Aerodynamic questions | Rocket body discussion | Forum

Hi,

I'll answer the questions in parts, here's some pointers for the nose cone design.  In general I would suggest looking at existing sounding rocket designs for ideas.  A lot of articles on sounding rockets and measurements of them are available on the NASA technical reports server (http://ntrs.nasa.gov/).

As in everything, there is no one single optimal nose cone shape for everything.  Since OHKLA is flying so high, I assume most of the distance will be in the supersonic region, and the nose cone should be optimized for supersonic (and maybe transonic) flight.

There's an article on Wikipedia on nose cone design giving a good overview on the topic:  http://en.wikipedia.org/wiki/N…..e_design  However, I'm not totally convinced by its diagram comparing the drag of the shapes, as I've encountered some seemingly conflicting data while developing OpenRocket.  I have the document from which the diagram has been taken, but haven't been able to verify the source used therein (Chinn, S.S.; Missile Configuration Design, McGraw-Hill Book Co., Inc., New York, 1961).

The Haack series is produced by minimizing the wave drag caused by the nose, and as a special case the von Karman nose cone optimizes the drag for a fixed length and diameter.  This could therefore be a good option for the nose shape.  Many sounding rockets flying at high supersonic velocities have conical nose cones, and I believe at these speeds it is good enough and easier to manufacture.

The tip of the nose does not need to be sharp.  In fact, with a fixed length a nose cone with a rounded head produces less drag.  The rounding should be about 15% of the entire nose diameter.

The shape of the nose has only marginal effect on the stability.  In the Barrowman equations the pitching moment coefficient derivative CNa is the same for all nose cone shapes, and only the CP position changes slightly.  Since the nose overall has a much smaller effect than the fins, the effect is negligible.

More later on.  Now back to getting our hybrid rocket ready for tomorrow's launch.  :)

Body shape:

The length/shape may have a significant impact on a rocket's performance.  If I remember correctly, increasing a rocket's fineness ratio (length-to-diameter ratio) generally decreases the wave drag to some extent, but on the other hand more overall body area translates into increased friction drag.  Again I'd refer to examining existing sounding rocket designs for guidance.  Many sounding rockets also have various configurations, often with the length a variable parameter, and experimental data may be available for multiple configurations.

The body itself has little direct impact on the stability, but since it changes the positions of the fins and affects the CG, it may overall affect the stability a lot.  Extremely long and slender rockets may also lose a significant amount of stability at even a few degrees angle of attack.  This shouldn't be an issue at high velocities, but may be significant at launch.  "Normal looking" rockets shouldn't have much of this problem.

One thing that can reduce base drag significantly is a boattail at the end of the rocket.  This both reduces the base area and also directs the airflow into the low-pressure area generated at the base.  OpenRocket does not currently simulate this totally accurately; it only takes into account the reduced base area but not the directed flow.  For more information you can refer to the bibliography in my thesis in the section about base drag.

One general comment about stability: While a statically stable rocket flies straight in the atmosphere, the rocket may start rolling around any axis after the atmosphere has become thin enough not to produce significant correction forces (especially near apogee where the rocket's speed is low).  This needs to be taken into account when designing payloads, apogee detectors etc.

A good place to discuss advanced rocketry issues such as these is the arocket mailing list and browsing its archives.  There are a lot of professionals there as well, including several X-Prize and Lunar Landing Challenge contestanst (for example John Carmack from Armadillo Aerospace).  They've also got some info about an open-source rocket design, though it seems it hasn't been updated for several years.  http://www.arocketry.net/

Our hybrid launch went extremely well, for the first time all the parachute deployment systems worked 100% correctly.  The weather forecast had promised clouds and earlier even rain, but it was a hot and sunny day.  The motor is a standard HyperTek motor, with plastic fuel and N2O oxidizer.  The flight altitude was 650m, which was lower than expected probably due to the warm N2O.  (The N2O temperature can have a drastic effect on the motor performance and thrust curve shape.)

Cheers,

    Sampo

Fin shape:

Regarding the planform fin shape, traditional trapezoidal fins should be good enough for everything.  There probably isn't any real reason to use other fin shapes except aesthetics.  Their shape is then again a matter of debate.  Their shape, size and positioning have the greatest effect on the stability.  There may also be some surprising factors involved, such as stability decreasing when the fin size is increased due to the CG moving rearwards or the CP of the fins moving at the same time.

The fins must be very tough to withstand the transonic forces.  I don't remember where it was, but I've seen a video where composite fins flutter like flags when the rocket climbs through and slows down over the transonic range.  Afterwards the fins seemed perfectly normal, but further investigation showed that the internal material structure had been completely destroyed.

The fin profile should also be shaped appropriately.  The typical advice is to make a symmetric airfoil shape, with a rounded front and tapered trail, however it seems that many supersonic sounding rockets use a wedge-like front and trailing edge.  I don't think the exact shape has much effect, as long as its about right.

Spin stabilization:

I wouldn't really recommend spin stabilization.  It may increase drag (possibly significantly – I'm unsure about that) and I'm dubious about how they would affect the supersonic flight.  It may also pose problems to the payload and structural integrity.  I also don't recall seeing many sounding rockets with canted fins except in wind tunnel tests.

The only reason I can think of using spin stabilization is, as you mentioned, to prevent pitching after exiting the atmosphere.  This might work, but it might even make things worse if the rocket starts precessing.

As a side note, while spinning may make an unstable rocket fly almost straight, it does not actually remove the instability.  Instead it just slows down the increase of instability so that it doesn't have time to affect the flight.  This is analyzed in more depth in Mandell, Caporaso, Bengan: Topics in Advanced Model Rocketry (quite an old book, but one of the most comprehensive analyses of many topics of interest for model rocketeers).

Hope these help along.  Feel free to ask more info if needed.

- Sampo

Hi,

I found the video on YouTube:  http://www.youtube.com/watch?v…..zQbO1D9d_o

There it states that the fins were aluminum, which would definitely be warped if it really fluttered.  After checking the video again I'm pretty convinsed it's a combination of the fins oscillating and the video scanning frequency.  So the fins don't actually wave, but still oscillate pretty violently.  I'm not sure where I read about the damage to composite fins.

More info can be found by searching for fin flutter.  On one page the recommended material is "fiberglass, fiberglass
coated plywood or graphite/epoxy composite for stiffness to resist
flutter" (http://mcfisher.0catch.com/oth…../mach1.htm).  I haven't got any personal experience in this area.

-Sampo