I'm confused...

[matt38]

Hello all,

New to the hobby but fully immersed. I just completed an Apogee Saturn V and bought 3 AT G79-4 motors as recommended on the Apogee site. I downloaded RockSim which shows I need a 77" launch rod...Is this accurate or a "suggestion". My Mantis pad came with a 57" rod; will this be OK and more importantly, safe?

 

[Sailorbill]

If you don't have a long enough rod the rocket will not reach sufficient speed to be stable before it leaves the rod.

 

[KerryQuinn]

Matt -
The Mantis pad is the launch pad Apogee recommends for that rocket on their site (there's even a note in that section of the websie from Tim talking about using that pad for the Apogee Saturn V). Apogee makes their engine recommendations on their site using Rocksim (suprise) - and they make the Rocksim files available on there as well -but in their own rocksim file they have the launch guide length set to 914mm (36") which would be too short for this size rocket (I think it is the default for Rocksim).
Still, I ran a quick simulation of a 5.6" dia rocket @ 3lbs on a G79W engine.
It shows 42 fps at a height of only 36" (so the launch rod would have to extend 36" above the top of the upper launch lug). 42 fps should be plenty fast enough to have aerodynamic stability if there is no wind. You should be good on the Mantis 57" x 1/4" rod without wind. On a breezy day, I'd launch a different rocket.

-Kerry

 

[matt38]

Thanks for the input. Like I mentioned, I am new to this hobby and jumped in with both feet without realizing the complexity of flying large rockets. The rockSim software is cool but a little overwhelming.
p.s.: I have gotten conflicting info on locating the CP on the Apogee Sat V and Semroc Sat1B: Do I measure from the tip of the capsule or tip of the tower?

 

[delta22]

Definitely only launch a Saturn V in very low winds. The short fins and blunt shape react very badly to crosswinds. I have a Sirius Rocketry Saturn V which does well in light winds and veers off course in moderate crosswinds. Click on my website for interesting pictures of one such flight.

Describing the location of CP is up to whoever is providing the information. If a manufacturer is providing CP for a complex model, hopefully they are making it clear what the reference point is.

I would encourage you to consider buying RockSim. Fun to use and a very powerful tool. Then you can calculate your own CPs and virtually try many different engines.

 

[matt38]

Very cool website. I look forward to learning a lot and getting my L1. You folks have been a wealth of knowledge.

 

[luke strawwalker]

Thanks for the input. Like I mentioned, I am new to this hobby and jumped in with both feet without realizing the complexity of flying large rockets. The rockSim software is cool but a little overwhelming.
p.s.: I have gotten conflicting info on locating the CP on the Apogee Sat V and Semroc Sat1B: Do I measure from the tip of the capsule or tip of the tower?

One caveat about Rocksim that can turn into a BAD "gotcha" moment if you're not aware of it...

MAKE SURE that when Rocksim is displaying the stability in calibers, that it is using the LARGEST FRONTAL DIAMETER for the stability calculation and NOT the diameter of the first tube. On rockets with towers, this can cause a situation where it will display something like 3 calibers of stability, but if it's using the tube size of the tower or a narrow upper section (for a small nosecone or something above the main body of the rocket) it is showing the stability margin in terms of THE SMALL TUBE'S CALIBER (Diameter) rather than the rocket. If this is the case, and say it shows your rocket having 3 calibers of stability, if the tower is say 1/2 inch diameter, that would equate to the CP being ONLY 3/4 OF AN INCH behind the CG-- FAR, FAR too close for a stable flight! If launched the rocket would likely get tipped off by the breeze and go unstable and crash, since CP moves FORWARD with angle of attack!

On one of the menu's at the top of the Rocksim screen when building a rocket, there is a selection box with a drop down menu to select what method Rocksim uses to calculate CP-- Rocksim, Barryman, or Cardboard cutout method. Just below it should be a box/menu to select HOW to display the CP/CG relationship, select "use maximum frontal diameter" or "maximum tube diameter" (can't remember exactly how it's worded at the moment) and it will calculate the stability and display the margin in calibers of the main body tube diameter, which is what you want to get a TRUE measure of the stability margin of the rocket.

This very thing caused one of the Mercury Joe rockets to crash when they put a bigger engine in it-- Rocksim showed 5 caliber stability or something, but it was 5 times the tiny diameter of the tower tube, not the main rocket, so the rocket was actually neutrally stable with the selected engine, or close to it, and when the bigger (heavier) engine was installed, it moved the CG BEHIND the CP and made the rocket totally unstable, leading to a BAD DAY...
(Google "mercury joe" for more info/cool vids)

Later and good luck! OL JR

 

[matt38]

A little off subject but in the results column on Rocksim, there are some parachutes shown for successful launches and some with black triangles also for successful launches. The difference?

 

[Mike Di Venti]

the black triangle tells you which way the rocket was going when the chute deployed.

If the triangle point is up, it means the rocket was still travelling up(early deployment)
If the triangle point is down, it means the the chute deployed after apogee(late deployment)