EZI-65 (heavily customized) on I115; is it too heavy?

[Aeva]

Barring a few things, I might be launching my EZI65 soon for an L1 cert. I definitely overdid it because I wanted it to be durable (it's fiberglassed), but also have some extra features (a go pro, and custom flight computer). The final result weighed in at about 2.28kg or about 5lbs without the motor. On paper it seems fine but from what I have been reading, my TWR may be a bit low (https://www.rocketryforum.com/threads/thrust-to-weight-ratio-5-1-or-3-1.14552/ suggests 5:1, mine is about 3:1 on average which is lower than I wanted.) Assuming that the club has an 8ft launch rail, the velocity off the rod will be just shy of 13m/s or 29mph.

I already purchased an engine from RCS because my original weight estimate was a bit lower (2kg no motor) and that seemed to meet the needs of my rocket... but I'm now I'm concerned that I picked an engine with too low average thrust. I know, I can just look at the information in other posts, I just want some feedback because I'm a bit committed to this rocket and the engine (since I already bought it <.<).

The last time I underdid the TWR it was a low powered rocket that turned ballistic missile... never again lol

 

[jmasterj]

Run a sim in OpenRocket or RockSim. Looking at the thrust curve, the I115 starts out a little wimpy, so you're right to be concerned. A longer rail might help, but the best answer is going to be provided by the sim.

 

[Aeva]

It will be at MDRA, so if you know their launch rails (the 1010 rail) it will probably be like 8ft. Think that will be enough? TWR will be about 3.5 at the point it leaves the rail, and will increase. It will be going about 12m/s or 30mph (still sounds slow...)

The sims in openrocket also put the rocket at about 600m pretty consistently even with wind and high turbulence; so it would suggest that it's stable enough. I'm still dubious on whether's "it's enough".

 

[Rocketjunkie]

As a rule of thumb, you can use the average thrust (115N)/20 for max liftoff weight in lb. This gives about a 4.5 thrust to weight ratio. If you have the thrust curve, use the 1st 1/2 sec. for the thrust value.

 

[jmasterj]

It will be at MDRA, so if you know their launch rails (the 1010 rail) it will probably be like 8ft. Think that will be enough? TWR will be about 3.5 at the point it leaves the rail, and will increase. It will be going about 12m/s or 30mph (still sounds slow...)

The sims in openrocket also put the rocket at about 600m pretty consistently even with wind and high turbulence; so it would suggest that it's stable enough. I'm still dubious on whether's "it's enough".

12m/s off the rail sounds pretty questionable. I'd recommend using that motor in another rocket.

 

[Aeva]

As a rule of thumb, you can use the average thrust (115N)/20 for max liftoff weight in lb. This gives about a 4.5 thrust to weight ratio. If you have the thrust curve, use the 1st 1/2 sec. for the thrust value.

The thrust at 0.5seconds on the curve is about 130N (marginally higher than the average). Even if that gets me to the 5 TWR, it still doesn't give me confidence.

@jmasterj Okay... that seems fair. Glad I brought it up here, thanks guys.

Relevant question: since I already about a rocket for my L1 cert from aerotech, would they prohibit me from buying another high powered motor? I don't have any contacts in the local rocket community so I can't have someone else order it for me. Alternatively, I guess I could just buy one from apogee or something. Thoughts?

 

[jmasterj]

Relevant question: since I already about a rocket for my L1 cert from aerotech, would they prohibit me from buying another high powered motor? I don't have any contacts in the local rocket community so I can't have someone else order it for me. Alternatively, I guess I could just buy one from apogee or something. Thoughts?

I don't think you'll get any pushback, but ordering from another retailer will save you potentially having to explain your mistake.

If you interested, you can PM me where you are in MD and if it's not too inconvenient I could even sell you one to avoid shipping charges. I think I have an I175WS DMS motor that wouldn't require you to buy hardware now.

 

[Tractionengines]

For a cert flight faster off the rail is better... don't get caught by a chuff, or other anomaly. You can go slow later...7.5:1 or 10:1 (or more)

The thrust at 0.5seconds on the curve is about 130N (marginally higher than the average). Even if that gets me to the 5 TWR, it still doesn't give me confidence.

@jmasterj Okay... that seems fair. Glad I brought it up here, thanks guys.

Relevant question: since I already about a rocket for my L1 cert from aerotech, would they prohibit me from buying another high powered motor? I don't have any contacts in the local rocket community so I can't have someone else order it for me. Alternatively, I guess I could just buy one from apogee or something. Thoughts?

You can buy one at a time over and over, just noting it is for certification flight. The BIG issue is your hazmat fees adding up shipping that.

Where do you live? Find someone who will be at MDRA the same day you want to fly and work a trade, or buy one from someone.

Most of us have a range of motors and will "horse trade" and get you something more applicable.

 

[Aeva]

For a cert flight faster off the rail is better... don't get caught by a chuff, or other anomaly. You can go slow later...7.5:1 or 10:1 (or more)



You can buy one at a time over and over, just noting it is for certification flight. The BIG issue is your hazmat fees adding up shipping that.

Where do you live? Find someone who will be at MDRA the same day you want to fly and work a trade, or buy one from someone.

Most of us have a range of motors and will "horse trade" and get you something more applicable.

Laurel Maryland, basically next to DC. I'm trying to go to the launch this weekend, but even if I get a motor the works, I'm not sure I will be able to launch because I need to get the FAA COA for that rocket. I'd gladly trade it though if only to save on hazmat fees. I'm just still very new to the club and haven't gotten to know anyone lol. Think if I put something out on the forums I can possibly find someone?

 

[jmasterj]

I'm not sure I will be able to launch because I need to get the FAA COA for that rocket.

Not sure what you mean here. The FAA waiver is held by the club. You could, in theory, get your own, but for the vast majority of people and places it's not worth the effort.

 

[Aeva]

AHHH I misunderstood that... I thought it was something you had to get individually lmfao. I think the only reason I'd like it for myself is so I can launch outside the club, but that's not a concern right now.

Isent a message about the I175, and that should get me a TWR of 7.8:1 which I'm much happier with.

 

[jmasterj]

AHHH I misunderstood that... I thought it was something you had to get individually lmfao. I think the only reason I'd like it for myself is so I can launch outside the club, but that's not a concern right now.

Finding HPR space that's kosher with both the landowner and the FAA in MD is going to be pretty tough. Flying with MDRA is the way to go.

 

[waltr]

It will be at MDRA, so if you know their launch rails (the 1010 rail) it will probably be like 8ft. Think that will be enough? TWR will be about 3.5 at the point it leaves the rail, and will increase. It will be going about 12m/s or 30mph (still sounds slow...)

The sims in openrocket also put the rocket at about 600m pretty consistently even with wind and high turbulence; so it would suggest that it's stable enough. I'm still dubious on whether's "it's enough".

MDRA requires a minimum 5:1 TTW ratio based on Average thrust. Know this since a 4:1 TTW was NOT approved by the RSO...
Getting a different motor is your best bet and for a Cert flight why take chances. Get that rocket off the rail with plenty of velocity. I would go for a high thrust H, good off the rail speed but not going too high. An H283T, 38mm DMS is popular for L1 certs.

I am planning to be at MDRA this Sunday.

 

[Aeva]

Yeah, I'm definitely gonna up the thrust with another motor. The main reason I "overdid" my rocket was to minimize chances damage to it if something going wrong... it would a shame to lose it because beefy additions weighed too much.

Also heck yeah, that's the day I'm going too. See you there!

 

[user 30299]

I ran your motor selections through my RockSim simulation for my EZI-65. The I-115 will barely get you to 40 fps
if you had a full 8 ft. of rail to work with. That's 8 ft. measured from the top rail button. That's a little too slow for
me with the EZI-65.

The I-175 will get you 81 fps if you had a full 8 ft. of rail to work with. If you only have 6 ft. of rail, from the top
rail button - you're looking at 58 fps. Works for me. It will be slow looking, but stable.

 

[cls]

remove the gopro! those are heavy. you want success on your L1, not one more thing to fail. the H115 is marginal, remove all extra weight.

fly the camera after you certify.

 

[OzHybrid]

The 4:1 TTW only gives you an acceleration vertically of 3G. One G of thrust balances the rocket in gravity. This is why although Tripoli permits 3:1, a 2G acceleration, you need to really consider your speed off the rail and the rail would have to be really long. Anyone using that value would have to make a good case to the RSO that it was safe.
For that reason, 4 or 5:1 is more generally used as a rule of thumb. Between 5 and 10 is really quick off the rail and above that you need to start considering what the acceleration will do to your other components. It might cause your chute to move rearwards affecting CG position, batteries to move.
What would the weight of a camera be under 10G of acceleration? What would the weight of a battery be under that acceleration.

Good luck with the cert.
Norm

 

[Aeva]

The 4:1 TTW only gives you an acceleration vertically of 3G. One G of thrust balances the rocket in gravity. This is why although Tripoli permits 3:1, a 2G acceleration, you need to really consider your speed off the rail and the rail would have to be really long. Anyone using that value would have to make a good case to the RSO that it was safe.
For that reason, 4 or 5:1 is more generally used as a rule of thumb. Between 5 and 10 is really quick off the rail and above that you need to start considering what the acceleration will do to your other components. It might cause your chute to move rearwards affecting CG position, batteries to move.
What would the weight of a camera be under 10G of acceleration? What would the weight of a battery be under that acceleration.

Good luck with the cert.
Norm

I mounted the GoPro internally and was factored into the CG, removing it isn't a problem but would require some changes to the sled and a few other internals so that it would maintain a CG centered over the CP. That would save me about 153g (not including some plastic for the frame holding it inside the rocket). I designed the rocket with that in mind so that it wouldn't be a problem.

Sorry I may have misread what you said. But rest-assured. literally everything is confined within the avionics compartment. Even if the drogue chute moves completely to the back of the rocket, the CG is still going to be more toward the front because of the weight in the avionics. The main chute has less room to move, it won't effect the CG during flight.

Honestly, I want to launch with the GoPro. I know that's not advisable, but I did plan for it (with the exception of overlooking the engine TWR). Otherwise, I could have saved myself the trouble and gotten a 29mm G138 for another rocket that has flown already. Appreciate it though!

 

[pugachu]

Be glad you fiberglassed your EZI... I got my L1 on an EZI-65 built mostly stock (I fiberglassed the fins) on an H128W. I zippered it on the 3rd flight on an I210R. The 4" LOC BT is very thin, which is good for weight, but not so much for durability. Just make sure you're using a large enough parachute, and don't launch if the wind is not calm.

Good luck on your cert this weekend!

 

[OzHybrid]

I mounted the GoPro internally and was factored into the CG, removing it isn't a problem but would require some changes to the sled and a few other internals so that it would maintain a CG centered over the CP. That would save me about 153g (not including some plastic for the frame holding it inside the rocket). I designed the rocket with that in mind so that it wouldn't be a problem.

Sorry I may have misread what you said. But rest-assured. literally everything is confined within the avionics compartment. Even if the drogue chute moves completely to the back of the rocket, the CG is still going to be more toward the front because of the weight in the avionics. The main chute has less room to move, it won't effect the CG during flight.

Honestly, I want to launch with the GoPro. I know that's not advisable, but I did plan for it (with the exception of overlooking the engine TWR). Otherwise, I could have saved myself the trouble and gotten a 29mm G138 for another rocket that has flown already. Appreciate it though!

There are much lighter cameras available. But the Go Pro is top notch. But also expensive when you crash. Of course that would never happen.......

Good luck with the flight. I have no on board footage of any of my cert flights. When you are doing a cert, it's much more pressure than you'd think. Adding a camera increases that.

Maybe we should start a GoPro carnage thread. I'll do it now.....

 

[jmasterj]

I mounted the GoPro internally and was factored into the CG, removing it isn't a problem but would require some changes to the sled and a few other internals so that it would maintain a CG centered over the CP. That would save me about 153g (not including some plastic for the frame holding it inside the rocket). I designed the rocket with that in mind so that it wouldn't be a problem.

Sorry I may have misread what you said. But rest-assured. literally everything is confined within the avionics compartment. Even if the drogue chute moves completely to the back of the rocket, the CG is still going to be more toward the front because of the weight in the avionics. The main chute has less room to move, it won't effect the CG during flight.

Honestly, I want to launch with the GoPro. I know that's not advisable, but I did plan for it (with the exception of overlooking the engine TWR). Otherwise, I could have saved myself the trouble and gotten a 29mm G138 for another rocket that has flown already. Appreciate it though!

I don't think the lateral CG matters as much as you think. People tape cameras to the side of rockets all the time without much performance difference.

That said, you should definitely do your cert your way as long as it's safe. The camera isn't making things less safe so I'd stick with it, especially with the cool internal mount.

 

[T-Rex]

would maintain a CG centered over the CP.

I really hope I am misinterpreting that statement. You do have >1 caliber of stability, i.e. CG is on the nosecone side of your CP, correct?

 

[user 30299]

I really hope I am misinterpreting that statement. You do have >1 caliber of stability, i.e. CG is on the nosecone side of your CP, correct?

Good question!

What is the distance from the tip of the nose cone to the CP location?

And the distance from the tip of the nose cone to the CG location?

 

[jmasterj]

I really hope I am misinterpreting that statement. You do have >1 caliber of stability, i.e. CG is on the nosecone side of your CP, correct?

Radially centered, hence the cross-section with the masses.

 

[Aeva]

I really hope I am misinterpreting that statement. You do have >1 caliber of stability, i.e. CG is on the nosecone side of your CP, correct?

To clarify for those confused: in the vertical axis, they're about 25cm (10in) apart. The weight of the GoPro and avionics shifts the CG well forward, and the EZI's big fins make the CP stay low vertically. In the lateral axises or from a radial cross section, those points are virtually centered over each other thanks to how I balance the avionics and camera (the red circles outlining the "mass objects" I added in openrocket). The cal ends up being about 2.38 when at speed.

 

[kjhambrick]

That's a pretty EZI, @Aeva !

-- kjh

 

[user 30299]

Aeva, do you have motor adapters for going down to 38mm and 29mm motors from the 54mm motor tube?

 

[Aeva]

Not yet. I was talking with jmasterj about it though, and I'll be printing an adapter, at least for 38mm motors.

 

[kjhambrick]

Not yet. I was talking with jmasterj about it though, and I'll be printing an adapter, at least for 38mm motors.

Wow !

I never thought of printing a motor adapter.

Can the materials take the load and the load with the heat ?

-- kjh( dang ! I am so 20th century ! I gotta get me one o' them 3D printers )

 

[Aeva]

Wow !

I never thought of printing a motor adapter.

Can the materials take the load and the load with the heat ?

-- kjh( dang ! I am so 20th century ! I gotta get me one o' them 3D printers )

Lol, if it's beefy enough, it should handle the load, and petg has been pretty good to me so far as far as heat tolerance. I should probably ground test it first. I'd probably just make a rig to put weight on it, and then use my heat gun to simulate the heat of the engine. I'll bring it to the edge of failure and if it holds up to that abuse for more than a minute, and if it does, it should be more than enough.

Printers are so worth it as a rocketeer. My printed designs are very modular so I can reuse parts like rocket legos almost lol. It makes it easier to replace and modify parts, as well as just easier to plan arranging parts in tight spaces.




It also allowed me to place the gopro more predictably within the rocket so I could minimize the viewport.