OpenRocket or RocketSim: Effective Fin Speed or Launch Rod Optimization?

The Rocketry Forum

Help Support The Rocketry Forum:

eduncan911

Active Member
Joined
Jul 5, 2016
Messages
36
Reaction score
1
Hello:

Q1: With OpenRocket, how do you get information/a reading on rather your rocket is stable leaving the launch rod? IOW, does OpenRocket tell you the minimal speed at which the Fins are effective?

I've read a good bit about ensuring your rocket is travelling fast enough that the fins are functional before leaving the rod. Considering some of the custom builds we are thinking of, I'd like to figure this out with OpenRocket with some of the LPR we are thinking of.

I see the simulation parameters of where I can set the length, and it plots on the graph. But nothing says if the rod is too short or too long in the warnings.

I see in the simulation result, the speed leaving the rod... But how do I know this is a safe speed? Too slow/too fast?

Maybe I need to learn to interrupt the plotted graph? As an extreme example, I took a Baby Bertha and shortened the launch rod to something ridiculous: 20"






^- exaggerating rod length to just 20" to lower the rod exit speed vs fin effective speed.

This should be unstable when leaving the rod. But closing the plot, or looking at the simulation, i can't see any advice or real measurement to tell me.


Or Q2: Is there a way to see what speed the fins are effective at?

Maybe I can cross-reference this speed with the speed at which it leaves the launch rod? If the effective speed is greater than the speed at which it leaves the rod, I'll know I need a longer rod.

Does RocketSim provide more details on these?

Screen Shot 2017-04-10 at 10.50.55 AM.png


Screen Shot 2017-04-10 at 10.50.11 AM.jpg
 

Cabernut

Well-Known Member
Joined
Oct 15, 2015
Messages
1,384
Reaction score
3
I would say set the launch rod length to whatever it will be launched on such as 3ft 1/8" rod or 6ft rail. In OR, you can plot calibers of stability. Add it as an x-axis and make sure the check box for launch rod clearance is checked. In the plot you can see exactly what the stability is at the moment when it leaves the rod.

Adjustments are in the rocket or in motor choice. Leave the rod as what its going to be.

Edit - you can also plot angle of attack among just about any other value in the sim.
 

Nytrunner

Pop lugs, not drugs
TRF Supporter
Joined
Oct 15, 2016
Messages
8,001
Reaction score
3,719
Location
Huntsville AL
15 m/s, 40 ft/s, and 30 mph are all numbers I've heard thrown out for safe rod exit velocity.

The windier it is, the faster you want it to be going.
 

eduncan911

Active Member
Joined
Jul 5, 2016
Messages
36
Reaction score
1
I thought I read somewhere that these pieces of software (OR or RS) can calculate the minimal speed required before the fins are effective.


The reason this is important is I plan on heavily modifying my Saturn V kits (big and small), which has tiny fins in relation to the overall rocket. So I'll need to create some clear fin versions.

Maybe I should have asked it like: can OpenRocket tell me the minimal speed for Fins to be effective? if not, does RocketSim tell you?

This would be important for me for either piece of software, if one does or does not.
 

Nytrunner

Pop lugs, not drugs
TRF Supporter
Joined
Oct 15, 2016
Messages
8,001
Reaction score
3,719
Location
Huntsville AL
I haven't seen this functionality in OpenRocket, and I haven't read of something like that in Rocksim (only played with it over the trial period).

There is a plot option that shows positions of CG with respect to CP. It can provide more insight on how the rocket is being stabilized
 

Charles_McG

Ciderwright
Joined
Sep 12, 2013
Messages
2,947
Reaction score
1,035
Location
SE Wisconsin
The rule of thumb, gleaned from other threads here that I don't have links to handy, is that most fins stall (and become ineffective) at an angle of attack of 15 degrees.

So if you draw a right triangle with one point at the base of the launch rod, one side going up the rod (and equal to your speed leaving the rod) and one side orthogonal to the rod (and equal to the cross wing speed), you've set up your angle of attack.

The sin of 15 degrees is .25, inverting that means that your speed off the rod should be at least 4x the wind speed.

I think I've seen slow rod speed warning in openrocket - but I'm not positive it's a real memory.
 

markkoelsch

Well-Known Member
Joined
Mar 18, 2009
Messages
4,364
Reaction score
150
I think the 4x the wind speed is pretty good. That said, velocity is your friend.
 

aerostadt

Lifetime Supporter
TRF Lifetime Supporter
Joined
Oct 27, 2009
Messages
3,581
Reaction score
433
Location
Brigham City, UT
In RocSim9 go to "Flight Simulations". Scroll down to the launch results that you are interested in. Right click on that flight. Then scroll down and choose "Display Details". then scroll down to "launch guide data".
 

eduncan911

Active Member
Joined
Jul 5, 2016
Messages
36
Reaction score
1
In RocSim9 go to "Flight Simulations". Scroll down to the launch results that you are interested in. Right click on that flight. Then scroll down and choose "Display Details". then scroll down to "launch guide data".
That looks closer to what I am after, yes...

Launch guide data:

  • Launch guide length: 36.0000 In.
  • Velocity at launch guide departure: 30.5667 ft/s
  • The launch guide was cleared at : 0.204 Seconds
  • User specified minimum velocity for stable flight: 43.9993 ft/s
  • Minimum velocity for stable flight reached at: 79.5926 In.

If I am reading this, it says, "User specified minimal velocity for stable flight." So, does that mean it is just a setting in the software somewhere? Not calculate based on fins?

That was for a stock Sat V rocketsim plan I found and imported, and stuck an E15W in to simulate.

EDIT:
I added some larger fins and played around with RocketSim. It looks like this "43.9993" is hardcoded no matter what. And based on that number, it does calculate a "minimum velocity for stable flight reached at" setting.

Then again, that can be seen in the OpenRocket graph as well if we are following this "43.9993" rule here. With the Baby Bertha example I plotted earlier, we can see a 20" rod length is just a bit shorter than we actually need, which would be about 24" instead.

So, I guess as long as we remember this "43.9993" rule, we can estimate this in OpenRocket as well.

@OpenRocket guys: maybe you can add a warning if the "Velocity off rod" < "43.9993".
 

Cabernut

Well-Known Member
Joined
Oct 15, 2015
Messages
1,384
Reaction score
3
That looks closer to what I am after, yes...

Launch guide data:

  • Launch guide length: 36.0000 In.
  • Velocity at launch guide departure: 30.5667 ft/s
  • The launch guide was cleared at : 0.204 Seconds
  • User specified minimum velocity for stable flight: 43.9993 ft/s
  • Minimum velocity for stable flight reached at: 79.5926 In.

If I am reading this, it says, "User specified minimal velocity for stable flight." So, does that mean it is just a setting in the software somewhere? Not calculate based on fins?

That was for a stock Sat V rocketsim plan I found and imported, and stuck an E15W in to simulate.

EDIT:
I added some larger fins and played around with RocketSim. It looks like this "43.9993" is hardcoded no matter what. And based on that number, it does calculate a "minimum velocity for stable flight reached at" setting.

Then again, that can be seen in the OpenRocket graph as well if we are following this "43.9993" rule here. With the Baby Bertha example I plotted earlier, we can see a 20" rod length is just a bit shorter than we actually need, which would be about 24" instead.

So, I guess as long as we remember this "43.9993" rule, we can estimate this in OpenRocket as well.

@OpenRocket guys: maybe you can add a warning if the "Velocity off rod" < "43.9993".
That 43.9993 is arbitrary. Nothing special about that specifically. Must be a setting somewhere since it's labelled User Specified.

Minimum velocity varies depending on today's on-field conditions. Windy day? Higher velocity needed. Calm day? Not so much.

You can plot stability at rod clearance in OR. Just add these in the plot setup screen:
Capture002.PNG
 

EeebeeE

Well-Known Member
Joined
Aug 7, 2011
Messages
1,427
Reaction score
49
15 m/s, 40 ft/s, and 30 mph are all numbers I've heard thrown out for safe rod exit velocity.

The windier it is, the faster you want it to be going.
And the larger your fins, the faster, due to weather cocking. No sim that I know of effectively calculates weather cocking because it is somewhat random. I like to keep my speed off the rod at 35 MPH, but if the winds are over 10 MPH, a try to keep off-rod speeds more toward 50 MPH.
 

Cabernut

Well-Known Member
Joined
Oct 15, 2015
Messages
1,384
Reaction score
3
Yep, that was checked in the graph I posted earlier. The line that says "Launch Rod Clearance". :)
Did you add stability margin as an x-axis? That will be much more useful for what you're trying.
 

Buckeye

Well-Known Member
TRF Supporter
Joined
Sep 6, 2009
Messages
2,787
Reaction score
571
Then again, that can be seen in the OpenRocket graph as well if we are following this "43.9993" rule here. With the Baby Bertha example I plotted earlier, we can see a 20" rod length is just a bit shorter than we actually need, which would be about 24" instead.

So, I guess as long as we remember this "43.9993" rule, we can estimate this in OpenRocket as well.
Yep, this is as good as it gets. Try something really crazy, like a 2" launch rod. Nothing bad happens in the simulation. The software isn't smart enough. You can edit the 43.9 ft/s setting to whatever you want in Rocksim.

Did you add stability margin as an x-axis? That will be much more useful for what you're trying.
Maybe not. Many rockets with wind included will show a brief negative stability margin at the launch rod, yet the simulation continues as normal. So does the real life flight. Maybe a wobble, but not unstable.

OR's Cp calculation is likely too conservative. It often gives a negative stability at the rail even though the velocity exceeds 4X the cross wind by a good margin (the angle of attack assumption mentioned above). I can't figure out how to rectify this contradiction, but I now tend to trust the velocity rules more. It is just easier to interpret, especially when Thrustcurve Motor Guide will give you the warning flag you are seeking.
 

cls

Well-Known Member
Joined
Jan 21, 2009
Messages
2,234
Reaction score
0
my dad always said "the reason for the 55 MPH speed limit is that's the knee in the drag curve." that's the speed where air foils start working. look at stall speeds for light aircraft, right around there.

so that's a good rule of thumb - 80 feet per second is for sure. but I use 35 MPH ~ 50 ft/sec. seems to work. as others posted above, pick a number but use a higher number in crosswind....
 

Buckeye

Well-Known Member
TRF Supporter
Joined
Sep 6, 2009
Messages
2,787
Reaction score
571
my dad always said "the reason for the 55 MPH speed limit is that's the knee in the drag curve."
Well, kind of. Drag varies by velocity squared, and the power needed to overcome drag varies by velocity cubed. Not really a "knee" in these smooth curves. 55 MPH is about where aerodynamic drag overcomes tire rolling resistance and becomes the dominant energy drain on the car at high speeds. This was back in the day when car drag coefficients were really bad (0.5), but today's average is closer to 0.3, and vehicles are much more fuel efficient.
 

bill_s

Well-Known Member
Joined
Jul 29, 2012
Messages
929
Reaction score
10
You can create unstable plots, I was able to change the max. altitude of an unstable flight by varying how unstable. Also can sim weathercocking and antiweathercocking. If you leave out the parachute you can even get weird glider effects on the way down, like drifting 3000' on a 1000' flight with no wind, or even making a loop or S curve -- this probably isn't realistic but does point out when you have a rocket that will fall sideways after a tailstand. Rod and wind condition are set at "simulate and plot" and then bottom left Edit. Also can be good to plot Flight Side Profile.

Another useful tool, I'd say essential to mess with, is seeing how angle of attack affects your CP and stability, this is under Component Analysis. A lot of long rockets will go from 2 cals margin to unstable in about 10 degrees, while shorter and less common designs may stay stable to a much greater angle despite a smaller margin.
 
Top