F,G Motors and Rocket Weight

[ryrego]

I am new to mid-power engines. I have never launched on a E, F, or G. Now I have some big engines and wondering what kind of weight these guys can handle.

Roger/Carl - about what is the weight of the rockets that you launch on your F/G motors? I think I have seen Akavish, Upscale Alpha, and Rubicon. I noticed Rubicon was pretty 'light'. Is there another/better way of estimating what a motor can handle lifting (or is it just guess work)?

Last question: what kind of motor mounts do you use? Will the typical cardboard tube and engine block with glue work?

THANKS!

[jadebox]

Here's a trick to estimate the maximum weight of a rocket for a specific motor. Take the average impulse of the motor (in Newtons) and divide it by 20. This gives you a rough value in pounds of the heaviest rocket you'll want to fly on that motor. For eample, given an Aerotech G38 motor ... 38/20 = 1.9. So, a G38 is good for up to about a two-pound rocket. This is just a general rule-of-thumb, but it's easy.

The formula generally works because you have to divide Newtons by 4.45 to convert to pounds. And you want the thrust in pounds to be about 4 to 5 times the weight of the rocket - 4.45 * 4 is about 20. Some people divide by 25, but I think 20 is a better value because the peak thrust of the motor is often much greater than the average thrust. This gets the rocket moving safely so that an average 4:1 thrust ratio is okay.

If the rocket is close to the calculated maximum weight or is very draggy, use the shortest delay available for the motor. Otherwise use the medium delay. Only, use the long delay for minimum-diameter, high-performance rockets (that you probably aren't going to launch on our field!).

Construction of mid-power rockets usually isn't much different from model rockets. About the only real difference is that you'll want to switch to using basswood or thin plywood for centering rings and you'll use stronger body tubes. I usually use epoxy to build mid-power rockets, but the reality is that wood glue is strong enough. For tubes, I usually use the ones from Loc Precision. They are heavier than typical model rocket tubes, but not too heavy or too expensive.

I'd recommend a kit for your first mid-power Rocket. The Aerotech kits are really easy to build (easier than most model rocket kits) and fly well. But, the techniques you use to build them aren't the same as you'll use for kits from other vendors or ones you build from scratch.

Warning, self-serving plug ... we have a couple of the Mercury RDR 1 kits in stock at JonRocket.com. It's a really nice mid-power kit that is built using standard techniques. We're having a 20% off sale right now (coupon code RELAUNCH), so it's priced right. You'll want to fly it on F motors in our field. A G would take it too high.

Other mid-power kits you might consider are the ones from Loc Precision. A larger one like the Iris would be good if you want to fly on a G motor in our field.

-- Roger


Other

[ryrego]

Roger - does that formula give you the targeted weight with or without the motor installed?

[Delta-IV]

Roger - does that formula give you the targeted weight with or without the motor installed?

That formula should include the motor as well, since the thrust/weight is being applied to the whole rocket in flight.

Roger has a very good formula there....I forgot about that when I tried to launch an egg with my OBV using a E9-4. Rocket ended up in one of the trees on the north end of the field...lasted over a year up there. That egg must have stunk pretty badly.