Need help with post launch analysis

To all:
I designed and built my own twin engine cluster rocket using rocksim. I originally wanted to fly it with Estes E9's for the "low and slow" effect. I then realized I was pushing my luck weight wise and asked for advice. See https://www.rocketryforum.com/showthread.php?25756-Maximum-safe-lift-off-weight-velocity-using-E9-s

Fast forward to now when I can fly it and the results have been odd. I have flown it four times. All four have had similar flight profiles. The flight starts off vertical and then goes horizontal and results in crashes or near crashes
1) Twin Aerotech D15-4T's. Up...horizontal..down...parachute deploys just above ground..broken fin
2) Twin Estes E9-4 Up higher...horizontal..down...parachute deploys just above ground
3) Twin Estes E12-4 Up even higher to about 300 feet...horizontal..down...parachute deploys at 200'
4) Twin Aerotech E18-4W Results similar to 3 BUT I almost take out a dog on the landing to the horizontal flight of about 400 yards.

The other odd thing is I used the Estes Sonic igniters on the Aerotech clusters. Both times the igniters went along for the ride!

Here are the launch photos of the E18's. Note the awesome flame and launch wire traveling upward.


I would like to know why my rocket likes to fly this way. I can not reproduce a near horizontal flight in Rocksim. I have accurately weighed the rocket when loaded and inputted that info to rocksim and it has around 1.5 calibers of stability.

I really like my launch field but want to protect innocent civilians and dogs. You can see my rocket almost landing on the dog at the 0:58 second mark.
[video=youtube_share;JtXm10yCDGI]https://youtu.be/JtXm10yCDGI[/video]

thanks
Jim

well I can't answer the odd flight profile question...but I can tell you that you were shoving those red caps on to far (aerotech motors). they only need go on far enough to hold the igniter in place until ignition. I have even on occassion used a strip of blue tape across half the nozzle.
rex

I am not an expert, but to me it looks like the rocket is under-powered. But that is just me, probably.

@blackbrandt:
I am also worried about under power...but..rocksim says I am fine. It says I should hit about 800' with E18's, which is exactly what I want, low and slow.

Jim

Couple things to consider:

1.) Did you both weigh and check CG with the motor loaded AND the camera.
2.) From doing some (very rough) timing on the video, it looks like your motors had about a 5 second delay - not uncommon for Aerotech motors - but at the point at which the motors would have fired a true 4 second delay, your rocket was already arced over and on its way down. Unfortunately, since 4 seconds is the shortest delay possible for the E18, I don't see a way around this short of modifying it for electronic ejection, which would add weight.
3.) You stated that your E18 flight was similar to the E12 flight, which you estimated at 300' AGL. If the E18 flight did actually only go 300', your Rocksim file is somehow very very off from your actual model, and you need to figure out where the discrepancy lies.

Personally, I think the most likely thing is simply that you can't get a delay short enough to be optimal for your rocket. Although it may be stable off the pad and fly well with the smaller motors you're using, if it's heavy and draggy enough it won't get up high enough for the delay to fire before it hits the ground, as you have already discovered. The only solution to that is to use motors with a higher total impulse, while still having enough oomph to get your rocket off the pad.

Did you check the "rocksim weight" vs. the actual weight of the rocket? That one has caught me before.

In my opinion - I think that you're underpowered as well but it is hard to tell from the video. Does the rocket travel horizontally, or is it just arcing over under power? Was there any wind?

In rocksim, I assume that you have the correct stability margins.

From what I can tell the initial thrust spike is enough to get off the pad and headed up, but then once the thrust starts to taper off, gravity begins to win the battle. I think that all of the motors you have shown are decent initial peak then a longer, lower thrust.

Maybe something with a touch more initial thrust like a E-28 and a 4 second delay might work.

Just some thoughts to consider. I am by no means an expert - just trying to help the conversation along.

With your fin configuration, you have the CP very high on the rocket. Did you use a lot of nose weight to get the stability right? You might want to check the CP Cg and make sure it's only 1 -2 calibers. Also, fly mostly F motors with this rocket. It looks like this rocket could easily have a very small margin of stability.

Rex R said:

well I can't answer the odd flight profile question...but I can tell you that you were shoving those red caps on to far (aerotech motors). they only need go on far enough to hold the igniter in place until ignition. I have even on occassion used a strip of blue tape across half the nozzle.
rex

Click to expand...

X2 on the igniters. Masking tape is your buddy here.

From the video your rocket isn't flying overly fast. I have the same issue with a BT-80 rocket powered with a C6. Very similar flight profile as yours. On my rocket it just doesn't get enough speed, eventho' I made this rocket as light as I possibly could.

I'd guess this a combination of weathercocking with insufficient thrust. You mentioned you had simmed this with better flights in Rocksim. In that case, I have two questions-- 1) Could you post the file? 2) Have you weighed the rocket?


Later!

--Coop

Looks like at least one ignitor got stuck (in the engine?) maybe it vectors the thrust or otherwise makes the thrust less efficient?

Just a thought, could your recovery package be shifting in flight due to accelleration, and if so, is there enought weight for it to make a difference? If you're under-powered and have marginal stability to begin with, combine with some wind and it could push it over the edge.

UPDATE:
Thanks for all the replies. It definitely appears to be a CG/CP issue. I reweighed my rocket fully loaded and it comes in a bit heavier at 28oz. Rocksim still says ok. When I override the CG and it put it rearward of the CP by about 1" (thus making it unstable) then the predicted flight profile nearly matches reality! When I measure the actual location of the CG, It is slightly in front of the predicted CP (by about a .25in).
Obviously I need to learn more about Rocksim but now the question is how to verify CP?

Jim