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Btw, you have now crossed into “the oddroc zone”
Dee dee, dee dee
Dee dee, dee dee.....
Dee dee, dee dee
Dee dee, dee dee.....
Lakeroadster's Lifting Rocket
- Thread starter lakeroadster
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lakeroadster
When in doubt... build hell-for-stout!
As always, thanks for the discussion and comments fella's.
I'll let Daddyisabar answer.
What I've read is, on a canted cluster, if the thrust C/L is ahead of the cg, the propulsion is relying on the pendulum theory for stability.... think about the safety towers on Gemini and Apollo.
Remove the pendulum (in this case the aft cargo bay) and things would tend to go a bit, as you would say, "Wackadoodle".
But you may be right, it may be stable as is... My comment was due to the thrust C/L and CG being close to each other (without the payload bay) the pucker factor is 9 out of 10
I'm thinking aerodynamic forces start to play a big role in stability the closer the thrust c/l and cg get to each other... just like when CG and CP get in close proximity.
Not sure where the CP is? I'm hoping I can created a decent Open Rocket Simulation to determine that.
I'll let Daddyisabar answer.
What I've read is, on a canted cluster, if the thrust C/L is ahead of the cg, the propulsion is relying on the pendulum theory for stability.... think about the safety towers on Gemini and Apollo.
Remove the pendulum (in this case the aft cargo bay) and things would tend to go a bit, as you would say, "Wackadoodle".
But you may be right, it may be stable as is... My comment was due to the thrust C/L and CG being close to each other (without the payload bay) the pucker factor is 9 out of 10
I'm thinking aerodynamic forces start to play a big role in stability the closer the thrust c/l and cg get to each other... just like when CG and CP get in close proximity.
Not sure where the CP is? I'm hoping I can created a decent Open Rocket Simulation to determine that.
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Yeah, that makes sense. I wasn't taking the pendulum effect into account.
lakeroadster
When in doubt... build hell-for-stout!
In regard to Pendulum Theory... it appears opinions vary. Here's a link to some interesting discussion:
https://space.stackexchange.com/questions/9682/why-are-rocket-engines-at-the-base-of-the-rocket
https://space.stackexchange.com/questions/9682/why-are-rocket-engines-at-the-base-of-the-rocket
lakeroadster
When in doubt... build hell-for-stout!
Finished up the Open Rocket simulations. The only thing I wasn't able to do was cant the clustered motors.
Every configuration seems to be stable. I ended up using (3) 18" chutes to get the ground hit velocities down to a realistic amount.
Next: Creating the drawing package
Every configuration seems to be stable. I ended up using (3) 18" chutes to get the ground hit velocities down to a realistic amount.
Next: Creating the drawing package
Remember that if you use those flat struts (have I mentioned they look really great!) to the cargo pod they will add to the stability too.
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Great thinking there. I love multipurpose structures, in this case something that provides material support and stability.. Given the size and position of these, if your stability is marginal, increasing the size of these even a couple of mm will buy you a lot.
Curious, will three 18” chutes take up more space than a single corresponding larger chute? All my experience is in low power, when I have tried even two chute is usually find one inflates and the other streamers.
lakeroadster
When in doubt... build hell-for-stout!
Multiple chutes tend to make for a more stable decent ... less spinning and twisting. Think Apollo capsule re-entry. But they indeed will take up more space.
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Definitely True if they have separate shock cord attachments AND lines don't tangle. I did a single 6 shroud line chute with two lines each going to each of three attachment points on the forward lip of the body tube. Added a spill hole. A little tricky to pack so it would open without twisting. Came down with no spin or wobble according to the camera.
lakeroadster
When in doubt... build hell-for-stout!
When I ran the simulations in Open Rocket the results showed a very low apogee and an error message said recovery device deployment at high speed?
For the cluster I had (2) booster engines and a delay engine. I changed that to (3) delay engines and then the simulation yielded no error messages.
So the simulation must have assumed the recovery device deployed as soon as the booster engine stopped... similar to what you pointed out.
So your saying just shove some wadding into the top end of the motor and hold the wadding by using tape on the i.d. of the motor housing.. that's adequate to contain the BP burn through flash?
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I just thought of something when you mentioned that OR showed deployment when the booster stopped: the blowby gases will pressurize the recovery compartment. Not as much as an ejection charge, but possibly enough to blow the chute early, especially if there are two -0 boosters. Solution would be either 3 motors with the same delay, plugged booster motors, or vented booster motor tubes with the top of the motor tube plugged. Actually a fourth solution would be plugged tube with motor eject, but that might not be permitted at some launches. Or for that matter self plugged booster motors, although many do that all the time using an epoxy plug over the front of the propellant. Depends if you're launching at a meet with a RSO or not.When I ran the simulations in Open Rocket the results showed a very low apogee and an error message said recovery device deployment at high speed?
For the cluster I had (2) booster engines and a delay engine. I changed that to (3) delay engines and then the simulation yielded no error messages.
So the simulation must have assumed the recovery device deployed as soon as the booster engine stopped... similar to what you pointed out.
So your saying just shove some wadding into the top end of the motor and hold the wadding by using tape on the i.d. of the motor housing.. that's adequate to contain the BP burn through flash?
Nytrunner
Pop lugs, not drugs
Very cool project.
Potentially, you could use one plugged, one good delay, and the next longest delay (if there is one) as a backup. [Backup events with blackpowder? Whatttt?]
Also, I recall a thread with canted motors where the flier was able to apply a a cosine factor to the motor files and account for the lost thrust for greater simulation accuracy
Potentially, you could use one plugged, one good delay, and the next longest delay (if there is one) as a backup. [Backup events with blackpowder? Whatttt?]
Also, I recall a thread with canted motors where the flier was able to apply a a cosine factor to the motor files and account for the lost thrust for greater simulation accuracy
lakeroadster
When in doubt... build hell-for-stout!
lakeroadster
When in doubt... build hell-for-stout!
<<<---- Lonewolf... no club.
rstaff3
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Yeehaw, finally a thread that awakened me from my slumber. I really love the design, especially the booster version!
While daddyisabar is the king of these tractor motor rockets, I have built a few and will offer a few comments.
1. You are on the right track with your sims. The idea of a 'pendulum' being a magical method of stabilization is incorrect. Stability, at least the static variety, is all based on the CP/CG relationship. Sticking your heavy motors up front helps with this. A lot.
2. Using motors with differing delays has some merit where over-pressurization may be an issue. I think I would reinforce the tube and use a loose nose cone. Nose weight shouldn't be needed and a light cone will not be that sensitive to drag separation. You can also use one motor for ejection and two pugged ones. I personally like plugging my -0 motors with a little 5-sec epoxy. You should check with you club's safety folks before trying this.
3. I have modified a motor file to account for canted motors but decided this was too much work. My rockets seldom align with available delays and if they do the delays can be inaccurate. Just know the actual performance will be degraded a tad and pick accordingly.
Looking forward to the build...and flight.
I really love the design, especially the booster version! While daddyisabar is the king of these tractor motor rockets, I have built a few and will offer a few comments.
1. You are on the right track with your sims. The idea of a 'pendulum' being a magical method of stabilization is incorrect. Stability, at least the static variety, is all based on the CP/CG relationship. Sticking your heavy motors up front helps with this. A lot.
2. Using motors with differing delays has some merit where over-pressurization may be an issue. I think I would reinforce the tube and use a loose nose cone. Nose weight shouldn't be needed and a light cone will not be that sensitive to drag separation. You can also use one motor for ejection and two pugged ones. I personally like plugging my -0 motors with a little 5-sec epoxy. You should check with you club's safety folks before trying this.
3. I have modified a motor file to account for canted motors but decided this was too much work. My rockets seldom align with available delays and if they do the delays can be inaccurate. Just know the actual performance will be degraded a tad and pick accordingly.
Looking forward to the build...and flight.
lakeroadster
When in doubt... build hell-for-stout!
Finished building the CAD model of the lifting rocket. I came up with a way to attach some external parachute chord mounts for the recovery bay, and external strut mounts for the cargo bay.
See detail B below. Basically it's quan. (6) 1/8" plywood centering rings that are notched to trap a 1/4" nylon lock nut, then the rings are glued together.
Should be hell-for stout.
See detail B below. Basically it's quan. (6) 1/8" plywood centering rings that are notched to trap a 1/4" nylon lock nut, then the rings are glued together.
Should be hell-for stout.
rharshberger
Well-Known Member
-0 booster motors are plenty to pressurize a recovery bay, I use them (D12-0) when I want a very short flight, like VERY short, my AeroDart clone with an egg gets about 50' max altitude before blowing the chute, and the chute has just enough time to inflate and slow the rocket enough that the egg is not broken. Entire time of flight less than 8.5 seconds. Why you ask would I want a short flight like this, for an egg drop contest aka quickest safe descent, it cut 2 seconds off the time.
If you want to prevent premature ejection...you will probably not want to use a -0 motor.
One concern I have for this rocket is: will it safely fly if only 2 of the 18mm motors light (without the 24mm pod or whatever it was)? Mindsim says its rather largish and that it will require all three motors to safely fly. Where is the CG and were are the thrustlines of the motor aimed to it? In the event one of the motors fails to light having the thrust of the other two motors can help keep the rocket stable (hope I stated that correctly). Clusters are something I don't fly a lot of (I have two clustered LPR rockets, a FlisKits Tres yet unflown, and a sport scale two stage Soviet SA-3/S-125 Perchora, flown a number of times).
lakeroadster
When in doubt... build hell-for-stout!
The rocket is of the tractor configuration. In every flight configuration the thrust line of the canted cluster is above the CG. Thus the canted cluster thrust lines don't aim at the cg.
The Open Rocket screen shots above show the CG, with 3xD12 and 4xD12 configurations. I ran a simulation with just 1 D12 in the lower cargo bay, it was stable with or without the lower cargo bay.
Since the cluster is canted, and Open Rocket doesn't allow "canting" of a cluster, I "reasoned" that running a simulation with just one or two of the cluster motors firing isn't real world for this rocket... but, based on your valued input above, I just ran simulations with (4) motors installed but only (1) D12 in the cluster fires... that was also stable with or without the Cargo Bay. I also ran the simulation with (2) D12's firing in the cluster, that was also stable.
That's the best I can do.. without a simulator that allows canting of the cluster.
rharshberger
Well-Known Member
Thats all we can ask...except for making the first flight from the Away Cell
(just kidding, but definitely move the first flight out to the HPR pad distance for safety).
rstaff3
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Stability is good but arcing over must be considered. Canting through the CP (or is it CG, I forget) helps but is not a guarantee as I've seen proven kits get squirrely when one motor doesn't go.
As Rich recommends, I fly my odd designs from HPR pads even when they are technically MPR or even LPR. My 11 motor Ekranoplan went off pads that could handle up to 'L' motors. This at least gives people on the flight line more time to react.
As Rich recommends, I fly my odd designs from HPR pads even when they are technically MPR or even LPR. My 11 motor Ekranoplan went off pads that could handle up to 'L' motors. This at least gives people on the flight line more time to react.
lakeroadster
When in doubt... build hell-for-stout!
Thanks for the advice fellas. I launch on BLM land. Nobody around except for me, my wife and lots of sage brush.
Here's an expanded view... gives a better feel for the main components. Still working to finalize the drawing set.
Here's an expanded view... gives a better feel for the main components. Still working to finalize the drawing set.
Sounds like it should be fine. Since canted motors lose some efficiency, have you tried the same sim with a correspondingly lower thrust motor file?
lakeroadster
When in doubt... build hell-for-stout!
Not yet... mostly when running simulations I'm just looking @ stability. I'll try it though and see what it spits out.
Apogee's website is full of useful stuff on canted motors and how to calculate decreased efficiency as cant angle increases.
Even if the pendulum theory turned out to be wrong it seemed to work for a thousand years and that ain't bad!
As for predicting what will happen if only two of the three light. Rocsim may calm your fear. Mindsim can set you upon a fantasy trip on the road to DeNile. But in the end all you have is prayer and the probability of safety in a wide open space. I have had two light on the undead Pict Warrior and it survived the most. If it leaves the rod with some speed in ideal conditions it might just arc nice and deploy above the ground. However many factors come into play. It might torque and seize on the rod and if that happens all bets are off.
Even one motor lighting a bit late can lead to unmitigated disaster, or have no real impact at all. Trust in science but appease the pagan rocket gods as well.
Cant angles meeting half way between the CP and CG is ideal. Canting above the CG is crazy...fun!
Even if the pendulum theory turned out to be wrong it seemed to work for a thousand years and that ain't bad!
As for predicting what will happen if only two of the three light. Rocsim may calm your fear. Mindsim can set you upon a fantasy trip on the road to DeNile. But in the end all you have is prayer and the probability of safety in a wide open space. I have had two light on the undead Pict Warrior and it survived the most. If it leaves the rod with some speed in ideal conditions it might just arc nice and deploy above the ground. However many factors come into play. It might torque and seize on the rod and if that happens all bets are off.
Even one motor lighting a bit late can lead to unmitigated disaster, or have no real impact at all. Trust in science but appease the pagan rocket gods as well.
Cant angles meeting half way between the CP and CG is ideal. Canting above the CG is crazy...fun!
Using different delays in a black powder cluster is simply called Poor Boy dual deploy. Works best when coming out of separate tubes like you might have on a silly airplane rocket like a B58. The second charge would just burn up stuff coming out the same tube. Find some D11Ps. I have never had an epoxy plug even come close to fail and then burn behind is so clean. You can get a little more umph and it almost weighs exactly what the delayed motors weigh. Burn so complete the epoxy forms a clear window on some where you can peer through the spent motor. Then the shame of breaking the rules sets in. Best to hide those motors with the bong stashed above the ceiling tiles in your parents basement.
Put one of those fancy gimbal jobs on the back motor, extend the landing tripod and set her softly down under power. All the rage these days. Tractor motors could give it some power going up and a second set some power going down. Leaves the gimbaled motor on the bottom to do its job and not waste all its power keeping things stable going up and down. It would look like something out of a 50's Sci-Fi movie.Thanks for the advice fellas. I launch on BLM land. Nobody around except for me, my wife and lots of sage brush.
Here's an expanded view... gives a better feel for the main components. Still working to finalize the drawing set.
View attachment 370304
rharshberger
Well-Known Member
Using different delays can also prevent overpressuring a tube...ask me how I know. My SA-3 used two 18mm motors and 24mm motor in the booster, two other 18mm tubes contained the chutes for the booster and were rear eject, after repairing the rocket twice for a burst central motor tube (the C6's were firing into a 3" and the 24mm central tube was for the gap staging of the sustainer), I switched to two different delays with the longer one basically becoming insurance as it was 2 seconds after the first one and after that never had to fix a burst central tube again. LPR tubing may or may not take the stress of 3 ejection charges firing at one time, just something to think about.
https://www.rocketryforum.com/threads/sa-3-goa-semi-scale-2-stage-with-cluster-booster.70827/
lakeroadster
When in doubt... build hell-for-stout!
I am at a loss? Why don't manufacturers sell "plugged" booster motors?
.... and why does NAR having an issue with us "plugging" a booster motor in order to address an obvious issue?
