Estimating Drag Force using Telemetry Data?

[phrajphogate]

Hello, recently, my rocket team launched a class-O rocket at Spaceport America acheiving 29000 ft and Mach 1.8. Right now, we are trying to conduct a post-flight analysis and part of that is comparing the simulated drag curves from CFD, OpenRocket, and RASAero to an estimation of the drag curve from the actual flight. I'm wondering whether using the acceleration data obtained from our onboard accelerometer and plugging it into the equation Thrust - Gravitational Force - Drag = mass*acceleration, given that thrust is similar to the one on thrustcurve.org and the reduction of mass calculated by OpenRocket, would give a reasonable estimate of the drag force? Also, is there another way to go about doing this kind of analysis? Thank you.

 

[mikec]

Certainly this will work. How else would you do this?

 

[asm109]

After motor burnout, the equation simplifies to -G-Drag= mass* acceleration.
No assumptions about thrust curve required.

 

[phrajphogate]

Certainly this will work. How else would you do this?

Thank you! I'm pretty new to this so I just wanted some assurance.

 

[phrajphogate]

After motor burnout, the equation simplifies to -G-Drag= mass* acceleration.
No assumptions about thrust curve required.

Thanks for replying! I believe it's still required in our case since we also want to look at drag at max velocity, which happens before burnout.

 

[Chuck Rogers]

To All:

As part of a 4-week short course I helped teach out of the Air Force Test Pilot School from 2000 through 2005, the students flew Caliber Isp high power rockets with Black Sky AltAcc accelerometers/barometric altimeters. Using the onboard acceleration data and barometric altimeter data, and a Matlab program run by the students, the students backed out power-off CD versus Mach number and the thrust curve of the rocket motor from their flights. Each team presented a briefing with their flight data and CD and thrust data a few days after the flights.

The slides from the class which include the AltAcc instrumentation and the equations and calculation techniques for backing out CD and motor thrust are attached.

View attachment Rocket CD and Thrust Data from Flight Data.pdf


Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[Chuck Rogers]

Some examples of backed-out in-flight CD measurements, compared to RASAero predictions, from the RASAero web site are presented below. They can be found on the RASAero web site on the Comparisons with Flight Data Page ( https://www.rasaero.com/comparisons-flight.htm ). Adrian Adamson flew the rockets and backed out the in-flight CD data. The analysis technique and the equations used are the same as in the PDF file included in my earlier post.

One note; when using this technique in the past I've found that it works best to have the rocket go out to Mach 1.5 to Mach 2, and then after burnout start a deceleration back through supersonic/transonic/subsonic Mach numbers. It's harder to get good data when you start the in-flight CD measurements right in the middle of transonic. With your rocket reaching Mach 1.8 this shouldn't be an issue.

One additional note; you'll need to use the RASAero predictions for the Power-On CD and the Power-Off CD to get an estimate for the change in CD between Power-Off and Power-On for the boost phase of the analysis.


Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[jderimig]

Thanks for replying! I believe it's still required in our case since we also want to look at drag at max velocity, which happens before burnout.

After motor burnout you can determine the drag force (or Cd) versus speed as the rocket decellerates. Then armed with this knowledge (drag force versus speed) you can then calculate the thrust curve of your motor. You do not need thrust curve data which likely will have error compared to the actual thrust curve for your flight.

 

[waltr]

Tim from Apogee rockets has written how to do this in one of Apogee's newsletters plus presented to a NAR conference. Both available online.

 

[JoePfeiffer]

Hello, recently, my rocket team launched a class-O rocket at Spaceport America acheiving 29000 ft and Mach 1.8. Right now, we are trying to conduct a post-flight analysis and part of that is comparing the simulated drag curves from CFD, OpenRocket, and RASAero to an estimation of the drag curve from the actual flight. I'm wondering whether using the acceleration data obtained from our onboard accelerometer and plugging it into the equation Thrust - Gravitational Force - Drag = mass*acceleration, given that thrust is similar to the one on thrustcurve.org and the reduction of mass calculated by OpenRocket, would give a reasonable estimate of the drag force? Also, is there another way to go about doing this kind of analysis? Thank you.

There's enough tolerance in a motor's thrust curve you don't want to look at the data under thrust. But yes, you can use the coasting data exactly as you're suggesting.

Something I'd like to see OR do some day is let the user input a table of altitude vs time and use the resulting drag estimate in simulations.

 

[Buckeye]

Something I'd like to see OR do some day is let the user input a table of altitude vs time and use the resulting drag estimate in simulations.

How do you get Cd from altitude? If you are thinking to differentiate twice to get acceleration, that doesn't work very well, especially with user supplied data.

 

[JoePfeiffer]

How do you get Cd from altitude? If you are thinking to differentiate twice to get acceleration, that doesn't work very well, especially with user supplied data.

I'm imagining (so hand waving follows) filtering the data, then fitting a curve, then the second derivative of the curve.

Yes, if you're using a flight computer that will give you acceleration directly it would be more direct.

 

[Buckeye]

Hello, recently, my rocket team launched a class-O rocket at Spaceport America acheiving 29000 ft and Mach 1.8. Right now, we are trying to conduct a post-flight analysis and part of that is comparing the simulated drag curves from CFD,

Can you post the details of your CFD analysis? Thanks.

 

[Buckeye]

I'm imagining (so hand waving follows) filtering the data, then fitting a curve, then the second derivative of the curve.

Yes, if you're using a flight computer that will give you acceleration directly it would be more direct.

OK, yes, FITTING a smooth curve is probably mandatory. Even with accel data, I try to fit a curve from burnout to apogee. However, I can't seem to get CD vs. Mach that looks like post #7 when crunching my Raven accel data. Not sure what I am doing wrong. The flight is not perfectly vertical, so perhaps the lateral accel needs to be factored in as well. I haven't worked through that, yet.

 

[phrajphogate]

Can you post the details of your CFD analysis? Thanks.

Of course but it's not finished. I'll post it when it is.

 

[phrajphogate]

Some examples of backed-out in-flight CD measurements, compared to RASAero predictions, from the RASAero web site are presented below. They can be found on the RASAero web site on the Comparisons with Flight Data Page ( https://www.rasaero.com/comparisons-flight.htm ). Adrian Adamson flew the rockets and backed out the in-flight CD data. The analysis technique and the equations used are the same as in the PDF file included in my earlier post.

One note; when using this technique in the past I've found that it works best to have the rocket go out to Mach 1.5 to Mach 2, and then after burnout start a deceleration back through supersonic/transonic/subsonic Mach numbers. It's harder to get good data when you start the in-flight CD measurements right in the middle of transonic. With your rocket reaching Mach 1.8 this shouldn't be an issue.

One additional note; you'll need to use the RASAero predictions for the Power-On CD and the Power-Off CD to get an estimate for the change in CD between Power-Off and Power-On for the boost phase of the analysis.


Charles E. (Chuck) Rogers
Rogers Aeroscience

After looking into your slides I have two questions.
1. Therorectically, because the flight is not vertical, I would also need to factor in the tilt angle of the rocket by adjusting the gravitational force as well right? (multiplying gravitational force with the cosine of tilt angle)
2. When calculating for Cd, do I need to also consider the compressibility effects on air density? Thank you.

 

[Chuck Rogers]

After looking into your slides I have two questions.
1. Therorectically, because the flight is not vertical, I would also need to factor in the tilt angle of the rocket by adjusting the gravitational force as well right? (multiplying gravitational force with the cosine of tilt angle)
2. When calculating for Cd, do I need to also consider the compressibility effects on air density? Thank you.

Phrajphogate:


<< 1. Therorectically, because the flight is not vertical, I would also need to factor in the tilt angle of the rocket by adjusting the gravitational force as well right? (multiplying gravitational force with the cosine of tilt angle) >>

That is correct. You'll find it's a small correction for a nearly vertical flight, but if you have attitude data (pitch angle data) and/or flightpath angle data (based on vertical velocity and horizontal velocity data), then you can include the correction. If you were really arcing over on a trajectory for a lot of the flight, then you'll definitively need pitch attitude and/or flightpath angle measurements, which it sounds like you have.

I've not included the full set of equations here (I posted material previously prepared for the course), but basically, it's the equation of motion for a rocket.

In our Aerospace Vehicle Test Course rockets, we just had axial accelerometers and barometric altimeters. So, the equations presented were just for a straight-up flight. (This assumption is of course shown in the Charts.) We did do a secondary visual check of the flights, "were they basically straight-up?".

As noted previously, if you have a straight-up or nearly straight-up flight, the analysis technique works well. The question for smaller rockets is how small of an instrumentation package is available that has pitch attitude and/or flightpath angle measurements. It sounds like for your O Motor rocket you had the space for an instrumentation unit with the additional measurements. But for backing out CD for small rockets, you really don't need the additional measurements, you just need to measure axial acceleration, and you need to have a straight-up or nearly straight-up flight.


<< 2. When calculating for Cd, do I need to also consider the compressibility effects on air density? Thank you. >>

No. The Drag Equation using the Drag Coefficient (CD) uses the freestream density, the atmospheric density. (See Chart 3; in hindsight I should have added the subscript "infinity" for the atmospheric terms.) So, it is atmospheric density. You'll of course need to vary the atmospheric density with altitude.



Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[Blast it Tom!]

Now I've been reading this lightly, but it just got real - my 9 yr.old granddaughter just saw the Flightsketch mini data from her MPC "Duck Dodgers" Star Traveler rocket's first flight - and I realized a good teaching moment with her, and as well realized that I had pretty good coasting data to back out a CdA value!

How's that for taking the thread right to the bottom of the rocketry ladder?

 

[xvlss]

Hello, recently, my rocket team launched a class-O rocket at Spaceport America acheiving 29000 ft and Mach 1.8. Right now, we are trying to conduct a post-flight analysis and part of that is comparing the simulated drag curves from CFD, OpenRocket, and RASAero to an estimation of the drag curve from the actual flight. I'm wondering whether using the acceleration data obtained from our onboard accelerometer and plugging it into the equation Thrust - Gravitational Force - Drag = mass*acceleration, given that thrust is similar to the one on thrustcurve.org and the reduction of mass calculated by OpenRocket, would give a reasonable estimate of the drag force? Also, is there another way to go about doing this kind of analysis? Thank you.

Hi, I am doing a CFD analysis in ansys of a model rocket, can you give some insights.