Chapter 1 Cont.
1.5 Airframe design
Most people would opt for the flying case design prior to
@watheyak 's L record. At the end of this thread he notes that in the future he will be utilizing a thin wall composite airframe from now on that matches the nosecone and creates a flush fit throughout the entire rocket body. The protruding lip produced by the fin can design probably does more harm than the missing airframe diameter does good. After talking with
@Loki Research over the phone last weekend, he pointed out that a student from Princeton attempted to break the L record by turning down the motor case to create a flush fit for the fin can to sit in. "One young customer of ours while attending Princeton a few years back would have captured the L altitude record, but they
deemed his modifications to go beyond the motor that I certified. He turned down the motor case by a depth of 0.020" where he then built up his 4 fin CF fin can. This way, the fin can diameter would not exceed the motor case OD of 2.127". Of course he made a custom NC to fit everything as well and he nearly broke 40,000' with it, something like 39,700ft AGL." Here is a photo of this rocket:
Scott Wathey's differs by being appearing to be much shorter but also having the step produced by the fin can:
A shorter rocket
should go much higher than one that is longer but Scotts didn't. There is a good chance this is due to the step of the fin can. Before anyone starts yelling at me, I know that there are a million other things that can affect two different launch altitudes including the humidity! Hear me out first.
Here are two different 38mm K627 designs:
Figure 6: Thin wall airframe, flush fit throughout entire body
Figure 7: Flying case with 1.6 OD fin can. All other geometries, weights, surface roughness, and simulation settings stay the same.
As you can see, the step in the fin can knocks off a few thousand feet compared to just utilizing a thin wall composite airframe. Note that the stability increases though and you may be able to make the fins smaller which can get you some extra altitude back:
Figure 8: Flying case figure 7 with reduced fin height to match stability of previous examples.
Even so, cutting the fins down so that the stability margin is similar to before results in less altitude than the streamlined design in figure .
The same thing can be seen in RASAERO:
Figure 9: Streamlined thin-wall composite airframe
Figure 10: Fin can with 0.125" shoulder
As you can see, the fin can loses a lot of altitude. IF you were to make the shoulder much longer (maybe 0.25 inches) then you appear to get your altitude back and maybe a little more (+100ft). The only issue is that this would need to be a perfectly tapered shoulder which just further complicates an already complicated design.
So with this in mind I decided to go with a thin wall carbon airframe purchased from mcmaster. Regarding the length, I needed it to be as small as possible. I wouldn't fully know this length until I received all of my avionics but I knew I needed to keep the Wildman nosecone nearly flush with the end of the motor tube. The Wildman nosecone shoulder has a length of about 2 inches while the loki motor is about 24 inches long. My guess-timated airframe length was 26 inches long.
I obviously wanted a Von Karman nosecone. One thing everyone should note though is that in the 38mm size, you should select Ogive in open rocket rather than VK. For some reason it drastically underestimates your VK altitude at this size. No idea why.
Here is RASAERO with VK for our control group:
Figure 11: RASAERO VK
Here is OpenRocket with Ogive:
Figure 12: Open Rocket Ogive
Here is Open Rocket with VK:
Figure 13: Open Rocket VK
Vk should definitely be going higher than Ogive for this supersonic flight profile if I am correct but it is the opposite here. If any of the developers see this maybe they can take a look at what's going on.
So I went with the Wildman nosecone but want to make my own custom cone for a future flight to give me a little more room since I would make it without a shoulder and use the motor tube as the coupler. This completes this section of airframe design.
HOWEVER... there is something I have been desperately wanting to look into.
If one person was to successfully create an epoxy bond between composite fins and the metal motor tube then no current record would be safe. I have some ideas that I plan on pursuing in the future. The reason no one does this now is these bonds typically fail. Bonding dissimilar materials turns out to be a pain in the *ss, especially when one of those materials oxidizes incredibly fast (the aluminum). There has been some successes by utilizing a wet sanding method with epoxy on the motor tube. This prevents the aluminum from oxidizing. In industry they would use some acid etching to prep the aluminum but that doesn't solve what I believe is the main reason these bonds fail on rocketeers.
I think it has to do with thermal expansion and vibrations. Epoxy is strong in compression and not so strong in tension. Fibers like fiberglass and carbon are strong in tension but weak in compression. When you mix the fibers and epoxy together it makes a composite which rivals the strength of metals. Aluminum does two certain things extremely well: Conduct heat and transfer vibrations.
My hypothesis is this: When the motor fires, the aluminum heats up which transfers heat directly to the epoxy bond which weakens it. The aluminum tube also experiences thermal expansion which puts the epoxy in tension causing cracking. The vibrations from the motor make this worse along with the forces of flight. This causes catastrophic failure of the bond.
