Low Flight, Stubby Rocket Caliber

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simplrocket

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My rocket is roughly 30.5" long and has a stability caliber of roughly 1.4 since it has some electronics in it pulling the CG up. I was wondering, for this size of a rocket and in Kansas windy weather for 200m apogee height-ish, what stability caliber should I am for?
 
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smstachwick

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1.4 is fine. Typically you’ll want to aim for somewhere between 1 and 2.

A bigger concern in the wind is your delay time. Sim it out if you can, err short if you can’t. Also remember the rules about wind speed: not more than 20 mph.
 
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rharshberger

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What diameter...a 30" long rocket has different requirements for stability based on its finess ratio. For 3" rocket (10:1 length x diameter) a 1.4 stabilty is good, on a 7.5" (4:1) its more than necessary as the short and fat rocket gets a solid boost to its stability from base drag so a .5 stability margin could be acceptable (the extra noseweight to go from .5 to 1.4 is usually pretty significant on short fat rockets). A 1.6" rocket is approaching 20:1 L x D and a 1.4 would be excellent. A 1.4 stability will work but sometimes its not always the best number to shoot for based on the individual rocket.
 

simplrocket

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What diameter...a 30" long rocket has different requirements for stability based on its finess ratio. For 3" rocket (10:1 length x diameter) a 1.4 stability is good, on a 7.5" (4:1) its more than necessary as the short and fat rocket gets a solid boost to its stability from base drag so a .5 stability margin could be acceptable (the extra noseweight to go from .5 to 1.4 is usually pretty significant on short fat rockets). A 1.6" rocket is approaching 20:1 L x D and a 1.4 would be excellent. A 1.4 stability will work but sometimes its not always the best number to shoot for based on the individual rocket.

My L/D is closer to 12, and my current cal. according to OpenRocket is 1.8 and we will verify the weight and location of the CG after. At launchrod clearance, it is only 1.2ish. We have a T/W ratio of 9 so I believe this is fine, but for our safety was wanting to double check.
 

simplrocket

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I am also aware that OpenRocket does not account for base drag and we have actually also developed our own model to simulate some special characteristics of our rocket.
 

rharshberger

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I am also aware that OpenRocket does not account for base drag and we have actually also developed our own model to simulate some special characteristics of our rocket.
Open Rocket can use the same base drag simulation trick as RocSim, an airframe diameter cone of Pi length behind the base of the rocket. There is an Apogee Peak of Flight newsletter about how to do, its pretty easy, just disregard the "airframe/rocket diameter discontinuity warning".
 
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My rocket is roughly 30.5" long and has a stability caliber of roughly 1.4 since it has some electronics in it pulling the CG up. I was wondering, for this size of a rocket and in Kansas windy weather for 200m apogee height-ish, what stability caliber should I am for?
The more important question is "what speed does your rocket leave the launch rod/rail?" The faster it is, the smaller the effect of windcocking will be. Which is what I assume you were asking for.
 

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Post the OpenRocket file so everyone can get the relevant info on your design.
 

simplrocket

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The more important question is "what speed does your rocket leave the launch rod/rail?" The faster it is, the smaller the effect of windcocking will be. Which is what I assume you were asking for.

Our rocket is leaving the launch rod at a T/W of roughly 9, but I dont remeber the speed off the top of my head. It also has a stability cal of 1.2ish which is what I am most concerned about just from what I know.
 

QFactor

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Our rocket is leaving the launch rod at a T/W of roughly 9, but I dont remeber the speed off the top of my head. It also has a stability cal of 1.2ish which is what I am most concerned about just from what I know.

Can you post the simulation file?
 
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Our rocket is leaving the launch rod at a T/W of roughly 9, but I dont remeber the speed off the top of my head. It also has a stability cal of 1.2ish which is what I am most concerned about just from what I know.
As long as you are not getting into trans sonic speeds or above, there will not be any significant change in your CP unless you have high cross winds. As I said, the effect of a cross wind is dependant on the speed it leaves the rail. If you take the sum of the velocity vectors of the vertical and cross wind speed, this gives you the effective angle of attack. RAS Aero will give you the change in CP based on the angle of attack. @Chuck Rogers may be able to advise if RAS Aero will produce the correct base drag value for a stubby rocket. A 12:1 L/D ratio id not what I'd generally consider stubby.
Norm
 
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What diameter...a 30" long rocket has different requirements for stability based on its finess ratio. For 3" rocket (10:1 length x diameter) a 1.4 stabilty is good, on a 7.5" (4:1) its more than necessary as the short and fat rocket gets a solid boost to its stability from base drag so a .5 stability margin could be acceptable (the extra noseweight to go from .5 to 1.4 is usually pretty significant on short fat rockets). A 1.6" rocket is approaching 20:1 L x D and a 1.4 would be excellent. A 1.4 stability will work but sometimes its not always the best number to shoot for based on the individual rocket.
When you are figuring the length, do you do the length of the entire rocket or just the body tube?
 

simplrocket

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When you are figuring the length, do you do the length of the entire rocket or just the body tube?

The original length I was talking about was prior to some revision to adjust for stability. It should be a 3" longer than the original.
 

QFactor

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I looked over your OR design. Is this a TARC rocket? It has all the makings of a rocket designed to carry eggs.

Also the F42 motor puts it near the TARC altitude.

I don't see any real stability issues. You're right at 47 feet per second when it exits the 4 ft. rail you used in the simulation.
I added two TARC egg weights and the exit velocity dropped to 45 feet per second. Either way you're fine, but just on the
line for an acceptable rail velocity.

A 6 ft. rail boosts the exit velocity to 55 feet per second. 50 and above is a good number.

The stability margin is always between 1.2 and 2.0. So you're fine there.

I would be a little concerned about about the amount of compartment space you allowed for your main chute and harness.
The deloyment charge is mighty close in that small an area. Be sure to protect the chute well so that it does not get scorched.

I'm not up on the use of 3D printed components for a motor mount. You show PLA as the plastic. I'll let someone else comment on that.

Don't overthink the base drag. But if you want to chase that issue, then you need to read up on Base Bleed first.

And if you have an experienced rocketry mentor/advisor - be sure to include him/her/them in your design decisions.
 

QFactor

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One quick note on your OR file. Be sure to select the correct "Component Material". This affects the mass calculation by OR, and it also allows a person
reviewing your design to easily see what material you are using in the construction. In your design - are you using basswood for the nose cone, fins,
centering rings, and cardboard for a bulkhead?
 

simplrocket

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One quick note on your OR file. Be sure to select the correct "Component Material". This affects the mass calculation by OR, and it also allows a person
reviewing your design to easily see what material you are using in the construction. In your design - are you using basswood for the nose cone, fins,
centering rings, and cardboard for a bulkhead?

I am using the component materials for all the items in the OR file with the exception of the added masses for things like sand and assemblies. I am using the new version (beta 22.0.2) so it may not have transferred over well to an older version if that is what you are using. I double checked and my design does have the materials picked out to the best of my knowledge and their weights appear to match my CAD model as well.
 

simplrocket

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I looked over your OR design. Is this a TARC rocket? It has all the makings of a rocket designed to carry eggs.

Also the F42 motor puts it near the TARC altitude.

I don't see any real stability issues. You're right at 47 feet per second when it exits the 4 ft. rail you used in the simulation.
I added two TARC egg weights and the exit velocity dropped to 45 feet per second. Either way you're fine, but just on the
line for an acceptable rail velocity.

A 6 ft. rail boosts the exit velocity to 55 feet per second. 50 and above is a good number.

The stability margin is always between 1.2 and 2.0. So you're fine there.

I would be a little concerned about about the amount of compartment space you allowed for your main chute and harness.
The deployment charge is mighty close in that small an area. Be sure to protect the chute well so that it does not get scorched.

This is actually an undergrad project that is not involved with TARC, but just local to our school.
I'm not up on the use of 3D printed components for a motor mount. You show PLA as the plastic. I'll let someone else comment on that.

Don't overthink the base drag. But if you want to chase that issue, then you need to read up on Base Bleed first.

And if you have an experienced rocketry mentor/advisor - be sure to include him/her/them in your design decisions.

I am also in contact with a few of our Rocket Club experts who are giving me advise. On another note, I did change the motors to a F67-4T so that the parachute deployed closer to apogee and gave me a better speed leaving the launch rod as well.

For the parachute compartment, I do realize this is small (and it used to be smaller), but we downsized our parachute from a 32" to a 24" as well and we will be using plenty of wadding and possibly a parachute protector so that it doesn't get burnt up.
 

QFactor

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I am using the component materials for all the items in the OR file with the exception of the added masses for things like sand and assemblies. I am using the new version (beta 22.0.2) so it may not have transferred over well to an older version if that is what you are using. I double checked and my design does have the materials picked out to the best of my knowledge and their weights appear to match my CAD model as well.

Sounds good !

You are correct - I have not loaded the new version yet.
 

simplrocket

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Sounds good !

You are correct - I have not loaded the new version yet.

If there is any other advice you would like to give my team, let me know. Will hopefully be able to launch this week/weekend as a test flight and then next week will be our drag flap demonstration.

Wish us luck!
 

QFactor

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If there is any other advice you would like to give my team, let me know. Will hopefully be able to launch this week/weekend as a test flight and then next week will be our drag flap demonstration.

Wish us luck!

Best of luck !
 

Chuck Rogers

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As long as you are not getting into trans sonic speeds or above, there will not be any significant change in your CP unless you have high cross winds. As I said, the effect of a cross wind is dependant on the speed it leaves the rail. If you take the sum of the velocity vectors of the vertical and cross wind speed, this gives you the effective angle of attack. RAS Aero will give you the change in CP based on the angle of attack. @Chuck Rogers may be able to advise if RAS Aero will produce the correct base drag value for a stubby rocket. A 12:1 L/D ratio id not what I'd generally consider stubby.
Norm

As noted, a 12:1 L/D ratio is not considered stubby. The Base Drag model in RASAero II is applicable over a very wide range, and a 12:1 L/D is certainly within the range of a normal rocket.

Note that RASAero II does not include any stability effects of the Base Drag.

Take a look at the Angle of Attack from RASAero II or the other software packages, or you can calculate the Angle of Attack by hand. You can assume a Launch of Angle of zero degrees from vertical to simplify the calculation. With that Angle of Attack look at the forward shift in the CP in RASAero II. (Or RASAero II can do all this for you automatically, including the more complex angle of attack calculations when the launch rail is not directly vertical, but is some angle from vertical.)

A rule of thumb I like to use is that the Angle of Attack just off the Launch Rail should not exceed 12 deg. Most airfoils will go into a leveling off of Lift Coefficient (CL) at 12-15 deg Angle of Attack, and then the CL really falls off and the airfoil stalls. But it is pretty much a straight CL (and a straight CNalpha) with Angle of Attack up to 12-15 deg, with 12 deg the low (conservative) value to use as an Angle of Attack limit.

Note that the Fins can temporarily stall and Jet Damping (included in RASAero II) will keep the rocket pointy-end forward, or at least greatly slow the rotation of the rocket if it is unstable or marginally stable, and during that time period more propellant is being burned off and the CG moves forward. Again, all included in RASAero II.

I've proposed in the past an Angle of Attack limit when leaving the Launch Rail of 12 deg.

As others have noted, you can lower the angle of attack by flying in a lower wind, or by having a higher initial thrust to weight ratio to exit the Launch Rail with a higher velocity.


Charles E. (Chuck) Rogers
Rogers Aeroscience
 
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