APE - Minimum diameter Loki 54mm M project

Attached below are RASAero II predictions for Subsonic, Transonic, and Supersonic Center of Pressure (CP) compared to wind tunnel data for two configurations of the ARCAS sounding rocket (ARCAS Short and ARCAS Long). The Supersonic wind tunnel data is up to Mach 4.63, approaching Hypersonic (Mach 5). The wind tunnel data is from NASA TN D-4013 and TN D-4014.

Slides 2-4 present the wind tunnel model configurations, and Slide 5-6 show the two ARCAS configurations entered into RASAero II.

The RASAero II CP (and CD) predictions are compared to the Subsonic, Transonic, and low Supersonic wind tunnel data for the ARCAS Short and ARCAS Long configurations on Slides 8 and 9. With the boattail only on part of the fin root, the exact way to implement the Barrowman Method is somewhat up to debate. The method used in RASAero II is to calculate the volume of the cylinder and the part of the boattail that are under the fin root, and then calculate the diameter of a cylinder of the same length that has the same volume. The diameter of this cylinder is now the diameter of the rocket body under the fins, and the fins are projected toward the centerline of the rocket until they intersect that cylinder. Thus the RASAero II Barrowman Method results are labeled on the graphs on Slide 8 and 9 as “RASAero II Implementation of the Barrowman Method”.

Note on Slides 8 and 9 that the Rogers Modified Barrowman Method and the Barrowman Method (as implemented in RASAero II) give similar results, very similar results in the case of the ARCAS Long configuration. Generally the Rogers Modified Barrowman Method is more accurate than the Barrowman Method, but in many cases the Rogers Modified Barrowman and Barrowman Methods will produce similar results, as shown here. This is because Barrowman left off additional normal force at the nose and the tail, but since he missed additional normal force at both the nose and tail, more accurate methods will in many cases produce surprisingly small changes in the CP. For these particular two cases (ARCAS Short and ARCAS Long), the Barrowman Method was more accurate, although the two predictions were close, very close for the ARCAS Long configuration. Both methods got very close to the Subsonic CP for the ARCAS Long configuration.

Note for the ARCAS Long configuration wind tunnel data on Slide 9 the unusual forward movement of the CP at Transonic, and then the CP moves aft again going into low Supersonic. The ARCAS Short configuration wind tunnel data on Slide 8 shows a more typical looking CP curve with Mach number, where the CP moves aft Transonic, and then for Supersonic the CP starts moving forward with increasing Mach number. This illustrates why you want to have an additional 1.0 calibers stability margin on top of the bare minimum 1.0 calibers stability margin, for a total stability margin of 2.0 calibers, at Supersonic Mach numbers to cover for possible CP mispredictions like shown on Slide 9. Hence the 2.0 calibers stability warning message for Supersonic Mach numbers built into RASAero II.

Slides 10 and 11 show the RASAero II predictions for the Supersonic CP compared to the wind tunnel data for the ARCAS Short and ARCAS Long configurations. RASAero II produced very accurate predictions for the Supersonic CP from Mach 1.5 to Mach 3, the area of interest for the forward movement of the CP for high power rockets. Again, this is a comparison with actual wind tunnel data for a sounding rocket configuration for Supersonic CP.

The warning message in RASAero II when the stability margin falls below 2.0 calibers for supersonic Mach numbers is there because if the stability margin falls below 1.0 calibers coning, pitch-roll coupling, and other effects seen on some of the prior flights mentioned in this thread can occur. With even the best supersonic CP prediction methods there can be errors, and the unusual CP shift for the ARCAS Long Configuration for Transonic Mach numbers shows why you want to have an additional 1.0 calibers stability margin on top of the bare minimum 1.0 calibers stability margin, for a total stability margin of 2.0 calibers at Supersonic Mach numbers. Thus the warning message included in RASAero II.


Chuck Rogers
Rogers Aeroscience

View attachment RASAero II Comparisons with ARCAS CP and CD Data.pdf

Chuck Rogers said:

Attached below are RASAero II predictions for Subsonic, Transonic, and Supersonic Center of Pressure (CP) compared to wind tunnel data for two configurations of the ARCAS sounding rocket (ARCAS Short and ARCAS Long). The Supersonic wind tunnel data is up to Mach 4.63, approaching Hypersonic (Mach 5). The wind tunnel data is from NASA TN D-4013 and TN D-4014.

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Chuck Rogers
Rogers Aeroscience

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Thanks for the great info and explanations. One thing I did discover last night while reading though the documentation is that I had been running 2-DOF simulations as my launch site conditions had been left at 0 wind . Changing the wind to realistic values I did then get the subsonic Cp with dynamic stability and weathercocking effects similar to OR. In the next few days I'd like to do a more in-depth comparison of the subsonic Cp predictions, as I trust OR very much to predict marginal stability and/or weathercocking off the pad as I have had a much larger test basis for this (all my 'sport' rockets) but would like to see if RASaero II gives similar results .

For supersonic Cp prediction, I am now pretty convinced of RASaero II's superiority. Though OR has not yet failed me, I have a limited sample size and given that it is probably over-estimating Cp shift forward, my flights were probably much further from the bare minimum stability at higher mach numbers than I had thought. I will probably go back and re-tweak my fins based upon RASaero II's supersonic results. Interestingly, the small-scale long ARCAS is actually very similar in size to a 54/4000 MD - similar length and diameter, and 4 fins with ~80% the body diameter semi-span.

I have two questions though about supersonic stability: first, I would guess that the nose cone profile has an increasingly larger effect on the Cp at higher mach numbers; do you think the accuracy of RASaero as shown with the ARCAS and its tangent ogive cone is maintained for other profiles, like LD-Haack?

Second, are smaller semi-span fins less effective than the software might predict for higher mach numbers? The major difference between my rocket design and the ARCAS long configuration is that my CG is much farther forward. This theoretically means that I could get away with fins with a semi-span a good amount less then 80% of the rockets diameter. However, due to the turbulent flow near the fins (like below) could they in fact be less effective than simulated, meaning I should stay closer to the ARCAS's 80% semi-span ratio?

RocketHunter said:

For supersonic Cp prediction, I am now pretty convinced of RASaero II's superiority. Though OR has not yet failed me, I have a limited sample size and given that it is probably over-estimating Cp shift forward, my flights were probably much further from the bare minimum stability at higher mach numbers than I had thought. I will probably go back and re-tweak my fins based upon RASaero II's supersonic results. Interestingly, the small-scale long ARCAS is actually very similar in size to a 54/4000 MD - similar length and diameter, and 4 fins with ~80% the body diameter semi-span.

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To my knowledge, and admittedly a small sample size (Class 3 and some two stage Class 2 rockets), no rocket analyzed using RASAero II that maintained the minimum 2.0 calibers supersonic stability margin has had supersonic stability issues. As you can tell with the comparison with wind tunnel data, we really worked the supersonic CP issue hard. Not only to make sure the rockets worked, but to avoid unnecessarily large fins.

RocketHunter said:

I have two questions though about supersonic stability: first, I would guess that the nose cone profile has an increasingly larger effect on the Cp at higher mach numbers; do you think the accuracy of RASaero as shown with the ARCAS and its tangent ogive cone is maintained for other profiles, like LD-Haack?

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The nose cone shape doesn't have a strong effect when the shape is similar to a tangent ogive. The fact that the nose cone is followed by a cylinder, and the effect of the cylinder length on the nose cone-cylinder CNalpha and CP is a stronger effect, and is included in the RASAero II supersonic CNalpha and CP calculations.

RocketHunter said:

Second, are smaller semi-span fins less effective than the software might predict for higher mach numbers? The major difference between my rocket design and the ARCAS long configuration is that my CG is much farther forward. This theoretically means that I could get away with fins with a semi-span a good amount less then 80% of the rockets diameter. However, due to the turbulent flow near the fins (like below) could they in fact be less effective than simulated, meaning I should stay closer to the ARCAS's 80% semi-span ratio?

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The RASAero II supersonic fin CNalpha and CP models in particular do a good job of penalizing short semi-span fins, including taking into account how far down the rocket from the nose cone the short semi-span fins are mounted. I recommend that the fin semi-span never be less than the body diameter, but for fin semi-span divided by body diameter ratios of under 1.0 (like the ARCAS), 1.0, and just over 1.0, the RASAero II supersonic fin CNalpha and CP models appropriately model the effect of the short fin semi-span.


Chuck Rogers
Rogers Aeroscience

Chuck,

Thanks for replying to the thread. I've been sick the last couple days. I'll dive into this project again in the next week or so.



Chris

ChrisAttebery said:

I like the idea of being able to replace a fin and fly again the next day.

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As someone who flew a RATT M900 minimum diameter for my L3 with a metal fincan, I can say that it is optimistic to just be able to replace a single fin. Twice on landing I bent a fin. Didn't bend one on my third flight.

Each time the fin bent, it bent the fincan fixture with it as well. This was an aluminum welded fincan. Had to remake it each time. Now I just make a composite fincan for my flights.

Just a data point.

Edward

Worsaer said:

True - can you fly Loki motors at FAR (not California 'CSFM classified') ?

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Yes, but physically obtaining the motor is still an issue.

Edward,

Thanks for the input. I'm going to make 3 fin cans at the same time. With CNC it usually takes more energy to set up the mill than it does to do the actual machining. I'm hopeful that having a radius at the fin root and ~3x the material at the bottom of the fin will make the fins the weak point. There's only one way to find out.

FAR is about the same distance from here as Black Rock. I'd rather go to Black Rock if I can.

I ordered a couple nose cones from Wildman on Friday. Hopefully they will be here this week. Once they are I can finalize my plans for the motor/airframe adapter.

I've been quietly working on this project in the background. I thought it was time for an update.

I bought a 54mm FG FW nosecone, a section of 54mm tubing and a 54mm CF coupler from Wildman. The NC and tubing showed up a couple days later. I used my CNC mill to cut a 3/8" thick centering ring for the tubing. I slipped the CR into one end of the tubing and the NC into the other. This jig allows me to cut the NC square and exactly the same OD as the coupler.

I have an adapter designed to attach the NC to the CF coupler but I'm waiting for it to show up. It was drop shipped from their supplier.

I also have the adapter for the motor to coupler designed. Again, I'm waiting on the coupler to finalize the design.

Once I get the designs finalized I will have all of these parts turned and I'll start working on the fin cans.

Here's the motor closure to coupler adapter. The adapter will thread onto the forward closure and be secured with a pair of 8-32 setscrews. The CF coupler will slide over this adapter and be held in place with a 2-56 shear pin.

ChrisAttebery said:

I've been quietly working on this project in the background. I thought it was time for an update.

I bought a 54mm FG FW nosecone, a section of 54mm tubing and a 54mm CF coupler from Wildman. The NC and tubing showed up a couple days later. I used my CNC mill to cut a 3/8" thick centering ring for the tubing. I slipped the CR into one end of the tubing and the NC into the other. This jig allows me to cut the NC square and exactly the same OD as the coupler.

I have an adapter designed to attach the NC to the CF coupler but I'm waiting for it to show up. It was drop shipped from their supplier.

I also have the adapter for the motor to coupler designed. Again, I'm waiting on the coupler to finalize the design.

Once I get the designs finalized I will have all of these parts turned and I'll start working on the fin cans.

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very clever! This sort of set up could also be used to cut the tips off plastic nose cones.

Thanks for sharing!

Tony

Thanks Tony. You're right, it would work for that task too.

tfish said:

very clever! This sort of set up could also be used to cut the tips off plastic nose cones.

Thanks for sharing!

Tony

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ChrisAttebery said:

I've been quietly working on this project in the background. I thought it was time for an update.

I bought a 54mm FG FW nosecone, a section of 54mm tubing and a 54mm CF coupler from Wildman. The NC and tubing showed up a couple days later. I used my CNC mill to cut a 3/8" thick centering ring for the tubing. I slipped the CR into one end of the tubing and the NC into the other. This jig allows me to cut the NC square and exactly the same OD as the coupler.

I have an adapter designed to attach the NC to the CF coupler but I'm waiting for it to show up. It was drop shipped from their supplier.

I also have the adapter for the motor to coupler designed. Again, I'm waiting on the coupler to finalize the design.

Once I get the designs finalized I will have all of these parts turned and I'll start working on the fin cans.

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i dont understand. Why will u have a coupler over a nose cone?

Why is a nose cone inside a coupler?

Any updates?

ColumbiaNX01 said:

Why is a nose cone inside a coupler?

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Read the last sentence of the second paragraph, its a jig for cutting the nose cone square.

Unfortunately, no. I decided to shelve this project. I've had a hard time even attending the local launches for the last year or so. It's hard to work on a project like this not knowing if I'll ever be able to fly it.

I have a(nother) 4" Punisher that I should put together first. I also have a 3" CF Blackhawk that I machined a set of fins for. Those two will take priority for the foreseeable future.

TRFfan said:

Any updates?

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