An "R"-powered rocket build

[Theory]

Correct that when properly adhered the glass won’t add much strength, but it also won’t hurt.

Agreed that the fabric should be fully wet out before hand as trying to saturate it completely while inside the airframe would be very hard.

And x2 on “cross linking” epoxy. There aren’t many opportunities to really do that in a rocket build, but when you can, take advantage

 

[Rail Dawg]

Chuck: Simply rotating those stringers 90 degrees will vastly increase their stiffness in the desired plane. Simple test is to attempt bending an individual stringer in either "direction".

Completely agree Wallace. I've only got 1 1/2" max width to work with and if I'm going to glass the stringers that knocks down the available width even more. That's why I went ahead and placed them the way I did.

The stringers aren't that heavy and I'm giving consideration to placing 4 more in the position you indicated and then NOT fiberglassing. That would be some heavy-duty reinforcement.

That decision is being mulled right now.

Chuck C.

 

[Rail Dawg]

I'm going out on a limb here, but I'm not sure I see the benefit of the glass on the interior. With the stringers solidly glued in, double wall (main tube and coupler), and centering rings, that thing is already bombproof without adding glass. It's going to be kind of a bastard to get the glass fully wetted out, wrapped around those stringers, and still get the voids out. Not impossible, but difficult. With voids, the glass won't add much strength.

That said, if you still want to go ahead with the glass, I'd recommend pre-wetting it, rolling it on a reasonably-sized dowel, and then unrolling it into place. You'll need some extra hands and another dowel (or possibly other tools, maybe a chip brush 90 degrees to a stick) to get the voids out. I'd also rout the plywood to a 1/2" radius and try to get a 1/2" radius fillet between it and the inside of the coupler. That will make your life much easier in getting the voids out. If you lay down the glass while the fillets are still wet/barely set, the epoxy will cross-link without having to sand more.

Disclaimers: YMMV, this isn't my project, and I've never flown more than a baby L.

It's a tough decision to not fiberglass because I agree it's not an easy task.

The problem is things don't scale as the thrust increases. A 575 lb rocket under 12 G's is putting a lot of weight (3 tons possibly) onto these lower coupler assemblies. So I'm batting around replacing the glass with say 4 additional stringers. Putting in stringers is easy.

Should have a decision soon.

Chuck C.

 

[Rail Dawg]

Well I'm going to add (4) stringers to the coupler assembly I made today because the stringers really need to be oriented correctly.

I had pre-fabricated the stringers without knowing exactly the coupler ID. After receiving the airframes and couplers I realized I needed to go to 1.40" wide instead of 1.50". At 1.40" they will fit perfectly while also allowing for the fiberglass.

One of many mistakes but you take your hits and move on. I'll post a pic of the updated coupler assembly once the new stringers are in. Thing is going to be bulletproof.

Chuck C.

 

[Theory]

Can’t say that is a mistake at all, more a “real time design modification.”

 

[Rail Dawg]

Can’t say that is a mistake at all, more a “real time design modification.”

Yep that’s exactly how I look at it too. You get an idea of how to build but mistakes and corrections will be made.

Thanks for the positive support!

Chuck C.

 

[Paulb06]

I didn't think of rounding the corners or doing fillets... that's good advice.

Let me give that some thought. Doing it the way you mentioned would definitely increase strength. But it's 13 oz twill carbon and just laying it in as flat as I can without the rounding and fillets should make it plenty strong.

Again though good advice.

Chuck C.

Your router with a 1/4" roundover bit will make short work of rounding the stringers

 

[Wallace]

Chuck: Maybe take a step back and work some of these things out beforehand. It appears as though you have some quality materials and are "collecting" tools, just try to not rush the project. I can't tell you how many times I've done things in haste that ended up being a thousandfold more difficult/ time consuming and costly to fix.

 

[boatgeek]

It's a tough decision to not fiberglass because I agree it's not an easy task.

The problem is things don't scale as the thrust increases. A 575 lb rocket under 12 G's is putting a lot of weight (3 tons possibly) onto these lower coupler assemblies. So I'm batting around replacing the glass with say 4 additional stringers. Putting in stringers is easy.

Should have a decision soon.

Chuck C.

Well I'm going to add (4) stringers to the coupler assembly I made today because the stringers really need to be oriented correctly.

I had pre-fabricated the stringers without knowing exactly the coupler ID. After receiving the airframes and couplers I realized I needed to go to 1.40" wide instead of 1.50". At 1.40" they will fit perfectly while also allowing for the fiberglass.

One of many mistakes but you take your hits and move on. I'll post a pic of the updated coupler assembly once the new stringers are in. Thing is going to be bulletproof.

Chuck C.

I think that's a good decision. You could probably use the part you've already built up higher in the airframe where it's not under so much load. I'm running off the back of an envelope, but 8 stringers alone are plenty to take the load you're talking about here. 575 lbs * 12 G = 6900 lbs. 8 * 1.4" * 0.75" thickness = 8.4 square inches of compression area, for a total calculated compressive stress of 821 psi even before you account for a fair amount of the weight being in the motor/casing and therefore not carried by the framework. Breaking strength of Douglas fir plywood is around 9,000 psi, so you have a factor of safety around 10.

I'll grant that's theory, and in theory there's no difference between theory and practice. However, the big concern I would have is that the stringers would buckle. Since these are very well supported by glue all down their lengths and you have some serious additional strength in the main body tube and the couplers, this engineer wouldn't worry.

Again, your rocket and your decisions. It's definitely not a bad idea to add some stringers in the bottom sections, but I also wouldn't throw away the section you have glued already. I also agree that it's a good idea to look at this section and see where you want to make changes before you get into the full swing of production on the rest of the rocket.

 

[Wallace]

Chuck; just a thought on my end, but have you considered recessing/adding a lip to your centering rings so they at least partially sit inside the coupler(s)? Additionally, if you were to add dadoes on the faces where the stringers fit it would make assembly and potential accuracy much easier and or "better". Wouldn't suggest it were you only doing a couple (pun intended), but seeing as you have a ton (pun also intended) to fabricate you'd probably find it time well spent to create a jig and make all the centering rings and stringers to close tolerance all at once.

 

[Rail Dawg]

I think that's a good decision. You could probably use the part you've already built up higher in the airframe where it's not under so much load. I'm running off the back of an envelope, but 8 stringers alone are plenty to take the load you're talking about here. 575 lbs * 12 G = 6900 lbs. 8 * 1.4" * 0.75" thickness = 8.4 square inches of compression area, for a total calculated compressive stress of 821 psi even before you account for a fair amount of the weight being in the motor/casing and therefore not carried by the framework. Breaking strength of Douglas fir plywood is around 9,000 psi, so you have a factor of safety around 10.

I'll grant that's theory, and in theory there's no difference between theory and practice. However, the big concern I would have is that the stringers would buckle. Since these are very well supported by glue all down their lengths and you have some serious additional strength in the main body tube and the couplers, this engineer wouldn't worry.

Again, your rocket and your decisions. It's definitely not a bad idea to add some stringers in the bottom sections, but I also wouldn't throw away the section you have glued already. I also agree that it's a good idea to look at this section and see where you want to make changes before you get into the full swing of production on the rest of the rocket.

Hey thanks for running the engineering numbers. It’s always good to have verification that there’s a solid safety margin.

I had already CNC’ed the CR’s and stringers. The glitch came from the fact that the tubes and couplers were not the advertised inner and outer diameters. So I’ve had to work on the CR’s to get them correct. Not a big deal with the belt sander.

By shaving .10” off the stringers that are already made they’ll fit perfectly now into the coupler assembly.

I expect to make mistakes but will always correct them without sacrificing the quality control. Am not going to shortcut anything.

Thanks again for looking at this with an engineering eye. That’s awesome.

Chuck C.

 

[Wallace]

Carry on then...

 

[Rail Dawg]

Chuck: Maybe take a step back and work some of these things out beforehand. It appears as though you have some quality materials and are "collecting" tools, just try to not rush the project. I can't tell you how many times I've done things in haste that ended up being a thousandfold more difficult/ time consuming and costly to fix.

Yep there’s no rush and you’re right it’s better to take it slow so costly mistakes aren’t made.

The dadoes idea is a good one Wallace. Fortunately the stringers fit really nice into the assembly already. One thing great about CNC is the tolerances are tight when assembling the pieces.

This is turning into a really fun build. The next rocket will be a 16” G-12 and will be built with the same design as this 12” rocket. As I get smarter/wiser I expect things to go smoother. The next coupler assembly is 30” long and it should put together nicely.

Thanks for the inputs and suggestions. Keep them coming!

Chuck C.

 

[Rail Dawg]

Your router with a 1/4" roundover bit will make short work of rounding the stringers

That’s a good idea Paul. Am just now learning the capabilities of my table router. It sure is a great tool for building rockets. Am picking up a 1/4” roundover bit as you suggested.

Thanks!

Chuck C.

 

[Ez2cDave]

Chuck,

It looks like things are coming together, nicely.

My area of emphasis, at this point in time, is turning to Recovery. That, in itself, may likely be the most complicated phase of the project.

Have you given any more thought to contacting a local sky-diving business about doing an "all-up" drop test of the electronics and parachutes
attached to a weight, equal to the recovery weight of the rocket ? It would be cool if they could free-fall along with it, at a safe distance,
of course, shooting Go-Pro video of the deployment.

Is there any update on when a static test of the motor will take place ? The sooner we get "real world numbers", establishing an accurate picture
of actual performance, will be very helpful.

I am looking forward to seeing the Fin Can, too !

Dave F.

 

[Rail Dawg]

Thanks Dave.

Recovery is definitely the 800 lb gorilla. In order to maintain some degree of order I’m concentrating on the bottom of the rocket and building up from there.

Parts like the fin can placement, motor fit and rail lug. Those items of course alongside the coupler assemblies’ work.

The motors (2 Q’s and an R) will be built starting the middle of next month. One of the Q’s will be our test motor which I suspect will be tested in May.

Thanks for staying on top of things.

Chuck C.

 

[Ez2cDave]

Thanks Dave.

Recovery is definitely the 800 lb gorilla. In order to maintain some degree of order I’m concentrating on the bottom of the rocket and building up from there.

Parts like the fin can placement, motor fit and rail lug. Those items of course alongside the coupler assemblies’ work.

The motors (2 Q’s and an R) will be built starting the middle of next month. One of the Q’s will be our test motor which I suspect will be tested in May.

Thanks for staying on top of things.

Chuck C.

Chuck,

Another issue is the "heat cycling" that will occur when the temperature of the airframe, prior to launch, is raised rapidly due to aerodynamic friction and then rapidly cooled by the extreme cold at 40,000 - 45,000 ft AGL.

It will probably not cause an immediate failure during the initial flight, BUT the effect will, most likely, be cumulative on subsequent flights, possibly compromising structural integrity, at some point. Contacting the companies that manufacture the various adhesives and resins being used might be advisable.

Dave F.

 

[boatgeek]

I'm 100% not the person to do the analysis, but it would be interesting to know how much the surface temperature and fin core temperature change during flight. Sure, it's awfully hot at Mach 2.X, but not for very long. Likewise, it's cold at apogee at 40K feet, but rapidly warms as the rocket descends under drogue.

 

[Steve Shannon]

Due to the short time spent heating and the thin atmosphere at 40k, I doubt whether heating or cooling will affect the adhesives at all.

 

[pondman]

Saw no issues from heating last year at BALLS with my flight to 41K and Mach 2.5.

 

[Theory]

Saw no issues from heating last year at BALLS with my flight to 41K and Mach 2.5.

One of the best data points yet!

True Chuck wants to fly his rocket again, but I doubt (and no offense here) that it will see enough action to experience a failure due to thermo-cycling epoxy

 

[Ez2cDave]

Saw no issues from heating last year at BALLS with my flight to 41K and Mach 2.5.

What adhesives, resins, and materials were used and in what locations on the rocket ?

Dave F.

 

[blackjack2564]

64,000 M-3.6 Boat glue...no issues. Carbon fiber fin can glued on motor case.
Not mine, but I was there. Only a few pinholes in paint. Auto heat cured 2-part clear. Cured in oven for 6 hrs at 165.
Stainless cuffs on leading edge.

 

[darkhelmet]

Heating is not a concern for a Mach 2 heavy rocket. It will spend only a few seconds at >M1. The skin temperature will barely increase then cool quickly. Certainly, it will get nowhere near the epoxy's glass transition temperature. The fin leading edges are aluminum, an excellent heat sink with significant mass; again barely measurable increase.

Cooling during coast and drogue recovery is far more significant due to the time spent at <-30 degrees F. After a minute, and with airflow, the airframe surface will be near freezing and the aluminum possibly lower. Most epoxies are good to -60F for long term exposure. No problem for this duration.

The only potential temperature issue is at the couplers. The airframe will cool faster than the coupler inside. Depending on the temperature coefficient for G12 and the temperature gradient, the separation points will be tighter than at ambient ground temperature.

 

[Ez2cDave]

Heating is not a concern for a Mach 2 heavy rocket. It will spend only a few seconds at >M1. The skin temperature will barely increase then cool quickly. Certainly, it will get nowhere near the epoxy's glass transition temperature. The fin leading edges are aluminum, an excellent heat sink with significant mass; again barely measurable increase.

Cooling during coast and drogue recovery is far more significant due to the time spent at <-30 degrees F. After a minute, and with airflow, the airframe surface will be near freezing and the aluminum possibly lower. Most epoxies are good to -60F for long term exposure. No problem for this duration.

The only potential temperature issue is at the couplers. The airframe will cool faster than the coupler inside. Depending on the temperature coefficient for G12 and the temperature gradient, the separation points will be tighter than at ambient ground temperature.

Darkhelmet,

On Chuck's rocket, his entire Fin Can is Aluminum, with a "Nike-style" Trapezoidal fin planform . . .( the pic above with the thermal-cured Carbon Fiber Fin Can & S.S. Fin Cuffs is a different rocket ).

What you said about the separation points being tighter is a critical piece of data for successful Recovery Deployment.

Dave F.

 

[Rail Dawg]

For the record I’m not concerned about heating. Lots of rockets fly all the time at Mach 2 and above. The epoxy seems to do just fine.

Chuck C.

 

[blackjack2564]

That was my point.

 

[pondman]

What adhesives, resins, and materials were used and in what locations on the rocket ?

Dave F.



View attachment 377763

Wildman Competitor V; West Systems throughout; automotive paint.

 

[Ez2cDave]

Wildman Competitor V; West Systems throughout; automotive paint.

Gary,

Thanks for the update . . . That information is reassuring.

Dave F.

 

[Rail Dawg]

Wildman Competitor V; West Systems throughout; automotive paint.

Gary looking forward to flying this rocket at BALLS this year! Am completing the C3RC application and when it's done I'd like to send you a copy. As the Prefect for the really big launch prefecture (and being a TRA Director) you have a lot of responsibilities!

This is going to be fun.

Chuck C.