A better way to test stability of small rockets.

[Senior Space Cadet]

I know some of you love your computer programs, and maybe I'm a dinosaur, but I don't think there is any substitute for real world testing.
An actual launch, with a low power engine, is still risky, and when you move to a bigger engine, it changes the balance, so it may not be a reliable test.
I think the spin test is a useful tool. If done right, passing it is probably proof of stability, but flunking it may not be proof of instability.
My proposal is the drop test. From a balcony or elevated deck or, maybe, a second story window, holding the rocket (ready for launch) horizontally, at the balance point, drop it into something that will prevent damage to the rocket. I'm thinking stretched out mosquito netting or a box full of fiber fil insulation.
If the rocket lands on it's tail, that's a sure fail.
If it lands on it's side, that's worrisome.
If it lands nose first, that's a sure pass.

 

[dhbarr]

You may find that ~15 feet at only 1g of acceleration isn't much time for the fins to do their work. Let us know what you find out!

 

[gldknght]

I know some of you love your computer programs, and maybe I'm a dinosaur, but I don't think there is any substitute for real world testing.
An actual launch, with a low power engine, is still risky, and when you move to a bigger engine, it changes the balance, so it may not be a reliable test.
I think the spin test is a useful tool. If done right, passing it is probably proof of stability, but flunking it may not be proof of instability.
My proposal is the drop test. From a balcony or elevated deck or, maybe, a second story window, holding the rocket (ready for launch) horizontally, at the balance point, drop it into something that will prevent damage to the rocket. I'm thinking stretched out mosquito netting or a box full of fiber fil insulation.
If the rocket lands on it's tail, that's a sure fail.
If it lands on it's side, that's worrisome.
If it lands nose first, that's a sure pass.

No.
When we launch a rocket, we generally use a velocity of 45 feet per second, as a guideline minimum velocity for stability at the point of launch rod departure.
Using your 'drop test' method, your rocket will essentially be departing "the launch rod", (i.e. your hand) with a zero feet per second velocity.
Using gravity only for acceleration your rocket will need to fall AT LEAST 31.4 feet BEFORE it will become stable. It should also be noted that the math for determining this distance assumes a free-fall state. So with air resistance, your rocket will actually need more than 31.4 feet to become stable.

Again, by all means, go ahead and drop your rocket. Don't be surprised if it fails this test every single time.

Also, I strongly recommend against dropping a rocket with a loaded motor. It won't take much to crack the propellant inside the motor. If you then try to fly the motor, spectacular things will happen at ignition.

 

[Steve Shannon]

Another problem with what you propose is that you also need to have enough distance for your rocket to react realistically, even if that means going unstable. A wind tunnel works well. You can duplicate that by attaching a low friction pivot at the CG of your rocket and driving with the rocket in the airstream above your vehicle.

But, simulation software works really well. I’ve been able to teach students how to use it starting at the sixth grade. They pick it up very quickly. By the way, the change due to the weight of larger motors is handle well by the software.

 

[dr wogz]

Senior, I think your thought process is valid, and scientifically make sense. But in practical terms, it comes out to be more of a guess. And, as my dad said to me (numerous time!!) >> Don't guess, make sure<<

The absolute best way to validate your design is to crunch the numbers. The Barrowman equation & simple physics equations will get you to the point of "yes, this is valid / a viable design". Then a true physical test is in order to validate those numbers. This will either validate or disprove your calculations. Then repeat. Tweak as needed, adjust certain parameters, know what the % error is, and what is an acceptable margin of error..

You can do all this by hand, and maybe with a calculator.
You can plug the numbers into an Excel spreadsheet.
You can also start to do fluids calculations to see how your design will pass thru air, how the fins react, drag coefficients, areas of high pressure, etc..
You can even go as far as FEA your design to validate its structure and whether it can withstand the forces you intend to apply.
You can use a tool, a program designed with this all in mind takes out a lot of the trial & tedium. Many people have added to, validated, and suggested improvements to the program. Therefore, we are very sure of it's accuracy. Rocsim or Open rocket will do a very good job at validating your design over a range of criteria, and will report back its findings.

Remember, the Computer is a tool, a glorified calculator that has been tailored to solve a very specific (and expansive) set of criteria to achieve a result.

We will all stand behind Rocsim or OR for validation.

And, i'm sure 30 Helens will also agree!

 

[Daddyisabar]

Build a wind tunnel!

 

[Senior Space Cadet]

Another problem with what you propose is that you also need to have enough distance for your rocket to react realistically, even if that means going unstable. A wind tunnel works well. You can duplicate that by attaching a low friction pivot at the CG of your rocket and driving with the rocket in the airstream above your vehicle.

But, simulation software works really well. I’ve been able to teach students how to use it starting at the sixth grade. They pick it up very quickly. By the way, the change due to the weight of larger motors is handle well by the software.

I was considering finding some sort of pivot and putting it in front of a high speed fan.

 

[Steve Shannon]

I was considering finding some sort of pivot and putting it in front of a high speed fan.

If you make a scale model, a leaf blower will give you the velocity you need.

 

[Daddyisabar]

The best thing to do is to get some experience and fly with experienced people. Launch from the far away pads. Use a conservative CG location then dial back on nose weight. You will then get a sense to mindsim. As your rockets become more odd stability will be a minor worry. Inexperience with a too complex rocket is the real danger.

Do not get in the back of Cooter's old pickup with the rocket on the string and tell "give her all she's got!"

 

[Senior Space Cadet]

No.
When we launch a rocket, we generally use a velocity of 45 feet per second, as a guideline minimum velocity for stability at the point of launch rod departure.
Using your 'drop test' method, your rocket will essentially be departing "the launch rod", (i.e. your hand) with a zero feet per second velocity.
Using gravity only for acceleration your rocket will need to fall AT LEAST 31.4 feet BEFORE it will become stable. It should also be noted that the math for determining this distance assumes a free-fall state. So with air resistance, your rocket will actually need more than 31.4 feet to become stable.

Again, by all means, go ahead and drop your rocket. Don't be surprised if it fails this test every single time.

Also, I strongly recommend against dropping a rocket with a loaded motor. It won't take much to crack the propellant inside the motor. If you then try to fly the motor, spectacular things will happen at ignition.

If you did the test the way I said, there would be zero chance of damaging the engine. You could drop an egg and it wouldn't break. As for the stability part, I don't have a rocket ready that I can test. But I could go drop an arrow right now. If it doesn't land point first, then you are most likely right. Let me go do it right now (sound of footsteps walking away). (sound of footsteps coming back). Well, you are probably right. The arrow hit the ground at about a 45 degree angle. Thanks for straightening me out, but I'm sure I would have figured it out the first time I dropped a rocket. I guess I'll have to work on that wind tunnel. I was thinking about a design a couple weeks ago. I already have a high speed fan.

 

[BEC]

Estes Technical Report TR-5 (from 1965) details how to build and use a wind tunnel powered by a furnace blower with a 12x12 inch test section.... It starts on page 13 of this document.

 

[Senior Space Cadet]

The best thing to do is to get some experience and fly with experienced people. Launch from the far away pads. Use a conservative CG location then dial back on nose weight. You will then get a sense to mindsim. As your rockets become more odd stability will be a minor worry. Inexperience with a too complex rocket is the real danger.

Do not get in the back of Cooter's old pickup with the rocket on the string and tell "give her all she's got!"

I'm going for the conservative CG already. As for the rest, pretty hard when no one is launching. Most likely I'll launch with the Northern Colorado club, if they will have me. They launch in Pawnee National Grasslands. If you haven't been there, it's an experience. If you have a fear of open spaces, it's not for you. The government has a free shooting range there, that I've been to a few times. Great place for it. Nothing to hit.

 

[Senior Space Cadet]

I think it's time I posted some photos of the boat I designed and built. It might explain why I feel pretty confident I can handle designing and building a rocket. I guarantee that any rocket I actually take to the launch pad will fly just fine. I'll get some photos ready.

 

[gldknght]

If you did the test the way I said, there would be zero chance of damaging the engine. You could drop an egg and it wouldn't break.

The problem with that is if you do manage to crack the propellant, you will not be able to tell, until the launch button is pushed.
Why not simulate the weight of the motor? According to Estes, their B6-4 weighs about 18 grams. It should be easy to add the correct amount of weight inside a wrap of paper towel and stuff it in the backside of your rocket.

After reading most or all of your posts, it's easy to come to the conclusion that you are really just trying to "re-invent the wheel" with all of this. I understand not wanting to fly the same thing as everyone else, and the desire to experiment with different designs. This drive is exactly why programs like Rocksim exist. Why not use the tools that exist and stop second guessing everything? People have been doing model rocketry for a very long time, (Estes has been in business for over 60 YEARS!) and the reason we do things the way we do is because IT WORKS! We know we can't put the main fins at the front of a body tube without substantial design mods. We know we can't safely reload a spent single use motor case. We know that fins must be a certain size to establish the correct CP/CG relationship.

Oh, and by the way, what the h**l does boat design have to do with model rocketry?

 

[Senior Space Cadet]

Here are three photos. One of my boat while under construction. One of my finished boat, and one a couple of my friends at the Pawnee National Grasslands shooting range. As I said, one of the clubs launches in the Grasslands.
Remember, I didn't just build this boat, I designed it with a boat CAD program. I'm no genius, but I'm no idiot either.

 

[Senior Space Cadet]

The problem with that is if you do manage to crack the propellant, you will not be able to tell, until the launch button is pushed.
Why not simulate the weight of the motor? According to Estes, their B6-4 weighs about 18 grams. It should be easy to add the correct amount of weight inside a wrap of paper towel and stuff it in the backside of your rocket.

After reading most or all of your posts, it's easy to come to the conclusion that you are really just trying to "re-invent the wheel" with all of this. I understand not wanting to fly the same thing as everyone else, and the desire to experiment with different designs. This drive is exactly why programs like Rocksim exist. Why not use the tools that exist and stop second guessing everything? People have been doing model rocketry for a very long time, (Estes has been in business for over 60 YEARS!) and the reason we do things the way we do is because IT WORKS! We know we can't put the main fins at the front of a body tube without substantial design mods. We know we can't safely reload a spent single use motor case. We know that fins must be a certain size to establish the correct CP/CG relationship.

Oh, and by the way, what the h**l does boat design have to do with model rocketry?

There would be zero (0) chance of damaging the motor, and motors aren't so expensive that it wouldn't be worth sacrificing one for a test, but it makes no difference one way or the other, since we've already determined that my idea doesn't work very well.

 

[watheyak]

Imagine being a test pilot for a new spy plane. Just before your first flight Kelly Johnson patted you on the back and said "I don't believe in math. Real world testing is where it's at. Hop in!"

The real world testing you're proposing is inaccurate verging on dangerous, maybe even reckless. Early rocketeers knew this was a bad idea. They validated their designs with "slide rules and cigarettes" before they ever put anything on the pad. These days we've got software to save is from the health issues caused by slide rules.

Please explain why you wouldn't use something like OpenRocket. They are accurate, safe and proven.

I feel that "I don't like computers" is not a valid argument.

 

[gldknght]

Imagine being a test pilot for a new spy plane. Just before your first flight Kelly Johnson patted you on the back and said "I don't believe in math. Real world testing is where it's at. Hop in!"

The real world testing you're proposing is inaccurate verging on dangerous, maybe even reckless. Early rocketeers knew this was a bad idea. They validated their designs with "slide rules and cigarettes" before they ever put anything on the pad. These days we've got software to save is from the health issues caused by slide rules.

Please explain why you wouldn't use something like OpenRocket. They are accurate, safe and proven.

I feel that "I don't like computers" is not a valid argument.

I could not agree with this more!

 

[lakeroadster]

Do not get in the back of Cooter's old pickup with the rocket on the string and tell "give her all she's got!"

May not be any room in the pickup... Bo and Luke are bumming a ride since the General Lee has been impounded and is headed to Earl Scheib for a roof re-paint.

 

[Daddyisabar]

I'm going for the conservative CG already. As for the rest, pretty hard when no one is launching. Most likely I'll launch with the Northern Colorado club, if they will have me. They launch in Pawnee National Grasslands. If you haven't been there, it's an experience. If you have a fear of open spaces, it's not for you. The government has a free shooting range there, that I've been to a few times. Great place for it. Nothing to hit.

Great idea. There are a lot of crazies in Colorado that can help. I usually try to show everyone what I am doing to tone down the scare. Then listen to what they say. NCR or Tripoli in Hartsel is a great place to launch. SCORE in Pueblo is also awesome. Depends on fire danger and conditions for launch. The top men have seen it all so hard to scare if it follows most of the basic rules and has a chance. Heck, I flew the Avro Lancaster for the first time at NCR and that was a bit odd.

 

[Daddyisabar]

May not be any room in the pickup... Bo and Luke are bumming a ride since the General Lee has been impounded and is headed to Earl Scheib for a roof re-paint.

Press the horn and it plays Lady Gaga!

I will wait to see your model rocket General with the correct paint scheme and good ole Dixie playing as she leaves the pad!

 

[Alan15578]

I know some of you love your computer programs, and maybe I'm a dinosaur, but I don't think there is any substitute for real world testing.
An actual launch, with a low power engine, is still risky, and when you move to a bigger engine, it changes the balance, so it may not be a reliable test.
I think the spin test is a useful tool. If done right, passing it is probably proof of stability, but flunking it may not be proof of instability.
My proposal is the drop test. From a balcony or elevated deck or, maybe, a second story window, holding the rocket (ready for launch) horizontally, at the balance point, drop it into something that will prevent damage to the rocket. I'm thinking stretched out mosquito netting or a box full of fiber fil insulation.
If the rocket lands on it's tail, that's a sure fail.
If it lands on it's side, that's worrisome.
If it lands nose first, that's a sure pass.

Your post was not well received. Nevertheless, you can go back to MRM and find a credible article on measuring the Cd by throwing your rocket out the window.

 

[Senior Space Cadet]

Imagine being a test pilot for a new spy plane. Just before your first flight Kelly Johnson patted you on the back and said "I don't believe in math. Real world testing is where it's at. Hop in!"

The real world testing you're proposing is inaccurate verging on dangerous, maybe even reckless. Early rocketeers knew this was a bad idea. They validated their designs with "slide rules and cigarettes" before they ever put anything on the pad. These days we've got software to save is from the health issues caused by slide rules.

Please explain why you wouldn't use something like OpenRocket. They are accurate, safe and proven.

I feel that "I don't like computers" is not a valid argument.

You guys are hilarious.
I didn't say I didn't like computers.
I didn't say computers have no place in rocket design.
What I did say is it can't replace real world testing.
Taking your Kellly Johnson analogy, you think they would send an astronaut up without testing an unmanned rocket first? How many rockets have blown up on liftoff, despite being designed on a computer?
You think Boeing puts a plane into production, straight off the computer, without doing wind tunnel testing and flight tests?
Ford may design it's cars on a computer, but anyone into cars knows they do thousands of miles of testing before putting into production. No car company depends solely on computers before going into production.
That's all I'm saying. Have your fun, but I'm right. Computers will never replace real world testing.

 

[Senior Space Cadet]

I might also point out that your computer programs predicted my arrow vanes wouldn't work as rocket fins, but, in the spin test they performed extremely well.
If your computer modeling is infallible, how do you explain that?
I think I've found a place high enough to do the drop test. Just hope I can hit the box from this height. If I were afraid of damaging the motor, I could, of course, I could use a used motor, filled with sand, or some other material, to bring the weight back up. I'm more concerned about the fins and nosecone.

 

[Senior Space Cadet]

Your post was not well received. Nevertheless, you can go back to MRM and find a credible article on measuring the Cd by throwing your rocket out the window.

You think?

 

[Senior Space Cadet]

You know, I was just trying to help. I thought I had a good idea. Obviously I was wrong, at least at the height I suggested. My intention wasn't to tick off every model rocket builder that uses Open Rocket or some other program.
I never proposed driving around in my pickup, but that might actually work. I'd probably have to mount it to my roof rack. I'd have to get my cousin, Billy Bob to hang out the window and take video. Or my wife, Billy Joe, who is also my cousin. I've seen tents tested this way and, I believe, Myth Busters has done stuff like this.
I really am thinking about building a wind tunnel. Have been for weeks. Not sure what to use as a gimbal. Should probably be on a stem, so as not to mess up the airflow too much. I could mount the rocket between two strings, one above and one below.

 

[Steve Shannon]

My own unhelpful comment removed.

 

[Antares JS]

Computers will never replace real world testing.

No, not completely, but a computer equipped with OpenRocket or Rocksim can make your life a lot easier. Rather than having to build a large number of test articles to figure out what works, you can validate a design using the software and then build a test article that you know will probably work. Boeing doesn't put a plane into production without real-world testing, but nor do they slap together a test article and hope it works without validating it first.

That said, having read a bunch of your threads, I really think you're overthinking this whole testing thing. If you have a standard-configuration 3 or 4FNC low-power rocket that's constructed properly, there's generally really no reason to believe it's going to create an unsafe condition when it flies. You can just fly it with a data-collecting instrument on board and see what happens.

However, maybe extensive testing is something you find fun, in which case, ignore me and do what you like.

 

[watheyak]

You guys are hilarious.
I didn't say I didn't like computers.
I didn't say computers have no place in rocket design.
What I did say is it can't replace real world testing.
Taking your Kellly Johnson analogy, you think they would send an astronaut up without testing an unmanned rocket first? How many rockets have blown up on liftoff, despite being designed on a computer?
You think Boeing puts a plane into production, straight off the computer, without doing wind tunnel testing and flight tests?
Ford may design it's cars on a computer, but anyone into cars knows they do thousands of miles of testing before putting into production. No car company depends solely on computers before going into production.
That's all I'm saying. Have your fun, but I'm right. Computers will never replace real world testing.

My point was that there were computers and slide rules used BEFORE real world testing. Skipping that first part is lazy.

Boeing already knew the 787 was going to be amazing, because the computers told them so. It took real world testing to find out the batteries catch on fire.

 

[JoePfeiffer]

Even assuming you drop your rocket from a high enough spot to let it get fast enough for the fins to become effective, dropping it horizontally won't give an accurate result -- the body tube has a negligible influence on stability at low angles of attack but a huge event at 90, so a rocket that would be stable under actual flight conditions could flunk the test. You'd need to point it down and use a sufficiently long rod to keep it pointed down until it was going fast enough to mimic actual flight conditions.

You're right that an actual physical test will give a more trustworthy result than a simulation, but a wind tunnel is the way to do it.