RockSim 9 Simulation - Nose Cone Drag

I am working to finalize my design for my rocket "Warp Test #1" and I have a question about my simulations. Originally I designed it with an Ogive nose cone which I prefer aesthetically on this rocket. I switched to Von Karman primarily because they seem to be more available in fiberglass and because I thought it would be easier to blend into my slightly larger pre-glassed PML tubing. Both of these are issues I have reconsidered after finder Performance Rocketry's 5:1 filament wound ogive, so I decided to rerun my simulations which produced confusing results.

I am of the understanding that a Von Karman nose cone is much better for supersonic flights, however on my maximum altitude simulation the ogive reduced my altitude by only 10 feet out of nearly 17,000. On my max velocity flight the ogive nose cone only slowed the rocket by 2ft/s of 2025ft/s. This means the ogive reduces performance by less than 1/100th or a percent. This seems to indicate that Rocksim is only taking the wetted area into account and not the actual nose cone performance. If this is the case, how much of a difference does the Von Karman really make over the ogive? How accurate are the RockSim results in the supersonic range?

In the transonic range (~ M 0.83 to 1.3) Von Karman's will perform better than just about any other nose cone. But, the caveat is this: how much does the nose cone weigh compared to others. If the NC's weigh the same or the ogive weighs more, the VK wins. If the ogive is lighter than the VK, there is a break even point where the ogive will perform better WRT velocity and altitude.

Regarding simulations. Rocksim is a great product for what it does. But others have noted that its accuracy is diminished in the transonic and supersonic flight regime. A sim that works well at higher velocities is RASaero.

Greg

GregGleason said:

Regarding simulations. Rocksim is a great product for what it does. But others have noted that its accuracy is diminished in the transonic and supersonic flight regime. A sim that works well at higher velocities is RASaero.

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I downloaded RASaero and roughly modeled the same rocket there (minus the nose fins and Dark Star style split fin) and while the rocket reached higher speeds and altitudes the difference between the two nose cones was virtually the same. At subsonic and supersonic speed the CDs were almost identical and in transition the ogive rockets CD was 3% higher. I tried the same experiment without the larger diameter fin can and the result was the same despite the overall CDs being lower.

Hmmm .... well, that is not what I expected. Perhaps there is something else going on.

Would be an interesting "real world" test is to have two nose cones of the same mass to see if the flight data matches the sims.

Greg

You might just be sitting at a "corner case." Try different rocket weights and diameters with and without the VC nosecone to compare. I would stay in RasAero as it has proved to be more accurate for me.

RasAero is the way to go from what I have read and concur with what was said about Rocksim 9. Of course you could try a real world test if you have one of each kind of nosecone. The only problem there is one would only be able to detect gross changes.
Even if one were to use the same commercial motor, there may be a little difference between the individual impulses. Best of luck. Kurt

Ravenex:

Nose cone wave drag is a function of the shape of the nose cone and the L/D of the nose cone. Nose cone wave drag is a strong function of L/D, try L/D's of 3, 4, and 5, and you'll see the difference.

For a given L/D, the Von-Karman ogive shape has a lower wave drag than the tangent ogive. But with a nose cone L/D of 5, you'll have a low nose cone wave drag, so the difference in CD between the two shapes will be less apparent.

Note that even at an L/D of 5, with the tangent ogive the CD is still 3% higher at transonic (Mach 1.05) compared to the Von-Karman ogive.

In the past most rocketeers used L/D = 4 nose cones, the thought was the decrease in nose cone wave drag was not worth the increase in weight for the longer nose cone. (An earlier comment noted you have to consider the weight effect.) Lately most rocketeers are using L/D = 5 nose cones, going for the lower wave drag.

If fabricating your own nose cone, one would have to weigh whether the lower wave drag at a given L/D of the Von-Karman ogive was worth the increased complexity of design and fabrication.

Once you're up to max Mach numbers of Mach 5 or so, the pure cone becomes the best shape. You can try different rockets on RASAero and see for yourself. Also some rockets reaching just over Mach 3 have gone to conic nose cones for ease of construction when the rocketeer is making his own nose cone.


Chuck Rogers
Rogers Aeroscience

Chuck Rogers said:

Note that even at an L/D of 5, with the tangent ogive the CD is still 3% higher at transonic (Mach 1.05) compared to the Von-Karman ogive.

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This makes sense, compared to the 25% increase in drag due to my larger diameter fin can the 3% difference in nose cones isn't a very significant portion of my overall drag, which explains my simulations. From some of the discussions I was under the impression the CD of the Von Karman had a much bigger difference from the ogive.

Ravenex said:

From some of the discussions I was under the impression the CD of the Von Karman had a much bigger difference from the ogive.

Click to expand...

It does for a nose cone L/D of 3. But much less so for a nose cone L/D of 5. With the nose cone L/D of 5, both the Von-Karman and tangent ogive nose cones will have low wave drag, so the reduction in drag in terms of the total rocket drag will be small. "Small" being relative though, as was once posted on this forum, "a little performance improvement here, a little performance improvement there, pretty soon you have the (fill in the blank) motor altitude record".

It's important to include the fin canister wave drag, which RASAero includes. The Frank Kosdon Full Metal Jacket rocket is a good example where a not very thick fin canister (4.25 in outside diameter on a 4.0 in diameter rocket), but with a steep fin canister shoulder; the fin canister had approximately the same wave drag as the L/D = 4 tangent ogive nose cone. So it's important to avoid a steep fin canister shoulder.


Chuck Rogers
Rogers Aeroscience

The fin can on this rocket is 3" on a 2" rocket, the transition is conical. Taken to a point the conical would have a length of 7.5", to much longer and it starts to not look like I wanted it to.

One of the weaknesses of Rocksim is the simulation of nose comes. It is essentially based on surface area of the nosecone. So, if you really want to have Rocksim simulate a Von Karmen nosecone accurately for transonic flights you need to substitute a conical nosecone of the same length, mass, and center of gravity. This works well.


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