Speed off the Rod Rule of Thumb and OpenRocket

[brockrwood]

I am simulating the Estes "Vapor" rocket in OpenRocket (see pic). I am using a 4 feet (122 cm) long launch rod. My simulations on both D and E motors show a speed off the rod of less than 15 meters/second. The D12-5 and E12-6 are Estes' recommended motors for this rocket. I read on this forum that a good rule of thumb for minimum speed "off the rod" is 15 m/s. Even the E12 engine doesn't quite hit that rule of thumb number. Maybe the simulation data is incorrect? I will check the materials in the data file for OpenRocket.

Maybe the Vapor is just a "little slow off the pad"? If so, I guess we will find out if that is a problem later this year when I build and launch it.

 

[Charles_McG]

My speed of the rod rule is 4X the wind speed. And I assume a minimum of 5mph, since wind become hard to detect by feel about there.

 

[brockrwood]

My speed of the rod rule is 4X the wind speed. And I assume a minimum of 5mph, since wind become hard to detect by feel about there.

So, with a wind speed of 5 mph that is 20 mph. That translates to 8.9 meters/second. Gotcha. The OpenRocket simulation is just fine if I use 8.9 m/s as the rule of thumb.

 

[brockrwood]

My speed of the rod rule is 4X the wind speed. And I assume a minimum of 5mph, since wind become hard to detect by feel about there.

Lessee. I avoid launching in wind greater than 7 mph or so. So, with the 4x rule of thumb, if the wind speed is 7 mph, then I need an off the rod speed of 7 x 4 or 28 mph. That is 12.51 m/s. The Vapor would still be OK on either the D12 or the E12.

 

[ZoomieG]

I am simulating the Estes "Vapor" rocket in OpenRocket (see pic)..... Maybe the simulation data is incorrect? I will check the materials in the data file for OpenRocket.

Check your weight. Estes advertised weight for Vapor is 3.5oz/99.2 grams. At 305 grams, you likely have some bad data for component materials.

 

[brockrwood]

Check your weight. Estes advertised weight for Vapor is 3.5oz/99.2 grams. At 305 grams, you likely have some bad data for component materials.

Thanks!

- Brock

 

[OverTheTop]

I will fly with a rail exit of 40fps if the wind is quite calm. My normal rail exit speed is above 50fps.

 

[Charles_McG]

Correction: My rule of thumb is >= 4x the wind speed. Slight distinction.

 

[brockrwood]

Thanks!

- Brock

I am trying to get weight down in OpenRocket but weight still says 7.09 ounces. That can't be right.

 

[brockrwood]

Check your weight. Estes advertised weight for Vapor is 3.5oz/99.2 grams. At 305 grams, you likely have some bad data for component materials.

I am trying to get the weight down in OpenRocket but it still says 7.09 ounces. That can't be right.

 

[brockrwood]

Check your weight. Estes advertised weight for Vapor is 3.5oz/99.2 grams. At 305 grams, you likely have some bad data for component materials.

That was it! Several components, including the parachute and the engine block had the mass set to "override". The mass was set FAR too high. OpenRocket now says the Vapor weighs 3.63 ounces, no engine installed. That's more like it!

The people who upload Rocksim files to rocketreviews.com do a great job but they are not infallible. Lesson learned: Check your input data before blaming the simulation software for weird numbers!

 

[ZoomieG]

That was it!

Glad I could help.

In the future, posting the open rocket file itself helps others troubleshoot.

As far as rock sim files on rocket reviews, anyone can upload them. Some can be rather error prone.

 

[bjphoenix]

The Vapor seems to be generic BT-60 rocket, just with a medium long tube. I seem to gravitate towards BT-60 rockets so consider some data points- I have some shorter ones- Estes ESAM and Estes Patriot Missile, both get off the pad well enough with B6. I have a HiFlier XL that is probably similar to the Vapor in weight and it gets off the pad pretty fast with a D12. I've noticed that the Estes Mean Machine flies well enough on D and E motors and it has got to be substantially heavier than the Vapor.
I happened to have just bought a Vapor kit yesterday, and another ESAM. I'm not sure if I will build the Vapor stock but I know I'll kit bash the ESAM.

 

[brockrwood]

The Vapor seems to be generic BT-60 rocket, just with a medium long tube. I seem to gravitate towards BT-60 rockets so consider some data points- I have some shorter ones- Estes ESAM and Estes Patriot Missile, both get off the pad well enough with B6. I have a HiFlier XL that is probably similar to the Vapor in weight and it gets off the pad pretty fast with a D12. I've noticed that the Estes Mean Machine flies well enough on D and E motors and it has got to be substantially heavier than the Vapor.
I happened to have just bought a Vapor kit yesterday, and another ESAM. I'm not sure if I will build the Vapor stock but I know I'll kit bash the ESAM.

BT-60 rockets are the “sweet spot” of Low Power Rocketry. Big enough to be fun and eye-catching. Big enough so that you can get a nice, nylon parachute and maybe even a baffle or nomex blanket in them with relative ease. Big enough to carry a fun payload. BUT, not so big that you need to expensive, composite motors to fly them. You can get C11’s, D’s, and E’s at your local hobby super-store. Not so big that you can’t launch them on an ordinary 3/16” launch rod on a garden variety launch pad (the Estes Porta-Pad E works nicely, thank you).
I have some BT-80 LPR rocket kits to build but I haven’t finished having fun with the BT-60 rockets yet.

 

[smstachwick]

I have a HiFlier XL that is probably similar to the Vapor in weight and it gets off the pad pretty fast with a D12.

I’ve landed mine in the water on a D12. I have yet to fly it on a C11 but it’s recommended for them, apparently. That would probably be the smart windy-day motor for it, to get it back on the field.

BT-60 rockets are the “sweet spot” of Low Power Rocketry. Big enough to be fun and eye-catching. Big enough so that you can get a nice, nylon parachute and maybe even a baffle or nomex blanket in them with relative ease. Big enough to carry a fun payload. BUT, not so big that you need to expensive, composite motors to fly them. You can get C11’s, D’s, and E’s at your local hobby super-store. Not so big that you can’t launch them on an ordinary 3/16” launch rod on a garden variety launch pad (the Estes Porta-Pad E works nicely, thank you).
I have some BT-80 LPR rocket kits to build but I haven’t finished having fun with the BT-60 rockets yet.

Agreed. BT-60 also seems to be a good starting point to practice for something bigger. My BT-60 Star Orbiter has proven a good platform to gain experience with fin papering, through-the-wall epoxy construction, screw-on motor retainers, and reloadable motors. I’ve also seen dual-deploy conversions and I-motor launches (the latter ending in re-kits, sadly) with Star Orbiters on the forum.

 

[Zeta]

The Vapor seems to be generic BT-60 rocket, just with a medium long tube. I seem to gravitate towards BT-60 rockets so consider some data points- I have some shorter ones- Estes ESAM and Estes Patriot Missile, both get off the pad well enough with B6. I have a HiFlier XL that is probably similar to the Vapor in weight and it gets off the pad pretty fast with a D12. I've noticed that the Estes Mean Machine flies well enough on D and E motors and it has got to be substantially heavier than the Vapor.
I happened to have just bought a Vapor kit yesterday, and another ESAM. I'm not sure if I will build the Vapor stock but I know I'll kit bash the ESAM.

I would like to see what you do with the ESAM ....

 

[bjphoenix]

I would like to see what you do with the ESAM ....

The easiest thing would be to ditch the small rear fins and mount the main fins at the rear of the tube. Another thing I'm wanting to do is to clone the old Centuri Javelin.

 

[Sooner Boomer]

Mine weighs in at 6.8 oz unloaded. I built it with a stuffer/motor tube that goes through the bottom tube almost to the coupler (it's 14" long). It also has a 3D baffle in the coupler, a screw-on motor retainer, and a bit of nose weight. The maiden flight was on an E9-6. It was a calm day. Velocity off the pad was a little slow, but it went straight up. The motor seemed like it burned *forever*! This is a good motor for a calm day. With the rather smallish fins, it shouln't weathercock too much, but a windy day calls for more motor.

 

[Pete.D]

I love my Vapor; it's a very light rocket for its size, very durable and very stable (maybe too stable). Mine weighs 4.9 oz unloaded with stock screw-in engine retainer, parachute, and 2 coats of paint but no filler other than sanding sealer for the fins. The unloaded CG is 24 inches, and the RockSim predicted CP is 35 inches. Its maiden flight was on an Estes E12-6, resulting in a CG at 30 inches and 37 MPH off a 4-foot launch rod. That was a little over-stable, so anticipating some weather-cocking I tilted the launch rod 5 degrees downwind. The flight went nearly straight up in a 10 MPH wind, ejecting at 965 ft at 13 MPH with apogee at 974 ft and maximum speed of 191 MPH, as recorded by a Jolly Logic Altimeter 2 and easily recovered well short of the trees.

I've got lots of bigger and smaller rockets, but I find that the BT60 and BT80 sized body tubes are a sweet spot for modest-sized flying fields, giving impressive flights without going too high to see and/or drifting into the trees. They fly great on cheap black powder motors, and on bigger fields or calmer conditions would also be ideal for smaller composites.

 

[Steve Shannon]

I am simulating the Estes "Vapor" rocket in OpenRocket (see pic). I am using a 4 feet (122 cm) long launch rod. My simulations on both D and E motors show a speed off the rod of less than 15 meters/second. The D12-5 and E12-6 are Estes' recommended motors for this rocket. I read on this forum that a good rule of thumb for minimum speed "off the rod" is 15 m/s. Even the E12 engine doesn't quite hit that rule of thumb number. Maybe the simulation data is incorrect? I will check the materials in the data file for OpenRocket.

Maybe the Vapor is just a "little slow off the pad"? If so, I guess we will find out if that is a problem later this year when I build and launch it.

View attachment 516722

RSOs may ask what speed you expect to attain. 45 ft/s is acceptable for most cases (15 m/s is closer to 50 ft/s) and could be used as a minimum. Rules that strictly tie the minimum speed to wind speed are very useful when used in conjunction with a minimum speed.
If your sim shows too low of a speed re-run it with a longer rod or rail.

 

[ghostfather]

Rocksim seems to use 44 ft/s (13.4m/s) as the default minimum speed for stable flight.
I find that the rail length where it achieves this minimum speed to be more important than the speed at the top of the rail.
I like to see that the minimum stable speed is achieved halfway up the rail, or with at least 2 rail buttons still on the rail. Say you use a 10 ft rail, and your rail buttons are 2 ft apart, then you need to achieve the minimum stable speed at 8 ft, and even better at about 6 ft for a factor of safety

 

[smstachwick]

Is there a way to calculate or estimate rocket’s precise minimum speed for safe flight? I’ve been using 30mph but I’m also simming stuff that can run well over 3 caliber stability in flight configuration.

 

[lakeroadster]

Is there a way to calculate or estimate rocket’s precise minimum speed for safe flight? I’ve been using 30mph but I’m also simming stuff that can run well over 3 caliber stability in flight configuration.

Since you prefaced your question with the words "precise minimum", that number will vary based on each day's flight conditions.

There are so many variables... that's the beauty of a simulation program. But it's only as good as the data that's input. The old adage of "Garbage In, Garbage Out" applies here.

Stick with your 30 mph off the launch rod, you'll be good to go.

 

[bjphoenix]

Here is another very unscientific observation-
I do a lot of launch photographs and I pretty much ignore anything BT50 or smaller because they are too fast to catch. Simple BT55 and BT60 rockets with B, C or D black powder motors are still fast but I can usually catch them. Something a bit larger such as a Big Daddy on a D12 gets going so slowly that they usually have not moved very far when I catch them. But still a Big Daddy on a D12 usually gets a good launch on a 4' rod.
Here are some examples from 2 days ago. First is an Estes ESAM on a B6. This is a BT60 rocket not much smaller than a Vapor. Second is a Big Daddy on a D12.