Difuminar: 9500 feet and Mach 1.5 on a CTI G150

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby TRFfan2 » Sat Apr 22, 2017 8:39 am

Viking wrote:Looking good!
Thanks!

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby drew » Thu May 04, 2017 3:50 pm

TRFfan2 wrote:Well honestly the G65 has an offset core which will result in corkscrewing.So i dont think ill try it since other people with optimized MD rockets have had issues with the offset core.


One person's optimized airframe is another person's unstable rocket. Just have a look at the carnage from all the "optimized" N5800 airframes on youtube. Speaking of youtube, I came across this G65 video earlier.



Looks like a pretty straight flying motor to me. Granted that's not a MD airframe; but still. Plus, you always get the opportunity to blame lunar coning if something does go wrong!

I actually had a look at the link you posted from maxvelocity on TRF. I'll quote the comment here.

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.


Underlining by me. A rocket pinwheeling in the air 100 ft agl is because the rocket's unstable with that motor choice, not because of a negligible imbalance from the offset grain configuration. My expectation is that the G65 MD rocket wasn't stable when it left the tower which resulted in a pinwheel at 100 ft agl. I did exactly the same thing last year at Williams with a 38mm MD airframe on a I59. I didn't blame the motor, I was cutting my margin of stability too tight. The difference is I knew my margin of stability was tight, I persisted anyway. Also, in another post the same guy stated he turned off his tracking receiver mid flight... Talk about a head scratcher! Not too sure I'd follow that person's advice wholesale.
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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby drew » Thu May 04, 2017 4:44 pm

And one other point, the I59 is an end burner, not a moonburner.
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Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby TRFfan2 » Sat May 06, 2017 10:01 am

drew wrote:
TRFfan2 wrote:Well honestly the G65 has an offset core which will result in corkscrewing.So i dont think ill try it since other people with optimized MD rockets have had issues with the offset core.


One person's optimized airframe is another person's unstable rocket. Just have a look at the carnage from all the "optimized" N5800 airframes on youtube. Speaking of youtube, I came across this G65 video earlier.



Looks like a pretty straight flying motor to me. Granted that's not a MD airframe; but still. Plus, you always get the opportunity to blame lunar coning if something does go wrong!

I actually had a look at the link you posted from maxvelocity on TRF. I'll quote the comment here.

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.


Underlining by me. A rocket pinwheeling in the air 100 ft agl is because the rocket's unstable with that motor choice, not because of a negligible imbalance from the offset grain configuration. My expectation is that the G65 MD rocket wasn't stable when it left the tower which resulted in a pinwheel at 100 ft agl. I did exactly the same thing last year at Williams with a 38mm MD airframe on a I59. I didn't blame the motor, I was cutting my margin of stability too tight. The difference is I knew my margin of stability was tight, I persisted anyway. Also, in another post the same guy stated he turned off his tracking receiver mid flight... Talk about a head scratcher! Not too sure I'd follow that person's advice wholesale.
Well i got the G150 already but even if i didnt, the g150 gets a faster speed and only about 200' less altitude than the G65. My stability is about 1.8 cal subsonic so the g65 could get some corkscrewing.

BTW, that guy set the all-time G altitude record. It's at 8700 ft.

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby drew » Mon May 08, 2017 2:01 pm

TRFfan2 wrote:]Well i got the G150 already but even if i didnt, the g150 gets a faster speed and only about 200' less altitude than the G65. My stability is about 1.8 cal subsonic so the g65 could get some corkscrewing.

BTW, that guy set the all-time G altitude record. It's at 8700 ft.


Ok, first off you need to understand that your stability margin isn't a static figure. My expectation is that your quoted stability of 1.8 calibers is from OpenRocket's summary on the right hand side of the side view of your rocket. Notice how it says "at M=0.30"? at the bottom of those figures? Do you expect to be doing Mach 0.3 as you leave your tower/rail? If not then that number is virtually useless to assess your margin of stability at liftoff.

What you need to do is Plot/Export your simulation with stability margin calibers. When you do this you want to look at the shift in your stability margin from liftoff to apogee. IF at any point you dip below 1.5 in there you're going to have a bad time. Lately I've been aiming for 2+ as I'm more comfortable with that number. YMMV.

Take my 24mm rocket design as an example. My OR sim places my Stability for a G150 at 3.03 calibers at M=0.30. BUT, if you look at the Simulation graph you'll notice at liftoff my Stability margin calibers are roughly 2.5 calibers. So while most people look at the static stability caliber figure in their sim they don't graph their stability over time which is the only real accurate way of assessing this over the entirety of your flight with an eye towards staying stable. Pretty graph below.

Image

Same rocket, this time with a G65. Straight up I notice my Stability for a G65 is 2.86 calibers at M=0.30. So straight away I can see that despite utilising the same casing the G65 provides 0.17 less calibers of stability at Mach 0.3 when compared to the G150. This is important as it highlights that you can't 100% assume a motor will be stable in your vehicle because you've already flown a different motor utilising the same case size in the same rocket. Next up I plot the data to see what my stability margin calibers are at liftoff. The answer? Roughly 2.2 calibers of stability. So 0.3 LESS than the G150.

Image

Finally, pinwheeling and corkscrewing are definitely two completely different things. If a rocket "cuts a flip" after leaving the tower or rail that's an unstable rocket/motor combination, plain and simple. Corkscrewing is most likely caused by a misalignment of fins, NOT moonburner grains. And just because a flyer holds a current altitude record doesn't make it an "All time record" or make that flyer an expert at assessing the margin of stability of their vehicle on any given motor combination as the case you point to over at TRF succinctly indicates.
Andrew Hamilton
AMRS 28 L3
AMRS Records Committee Chairman
Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby strud » Mon May 08, 2017 9:10 pm

CF nose cone is going to kill your transmission performance ie be next to no signal in or out.

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby martymonsta » Mon May 08, 2017 10:33 pm

Here's my 2 bob's worth.

I recently read, "The Right Kind of Crazy" by Adam Steltzner, which is the story of the NASA JPL EDL (Entry, Descent, and Landing) team for Curiosity. Adam spoke about the 4 states of knowledge,

1/ known knowns (KK), the hard facts, eg. the diameter of a rocket.
2/ known unknowns (KU), things that we know we don't know. eg. the exact amount of thrust a motor will produce.
3/ unknown knowns (UK), things that we think we know, but we don't. eg. we think we know a rocket is stable, but in certain situations, it actually isn't.
4/ unknown unknowns (UU), the things that we don't know that we don't know.

So here are some KK's;
the mass of the rocket, the rockets dimensions, gravity and the center of gravity prior to the motor's ignition.

These might be some of the KU's;
the thrust of the motor, the barometric air pressure, the Cd, the CP, the CG after motor ignition, the wind speed at the pad.

One UK that I thought of, (yes since I've thought of it, it is could be considered a KU, but I put it here because it hasn't been mentioned before and I leave it here because Open Rocket (OR) ignores it)
Wind speed at different altitudes.

UU's
You generally find these when your rocket experiences a RUD

So why have I focused on wind speed? If you are using OR you will find that your default wind speed is 2m/s +- 10% (7.2km/h or 4.5mph). Try for a minute changing your wind speed to 7m/s (25km/h or 15.6mph), this is about 80% of the Max wind speed allowed under the safety code. I'm going to take a guess that if you had 1cal of stability at takeoff with the default windspeed, it is now about 0.5cal. And then it gets worse. OR assumes that all the wind moves at the same speed regardless of altitude. This is not the case and it is quite common that even when conditions are calm at the surface, that at 100-300ft, the wind is quite strong.

What I'm trying to emphasize is that while a simulation might tell you a rocket is stable, based on KK's, assumed KU's and ignored UK's, it takes very little to shift the parameters outside those simulated and leave you with a very unstable optimized lawn dart.
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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby TRFfan2 » Tue May 09, 2017 12:27 am

strud wrote:CF nose cone is going to kill your transmission performance ie be next to no signal in or out.
Im not going to use a CF nose cone. Its 3d printed polyesterene.

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby TRFfan2 » Tue May 09, 2017 12:40 am

drew wrote:
TRFfan2 wrote:]Well i got the G150 already but even if i didnt, the g150 gets a faster speed and only about 200' less altitude than the G65. My stability is about 1.8 cal subsonic so the g65 could get some corkscrewing.

BTW, that guy set the all-time G altitude record. It's at 8700 ft.


Ok, first off you need to understand that your stability margin isn't a static figure. My expectation is that your quoted stability of 1.8 calibers is from OpenRocket's summary on the right hand side of the side view of your rocket. Notice how it says "at M=0.30"? at the bottom of those figures? Do you expect to be doing Mach 0.3 as you leave your tower/rail? If not then that number is virtually useless to assess your margin of stability at liftoff.

What you need to do is Plot/Export your simulation with stability margin calibers. When you do this you want to look at the shift in your stability margin from liftoff to apogee. IF at any point you dip below 1.5 in there you're going to have a bad time. Lately I've been aiming for 2+ as I'm more comfortable with that number. YMMV.

Take my 24mm rocket design as an example. My OR sim places my Stability for a G150 at 3.03 calibers at M=0.30. BUT, if you look at the Simulation graph you'll notice at liftoff my Stability margin calibers are roughly 2.5 calibers. So while most people look at the static stability caliber figure in their sim they don't graph their stability over time which is the only real accurate way of assessing this over the entirety of your flight with an eye towards staying stable. Pretty graph below.

Image

Same rocket, this time with a G65. Straight up I notice my Stability for a G65 is 2.86 calibers at M=0.30. So straight away I can see that despite utilising the same casing the G65 provides 0.17 less calibers of stability at Mach 0.3 when compared to the G150. This is important as it highlights that you can't 100% assume a motor will be stable in your vehicle because you've already flown a different motor utilising the same case size in the same rocket. Next up I plot the data to see what my stability margin calibers are at liftoff. The answer? Roughly 2.2 calibers of stability. So 0.3 LESS than the G150.

Image

Finally, pinwheeling and corkscrewing are definitely two completely different things. If a rocket "cuts a flip" after leaving the tower or rail that's an unstable rocket/motor combination, plain and simple. Corkscrewing is most likely caused by a misalignment of fins, NOT moonburner grains. And just because a flyer holds a current altitude record doesn't make it an "All time record" or make that flyer an expert at assessing the margin of stability of their vehicle on any given motor combination as the case you point to over at TRF succinctly indicates.
Yeah i know the 0.30 mach figure is just OR's standard setting. With wind at 10 mph my stability off the pad is about 1.2 cal. As it goes into the transonic region the stability gets higher and after about mach 1.1 it starts dropping.
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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby TRFfan2 » Tue May 09, 2017 12:45 am

Got the fillets done with JB weld. They are very hard now.
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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby strud » Tue May 09, 2017 6:24 am

You may have 'wasted' a few precious grams with the JB weld, since it is metal filled, unless you need the mass at the rear of the rocket for some reason.

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby TRFfan2 » Tue May 09, 2017 9:51 am

strud wrote:You may have 'wasted' a few precious grams with the JB weld, since it is metal filled, unless you need the mass at the rear of the rocket for some reason.
What is the JB welds density?

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby drew » Tue May 09, 2017 12:18 pm

TRFfan2 wrote:Yeah i know the 0.30 mach figure is just OR's standard setting. With wind at 10 mph my stability off the pad is about 1.2 cal. As it goes into the transonic region the stability gets higher and after about mach 1.1 it starts dropping.


Good luck!
Andrew Hamilton
AMRS 28 L3
AMRS Records Committee Chairman
Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby OverTheTop » Tue May 09, 2017 12:36 pm

Marty, I like your idea of putting more realistic values for wind into the sim.

Remember that sims are only approximations to reality (refer to my sig below). I would be using even higher values than the safety code, just to see what the sim produces when prodded.

A pass in one simulation (or many for that matter) does not give you any feel for how much margin the design has for performance. It can result in a false sense of security. Simulating failures will be a better indication of design margins.
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Re: Difuminar: 9500 feet and Mach 1.5 on a CTI G150

Postby strud » Tue May 09, 2017 1:23 pm

From here : http://site.skygeek.com/MSDS/j-b-weld-8265-cold-weld-compound.pdf

SG : 1.9313

http://www.fiberglasssupply.com/pdf/msds/3MGlassBubbles.pdf

SG: microballoons : 0.12 to 0.6

SG typical epoxy : (West Systems 105) 1.15

So assuming about 30% microballons by volume, overall SG for epoxy + microballon mix would be around : 0.9


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