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Why My Rocket Wont Reach Rocksim Projected Altitude

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pugsleyd

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I designed a rocket and put it into Rocksim, I'm using the F35W-5 motor for this rocket. It is 31.5in with a 3 in diameter and weighs 665g fully loaded with the motor and the payload. When I let Rocksim calculate the Cd it simulates the rocket going 887.25ft. However, when I actually launched the rocket it went 652ft so I adjusted the Cd and found that the Cd would be .56 if it were to go that high. So I tried removing 8in of the body tube in Rocksim leaving the same Cd .56. Now it weighs 603.32g fully loaded ready for launch and it still wont break 750ft in the simulation. It averages an altitude of 740ft. I'm not ready to cut 8 inches off of my rocket that I've been working on for months not knowing if it will reach the altitude I'm trying to achieve. Any advice would help with reaching the proper altitude or let me know if I'm doing something wrong or I need to change something. It would be greatly appreciated.
 

Antares JS

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What is your finish setting? Every part defaults to "polished" and I really don't think it should. If your parts are set to polished, it's going to think there is less drag than there is.

Cutting your body tube shorter is not a great way to reduce weight (tubes are light) and can screw up your stability.

Also, are you using the actual weight of the rocket in your simulation or letting Rocksim calculate the weight? If your rocket is built you should input the actual weight.
 

pugsleyd

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I attached both files, the one at launch, and the one where I have taken off the 8 inches to try and achieve the 800ft altitude. I have yet to make any changes to the rocket since launch. So nothing is set in stone yet! Also both files are using mass override. Thank you for looking into this!
 

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pugsleyd

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What is your finish setting? Every part defaults to "polished" and I really don't think it should. If your parts are set to polished, it's going to think there is less drag than there is.

Cutting your body tube shorter is not a great way to reduce weight (tubes are light) and can screw up your stability.

Also, are you using the actual weight of the rocket in your simulation or letting Rocksim calculate the weight? If your rocket is built you should input the actual weight.
All of the parts are set to polished, I launched with only primer on the rocket because I didn't have time to finish the paint job and I figured that was better than nothing.
Yes I did put the actual weight of the rocket into Rocksim once I finished building.
 

mbeels

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All of the parts are set to polished, I launched with only primer on the rocket because I didn't have time to finish the paint job and I figured that was better than nothing.
That'll make a big difference. I find I get the best agreement between simulation and reality when I set the finish to "matte" or even "unfinished". Probably says something about my painting skills. A rocket in primer is going to be really rough.
 

heada

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If you're using wooden fins and nosecone, actually finish those to get the most impact in changing your drag. Paper body tubes, plastic fins and nose cone are already fairly smooth and don't add much drag but wooden fins (plywood, basswood, balsa, etc.) and wood nose cone are very draggy until sanded smooth and painted. After wooden fins/nose cone, check your fillets. If they're rough, they'd be the next source of drag. If they're nice and smooth, then move on to launch lugs, paper tubes and plastic stuff.
 

RocketTree

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I designed a rocket and put it into Rocksim, I'm using the F35W-5 motor for this rocket. It is 31.5in with a 3 in diameter and weighs 665g fully loaded with the motor and the payload. When I let Rocksim calculate the Cd it simulates the rocket going 887.25ft. However, when I actually launched the rocket it went 652ft so I adjusted the Cd and found that the Cd would be .56 if it were to go that high. So I tried removing 8in of the body tube in Rocksim leaving the same Cd .56. Now it weighs 603.32g fully loaded ready for launch and it still wont break 750ft in the simulation. It averages an altitude of 740ft. I'm not ready to cut 8 inches off of my rocket that I've been working on for months not knowing if it will reach the altitude I'm trying to achieve. Any advice would help with reaching the proper altitude or let me know if I'm doing something wrong or I need to change something. It would be greatly appreciated.
Check your 'conditions at launch', temperature, altitude, baro, wind speed etc.

I would consider the RS max altitude best-possible-scenario. Weight reduction can decrease altitude depending on design/motor. Shortening the body may result in a less vertical flight, and lower apogee. As previously mentioned, changing your finish to "unfinished" will also make a difference. All these little things add up to better accuracy in the simulation. If we are comparing an altimeter to a simulation, both can have a margin of error.
 

Bat-mite

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Rod length.... Keep in mind that simulators are not perfect.
 

Jowayen

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From what I was ab le to dig up on certification of motor testing:
"... the total impulse must not have a standard deviation greater than 6.7%, and the average thrust must not vary by more that 20% between motors when corrected to sea level @ 20 degrees C..."

My conclusion is that motors are allowed to deviate 6.7% for the total impulse.
So if you change your simulation to say 5% less thrust, you're probably going to get a closer simulation.
As has been said above, actual surface finish matching simulated surface finish is the key to getting an accurate drag profile.
Add these two together and it's easy to see how simulations (based on best results) don't match real-world results (with many, many variables).

One thing not mentioned is in the real-world you'll rarely get a perfectly straight vertical flight with zero wasted energy contributing to rolling and pitching.
 

rcktnut

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From what I was ab le to dig up on certification of motor testing:
"... the total impulse must not have a standard deviation greater than 6.7%, and the average thrust must not vary by more that 20% between motors when corrected to sea level @ 20 degrees C..."
One thing not mentioned is in the real-world you'll rarely get a perfectly straight vertical flight with zero wasted energy contributing to rolling and pitching.
Beat me to it!! There are just too many variables in a real-world launch.
 

teepot

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When I first started using Thrustcurve it's predicted altitude was always higher than the actual height. I found if I increased the CD from .6 to .75 the predicted height and the actual were closer. So this is another example of reality being different than predicted.
 
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From what I was ab le to dig up on certification of motor testing:
"... the total impulse must not have a standard deviation greater than 6.7%, and the average thrust must not vary by more that 20% between motors when corrected to sea level @ 20 degrees C..."

My conclusion is that motors are allowed to deviate 6.7% for the total impulse.
So if you change your simulation to say 5% less thrust, you're probably going to get a closer simulation.
As has been said above, actual surface finish matching simulated surface finish is the key to getting an accurate drag profile.
Add these two together and it's easy to see how simulations (based on best results) don't match real-world results (with many, many variables).

One thing not mentioned is in the real-world you'll rarely get a perfectly straight vertical flight with zero wasted energy contributing to rolling and pitching.
You misunderstand standard deviation. Under the assumption (possibly unwarranted) that total impulse across all engines of a given type is normally distributed, about 95% of all engines will have a total impulse within plus or minus two standard deviations (13.4%) of the mean measured value. This has nothing to do with any "allowable" deviation. However, if you do not know how the measurements are distributed, the standard deviation by itself does not tell you anything really useful.

20% tolerance on average thrust is enormous, and could easily account for the differences between simulated and actual performance.

Drag is not a big concern for model rockets, because velocities are relatively low. Drag for projectiles becomes a dominant factor when velocity gets above 85% of the speed of sound.
 

manixFan

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What is your finish setting? Every part defaults to "polished" and I really don't think it should. If your parts are set to polished, it's going to think there is less drag than there is.

Cutting your body tube shorter is not a great way to reduce weight (tubes are light) and can screw up your stability.

Also, are you using the actual weight of the rocket in your simulation or letting Rocksim calculate the weight? If your rocket is built you should input the actual weight.
Ahh, but a shorter body tube reduces the 'wetted area' of the rocket which is one of the main components of drag. The best way to improve altitude is to make the rocket as short as possible. With many rocket/motor combos, you may actually have to add weight to get the best altitude, regardless of length.

With regards to the OP, the launch lug is way off - it's positioned well behind the rocket and is impossibly small. I would correct that. Unfortunately, large lugs can make a big difference in drag and are not always properly accounted for by RS. Here is an excellent article by Tim Van Milligan on the subject:


On all the minimum diameter rockets I fly, I use a tower so launch lugs/rail guides aren't an issue. You can use fly-away guides to achieve the same purpose, but they are expensive for a 3" rocket.

Finally, as has been suggested, you need to change the conditions to match your actual launch conditions and location. For example, in OpenRocket (I don't have RockSim) I uncheck 'Use standard international atmosphere' and enter the actual air temp and barometric pressure. Also, once I've set the wind speed and variability, I uncheck 'always launch directly up-wind or down-wind' and enter the actual launch angle and direction relative to wind. That can make a very big difference, especially if it is a windy day. Also, run the sim multiple times - based on the wind settings you will get very different results with each run. For example, in OpenRocket I set the wind speed to 15MPH and standard deviation for 5MPH (yes, extreme settings but to illustrate my point) and here are the altitudes I got from 10 simulations:

657 ft
679 ft
634 ft
606 ft
551 ft
632 ft
656 ft
691 ft
640 ft
596 ft

Low: 551, High 691, Average: 634, Variation: 140ft

I think in RockSim you can plot the angle of attach during the flight and it will show you the variability on successive runs.

Good luck,


Tony
 
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jderimig

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Why can you use a bigger motor? And how do you know the true altitude you achieved? What was the air temperature when you launched?
 

Jowayen

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From post #10
"" "... the total impulse must not have a standard deviation greater than 6.7%, and the average thrust must not vary by more that 20% between motors when corrected to sea level @ 20 degrees C..." ""

This which is quoted above is from certification testing guidelines - not my own opinion.
The guide says to use 2, or 3 motors max, per cert. This 6.7% and 20% variations can be in those 3 motors - not from hundreds and hundreds of tests. The 20% can be average thrust variation - but TOTAL THRUST to have 6.7% max variation.

In other words, the average thrust can vary by 20%, but the burn time will be longer or slower to match - so that TOTAL THRUST shall vary no more than 6.7%. It doesn't end up being a huge variation as suggested in Post #13.

But I do suggest the allowed 6.7% can be significant enough to make simulations being useful for a guide only.
That was really my only point.

Of course, you can change Cd numbers and get a sim to match after you fly a particular rocket as in Post #12.
That does not suggest every sim should have Cd number corrected to a certain number before flying.
 

jderimig

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As mentioned earlier you do not understand standard deviation. Motors are usually certified with a handful if that test firings. Hardly enough to estimate the standard deviation.

If the standard deviation of a motor was actually known, which it is not, 37% of the motors will have an impulse outside the one standard deviation number. 5% will be 2 std dev off.
 

Jowayen

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You do not understand my post.

"" "... the total impulse must not have a standard deviation greater than 6.7%, and the average thrust must not vary by more that 20% between motors when corrected to sea level @ 20 degrees C..." ""

The above quote is from the certification testing procedure.
They use 2 or 3 motors MAX.
I didn't make the numbers up that I quoted.
They are from the testing procedures.

I understand standard deviation and I understand what they have written in the testing procedures.
Maybe you're argument should be with the testing procedures, not with me.
 

jderimig

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You do not understand my post.

"" "... the total impulse must not have a standard deviation greater than 6.7%, and the average thrust must not vary by more that 20% between motors when corrected to sea level @ 20 degrees C..." ""

The above quote is from the certification testing procedure.
They use 2 or 3 motors MAX.
I didn't make the numbers up that I quoted.
They are from the testing procedures.

I understand standard deviation and I understand what they have written in the testing procedures.
Maybe you're argument should be with the testing procedures, not with me.
No argument with you, just that the there is no data on the actual impulse variation from motor to motor. Don't believe the spec which I am sure you agree with.
 

rklapp

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I think the SD comes into play when they QC the motors to determine if the batch is within tolerance. My question is what do they do with the 100 that were out of spec? Do they put them in a discount bag and sell them at the company store or do they dump them in a tub of water for disposal?

 
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jderimig

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Its a useless specification. What is the spec on the mean?
 

rklapp

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I think what @Jowayen is saying is that the OP is drawing the short straw. Not likely but possible...

I’ve also noted my rockets are 20% less altitude than projected. I figure it could be high humid air or could be the extra glue and paint I use.
 

jderimig

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Humid air makes less drag but high temperatures make baro altimeters read low.
 

manixFan

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Another variation in altitude has nothing to do with the motor impulse, rocket finish, temp, etc., but with the delay accuracy. For a science fair project years ago my son tested rocket motors that were stored at roughly 0° and 120°. There was not only a variation in thrust but also in the delay timing. It your delays are on the short side but still within limits the early ejection charge will limit your altitude.

My son’s TARC team found their predicted altitude was a lot more accurate when the delay was chosen to be past apogee. In the case of the OP, a delay slightly longer than 5 seconds is required for maximum altitude. So the delay itself may be limiting altitude regardless of any other changes that are made.

Tony
 
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You do not understand my post.

"" "... the total impulse must not have a standard deviation greater than 6.7%, and the average thrust must not vary by more that 20% between motors when corrected to sea level @ 20 degrees C..." ""

The above quote is from the certification testing procedure.
They use 2 or 3 motors MAX.
I didn't make the numbers up that I quoted.
They are from the testing procedures.

I understand standard deviation and I understand what they have written in the testing procedures.
Maybe you're argument should be with the testing procedures, not with me.
Two or three motors drives you into the realm of t-statistics and/or non-parametric statistics, both of which provide very weak predictions. Standard deviation has nothing to with "allowable" thrust, and, in the absence of knowledge of the true distribution of the measurements, provides little information on its own.
 
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I think the SD comes into play when they QC the motors to determine if the batch is within tolerance. My question is what do they do with the 100 that were out of spec? Do they put them in a discount bag and sell them at the company store or do they dump them in a tub of water for disposal?

You can't tell how much thrust a motor will produce until you fire it. Then you know, for that motor. Does not tell you what the next motor will do.
 

PayLoad

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Trying to bring this subject back around to OP;

You have all your settings at "polished" when it should be set at "Unfinished"
Your lug is set completely wrong, although that is just an FYI - it is not hurting performance - Set it at COG
Correct the fillet material

With your settings corrected to what the rocket actually is, you have a SIM altitude of 215 Meters (700 feet) via Open Rocket
Your fins are far too thick & too big (Stability of 6? Yowsa), all the surfaces should be painted & smooth. Airfoil the fins you have.

Do these things & you'll hit 800 feet

What altimeter are you using?
 
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