# Open Rocket Usage Questions

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#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
Greetings!
Returning back to the hobby since many years absent, will be enjoying building and launching rockets with my 5 year old son. I've taken an interest in building as my first mid/high power rocket the 4" Paper Madcow DX3. Since I couldn't find it new from a dealer, I purchased an Aerotech 29/40-120 RMS from ebay along with 6 reloadable motors. I downloaded and ran simulations with these motors, and all appears be good. I plan on obtaining the delay time tool to do any needed adjustment to the motors where the delay is too long. The velocity @ parachute deployment for all except the G64-4W is between 1.16 to 2.05 m/sec. I'm assuming that this is a good velocity range? For the G64-4W engine it states the velocity is 10.8 m/sec - is this too fast for this rocket? I have read elsewhere that 20 to 25 fps is the max . . . What is the generally recognized fastest velocity at deployment for 4" paper tube rockets?

G53-5FJ (works great as is)
G64-10W (drilled to 5 seconds)
G76-10G (drilled to 5 seconds)

Another question is the velocity off rod. For the G53FJ it is 13.4 m/sec from a 6' (184 cm) rod. Is this sufficient? What is the minimum? How can I find/calculate that minimum? I found the following statement on NAR from their launch safety pdf: "Best practice would indicate that rockets should be guided by launch rods, rails, or towers until they have attained a forward velocity of at least 4 times the velocity at which the wind is blowing (or gusting) at the launch site" If the wind is still or very low?

Thanks for any advice . . . feel free to point me to published tables and etc. . . .

Mark Sanderson

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#### boatgeek

##### Well-Known Member
Usually, you aim for deployment within 1-2 seconds of apogee, which works out to a maximum of 20 m/s. The delays can vary by something like +/- 10%, so don't expect 100% precision. Also, this isn't an issue on any of the motors you show above, but you shouldn't drill to below 4s of delay. Personally, I don't sim anything for less than 2.5 m/s wind speed, and usually plan for 5 m/s. Other people may have different approaches. If the wind in your area is commonly dead flat calm or blowing hard, you might change those up.

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
Usually, you aim for deployment within 1-2 seconds of apogee, which works out to a maximum of 20 m/s. The delays can vary by something like +/- 10%, so don't expect 100% precision. Also, this isn't an issue on any of the motors you show above, but you shouldn't drill to below 4s of delay. Personally, I don't sim anything for less than 2.5 m/s wind speed, and usually plan for 5 m/s. Other people may have different approaches. If the wind in your area is commonly dead flat calm or blowing hard, you might change those up.

#### Bat-mite

##### Rocketeer in MD
As for speed off the launch rail, 50 fps is about the bare minimum. Let's see, that's 15.2 meters/sec. So, the G53 might be a little weak. Definitely don't choose that motor on a windy day.

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
As for speed off the launch rail, 50 fps is about the bare minimum. Let's see, that's 15.2 meters/sec. So, the G53 might be a little weak. Definitely don't choose that motor on a windy day.
Cool thanks . . . according to open rocket if I run it from an 8 foot rail I'll get 15.7 m/s . . . barely enough to be safe . . . 13.9 m/s for a 6 foot . . . hmmm

Mark

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#### mikec

##### Well-Known Member
DX3 or Super DX3? I've flown my Super DX3 many times on G64-4, but never on G53.

Keep in mind that the Class 1 limit is 1500 grams loaded, and the Super DX3 will come close to that or exceed it depending on how heavily you build.

If you really meant DX3, that should be fine on a G64-7 or a G53-7, 5 seconds is way too short.

#### samb

Good advice so far. I'll add another tool that I found useful in learning how to choose motors that doesn't make use of a full featured sim program. John Coker's site has a number of tutorials on many areas of hobby rocketry. Choosing-motors is a good one. I almost always use Thrustcurve.org first before diving into Openrocket.

http://www.jcrocket.com/choosing-motors.shtml

The 2.6inch DX3 comes with a 29mm motor mount which matches the case you have. The 4inch Super DX3 comes with a 38mm motor mount and would require an adapter to use the 29/40-120 case. Which one is it ?

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#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
DX3 or Super DX3? I've flown my Super DX3 many times on G64-4, but never on G53.

Keep in mind that the Class 1 limit is 1500 grams loaded, and the Super DX3 will come close to that or exceed it depending on how heavily you build.

If you really meant DX3, that should be fine on a G64-7 or a G53-7, 5 seconds is way too short.
Mike,
Thanks! Super DX3 is the one I'm looking at. According to OpenRocket, the optimum delay is 5.12 seconds @ 2.5 m/s wind. You experience and observation says 7 seconds is where its supposed to be? I wonder how accurate the simulations are?

mark

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
Good advice so far. I'll add another tool that I found useful in learning how to choose motors that doesn't make use of a full featured sim program. John Coker's site has a number of tutorials on many areas of hobby rocketry. Choosing-motors is a good one. I almost always use Thrustcurve.org first before diving into Openrocket.

http://www.jcrocket.com/choosing-motors.shtml

The 2.6inch DX3 comes with a 29mm motor mount which matches the case you have. The 4inch Super DX3 comes with a 38mm motor mount and would require an adapter to use the 29/40-120 case. Which one is it ?

Sam,
Thanks for the data! I'll take a look for sure . . . It is the 4 inch Super DX3. Yup, the adapter would be in the cards. I'd also purchase a 38mm system to run it in the Level Category One when I'm ready to go for that certification. Of course buying both wouldn't be bad . . .hmmm

Mark

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#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
It would seem that 8 foot rails (244cm) seem to cure many ills . . . at least on dead calm days!
Mark

#### mikec

##### Well-Known Member
Mike,
Thanks! Super DX3 is the one I'm looking at. According to OpenRocket, the optimum delay is 5.12 seconds @ 2.5 m/s wind. You experience and observation says 7 seconds is where its supposed to be?
No, please read what I said. G64-4 on Super DX3. Weigh your rocket first to know if you can even fly legally without an FAA waiver.

#### samb

Hmmm... ok. Like mikec said, you're gonna have a challenge keeping the build light enough in order to keep the flight weight under the high power threshold. I don't know about down your way but here in DFW dead calm days are a myth. I think flights on mid-power G's would be "interesting " in the extreme and borderline unsafe if you're much below the 50fps rule of thumb that bat-mite mentioned. You said in another thread that you were going to attend a club launch which I think is a great idea. Talking to the locals should give you some more perspective and ideas on how to proceed.

#### Nytrunner

##### Pop lugs, not drugs
Mike,
Thanks! Super DX3 is the one I'm looking at. According to OpenRocket, the optimum delay is 5.12 seconds @ 2.5 m/s wind. You experience and observation says 7 seconds is where its supposed to be? I wonder how accurate the simulations are?

mark

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
I used it as is . . . good point! I need to adjust the required parameters for my specific build.

Thanks!

Mark

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
Hmmm... ok. Like mikec said, you're gonna have a challenge keeping the build light enough in order to keep the flight weight under the high power threshold. I don't know about down your way but here in DFW dead calm days are a myth. I think flights on mid-power G's would be "interesting " in the extreme and borderline unsafe if you're much below the 50fps rule of thumb that bat-mite mentioned. You said in another thread that you were going to attend a club launch which I think is a great idea. Talking to the locals should give you some more perspective and ideas on how to proceed.
Sam,
I'm still learning, and posts like this help me out greatly. My son and I are going to fire off a couple of small Estes rockets (assuming they let us, I'm sure they will) and I'm going to find out about the 'lay of the land' . . . I can purchase the Madcow DX3 in both 4 and 2.6" diameter versions for mid/high power . . . that will solve any borderline safety issues . . . at least I hope so! I won't be very popular firing off major safety violations on my first mid-power launch, for sure . . . Thanks again!

Mark

#### Nytrunner

##### Pop lugs, not drugs
I used it as is . . . good point! I need to adjust the required parameters for my specific build.

Thanks!

Mark
You got it, just want to head off any unintended miscalculations.

The built weight of your model including adhesives, paint, and any electronics or mods you add will probably not match the manufacturer file (I always do things to my rockets so I prefer to build up my Openrocket sims myself). Initial guesses from thrustcurve are good enough for me until I get something together.
The 2.6 should be good for those -10 delays and can be drilled accordingly (I just learned those are adjustable!).

-Either keep track of weight changes as you build, or just finish it up and weigh it at the end.
If it's a simple build, I won't even bother with internal components beyond a motor tube, I'll just use the CG override option to match the balance point and weight of the built model.
For more complex rockets (like my L2), I did track all components, then had a floating "misc" mass object that I used to account for glue/paint.

Have fun!

#### boatgeek

##### Well-Known Member
Your effective rail length is the distance from the top rail button to the top of the rail. Once the top button clears the rail, the rocket is free to point any which way. I usually use 6 feet in OpenRocket for a nominal 8' rail to be safe. You can always also ask your club how long the rails are above the stop at the bottom and subtract the distance from the bottom of your rocket to the top rail button.

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
No, please read what I said. G64-4 on Super DX3. Weigh your rocket first to know if you can even fly legally without an FAA waiver.
Mike,
Ok . . . I get it . . . I'm a bit thick at times - haha!

Thanks!

Mark

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
You got it, just want to head off any unintended miscalculations.

THANKS!!

-Either keep track of weight changes as you build, or just finish it up and weigh it at the end.

Great Idea! Makes complete sense . . . I've a kitchen scale that should do the trick . . . and for finer measurements I've reloading scale as well.

Have fun!
Thanks Again!

Mark

#### solid_fuel

The best way to deal with the motor issues you are having is to buy more than one rocket
Seriously

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
The best way to deal with the motor issues you are having is to buy more than one rocket
Seriously
Sounds like a plan!
Mark

#### Sabrina

##### The WildChild
TRF Supporter
Greetings!
Returning back to the hobby since many years absent, will be enjoying building and launching rockets with my 5 year old son. I've taken an interest in building as my first mid/high power rocket the 4" Paper Madcow DX3. Since I couldn't find it new from a dealer, I purchased an Aerotech 29/40-120 RMS along with 6 reloadable motors. I downloaded and ran simulations with these motors, and all appears be good. I plan on obtaining the delay time tool to do any needed adjustment to the motors where the delay is too long. The velocity @ parachute deployment for all except the G64-4W is between 1.16 to 2.05 m/sec. I'm assuming that this is a good velocity range? For the G64-4W engine it states the velocity is 10.8 m/sec - is this too fast for this rocket? I have read elsewhere that 20 to 25 fps is the max . . . What is the generally recognized fastest velocity at deployment for 4" paper tube rockets?

G53-5FJ (works great as is)
G64-10W (drilled to 5 seconds)
G76-10G (drilled to 5 seconds)

Another question is the velocity off rod. For the G53FJ it is 13.4 m/sec from a 6' (184 cm) rod. Is this sufficient? What is the minimum? How can I find/calculate that minimum? I found the following statement on NAR from their launch safety pdf: "Best practice would indicate that rockets should be guided by launch rods, rails, or towers until they have attained a forward velocity of at least 4 times the velocity at which the wind is blowing (or gusting) at the launch site" If the wind is still or very low?

Thanks for any advice . . . feel free to point me to published tables and etc. . . .

Mark Sanderson

Hi Mark - Welcome to the forum!!!

My name is Sabrina, I'm in High School now, but I have been launching rockets all my life.

I'm sure you and your 5yr old will be having a BLAST! Rocketry is a hobby the whole family can enjoy. My dad's been flying since he was a teenager. He introduced my mom to rocketry. She did Level 1 and 2 with the same 2.6" rocket. I attended my first launch as a baby.

I just want to remind you to pick up some "Ready-to-fly rockets" for your son. Show him how to put in the motor... "A" size motor is best because -nothing worse than losing a rocket that went too high. -Also streamers are so much easier than a parachute.

A Super Big Bertha - My favorite rocket - will be almost as big as your son. I bet he would love to help you build it (hint hint)

Wanna see a smile - help him push the button the first few times...

Oh yea... one more thing. PICTURES. You should share some pictures - of your rockets and launches and stuff.

Here's some pictures of me - when I was a "smaller" rocketeer.

#### SpaceManMat

##### Space Nut
Your effective rail length is the distance from the top rail button to the top of the rail. Once the top button clears the rail, the rocket is free to point any which way. I usually use 6 feet in OpenRocket for a nominal 8' rail to be safe. You can always also ask your club how long the rails are above the stop at the bottom and subtract the distance from the bottom of your rocket to the top rail button.
Actually open rocket will do this calculation for you provided you enter the launch lugs in correctly.

#### Buckeye

##### Well-Known Member
TRF Supporter
Actually open rocket will do this calculation for you provided you enter the launch lugs in correctly.
Never saw this option. Can you elaborate?

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
Hi Mark - Welcome to the forum!!!

My name is Sabrina, I'm in High School now, but I have been launching rockets all my life.

...

View attachment 368831 View attachment 368832
Sabrina,
Thanks for the encouragement! That's the plan, have him help me with some of the smaller Estes rockets and we'll graduate to the mid and hi power ones. I was thinking streamers and helicopter-style recovery as well to throw in something unusual . . . Oh yeah, he'll be helping with setting up the rockets (maybe not the parachute quite yet, but streamers . . . probably so), counting down and pushing the launch button. I hope that he continues his love for the sciences like you have; and I'll certainly post some pictures of him launching a rocket!

Many Thanks!

Mark

#### markfsanderson

##### New Guy and Wannabee
TRF Supporter
Never saw this option. Can you elaborate?
Buckeye,
Flight Simulations -> Edit Simulation (after selecting a specific simulation) -> Launch Conditions -> Launch Rod

There is a place to insert the launch rod length. I'm assuming that this is similar to effect to a launch lugs using the buttons? It would seem so . . .

Mark

#### SpaceManMat

##### Space Nut
Buckeye,
Flight Simulations -> Edit Simulation (after selecting a specific simulation) -> Launch Conditions -> Launch Rod

There is a place to insert the launch rod length. I'm assuming that this is similar to effect to a launch lugs using the buttons? It would seem so . . .

Mark
Plus you simply put you launch lugs on the rocket design. The calculation is done automatically subtracting the difference between the top and bottom lug from the rod length.

#### Buckeye

##### Well-Known Member
TRF Supporter
Plus you simply put you launch lugs on the rocket design. The calculation is done automatically subtracting the difference between the top and bottom lug from the rod length.
Still not seeing this capability in v15.03. The user inputs the launch guide length. Where is this automatic feature of which you speak?

#### dhbarr

##### Amateur Professional
Take a working sim that has lugs and observe the speed off the rail. Move the forward lug. Observe that the speed off the rail changes.

#### SpaceManMat

##### Space Nut
Still not seeing this capability in v15.03. The user inputs the launch guide length. Where is this automatic feature of which you speak?
It’s one of a gazillion calculations OR does on your behalf every time you ask it to do a simulation. Happens automatically - that is you get no say in it.

This is the section of code.
Code:
/*
* Calculate the effective launch rod length taking into account launch lugs.
* If no lugs are found, assume a tower launcher of full length.
*/
double length = this.simulationConditions.getLaunchRodLength();
double lugPosition = Double.NaN;
for (RocketComponent c : this.configuration.getActiveComponents()) {
if (cinstanceof LaunchLug) {
double pos = c.toAbsolute(new Coordinate(c.getLength()))[0].x;
if (Double.isNaN(lugPosition) || pos > lugPosition) {
lugPosition = pos;
}
}
}
if (!Double.isNaN(lugPosition)) {
double maxX = 0;
for (Coordinate c : this.configuration.getBounds()) {
if (c.x > maxX)
maxX = c.x;
}
if (maxX >= lugPosition) {
length = Math.max(0, length - (maxX - lugPosition));
}
}
this.effectiveLaunchRodLength = length;