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RasAeroII CG Calculations

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JackO

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Simulating my first large rocket on RasAeroII and am really paying attention to the details. After reading the whole manual, I know to enter takeoff weight, and CG under Flight Performance, double tapping the loaded sustainer motor (single stage). In order to get an accurate sim, I am assuming (hoping) the program subtracts spent propellant weight proportionately to thrust and based off the propellant weight entered into the .eng file in the header. This moves forward the CG, something I need to check the extent of when the CP moves forward past Mach 1. If someone could please let me know if this is the case I would appreciate it. Secondly, does anyone know if RasAeroII enables more detailed component based CG calculations? For example, the nozzle weighs a lot more than the motor forward bulkhead so the CG of the motor itself will move towards the nozzle end during the burn, while the whole rocket's CG will move nose end. Any input's appreciated.
 

JackO

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Also the CG vs CP plots show the CG is moving down the length of the rocket causing instability regardless of its initial position, anyone know why?
 

Buckeye

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Your understanding in post #1 is correct. More detailed CG calculations are not possible - use Rocksim or OpenRocket instead. What I hate about RAII is that it requires you to input the flight ready mass and CG, yet uses the .eng file to compute change in mass/CG during the burn. If it knows how to use the engine data, why does it require you to include the engine mass for flight? No other software does this. So, RAII is not really a good "design" software. It assumes you already built the rocket.

Post #2 - Don't know. More info needed.
 

JackO

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Your understanding in post #1 is correct. More detailed CG calculations are not possible - use Rocksim or OpenRocket instead. What I hate about RAII is that it requires you to input the flight ready mass and CG, yet uses the .eng file to compute change in mass/CG during the burn. If it knows how to use the engine data, why does it require you to include the engine mass for flight? No other software does this. So, RAII is not really a good "design" software. It assumes you already built the rocket.

Post #2 - Don't know. More info needed.
That is annoying. What software would you recommend for the design stage, OpenRocket? Faulty input from Burnsim for the .eng file was the issue behind the second post. Also, RAII will tell me to keep the CG at least a caliber above the CP at all times, but for a rocket going over Mach 2, that means I need the CG about 3 calibers ahead of the CP at launch, at how many calibers in this direction should I start worrying about over-stability?
 

Adrian A

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That is annoying. What software would you recommend for the design stage, OpenRocket? Faulty input from Burnsim for the .eng file was the issue behind the second post. Also, RAII will tell me to keep the CG at least a caliber above the CP at all times, but for a rocket going over Mach 2, that means I need the CG about 3 calibers ahead of the CP at launch, at how many calibers in this direction should I start worrying about over-stability?
You don’t ever have to worry about over-stability. Even if you’re going for altitude records you’re better off with more stability than the recommended minimums.
 

Chuck Rogers

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RASAero II assumes the motor propellant Center of Gravity (CG) is located at one-half the motor length from the bottom of the rocket, or the bottom of the individual stage. It gets this motor length from the rasp.eng motor data, so it is important that the motor length in the rasp.eng file is correct.

The initial CG for the rocket, or for a two stage rocket the initial CG for the configuration with the booster, and the configuration with the sustainer only, is then entered.

When developing RASAero II, rather than having just another rocket design program, we put our effort into more accurate aerodynamic predictions, and more accurate altitude predictions. A summary altitude prediction accuracy plot is attached, see the RASAero web site here:

http://www.rasaero.com/comparisons-alt.htm

and other examples at www.rasaero.com for more altitude prediction accuracy examples and comparisons with wind tunnel data and in-flight measured Drag Coefficient (CD) data.

Many rocketeers do the design lay-out in Open Rocket or RockSim to get the weight and CG, and then convert the rocket into RASAero II for the final aerodynamic predictions and altitude predictions.

Note that many of the airfoils used on high power rockets are not available in Open Rocket or RockSim, so when the rocket is converted into RASAero II the specific airfoils have to be added. Relative to the aerodynamic prediction accuracy, and altitude prediction accuracy, that’s something important to note.

One of the errors in Open Rocket and RockSim altitude predictions is that the predicted weight and CG are never updated with the actual values. Compare your predicted Open Rocket and RockSim weights to the actual weights when flown. In the end for the best accuracy you need to use the manual override values for weight and CG.

Again, many rocketeers do the design layout in Open Rocket or RockSim, and then convert the rocket to RASAero II for the final altitude prediction and final stability prediction. This is almost standard procedure on Tripoli Class 3 rockets. RASAero II reads RockSim files, Open Rocket files can be converted to a RockSim file, which then can be imported into RASAero II.


Charles E. (Chuck) Rogers
Rogers Aeroscience
 

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Chuck Rogers

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The RASAero II recommended stability margin for subsonic Mach numbers is a stability margin of 1.0 calibers, for transonic and supersonic Mach numbers the recommended stability margin is 2.0 calibers. The RASAero II Flight Simulation checks for these stability margins only at low angles of attack (less than 5 degrees angle of attack) because the rocket can have momentary marginal stability right off the launch rail with wind, and at high angles of attack at low velocity going through apogee; these two conditions can cause stability warnings in Open Rocket, especially the high angle of attack going through apogee, that aren't really of a concern.

For rockets exceeding Mach 2, and especially for rockets approaching and exceeding Mach 3, you need to maintain the minimum stability margin of 2.0 calibers at all transonic and supersonic Mach numbers. As Adrian noted, the excess stability margin at launch for maintaining a 2.0 caliber stability margin at Mach 2-3 is usually not a concern. Rockets of this type typically have high thrust and high acceleration off the launch pad, and thus typically don't have issues with weathercocking with a wind present at launch.


Charles E. (Chuck) Rogers
Rogers Aeroscience
 

JackO

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RASAero II assumes the motor propellant Center of Gravity (CG) is located at one-half the motor length from the bottom of the rocket, or the bottom of the individual stage. It gets this motor length from the rasp.eng motor data, so it is important that the motor length in the rasp.eng file is correct.

The initial CG for the rocket, or for a two stage rocket the initial CG for the configuration with the booster, and the configuration with the sustainer only, is then entered.

When developing RASAero II, rather than having just another rocket design program, we put our effort into more accurate aerodynamic predictions, and more accurate altitude predictions. A summary altitude prediction accuracy plot is attached, see the RASAero web site here:

http://www.rasaero.com/comparisons-alt.htm

and other examples at www.rasaero.com for more altitude prediction accuracy examples and comparisons with wind tunnel data and in-flight measured Drag Coefficient (CD) data.

Many rocketeers do the design lay-out in Open Rocket or RockSim to get the weight and CG, and then convert the rocket into RASAero II for the final aerodynamic predictions and altitude predictions.

Note that many of the airfoils used on high power rockets are not available in Open Rocket or RockSim, so when the rocket is converted into RASAero II the specific airfoils have to be added. Relative to the aerodynamic prediction accuracy, and altitude prediction accuracy, that’s something important to note.

One of the errors in Open Rocket and RockSim altitude predictions is that the predicted weight and CG are never updated with the actual values. Compare your predicted Open Rocket and RockSim weights to the actual weights when flown. In the end for the best accuracy you need to use the manual override values for weight and CG.

Again, many rocketeers do the design layout in Open Rocket or RockSim, and then convert the rocket to RASAero II for the final altitude prediction and final stability prediction. This is almost standard procedure on Tripoli Class 3 rockets. RASAero II reads RockSim files, Open Rocket files can be converted to a RockSim file, which then can be imported into RASAero II.


Charles E. (Chuck) Rogers
Rogers Aeroscience

Fantastic, thanks very much for the very detailed response Chuck. I will finish the design on OpenRocket and then move to RasAero II. You are correct about the high thrust margin, I will keep the CG at least 2 calibers in front throughout the launch. This is an experimental O motor, so I'm going to be playing things on the safe side for sure in case we get higher thrust than expected. I heard early on that RASAero was the best for aerodynamics and accuracy, so its great to have the program combination with OpenRocket for design and accurate flight simulation. Appreciate it.
 

JackO

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RASAero II assumes the motor propellant Center of Gravity (CG) is located at one-half the motor length from the bottom of the rocket, or the bottom of the individual stage. It gets this motor length from the rasp.eng motor data, so it is important that the motor length in the rasp.eng file is correct.

The initial CG for the rocket, or for a two stage rocket the initial CG for the configuration with the booster, and the configuration with the sustainer only, is then entered.

When developing RASAero II, rather than having just another rocket design program, we put our effort into more accurate aerodynamic predictions, and more accurate altitude predictions. A summary altitude prediction accuracy plot is attached, see the RASAero web site here:

http://www.rasaero.com/comparisons-alt.htm

and other examples at www.rasaero.com for more altitude prediction accuracy examples and comparisons with wind tunnel data and in-flight measured Drag Coefficient (CD) data.

Many rocketeers do the design lay-out in Open Rocket or RockSim to get the weight and CG, and then convert the rocket into RASAero II for the final aerodynamic predictions and altitude predictions.

Note that many of the airfoils used on high power rockets are not available in Open Rocket or RockSim, so when the rocket is converted into RASAero II the specific airfoils have to be added. Relative to the aerodynamic prediction accuracy, and altitude prediction accuracy, that’s something important to note.

One of the errors in Open Rocket and RockSim altitude predictions is that the predicted weight and CG are never updated with the actual values. Compare your predicted Open Rocket and RockSim weights to the actual weights when flown. In the end for the best accuracy you need to use the manual override values for weight and CG.

Again, many rocketeers do the design layout in Open Rocket or RockSim, and then convert the rocket to RASAero II for the final altitude prediction and final stability prediction. This is almost standard procedure on Tripoli Class 3 rockets. RASAero II reads RockSim files, Open Rocket files can be converted to a RockSim file, which then can be imported into RASAero II.


Charles E. (Chuck) Rogers
Rogers Aeroscience

One question for you Chuck, is conversion of an Open Rocket file to RockSim only possible through purchase of RockSim? Seeing as the price of that software is pretty hefty just to convert file types, would you recommend just borrowing the CG from OpenRocket?
 

timbucktoo

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One question for you Chuck, is conversion of an Open Rocket file to RockSim only possible through purchase of RockSim? Seeing as the price of that software is pretty hefty just to convert file types, would you recommend just borrowing the CG from OpenRocket?
No need. Save your open rocket file with .RKT extension instead of the default .ORK and then you can import it into rasaero.
 
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