Major differences between RasAero and OpenRocket

MasonH said:

Me and AKSrockets have been corroborating all afternoon on an N1100 minimum diameter design. Right now, we aren't sure if it is just for giggles, or if we might do this for real. Either way, we ran through about a dozen fin designs and finally landed on one we can both agree on, I think.

But there is a catch.

OpenRocket says the rocket should have an apogee of almost 51,000' AGL. RasAero is saying the rocket should have an apogee closer to 48,000' AGL. That is fairly reasonable I think, as it is only about a 6% difference either way.

The BIGGER problem is that the rocket sims to a max speed of roughly M2.1. At this speed, OR says that the rocket will have a CP about 53" from the tip of the cone, and have a stability of 0.272 cal. RASAero, on the other hand, says that M2.1, the rocket should have a CP about 60.25" from the nosecone and stability of over 2 cal. Thats a 7.25" difference, and a 1.75 cal difference in stability.

I am more inclined to believe RasAero in both areas of the simulations (apogee and stability), but that is still a HUGE step. Are we perhaps missing a step?

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MasonH said:

The BIGGER problem is that the rocket sims to a max speed of roughly M2.1. At this speed, OR says that the rocket will have a CP about 53" from the tip of the cone, and have a stability of 0.272 cal. RASAero, on the other hand, says that M2.1, the rocket should have a CP about 60.25" from the nosecone and stability of over 2 cal. Thats a 7.25" difference, and a 1.75 cal difference in stability.

I am more inclined to believe RasAero in both areas of the simulations (apogee and stability), but that is still a HUGE step. Are we perhaps missing a step?

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Chuck Rogers said:

The rocket CP moves aft from its subsonic value from Mach 0.9 to Mach 1.05, and then moves forward at increasing Mach numbers until its back to its subsonic value between Mach 2 and Mach 3. Thus generally, if you have a good subsonic CP (and you can just use Barrowman), the rocket CP shouldn't be farther forward than that CP until Mach 2-3. The supersonic CP methods for the rocket components used by RASAero determine the total rocket CP and hence this cross-over point.

An example is the Aerobee sounding rocket CP plot on the RASAero web site;
https://www.rasaero.com/img/Aerobee_150A_Sustainerc.png

where the cross-over of the wind tunnel data CP with the Barrowman subsonic CP is about Mach 2.75.

I've also looked up some Arcas wind tunnel data, and the cross-over for the Arcas is about Mach 2.

So with a max Mach number of Mach 2, you should be fine.

But, in my Class 3 rocket and over 50K 2-stage rocket analysis work, as others have noted, I'm starting to see some really short fin spans on some minimum diameter rockets. (Fin span measured from the root of the fin to the tip of the fin.) The Kosdon Full Metal Jacket rockets had a fin span to body diameter ratio of 1.1 (they flew fine), and the Arcas had a fin span to body diameter ratio of 1.0 (it flew fine although with canted fins for spin). So I'm not sure the root area of the fins being in disturbed flow from the long body at an angle of attack is that severe (an effect I'll be researching), although for other reasons the minimum diameter N motor rockets may have been hitting higher angles of attack making the effect more severe.

I would recommend that fin span to body diameter ratios not be under 1.0.

I also recommend that using the supersonic CP predictions in RASAero, that the maximum Mach number of the rocket be determined, and the rocket have at least 2.0 caliber stability margin at that most forward CP point. Why 2.0 caliber instead of 1.0 caliber? Various uncertainties could use up 0.25 caliber, 0.5 caliber of your stability. You want to make sure you have a solid 1.0 caliber of stability left after the various uncertainties, i.e., margin.


Chuck Rogers
Rogers Aeroscience

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bobkrech said:

I have a few more questions for you.

There's a good report on the Arcus. www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0437681 I think the Arcus is a fair comparison to Chris Cotners's Bare Necessities but there are some significant differences that might explain his and others in-flight instabilities using a Pro98 N5800.

On page 12 there is a mention of a pitch roll coupling at a spin rate around 5 Hz that was circumvented by increasing the fin cant to increase the spin rate to 15-20 Hz. I find this interesting as the Arcus is about the same size as Bare Necessities.

A difference between the Arcus and Bare Necessities is that the Arcus had 4 fins and a boat tail whereas Bare Necessities had 3 fins and no boat tail. The fin height of the Arcus is 0.94 diameters. Am I correct in my believe that you need to increase the fin height to (4/3)*.94=1.25 diameters for 3 rocket fins to have the same aerodynamic effect as the 4 fins on the Arcus? Also will the boat tail move/hold the CP aft compared with no boat tail?

The Arcus motor is a 42kNs end-burner that consumes it's propellant in 29 seconds with an average thrust of 1445 N. The N5800 has 4 times the average thrust of the Arcus but a short 3.5 second burn time with about half the Arcus total impulse. Will the higher thrust of the N5800 enhance a pitch-roll coupling instability compared to a lower thrust engine?

Bob

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Chuck Rogers said:

Bob:

Great report on the Arcas! I'm using NASA wind tunnel reports, the models used in the wind tunnel tests had a fin span to diameter ratio of 1.0 (measured from the body tube, not the top of the fillet structure), so I'm not sure where the discrepancy comes from.

3 fin versus 4 fin, boattail present or not present, RASAero takes all this into account. Although the boattail supersonic CNalpha and CP models are not as accurate as other models in RASAero.

The question is the effect of the fin span. You could lengthen the chord of the fins for the 3-fin rocket, until the CP matched the 4 fin rocket, with both having the same fin span. The question is, for short fin spans (low fin span to diameter ratios), is the fin effectiveness reduced? I plan to look further at CP wind tunnel data with angle of attack, but the primary effect is viscous crossflow on the body, which RASAero does include in the supersonic methods, and in the Rogers Modified Barrowman Method for subsonic CP.

The pitch frequency/roll frequency coupling is an issue for spinning sounding rockets, the two frequencies should be spread apart from each other as much as possible. If the fins were misaligned, a low (or maybe not so low) roll frequency might have coupled with a low pitch frequency. Again, this is normally an issue only when you are purposely spinning a sounding rocket.


Chuck Rogers
Rogers Aeroscience

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Chuck Rogers said:

Bob:

Great report on the Arcas! I'm using NASA wind tunnel reports, the models used in the wind tunnel tests had a fin span to diameter ratio of 1.0 (measured from the body tube, not the top of the fillet structure), so I'm not sure where the discrepancy comes from.

3 fin versus 4 fin, boattail present or not present, RASAero takes all this into account. Although the boattail supersonic CNalpha and CP models are not as accurate as other models in RASAero.

The question is the effect of the fin span. You could lengthen the chord of the fins for the 3-fin rocket, until the CP matched the 4 fin rocket, with both having the same fin span. The question is, for short fin spans (low fin span to diameter ratios), is the fin effectiveness reduced? I plan to look further at CP wind tunnel data with angle of attack, but the primary effect is viscous crossflow on the body, which RASAero does include in the supersonic methods, and in the Rogers Modified Barrowman Method for subsonic CP.

The pitch frequency/roll frequency coupling is an issue for spinning sounding rockets, the two frequencies should be spread apart from each other as much as possible. If the fins were misaligned, a low (or maybe not so low) roll frequency might have coupled with a low pitch frequency. Again, this is normally an issue only when you are purposely spinning a sounding rocket.


Chuck Rogers
Rogers Aeroscience

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bobkrech said:

Thanks for the response Chuck,

All unguided sounding rockets are required by DoD and NASA range safety rules to have canted spin fins to minimize dispersion and keep the missiles pointed in the desired direction by averaging out the thrust axis/aerodynamic axis/CG misalignments. The Arcas has such a poor T/W ratio that it needs a very long launcher and a high spin rate to maintain a stable trajectory with minimum dispersion and avoid pitch-roll coupling instability.

The 0-angle of attack boundary layer at the fins will be about 0.8" +/- in the Mach 1 to Mach 2 flight regime. If there is any pitching due to thrust axis misalignment or CG asymmetry, or spinning due to aerodynamic asymmetry in an "unspun" unguided missile, the boundary layer thickness at the fins will increase, and as a SWAG increase pitch-roll coupling rapidly when the fin number and height is minimized to reduce drag leading to the destruction of the rocket. I think caused the recent spat of N5800 flight failures at Black Rock.

I don't think RASAero has asymmetry dynamics leading to pitch-roll coupling programed in, so I don't know how or if it can predict dynamic stability which is what this problem is all about.

Thanks in advance for your comments.

Bob

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Chuck Rogers said:

Bob:

RASAero doesn't include pitch-roll coupling or asymmetry dynamics. While I believe study of wind tunnel data would be required to verify whether this boundary layer effect, or other effects on the fins, is present, since RASAero doesn't include pitch-roll coupling or asymmetry dynamics, any developed model couldn't be added to RASAero at this time.

But you had hit on something in one of your original posts:

<< I'm not sure I believe either one when it comes to stability, because I doubt either one is sophisticated enough to calculate the dynamic stability which is the only one that counts when you're near the stability limits as evidenced by the N5800 flights at Black Rock. >>

And putting in context the original rocketeers question; he wasn't sure what his supersonic CP was going to be, and in more general terms, he wanted to make sure his N motor rocket would work.

The answer; don't be near stability limits.

These pitch-roll coupling and asymmetry dynamics effects may be out there. But high fineness ratio High Power rockets have successfully flown at Mach 3; Proteus 6, Qu8K, Don't Debate This on an N5800. And a rocket similar to the rocket the rocketeer was asking the CP question on, flown on an N1000, detailed in this thread:
https://www.rocketryforum.com/showthread.php?57262-N-altitude-attempt-and-V2-0-quot-build-quot

I believe RASAero has the most accurate supersonic CP. So I'd use the RASAero supersonic CP, and make sure you always have at least 2.0 calibers stability.

Is a 0.8 in thick boundary layer reducing the effectiveness of some of the fins (would need to be confirmed by wind tunnel data), exciting various asymmetry dynamics and potentially pitch-roll coupling? The following rockets had the following fin span to diameter ratios; Qu8K - 0.83 (an outlier), but Proteus 6 - 1.5, the recent N1000 flight detailed on a Forum thread similar to the rocketeers rocket - 1.25. 0.8 in of boundary layer on your 4 in span fin? Maybe your fin span should be 5 in to 6 in? (Fin span to diameter ratio of 1.25 to 1.5.)

So answering the rocketeers original question; use the RASAero supersonic CP, always have at least 2.0 calibers stability at all Mach numbers, have a fin span to diameter ratio of at least 1.0, but it's probably better to have 1.25 to 1.5. And your rocket should fly fine (at least from a stability standpoint). You won't be near stability limits, and so this potential pitch-roll coupling won't put you over the edge.

To further explore the potential pitch-roll coupling effect, a comparison would need to be made of the pitch frequency and the potential roll frequency one could get from fin misalignment. Once those frequencies are compared, then one would get an idea of whether this potential effect would need further research.


Chuck Rogers
Rogers Aeroscience

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