Lakeroadster's "Level One"
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If you can stretch it a little bit longer, 14.5" is the most space you need for commercial 29mm motors (CTI 6G XL), or 12.75" will get you an AT 29/360. Won't you want to keep flying it? I'm always tempted to keep going bigger, and those 29/360 AT loads are still HAZ-free (your H97J is not, like the G75J; Blackjack is unique among AT 29mm HP loads).
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Wanna bet he's planning on flying it only long enough to get his L1 and then retire this rocket?
lakeroadster
When in doubt... build hell-for-stout!
I design these desk top sized rockets so they apogee at a height where they can still be clearly seen, makes for a more exciting flight visually.
800 feet to 1,200 feet is optimal, in my opinion. YMMV.
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Overall, I gotta say that I like this design. I happen to have a Mercury/Gemini capsule in balsa laying in my parts box that fits a BT-60. I might make a downscale of this as I like the 6 fins look. Sort of a heavily modded "Little Joe" is what it reminds me of. If I still have the centering rings from my citation patriot, I have enough bits to build this for an 18mm motor. Can't do the rear eject at that scale, as there isn't enough room for a chute though...
I wouldn't call this an oddroc; I don't see any problem with a 6FNC and a few artistic touches. That said, rear ejection does seem to deviate from the KISS philosophy. But lakeroadarer skipped philosophy class the day they covered KISS, and he seems to do OK anyhow.
Not if he stays with rear ejection.
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boatgeek
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I'm a little late to the party, but somehow I missed this one. I need to troll the Scratch Built subforum more often. Anyway, a couple of thoughts:
The fins look a little small to me. IIRC, there have been a number of times when people had less stability than they expected because the fins were stubby and never made it out of the boundary layer into clear air. I think that's a higher risk on this particular rocket because the conical nose cone with the extra flat plate will tend to deflect air out away from the body tube. I don't know if the air will have time to get back down to the fins. I don't know that there's an easy way to test this (short of CFD!), but a typical recommendation is that fin span should be at least the body tube diameter. I don't know if a swing test will be close enough to real flight to accurately sim the flight.
I can't tell how you're planning on launching this. Is there space between fins/pods for a rail? If not, you might want to check with the club you'll fly with to see if they'll buy off on flying an H from a rod. Some clubs don't like to fly anything G or bigger off of a rod because of rod whip.
For the simulation purposes only, you might want to eliminate half of your pods and see if it has an appreciable effect on CP. I don't know if OpenRocket can tell that the fin root pods are half shaded by the body tube and therefore not contributing drag.
Looking forward to see how this comes out!
The fins look a little small to me. IIRC, there have been a number of times when people had less stability than they expected because the fins were stubby and never made it out of the boundary layer into clear air. I think that's a higher risk on this particular rocket because the conical nose cone with the extra flat plate will tend to deflect air out away from the body tube. I don't know if the air will have time to get back down to the fins. I don't know that there's an easy way to test this (short of CFD!), but a typical recommendation is that fin span should be at least the body tube diameter. I don't know if a swing test will be close enough to real flight to accurately sim the flight.
I can't tell how you're planning on launching this. Is there space between fins/pods for a rail? If not, you might want to check with the club you'll fly with to see if they'll buy off on flying an H from a rod. Some clubs don't like to fly anything G or bigger off of a rod because of rod whip.
For the simulation purposes only, you might want to eliminate half of your pods and see if it has an appreciable effect on CP. I don't know if OpenRocket can tell that the fin root pods are half shaded by the body tube and therefore not contributing drag.
Looking forward to see how this comes out!
The old dudes around here fly HIPO off of rods all the time. Get out yer 1/2" stainless steel rod and party like it is 1999! Rod whip is not the rod's fault, it is the flyers and RSO, LCO fault. Oopsie, that probably PO'ed some folks.
boatgeek
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Apparently, our individualism isn't quite rugged enough here in the upper left.The old dudes around here fly HIPO off of rods all the time. Get out yer 1/2" stainless steel rod and party like it is 1999! Rod whip is not the rod's fault, it is the flyers and RSO, LCO fault. Oopsie, that probably PO'ed some folks.
I don't think I've ever seen a 1/2" rod in the wild.
lakeroadster
When in doubt... build hell-for-stout!
Thanks for the comments.
My plan was to build it and fly it on a G motor here in the spring at my local launch site. If that's successful, then go to an event later in the year, install the H motor and try the L1 attempt.
Other than shredding fins, if a rocket flies on a G motor I would think it would also be stable with an H motor. Is that flawed logic?
I was planning to use my 5/16" dia x 6 ft launch rod. But I could always upgrade to something else. I'll think about that, might be time for me to go with a rail.
I'm not sure there is a way to do that. But the swing test should help to confirm stability
.The conical fairings at the base of the fins will be made to slide into place after the fins are installed. I could temporarily install them , swing test, remove them and swing test again. Then compare the results.
Again, thanks for the input, gives me some things to think about.
Kind of funny that I'm replying to this thread, the USPS mail carrier just brought the 4" Shipping Tubes...
It cannot.
boatgeek
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Always a pleasure!
No, that's a reasonable approach. The H is enough longer that it doesn't move the CG as much--the added weight compared to a G is pretty close to the CG. With your solid materials and attention to detail, I wouldn't worry about shreds either. Unless you go an stick an H550 in there or something.
That was a purely OpenRocket simulation item. Per @neil_w above, OR will assume you have the full 360-degree pod at each location instead of only the 180* degrees that sticks out from the airframe. If the fins aren't integrated with the pods in your OR model, then just change the pods from 6 to 3 and call it good. If the fins are integrated, it's a little harder. Once it's built, swing test it like you'll fly it. No need for removable pods.I'm not sure there is a way to do that. But the swing test should help to confirm stability
The conical fairings at the base of the fins will be made to slide into place after the fins are installed. I could temporarily install them , swing test, remove them and swing test again. Then compare the results.
Looking forward to seeing this build go forward.
* Actually a hair more, but that's Captain Technicality territory.
The upper left is no country for old men. Come on down to South Park, see where the 1/2" stainless rods grow, have a real good time. High power rocket has too many fins or is a bit odd and cannot fit on the rail with buttons, no problem, we can launch it with out the rod whip.Apparently, our individualism isn't quite rugged enough here in the upper left.
lakeroadster
When in doubt... build hell-for-stout!
I don't know. The last time I was hip enough to know we had nine inch nails, industrial strength heavy metal. A rocket powered flying Emu would be cool.
lakeroadster
When in doubt... build hell-for-stout!
So says the "Swooshing Crane" Grand Poohbah.
Yes Mr. RSO. Here is my high power, rear ejecting Emu rocket with feathers. Got a little bored after the 3-4FNC level one certification flight.
Quarter inch is the biggest I've seen here in the upper right. And my own local club doesn't use those; if you think you need it, go off a rail.Apparently, our individualism isn't quite rugged enough here in the upper left.
It's probably good logic. I think. I'm not saying it's not, but I don't think it's air tight (so to speak).
The greater mass of the H can sometimes make an appreciable difference to the GG, though Boatgeek is, of course, right about the compensatory effect of its greater length. Of course, OR will easily tell you about that. And your CG to CP distance as a fraction of the rocket's length is quite large, so I'd bet good money you're OK on that.
What really concerns me - and again, I don't know if it's a problem or just something to consider - is that the CP can change with speed, and you'll presumably be going faster on an H motor than on a G. For how it changes, I'm totally out of my depth. With the somewhat stubby fins and the question of whether or not they get into enough clean air, the increased speed with the H motor could potentially have a big effect. Maybe. And here's the rub: a swing test can't check for that since it can't reach anything near actual flight speed.
Food for thought.
I thought CP shift was only an issue once you approach the transsonic region, and this rocket won't be getting anywhere near that.
Not sure why that would be a particular point of concern here.
Not sure why that would be a particular point of concern here.
As I wrote, I'm out of my depth, it was just something I'm aware sometimes needs to be considered. And the depth of the boundary layer, i.e. how much fins span is needed to reach the clean air, is probably affected at least a little by lower than transonic speeds. How much? 
lakeroadster
When in doubt... build hell-for-stout!
Always a pleasure!
No, that's a reasonable approach. The H is enough longer that it doesn't move the CG as much--the added weight compared to a G is pretty close to the CG. With your solid materials and attention to detail, I wouldn't worry about shreds either. Unless you go an stick an H550 in there or something.
That was a purely OpenRocket simulation item. Per @neil_w above, OR will assume you have the full 360-degree pod at each location instead of only the 180* degrees that sticks out from the airframe. If the fins aren't integrated with the pods in your OR model, then just change the pods from 6 to 3 and call it good. If the fins are integrated, it's a little harder. Once it's built, swing test it like you'll fly it. No need for removable pods.
Looking forward to seeing this build go forward.
* Actually a hair more, but that's Captain Technicality territory.
Per Open Rocket:
- The stability with the MPR G74-6 motor is 1.54 calibers, max velocity 150 mph
- The stability with the HPR H135-8 motor is 1.44 calibers, max velocity 253 mph
- Without The Base Drag Hack, the H135-8 design drops to 0.638 calibers, the G74-6 drops to 0.734 calibers.
- Deleting the conical fairings increases stability by 0.02 calibers
- Changing the number of fins from 6 to 3 does not change the stability at all.
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Base drag hack for the faircones would add even a tad more. That's a very rockety rocket.
lakeroadster
When in doubt... build hell-for-stout!
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there is a cheat that helps you do this when space is tight.
BT-20 motor mount tube, just long enough for motor,
motor hook,
BT-20 to BT-5 centering ring which will also serve as your engine block.
two centering rings BT-20 to BT-60,
one centering ring BT-5 to BT-60.
section of BT-5 tube that runs from the motor block up to base of nose cone, (or if nose cone hollow, cut off base bulkhead and extend the tube INTO the nose cone. Smear some JB Weld on inside of cone to protect it.) With plastic nose cones, this is a nice trick, as you can put your nose weight on the forward end of the BT-5 chimney so it ejects from body at apogee. Reeeeaaaally nice for internal pop rocket gliders.
aluminum can piece almost the length of the can, width just enough that it rolls up inside the BT-20 with a slight overlap. This goes into the BT-5 rear end all the way to the edge inside the motor block. Reason: the concentrated ejection charge from the 18 mm motor will char and quickly burn through a smaller unprotected paper “chimney” or “stuffer” tube. Because this isn’t structural, IMO it doesn’t violate the low power NAR “no metal rule” (any more than a metal engine hook, a nose cone screw eye or ballast washer, or a snap swivel.) alternative is smear the tailward internal 3 inches with JB Weld.
you DO need to run your shock cord from nose cone THROUGH the forward centering ring to attach to the BT-20 motor mount, the BT-5 will likely bend On recovery if it is the attachment point.
going BT20 to BT-5 gives you over an extra centimetre circumferentially in laundry space for rear ejection, which turns out to be quite helpful.
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lakeroadster
When in doubt... build hell-for-stout!
A BT-60 will work for a rear eject. Here's a link to my BT-55 rear eject on the "I Was Inverted" Rocket.
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Neutronium95
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I'm doubtful that the swing test will provide useful information. Some friends of mine swing tested a Falcon 9 with the legs deployed, and it failed in several configurations. I've even heard that an Estes Alpha 3 will fail the swing test, although I haven't seen that in person. The simulations indicated that it would be fine, and it was perfectly stable in flight. I think that one major reason is that during a swing test, the rocket will always be at some angle of attack, instead of starting off near zero angle of attack like in a normal flight.
I'd also suggest you start looking at stability in terms of percent of body length, instead of in calibers. Calibers are a rule of thumb that were intended for normal shaped rockets. The 1 caliber rule of thumb breaks down with very long or very short rockets. I tend to try to keep my CP and CG separated by around 8-18% of the overall rocket length. The beta version of Openrocket actually has an option to view stability in percent of rocket length now.
I'd also suggest you start looking at stability in terms of percent of body length, instead of in calibers. Calibers are a rule of thumb that were intended for normal shaped rockets. The 1 caliber rule of thumb breaks down with very long or very short rockets. I tend to try to keep my CP and CG separated by around 8-18% of the overall rocket length. The beta version of Openrocket actually has an option to view stability in percent of rocket length now.
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As I understand it, that's exactly the right explanation. And it means that the swing test is conservative, sometimes overly so. Which means that a successful swing test is something you can generally hang your hat on, and it's only the failures that need closer examination.
And the fact that this design's CP and CG are so far apart as a fraction of its length is why I for one have no worries about the static margin, even if an H motor were to weight twice a G motor without being longer.
lakeroadster
When in doubt... build hell-for-stout!
you never cease to amaze me.
Oh great, now my brain hurts.
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