RockSim Booster going higher than the sustainer! Playing with a clustered + staged

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snrkl

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So I have been intrigued with the clustered and staged design posited by TVM in his "Model Rocket Design and Construction" book: it suggests combining booster pods and a booster stage for a more visually spectacular staging for LPR:

IMG_6920.jpgIMG_6919.jpg

So I duly designed and simulated a design of this elk in RockSim:
rocksim002.jpg

Hilarity ensued - I am not sure that RocSim is properly ejecting the pods at booster burn out... When you watch the flight profile, the booster actually overtakes the sustainer...

rocksim001b.png

Inspection on the chart shows the booster beating the altitude of the sustainer by a good 60m!!

rocksim003.png
 

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Woody's Workshop

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Feet is what you walk on.
Inches is measurement of your feet.
And you drive on the wrong side of the road too.
Don't get mad, just poking fun.
Wish I could help you with your RocSim problem, but I never incountered that before in RocSim.
Perhaps go back to your build process and challenge your procedures.
You might find a something simple.
 

Tinker

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I am working on a similar design from Tim's book. I am going lazy though. I am using an Estes Mamoth, with the PSII booster, and two Estes Orange Crush rockets as the boosters. I am using the glider hooks that apogee sells for the hooks. The only issue I think I have yet to work out is stability between the boosters and the sustainer. Just connecting the hooks does not give me a warm and fuzzy that it will survive. I am thinking about a strip of balsa between the sustainer and boosters to provide stability for the other forces that the hooks aren't designed to deal with. This would be between the hooks.

Tinker
 

Tinker

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Have you taken a look at the strap on booster kit from apogee? https://www.apogeerockets.com/Rocket-Kits/Skill-Level-2-Model-Rocket-Kits/Strap-on-Booster-Pods

They have booster alignment rails..

View attachment 325535

I don't think these will work. As the center booster blows, it would separate but the strap on boosters will still be hooked on at the nose cone because the hooks go up, and not down. If I use the glider hooks, I have a pair at the top of the boosters with the hooks down, and at the bottom with the hooks up. That way when the center booster separates, the side booster should slide off their upper hooks at the same time. As long as the burn time for the strap on is shorter that the center, one should be good to go. I was going to use a E16-0 in the center with D12-3's in the boosters. The sustainer will have a F15-8. (I didn't want 1.5 seconds of flight with a F15-0 with the added drag of the boosters) Unlike this kit, I need to come up with support for between the booster as the hooks together will create a gap of about 1.25cm between the sustainer and the booster.


Tinker
 

snrkl

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Apologies - I was unclear in my post: I was more referring to a design for the support rail that you could reference.
 

Scott_650

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Tinker - that's not lazy, that's just a good use of available resources! Please post some pictures of the build, sounds fascinating. And like a good reason for buying Mammoth number three. Though I'm not sure where I'd launch such a beast - that combo would over fly my little local launch site.
 

les

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Would you please post your Rocksim file to get a better look at the simulation?
Thanks
 

vic20owner

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So I have been intrigued with the clustered and staged design posited by TVM in his "Model Rocket Design and Construction" book: it suggests combining booster pods and a booster stage for a more visually spectacular staging for LPR:

View attachment 325043View attachment 325044

So I duly designed and simulated a design of this elk in RockSim:
View attachment 325045

Hilarity ensued - I am not sure that RocSim is properly ejecting the pods at booster burn out... When you watch the flight profile, the booster actually overtakes the sustainer...

View attachment 325048

Inspection on the chart shows the booster beating the altitude of the sustainer by a good 60m!!

View attachment 325047
I don't see any thrust from the sustainer? If the sustainer is actually just a dart (no motor), then it must be lighter than the boosters, or larger in diameter, or both.

Just my guess!
 

snrkl

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I don't see any thrust from the sustainer? If the sustainer is actually just a dart (no motor), then it must be lighter than the boosters, or larger in diameter, or both.

Just my guess!
Ok, so here is where we fall prey to the limitations of different simulation software:

OpenRocket will let you graph lots of things simultaneously but doesn't yet support pods. (Hint hint if anyone wants to clue me in on how to get beta access to the next version!)

Rocksim supports pods but is limited to 4 (or maybe 5) items on a single chart, so in that screenshot, from memory, I couldn't get the booster and sustainer velocity on the same graph without dropping something else.

I confirmed with several charts with different items graphed that:
The pods are dropping off (weight drops the right amount on booster separation)

All the motors are firing as they're supposed to.

One thing that I may have stumbled onto:

I ticked the option for rocksim to actually generate a 3D model of my actual rocket for the flight animation, and I noticed that the model kept the booster pods on the actual booster on separation, which promptly inverted (CG behind CP once separated) and that was the configuration that out flew the booster.

I'm wondering if the CG/CP calculations in rocksim aren't taking the dropped pods into consideration, ie: keeping them on the booster model post separation and just adjusting the weight and CG/CP of the sustainer?
 

snrkl

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That also being said - I've posted to the apogee website support form and emailed them directly asking how to raise support issues with rocksim - I've had no response to either hail yet...
 

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