Mariah-54 Rocksim: Why can't I replicate the results?

I am modifying a Fiberglass Mariah-54, and intend to use RocSim to determine the performace impact of using some heavier components. After taking the "non-glass" version of the file and changing the relevant parts to be fiberglass instead of magnaframe, I noticed I was only getting half the altitude of the original sims. Really?!?!

So thinking I was doing something wrong I grabbed the unmodified file and re-ran some of the sims. To my surprise, all of those seemed to be way under-performing as well.

The RockSim file I obtained here on this forum is named "GLH Mariah 54 B3.rkt". I have made no changes to the file, and configured the starting conditions to be the same as those selected in the simulation I am trying to replicate. The simulation in question used a CTI L640-DT to achieve a maximum altitude of ~23696 ft. When I load the same engine (using the latest available engine file for the L640-DT), the result I get is only 13044 ft. !!! How is it that with the same starting conditions, same rocket and same engine I get only half the altitude?

Surely I am doing something wrong here. Maybe some kind soul with Rocksim (I am using version 9.1.4f145) could try this also and see if you can replicate the results given in the sims that come with the file?

GLR Mariah-54 B3.rkt

I'd guess your 13,044 is the more realistic sim. 23,000 seems like an awful lot, and I suspect that's in error, somehow.


Later!

--Coop

Coop said:

I'd guess your 13,044 is the more realistic sim. 23,000 seems like an awful lot, and I suspect that's in error, somehow.

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That's what I thought, as well. I used to fly a somewhat heavy fiberglass 3" diameter Talon-3, and the AMW L-1100 pushed it to a bit over 10K on numerous flights. If I sim that L1100 in this Mariah-54 it projects 14K. That seems reasonable to me.

RocSim seems to under "guess" any flight that stays in mach. The L640 in a MD 54 will do much closer to 23,000 than 13,000. I have compared several of the RocSim "guess" from a Space Cowboy (MD 54) to a real flight and it was WAY under estimated. I think any of the 6XL 54 motors will push a MD54 to close to if not over 20K. Not sure why RocSiim is like that. For that reason, I really don't use it any more.

Rocksim has a Mach 2 cap, meaning once the rocket crosses that speed. The software stops calculating Coefficients of Drag. This leads to the bad altitude calculations.

I have been using rocksim and RASAero together in order to get the calculations correct. The estimate on most L motors push the rocket to 20K.

If I am not mistaken that file is the one I posted. The reason those altitude are there is, because I messed with the Cd over ride which lets me put a certain value to the Cd. The more I lowered it along with the rocket loaded with an L motor the more it increased in speed and altitude. I pick the value closest to what real life flights have achieved and input them in rocksim. Of course I do not know if the L640 on there is correct at 23K. However, I am convinced since RASAero calculated 21K. You wonder how I think this is correct? Well one important factor I cant do in RASAero is make curved fins. I do not know how much drag a curved fin produces, but I am certain it is less than that of a trapezoid fin shape. That said, you are still expected to go over 20K on L motors.

I have spoken with Apogee Rockets about this issue with rocksim. I was disappointed for them to tell me that we have to get Rocksim Pro, which they can bite me, because that software is $1,000. In all honesty for the $300 you all paid for rocksim should have a greater cap of up to at least Mach 5. Given that the N5800 motor, when going very minimal and high performance that motor pushes rockets to Mach 4.

I apologize if the altitudes seem too much of a calculation. What I could do is use RASAero and recalculate the ones in rocksim and repost on the thread I made. Let me know if you want me to do this or you could do it yourself?

Thanks, SDL!

Which of the sims in the file did you have actual flight data by which to compare?

sti_ffy said:

Thanks, SDL!

Which of the sims in the file did you have actual flight data by which to compare?

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The only flight thats correct was the K300 motor, which the altitude should be in between 19k-20k. Honestly I think I need to check on each sim and get the altitudes correct. Using RASAero should help me get the altitudes more correctly. Once I get all rocket motor sims I will fix them in rocksim and post the new file on my build thread tomorrow along with the Cd values of each motor so in case you need to run a new simulation you'll know which Cd value to use.

Functionally, Rocsim has a mach 2 cap. Practically, rocksim has a mach ~mach 1.25 cap; I have never seen it give 'accurate' results when the top speed is above that for any length of time. On any rocket sim file posted to the forum or flown in person. OpenRocket does somewhat better.

It's important to note, however, that all three hobby rocket simulators that are in wide use (Rocksim, Openrocket, and RASAero) have the same limitation; they are only as accurate as the inputs, and the definitions of the inputs can be a little odd, or at least unintuitive. What I would recommend regardless of simulator is to read through the manual for it like was a holy book, enter everything as carefully as possible following the instructions, and then see what it looks like. Most of the time I see students with inaccurate simulation results it's because they're entered some part of the geometry or mass in a normal, intuitive way, instead of whatever the programmer was thinking at the time.

SinfulDarkLord said:

I have spoken with Apogee Rockets about this issue with rocksim. I was disappointed for them to tell me that we have to get Rocksim Pro, which they can bite me, because that software is $1,000. In all honesty for the $300 you all paid for rocksim should have a greater cap of up to at least Mach 5.

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I never thought I'd defend rocksim, but I think it's only $125, not $300.

Of course, Openrocket is more accurate at high speeds and allows for everything rocksim does except for tube fins (I think), and is free. And RASAero is far more accurate especially at higher mach, allows for simple geometries, and is free.

CCotner said:

Functionally, Rocsim has a mach 2 cap. Practically, rocksim has a mach ~mach 1.25 cap; I have never seen it give 'accurate' results when the top speed is above that for any length of time. On any rocket sim file posted to the forum or flown in person. OpenRocket does somewhat better.

It's important to note, however, that all three hobby rocket simulators that are in wide use (Rocksim, Openrocket, and RASAero) have the same limitation; they are only as accurate as the inputs, and the definitions of the inputs can be a little odd, or at least unintuitive. What I would recommend regardless of simulator is to read through the manual for it like was a holy book, enter everything as carefully as possible following the instructions, and then see what it looks like. Most of the time I see students with inaccurate simulation results it's because they're entered some part of the geometry or mass in a normal, intuitive way, instead of whatever the programmer was thinking at the time.

Click to expand...

When it comes to rocksim I am a perfectionist when using it. All my measurements are always accurate to the design.

CCotner said:

I never thought I'd defend rocksim, but I think it's only $125, not $300.

Of course, Openrocket is more accurate at high speeds and allows for everything rocksim does except for tube fins (I think), and is free. And RASAero is far more accurate especially at higher mach, allows for simple geometries, and is free.

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They must have lowered it, I do recall seeing it for $300. Even Openrocket has its shares of bad calculations. If I'm not mistaken you can load a rocksim file on there. Well I did just that with my Mariah 54 file and loaded the L640 dual thrust motor and the calculation came out at 23,012 feet. Well what do you know, it matches closely to the simulation that I had going at 23k. I had to load another motor and see if the results come out the same. This time the K300 LB was chosen and the results were just bad at 10,932 feet. RASAero calculates 20K. Man I wonder what happened to Openrocket right there. At any rate I'll just leave my accurate calculations to RASAero. I use rocksim mainly, because of the degree of designing freedom.