that had the right tool for it and would be well worth the expense.

This is good advice also. I have a couple of friends with shops who would at least be able to recommend someone.

My second truss section came in last night, and today I did a test fit. The faces are not very flat but were surprisingly square once I bolted them together. I couldn't detect any bend when looked down the corner tubes. I also supported it at each end and bounced on it in the middle, and the flex was almost undetectable. It seemed like it was stiffer than the spec value of 0.67" deflection for 220 lbs in the middle of a 10' span. It's possible I'm over-thinking this.

 

[mrwalsh85]

It's a good question. Flying my 38mm space shot is the requirement. Fitting a possible future 54mm rocket is a goal, and fitting a 3" rocket would be nice to have but not necessary. The 6.5" span across the corners is fine for 38mm. For 54mm, it's probably still adequate for a 54mm booster in a 54mm to 38mm rocket. Unlike sustainer fins, I could design booster fins to fit with little/no performance penalty. Fitting a 3" rocket in there is out of the question, but I don't see a need for a 3" rocket for my foreseeable future, and I'd rather keep this as portable as it is.

Ah, thanks for the clarification.

In any case, this will be cool to see unfold.

Best of luck!!!