Not as simple as it would seem

In this post I talked a little bit about the task for the Team America Rocket Challenge for 2005:

Instead of a target altitude, the kids will have to design a rocket for a target duration. In other words, the rocket from lift-off to touching ground again will be timed, and that's the parameter they're trying to zero in on.

Stephen remarked:

It would seem to me that if was total time from take-off to landing a big chute would have big advantage.

Absolutely correct, except that instead of total time the goal is a targeted time. The National Association of Rocketry holds many types of competitions, including duration events, and when you're going for maximum duration a great big chute is indeed the way to go. Imagine a 30" diameter parachute made from that ultra-thin plastic you get from the dry cleaners, marked with a few spots of hi-lighter for visibility in the air, and folded down to fit inside a 1/2" diameter tube. It's possible - with care - and I've seen many of these drift away over the horizon. They don't always come down either, because they'll catch a thermal column of rising air. I lost a rocket once under chute that was still going up as it drifted out of sight.

But for the Challenge, these won't be featherweight rockets. They'll be carrying an egg or two (26 - 52 grams total) that'll have to be brought back safely to earth. They'll also be carrying the expended motor casings because the rules allow for no ejection of mass for a gentler descent.

Like any other engineering task, it's all about finding a trade-off that works best under a variety of conditions. Let's consider two hypothetical rocket designs, both of which will perform as required. For the sake of argument, lets say the target time is 45 seconds (I have no idea what the actual target time will be, it hasn't been announced yet).

Rocket 1 is a fat draggy brute of a rocket. It's not going to go fast, it's not going to go high, and once the motors stop thrusting it's going to slow down pretty quickly. It uses a big chute to gently bring it down. Using the right motors, this rocket could easily meet the 45 second target from liftoff to touchdown.

Rocket 2 is thinner and more aerodynamic. It'll move faster and it'll go much higher than Rocket 1 on the same engines, but there’s nothing that says they can’t use smaller engines. It's also a lighter-weight rocket all around, so it doesn't need as big a chute to descend at a safe (for the egg) speed. This design could also be very successful for the challenge.

Under perfect conditions, both of these rockets will do the job equally well. So let's assume reality here, because things are never perfect. Cold dry air is less dense than warm moist air, so drag will be affected depending on local conditions. Because of the differences in design, they'll be affected in varying degrees, especially considering surface drag. The efficiency of the chutes will also be affected, and if they're skirting the edge of safe descent rates, a chilly day might cause the rocket to drop faster than expected, resulting in broken eggs.

If it's windy, there's a whole new set of variables to contend with. Weathercocking is the effect where a rocket veers away from vertical because of the breeze. Weathervanes that point out wind direction take advantage of this effect, and you've felt it yourself if you've ever stuck your hand out of a car window as it moves down the road. Fat rockets feel the wind more, because there's more area being affected by the breeze. Slow rockets are affected more, because the fins take longer to self-correct for weathercocking. But Rocket 2, the thin speed-demon design, while affected less (theoretically) by weathercocking, will be under chute longer in the wind, when it's not going fast and streaky and sleek. More time aloft equals more drift time. More drift time means longer in the air, because the rocket is going sideways instead of downwards.

Cold dry air, calm day.
Cold damp air, calm day.
Cold dry air, windy day.
Cold damp air, windy day.
Warm dry air, calm day... and so on. Each in infinite variety too.

The draft rules mention bonus points for a dual-egg payload and making your design two stages. Although it adds complexity and thus more chance of failure, both should be planned for. To me, you don’t turn down free points, but you’d better have a reliable staging design.

The answer is simple. The winner is going to design a rocket that works best for them. If past contests are any indication, designs will be broken down into two basic types: designs that are conventional, and designs that are ingenious. The ingenious ones are the most fun, because I've seen some real Frankenstein engineering. Sometimes it works, sometimes it doesn't. I also think a key for this event is going to be to learn to adjust parachute sizes up or down according to local conditions.

Two ideas have already jumped out in my mind as potential winners, both unconventional but not original (there’s very little new under the sun). First, a radio-controlled glider carried to altitude by a rocket, which releases and is flown back to the ground. With a nicely trimmed design and a good pilot, timing the landing could prove to be very accurate and gentle on the eggs.

My second idea – and the one I like – is to build a UFO style design. Something saucer, cone or pyramid shaped. These designs have incredible drag, and go low and slow as long as the motor keeps thrusting, and stopping almost immediately when the motor burns out. A simple timer circuit or magnetic field apogee detector would throw the parachute out at the proper time. Plus by keeping the altitude down below 300 feet, wind drift would be minimized and more uniform descent times should be possible.

I may have to build one of these, just to see if my ideas work.

Posted by Ted at April 19, 2004 05:50 AM
Category: Rocketry