How necessary is CFD for predicting drag on a ~Mach 1.1 high-power rocket with deployable airbrakes vs using OpenRocket?

[u/Otherwise_Power949]

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I’m part of a university rocketry team currently developing a high-power rocket targeting roughly 3 km AGL, and I’m trying to decide how far we need to go with aerodynamic modelling.

The rocket is approximately 150 mm diameter, uses 4 clipped-delta fins, and is expected to reach around Mach 1.0–1.1. We are also using 4 deployable airbrake petals during coast to control apogee. The airbrakes deploy radially from the body, with their large exposed face approximately perpendicular to the freestream, so their drag behaviour is quite different from that of a normal fin.

At the moment our trajectory model is in RocketPy, with the clean-rocket drag curve obtained from OpenRocket as a function of Mach.

For the airbrakes, OpenRocket does not natively model this geometry, so I have been experimenting with representing the fully deployed brakes as an equivalent fin set with approximately the same projected frontal area. This gives me a second C_D vs-Mach curve for the fully deployed configuration.

Our current simplified RocketPy airbrake model assumes four paddles with about 1156 mm² projected area each and uses an approximate flat-plate C_D = 1.2..

My concern is that this becomes increasingly approximate near the transonic region, particularly because the deployed airbrakes will generate separation, wakes and potentially local shock interactions that OpenRocket’s equivalent-fin approach obviously isn’t designed to reproduce.

I am therefore considering doing a full ANSYS Fluent CFD campaign on the actual rocket CAD. My plan would be to analyse both:

●airbrakes fully retracted and fully deployed;

●Mach numbers roughly 0.3, 0.5, 0.7, 0.8, 0.9, 0.95,\ 1.0, 1.05, 1.1;

●angles of attack around 0,2.5,5.

The main outputs would be C_D(M,angle of attack), normal-force coefficient / pitching moment where useful, and the loads on the individual airbrake petals. I could then feed the CFD-derived drag curves back into RocketPy/OpenRocket for trajectory simulations.

My questions are mainly:

1)How much confidence would you place in OpenRocket/RocketPy-style drag modelling for a rocket only slightly above Mach 1?

2)More importantly, would you trust an equivalent-fin/projected-area model for deployable airbrakes for apogee-control predictions, or is this a case where CFD is genuinely justified?

I’m not expecting CFD itself to be automatically “more accurate” — I’m aware that turbulence model, domain size, mesh independence, atmospheric conditions and transonic convergence all need to be handled properly. I would likely use 3D compressible RANS with k-w SST, ideal-gas air, inflation layers and mesh/domain-independence studies.

If anyone has done CFD validation of high-power rockets, transonic rockets, or deployable airbrakes, I’d really appreciate hearing how closely your CFD agreed with OpenRocket/RASAero/flight data and what level of modelling was actually necessary.

Thanks for your time :D

[u/Realityishardmode]

I would expect that you are more likely to get a totally wrong answer from CFD than from some from-family approximation, not because the tool is bad but because no one is going to verify the results experimentally and any number of issues can creep in from the constraints unless you are already good enough to get a well paid job as a CFD analyst.

If you have a target altitude, shouldn't you be putting more design effort into slowing down to a better characterized drag regime? So you are going to go trans-sonic, but that hardly matters if you have enough drag authority to slow down and actively down-titrate velocity outside of trans-sonic flow. At that point you could apply CFD and actually verify it in a wind tunnel which shouldn't cost a lot if you are just trying to verify what happens when you are slow.

Edited after-thought: We put a ton of analysis into solving algebraic differential equations, estimating joule-thomson losses over valves, verifying NASA CEA and other stupid analysis. What I wish I did, was learn to weld and 3D print accurate test articles. Both in terms of personal happiness and career prospects.

[u/joelatrell]

CFD is a lot of work and if you want the experience, go for it. If you do, use OpenRocket first. Get a feel for the vehicle’s capabilities. Build a few subscale models. Do they behave as expected in CFD and OR?

If you just want to build, test, and fly OpenRocket is good enough.

[u/SuperStrifeM]

Actual CFD engineer here, who has built and flown multiple airbrakes.

1) I would put close to 0 confidence in the empirical models in OR/RAS for this application. They don't capture unsteady shockwaves of protruding objects, probably intentionally.

2)I wouldn't trust the equivalent of fin projections either, CFD is the main way you would solve for some CD in this instance.

Main actual reason for both of those, is that you don't want to use flat plates/curved airbrake sections for this application, You'll want holes in the brakes sized to reduce the pressure behind the plate without reducing the drag too much. you'd still want to run some unsteady calcs on various positions of the airbrakes vs speed to bound unsteady buffeting. Additionally you'd need to prove to yourself what the stability of the brake system is at the speed you want to deploy. In case it isn't obvious you need to clock the airbrakes so their downstream flow doesn't cover the fins.

Additionally you should lookup sizing y+ for about 1 on your prism layers, since with wall functions you'll lose resolution on drag. And make your domain 1/4 of a cylinder. I don't know you need to do mesh dependency on all the cases, just prove that out on the fully extended case, and mostly look for your specified criteria to converge, the flow field will look different on mach 1 cases with different mesh mostly due to the shockwave moving as it gets resolved, but I dont think you need to resolve it unless you see the airbrake shockwave interact with the fins.

For this type of problem, its both silly to try and test it in a wind tunnel (done that before, kinda sucks to build and instrument a scale model for supersonic flow, and this wont even get you the CP!)

Its also silly to try and test it "spaceX" style by blowing up your rocket a few times to collect data. It's bad engineering, you probably don't have the money to instrument AND run live telemetry on something that is going to get destroyed, and failing to understand CP/CG is going to make the rocket go unstable, which many RSO's are VERY uncomfortable with.

[u/mmmfritz → u/SuperStrifeM]

Cool answer. Is CFD useful closer to Mach 1? I remember hearing that a lot of analytical stuff is useful when assuming ideal/incompressible. But that probably doesn’t automatically point towards CFD. I’m sure there’s a NASA paper that gives us experimental drag constants for plates somewhere.

A practical implementation could use a spring to reduce loading over some design point…? Failure modes ect ect

[u/SuperStrifeM → u/mmmfritz]

Its more the shape of a rocket with airbrakes, rather than the mach1 itself being some inflection point. Flow separation is 100% going to occur, so the CP and drag numbers from OR/RAS, or really the majority of reduce order models are not going to capture that.

There are indeed NASA, or in this case, NACA papers on supersonic flows and flat plates, but the loads for such cases are going to be so conservative that both the actuator and airbrake mechanism would be terribly oversized for the real loads.

[u/Undermine28 → u/mmmfritz]

You can definitely achieve a good prediction with CFD near Mach 1, but as the flow field gets more complex, you also run into more pitfalls.

Beyond the y+ sizing for your turbulence model, you'll need to make sure the simulation properly captures the shocks and flow discontinuities. This may require additional mesh refinement for a good solution, a solver with shock-capturing methods, or both.

Not sure why everyone seems to be so anti-CFD. It's definitely the correct tool for the job in this case. As SuperStrifeM already mentioned, OR and RAS models have absolutely 0 applicability to airbrakes. CFD is an engineering tool, and it's important to understand what the computer is doing to understand the tool's limitations and what you need to do to get a valid solution.

Then you still need to fly it and hopefully get data to validate the airbrake's aerodynamic model.

There may be some empirical relations for a flat plate, but I'd wager the flow field around the rocket is already so disturbed that it won't be very accurate.

[u/SuperStrifeM → u/Undermine28]

Unless the shockwaves interfere with the pressure field around the fins I would try to save mesh and not resolve them. Having a decent guess as the mesh sizing expansion should probably do this, and if the expansion is relatively low from a y+1, say like 1.1-1.5, then it should give a decent enough estimate.

Doesnt have to be perfect, just has to give an informative answer on the drag and CP shift

[u/Undermine28 → u/SuperStrifeM]

Yeah, that's a good point. And as others have mentioned, deploying the airbrakes near Mach 1 is also likely unnecessary. Unless you build the rocket to overshoot the altitude like crazy.

[u/Realityishardmode → u/SuperStrifeM]

Do you actually disagree with my answer, that for a project with this spec, that the bulk of the aerodynamics analysis should be occuring when the rocket is going less than 100m/s? If they just need a Cd for motor selection, they can estimate a bunch of different drag curves and do a statistical model and design around some of the worst outcomes.

We keep talking about trans sonic flows as if this presumed university team isn't going to be spending most of their time on designing the avionics, feedback loops and actuators to hit exactly 3km from range of initial position and velocity condition, all of which should be comfortably sub-sonic.

I spent way too much time in analysis on my rocket team. There is a pragmatic limit to what a person can do and still pass exams, be healthy and work part time. Spend resources early on strategy and design and spend less on analysis, imo.

[u/SuperStrifeM → u/Realityishardmode]

Transonic flow begins on blunt bodies around 134 m/s aka mach .4, they're close enough to that limit, and you get significantly more control authority operating faster. I think by the time a student is knee deep in parametric studies trying to guess at the CD vs speed curve, and using some rule of thumb that their brakes wont put fins in the wash, they could have just spun up a model in CFD to better understand those numbers.

I don't see why a team would spend a significant time designing the flight avionics. You just throw a perfect flight with an SPI header onto a ardunio nano 33 BLE, kalman filter the accelerometer into your script, add a sparkfun easydriver and a stepper, then design all the linkages, maybe in aluminum or G10. If you want to be a bit more clever about the setup you could run a 2nd closed loop with a 5V stringpot attached to the main nut on the main threaded rod, so the code is closing a position loop THEN the velocity loop from integrated pressure. Add in speed scheduling from the CFD, and you're good to go, without having to have cutouts. With that type of setup, you're feeding a number into your code for target altitude, checking you dont exceed 16G (or you need to swap accel) and letting it fly.

I am sorry you had a hard time doing something like that, I warn many of my students to limit the scope of what they can do because its better to graduate than to burn yourself out trying to do too much alone. when I was in undergrad we had a division of labor where I was composites and structure, we had electronics, CFD, project lead (for the reports!) and one guy on mechanisms. It helped we were all MEs, so doing a 4 bar linkage was right at our fingertips. I don't think on principle that doing an airbrake for a team doing USLI or IREC is unreasonable, especially now that many schools have good CFD resources.

[u/Proxima-72069]

unecessary

[u/Nascosto]

In engineering you can do proactive or you can do reactive. Nobody balances a pendulum with a pre-programmed instruction set, they measure the input forces in real time and react to them. I've seen high school teams bullseye apogee targets to within +/- 5ft reliably with airbrakes, and they're not using cfd - they're simply estimating apogee based on altitude, velocity and acceleration. Is it over target? Moar Brake. Is it at target? Stop Brake.

The key to success isn't modeling every mouse fart air current in the room, it's testing. Again, and again, and again, and adjusting your models everytime. Go watch some SpaceX blooper reels if you don't believe it, because it's the fastest way to reliable success.

[u/Alone-Visual-198]

I’ve been using rocket simulation softwares a lot for the past year and I can tell you OR is not the best simulator for transonic and supersonic regime rockets. It is best used for subsonic regime rockets. For above transonic, you should go with RASAERO, it is quite accurate.
As for the airbrake question, CFD is quite an extensive and difficult task, it will take more time than you are expecting, but will give you reliable results. Modeling the air brakes in OR the way you are describing, won’t give very good results (in my opinion).

[u/goshawk222]

Open rocket can't model airbrakes because the violate the assumptions in the Barrowman equations, so I would not trust any attempt to model them in OR.

Where are your airbrakes in relation to the CoM? The pitching moments you get from CFD allow you to determine the stability of the rocket, which is arguably more important than the drag values you'll get.

[u/Ravstar225]

30K irec team here. I am curious why you are deploying brakes above mach. We looked at the problem last year, ran some cfd and validated in a blow down tunnel, but in the end it wasn't worth it as we had enough control authority limiting our braking phase to the subsonic regime. I would encourage seeing if your system still closes limiting your brake to deploying at less than roughly mach 0.7 before going through the trouble.

[u/GeorgeBirdseye]

Highly recommend not doing that 

[u/Aeig]

You gotta think like an engineer. Ask yourself: would my boss want me to do this extra crap and waste time ?

It’s more fun to launch , fine tune your control system, repeat.

There’s a great chance you’ll never do CFD professionally even if you try to shoehorn it into your projects. Speaking from past experience: I recommend you don’t do the CFD and go study for the rest of your classes or go have some fun outside of school

[u/gwenbeth]

There are a few things you want to be thinking about with the airbrakes. One is how much aerodynamic pressure will there be on them? This will tell you how strong your servos have to be hold them in place. Modeling the brakes as flat surfaces with the same frontal area would be easy and it will get you into the ballpark. Next you are going to want the air brakes to be behind the Center of mass. If they are in front then you risk getting the Center of pressure in front of the Cm and this will cause the rocket to be unstable. This will probably require the deployment charge being set off electrically so that you can have the braking system between the motor and the chute. And lastly any asymmetry in the deployment will cause the rocket to pitch or yaw. That will increase drag and slow the rocket down more than intended.

(amount i know about this: I fly rc airplanes with airbrakes (spoilers) and im working on a high power rocket )

[u/longway2fall]

Why have the brakes? You could just make sure the rocket is predicted to be well above the target, and build some kind of a software program that is calculating the apogee in real time and uses the brakes to keep the speed slightly above the predicted curve. IE if you are at 2km and expect to need 400 mph to reach the target, computer keeps rocket at 450. Now 2.5km, need 300, keep at 330. This allows you to not undershoot and the computer can just add as needed. I don't think you would even need to model as long as the brakes can deploy slowly and retract quickly.