Today I learned 55ft/s off the rail is kind of slow

Today I launched my PML Endeavor with a CTI I140 Skidmark. Thrustcurve put the rail speed at 55ft/s, which is technically safe although it's the lowest rail speed I've ever tried. Wind was light (5-8 mph).

When it left the rail, however, it probably ended up rotating at least 30 degrees from vertical before it built up enough speed to continue straight. Held my breath for a moment. I think in the future I'm going to shoot for a minimum of 60ft/s, even more in higher winds.

In the end it was a successful flight, aside from the long hike to where it ended up landing. The Skidmark was a nice loud and dramatic motor.

Velocity off the rail is important. Most flying I am doing starts at 8:1 thrust to weight on the pad, and most is higher than that.

Don't forget to account for a bit of rail friction.

mpitfield said:

Don't forget to account for a bit of rail friction.

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+1 The launch should provide cleaning pads at the rails and either Simple Green or WD40. I clean the rail and make sure there are no catches before loading my rocket.

Can always use a longer rail if you have that option, personally i've never used thrustcurve to simulate, i'll use rocksim and OR for that kind of info there are other variables than just wind.

A quick sim in OpenRocket shows the stock PML Endeavor with a CTI I 140 leaves the rail at 30 fps. I think that's the real issue. A big 4" rocket with a small motor.


Tony

ps: the accepted min speed is typically between 30 - 50 fps depending on windspeed. So very unlikely you were at 55 fps.

Yeah you have to be careful with those Skidmark motors, they make a lot of noise and sparks and smoke but they don't make a lot of thrust.

No, 55 ft/s is not slow. Thrustcurve defaults gave you a bogus answer. Indeed, a stock PML at 3.9 in, 71 oz, and 0.6 Cd will calculate as 55 ft/s on the I140. However, you don't know the launch guide length unless you log in and edit your own rocket. I use a 6' rail in my sims, and this setup gives just 42 ft/s on Thrustcurve. The only way I could duplicate the 55 ft/s was to use a 10 ft rail. 10 ft is a horrible default, or maybe there is some other bug in Thrustcurve for this scenario.

Yes, there is no way you were at 55 ft/s on this flight. At best, 42 ft/s, and probably much less considering your build is likely much more than 71 oz in mass. Moral of the story: Edit your rockets in Thrustcurve carefully.

50fps is not even a great threshold unless there is almost no wind. At launch guidance exit, the angle of attack becomes the result vector of the rocket velocity + the wind vector, this is just arctan( w / v ) where v = rocket velocity and w = crosswind. With a 10fps wind (around 7 mph), the angle of attack for 50 fps rocket velocity is 11.3 degrees. This will cause a serious rotation into the wind. Watching hundreds of TARC flights on lower thrust (F30) vs higher (F59/F79) thrust motors has fully confirmed this.

Nathan said:

Yeah you have to be careful with those Skidmark motors, they make a lot of noise and sparks and smoke but they don't make a lot of thrust.

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One word, "Loki K-830 Spitfire"

Okay that's more like two words and a code.

Although I agree relativity speaking.

Buckeye said:

No, 55 ft/s is not slow. Thrustcurve defaults gave you a bogus answer. Indeed, a stock PML at 3.9 in, 71 oz, and 0.6 Cd will calculate as 55 ft/s on the I140. However, you don't know the launch guide length unless you log in and edit your own rocket. I use a 6' rail in my sims, and this setup gives just 42 ft/s on Thrustcurve. The only way I could duplicate the 55 ft/s was to use a 10 ft rail. 10 ft is a horrible default, or maybe there is some other bug in Thrustcurve for this scenario.

Yes, there is no way you were at 55 ft/s on this flight. At best, 42 ft/s, and probably much less considering your build is likely much more than 71 oz in mass. Moral of the story: Edit your rockets in Thrustcurve carefully.

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Thanks for pointing that out. I didn't even realize you could save rockets on Thrustcurve, much less change the rail length. Sure enough the 6 foot actual rail length gives wildly different numbers...I never knew it was assuming a 10 foot rail.

I also never realized the actual dry weight of my Endeavor is significantly higher than advertised. I figured my liberal epoxy fillets only added a few ounces and never thought to actually weigh them. So now I've learned my Ariel is 53 ounces, only 2 ounces more than advertised. My Warlock is 112 ounces and not 100. But holy crap my Endeavor is 95 ounces...way more than 71. In retrospect I did get 3/16" instead of 1/8" fins, and I really went to town on all the fillets.

Using the corrected numbers my Endeavor left the rail at 36 ft/s.

Thanks for getting me straightened out on this before I did something really stupid.

Glad you figure it out. Thrustcurve is awesome for quickly comparing motors, especially for launch speed. Not sure what is going on with the default rail length. Maybe John Coker will chime in.

You can weigh your component mass before you attach every component to rocket. Then after you assemble and epoxy it you can have the epoxy mass. On OR you can add a mass like payload and specify it for the length of the fin tab if you feel like it's a rocket that had globs of epoxy on it. Some of the open rocket material densities don't match the rocketry airframe density of actual specified material. There's always slight variations. Sim the rail height value you will fly off of. I've done OR with four feet rail lengths and 70 fps off pad. Where details count are when the fifteen mile per hour gusts hit right off the pad unexpected. From then on I thought a four feet rail was too short and seventy feet per second while okay with RSO wasn't fast enough in my gut feeling. It tilted six degrees off pad and had to walk far. A little bit of variable crosswind will really test the exit velocity and stability behavior of a design. If your flying close to waiver allowable height the RSO may start caring a lot more of the coordinates and elevations entered as this will matter to altitude performance based on altitude densities. If you really want that sim spotless, care for the temperature,winds, and humidity before launch.

With all that yip yap said you've made a good point that thrust curve program has a lower default rail length than other programs used in rocketry. This default thrust curve rail length is not consistent with common 1010 or 1515 rail lengths of six to eight feet. This is poor coding choice of thrust curve program and could be corrected but the designer of program expects you to enter a realistic value. And a lot of people build better rockets than I do. It's easy to make an entry error into a program. You've learned never to underestimate a default program value. And luckily no one was harmed.

muddymooose said:

...<sniped>Using the corrected numbers my Endeavor left the rail at 36 ft/s..<snipped>

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Your new numbers are still over by about 10%, which is significant. I don't understand why aren't using something like OpenRocket. It's free and all the PML kits are already built for you. All you have to do is plug in your numbers. Now that I know the correct weight, OpenRocket shows your speed off the rail was much closer to 27ft/s, not 36ft/s. I think most everyone would agree anything under 30ft/s is an unsafe speed.

All I had to do in OpenRocket was add a mass object of 22.8 oz to bring the model up to your actual weight. The default rod/rail length is already 6', which is what most clubs use.

I don't have RockSim but it would be interesting if someone ran the sim with your weight and compared the results.


Tony

manixFan said:

Your new numbers are still over by about 10%, which is significant. I don't understand why aren't using something like OpenRocket. It's free and all the PML kits are already built for you. All you have to do is plug in your numbers. Now that I know the correct weight, OpenRocket shows your speed off the rail was much closer to 27ft/s, not 36ft/s. I think most everyone would agree anything under 30ft/s is an unsafe speed.

All I had to do in OpenRocket was add a mass object of 22.8 oz to bring the model up to your actual weight. The default rod/rail length is already 6', which is what most clubs use.

I don't have RockSim but it would be interesting if someone ran the sim with your weight and compared the results.


Tony

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When I got back into rocketry last summer I started working with the trial of RockSim. When it came time to buy half a dozen people told me RockSim and even OR was overkill as a motor guide and I should just use Thrustcurve. Thrustcurve worked fine until I started flying bigger rockets, although I now see what I've been doing wrong. Regardless I'll give OR a whirl.

Buckeye said:

Glad you figure it out. Thrustcurve is awesome for quickly comparing motors, especially for launch speed. Not sure what is going on with the default rail length. Maybe John Coker will chime in.

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I don't remember the default off hand, but this is indeed an area that could be better. Maybe there should be no default and the user required to enter a guide length?

The new site (if I ever finish it), prompts you up front for the guide length:
thrustcurve.herokuapp.com/motors/guide.html
It still has a default, but at least it's a clear input on the initial page.

manixFan said:

Your new numbers are still over by about 10%, which is significant. I don't understand why aren't using something like OpenRocket. It's free and all the PML kits are already built for you. All you have to do is plug in your numbers. Now that I know the correct weight, OpenRocket shows your speed off the rail was much closer to 27ft/s, not 36ft/s. I think most everyone would agree anything under 30ft/s is an unsafe speed.

All I had to do in OpenRocket was add a mass object of 22.8 oz to bring the model up to your actual weight. The default rod/rail length is already 6', which is what most clubs use.

I don't have RockSim but it would be interesting if someone ran the sim with your weight and compared the results.


Tony

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We don't use OpenRocket because it isn't necessary. All the stuff OR and RS are good for - like detailed Cd, weather conditions, flight profile - are meaningless in the first 6 feet of rocket travel. If stability is known, and not an issue, then the Thrustcurve calculation is perfectly fine by itself for launch speed.

In far less time than you spent downloading the file from PML, adjusting the weight, futzing with the motor configuration, and setting the simulation to get one result for the I140, I got 130 motor results from Thrustcurve. Efficiency.

Care to post your .ork file and motor file, because I think there is something wrong/inconsistent with your result. I would expect TC, OR, and RS to give the same result for launch guide velocity, and they do. I got 36 ft/s in all three cases.

muddymooose said:

When I got back into rocketry last summer I started working with the trial of RockSim. When it came time to buy half a dozen people told me RockSim and even OR was overkill as a motor guide and I should just use Thrustcurve. Thrustcurve worked fine until I started flying bigger rockets, although I now see what I've been doing wrong. Regardless I'll give OR a whirl.

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Who were the people who told you Rocksim and OR are overkill? If I were you so would stop listening to them immediately.

Thrustcurve is a useful application at basic prediction especially on the field. That said, it is not a replacement for either Rocksim or OR.

markkoelsch said:

Who were the people who told you Rocksim and OR are overkill?

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Buckeye said:

I would expect TC, OR, and RS to give the same result for launch guide velocity, and they do. I got 36 ft/s in all three cases.

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I am probably one of those people. For launch velocity, I rest my case. oint:

The more you fly high, fast, in wind, and away from sea level standard conditions, then OR and RS will be a better tool for detailed analysis. For quickly sorting through motors, nothing beats TC.

Regardless, kudos to the OP for putting some science and personal analysis into his flights, rather than "the motor vendor at the launch said this sparky should work in my rocket."

Buckeye said:

For quickly sorting through motors, nothing beats TC.

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Over the last year all of the TC numbers for my Ariel and MPRs closely matched my Altimeter3 numbers. Flights all went perfectly.

My recent problems were due to misunderstanding TC settings and underestimating the weights on my two newest/biggest rockets. Now that that's sorted out, the TC numbers should be relatively accurate.

From what I learned working with RS for a month, it really is unnecessarily complicated for simply picking motors for kit-built rockets. There are a lot of variables that in the end don't mean much, and because of all the options it's easy to enter something wrong. If I did a scratch-build I would use it (or OR) for sure, but TC seems reasonably accurate otherwise (provided you put in the right numbers...lesson learned).

Buckeye said:

Regardless, kudos to the OP for putting some science and personal analysis into his flights, rather than "the motor vendor at the launch said this sparky should work in my rocket."

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People do that? Half the fun is figuring out what motors will do what you want.

muddymooose said:

From what I learned working with RS for a month, it really is unnecessarily complicated for simply picking motors for kit-built rockets. There are a lot of variables that in the end don't mean much, and because of all the options it's easy to enter something wrong. If I did a scratch-build I would use it (or OR) for sure, but TC seems reasonably accurate otherwise (provided you put in the right numbers...lesson learned).

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I agree. 90% of my flights need just an estimate of launch speed, altitude, and delay so I can choose a good motor from my stash for the field conditions. TC does this just fine and only requires about 5 inputs on one page.

My other 10% need stability analysis, altimeter comparison, or have a specific performance objective. Then, I'll model the rocket in the more detailed, CAD-like simulators.

Buckeye said:

<snipped>....Care to post your .ork file and motor file, because I think there is something wrong/inconsistent with your result. I would expect TC, OR, and RS to give the same result for launch guide velocity, and they do. I got 36 ft/s in all three cases.

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Ok, so looks like now I'm the one goofing up. My OR sim shows 36.8 ft/s, which I intended to round to 37 but wrote down 27 instead! Dang. So of course the Thrustcurve numbers are correct as are all the others.

More importantly, when I logged in and simmed the OP's rocket as built, the CTI I140 was listed in the 'FAIL' section indicating it did not have enough velocity off the rod for a safe flight. So it really goes to show how sims are only as good as the information fed into them.

This whole thread has given me a renewed appreciation for the Thrustcurve sim tool. While I use the other parts of the website extensively, I hadn't used the sim function in years, if really ever. I do like how it can quickly give you a range of motors as a starting point and the numbers were certainly close enough on the few sims I ran. But the big caveat is you really need to login to be able to get an accurate launch velocity off the rail.

So, there you go, from a doubter to a believer. But the OP really needs to weigh his rockets!


Tony