Cutting stability to the bare minimum. 54mm minimum diameter design

Since becoming a REBAR last year I've been flying low and slow. I plan on making the trip to Black Rock for the first time in 15 years this year. I'd like to build something worthy of the trip. I can't afford to build an L3 project so it's going to have to be a minimum diameter 54mm rocket to fly at the extreme edge of the envelope. My personal altitude record is 8500' on an L850 in a 5" 35lb rocket. I'd like to break into the 25k region this time around. That should be reasonable with the motors and materials available now.

I plan to use the new thin wall fiberglass and an injection molded nose cone from Wildman. I've been tinkering with the design in OpenRocket and I've got it all worked out except the for the fins. The design I have now has just over 1 caliber of stability with the K300 loaded. All other motors will need ballast in the nose cone or forward body tube to be stabile. Since I won't be shooting for a record I don't see that as a problem. I'll probably just figure out what the absolute worst case is and make a removable ballast weight to suit.

The design I have now uses 3 small fins with a span of 2". According to OpenRocket there is ~1.1 calibers of stability. So the question is, how small of fin can you really get away with? Are there any better simulation tools for the mach 2+ region?

View attachment Attebery 54mm Minimum Diameter 3 Fin.ork

Small margins of stability will bite you with the long burn motors. You will be better off giving up a little altitude to gain stability margin because in the end flying with less stability with long burn motors will cost you more altitude because most of the time they will not fly straight at all. I flew a long burn G65 in a min dia with a small margin of stability. It did a loop about 100 feet up then kept going straight up. Needless to say I lost a little altitude on that one. That same rocket had flown on higher thrust motors perfectly straight every time. Same margin. the issue is the offset core.

watching...... Which Black Rock date are you looking at?

I would suggest a bit more fin. Increase the static margin to about 1.5. Remember the Cp will move a bit as the velocity changes. So, I would see if you can graph the margin vs velocity to make sure you stay stable.

I'd suggest increasing the fin span to 1 cal. Through personal experience, I've found that OR isn't necessarily extremely accurate when it comes to high speed stability and short fin spans (under 1 cal). Interesting looking build! Hope you document it some way or another.

Alex

So where do you think the limit is?

OpenRocket has a plot for stability margin vs time. It looks like my sim above dips to about .2 calibers. That seems sketchy. I resized the fins so that the worst case is 1 caliber. That seems a little more reasonable.

maxvelocity said:

Small margins of stability will bite you with the long burn motors. You will be better off giving up a little altitude to gain stability margin because in the end flying with less stability with long burn motors will cost you more altitude because most of the time they will not fly straight at all. I flew a long burn G65 in a min dia with a small margin of stability. It did a loop about 100 feet up then kept going straight up. Needless to say I lost a little altitude on that one. That same rocket had flown on higher thrust motors perfectly straight every time. Same margin. the issue is the offset core.

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Probably Aeronaut.

Salvage-1 said:

watching...... Which Black Rock date are you looking at?

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OK, round two. Fins are 2.25" span, moved them all the way to the back and swept the trailing edge forward slightly. 1.88 calibers loaded. Worst case is still over 1 caliber.



View attachment Attebery 54mm Minimum Diameter 3 Fin.ork

I can't really say for sure. I mostly abandoned the long burn motors in my G project. I will go back and look through my sims at past projects that did not fly straight and what margins they had. I do know that the rocket that made me abandon the longburn had a decent margin of stability. I started out with really small fins and probably just over a caliber, the rocket flew fine on motors without the offset core. Then I went with the offset core same rocket same margin and it didnt fly straight. So I went with larger fins, still didnt fly straight. I also recall a story that Jack G told me the day my rocket did a loop about a guy that flew a K300 in a rocket that had only a small margin of stability. the rocket had flown fine on motors without the offset core but Jack warned him that the K300 was different. He went ahead and flew it anyways and the rocket went unstable and crashed. I think you will want your span at least 1 caliber +. I always go just a little over to be safe, because again, if your rocket doesnt fly straight you will lose more altitude than these little additions for stability will cost you. I learned this the hard way so now just pretty much made it a rule. For example on 38mm I will use a span of 1-5/8". this seems to work fine. May be a different story with the offset core, I couldn't tell you because I have not had one fly straight. if I had to guess I'd shoot for 2.5 calibers in a rocket flying on a motor with an offset core and going above mach... More data later tonight.

For better sims switch to Ras-Aero.

Open Rock is a poor mans sim.
You want something that will do Mach+ flights.
Use Ras-Aero or RockSim Pro.

JD

ChrisAttebery said:

So where do you think the limit is?

OpenRocket has a plot for stability margin vs time. It looks like my sim above dips to about .2 calibers. That seems sketchy. I resized the fins so that the worst case is 1 caliber. That seems a little more reasonable.

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Chris, Is this rocket going to be used for a single flight? The reason I ask is that I've seen a few rockets with fin spans less then one caliber not fly so good. Loops or off vertical flights. Some times they work. You'd be surprised at how many flights Curt Von Delius has to do, to get one clean flight. Curt tried swept fins like you are showing. Those usually came back with the tips missing or the tips shreaded. I always tell guys to at least build them with fins that have at least one caliber spans (if not a tad more) and stability of 1.5. In 2011 I flew a slightly modified Kestrel in the XPRS Altitude contest. I used a K300 in it and came in 2nd place with 29,456' Last year one of my Rx motors lost one side of the exit cone on the nozzle. Photo below. This put a pretty good kink (do to lack of a better word) into the rockets flight path. The rocket was able to recover from the mishap and continued on straight to 26K, This rocket had enough fin spam, stability and material strength to over come this mishap. I've been calling these types of rockets "AeroPac Sport Fliers". I do loose some altitude and speed but they are not right on the ragged edge all the time.



Tony

Interesting. Thanks Chris.

BTW: I'm not dead set on flying an offset core long burn. I'm just as likely to use a dual thrust. I just figured the long burn would need ballast so I'd start by making it barely stable.

JDcluster said:

For better sims switch to Ras-Aero.

Open Rock is a poor mans sim.
You want something that will do Mach+ flights.
Use Ras-Aero or RockSim Pro.

JD

Click to expand...

For above mach flights I've found that Open Rocket is more accurate than RasAero and Rocksim when it comes to stability. There's been a few very high performance flights that have gone unstable exactly when Open Rocket said they would, while RasAero and Rocksim said they would be fine. I can cite specific examples but I'd like their permission before I name names!

I've been using Open Rocket for stability and RasAero for Cd. Rocksim is a waste of money.

JDcluster said:

For better sims switch to Ras-Aero.

Open Rock is a poor mans sim.
You want something that will do Mach+ flights.
Use Ras-Aero or RockSim Pro.

JD

Click to expand...

I've seen first hand on many occasions that this isn't true. (as far as stability is concerned)

Probably Aeronaut.

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Cool! See you out there!

Alex

Chris,

I am doing something similar so will be following your thread. A couple of things to consider. My simulations are based on everyday rockets and real flight info. The Madcow Tomach has been flown on a K300 to 23 k or so. It was a shortened version, 48 inches or so. I can dig up the info. if you like. You can buy the Madcow Tomach fins for $5.00 or so. They are unbevelled. These might be a good starting point for your fin simulations and there is flight data with them. I have both the CTI nosecone and the standard FW FG nosecone. I am also using regular FG tubes, not thin walled. My opinion is that if you have to add ballast to the CTI nosecone, you might go for the heavier standard nosecones, although fitting the thin walled tube might be an issue. Also keep in mind that if you go for altitude, a little heavier might not be a bad thing. You get a better coast after burnout. Best of luck. I might see you out there.

you might look into the L640 DT it will help you come off the pad a little faster and for sure be stable up until its not

I flew a pretty similar flight last year. If you want I can dig up the OR file I also used RAS

https://www.rocketryforum.com/showthread.php?55925-What-is-this-Flynfrog-s-carbon-aero

There is a fin flutter spreadsheet out there somewhere to help with your fin shape and matl.

The hard part is going to be recovery.

Very cool, are you planning any lower altitude flights at Snow Ranch or TCC? I probably won't make it to Black Rock this year.

What's the recovery system, are you using the nose cone altimeter sled you posted for this build? I'm thinking of possibly building a 38mm build similar to this (MD, minimum length, probably single deploy with tracking) to fly the I216 or I125.

I'm subscribing, can't wait to see how this works. Any plans to eventually fly an L935 in it?

Minimum stability works great in sims, not so well in reality.
I do not like using some number of calibers to measure stability, I prefer it based on the length rather than diameter.
Using 10% of the length is good.
Short span fins also have issues at high speed, and it is aggravated by a long rocket

M

No, it's not. However, I also don't intend to put too much effort into it either. I'm going for a phophecy style minimalist approach. No tip to tip, just G10 or carbon fins with generous Rocketpoxt fillets. I like the Aeropac sport flier concept. High flying without being high maintenance. Thanks your input Tony.

tfish said:

Chris, Is this rocket going to be used for a single flight?

Tony

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I looked back at my sims, now I know this is a different size but it is similar otherwise, both rockets had over 2 calibers stability. the rocket I still have and have flown many times successfully, twice on the G65 (offset core) has 2.14 calibers of stability and is 23.75" long overall. It has not flown straight on the G65. The rocket that did the loop had 2.75 calibers of stability and was 24.5" long. The fins were smaller but the rocket had more weight up front. The original version of the first rocket had less stability, probably around 1.5 cal, but I'm not 100% sure because when that rocket didn't fly straight on the G65 I cut the fins off and went to a larger fin to add stability. That still didn't work. I have tried for a while now to figure out how to get these to fly straight because they sim significantly higher with the longer burn but I just cant make it happen so I have settled for the faster burning motors. Keep in mind these rockets and other similar versions to them have all flown perfectly straight on faster burning motors which is why I can confidently say it is the core that causes the issues, not the rocket design.

Don't forget the new kid on the block... L-265 Mellow long burn. it's a bates grain, so no off center [coning issues] thrust.

About 100 newtons more [98], much flatter thrust curve, tad longer burn, bet this one will do some altitude. If it's calm enough for the K-300. you ought to be able to fly it.

I'm dying to try one of these myself.

Chuck Rogers is working on RASAero 2.0 which will improve supersonic CP estimates. I emailed him questions about a sim and he gave some pointers. Basically he said version 2.0 will move the CP forward about one caliber in the Mach 1 to 3 range. To be safe he said to shoot for a caliber of 3, up to burnout, for supersonic flights in RASaero 1.0.

watheyak said:

For above mach flights I've found that Open Rocket is more accurate than RasAero and Rocksim when it comes to stability. There's been a few very high performance flights that have gone unstable exactly when Open Rocket said they would, while RasAero and Rocksim said they would be fine. I can cite specific examples but I'd like their permission before I name names!

I've been using Open Rocket for stability and RasAero for Cd. Rocksim is a waste of money.

Click to expand...

MarkH said:

Chuck Rogers is working on RASAero 2.0 which will improve supersonic CP estimates. I emailed him questions about a sim and he gave some pointers. Basically he said version 2.0 will move the CP forward about one caliber in the Mach 1 to 3 range. To be safe he said to shoot for a caliber of 3, up to burnout, for supersonic flights in RASaero 1.0.

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Only in the Mach 1-3 range? That doesn't exactly help with Bare Necessities (mentioned earlier in the thread but not by name) which went unstable at about M4 exactly when OpenRocket predicted.

Clearly, based on the coning nature of the instability, it's something that has to be done using 6 degree of freedom simulation. Fudge factors for stability in a 3dof simulation aren't my cup of tea.

CarVac said:

Only in the Mach 1-3 range? That doesn't exactly help with Bare Necessities (mentioned earlier in the thread but not by name) which went unstable at about M4 exactly when OpenRocket predicted.

Clearly, based on the coning nature of the instability, it's something that has to be done using 6 degree of freedom simulation. Fudge factors for stability in a 3dof simulation aren't my cup of tea.

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Add James Donald's Minfluous and Curt Von Delius' large 2-stage projects to that list of Open Rocket told-ya-sos.

CarVac said:

Only in the Mach 1-3 range? That doesn't exactly help with Bare Necessities (mentioned earlier in the thread but not by name) which went unstable at about M4 exactly when OpenRocket predicted.

Clearly, based on the coning nature of the instability, it's something that has to be done using 6 degree of freedom simulation. Fudge factors for stability in a 3dof simulation aren't my cup of tea.

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I was running a sim which peaked at Mach 3.0 so that is my point of reference. I'd imagine version 2.0 will have improved CP beyond Mach 3.

watheyak said:

Add James Donald's Minfluous and Curt Von Delius' large 2-stage projects to that list of Open Rocket told-ya-sos.

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Interesting they others have experienced it as well.

When I was running the simulations in OpenRocket, it was giving weird results where it went less than 20000 feet, which I couldn't believe at the time, and I wrote it off since the program itself says not to trust it for supersonic flights. Well, now I know that I can trust it.

I wonder if there is a way to detect these stability problems and report them better, since OpenRocket didn't report anything unusual about the simulations... Maybe have the high angle of attack warning much stricter (5 degrees?) when supersonic?

CarVac said:

Only in the Mach 1-3 range? That doesn't exactly help with Bare Necessities (mentioned earlier in the thread but not by name) which went unstable at about M4 exactly when OpenRocket predicted.

Clearly, based on the coning nature of the instability, it's something that has to be done using 6 degree of freedom simulation. Fudge factors for stability in a 3dof simulation aren't my cup of tea.

Click to expand...

You guys ended up getting data from that flight?

The improved Fin supersonic CNalpha and CP models in RASAero Version 2.0 shift the rocket CP forward about 1.0 calibers from Mach 1.05 to Mach 4-5. See the attached RASAero Version 1.0 CP versus Mach number plot, just shift the supersonic CP curves forward by 1.0 calibers.



Above Mach 4-5 the new CP versus Mach number curves start to blend into the old hypersonic CP curves at higher Mach numbers.

So for Mach 1.05 to Mach 4-5, move the RASAero Version 1.0 CP's forward 1.0 calibers at all Mach numbers. Use the angle of attack 0-4 deg CP curve. (Your rocket's angle of attack won't be exactly zero, the CP will move forward with the small angles of attack, 0-4 deg, present in supersonic flight.) This will be the updated RASAero CP prediction. Then have the CG be 2.0 calibers ahead of the CP for margin.


Chuck Rogers
Rogers Aeroscience

If the rocket is a long-necked one be sure to read this article for further insight:

https://argoshpr.ch/joomla1/articles/pdf/sentinel39-galejs.pdf

I think trying to design the stabilty on the edge, it more prudent to err on the side of stable/safe flight.

Kurt

Thanks for the input everyone. I downloaded RASAero this morning and played with a couple fin designs. Just doing some quick comparisons it seems to line up pretty well with OR if you take Chuck's advice above and move the RAS CP forward by one caliber over mach.

I think I'll stick with the core burning motors with this one. I can always try the L265 if I want a long burn. I really like the idea of using a Dual Thrust motor though.