How do you obtain the coeeficient of drag for a rocket?

I have always used ThrustCurve.org as a guest, but yesterday I registered and started saving my rockets. In order to save a rocket, to have to enter the drag coefficient. So I have two questions?

Is that a flat value based on the size/shape of the rocket? I always thought it changed based on windspeed, altitude, etc.

If the above is a "yes," then how do you calculate it?

Okay, third question, anyone got a spreadsheet or web site that will do it?

Thanks.

Well ... it's complicated.

https://en.wikipedia.org/wiki/Drag_coefficient

It is my understanding that a "standard" run of the mill 4FNC rocket will have a Cd of around 0.7, with a lower value if it has a very smooth surface finish. A 3FNC rocket will have a lower Cd than a 4FNC rocket (one less fin to disturb the air around it).

Without expensive simulation software, it's hard to know what the Cd will be.

There was some discussion on another thread that you could calculate the Cd of a rocket after burnout if you had a data logger recording altitude at regular intervals.

If you want to look at the math of aerodynamics, download DESIGN OF AERODYNAMICALLY STABILIZED FREE ROCKETS (MIL-HDBK-762) and look at chapter 5.

Greg

Bat-mite said:

I have always used ThrustCurve.org as a guest, but yesterday I registered and started saving my rockets. In order to save a rocket, to have to enter the drag coefficient. So I have two questions?

Is that a flat value based on the size/shape of the rocket? I always thought it changed based on windspeed, altitude, etc.

If the above is a "yes," then how do you calculate it?

Okay, third question, anyone got a spreadsheet or web site that will do it?

Thanks.

Click to expand...

The real answer is to fly a rocket with an accelerometer and back out a Cd/velocity curve. You can get a single flight summary drag coefficient by flying a rocket with an altimeter and backtracking the number from a simulation. (You will need to average values from several flights, since the backtracked number is very sensitive to noise and other measurement errors.) Flight summary values can vary a lot with different motors and payloads.

If you want a working value _before_ a flight, I suggest you use a credible interval of Cd’s, and plan around the variability in altitude and coast time. A typical such interval might be .4 - .8.

(Oddly, long thin rockets tend to have higher Cd’s than short wide rockets – though the latter have greater drag.)

There are programs out there (e.g.; RocSim, Open Rocket, and Rogers Aerospace) that claim to predict Cd curves. In my experience (with simple configurations), they tend to predict the subsonic plateau reasonably well, and many (Not RocSim) predict supersonic Cd’s pretty well too. They tend, IMHO, to do a poor job on traditional model rockets, which spend a lot of time in the portion of the Cd/speed curve before the subsonic plateau is reached. Also, physically larger rockets tend to be better represented than smaller rockets, since larger rockets can be modeled as having consistently turbulent flow.

Wayco said:

I do it backwards. I set it at .7 initially, and after each flight, I check the estimated altitude against the actual altitude attained. If the rocket went higher than the Thrustcurve estimate, increase the drag until the estimated altitude matches what the rocket actually did, or decrease drag if it didn't go as high as TC estimated. Most of my rockets end up somewhat less than .7. "Rapiddity", my RW X-Celerator is set at .4, but has a pretty slick paint job on it.

Click to expand...

That's pretty much what I did, too.

I also do it in that manner. I fly a rocket with a few different motors, then adjust the Cd until the sims match up to around what the rocket actually flew.

you can also use the software with a MARSA 54 to ballpark it. (worked fairly well with my 3" darkstar)

DizWolf said:

I also do it in that manner. I fly a rocket with a few different motors, then adjust the Cd until the sims match up to around what the rocket actually flew.

you can also use the software with a MARSA 54 to ballpark it. (worked fairly well with my 3" darkstar)

Click to expand...

The advantage of an interval in the first or second flight prediction is that it shows you how sensitive the predictions are to error in the drag coefficient. You don't get that with a point value.
When you fit the Cd to the various results, you are, in effect backtracking a Cd from altitude.

Turns out that flights with apogee values sensitive to Cd are poorly predicted but they give you less noisy (more accurate) Cd values in backtracking actual data.
Flights with apogee values insensitive to Cd give good predictions, but they give you less accurate Cd values from backtracking actual data.

As you undoubtedly know, flight summary Cd's vary in dissimilar flight profiles. Cd curves from accelerometers are preferable.

Best Regards,
-LarryC

calculation of Cd is pretty much as much "black art" as it is science. And IME most folks flatter themselves to think that their baby has a low Cd, when in actuality other factors can enter into the detailed calcs and confuse what is really going on.

Track your vehicle WEIGHTS as carefully as you can, at launch, during burn, after burnout, etc. Weight is a far bigger influence on flight performance than drag. Analyze your flight profile carefully, and if you don't know what your canned software simulation accounts for, it's time to write your own spreadsheet. I think you will gain at least as much insight as you might from sweating the possible values of Cd.

Basically yes the Cd changes and is not a "flat value," and depends on the drag over the density of the fluid times 1/2 the (velocity)^2 times the surface area of the vehicle. Seeing as the Cd of the vehicle is a very important design parameter (for me at least), I tend to do it before the rocket is even built. I draw up the rocket and run a computational fluid dynamic study on the vehicle with a drag force goal and a equation based goal to find the Cd. This is much more reliable than other methods but for the average hobbyist it may not be a possibility. This is essentially a computerized wind tunnel.

Lots of good info here. Note that the ThrustCurve.org Motor Guide estimates the Cd for you, in the range mentioned, from the "complexity" and "surface finish" entries. For more info, see the explanation.

The primary goal of ThrustCurve.org is to help you find motors for your rocket. In particular, it does not attempt to complete with full-fledged flight simulators such as those mentioned here. Its algorithms are the simplest form of simulation, using a static Cd value, which is sufficient for a rough approximation only.

A5tr0 An0n said:

Basically yes the Cd changes and is not a "flat value," and depends on the drag over the density of the fluid times 1/2 the (velocity)^2 times the surface area of the vehicle. Seeing as the Cd of the vehicle is a very important design parameter (for me at least), I tend to do it before the rocket is even built. I draw up the rocket and run a computational fluid dynamic study on the vehicle with a drag force goal and a equation based goal to find the Cd. This is much more reliable than other methods but for the average hobbyist it may not be a possibility. This is essentially a computerized wind tunnel.

Click to expand...

The poor man's CFD is DATCOM. You can find a very nice reference for that here

https://sargrocket.org/documents/estes/tr-11.pdf

It's even in a model rocket context. This is a classic Estes technical report written by Professor Gerald Gregorek.

DATCOM results are variable (as even the above report demonstrates). The attached shows a smooth DATCOM curve for the LOC Weasel along with several accelerometer curves.
Again, the bigger the rocket, the better the results.

Note: IIRC, Gregorek sometimes accidentally uses root chord for mean aerodynamic chord. (Or something like that.) His results are a little better than they should be.
Also, today you can use the entire DATCOM curve in a simulation, rather than a flight summary value.


More information is to be found here:
https://www.amazon.com/dp/0262632780/?tag=skimlinks_replacement-20

The DATCOM experiment in the above didn't work out that well.

Chuck Rogers has provided some excellent references in the past. Perhaps someone can provide pointers?


Should say that the difficulties with DATCOM in both references (and in my own experience) are at least as much concerned with experimental measurement as they are with the method. (See the attached accelerometer curves) Either way, the difficulties are... difficult.


Regards,
-LarryC