Why My Rocket Wont Reach Rocksim Projected Altitude

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Interesting debate. I'm gonna say that humidity increases drag because 85% humidity feels heavy like the rocket is having to push through 3 gallons of water (give or take).

Actually it is exactly the opposite.
The more humid the air, the less dense it is. You're rocket will fly higher in humid, less dense air.
 
Humidity, temp, wind speed - all have nothing to do with original OP or why it won't reach altitude. Important factors, sure, but not in this case
 
I've had Rocksim, Open rocket and RAS get within 3% on some flights. Certainly not consistently and I really don't care. 10% is fine by me as I just want to get an idea of how far the rocket is going to be when it's down. In that regard, I'll do a lot and I mean a lot (25 to 50 sims) to get an idea of how the rocket will perform in wind. There are a lot of variables here.
I always (except in tight fields) angle the rail/rod a few degrees downwind as the rocket will have a tendency to weathercock into a slightly curved flight to a low energy apogee deployment. With electronics this results in low stress on the airframe. With motor deploy, I've gotten good at drilling delay grains to shorten a delay as needed.
Modrocs can get away with upwind angles as long as the shockcord and take it. I've resorted to kevlar cord of various thicknesses where I've tied two loops in the kevlar then attach appropriate lengths of rubber so there is some slack in the kevlar.
The rubber stretches and helps absorb some of the energy but one is depending on the kevlar for strength and the rubber for energy dissipation. One has to be careful with packing this as there is a loop of kevlar between the rubber. When packed carefully works fine.
I hate zippers and had my share of them over time and this seems to help if a flight goes off nominal.
Kurt
 
So rockets in the desert have more drag than in the tropics? They do seem to hang in the air more during descent.

Nope, Warm dry air is less dense (thinner) resulting in less drag.
probably won't affect the apogee that much. High humidity air would be more dense at a given temperature and hence more drag.
The affect is likely not that great and probably not much noticeable.

Kurt
 
My input is based on quite a few years of Rocksim use.
I find that on smaller rockets say 24mm It overestimates the altitude by 15%. On bigger stuff like 3" it seems to come a lot closer, maybe 5%. If I add a boat tail it underestimates by 5%.
That is assuming that all of the parameters are input correctly.
On smaller rockets that I airfoil the fins I input square leading edges. That usually gets me close.
 
Nope, Warm dry air is less dense (thinner) resulting in less drag.
probably won't affect the apogee that much. High humidity air would be more dense at a given temperature and hence more drag.
The affect is likely not that great and probably not much noticeable.

Kurt
Water vapor is less dense than dry air, which is why clouds are up there and not down here. Higher rh less dense lower drag. More baseball home runs in hot humid games. It also throws off altimeter readings because high rh is not the SAM.
 
1619309660858.png
from----- https://www.engineeringtoolbox.com/density-air-d_680.html
The Graph say Moist Air but actually covers all dry to moist conditions. The full calculations are available on the website for anyone interested.

The truth about air density is neither explanations are true. Depends on the temperature. And the graph here doesn't tell you what this means to the weather and if you'll actually be able to launch. At lower temps with high humidity it will probably be close to snowing or sleet or fog. At higher temps above 40 I'm going to be seeking aircon and should the world get to 60, we're all dead, probably. In between a higher humidity is going to increase cloud cover.
So the answer in what humidity means to your expected altitude is......... It depends. And it's going to change with altitude. So what's the full humidity for your full launch profile from ground to apogee????? Who knows.

When considering all rocket related factors it's important to try to understand what the data is saying in the real world.

You might not even be able to launch due to real world factors.

Norm
TRA12957 L3
 
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The truth about air density is neither explanations are true. Depends on the temperature.
That would explain why half say yes and half say no. Hawaii runs between 25C and 30C most of the year. Humidity averages 60% but can be 85% in the morning when the winds are lighter. I agree that it feels heavier because sometimes it's hard to breathe in the humid air.
 
View attachment 461449
from----- https://www.engineeringtoolbox.com/density-air-d_680.html
The Graph say Moist Air but actually covers all dry to moist conditions. The full calculations are available on the website for anyone interested.
Norm
TRA12957 L3

The bigger point what does everyone mean by altitude? Is it the true altitude (which is largely unknowable unless you have some pretty sophisticated equipment or analysis technique) or the altitude reported by your altimeter? Most people confuse the two.

If its the latter, what your altimeter beeps out, you can maximize your altitude by launching on cold dry day. I recommend that if you are going for a record, the altimeter error will be in excess of 5% in your favor.
 
The bigger point what does everyone mean by altitude? Is it the true altitude (which is largely unknowable unless you have some pretty sophisticated equipment or analysis technique) or the altitude reported by your altimeter? Most people confuse the two.

If its the latter, what your altimeter beeps out, you can maximize your altitude by launching on cold dry day. I recommend that if you are going for a record, the altimeter error will be in excess of 5% in your favor.
Johns answer is of course correct and wrong. :) It depends. What record are you going for? How much "thick atmosphere" are you going through. Will that give you surface heating due to your Mach profile that will cause your beautiful tip to tip laminations to de-laminate giving you fuzzy fins. :) Looking at the effect from just 1 variable without taking into account the other consequences of it won't give you the correct answer.
This is what I meant by you have to take into account what the data is saying to you in the real world. In the rocksim world fins don't de-laminate. In the real world they do. If you are going for a record you need to take a look at every photo you can find of previous attempts and everything you can read on what people have done before then distill from that what you are going to do that will be better while trying to avoid every mistake everyone else,( many of them very smart people) has made.
This will give you your known knowns, your known unknowns and after recovery you find out your unknown unknowns and learn.
Many cafes cannot make a decent coffee with the 3 basic ingredients water milk and coffee. When you launch a rocket you have a lot more ingredients, you have to understand them all and you hope for a perfect flight. Tricky and wonderful when it goes right.
 
I've had a look at your design.
You're creating a vacuum area with the large flat end. This is a 3" tube so you're creating a 3" hole in the sky with point thrust from the 24mm motor. This will create a vacuum at the rear with turbulent airflow at the rear and I'm uncertain how RockSim or OR handle this especially with such a large difference in the motor to tube size. RasAero deals with turbulent airflow. You could try running a sim there. Might be interesting to see what it gives.
A quick look tells me you're sub-sonic. No need to worry about skin heating destroying your fins...... You're using a polished finish on 3/16 Birch plywood fins with a square finish on the LE. Not great.
A Von Karmen nosecone will not be optimal but is not massively significant.

At present, my best advice for matching the rocksim simulated data with real world figures would be to launch it on a Tuesday. There won't be many people around and you can just say it went to whatever altitude you wanted. :)
 
Start with a rocket that will actually exceed your required altitude for the comp by lets say 20%. Then add DRAG or weight to trim it to where you need to be. Or treat the whole thing as fun and just see what you get. Sometimes it's nicer to let the other people who need to win, win.
 
Humidity has very little effect on drag. Temperature and altitude, both of which strongly affect air density, are much more significant than humidity.
Drag will be determined by air density, so I believe this is correct. Humidity is a relatively smaller effect. Still, at the SAME temp and pressure (altitude), humid air will be less dense since a water molecule weighs less than N2 or O2. Adair's book "The Physics of Baseball" explains this nicely.
 
Interesting debate. I'm gonna say that humidity increases drag because 85% humidity feels heavy like the rocket is having to push through 3 gallons of water (give or take).

https://www.quora.com/How-does-air-...pared to cold air,This means, lesser air drag.
My understanding is that when all else is equal, humidity will decrease air density because of its composition (H2O). The partial pressure of water vapor will decrease the molar weight of the ambient atmosphere since N2 has considerably more mass than H2O.
My conjecture does NOT take into account any mass gains the rocket may accrue through absorption, or the fog of decades since my last chemistry class.
 
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?
The temp was about 42F at launch and I used an altimeter to determine the height.
 
The humidity, air temp, phase of the moon or your wife's mood has little to nothing to do with why your rocket sims at 20% higher than it actually goes.

The famous phrase "garbage in, garbage out" applies here and, although you did not necessarily enter garbage, something entered in the sim program is incorrect. Find out what that piece of data is, and you will have your answer.
 
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?
Sorry for such a late reply, How would I go about applying the airfoil to the fins? That's the one reason I haven't tried that yet because I didn't want to do it wrong. Also how much smaller should I go with the fins?

I have done everything else you said besides paint because I'm going to wait until I remove the 8 inches from the rocket so it's a smoother process. Thank you so much for the advice!!!
 
Sorry for such a late reply, How would I go about applying the airfoil to the fins? That's the one reason I haven't tried that yet because I didn't want to do it wrong. Also how much smaller should I go with the fins?

I have done everything else you said besides paint because I'm going to wait until I remove the 8 inches from the rocket so it's a smoother process. Thank you so much for the advice!!!
I'm also using the FireFly altimeter.
 
Pictures are worth 1000 words - another useless cliche - but if you have a good picture that won't hurt. Air-foiling the fins is basically putting a knife edge on the exposed edges using a sanding block.
 
Going back to the delay issue, if you have video of your flights, time the delay portion - from after burnout to ejection charge - and see how short/long it is to the correct 5 secs. Also, you should be able to tell if the delay is before or after apogee. As I mentioned before, you need a slightly longer than 5 second delay to reach maximum.You should be able to tell pretty quickly if it is the delay that is limiting your altitude. If the delay is firing before apogee, you've likely identified a big reason you are not matching the sim.


Tony
 
Interesting debate. I'm gonna say that humidity increases drag because 85% humidity feels heavy like the rocket is having to push through 3 gallons of water (give or take).

https://www.quora.com/How-does-air-...pared to cold air,This means, lesser air drag.
Humid air (say ~ 5% of the molecules) is lighter because a water molecule is lighter than nitrogen or oxygen. Feels subjectively heavier but is actually less dense. See "The Physics of Baseball" by Robert Adair.

Note: w/ 100% relative humidity at 95F (35C), water molecules count up to ~ 5.5% of the total.
A water molecule is ~ 2/3 the mass of average air (N2/O2), so this saturated air is about 2% less dense than dry (0% RH) air at the same temp. Small effect.
 
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Reopen the Sim you ran. Input all the particulars of your flight on "Launch Conditions" tab. Enter your rod/rail length on the "Starting State" tab. Rerun the sim. Adjust the Cd. Rerun. Repeat until your altitude corresponds with your measurement. Save. From then on, all your sims should be good
 
Humid air (say ~ 5% of the molecules) is lighter because a water molecule is lighter than nitrogen or oxygen. Feels subjectively heavier but is actually less dense. See "The Physics of Baseball" by Robert Adair.

Note: w/ 100% relative humidity at 95F (35C), water molecules count up to ~ 5.5% of the total.
A water molecule is ~ 2/3 the mass of average air (N2/O2), so this saturated air is about 2% less dense than dry (0% RH) air at the same temp. Small effect.
I'm not a physics expert so don't shoot the messenger. Talking to my physics professor friend, this is a myth. Water vapor is immensely bigger than air molecules. The rocket has to push the water vapor out of the way. As mentioned before, would have to be over 50°C for there to be a significant difference in the air density, but the water vapor in the air remains. Yesterday, it was humid as balls here. I could feel it and see it hanging in the air. Sometimes, it produces a rainbow with no rain. Yes it is lighter than air but it's still visible.

1666571293410.png
 
You point out an important distinction! Water vapor, a gas of individual molecules, is invisible. But mist (or clouds or fog), is liquid water in small droplets (hugely massive compared to an individual molecule). If you can see it, it's not vapor (i.e. a gas).
 
If it hasn't been said already. your enemy is DRAG. If the fins are squared and not sanded to a perfect shape, that's drag. If the finish on the rocket is scruffy and not perfectly smooth, that's drag. If one of your fins is not 100% accurate, square on the tube, and is off by a degree, that's DRAG. If the nosecone still has a bit of rough surface or flash plastic from the molding process that's more drag. Add up all that drag and that's why the rocket doesn't match the sim. the sim assumes the entire rocket is PERFECT. And there's no wind, and probably the atmosphere is slightly less dense. Are you flying on a humid day?
 
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