Help with my long model rocket please?

Question

Im building a model rocket that is 6 feet long, diameter is 1 1/2inches, it weighs around 2lbs, I only need the rocket to fly 100feet without a nose cone. My father said I cant do it and If I can do it, hes giving me $1000. Im thinking of putting on 3 fins and 3 Dsize engines, one inbetween each fin. Im just wondering about the open tube, what affect will this have on the center of pressure? are 3 D engines overkill? I dont need it to go that high but I wont get the money if it cant take off. Thanx guys.

it is made out of a thick cardboard tube. Yes, its far too heavy and far too long, thats why Im getting $1000 to make it fly.

Answer 1

The open tube will have only a small effect on the center of pressure. The length of the tube will be critical, though, so be sure to make those fins BIG.

I would also use a guiding rod for launch, otherwise you run the risk of having the engines ignite at different times, causing an imbalance at the point of launch.

Answer 2

D is way over kill.... Start small and work up like a B4-3

EDIT: 2 pounds!!! seems a little heavy you should be using carboard tubes. What materials are you using??

Your father has a point, the lack of a nose cone will severly degrade performance by increased drag. Center of pressure will mainly be a function of fin size and position. However you will be designing a model without a nose cone so you should have a stable model at launch. (Its not like having the nose cone fall off during flight causing instability)

Rocket stability is a function of where the CP is in relation to the CG (center of gravity) Stable when the CP is Behind the CG

You can perform a simple test to determine if a home designed modle will be stable. Do a swing test....

Tie a string (4 to 5 feet long or longer) to the modle at its balance point (the CG with a flight payload, engine parachute ect) adjust the string position on the CG so the model is horizontal when held by the string. You then use the string to swing the model in a circle. This will give you some air flow and generate a CP ... If the model is unstable it will swing around tail first (Fails - do not launch!!!!) A stable model will fly nose first and a neutral focket will fly at an angle as there is not enought moment generated between the CP and CG to correct the flight.


To get a stable rocket Larger fins are placed at the rear to increase the CP and move it behind the CG

EDIT:

Keep the engines inline mounted within the tube to prevent asymetrical thrust. Avoid the use of clustered engines to avoid ignition delay issues between engines

Answer 3

I think you have to weight the nose and get it nose-heavy to help stabilize it before you build enough speed for the fins to work.

Its easier to balance a vertical baseball bat in your hand with the barrel end up, not the grip end.

I predict the rocket falls over sideways before it finally stabilizes, and puts some neighbor's Pontiac out of it's misery....

It will be hard to get multiple engines lit simultaneously.

If you are a real stud, you could do a Bob Goddard-type passive 2-axis gimbal with a pendulum weight in the nose, with the gimbals tied to the fins to actively control your rocket. Your Dad should cough up $5k for that feat.