Need Advice on "H" Motor for Level 1 Cert

[loopy]

I've heard the H550ST to be called a "blow the fins off" motor. Might need to fiberglass the fins to prevent flutter of the large Goblin fins.

H550 doesn't burn long enough to worry too much about that, honestly. It hits hard, but is over quick, so it should be fine. It definitely gets peoples attention when it fires though...lol

 

[dmaaero]

Here is a comparison of the H115 to the H550... quiet a bit if difference there.

 

[Ronz Rocketz]

H550 doesn't burn long enough to worry too much about that, honestly. It hits hard, but is over quick, so it should be fine. It definitely gets peoples attention when it fires though...lol

Becky told me this but may have been confused with the K550. Dr Hudson said if the fins are surface mounted, they'll leave the fins behind.

I ended up using an I285GR for my second flight but added too much love to the ejection charge which blew off the chute.

 

[Neutronium95]

I've heard the H550ST to be called a "blow the fins off" motor. Might need to fiberglass the fins to prevent flutter of the large Goblin fins.

Airspeed is what tears fins off, not thrust or acceleration. While the H550 has a lot of thrust, it doesn't have any more impulse than other motors the same size. If your rocket won't shred on a normal I motor it won't shred on a H550.

 

[2-0turbo]

Airspeed is what tears fins off, not thrust or acceleration. While the H550 has a lot of thrust, it doesn't have any more impulse than other motors the same size. If your rocket won't shred on a normal I motor it won't shred on a H550.

I tend to agree with this. I flew rockets in my teens and didn't understand the engineering concepts. Now, as an engineer, let’s just say it makes more sense. The last rocket I put together, all the fins (3 of them) weighed 7 ounces. So, 2.33 oz each. If you assume 75 G’s of acceleration, that means the fins sees 75 * 2.33 oz or 10.9 lbs of force from accel only. You’d have to add aero drag in there too, but just say 15 lbs. Center of mass of the fin is usually mid-span. So take half that weight and add it to the tip. Or just go pull on your fin with ~7.5 lbs. Did it come off? If you can build a decent rocket, the answer is usually NO.

But go do some flutter analysis and it’s a whole different story. At the resonant frequency, it takes VERY LITTLE force to get the fin (or fins) oscillating back and forth and the loads are way higher and have a larger moment arms than the accel loads. It’s hard to know for certain, but I venture to say that most shreds are structural failures of a fin, tubing or coupler which then cascades into complete vehicle failure by the rocket trying to fly sideways at 700 mph. It doesn’t end well.

 

[FlyBy01]

Hi!

Going for my Level 1 on March 9th and I'm in the middle of building a 4" LOC Goblin. I don't want to get too crazy with 54mm or 38mm motors just yet, so looking for suggestions on a good "H" motor. I thought about getting an H115-DM sparky motor, but OpenRocket is telling me it'll only get up to maybe 900ft? Maybe less considering all the epoxy I put on it today lol.

I'm not sure what I'm after really, but maybe someone can suggest a motor that would be the best for my build? I'm still familiarizing myself with the different types. Any advice would be amazing and thank you in advance!

Speaking from experience if you have hardware or use a one time use motor choose a motor that will get off the rail safe and has a delay close to an easy to drill number. RocSim tends to over estimate altitude if your weight is not the same or similar to the sim. I back mine up with thrust curve the website.
The Goblin is a strong rocket and I have flown mine on a J800 so good luck and have fun!

 

[2-0turbo]

I tend to agree with this. I flew rockets in my teens and didn't understand the engineering concepts. Now, as an engineer, let’s just say it makes more sense. The last rocket I put together, all the fins (3 of them) weighed 7 ounces. So, 2.33 oz each. If you assume 75 G’s of acceleration, that means the fins sees 75 * 2.33 oz or 10.9 lbs of force from accel only. You’d have to add aero drag in there too, but just say 15 lbs. Center of mass of the fin is usually mid-span. So take half that weight and add it to the tip. Or just go pull on your fin with ~7.5 lbs. Did it come off? If you can build a decent rocket, the answer is usually NO.

But go do some flutter analysis and it’s a whole different story. At the resonant frequency, it takes VERY LITTLE force to get the fin (or fins) oscillating back and forth and the loads are way higher and have a larger moment arms than the accel loads. It’s hard to know for certain, but I venture to say that most shreds are structural failures of a fin, tubing or coupler which then cascades into complete vehicle failure by the rocket trying to fly sideways at 700 mph. It doesn’t end well.

I had to go and find the link for @Art Upton 's rocket--found it on his youtube channel: Fins waving bye-bye I don't think you're going to find a better video of fin flutter leading to structural failure.

But, back to the thread. I often forget to use Thrustcurve.org as I gravitate towards OR, but I've used it a couple times. It's much faster than OR for commercially available rockets that are already in the database. Bottom line, there are some great simulation aids out there. Plug in some criteria, go pick a motor and fly it.

 

[Ronz Rocketz]

That's why I'm thinking now it was the K550, not the H550 that would blow the fins off.

 

[OverTheTop]

Check out the red motors. Nice an punchy without being too wicked. Nice red flame too. My favourite

 

[2-0turbo]

That's why I'm thinking now it was the K550, not the H550 that would blow the fins off.

This is so cool. Great example of one that fluttered but stayed together. If that motor had burned for another second, it might not have. It would be interesting to know if the cameras and shrouds that show some of these videos perhaps contribute to this phenomenon by shedding vortices that excite the fins. I kind of doubt it, but I'd have to run some numbers to confirm.

 

[Stargeezer]

I apologize I just now had time to read through all of these comments. And now I am feeling a little overwhelmed. I just finished my build this morning but have yet to sand and paint. I am wondering HOW I can properly weigh this rocket? I don't have a large enough scale....I'm wondering what I could use to get a proper weight or where I might be able to go?

That said...I'm not sure if I want to "blow my fins off" lol.......there are so many options I don't know what to choose. And I'll be perfectly honest, someone suggested I buy an Aerotech Universal Delay Tool though I'm not sure how to use it?

I just want to make sure this launch goes off without a hitch, and without any hiccups or it nosediving. I've had 20 different people suggest 20 different motors so now I'm really uncertain what to buy.

 

[smstachwick]

I apologize I just now had time to read through all of these comments. And now I am feeling a little overwhelmed. I just finished my build this morning but have yet to sand and paint. I am wondering HOW I can properly weigh this rocket? I don't have a large enough scale....I'm wondering what I could use to get a proper weight or where I might be able to go?

That said...I'm not sure if I want to "blow my fins off" lol.......there are so many options I don't know what to choose. And I'll be perfectly honest, someone suggested I buy an Aerotech Universal Delay Tool though I'm not sure how to use it?

I just want to make sure this launch goes off without a hitch, and without any hiccups or it nosediving. I've had 20 different people suggest 20 different motors so now I'm really uncertain what to buy.

The delay tool is basically a calibrated drill, which reduces the delay’s burn time from the supplied maximum. Ensure that the tool is compatible with the motor you have (they’re not truly universal, despite the name) and follow the enclosed instructions to drill down to the delay time you want.

 

[kjhambrick]

@Stargeezer --

I've got a portable 5-lb / 2.5 Kg scale and a rocket that exceeds the capacity of my scale ( about 2.6 Kg without a motor ).

In this case I weigh my parts and sum them up:

Nose,
Payload Section ( Main Chute Tube + Main Chute + AV-Bay )
Fin Can + Drogue + Shock Cord.

Drogue ematch + BP
Main ematch + BP

Weigh the motor before flight,

I bring a plastic grocery bag full of dog barf to the launch and weigh the bag, prep the rocket and then weigh the bag after the rocket is prepped.

( I usually weigh it after loading the fin can and again after loading dog barf in the payload section )

The sum of those parts is my liftoff mass.

After the flight, I weigh my spent motor so I can get a propellant mass / rocket coast mass.

Maybe something like this might work for you ?

HTH and good luck with your L1 !

-- kjh

 

[Stargeezer]

I was looking into the 219T but it appears as though they are out of stock sadly.

 

[Stargeezer]

Here is a comparison of the H115 to the H550... quiet a bit if difference there.

So would you suggest an H115 for me? I have yet to weight my final build but will be doing so in the next day or so. When I ran schematics in OR it told me closer to 900ft.....hmmmm.

 

[tsmith1315]

I've had 20 different people suggest 20 different motors so now I'm really uncertain what to buy.

That's often the problem with online advice of any sort.


Talk to your motor dealer, you can depend on their knowledge. If they will be at the launch, you can run sims on several motors now and wait until the launch to choose one with their advice. This way, field conditions can be taken into account on flight day.

 

[Stargeezer]

That's often the problem with online advice of any sort.


Talk to your motor dealer, you can depend on their knowledge. If they will be at the launch, you can run sims on several motors now and wait until the launch to choose one with their advice. This way, field conditions can be taken into account on flight day.

The problem is I'm not sure if there will be motor dealers at this launch. They told me there "might" be......which I don't like that uncertainty.

 

[smstachwick]

The problem is I'm not sure if there will be motor dealers at this launch. They told me there "might" be......which I don't like that uncertainty.

Remember, you can always pick a different day to certify. The only deadline is your own.

The first problem to solve is measuring the rocket’s weight. Focus on that and only think about the next step when you’ve solved it. One step at a time.

 

[dmaaero]

So would you suggest an H115 for me? I have yet to weight my final build but will be doing so in the next day or so. When I ran schematics in OR it told me closer to 900ft.....hmmmm.

That"s why you need the weight and CG of "your" rocket. Until you get those, we are Siming someone else's rocket. I was just showing you the difference in what you chose (H115) and what others had chose (H550).
Someone once told me, if you walk down this flight line an ask 10 people the same question, you'll get 10 different answers. It"s up to you to figure out which one is correct. Get some numbers and we"ll see what RockSim recommends.
Harbor Freight sells a digital scale that goes to about 11 pounds for $20 grams, ounces, or pounds.

 

[Stargeezer]

That"s why you need the weight and CG of "your" rocket. Until you get those, we are Siming someone else's rocket. I was just showing you the difference in what you chose (H115) and what others had chose (H550).
Someone once told me, if you walk down this flight line an ask 10 people the same question, you'll get 10 different answers. It"s up to you to figure out which one is correct. Get some numbers and we"ll see what RockSim recommends.
Harbor Freight sells a digital scale that goes to about 11 pounds for $20 grams, ounces, or pounds.

I'm borrowing one of those scales this evening and will have some sort of numbers in the next coming days. I hope someone out there can help me figure some things...and if I need to shorten my delay time as well if I do use the H115. I'll write back soon.

 

[FlyBy01]

The problem is I'm not sure if there will be motor dealers at this launch. They told me there "might" be......which I don't like that uncertainty.

Email your club / prefect person who does the certification process. Club members help each other out by selling each other motors at cost or on motor selection all together.

Why not ask the person who will be signing your certification form?

They will appreciate you reaching out and can give solid advice for the field you are flying on.

 

[Stargeezer]

I've already sort of initiated that process with the help of the former club president who ironically lives here in my town (launch site is 2.5 hours west of here). Waiting on a reply and contact for motor vendor(s) who may be going to this March launch.

 

[FlyBy01]

I've already sort of initiated that process with the help of the former club president who ironically lives here in my town (launch site is 2.5 hours west of here). Waiting on a reply and contact for motor vendor(s) who may be going to this March launch.

Most clubs have a great support structure. I offered a motor and case for use just a few weeks ago to another club member going for a level 2.

I have my level 2 but I am always asking those with more experience for a sanity check to avoid turning my nice rocket into a kit again.

 

[Handeman]

Don't get too tided up with motor selection. You have a L1 rocket that should be able to handle baby H to full I motors. The only difference between all of those is going to be the final altitude. Pick any motor that will get you to an acceptable altitude for your flying field.

If you are unsure of the final weight, over estimate and pick a motor that will work with that over estimated weight. As long as that motor will work with an underestimated weight and resulting altitude at your field, you are inside the envelope and good to go.

Don't overthink it.

Good luck!

 

[cbrarick]

Don't worry about the grams, and probably not the ounces. when you've hit high power, pounds matter because that's what the motors are doing.
Some people like to get waaaay deep into the data, good for them, rock on. As we say on the trail, "hike your own hike."
but it's not necessary. I'm willing to bet even your bathroom scale would give you a "good enough" answer.
You will always be able to get an exact weight at the RSO. I have a friend who uses this to tune his rocksim numbers. He's amazingly accurate
I, on the other hand, just want to get the darn thing in the air, preferably fast. You might even accuse me of abusing my rockets.

bottom line: Handeman is right, get a motor big enough to get the rocket gone safely on that overestimated weight. Everything else falls into place.

 

[thzero]

Post office or UPS store and have them weigh it

 

[dmaaero]

Post your numbers and i will run some sims an post a screen capture. That will show you some options you have. All will have safe rod speed, safe deploy speed, correct delay and altitude. All you have to do is decide how far you want to walk to go get it.
Make sure you mark the CG and CP on your rocket, If they ask how you came up with what engine to use, having a copy of the sims would help. Just tell them why you chose that motor. (altitude) It would be better than saying cause that's what everyone on the internet suggested. You can also rerun the sims on OR an compare to RS.

 

[cbrarick]

Dmaaero:

I'd let the op run them. It's his cert and he should learn how to do it, not ask us to do it for him.

JMHO

 

[smstachwick]

Dmaaero:

I'd let the op run them. It's his cert and he should learn how to do it, not ask us to do it for him.

JMHO

Agreed. He can always send out an .ork file so we can double-check.

 

[dmaaero]

I"m going to agree with @cbrarick you should really learn to run them yourself. There"s a lot of good motor recommendations in this thread. Let us know what motor you chose an the sim results an someone can double check it for you.