Two stage limitations...

[pr_rocket04]

Right now I'm being faced with some big time issues concerning motor weight in the booster of my two stage project Armageddon. I'm trying to add larger motors to my booster (this thing is going to have so many motors it's not even funny), but the rearward weight is taking away all stability. I have to add like 40lbs to the nose tip in Rocksim to get it anywhere close to where I'd like to be, so obviously this isn't an option.

What do you guys recommend to correct my improper gravity center without just adding nose weight? I know there is a combination of things I can do, but I want to know which methods are most efficiently used together to gain stability. Thanks, couldn't build this without you guys!

 

[thomasrau]

You could lengthen the rocket.

 

[gerbs4me]

I agree with thomasrau

 

[pr_rocket04]

That's what I will do. Thanks.

 

[powderburner]

I don't have a clue what a 'project Armageddon' looks like but I will take a shot at a suggestion:

You could try adding some fin area on the booster, made more effective by first adding some spruce struts (maybe 1/4 x 1/4 inch, a fairly easy to find size) projecting aft from the booster, and then putting the fins on the far aft end of the struts?

Yeah, I know this approach is butt-ugly

 

[pr_rocket04]

Here is a Rocsim file just so you can get an idea of the design, however note the end design will have external motors on the sustainer and booster section. I didn't get to produce a sim file with my latest motor configuration yet so this is all I have so far. I have changed my mind on external motors quite a few times, but the latest configuration I have come up with is: a 75mm central sustainer motor with 8 external 38mm's, and a 150mm booster motor with 4 external 54mm's (I will probably end up using a 98mm, but the 150mm tube will just give me the option to go bigger). The booster will have a 7.5" diameter airframe, not a 5.5" as in the file. Hopefully I can play around with this the next few days and come up with a final design file to post instead of this one.

 

[GL-P]

Here is a Rocsim file just so you can get an idea of the design, however note the end design will have external motors on the sustainer and booster section. I didn't get to produce a sim file with my latest motor configuration yet so this is all I have so far. I have changed my mind on external motors quite a few times, but the latest configuration I have come up with is: a 75mm central sustainer motor with 8 external 38mm's, and a 150mm booster motor with 4 external 54mm's (I will probably end up using a 98mm, but the 150mm tube will just give me the option to go bigger). The booster will have a 7.5" diameter airframe, not a 5.5" as in the file. Hopefully I can play around with this the next few days and come up with a final design file to post instead of this one.

HOLY COW!!!!

 

[pr_rocket04]

14 motors isn't over doing it now, is it? Hehe.

 

[GL-P]

Nah!

What will the expected altitude be when it's full of motors?

 

[pr_rocket04]

An old simulation on much much less power simmed 35,000 feet. I haven't got anything created yet that comes close to simming the real design, so I can't really make an accurate estimate just yet. There are so many configurations the motors can be ignited in I'm not sure yet whether to go for a slower launch or maximum altitude.

 

[GL-P]

WOW!!!

I'd personally got for a slower but higher launch so you can actually see the thing!!!

Is it going to be dual deploy? R U going to have an RF locator (methinks you might want one)

Sorry to be the Spanish Inquisition!!!
D)

 

[pr_rocket04]

It will have dual gps transmitters which will transmit a live signal to my laptop and show me the booster and sustainer position on a map. It will also house a DV camcorder.

What I was basically infering about the slower launch deal is I don't know how many motors to ignite on the pad, I could get a spectacular effect by lighting all four 54mm's and the 150mm(or 98mm) on the pad, but I could easily reach a higher altitude if I air started the 54mm's after the core motor.

 

[GL-P]

I think it would be a better (ie simpler) idea to ignite everything at once. When you add timers into the mix you start to increase the complexity greatly. Then you can get into some serious problems.

Disregard my previous opinion on motor combo..

 

[pr_rocket04]

Even if everything in the booster was ignited on the pad, there are still 9 more motors or 3 more stages of ignition, so the complexity still exists. I don't feel it's too much of a task for the air starts, but 5 motors roaring off the ground sure is a lot more fun now isn't it? I think thats what I'll end up doing to get things moving. I will then ignite the 75mm M at booster seperation, followed later by four 38mm's, then finally the last set of 38mm's.

 

[pr_rocket04]

Alright my latest sim pushes 80,000 feet easily. I didn't just stick the biggest motors I could find in it either. I played with ignition timing somewhat to find the upper limits this thing is capable of. I haven't tinkered with mass yet but I'll get to that soon. I tried posting my sim file but apparently it's too big.

Here is my email: [email protected] I will be glad to send the Rocsim file to anyone who is curious to see this monster.

Like I said, I have changed my mind with engine configurations so many times, and today I realized if I used 38mm's the design would not work out so great. You see the transition has to be slotted to slide past the outboards, 54mm's will allow there to be more larger slots instead of many very small ones. These give some support on top of the shoulder so strength here is somewhat important. A larger impulse will also be cheaper in a 54mm. 75mm's is also the only way to go for the outboards on the booster, as this thing needs serious impulse to get moving!

Once I order a few more parts you guys should start seeing some pics appear, hope you guys like the design.

 

[GL-P]

Holy jeez!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[r1dermon]

geez man, whats the bill for 14 motors? lol. to correct an unstable rocket, you can also add fins to the nose. as long as your launch rail is long enough, it works pretty good, in my experience with it. im pseudo working on a 2 stage estes wizard which would definately need a ton of nose weight, so im putting on little plastic fins, 4 in total. i did it before on a 24mm scratch built that i had. i made it too short. the nose fins made it a lot better after it reached enough speed. it still weathercocked hard after it cleared the rail because it was a small pad. with a HPR bird going 80,000' i have no idea where i'd even start on stabalizing. maybe create longer sections. like the booster section could house the main motors and then past the motors there could be just empty space. and then the second stage, sortof like

|___------|___-------|___---------------------------------------------NC

where the underbar is the motor and the dotted lines are space. as opposed to
|___--|___--|___------------------------------------------------------NC
which would be a lot less stable without nose weight.

 

[xenon]

Originally posted by r1dermon
geez man, whats the bill for 14 motors? lol. to correct an unstable rocket, you can also add fins to the nose. as long as your launch rail is long enough, it works pretty good, in my experience with it. im pseudo working on a 2 stage estes wizard which would definately need a ton of nose weight, so im putting on little plastic fins, 4 in total. i did it before on a 24mm scratch built that i had. i made it too short. the nose fins made it a lot better after it reached enough speed. it still weathercocked hard after it cleared the rail because it was a small pad. with a HPR bird going 80,000' i have no idea where i'd even start on stabalizing. maybe create longer sections. like the booster section could house the main motors and then past the motors there could be just empty space. and then the second stage, sortof like

|___------|___-------|___---------------------------------------------NC

where the underbar is the motor and the dotted lines are space. as opposed to
|___--|___--|___------------------------------------------------------NC
which would be a lot less stable without nose weight.

Ummm.........could you explain exactly how the heck fins on the nose make it more stable.

From my experiance the fins on the nose would bring the CP forward making it less stable. That is unless you make your fins out of Plutonium or something like that. In that case, the extra weight would make it more stable.


pr_rocket04 - this thing sounds crazy, 80,000 feet geez thats high. Very Cool!

 

[daveyfire]

Originally posted by r1dermon
to correct an unstable rocket, you can also add fins to the nose. as long as your launch rail is long enough, it works pretty good, in my experience with it.

Um, that scares me.

Remember the little alphabetical rule?

G before P?

That means your CG needs to be *ABOVE* your CP for your rocket to be stable.

Fins up front = CP up front.

Why do you think that AMRAAMs always do that little spiral on the way up? Or Sidewinders? Or Phoenixes? Why do scale models of missiles always require nose weight?

Scary stuff. Keep those rules in line, especially in HPR!


BTW, pr_rocket04, you gonna bring that out to BALLS to fly it? Sure as heck would be a cool flight to watch! Good luck with it

 

[r1dermon]

it's worked for me in the past marginally. and as long as you keep em small. im not saying that it will definately work. im just saying that it HAS worked alright for me in the past.

 

[pr_rocket04]

Originally posted by daveyfire
BTW, pr_rocket04, you gonna bring that out to BALLS to fly it? Sure as heck would be a cool flight to watch!

I don't know where it will end up flying right now, just maybe it will end up being there. Currently my big concern is money and time, this project has been moving at a much slower rate than I planned.