Long burn motor (I65) and thrust to weight/safety

[Zyzzyva1000]

So I have never used one of these longer burning motors before, but have always wanted to do so. I have an SBR 3" Nike smoke that is a reasonable candidate. It has a 54 mm MMT, and has an ebay with altimeter (egg timer apogee) in the nosecone for electronic deployment. Fully loaded the rocket weights 1400 g, with an I65 it would be 2150g (so like 4.75 lbs).

Open rocket says thrust:weight is 4.15: 1 (which I presume is based on the average thrust), but since this has a 'kick' off the pad with higher thrust for a few seconds before tapering off (and open rocket sims at 71 fps rail exit velocity), is this a reasonable rocket and motor combination? Would love to try this out at LDRS, but I want to make sure I am not thinking about this wrong.

 

[Kane]

I recently flew an Aerotech I59 in a rocket roughly 5.8 lbs wet, also a 3" diameter, but longish so the stability margin is on the higher side.
Similarly, the TTW using average thrust was roughly 4.9:1. TTW using initial thrust was around 5.7:1. Super low winds on the day of the launch. I discussed the flight with our club's RSO prior to the launch date.
Initial thrust was good, but about 200' up she hit a layer of higher winds and severely weather cocked. Less than nominal flight even though the rocket was recovered.

First piece of advice: simulate using multiple wind speeds and establish your scrub limit. Second, discuss with your local club's RSO. Third, check your static margin. Higher margins (i.e., 2.5+) are going to suffer from weathercocking due to the increased lever arm between CG and CP. The higher the margin, the more susceptible to weathercocking.

 

[tsmith1315]

First piece of advice: simulate using multiple wind speeds and establish your scrub limit. Second, discuss with your local club's RSO.

And download the Windy app. Real- time winds at various altitudes in your location.

 

[Zyzzyva1000]

I recently flew an Aerotech I59 in a rocket roughly 5.8 lbs wet, also a 3" diameter, but longish so the stability margin is on the higher side.
Similarly, the TTW using average thrust was roughly 4.9:1. TTW using initial thrust was around 5.7:1. Super low winds on the day of the launch. I discussed the flight with our club's RSO prior to the launch date.
Initial thrust was good, but about 200' up she hit a layer of higher winds and severely weather cocked. Less than nominal flight even though the rocket was recovered.

First piece of advice: simulate using multiple wind speeds and establish your scrub limit. Second, discuss with your local club's RSO. Third, check your static margin. Higher margins (i.e., 2.5+) are going to suffer from weathercocking due to the increased lever arm between CG and CP. The higher the margin, the more susceptible to weathercocking.

I talked with my local RSO about it previously, he didn’t see a problem but I just wanted to get some other thoughts (to make sure I wasn’t doing something stupid at a national launch). I will sim again with multi level winds. Loaded with an I65 I am at 1.2 calibers of stability so hopefully less risk of weather cocking.

 

[waltr]

Kane's advice is perfect.

Another thing to consider is the rail length, longer allows higher velocity when exiting the rail.
I made a 1010 5 foot extension for lower TTW rockets. This and low wind speed makes for a nice safe flight.

 

[Steve Shannon]

Kane's advice is perfect.

Another thing to consider is the rail length, longer allows higher velocity when exiting the rail.
I made a 1010 5 foot extension for lower TTW rockets. This and low wind speed makes for a nice safe flight.

This^

The best thing to do is to run a simulation at different rail lengths and make sure that you’re going at least 45 feet per second when your rocket leaves the rail. If you have that information you’ll be able to help the RSO.

 

[Zyzzyva1000]

Ok, well open rocket seems to indicate I will be fine at 5 and 10 mph, and with a 10 ft rail I will exit at 71 feet per second, so I guess I have just been second guessing myself, time to make this happen.

 

[Kane]

Kane's advice is perfect.

Another thing to consider is the rail length, longer allows higher velocity when exiting the rail.
I made a 1010 5 foot extension for lower TTW rockets. This and low wind speed makes for a nice safe flight.

Yes! I forgot to mention rail length.

 

[SolarYellow]

I talked with my local RSO about it previously, he didn’t see a problem but I just wanted to get some other thoughts (to make sure I wasn’t doing something stupid at a national launch). I will sim again with multi level winds. Loaded with an I65 I am at 1.2 calibers of stability so hopefully less risk of weather cocking.

Run your sims with a variety of wind conditions. Do a search and read up on "wind caused instability." You can lose a lot of stability due to AOA off the rod. Your rail clearance velocity is good, but still worth checking. On a rocket likely to have an I65 motor loaded, 1.2 calibers isn't actually very much. I'd be a lot more comfortable with the 8-15 percent of overall length rule.

There's a lot of talk about weathercocking, a lot less about wind-caused instability. They can look the same when watching, but have opposite causes. The answer to both is rail/rod exit speed, but slow-burn motors are often challenged there.

If you're looking for a record (even just personal maximum achievement), it's worth recognizing that's not going to happen if conditions aren't close to optimum. Use the sim work to generate some go/no-go criteria for launch conditions and take along some other rockets that will still be fun to fly if the important one stays on the ground.

 

[Zyzzyva1000]

Run your sims with a variety of wind conditions. Do a search and read up on "wind caused instability." You can lose a lot of stability due to AOA off the rod. Your rail clearance velocity is good, but still worth checking. On a rocket likely to have an I65 motor loaded, 1.2 calibers isn't actually very much. I'd be a lot more comfortable with the 8-15 percent of overall length rule.

There's a lot of talk about weathercocking, a lot less about wind-caused instability. They can look the same when watching, but have opposite causes. The answer to both is rail/rod exit speed, but slow-burn motors are often challenged there.

If you're looking for a record (even just personal maximum achievement), it's worth recognizing that's not going to happen if conditions aren't close to optimum. Use the sim work to generate some go/no-go criteria for launch conditions, and take along some other rockets that will still be fun to fly if the important one stays on the ground.

So the rocket isn't that long, 1.3 calibers (I misread it earlier) is 9.3% (and this isn't taking into account my GPS that I will likely add into the nosecone, minimal weight but still some nose weight). I have flown this rocket before many times, so at the very least I know that its super stable (always goes straight as an arrow, a few times even backslid at apogee when there wasn't much wind). Not looking for any records here, just wanted to have a long burn high power launch. Hopefully the current weather forecast for LDRS will improve some so I can give this a try.

 

[mikec]

Having been embarrassed flying a low-thrust motor on my Punisher 3 that weathercocked badly at an LDRS, you might want to consider doing this at a smaller venue depending on how you feel about public shaming

Off-nominal flights are hard to rule out completely, though all of the advice here is great.

 

[Zyzzyva1000]

Having been embarrassed flying a low-thrust motor on my Punisher 3 that weathercocked badly at an LDRS, you might want to consider doing this at a smaller venue depending on how you feel about public shaming

Off-nominal flights are hard to rule out completely, though all of the advice here is great.

Fair enough. Not terribly concerned about being embarrassed, just wanted to try and make sure I wasn't about to do something dangerous.

 

[ECayemberg]

So the rocket isn't that long, 1.3 calibers (I misread it earlier) is 9.3% (and this isn't taking into account my GPS that I will likely add into the nosecone, minimal weight but still some nose weight). I have flown this rocket before many times, so at the very least I know that its super stable (always goes straight as an arrow, a few times even backslid at apogee when there wasn't much wind). Not looking for any records here, just wanted to have a long burn high power launch. Hopefully the current weather forecast for LDRS will improve some so I can give this a try.

I love a good long burning combo! Just an FYI, most rails around here (and at LDRS) will be 8' in length, not 10'. That said, you may be able to find a longer 1515 rail on the range, and 1010 buttons work in 1515 rails.

 

[Charles_McG]

That said, you may be able to find a longer 1515 rail on the range, and 1010 buttons work in 1515 rails.

FYI, I checked the 1515 rail at the last WOOSH launch, and the 3D printed conformal rail guides Wildman sells (not buttons) DON'T work on it. But the version I use from Thingyverse should. I hope. The issue is the height, not the width.

 

[n3tjm]

I65 is a great motor. I flew one at a church event in a LOC Caliber ISP using motor eject two years ago. Got it back too. Amazingly I'm 3/3 on all my long burn flights. J90 in a EZI-65, H45 in a LOC Legacy, and the I65 in the Caliber

 

[Rocketjunkie]

Look at the first 1/2 second of the thrust curve and use that as the thrust/weight calculation.

 

[Zyzzyva1000]

Look at the first 1/2 second of the thrust curve and use that as the thrust/weight calculation.

Which is what I had figured, so that would put me around 5.5:1.

And I have regular delrin rail buttons, so hopefully I can snag a longer 1515 rail (or I thought in one of the LDRS threads someone indicated that might be a longer 1010 rail there)

 

[Art Upton]

So I have never used one of these longer burning motors before, but have always wanted to do so. I have an SBR 3" Nike smoke that is a reasonable candidate. It has a 54 mm MMT, and has an ebay with altimeter (egg timer apogee) in the nosecone for electronic deployment. Fully loaded the rocket weights 1400 g, with an I65 it would be 2150g (so like 4.75 lbs).

Open rocket says thrust:weight is 4.15: 1 (which I presume is based on the average thrust), but since this has a 'kick' off the pad with higher thrust for a few seconds before tapering off (and open rocket sims at 71 fps rail exit velocity), is this a reasonable rocket and motor combination? Would love to try this out at LDRS, but I want to make sure I am not thinking about this wrong.

On my 3" ISP Loc Rocket that I designed like a D-region tomahawk, 3 pounds build gives the best performance.

Others flying the I65 at NSL 2018 were above that buy about 1/2 a pound or more but they got better altitude then I did as they were allowed to flying straight up.

For some reason after a few of those contest rockets flew, the RSO said all rails must be 20 degrees into the wind.

 

[Zyzzyva1000]

On my 3" ISP Loc Rocket that I designed like a D-region tomahawk, 3 pounds build gives the best performance.

Others flying the I65 at NSL 2018 were above that buy about 1/2 a pound or more but they got better altitude then I did as they were allowed to flying straight up.

For some reason after a few of those contest rockets flew, the RSO said all rails must be 20 degrees into the wind.

So my build weight is right at 3 lbs (I assume you mean without the motor's weight?) - hopefully means this will perform well too.

 

[Art Upton]

So my build weight is right at 3 lbs (I assume you mean without the motor's weight?) - hopefully means this will perform well too.

Yes, no motor weight. Don't use huge amounts of epoxy on the fin can. I used gorilla glue. I build the fin can, painted it, then painted the body tube, Then slit the slots to the bottom of the tube so I could slip it over the motor mount then seal and secure Fin-Can/Motor mount in place.

 

[Zyzzyva1000]

Well I went for it, and actually it was a pretty good flight. Slight weathercocking, but not awful (still got to 90% of predicted altitude), Main problem was its a white rocket, and it got lost against the sun/clouds. However, I had GPS on board. I checked where the last coordinates were, and it was literally in the middle of the wildlife reserve pond/marsh. So I went over there, couldn't get closer than 1000 ft (but was still receiving signal from GPS - in fact as I drove around I got as far away as 8500 ft and still had signal, but far a new best for me with the eggfinder). Anyway, went to the main table and immediately found some help - there was a boat. We loaded it into a truck and found a place to put into the water. My dad and I rowed out and followed the eggfinder signal right to the rocket. The nosecone was bobbing upright so things were only a bit wet. Chute had opened and JLCR had released, so other than the water recovery, was actually a great flight. The long burn looked great and was a crowd pleaser. Rocket is mostly dry now and should be good to fly again one day. Glad I decided to just go for it. Also super thankful for all the help from the Tripoli and local folks who facilitated the rescue.

 

[StreuB1]

If you are using a rail, make sure the rail is CLEAN. If not, with a low TTW ratio, the rocket can actually hang-up ON THE RAIL due to corrosion. Bring a green scotchbrite pad to your launches in a ziplock baggie and carry with you to the pads. If the rail is corroded a bit and its rough, take a minute to scrub the rail channel and the outside of the rail with the scotchbrite to knock down the buildup. HC acid from our motors exhaust corrodes the anodizing on the rails and leaves a layer that can build up pretty heavy at times. Some of the rails at my club and Bong need a good scrubbin once and a while so in this case, I would make sure you give it a look.