Kit recommendations for 38mm Min Diameter

[AeroAggie]

I thought the K required that one ask for a longer sustainer tube to accommodate that motor?

The Loki K627 fits in the 38/1200 case, which fits into the Go Devil 38 just fine.

You might be thinking of the 54/2800 case where the K350 uses a standard forward bulkhead but the L1040 uses a longer bulkhead on the same case to get extended tracking smoke.

 

[plugger]

Correct about this.
When you factor in BlackHawk coming with Kevlar shock cords, aluminum BP's, Chute, and high temp Epoxy, alignment guides that ARE accurate, and thicker fins 3/32. Makes the extra few bucks worth it.

I also have one, I've beat the hell out of. By the way, fins are same as Mongoose. For all intents, it's a Mongoose clone, made of fiberglass, so no worries about RF transparency.

I'd come in on the Blackhawk 38 side as well. I've owned both rockets (well, my Mongoose wasn't a Madcow variant as Curtis didn't sell to them at the time?) and found out some interesting info when comparing the airframes earlier in the year. TLDR; the thin walled FWFG airframe the Blackhawk leverages is actually lighter than the FWCF airframe that comes with the Mongoose. The difference came out to be the difference in thickness of the airframe, the Mongoose was nearly 1mm larger than the Blackhawk in OD. Assuming the CF airframe that comes with the Mongoose kit hasn't been slimmed down you'd be better off with the BH38 kit, at least from a performance perspective.

More info here.

https://forum.ausrocketry.com/viewtopic.php?f=10&t=5886

 

[dixontj93060]

I'd come in on the Blackhawk 38 side as well. I've owned both rockets (well, my Mongoose wasn't a Madcow variant as Curtis didn't sell to them at the time?) and found out some interesting info when comparing the airframes earlier in the year. TLDR; the thin walled FWFG airframe the Blackhawk leverages is actually lighter than the FWCF airframe that comes with the Mongoose. The difference came out to be the difference in thickness of the airframe, the Mongoose was nearly 1mm larger than the Blackhawk in OD. Assuming the CF airframe that comes with the Mongoose kit hasn't been slimmed down you'd be better off with the BH38 kit, at least from a performance perspective.

More info here.

https://forum.ausrocketry.com/viewtopic.php?f=10&t=5886

I owned and built a Blackhawk 38mm. It wasn't less than 1lb dry weight (like the Mongoose 38mm). Also did you see the stats published by Madcow? One hundred (100) 38mm Mongoose kits sold in 7 minutes?! OMG! NICE!! And guess what, if you are maximizing velocity the Mongoose wins. If you are wanting to win an altitude contest, then just add ballast to the NC to optimize any given motor selection.

 

[plugger]

I owned and built a Blackhawk 38mm. It wasn't less than 1lb dry weight (like the Mongoose 38mm). Also did you see the stats published by Madcow? One hundred (100) 38mm Mongoose kits sold in 7 minutes?! OMG! NICE!!

I did not see the stats published by Madcow. Got a link?

And guess what, if you are maximizing velocity the Mongoose wins.

The lighter vehicle will win. My data contradicts you assertion, at least from a airframe perspective. BH38 = lighter airframe = higher max V. And that doesn't even take into account the fact that a larger OD airframe will be slower when compared to a smaller OD airframe.

 

[dixontj93060]

I did not see the stats published by Madcow. Got a link?



The lighter vehicle will win. My data contradicts you assertion, at least from a airframe perspective. BH38 = lighter airframe = higher max V. And that doesn't even take into account the fact that a larger OD airframe will be slower when compared to a smaller OD airframe.

The weight of my BH38 is greater than 1lb. The Mongoose 38 from Madcow is advertised at <1lb, thus velocity is greater for Mongoose 38.

 

[dixontj93060]

The weight of my BH38 is greater than 1lb. The Mongoose 38 from Madcow is advertised at <1lb, thus velocity is greater for Mongoose 38.

Also a comment on diameter/weight stats...

Mongoose CF: Outside diameter 1.645 inches and Weight: 0.26 oz / foot

Thin wall FG: Outside diameter 1.600 inches and Weight 2.570 oz / foot

10x savings on weight swamps 2.7% differential on diameter.

 

[dixontj93060]

I did not see the stats published by Madcow. Got a link?

Madcow customer email:

"The second Door Buster deal will start Friday, November 10th, at 7 AM Pacific Time, 10 AM Eastern Time. The deal will continue until we run out of stock or until the Black Friday sale ends - whichever comes first. When we run out of stock the deal is over. You need to get your order in quick. Last week we sold out of 100 of the Mongoose 38 kits in 7 minutes."

 

[plugger]

The weight of my BH38 is greater than 1lb. The Mongoose 38 from Madcow is advertised at <1lb, thus velocity is greater for Mongoose 38.

:no::no::no:

Madcow lists the 1.1" and 1.6" Carbon Fibre Mongoose kits as both "Built Weight Approximately 1 lb". Do you honestly believe the 29mm and 38mm Mongoose are identical in weight? Furthermore approximately =/= <1lb.

Just because you read something on the internet doesn't make it true. I prefer actual data, not marketing fluff. All of my points still stand and are based on actual data I generated.

 

[plugger]

Mongoose CF: Outside diameter 1.645 inches and Weight: 0.26 oz / foot

7ish grams / foot?!?!?! :surprised: :surprised: :surprised: :lol: :lol: :lol:

 

[plugger]

As you can see from the link I shared earlier i roll my own CF airframes with 2x2 twill 200gsm CF. I use 3 layers + epoxy to roll airframes and my 38mm airframes come in at roughly 50 grams / foot, or roughly 1.75 oz / foot. Honestly, I believe your decimal point is in the wrong place because what you're describing doesn't exist.

 

[dixontj93060]

7ish grams / foot?!?!?! :surprised: :surprised: :surprised: :lol: :lol: :lol:

OK, yes, maybe the specs are wrong. I should have ordered one to compare.

I will say that I am unimpressed with the Profusion FG used in the Blackhawk design (and some Madcow kits). Outside of wall thickness, there are no real gains in weight and performance. I owned a 29mm CF Mongoose equivalent and have both built/flown a 2.1" CF and own a 3" and 4" full CF minimum diameter builds they are less than half the weight of thin wall of the equivalent.

I cannot believe that thin wall FG is less than a CF tube of the same inner diameter. I have the full spec sheet from Curtis (unfortunately not on this computer). I will post later this week and compare the real oz/ft data.

 

[plugger]

Mongoose CF: Outside diameter 1.645 inches

Thin wall FG: Outside diameter 1.600 inches

Thanks for those data points dixontj93060, they're quite comparable to my measurements. Allow me to convert them to metric for easier comparison with my figures.

Mongoose 38
1.645 inches = 41.783 mm

Blackhawk 38
1.6 inches = 40.64 mm

My measurements are as follows.

38mm fwcf (Mongoose 38)
41.7mm OD

38mm fwfg (Blackhawk 38)
40.5mm OD

My measurements are roughly within 0.1mm of what you've reported.

I'm calling it now, if the Mongoose 38 is still utilising this "thick walled" fwcf for its airframe it would be the slower kit when compared to a Blackhawk 38 due to higher overall vehicle weight. And my comment about OD drag still stands.

 

[plugger]

I cannot believe that thin wall FG is less than a CF tube of the same inner diameter.

I was once like you, but the numbers don't lie. The old adage of "CF is faster" does not hold up in this instance. I never even considered it myself until I ran the numbers. The 0.7mm extra thickness of the FWCF completely negates the lighter CF material.

 

[dixontj93060]

I was once like you, but the numbers don't lie. The old adage of "CF is faster" does not hold up in this instance. I never even considered it myself until I ran the numbers. The 0.7mm extra thickness of the FWCF completely negates the lighter CF material.

Material density is well-known:

Material Density, Lb/in3
Fiberglass 0.072
Carbon Fiber 0.058

So for any effective weight gain fiberglass tubes would have to be 16% less radius, 33% less net diameter (24.1%=pi*r-squared).

Of course this whole discussion is ridiculous as the composite CF material is orders of magnitude stronger than the fiberglass. Current rocketry vendors are at the mercy of their suppliers. So the fact of the matter is that CF tubes could/should be offered in a thin wall format, and in fact, could be much thinner than the fiberglass equivalent and achieve the same strength. But product demand and the willingness on the part of the small rocketry consumer base removes the focus and urgency of offering optimum filament wound CF tubes. In fact, the whole filament wound technology seems to be a passing fad. Note how Public Missiles, Carolina Composites and MAC Performance simply ignore FWCF and concentrate on the superior fabric-based airframe designs.

 

[plugger]

In fact, the whole filament wound technology seems to be a passing fad. Note how Public Missiles, Carolina Composites and MAC Performance simply ignore FWCF and concentrate on the superior fabric-based airframe designs.

Yep. I've been rolling my own CF airframes since 2013 and haven't looked back. Cheaper, lighter, and easy to make. I've not played with any of the Prepreg stuff before though.

Charlie's airframes are arguably the prettiest I've ever seen. Added bonus they're super light and hard as nails.

 

[plugger]

Of course this whole discussion is ridiculous as the composite CF material is orders of magnitude stronger than the fiberglass.

I'm not so sure I'm in agreement with that statement. Fiberglass is plenty strong for most of the rocketry projects that we engage in. It works fine for most use cases. Think of it this way, we use FG NCs pretty much constantly. Given the collapsing of shockwaves on a nose cone at Mach+ speeds when compared to the forces imparted on the airframe at the same speed the NC would be the first to fail under a high stress flight. When was the last time you saw a FG NC fail from a lack of strength in an otherwise nominal flight?

 

[AeroAggie]

I believe your decimal point is in the wrong place because what you're describing doesn't exist.

I believe this to be true. I tabulated all of Madcow's carbon tubes and the 38mm tube needs to be 2.6oz/ft to be consistent.

 

[AeroAggie]

I'm not so sure I'm in agreement with that statement. Fiberglass is plenty strong for most of the rocketry projects that we engage in. It works fine for most use cases. Think of it this way, we use FG NCs pretty much constantly. Given the collapsing of shockwaves on a nose cone at Mach+ speeds when compared to the forces imparted on the airframe at the same speed the NC would be the first to fail under a high stress flight. When was the last time you saw a FG NC fail from a lack of strength in an otherwise nominal flight?

The issue isn't really one of strength, but of stiffness. Most structural failures tend to be a form of column buckling for really long tubes, or some kind of local instability (compression crippling) of shorter tubes. Stability is proportional to E*I of the cross section, and inversely proportional to the Length squared. Nose cones don't generally fail because the effective column length is very short due to the taper and stiffness of the pointy end. The most common structural failure I've seen is buckling around the coupler. If there's any wobble between the two sections of tube, you're asking for trouble.

I believe the Madcow thin wall glass tubes to strong enough for most high performance 38mm flights, provided you're not making very long unsupported tubes. A rocket with 80% of its length being supported by motor case topped off by a nose cone and coupler should be perfectly fine. And there are plenty Go Devils and Blackhawks flying around with no problems. Go much longer than that, and you'd be wise to move to carbon for stiffness.

The thin wall glass is lighter than the thicker carbon tube, therefore should be faster assuming all other things being equal. It just depends on what your goal for the rocket is.

If you're looking to set altitude records, then you already know that the optimum weight may be higher than that for maximum speed. Drag reduction also comes at a price - reduced aerodynamic stability (assuming you're going to cut the fins down to bare minimum). Lower aerodynamic stability can lead to higher side loads on the airframe, so a slightly heavier but stiffer carbon tube can pay off here. There are a lot of variables to consider.

I don't think the filament wound tubes are a passing fad because something else is better; I think it's simply economics. If those companies can get a fabric rolled tube cheaper than a filament wound tube, they'll do that. The rest is marketing.