Motor size questions

[Pyra]

Hello all,
I'll start by introducing myself as I am new to this forum.

I started launching rockets when I was 13yo and continued until around the age of 16. I have never owned a kit and never really want one, the whole idea of building from scratch was 75% of the appeal to me. during these three years I had a lot of failures and a couple of wonderful successes.
I have only used estes engines (mainly D)

Which brings us to my question... is there an ideal minimum weight for the rocket for each type of engine? I currently have a rocket designed (using openrocket) which is 140g, would it be ok to shove a D12 in a rocket so light?

[karlbaum]

I don't know that there is a minimum total weight but you must keep the center of gravity forward of the center of pressure as you may well know. Also the rocket has to be strong enought to withstand the flight speed. My guess is that you are probobly fine with three fins and a nose cone.

[scsager]

A very popular D powered rocket is the Estes Goblin which weighs about 2 ounces or 57 grams. I'm sure you could build something even lighter.... keeping in mind that stability and "shreadability" might start to be a factor.

[Gnomad]

There is a minimum weight for any given motor below which will show no increase in altitude. However, in practice, it is generally difficult to build light enough to reach this weight.

It is also worth noting that when determining the optimum minimal weight, you must consider the thrust curve and not just the total impulse. The optimal weight for a motor with a short, high thrust burn will be greater than that for a longer burn at lower thrust.

You should be able to demonstrate this in OpenRocket. Override the overall weight down in increments of 10 grams and plot the simulated apogee.

[Pyra]

Thanks for the input guys, I'll be posting more about this rocket closer to the build time so I can pick your brains further

Thanks again.

[luke strawwalker]

Hello all,
I'll start by introducing myself as I am new to this forum.

I started launching rockets when I was 13yo and continued until around the age of 16. I have never owned a kit and never really want one, the whole idea of building from scratch was 75% of the appeal to me. during these three years I had a lot of failures and a couple of wonderful successes.
I have only used estes engines (mainly D)

Which brings us to my question... is there an ideal minimum weight for the rocket for each type of engine? I currently have a rocket designed (using openrocket) which is 140g, would it be ok to shove a D12 in a rocket so light?

There is an IDEAL weight-- but it varies considerably depending on the design. That's where computer simulation programs can help you zero in on that particular weight for the particular airframe design your contemplating.

A rocket CAN be too light-- so light that it "flies like a feather", and as we all know, you can't throw a feather very far! (Which is essentially what a rocket engine does-- "throw" the model into the air during powered flight, building acceleration to the maximum velocity attained, which then is exchanged for altitude during deceleration.)

Remember that the lighter a rocket is, the faster it will accelerate (F=MA) and that drag squares as the velocity doubles, meaning it will experience SIGNIFICANTLY higher drag at higher speeds. Of course too little velocity due to being heavy reduces the maximum altitude as well, since the rocket doesn't have as much stored momentum to exchange for altitude during the coast phase.

Good luck and hope this helps! OL JR

[spacecadet]

A rocket CAN be too light-- so light that it "flies like a feather", and as we all know, you can't throw a feather very far!

! OL JR

I can't agree with this proposition.
When you throw a feather you impart force for a very short period of time. A better analogy would be to run with the feather until you had accelerated it to a velocity equivalent to that it would acquire under thrust from a given rocket motor before releasing it. I doubt if you could do it, but if you could, I think you'd find the feather would fly a long, long way. Assuming it survived structurally, but that's a separate matter.
The lighter the rocket, the better the mass ratio and the higher it goes. Simple.
You might want it heavy enough not to lose, though. I've had one or two go that way. A 1g feather on a C6 (assuming no other mass) would accelerate to about 250 m/sec and go to about 3km by my calculations.

[RandyT0001]

I can't agree with this proposition.

The lighter the rocket, the better the mass ratio and the higher it goes. Simple.

You might want it heavy enough not to lose, though. I've had one or two go that way. A 1g feather on a C6 (assuming no other mass) would accelerate to about 250 m/sec and go to about 3km by my calculations.

Did you take into account the aerodynamic forces? The aerodynamic forces can overwhelm the momentum (mass times velocity) forces making the rocket fly lower than expected. This is the premise behind 'drag racing' rockets where points are given for first off the pad, lowest altitude, and last to land. For a rocket design and motor combination there is an optimal mass where the rocket achieves it's maximum attainable altitude.
Handbook Model Rocketry 6th ed, p 131
Model Rocket Design and Construction 3rd ed, p52, p320

[Gnomad]

I can't agree with this proposition.
When you throw a feather you impart force for a very short period of time. A better analogy would be to run with the feather until you had accelerated it to a velocity equivalent to that it would acquire under thrust from a given rocket motor before releasing it. I doubt if you could do it, but if you could, I think you'd find the feather would fly a long, long way. Assuming it survived structurally, but that's a separate matter.
The lighter the rocket, the better the mass ratio and the higher it goes. Simple.
You might want it heavy enough not to lose, though. I've had one or two go that way. A 1g feather on a C6 (assuming no other mass) would accelerate to about 250 m/sec and go to about 3km by my calculations.

Aah, but think about this for a minute. Your description is completely apt for the relatively long burning, low-impulse, Estes motors most people are familiar with. However, I have some much higher impulse sugar core-burners that burn E-F class in in less than 1/2 second. These motors are better served by a rocket with some mass to keep it stable at the high speeds it must attain early in flight, the speed it needs to coast cleanly to apogee.

The correct answer, as the case most often is, "it depends." ;-)

-g.

[edwinshap1]

woo, jump in time. i modified an estes monarch (version one with the swept back fins). i lengthened the rocket and added a little nose weight to move the CG forward, which also increased the predicted altitude.

In a world with no air drag, yes, two objects of equal mass will achieve the same speed and decelerate at the same rate, but here on earth , we have to consider drag forces, which, on a heavier object means it takes more to get it moving, but also moves it will take more air drag to slow it down.

[Tronman]

Didn't Estes sell lead nose cone weights in the 60's? I vaguely remember seeing them in their catalog.

[Rex R]

yes, 0.12oz each. they also sold a 'payload' (1oz).
rex

[Tronman]

yes, 0.12oz each. they also sold a 'payload' (1oz).
rex

Another old memory jogged back to the front!

[Rex R]

well having the 1974 catalog right at hand helps
rex

[MarkII]

The weights that Estes used to sell had nothing to do with providing the rocket with optimal mass, and everything to do with improving the stability ratio. In a few instances, bits that would be carved off of a lead weight were used to improve the glide balance for rocket-boosted gliders. A rocket that is unstable won't fly very high, no matter what its weight.

To my mind, spacecadet's position makes more sense. A lighter rocket will fly better than a heavy rocket on the same motor. The rocket designers for NASA and other agencies and private corporations work very hard to pare as much weight as possible from a design in order to improve its performance. Rockets (boosters) that are intended to place objects in orbit have to be built as light as possible. But in the case of very clean, streamlined rocket designs, adding a small amount of weight to the nose or forward payload section can help it achieve a higher apogee because it increases the coast time following motor burn out in my experience. The key factors here are streamlined design and small amount of added weight. I don't think that adding a bit more nose weight will help a rocket that has a lot of drag-inducing surface treatments (often the case in futuristic designs, for example) or a rather draggy profile.

[dpower]

yes, 0.12oz each. they also sold a 'payload' (1oz).
rex

Was that the NCW-1, or NCW-1A? Extra points for knowng the difference from memory.

[Rex R]

durned if I know , twas listed as pl-1
rex